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1 CONJUGATED POLYMER ELECTROCHROM IC AND LIGHT-EMITTING DEVICES By AUBREY L. DYER A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLOR IDA IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY UNIVERSITY OF FLORIDA 2007
2 2007 Aubrey L. Dyer
3 To Nathan
4 ACKNOWLEDGMENTS I can honestly say that this process has been one of the most difficult that I have yet to go through. Its not that the Ph.D. process is inhere ntly hard, just the emo tional toll that conducting research while trying to obtain a degree, and remain a sane person, takes on you. Theres the good days, the bad days, the times of self-doubt, the considerable amount of stress you put on yourself, and the challenges of working so cl osely with other people who, while going through the same process, are so different from yourself. I feel that to successfu lly get through this with your spirit still intact, you need strong support from others in your life. I am happy to say that there have been many people along the way who have fulfilled that role and I would like to say thanks. The one person who has been there through it all and never once let me give in and give up has been Nathan Dyer, my husband. He underst ood how important this degree is and, while not able to relate to what I was going through, stood by me and did everything he could to show support, understanding, and most of all, patience. Another pers on who has been there and who I could not have done this without is my advisor, John Reynolds. His ability to relate with every single person in the group on a personal level is on e of the things that sets him apart from most advisors. He not only keeps the group running by securing funding and keeping the research we do relevant, but also ensures we, as individuals, are making progress in our research, our professional lives and our personal lives. He is the group cheerlead er, and his level of enthusiasm for what he does and we do is infecti ous and its what makes this group great to work in. I would like to thank the many people who Ive worked with on various projects over the years: Christophe Grenier, Ben Reeves, a nd Bob Brookins. Thanks for handing over your babies to me to tinker with in the lab. Id also like to thank Dr. Tanner for the optics
5 discussions, Evan Donoghue and Ken Graham for pu tting the time into designing and fabricating the black box. Special thanks to Nate Heston for the hard work and effort in keeping the glovebox in such good shape and the evaporator r unning for me. A final thanks to Evan, and Eric Shen for volunteering their expertise at the 11th hour in order to help me make sure I get this done. Thanks Evan for the AFM measurements, and Eric for the distillation. You guys were there when I really needed help. Also, thanks to my friend Jack for the early morning coffee breaks. Over the past five years, there have been a large number of people come and go from the group and each individual has leant their persona lity to the dynamics of the Reynolds group and the Polymer Floor in general. I would like to tha nk those who have been there for me, as either a shoulder to cry on, an ear to list en, or just to make me laugh. These people include the MCCL orphans (Nate Heston, and Ece Unur), Emilie Ga land, Sophie Bernard, Maria Nikolou, Cheryl Googins, Genay Jones, and Gena Borrero. In addition, Id like to thank my friends Heshan Grasshopper Illangkoon, Dr. Mike Bowen, and Susa n Bongiolatti. Looking back, I still cant believe some of the things weve been a part of on this campus. It was fun and I could honestly say, a valuable eye-opening experience. Id also like to thank the many people who Ive encountered over the past five years (in the re search lab and across campus) who have, in some way or another, taught me valuable lessons. So me have taught me how to be a good person and a great friend, others have taught me patien ce and understanding, probabl y some of the most valuable things Ive received from this educa tion and of which Im still learning. Finally, Id like to thank my family: my mother, Sheila Dedrickson, my sister, Stevie, and my brother, Shaun.
6 TABLE OF CONTENTS page ACKNOWLEDGMENTS...............................................................................................................4 LIST OF FIGURES................................................................................................................ .........8 ABSTRACT....................................................................................................................... ............11 CHAPTER 1 DOPED CONJUGATED POLYMERS IN DISPLAY DEVICES........................................15 Introduction................................................................................................................... ..........15 Electrochromism................................................................................................................ .....17 Electrochromic Devices......................................................................................................... .22 Light-Emitting Devices of Doped Polymers..........................................................................26 Dual-Purpose Devices........................................................................................................... .32 2 REFLECTIVE ELECTROCHROMIC DEVICES OF DISUBSTITUTED POLY(PRODOTS) AS VARIABL E OPTICAL ATTENUATORS......................................36 Reflective Device Construction..............................................................................................36 Unsymmetrical Switching......................................................................................................39 Conductive Front............................................................................................................... .....44 Reflective ECDs as Variable Optical Attenuators..................................................................50 Conclusions.................................................................................................................... .........58 3 ELECTROCHROMIC DISPLAYS OF MEH-PPV AND CARBAZOLECONTAINING COPOLYMERS...........................................................................................60 Electrochromism of MEH-PPV..............................................................................................62 Electrochromism of Cbz2-Fl2................................................................................................65 Electrochromism of Cbz2-Ph3...............................................................................................68 Reflective Electrochromic Displays.......................................................................................70 Conclusions.................................................................................................................... .........78 4 DUAL ELECTROCHROMIC/ELECTROLUMINESCENT DISPLAYS............................80 Polymer LEDs................................................................................................................... .....83 Polymer LECs................................................................................................................... ......89 Dual EC/EL..................................................................................................................... .......99 Overview and Future Directions...........................................................................................104 5 INSTRUMENTATION AND EXPERIMENTAL TECHNIQUES.....................................107 Chemicals and Materials.......................................................................................................107
7 Device Construction............................................................................................................ .109 Electrochromic Displays...............................................................................................109 Polymer Light-Emitting Diodes....................................................................................111 Polymer Light-Emitting Electrochemical Cells............................................................111 Dual Electrochromic/Electroluminescent Devices........................................................112 Electrochemical Methods.....................................................................................................113 Electropolymerization...................................................................................................113 Polymer Electrochemistry.............................................................................................114 Optical Methods................................................................................................................ ....114 Spectroelectrochemistry................................................................................................114 Electroluminescence Measurements.............................................................................116 APPENDIX A POLYMER STRUCTURES.................................................................................................117 LIST OF REFERENCES.............................................................................................................117 BIOGRAPHICAL SKETCH.......................................................................................................129
8 LIST OF FIGURES Figure page 1-1. Geometric structure of a thiophene trimer showing the allowed el ectronic transitions....18 1-2. Spectroelectrochemical series for a film of poly(3,4-ethylenedioxythiophene) (PEDOT)........................................................................................................................ ....19 1-3. Schematic of a typical absorptive/tran smissive electrochromic polymer window............23 1-4. Schematic of a typical absorptive/refl ective electrochromic polymer display..................25 1-5. Schematic diagram of a PLED in forwar d bias showing charge injection, carrier transport, and recombination, with a focu s on the two possible methods for charge injection...................................................................................................................... ........27 1-6. Energy-level diagram, relative to vacuum, for the materials used in a typical PLED.......28 1-7. Schematic of a typical PLED.............................................................................................29 1-8. Schematic of PLEC device operation................................................................................31 2-1. Schematic of a reflective ECD and photographs of a PProDOT-(CH2OEtHx)2 reflective ECD when fully reduced and fully oxidized.....................................................37 2-2. Reflectance spectroelectrochemist ry of a 236 nm thick PProDOT-(CH2OEtHx)2 film in a reflective ECD............................................................................................................ .39 2-3. Reflectance spectroelectrochemist ry of a 750 nm thick PProDOT-(CH2OEtHx)2 film in a reflective ECD............................................................................................................ .40 2-4. Percent reflectance versus cell pot ential for a 450 nm thick PProDOT-(CH2OEtHx)2 film reflective ECD, and transmittance versus potential for a film of the same polymer and thickness on ITO-glass in 0.2 M TBAPF6/PC electrolyte............................41 2-5. Percent reflectance versus cell pot ential for a 181 nm thick PProDOT-(CH2OEtHx)2 film reflective ECD demonstrating the la ck of unsymmetrical switching in the thinner films.................................................................................................................. .....42 2-6. Reflectance spectroelectrochemistry for a 998 nm thick electropolymerized PProDOT-(CH2OEtHx)2 film in an ECD and transmittance spectroelectrochemistry for a film of the same thickne ss on ITO-glass in 0.2 M TBAPF6/PC................................43 2-7. Percent reflectance versus cell potential for reflective ECDs of electrochemically polymerized PProDOT-(CH2OEtHx)2 of thickness 111 nm, 538 nm, and 998 nm...........43
9 2-8. Percent reflectance versus cell potential for a 351 nm spray-cast film of PProDOTHx2 in a reflective ECD and percent transmittance versus potential for a film of the same thickness on ITO-glass in 0.2 M TBAPF6/PC..........................................................44 2-9. Conductive front model...................................................................................................454 2-10. Schematic of multilayer geometry used in analysis of penetrat ion depth of optical radiation through the polymer sample...............................................................................48 2-11. Optical attenuation at va rious applied potentials for a 750 nm thick film of PProDOT-(CH2OEtHx)2 in a reflective ECD as an EC-VOA at the wavelengths of 550, 1310, and 1550 nm.....................................................................................................55 2-12. Schematic of setup to measure reflective EC-VOA using fiber-optic spectrophotometer and photograph of actua l holder with fibe r-optics in place.................56 2-13. Optical attenuation across the wavelengt h range of 1.3 m and 1.55 m for a 450 nm thick film of PProDOT-(CH2OEtHx)2 in a reflective ECD as an EC-VOA measured using the fiber-optic setup..................................................................................................57 2-14. Optical attenuation at 1. 55 m and 550 nm of a 450 nm thick film of PProDOT(CH2OEtHx)2 in a reflective ECD as an EC-VOA............................................................58 3-1. Diagram of oxidation potentials (vs. SCE) for several electrochromic and electroluminescent polymers re lative to the level required for air st ability along with optical bandgap values.......................................................................................................61 3-2. Structures of polymers studied...........................................................................................62 3-3. Cyclic voltammograms of a drop-cast film of MEH-PPV on ITO/glass at a scan rate of 30 mV/s switched in 0.2 M LiOTf/ACN for 50 cycles.................................................63 3.4. Spectroelectrochemistry of a drop-cast film of MEH-PPV on ITO/glass switched in 0.2 M LiOTf/ACN and photographs of the same film held at potentials indicated...........64 3-5. Cyclic voltammograms of a spray-cast f ilm of Cbz2-Fl3 on ITO/glass switched at 30 mV/s in 0.2 M LiOTf/water for 50 scans...........................................................................66 3-6. Spectroelectrochemistry of spray-cast film of Cbz2-Fl2 on ITO/glass in 0.2 M LiOTf/water and photographs of the same film held at various potentials........................67 3-7. Cyclic voltammograms of a spray-cast film of Cbz2-Ph3 on ITO/glass at a scan rate of 30 mV/s in 0.2 M LiOTf/water for 5 scans...................................................................68 3-8. Spectroelectrochemistry of a spray-ca st film of Cbz2-Ph3 on ITO/glass in 0.2 M LiOTf/water.................................................................................................................... ...69
10 3-9. Schematic of reflective electrochromic di splays with porous white diffuse reflector and aluminum-coated porous membrane...........................................................................71 3-10. Reflectance spectroelectrochemi stry of spray-cast PProDOT-(CH2OEtHx)2 in an electrochromic display with a porous wh ite reflector and aluminum-coated porous membrane....................................................................................................................... ....73 3-11. Spectroelectrochemistry and photographs of MEH-PPV/PEO/LiOTf blend reflective ECD with porous white reflector and sp ectroelectrochemistry and photographs of MEH-PPV/PEO/LiOTf blend reflective EC D with aluminum-coated membrane............76 3-12. Spectroelectrochemistry of Cbz2-Fl2/L iOTf blend reflective ECD with a porous white reflector and aluminum-coated membrane..............................................................78 4-1. Schematic of measurement geometry be tween fiber-optic probe and PLED pixel...........85 4-2. Spectral irradiance of an ITO/PE DOT:PSS/70 nm MEH-PPV/Ca/Al PLED at different applied voltages. Inset shows a photograph of pixel at 9 V...............................84 4-3. Current density and luminance versus appl ied voltage for an average of 4 pixels of an ITO/PEDOT:PSS/70 nm MEH-PPV/Ca/Al PLED.......................................................89 4-4. Current density and luminance versus a pplied voltage for an ITO/blend/Al PLEC with an active layer of a blend of MEH-PPV :PEO:LiOTf in a weight ratio of 10:3:1......92 4-5. Current density and luminance versus appl ied voltage for an average of 3 pixels of an ITO/MEH-PPV blend/Al PLEC....................................................................................93 4-6. Spectral irradiance of an ITO/170 nm MEH-PPV:PEO:Li OTf blend/Al PLEC pixel at different applied voltages. Inset s hows photograph of pixel operated at -6 V..............94 4-7. Luminance and current density as a functi on of applied voltage for an ITO/blend/Al PLEC with an active layer of a blend of MEH-PPV:PEO:LiOTf cycled between 0V and -8 V for three cycles....................................................................................................95 4-8. Integrated spectral irradiance over the wavelength range of 400 and 800 nm for a LEC pixel operated for 13 continuous hours at -6V. The inset shows the spectral response in the first 2 minutes of operation.......................................................................96 4-9. Comparison of the J-V and L-V response of two LECs constructed with PEO dried to different extents........................................................................................................... ..99 4-10. Schematic of dual EC/EL devi ce and photograph of actual device.................................101 4-11. Photographs of a dual electrochromic and light-emitting device with MEH-PPV as the active material............................................................................................................103 4-12. Schematic of proposed lateral dual EC/EL device..........................................................106
11 LIST OF ABBREVIATIONS ACN: Acetonitrile. Eg: Bandgap energy. Cbz: Carbazole. CB: Conduction band. EC: Electrochromic. ECD: Electrochromic display. EL: Electroluminescent. Fl: Fluorene. ITO: Indium-doped tin oxide. LiOTf: Lithium trifluoromethanesulfonate. MEM: Microelectromechanical. OLED: Organic light-emitting display. Ph: Phenylene. PAc: Polyacetylene. PEDOT: Poly(3,4-ethylenedioxythiophene). PEO: Poly(ethylene oxide). PLED: Polymer light-emitting diode. PLEC: Polymer light-emitting electrochemical cell. PMMA: Poly(methylmethacrylate). PPV: Poly(para-phenylenevinylene). PProDOP: Poly(3,4-propylenedioxypyrrole). PProDOT: Poly(3,4-propylenedioxythiophene). PSS: Poly(styrenesulfonate).
12 PC: Propylene carbonate. SWNT: Single-walled carbon nanotubes. VB: Valence band. VOA: Variable optical attenuator.
13 Abstract of Dissertation Pres ented to the Graduate School of the University of Florida in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy CONJUGATED POLYMER ELECTROCHROM IC AND LIGHT-EMITTING DEVICES By Aubrey L. Dyer December 2007 Chair: John R. Reynolds Major: Chemistry Conjugated conducting polymers are among some of the most versatile materials to emerge in the past 20 years. They have been researched for applications that range from photovoltaics, to light-emitting diodes, electroc hromic windows, actuators, and field effect transistors. This work details the analytical characterization of -conjugated polymers to understand their redox and optical properties, alo ng with the development of several new types of devices that employ these polymers as the activ e materials for both variab le optical attenuators and as dual electrochromic/elect roluminescent displays. Within this work, a phenomenon that we term unsymmetrical switching has been unc overed for the first time in reflective electrochromic cells. The unsymmetrical switchi ng in these displays is explored and a model devised to illustrate that the presence of a conductive front pr opagating through the polymer film on electrochemical oxidation is the contribu ting cause for this observation. For these experiments, a variety of analytical techniques ha ve been utilized to characterize the devices and materials contained therein and include elec trochemistry, spectroelectrochemistry, and photometry. The first application utilizes the large near infrared electrochromic contrasts of some dioxythiophene-based polymers when incorporated in reflective electrochromic displays. For the
14 first time, it is demonstrated that these devices ca n be utilized as electroc hromic variable optical attenuators, important compone nts for the optical telecommuni cations industry, exhibiting optical attenuation values of 12 decibels while main taining optical loss values, in the off state, as low as 0.1 decibels. These devices offer the benefit of yielding m echanically flexible, miniaturized electrochromic variab le optical attenuators that opera te with a low drive voltage ( 1.2 volts). A new concept is also introduced for c onjugated conducting polymers as the active material in dual electrochromic/electroluminescent displays that are unique in that a single active polymer film is employed as both the emitter an d electrochrome. In using a soluble paraphenylene-vinylene-based polymer a device is developed that shows orange to blue electrochromism and orange-red electroluminescence from the same pixel. This type of device potentially fulfills a need in the display industry for a device that exhibits light emission in low ambient lighting while maintaini ng a high optical contrast, without sacrificing image quality, in directly lit situations. This devi ce also offers the possibility of a flexible, patterned display as desired in the consumer electronics industry.
15 CHAPTER 1 DOPED CONJUGATED POLYMERS IN DISPLAY DEVICES Introduction The field of conducting polymers can be date d to as far back as 1862 when Letheby described the production of a dark green film on an electrode when oxidizing aniline under acidic conditions.1 The most studied conducting polymer, polyactetylene (PAc), can even be dated to 1955 when Natta produced the polymer using the coordinati on catalyst system commonly used to polymerize ethylene and propylene.2 PAc was even found to behave as a semiconductor by various researchers in the 1960s on treatment with reagents such as boron trifluoride and ammonia, producing dramatic changes in conductivity.3-6 It was not until the 1970s when Shirakawa and coworkers improved the procedure for the synthesis of PAc,7, 8 combined with further work by Heeger and MacDiarmid,9-11 that the field of conducting polymers really emerged with PAc establis hed as the first conducting organic polymer. In their work, Heeger, MacDiarmid, and Shirak awa demonstrated that the conductivity of PAc films increased many orders of magnitude on exposure to halogen vapors.10 Other researchers have also demonstrated that reacting PAc w ith oxidants (e.g., I2, Fe(ClO4)3) or reductants (e.g., NaC10H8), forms highly conducting derivatives of PAc. 12, 13 These reactions are called doping, analogous to terminology used in semiconductor physics with p-doping an oxidative reaction, with inser tion of charge-balancing anions and n-doping a reduction with insertion of charge-balancing cations. The tw o most commonly used methods for chemical doping are exposing the polymer to a gas-phase dopa nt, such as iodine vapors, or a solution of the dopant, such as SbCl5, diluted in an appropriate solvent.14 The rate of doping and maximum doping level can be controlled by dopa nt concentration and exposure time.
16 While chemical doping has been instrumental in initial demonstrations of the theoretical and technological promise of PAc by allowing tunable conductivity of the polymer film from the semiconducting to the metallic regime, the applic ability of this method is lacking. In many cases, the chemical dopants are highly reactive and difficult to handle, not allowing for the utilization of these polymers outside the res earch laboratory. In 1979, Nigrey and coworkers demonstrated that PAc could be reversible por n-doped electrochemica lly, but the materials instability remained as an obstacle to commercial applications.15 The neutral polymer is not air stable and decomposes quite rapidl y and therefore must be handled in an inert atmosphere and is additionally difficult to pr ocess with melt processing resulting in degradation. It was not until that same year when Diaz et al., electrochemically polymerized pyrrole to produce free-standing, air-sta ble polymer films that had conductivities of 100 -1cm-1.16, 17 The method they used was a modification of that or iginally reported by Dall Olio in 1968 with the main difference being that Diaz et al, obtained continuous films that could be peeled off the platinum electrodes as opposed to a powdery, insoluble precipitate.18 Not only did this demonstrate the ability to electrochemically polym erize directly onto an electrode and reversibly switch the polymer between a neutral and oxidi zed state, but it introduced the field to polyheterocycles, which would become the most im portant class of conducting polymer to date. The impact of pyrrole systems stems from seve ral factors that include chemical and thermal stability of the polymers, the ease of preparation of polymer films, and th e ability to synthesize derivatives that allow for modifi cation of the electrical and ph ysical properties of the final polymer. This opened the door for other electrochemically active polymer systems to enter the field and demonstrate their applicability as the activ e materials in areas ranging from batteries,
17 capacitors,19 light-emitting displays,20 field-effect transistors,21 electrochromic displays,22 solar cells,23 and actuators.24 Conducting polymers hold much promise for these, and many other, applications as they allow tuning of physical and electronic propert ies through structural modification of the polymer backbone; a feature not available in any other material. This introduction will cover two applications of doped conjugated polymers (electrochromism and light-emission), the methods by which these physical processes occur and the types of devices in which they are employed. Electrochromism Electrochromics are materials that can indu ce a reversible change in the absorption, reflection, or transmission of optical radia tion when the material is involved in an electrochemical reductiv e or oxidative process.25 Electrochromism not only includes changes in the visible region, but also in the ultraviolet (UV) infrared (IR), and even the microwave regions of the spectrum.26-28 Some of the most commonly resear ched electrochromes are those based on transition metal oxides (e.g., tungsten oxide, WO3), 4,4-bipyridinium salts (e.g., methyl viologen), and polynuclear transition metal hexacyanometallates (e.g., Prussian Blue, [FeIIIFeII(CN)6]-).29-35 Over the past two decades, conjugated polymers have emerged as one of the most promising electrochromic materials available.22 This is due to the fact that they offer color tunability, faster switching, and improved pro cessability over the inorganic and molecular electrochromes mentioned. Electrochromism in conjugated conducting polym ers arises from the polymers bandgap and any midgap states created on doping. The optical bandgap is determined by the onset of the transition of the polymer in th e neutral, undoped state and is the electronic transition, as shown in Figure 1-1A, th at occurs with the minimum energy difference from the valence band to the conduction band.
18 Figure 1-1. Geometric structure of a thiophene trimer showing th e allowed electronic transitions. A) Neutral; B) Polar on; C) Bipolaron states. Note: The forbidden transitions are not shown for clarity. As the polymer is oxidized, electrons are re moved from the valence band and lower energy absorptions begin to emerge in the el ectronic spectra at the expense of the transition. This is due to the removal of the -electrons from the valence band, creating half-filled polaron levels that are symmetric about the bandgap center. Figure 1-1B shows the allowed electronic transitions for these new lower energy states. This newly formed radical cation is delocalized over a polymer segment and behaves as a polaron ic charge carrier. This oxidation induces a relaxation of the aromatic geometry to a quinoi d-like structure in the polymer chain and is accompanied by a charge balance by the electrolyte anions. On further oxidation, a dication is formed (with the cations coupled to one another) and is a bipolaronic charge carrier delocalized over the sa me polymer segment. As is shown in Figure 1-1C, only electronic transitions from the top of the valence band occur as the bipolaron levels are unoccupied. At this point, onl y lower energy absorptions are se en in the electronic spectra, and the is depleted. This can be seen in Figure 1-2 for the spectroelectrochemical series of poly(3,4-ethylenedioxythiophene) or PEDOT. The neutral polymer is dark blue as the bandgap
19 is in the visible region at 1.65 electron volts (eV). As the polymer is p-doped, an absorption emerges in the NIR and the color of the polymer begins to bleach, finally becoming a highly transmissive sky blue. Figure 1-2. Spectroelectrochemi cal series for a film of poly(3,4-ethylenedioxythiophene) (PEDOT). Dashed arrows show direc tion of spectral gr owth or recession. As mentioned previously, conducting polymers of fer the advantage of color tailorability in the neutral and doped states th rough synthetic methods. This can be seen when comparing polythiophene to PEDOT. Unsubstituted poly thiophene has a bandgap of 2.0-2.2 eV, switching from red in the neutral state to blue upon oxidati on. However, it is difficult to obtain uniform polymer films because the polymer overoxidizes at the potentials required for electropolymerization and side reactions can occur at the -position causing undesirable structural defects.36-39 To overcome these issues, research became focused towards substitution at the 3-position or the 3and 4-positions of th e thiophene ring yielding polymers with bandgaps that ranged from less than 1 eV to higher than the parent thiophene.
20 One of the most commercially successful pol ymers to date, PEDOT was produced with the goal of not only lowering the polymer bandgap, but also exhibit a lower oxidation potential and higher stability. PEDOT not only has a lower bandgap than the unsubstituted thiophene, it is also easily oxidized and highly st able in the doped state. This is due to the presence of the electron-donating oxygens adjacent to the th iophene ring. On oxidation, PEDOT becomes highly conducting and transmissive in the visi ble region. However, this polymer has the drawback of insolubility. To overcome this lim itation, the aqueous disper sion of the doped form of PEDOT with the polyelectrolyt e, poly(styrenesulfonate) (PSS), was produced. Cast films of PEDOT:PSS have greater than 75 % transmissivity in the visi ble region and greater than 120 S/cm film conductivity.40 Applications for this polymer ha ve been mostly for its use as a conductor for antistatic coatings, solid state capacitors, organic elec tronic devices, and even as a transparent electrode material.19, 41-44 Recent synthetic effort has been towards pr oducing polymers, especially those based on the alkylenedioxythiophenes, that are processabl e. Through the introduction of alkyl and alkoxy chains on the propylene bridge, a family of poly(3,4-propylenedi oxythiophenes) (PProDOTs) has been synthesized that is solubl e in common organic solvents such as toluene and chloroform.45-48 This allows uniform, thick films to be cast fr om solution using methods such as drop-casting, spincoating, spraycoating, and inkjet or sc reen printing. In addition to the improved processability, the PProDOTs have faster switch ing speeds and larger optical contrasts. The tetrahedral substitution pattern on the propylene bridge causes the alkyl and alkoxy groups to be positioned above and below the plane of the polymer chain. This allows the polymer chains to separate, opening up the film for more facile counter ion moveme nt and leading to higher doping levels. This also hinders -stacking, causing a decrease in the polymer conductivity.
21 The dioxythiophene family of conjugated polym ers are colored in th eir neutral state and switch to a highly transmissive state on oxidation and are th erefore cathodical ly coloring. Another class of polymers are those that are anod ically coloring, and high ly transmissive when neutral, switching to colored on oxidation.49-53 The N -substituted alkylenedioxypyrroles are such a family of polymers with the polymer bandgap shif ted into the ultraviole t region of the spectrum allowing for near-transparency in the neutral state. Substitut ion at the N-position introduces steric interactions that vary the degree of -overlap along the polymer backbone. This induces a twist in the polymer backbone that causes a decr ease in the effective c onjugation length leading to an increase in the bandgap.22, 54, 55 As previous calculations have shown,22 substitution with the bulky ethoxyethoxyethanol at the N-position in poly(3,4-pr opylenedioxypyrrole) (PProDOPN -Gly) causes a twist angle be tween the pyrrole rings of 71.6 whereas the unsubstituted PProDOP is fully planar. The unsubstituted PProDOP has a bandgap of 2.2 eV and is orange in the neutral state switching to light gray-blu e on oxidation, while PProDOPN -Gly has a bandgap of 3.4 eV and is colorless in the neutral state and blue-gray when oxidized. The electron -rich character of the pyrrole along with the presen ce of the alkylenedioxy bridge reduces both the monomer and polymer oxidation potentials. This allows for polymerization to occur under relatively mild conditions and the polymer to have an increased stability under ambient conditions in the doped conducting state with the family of PProDOPs having the lowest reported polymer oxidation potentials to date.50 The anodically coloring properties of the N -substituted PProDOPs along with the low polymer oxidation potential makes them complementary to the PProDOTs when both are incorporated into absorptive/transmissive electrochromic windows.53 Additionally, the
22 fast switching speeds, large optical contrasts (i n the visible and NIR), and high redox stability of the PProDOTs makes them ideal for absorptive/reflective electrochromic displays.56, 57 Electrochromic Devices Electrochromic devices are electrochemical cel ls in which a redox reaction occurs at a working electrode and counter elec trode by application of an elec tric field across the device. Typically, one or more electrochromic materials are deposited on both the working electrode and counter electrode and the electromagnetic radiat ion absorbed, transmitted, or reflected from the device is modulated.29, 58-60 Most commonly, electrochromic ma terials are deposited as films on both the working and counter electrodes with th e material on the counter electrode acting as a charge-balancing source for the redox reacti on and, in some cases, as a complementary electrochrome to that at the working electr ode. In dual polymer EC Ds, both materials are electrochromic polymers, while in hybrid devices, the material at the counter electrode can be any other class of electrochromic material and has included Prussian Blue, viologens, and WO3.61-66 An electrolyte layer is deposited between the two electrodes with the requirement that the material used have a high conductivity, transpar ency in the wavelength range measured, wide window of electrochemical stability, and low volati lity. Materials used in clude gel electrolytes, solid electrolytes, and ionic liquids.53, 67-71 The electrodes utilized in these devices are dependent on how the light is to be modulated with reflective metallic electr odes in the absorptive/reflective devices and transparent electrodes utilized in the absorptive/transmissive devices. The reflective electrodes can be metallized, porous membrane s that allow for ion transport through the membrane while exhibiting a high specular reflectance.56, 72-74 Metals used include gold, platinum, and aluminum. Transp arent electrodes include indium -doped tin oxide (ITO) on glass,
23 fluorine-doped tin oxide (SnO2:F) on glass, PEDOT:PSS on glass or plastic, and single-walled carbon nanotubes (SWNT) on glass or plastic.75, 76 Absorptive/transmissive, or window-type, disp lays operate by reversibly switching the device between a highly absorptive colored state and a highly transp arent, bleached state. Both the working electrode and counter are transparent for light to pa ss with the conductive materials (ITO, SnO2:F, PEDOT:PSS, or SWNT) deposited on glass or plastic substrates. As is shown in Figure 1-3, films of two complementary electroch romic materials, one anodically coloring, the other cathodically coloring, ar e deposited on the electrodes w ith a layer of electrolyte sandwiched in between. Figure 1-3. Schematic of a typical absorptiv e/transmissive electrochromic polymer window. The device becomes highly absorptive when bias ed with a negative potential applied to the cathodically coloring electrode and becomes bleached when the bias is reversed. The colors of the device are additive in that they are a combin ation of those exhibited by both materials. The absorptive/transmissive window devices typically operate in the visible region, but recent
24 research towards the production of IR-transpare nt electrodes (e.g., SWNT) has allowed for the fabrication of dual polymer devices that opera te in the NIR through far-IR regions of the spectrum.76 The absorptive/reflective electrochromic displays can also operate not only in the visible region, but also at the longer wavelengths of the NIR and far-IR as most metals utilized as the working electrode are highly reflective through those regions of the spectrum.47, 56, 57, 72-74 One type of electrode demonstrated is a gold-coated ion permeable porous membrane. As is shown in Figure 1-4, this electrode is outward facing, with the active electrochromic material deposited on top. Behind the electrode is the electrolyte laye r and counter electrode. The counter electrode in this case only acts as a redox-balancing layer, not contributing any optical properties to the electrochromism of the device. This allows fo r probing of the electrochromic properties of the active layer alone. The en tire device is encapsulated with a cover window that is transparent to the wavelengths measured with ZnSe utilized in the NIR to mid-IR, glass for visible to NIR, and polyethylene for visible to mid-IR.56, 57 As an example, when the active polymer layer is a cathodically coloring polymer, the device is highly absorptive in the visible region and highly reflective in the NIR when a negative bias is a pplied. When the bias is switched, the polymer becomes oxidized and the device is highly reflect ive in the visible region, with the metal electrode visible underneath, and absorptive in the NIR. Both types of device platforms allow for pattern ing of not only the electrodes but also the active electrochromic layer. In window-type devices with PEDOT:PSS as the transparent conductor, the electrode can be patterned by scr een printing, inkjet prin ting, and micro-contact printing, not only on glass, but also on a flexible substrate such as plastic.
25 Figure 1-4. Schematic of a typical absorptiv e/reflective electrochromic polymer display. Similarly, the metallic conductor in the reflec tive devices can also be patterned by metal vapor deposition, or line patterni ng and micro-contact printing followed by electroless plating. The polymer layers can then be selectively de posited by electrochemical polymerization onto the patterned electrode or, with the increasing number of soluble polymers becoming available, printing by inkjet or screen printing.77-80 The combination of the various device platforms available, along with the versatility of the electrochromic polymers and the large variety of colors available makes them ideal for most display applications. While these devices and materials offer enhanced performance over other electrochromic materials in co lor tailorabilit y, subsecond switching speed s, optical memory, and low power consumption, some challenges still exis t. These include incr easing device lifetime to that needed in commercial applications by im proving environmental stability and decreasing
26 switching speeds to below millisecond rates. Meanwhile, there exist applications where these issues are not imperative, one of which is applic ations where the display or device is disposable, being discarded after only a few uses, for example, and the other is the vari able optical attenuator that will be discussed in de tail in this dissertation. Light-Emitting Devices of Doped Polymers One of the most highly researched and public ized uses of conjugated conducting polymers is as the active material in polymer light-em itting displays. In 1987, it was demonstrated that display-brightness could be achieved for organic light-emitting diodes (OLEDs). These devices consisted of vacuum-deposited layers of an ar omatic diamine and the fluorescent metal chelate complex 8-hydroxyquinoline aluminum (Alq3).81 A drawback to these materials is that the requirement of vacuum sublimination of films is difficult to translate to practical commercial devices. It was not until three years later when the first conjugated polymer LED (PLED) was introduced with high quality film s being prepared by thermal treatment of a solution-processable precursor polymer.20, 82 These PLEDs utilize the undoped, pristine form of the polymer with the mobile charge carriers (electrons an d holes) supported by the delocalized -bonding along the polymer chain. The electrons are in jected from the cathode into the band of the semiconducting polymer and holes are injected into the band from the anode. The oppositely charged carriers in the two bands capture one an other (recombination) within the polymer film, form neutral bound excited states (excitons), and radiatively decay as is shown in Figure 1-5. Ideally, the work functions of the metal contacts are perfectly matched to the and bands; however, in real devices the match is im perfect, requiring hole a nd electron injection by tunneling through or thermal activation over the energy barriers formed at the polymer/metal interface.83 The energy levels of a conjugated lu minescent polymer (poly(2-methoxy, 5-(2-
27 ethyl-hexyloxy)paraphenylene viny lene) (MEH-PPV), and the electr odes used in a typical PLED are shown in Figure 1-6. Figure 1-5. Schematic diagram of a PLED in forw ard bias showing charge injection (a. and c.), carrier transport (b. and d.), and recombination (e.), with a focus on the two possible methods for charge injection, t unneling and thermal activation. In order to have efficient and balanced dua l-charge injection, a low work-function cathode (e.g., calcium) and high work-functi on anode (e.g., ITO) are require d to match the energy levels of the luminescent polymer. Th e most commonly utilized materi als are calcium, magnesium, or lithium as the cathode and ITO coated with a thin layer of PEDOT:PSS as the transparent anode. The PEDOT:PSS acts to not only lower the hole-i njection barrier by ~0.5 eV to better match with the polymer highest occupied molecular or bital (HOMO), but has also been suggested to smooth the relatively rough ITO surface, providi ng a better surface for the active polymer layer to be deposited.43, 84-86
28 Figure 1-6. Energy-level diagram, relative to vacuum, for the materials used in a typical PLED. The HOMO and LUMO levels of MEH-PPV87 and the Fermi level positions of the electrodes are shown. The high-lying lowest unoccupied molecula r orbital (LUMO) levels of luminescent conjugated polymers requires the use of low work -function, and therefore highly reactive, metals such as calcium, magnesium, or cesium. These metals have been shown to interact with the organic polymer layer and quench luminescence in addition to interacting with the environment leading to quick degredation.88, 89 In most cases, the reactiv e cathode layer is capped with another metal layer, such as aluminum, to protect the cathode from exposure to oxygen and moisture. In addition, the opera ting voltage and efficiency of the device is determined by the active polymer layer thickness with ideal thic knesses under 100 nm. The typical device layout for a PLED is shown in Figure 1-7. The device c onsists of the transpar ent anode, onto which the luminescent polymer layer is cast. The metal cathode contacts are then thermally evaporated onto the polymer layer and the entire device encapsulated to exclude moisture and oxygen.
29 Figure 1-7. Schematic of a typical PLED. The intense interest in PLEDs for use in disp lay applications results from the processing advantages, opportunities for the use of flexible substrates, and with a significant number of todays conducting polymers being highly luminescent, there is an availabil ity of emission colors that span the entire visible spectrum and beyond.28, 90-96 Results have been reported for devices with brightness values comparable to color televisions (100 cd/m2), fluorescent lamps (4,000 cd/m2), and even in excess of 10,000 cd/m2.91 In the PLEDs described above, the semiconducti ng polymer layer is oxidized at the anode (holes are injected) and reduced at the cathode (electrons are in jected); however, doping does not take place as there are no ionic species in the polymer layer to compensate the charges on the polymer chains. An interesting alternativ e to the PLED is the polymer light-emitting electrochemical cell (PLEC) introduced by Pei et al.97 The PLEC is a solid state electrochemical cell that is a blend of a conjuga ted luminescent polymer and a solid electrolyte that acts as the active layer for light-emission. Since that semina l publication demonstrating these devices, there has been much disagreement on the fundamentals of the device operation.98-100 The argument centers around whether the formation of an electro chemical junction is th e operating principle of
30 the device. While not entirely resolved, much recent research has been focused towards this effort with experimental observations agreeing with this initial model.101, 102 In general, as is shown in Figure 1-8, when a sufficiently high volta ge is applied between the cathode and anode, charges are injected from the electrodes into th e luminescent polymer. Counter ions from the solid electrolyte redistri bute to compensate the charges on the oxidized and reduced polymer chains, and simultaneous el ectrochemical pand n-doping occur at the anode and cathode, respectively. Since the elec trical conductivity of doped conjugated polymers increases significantly on doping, the polymer/e lectrode interfaces become low-resistance contacts. Once electrochemical equilibrium is r eached in the cell, a p-n junction is formed and the ionic contribution to the current goes to zero The electronic contri bution to the current, on the other hand, continues under th e influence of the applied vo ltage with the holes in the -band (p-type carriers) and electrons in the *-band (n-type carriers) migrating through the highconductivity doped regions. After a turn-on time, a thin insulating region is formed separating the doped regions, creating a p-i-n junction, where the charge carrier s recombine to form neutral charge carrier pairs that radi atively decay to the ground state.97, 100, 103-110 Unlike conventional inorganic LED s, the p-n junction is dynami c in that the device will discharge after the external bias is removed and the junction will need to be reestablished when the device is to be once again turned on. A dditionally, given the electrical conductivity of conducting polymers on doping, most of the external bias is applied acro ss the p-i-n junction. The minimum bias required to maintain the juncti on is equal to the built-in potential difference of the p-n junction and is ideally equal to the HOMO-LUMO gap of the polymer. It can be seen that the turn-on time of a LEC is dictated by the speed of electrochemical doping of the conjugated polymer and the rate of formation of the p-i-n junction, which are
31 Figure 1-8. Schematic of PLEC device operation. A) Unbiased device, B) Low voltage applied with p-doping of the polymer at the anode n-doping at the cathode, and an undoped insulating region, C) Charge migrati on with emission of a photon on carrier recombination. directly related to the redistribut ion of ions and initial injection and transport of electronic charge carriers. Given that ion mobilities are significa ntly lower than mobilitie s of electronic charge carriers, it is the ion mobility that then limits the device turn-on time. The solid electrolyte component of the active layer blend typically co nsists of an ionically conductive polymer or liquid additive that can effectivel y complex with ions and separate them from the counter ions allowing them to move within the polyme r matrix. Among various ionically conductive polymers, poly(ethylene oxide) (PEO) has been the most widely studied in combination with a variety of lithium salts, such as lithium trifluor omethanesulfonate (LiOTf). However, conjugated polymer and PEO/lithium salt mixtures have been observed to phase separate into a conjugated polymer phase and a PEO/LiX phase, due to th e entropy of mixing polymer/polymer blends being quite low and the mismatch in polarity between the typically non-polar conjugated polymer, such as MEH-PPV, and the polar PEO/LiX complex.111-114 Given that this can limit device performance, much effort has been dir ected towards other materials that show an
32 improved morphology on blending with luminescent polymers.115-119 Such materials include crown ethers complexed with various salts. Th ese devices have been de monstrated to show a higher luminance and longer lifetimes than devices containing the PEO/LiOTf solid electrolyte. In addition, binary systems of conjugated lumine scent polymers containing ion solvating side groups complexed with a salt and luminescen t/ionic liquid blends have gained increasing attention as there would be no need for an addi tional ion solvating material and complications arising from phase separation are decreased.120-126 An important characteristic of the PLEC is that, due to the low resistivity of the doped polymer and the tendency to form ohmic contacts with the metal electrode s, the performance of the device is insensitive to the electrode material s. Not only can the use of highly reactive, low work function cathodes be avoided, but the same metal can be utilized for both anode and cathode.127-129 Another unique property of LECs includes the fact th at light emission can occur for both forward bias as well as for reverse bias with onset voltages equal in both directions. Onset voltages have also been demonstrated to be nearly independent of the active layer thickness. This has been effectively demonstrated by the use of a planar electrode configuration with two parallel gold electrodes of 1 mm spacing.129 In this case, the light is emitted from within the zone between the metal contacts.128, 130 Even though these devices have turn-on voltages at or near the polymer bandgap, while be ing independent of film thickness and without the need for highly reactive metal contacts, quantum efficiencies can be achieved that rival those of PLEDs. As the switching speed s and lifetimes are improved, their impact in the field of lightemitting displays will be distinct. Dual-Purpose Devices Electroactive conjugated polymers can exhibit electrochromism and light-emission, as has been discussed, but they also have been demons trated as the active component in many devices
33 such as photovoltaics, actuators, field-effect tran sistors, and supercapacito rs with more than one of these devices utilizing the sa me family of polymers. Many of these properties operate on similar principles or by similar mechanisms. This opens the door for dual purpose devices or displays that perform multiple functions with th e same active material or same device by the fabrication of creative device architectures. For example, both electrochromic devices and actuators operate by electrochemical oxidation or reduction of the polymer film with simultaneous ingress/egre ss of charge balancing counterions. With the electroch romic devices, the property desi red from this action is the resulting color change on oxidation/reduction. For the actuator devices, th e action is contraction or expansion of the polymer film due to a vol ume change from the counter ion movement. A device demonstrated in the literature that takes advantage of this dual property exhibited within the same material was an electrochromic moveable pixel.24 This device utilized polypyrrole as the active layer in which the polymer acted as an actuator joining two electrodes. The volume change induced on oxidation of the polypyrrole layer causes a rotation be tween the electrodes, creating a hinge-type device. A dditionally, polypyrrole layers are deposited on the electrodes where their electrochromic response can be a ltered along with the position of the electrode giving a color-changing movable pixel.24 Similarly, Andersson et al. have demonstrated a smart pixel that operates in a similar fashion in that the electrochemical reacti on of a polymer film is used for multiple purposes. In their devi ce, a film of PEDOT is used as the active electrochromic layer and also as an electrochrom ic transistor for matrix addressing switches. The result is an electrochemical active matrix addressed electrochromic display131 Another type of dual-purpose device is one that operates as both a light-emitting display and a photovoltaic device. These types of device s offer the possibility of an electroemissive
34 display that can be powered by re versing the operation of the de vice from light-emission to lightharvesting within the same active layer.132-135 Yet another dual-purpose device is an electrochromic/electroluminescent (EC/EL) display. This device can operate as a color changing display in ambient lighting conditions where the electrochromic contrast is sufficient while operating as an electroemissive display when lig hting conditions are low. A dual EC/EL display would offer advantages over curren tly utilized emissive devices th at operate with the use of a liquid crystal display or light-emitting diodes. Both liquid crystal and LED devices have a low display contrast in direct or bright light situations sacrific ing image quality. A device that operates in this manner has been demonstrated in the literature.136 However, the electrochromism and electroluminescence occur from different layers within the device and not from the same material. It is our goal to de monstrate a dual EC/EL display utilizing the same active layer for both electrochrom ic and electroluminescent opera tion of the device, simplifying device fabrication and operation. Before the concept of a dual EC/EL device is discussed, we investigate unique properties of reflective electrochromic displays utilizi ng soluble propylenedioxythi ophene polymers as the active electrochromic layer. The phenomenon of unsymmetrical switching is introduced and the factors contributing to this pr operty are explored with a mode l proposed. In addition, we study the possibility of an application of these ECDs as electrochromic variable optical attenuators due to the high optical contrasts seen in the NIR region. The next two chapters will focus towards th e goal of creating a dual EC/EL device with Chapter 3 directed towards the examination of both MEH-PPV/solid electrolyte blends and carbazole-based copolymers with i on solvating groups as the active layers in reflective ECDs. In this chapter, two types of electrochromic devi ce configurations will be analyzed with the
35 carbazole-based copolymers and MEH-PPV as th e active electrochromic layers. One device contains an ITO/glass working electrode and an aluminized porous membrane as the reflector while the other has an ITO-glass working electr ode with reflection occurring off a porous white reflector. In Chapter 4, the fa brication of model ME H-PPV PLEDs devices is described which allow for the optimization of device fabrication a nd characterization protocol s. This then leads to the fabrication of MEH-PPV/PEO/LiOTf ble nd PLECs followed by the combination of both ECD and LEC concepts into a dual EC/EL device with MEH-PPV as the active layer. We will show that, depending how the electrodes are bias ed, the device can exhibit electrochromism and light emission from the MEH-PPV layer. The dissertation will be conc luded with a chapter detailing the instrumentation and materials used throughout this work in Chapter 5. The unifying theme throughout this dissertation is the utilization of soluble electrochemically doped polymers in both electrochromic and light-emitting devices for both NIR applications (as with the EC/VOAs) and visible light displays
36 CHAPTER 2 REFLECTIVE ELECTROCHROMIC DEVICES OF DISUBSTITUTED POLY(ProDOTS) AS VARIABLE OPTICAL ATTENUATORS Reflective electrochromic devices act to modul ate incident electromagnetic radiation by varying the amount of light reflected from th e device. This is accomplished through employing an electrochromic material at the active worki ng electrode. Traditionall y, the focus has been on visible light electrochr omism with the device acting as a di splay or electrochromic mirror, switching between highly colored and highly reflectiv e states. Recent attention has been paid to the possibility of modulating light at longer wavelengths extending from the NIR into the far IR regions of the spectrum.26-28, 57 Of the available electrochromic materials, conjugated conducting and electroactive polymers hold the most promise due to their pr ocessability, fast swit ching speeds, and high optical contrasts. Many conjuga ted polymers synthesized in recen t years are soluble in common organic solvents, allowing for processing through spray-casting or roll-to-roll fabrication of devices. Switching speeds considerably improved over other electrochromic materials, such as WO3, have been demonstrated for several conjugat ed polymers in the reflective device platform with full optical contrast achieve d in a tenth of a second. Optical contrasts in the visible region in excess of 50%, 80% in the NIR and 50% in the mid-IR have been demonstrated.56, 57 Reflective Device Construction The reflective electrochromic devices were constructed as shown in the schematic and photographs in Figure 2-1. The polymer-coa ted counter electrode was fabricated by electrochemically depositing PEDOT ont o a square of gold-coated Kapton contacted with copper tape. The next layer of the device consis ts of three pieces of porous separator soaked with gel electrolyte. The top layer is a gol d-coated porous membrane onto which the active electrochromic polymer layer has been either electrochemically deposited or spray-cast. The
37 entire device is then encapsulated by sandwichi ng between a back support layer and a transparent window and sealed on all four edge s with transparent tape. A mo re detailed description of the materials utilized in fabricating the devices can be obtained in Chapter 5. Figure 2-1. Schematic of a refl ective electrochromic display (A). Photographs (B) of a PProDOT-(CH2OEtHx)2 reflective electrochromic device when fully reduced (top) and fully oxidized (bottom). The PEDOT layer is polymerized to a thickness su ch that the charge to switch is greater for the counter electrode than for th e working active electrode. This ensures that the electrochromic switching at the working electrode is the most efficient, yielding a full optical contrast. As the counter polymer layer does not lend any of its opti cal properties to the device, only acting as a charge-balancing layer in the electrochemical cell, practically any electroc hromic polymer can be used. The properties of interest for this polymer layer are electrochemical stability and relative switching speed over the potential range the device is switched. The porous separator acts to physically sepa rate the working electrode and the counter electrode, preventing electrical shor ts in the device. This layer also prevents any optical changes that occur at the counter electr ode from being seen and, as a result, spectral changes from being
38 measured at that electrode. The gel electrolyte utilized consists of an appropriate salt (typically TBAPF6) dissolved in a propylene carbonate (PC) swollen PMMA matrix. The addition of PMMA to the electrolyte increases the viscosity of the solution, preventing leaks, allowing for long-term storage and testing of the device, while not affecting electrochemical stability. The working active layer consists of a por ous polycarbonate membrane that has been metallized by thermal evaporation of gold to a th ickness of 60 nm. This thickness provides for a sufficiently conducting electrode that is highly reflective. The act ive polymer layer is deposited by electrochemical polymerization from a monomer solution at a constant potential, or spraycasting from solution using a commercial ai rbrush. The device is placed on a plastic transparency film support and a transmissive wi ndow is placed on top, also composed of plastic transparency film for this st udy. The device is sealed by eith er using epoxy or wrapping transparent tape around the edges. With the ma terials utilized in this particular device construction, the entire device is flexible and, in addition, can be patterne d to a variety of sizes and shapes. The reflectance spectroelectrochemistry for a device containing bis(ethylhexyloxy)substituted poly(3,4-propylen edioxythiophene) (PProDOT-(CH2OEtHx)2) as the active electrochromic layer is shown in Figure 2-2. The spectroelectrochemistry of the device is measured by applying a potential between th e two electrodes (counter and working) and measuring the reflectance spectra at each applie d potential. The reflectance measured is both specular and diffuse by utilizing an integrating sphere attachment to the spectrophotometer. As each potential is applied and held (typically in increments of 50 to 200 mV), the reflectance of the device is measured in the waveleng th range of 2.0 m to 350 nm. For all spectroelectrochemical measurements, the reflecta nce is taken as a difference from that of a
39 reference device that contains all of the same components as the device measured except for the active polymer layer. For the disubstituted PProDOT devices, the active layer is fully neutralized at -0.8 V and fully oxidized at +1.2V. Figure 2-2. Reflectance spect roelectrochemistry of a 236 nm thick PProDOT-(CH2OEtHx)2 film in a reflective ECD. Unsymmetrical Switching On switching reflective electroc hromic devices with PProDOT-(CH2OEtHx)2 as the active electrochrome, it was realized that at low a pplied potentials, there were large reflectance contrasts in the NIR with little to no change in th e visible region. This is illustrated in Figure 23A, where the spectroelectrochemi cal series is shown for the pot ential range of -0.8 V and +0.4 V for a device having a 750 nm thick film. In the visible region, there is a 2.5% reflectance change, whereas a large 70-90% change occurs in the NIR extending from 0.8 to 2.0 m. When higher potentials are a pplied to the device, +0.5 V to +1.2 V, the visible region reflectance
40 begins to increase with a cont rast of 30-40% occurring between 500 and 550 nm as is shown in Figure 2-3B. This phenomenon is unexpected given that the intr oduction of the mid-gap states, and hence increase in absorbance at longer wa velengths, occurs at the expense of the transition as is shown in Figure 11 and as seen in the transmittance spectroelectrochemistry of a similar polymer (PEDOT) in Figure 1-2. Figure 2-3. Reflectance spect roelectrochemistry of a 750 nm thick PProDOT-(CH2OEtHx)2 film in a reflective ECD between the applied pot entials of A) -0.8 V and +0.4 V and B) +0.5 V and +1.2 V. To determine the factors contributing to this phenomenon, a comparison of devices of two different disubstituted PProDOTs were made of various film thicknesses, and by different film deposition methods. The reflectance of these de vices were then compared to the transmittance spectra of films on ITO of the same polymers film deposition methods, and film thicknesses. Films of several different thicknesses (181, 236, and 450 nm) were spray-cast on the goldcoated membranes for the reflective ECDs and onto ITO-glass for transmittance measurements. As can be seen in Figure 2-4, on comparison of the percent reflectance versus applied cell potential (A) for the reflective device and percen t transmittance versus applied potential (B) for the film on ITO-glass, both with a polymer film thickness of 450 nm, the change in NIR
41 reflectance occurs 400 mV before any change in the visible region reflectance occurs in the device, exhibiting unsymmetrical switching. Figure 2-4. Percent reflectance versus cel l potential (A) for a 450 nm thick PProDOT(CH2OEtHx)2 film reflective ECD. Transmittance versus potential (B) for a film of the same polymer and thickness on ITO-glass in 0.2 M TBAPF6/PC electrolyte. On the other hand, for that same polymer thickness, the NIR and visible region transmittance change occurs at the same potential, as is seen in Figure 2-4B, not exhibiting any unsymmetrical switching. As the film thickness is decreased in the reflective ECDs, the extent of unsymmetrical switching decr eases, shown in Figure 2-5, for a 181 nm thick film with the device exhibiting no unsymmetri cal switching, indicating a polym er film thickness dependence of this phenomenon In order to determine whether this phenom enon is isolated to only the chemically polymerized spray-cast polymer, films of PProDOT-(CH2OEtHx)2 were electrochemically polymerized potentiostatically directly onto ITOglass. The final film thickness was controlled by setting the polymerization to terminate when a predetermined charge had passed at the working electrode. The resulting film thickness at the ITO electrode was then measured by profilometry. The polymer films on the gold-coat ed porous membrane working electrodes were
42 obtained by the same method. Given that the th ickness of the polymer f ilms on the gold-coated porous membranes could not be measured by pr ofilometry, it was assumed that the thickness produced on the membrane was similar to that on ITO with the charge passed for polymerization corrected for the electrode area differences. Figure 2-5. Percent reflectance versus cel l potential for a 181 nm thick PProDOT-(CH2OEtHx)2 film reflective ECD demonstrat ing the lack of unsymmetrical switching in the thinner films. The reflectance spectroelectrochemical series for a polymer film 998 nm thick in an ECD is shown in Figure 2-6A and that of the same film thickness measured by transmittance on ITOglass is shown in Figure 2-6B. The NIR reflect ance begins to decrease at -0.4 V while the visible region reflectance begins to increase at +0.1 V, 500 mV la ter for the polymer film in the reflectance device. On the other hand, the NI R transmittance decreases at the same applied potential for the polymer film on ITO-glass. Th is can be more clearly see in Figure 2-7 where the percent reflectance versus ce ll potential for the three differe nt film thicknesses (111, 538, and 998 nm) of electropolymerized polym er are shown. Similarly, with the spray-cast films, this
43 extent of unsymmetrical switching in the reflec tive ECDs shows a film thickness dependence on the electrochemically polymerized films. Figure 2-6. Reflectance spectroel ectrochemistry (A) for a 998 nm thick electropolymerized PProDOT-(CH2OEtHx)2 film in an ECD. Transmittance spectroelectrochemistry (B) for a film of the same thickne ss on ITO-glass in 0.2 M TBAPF6/PC with potentials referenced versus Fc/Fc+ Figure 2-7. Percent reflectance ve rsus cell potential for reflecti ve ECDs of electrochemically polymerized PProDOT-(CH2OEtHx)2 of thickness A) 111 nm, B) 538 nm, and C) 998 nm. A comparison was made between films on IT O-glass and in reflective ECDs of two different spray-cast polymers, PProDOT-(CH2OEtHx)2 and PProDOT-Hx2. Initial experiments where the unsymmetrical switching was eviden t were performed on devices containing the alkoxy branched polymer, PProDOT-(CH2OEtHx)2. The linear alkyl derivative was then investigated for the presence of unsymmetrical sw itching when incorporated in reflective devices
44 to determine whether this phenomenon is isolated to one specific polymer or is present in the family of disubstituted PProDOTs. As Figure 2-8 shows, unsymmetrical switchi ng is found to occur in the linear alkyl derivative as well when incorporated into reflective ECDs, but not when measured by transmittance on ITO-glass. This indicates that the phenomenon is not due to the branched alkoxy groups on the polymers, but rather to th e large optical contrast s in the NIR and the relative ease with which to prepare thick films, both spray-cast and electrochemically polymerized, for this family of polymers. Figure 2-8. Percent reflectan ce versus cell potential(A) for a 351 nm spray-cast film of PProDOT-Hx2 in a reflective ECD. Percent transmittance (B) versus potential for a film of the same thickness on ITO-glass in 0.2 M TBAPF6/PC. Potentials are referenced vs. Fc/Fc+. Conductive Front Here, a model is proposed for the effect of unsymmetrical switching in reflective ECDs of conjugated conducting polymers. As was discus sed previously in Chapter 1, a conjugated conducting polymer is insulating in its neutral, undoped state. Othe r researchers have shown that there is a process whereby the neutral polymer f ilm is oxidized electrochemically in a localized
45 area beginning at the electrode/polymer interface.137-143 This oxidized area is now conductive and acts as an extension of the metallic electrode to further oxidize to conductive domains farther from the electrode surface. This process cont inues until the conductive zone, or front, reaches the polymer film surface and is shown schematically in Figure 2-9. When the film is neutral and insulating, the polymer is highly absorptive in the visible region and highly transmissive in th e NIR. As can be seen in th e spectra in Figure 2-9A, a large amount of light in the visible region is absorbed (~90% at max) and therefore only a small amount is able to traverse the entire film and be reflected back by the gold electrode. On the other hand, ~90% of the light in the wavelength range of 800 to 2000 nm is measured, indicating that nearly all the light is able to penetrate through the entire film, be reflected by the electrode and travel through the film a second time to be measured. At low applied potentials, the film becomes oxidized to a small distance from the polymer/electrode interface. The concentration of conductive sites is low and the redox-doped front does not penetrate far into the insulating fi lm as shown in Figure 2-9B. Given that the NIR light is able to penetrate the en tire polymer film, this small increase in the concentration of oxidized species at the polymer/electrode interface is able to be probed at those wavelengths and a change in the spectra is observed. However, near ly all of the visible region light is absorbed by the polymer film before reaching the conductive front and no change in the redox state of the polymer is measured by this wavelength range. This can be seen in the re flectance spectra where an absorbance change is seen in the wavelengt h range of 700 to 2000 nm with no change in the visible region below 700 nm. As the doping leve l increases at higher applied potentials, the concentration of oxidized sites increases and the conductive front propaga tes farther into the polymer film, as shown in Figure 2-9C.
46 Figure 2-9. Conductive front model. A) Schematic of fully neutralized film showing penetration of visible (blue line) and NIR (red line) a nd respective spectra. Width and dashing of lines indicate intensity of light. B) C onductive front propagating through film at low applied potentials. C) Conductive front propagating further into film at higher applied potentials, past the penetration depth of visible light. D) Fully oxidized film.
47 An even higher concentration of oxidized spec ies is probed by the NIR light and, since the conductive front has now extended pa st the penetration depth of th e visible light, there is now a measure of the oxidized portion of the film by this wavelength range as shown with a change in the visible region absorbance. The NIR absorbance is constantly increasing as the concentration of doped polymer is increasing, but the measurem ent of optical change in the visible region cannot occur until the conductive fr ont extends past the penetrati on limit of visible light. Once the entire film becomes fully oxidized, as is shown in Figure 2-9D, the polymer is highly absorbing in the NIR and highly transmissive in the visible region, with the visible light now able to penetrate through the entire film to the exposed gold electrode underneath. The distance to which light can penetrate a material can be determined from the penetration depth, also known as the skin depth. This depth is dependent on the absorption coefficient of the sample and is defined as the de pth at which the intensity of radiation inside the material falls to 1/ e of the original value at the mate rial surface and is calculated by: = 2/ (2-1) where is the absorption coefficient and can be determined from the transmission through the sample of known thickness as shown schematically in Figure 2-10. To determine this value for the entire wavelength range, polymer films of several different thicknesses were deposited on glass and the transmittance through the polymer f ilm from the wavelength range of 350 nm to 2000 nm were measured. These films can be treate d as planar layered structures from which the optical transmission can be determined while taking internal reflections into considerations. For the case of an absorbing layer (polymer film) on a nonabsorbing substrate (glass), the relation between optical transmission and absorption co efficient has previously been determined144 and is as follows:
48 T = 1 R12R23A2R01( R12+ R232 R12R23)(1 R01)(1 R12)(1 R23) A (2-2) and the value of A is defined as A = et ( where t is the thickness of the absorbing polymer layer), and Rjk is the layer-layer power re flection coefficient for each layer noted in Figure 2-10. Figure 2-10. Schematic of multilayer geometry used in analysis of penetration depth of optical radiation through the polymer sample. Mate rial 1 is the absorbing polymer film, material 2 is the nonabsorbing glass substr ate. The reflections at each surface are indicated by Rjk. Given that the reflections at each surface are much less than 1, the denominator in equation 2-2 reduces to 1 and the tr ansmission then becomes: T =(1 R01)(1 R12)(1 R23) ea t (2-3) If the ratio of the transmission,T, for samples of two different thicknesses (A and B) are taken, the equation reduces further to: TA T B = ea tAea tB =ea (tBtA) (2-4) The penetration depth, is now calculated by:
49 d =a2 = ln TBln TA2( tBtA) (2-5) The penetration depth for sp ray-cast films of PProDOT-(CH2OEtHx)2 were determined by measuring the transmittance of samples of vari ous thicknesses (specifically 93, 133, and 189 nm) over the wavelength range of 350 nm to 2000 nm and calculating the penetration depth from equation 2-5. The average penetration depth at 1.55 m was calculated to be 15 m and that at 550 nm (the max for this polymer) was calculated to be 0.35 m. Therefore, film thicknesses of this polymer of 0.15 m or less should exhibit no unsymmetrical switching when incorporated into reflective ECDs as the light in the visible region is able to travel through the entire polymer film, be reflected off the gold electrode and travel back to be measured. In films thicker than 0.35 m, the penetration depth is not far enough fo r the light to reach the polymer/electrode interface to measure any optical changes that occur close to the electrode at low oxidation potentials, which is the case as was shown in Figure 2-7 for three different film thicknesses of PProDOT-(CH2OEtHx)2 in reflective ECDs. This phenomenon of unsymmetrical switching exhibited by these devices has possible applications with one being variable thermal contro l. It can be imagined a reflective device that, when in the fully reduced state, allows for all NI R light to be fully reflected while the device is colored. By adjusting the device bias, it remains in that colored state in the visible region, but the NIR light is absorbed and can be collected and transferred using a heat conducting element. Another possible application for this feature is the utilization of this device in combination with a solar cell placed such that light is reflected off the ECD before being directed to the solar cell. The amount of light in the near to mid IR impinging on a photovoltaic device can be tuned while excluding light from the visible region. In effect, the device would act as a tunable filter for the
50 solar cell. Yet another possible application is in the area of optical telecommunications and will be discussed in the next section. Reflective ECDs as Variable Optical Attenuators A device that can take advantage of the la rge, tunable NIR contrasts exhibited with conjugated polymer ECDs is the variable optic al attenuator (VOA). VOAs are key components in fiber-optic telecommunications networks and are utilized to re duce signal power by inducing a fixed or variable loss.145, 146 VOAs are also often required in applications where photosensitive components require dissimilar incident optical signals, and hence variable sensitivities and saturation points. The attenuator regulates the light intensity to be within the dynamic range of the component, preventing any damage or overloading. In recent years, the use of wavelength division multiplexing (WDM) has become important to handle the increased demand placed on op tical telecommunications by fiber-optic transmission.147, 148 With WDM, a number of signals of differing wavelengths are combined onto a single fiber allowing for increased signal density without increasing the amount of optical fiber used. These systems require that the op tical power between all channels be equalized before combination. Direct control of the source fo r each signal is not practical as changes in the laser drive current can lead to an undesirable wavelength shift. VOAs allow for adjustment of the channel power while maintaining wavelength stability. Another application of VOAs is for channel balancing to optimize the optical power of signals at key points in the netw orks. Optical power levels ar e determined by the optical loss along the transmission line and can vary with th e length of the line, the number of connection points, and the efficiency of optical components (e.g., couplers, amplifiers, and source) and deterioration of the signal-tonoise ratio may reduce the optical transmission quality. A VOA is
51 capable of maintaining the light level acting as a noise discriminator, reducing the intensity of spurious signals to below a threshold value. Additionally, VOAs serve to c ontrol feedback in optical amplifier control loops to maintain a constant output (e.g., as an auto matic gain control element (AGC)). In WDM systems, transmission loss is compensated by utilizing erbium-doped amplifiers (EDFAs), which amplify the signal made up of numerous wavele ngths. However, EDFA gain is wavelengthdependent and a gain equalizer is needed that has a wavelength dependence profile that is the inverse of the EDFA in order to maintain a flat transmission bandwidth. However, when there is a change in the optical power to the EDFA, the wavelength depende nce of the EDFA gain profile changes. Since the profile of the gain equalizer is fixed, there is a shift in the flattening effect that it provides. The VOA allows for tuning of th e signal to compensate for this gain tilt and flattens the transmission profile. When considering the optical performance of a VOA, the most important specifications are insertion loss and range of opti cal attenuation. These values are typically reported for the most commonly used wavelengths, 1.55 and 1.31 m as these are the wavelengths where absorption and dispersion are the lowest for silica fibers. The range of optical atte nuation is the minimum and maximum attenuation values achievable by th e device with optical attenuation measured by: Optical Attenuation (dB) = 10 log10 (Ioff/Ion) (2.6) where Ioff and Ion are the intensity of the signal when the a ttenuator is in the off state and on state, respectively. Many VOAs either transmit or reflect light so intensity is the percent reflected or transmitted by the attenuator. For a variable optical attenuator, the on state can be tuned as a function of voltage applied to the device yielding a dynamic attenuation range. Typical VOA attenuation values range from 10 to 25 dB with many VOAs able to achieve up to
52 50 dB attenuation. To illustrate the significance of such values, a 3 dB loss leaves 50% of the original light signal, a 10 dB loss leaves 10%, and a 20 dB loss leaves 1%. Ideally, when the attenuator is in the off stat e, the amount of light transmitted or reflected would be 100%, very often this is not the case. The insertion loss (or opti cal loss) is the amount of light lost when the attenuato r is off and is calculated by: Optical Loss (dB) = 10 log10(100%/Ioff) (2.7) where again, Ioff is the intensity of the optical signal wh en the attenuator is off and is commonly expressed as percent transmittance or percent reflectance. Optical loss in a fiber-optic system is additive, therefore the loss due to the introduction of a VOA into the network is needed to be kept to a minimum. Given that the loss due to fiber attenuation alone can be as low as 1 dB/km, for many applications, the maximum insert ion loss tolerated is less than 1 dB. Optical attenuation and insertion loss values vary with the type of attenuator device utilized with there being a larg e number of devices that are eith er commercially available or currently researched. Many of these devices di ffer in their operating mechanism, which also determines other performance ch aracteristics such as drive vo ltage, switching speed, and size. The most common VOAs being developed are th ose that include liquid crystal devices, mechanical actuators, microelectromechanical (MEMs) devices, and electrochromic devices. The liquid crystal devices operate by passing the light through a liq uid crystal element.149, 150 When a voltage is applied to the device the liquid crystal mol ecules are oriented in such a way that light is attenuated. These devices a llow for efficient tunabl e attenuation; however, liquid crystals are temperature sensitive and as the temperatur e of the surrounding environment varies, the optical properties of the liquid crystal cha nge, resulting in higher insertion loss and inaccurate attenuation.
53 The mechanical attenuators operate by a va riety of methods to affect attenuation.147, 151 One method uses a rotatable filter placed between an input fiber and an output fiber and varies the attenuation by rotating the f ilter. Another mechanical at tenuator involves changing the alignment between the axis of input and output fibers, varying the quant ity of light coupled between the fiber ends. Yet another mechanical attenuator utilizes a moveable mirror, varying the angle at which an incident ra y is reflected, thereby tuning the amount of light directed to the input optical fiber, and causing attenuation. Th ese devices offer effective attenuation and low optical loss. Unfortunately, mechanical optical attenuators have the drawbacks of high operating voltages (10-20 V) and moving part s that can be relatively slow to engage and can wear and eventually fail. In addition, these devices do not lend well to mini aturization, a feature increasingly sought after for the realization of higher density networ ks and the ability to attenuate individual fi bers independently. On the other hand, research of MEMs devices as VOAs has become increasingly popular as these devices offer high attenuation, low optical loss, fast switching speeds and lend themselves easily to miniaturization.152-155 These devices are micr ostructures that act as actuators with the most common utilizing a mirror or rigid shutter to reflect or block a light beam when a variable voltage or current is applied. Th eir very elegant design, however, adds a level of complexity that makes manufacturing of these de vices on a scale outside the research laboratory impractical and cost prohibitive. Recent interest has grown in utilizing elec trochromic materials and displays for VOA devices.156-166 Many of the materials reported in th e literature are based on traditional electrochromic materials such as WO3, ruthenium complexes, and polynuclear molybdenum complexes. The molybdenum complexes, such as tris(pyrazolyl)boratomolybdenum (V), have
54 been reported to have the highest attenuation out of these with a value of 50 dB at a low applied voltage.156, 159, 165 However, the prototype devices are ba sed on a solution of the electrochrome sandwiched between two transpar ent electrodes. This limits the device architecture and switching speed, as the species is required to diffuse to the electrode to be redox switched. The transition metal oxide film devices based on WO3 have also shown impre ssive attenuation of 40 dB, reaching 20 dB in 2 seconds, but require an hour for the device to return to the transmissive state.158, 164 Dendritic mixed-valence ruthenium comp lexes have also been researched as VOA materials with films showing attenuation va lues of 2-7 dB, switching within 2 seconds.160-162 The first reported use of a conjugated polym er as a possible electrochromic VOA material were for films of PEDOT.163, 167 These devices were shown to ha ve an optical attenuation of 11 dB, switching within 5-7 seconds and a low optic al loss of 0.86 dB. Conjugated electrochromic polymers and their devices offer many advantages not available with the other devices and materials discussed. These include a low ope rating voltage (under 1.5 V), mechanical flexibility, improved film processa bility, and a variety of options in device design and patterning. It is demonstrated in this chapter the applicab ility of the previously introduced reflective electrochromic displays, with PProDOT-(CH2OEtHx)2 as the active layer, as an electrochromic variable optical attenuator (EC-VOA). The reflective ECD was constructed as previously described in this chapter. Briefly, the counter electrode consisted of PEDOT electrochemically deposited onto a gold-coated Kapton electrode. The next layer was comprised of porous filter papers soaked with a TBAPF6/PMMA/PC gel electrolyte. This was follo wed by the outward-facing working electrode that was a gold-coated porous polycarbonate membrane onto which the active polymer layer (PProDOT-(CH2OEtHx)2) was spray-cast. The entire device was encapsulated with a plastic
55 transparency film back support and cover, and se aled on all four sides with transparent tape. Initial measurements were performed with a UV/Vis-NIR spectrophotometer fitted with an integrating sphere as described in Chapter 5. Th e percent reflectance of the device was measured for each potential applied and optic al attenuation and insertion loss values were calculated using equations 2-6 and 2-7. As is shown in Figure 2-11, for a 750 nm thick film, the maximum attenuation achieved was 11.2 dB at an applied voltage of 400 mV for the wavelength of 1.55 m and 10.6 dB at 700 mV for 1.31 m. The optical loss when this de vice was fully reduced was low with a value of 0.1 dB at 1.55 m and 0.2 dB at 1.31 m. Figure 2-11. Optical attenuati on at various applied potentials for a 750 nm thick film of PProDOT-(CH2OEtHx)2 in a reflective ECD as an EC-VOA at the wavelengths of 550, 1310, and 1550 nm. To further demonstrate the possibility of th is device as an EC-VOA, a model fiber-optic setup was fabricated as shown schematically in Figure 2-12A a nd in the photograph in Figure 2-
56 12B. This setup allows the device to be demonstr ated in practical-use co nditions by coupling the light to the device using a fibe r-optic spectrophotome ter (described in Chapter 5) and not subtracting the absorbance of a reference de vice, which would have optimized the final reflectance by taking into account the absorbances from device components. The light from a halogen light source was brought to the device surface with a silica fiber-optic cable and the light reflected at a 45 angle was coupl ed into another fiber-optic cable and measured using an InGaAs detector in the wavelength range of 1.3-1.55m. Figure 2-12. Schematic (A) and photograph (B) of setup to measure reflective EC-VOA using fiber-optic spectrophotometer.
57 The potential across the device was varied and the reflected spectra recorded from which the optical attenuation was calculated as is shown in Figure 213. With this device, containing a 430 nm thick film, the maximum attenuation va lue achieved was 5.4 dB at 1.55 m and 5.0 dB at 1.31 m. The maximum optical loss values wh en the device was neutralized were 0.15 dB and 0.14 dB at the wavelengths of 1.55 and 1.31 m, respectively. These values could potentially be optimized by utilizing an antirefl ective coating at the NI R transparent window, to minimize scattering at that surface, and by varying the angle of the fiberoptics with respect to each other and the device to maximize light output. Figure 2-13. Optical attenuati on across the wavelength range of 1.3 m and 1.55 m for a 450 nm thick film of PProDOT-(CH2OEtHx)2 in a reflective ECD as an EC-VOA measured using the fiber-optic setup shown in Figure 2-12. To establish a possible applic ation for the unsymmetrical sw itching shown previously, the device was switched within a limited voltage rang e while the reflectance was measured with the integrating sphere setup. When the device is switched betwee n -0.8 V and 0.0 V, the attenuation at 1.55 m is modulated from 1.9 dB to 0.05 dB. Within the same voltage range, the attenuation at 550 nm varies only between 3.8-4.1 dB as shown in Figure 2-14A. When the voltage is
58 switched between 0.6 V and 1.2 V, the attenuation in the visible, at 550 nm, varies between 00.81 dB, while that in the NIR is between 6.4 and 6.6 dB, as shown in Figure 2-14B. This demonstrates the possibility of modulating the NIR and visible independent of each other by maintaining the switching volta ge within a specific window. Figure 2-14. Optical attenuation at 1.55 m (red line) and 550 nm (blue line) of a 450 nm thick film of PProDOT-(CH2OEtHx)2 in a reflective ECD as an EC-VOA when switched between the potentials of A) -0.8 V and 0.0 V, and B) 0.6 V and 1.2 V. Conclusions In summary, it has been demonstrated that at low applied potentials, the NIR reflectance can be modulated in a disubstituted-PProDOT refl ective electrochromic device with little to no optical change in the visible regi on of the spectrum. This can be attributed to the presence of a conductive front that propagates through the polymer film and grows as the potential applied to the device is increased. Given that the visible light cannot pe netrate as far into the entire polymer film as the NIR in devices above a spec ific film thickness, the NIR optical changes are observed to occur before that in the visible. This allows for modulating of the NIR independent of the visible region.
59 In addition, reflective ECDs containing spraya ble PProDOTs have been demonstrated as electrochromic variable optical attenuators. Not only do these devices offer effective optical attenuation and low insertion loss, but also the advantage of mechanical flexibility, low drive voltage and the possibility of patterning for the fabrication of multiple, individually addressable devices in a variety of architect ures. Future directions for th ese devices as VOAs would include optimization of the device performance. This would entail determining the optimum polymer film thickness for the highest attenuation while maintaining a low insertion loss. Additionally, coupling the fiber-optics to the device in such a way as to achieve the maximum reflectance of incident light measured. The possibilities for achieving this would include a fiber-optic holder with adjustable incident and out coupling angles. The current fibe r-optic holder is constructed such that the light measured is at a 45 degree an gle to the incident light, however, this may not be the optimum angle for the measurement of refl ectance from the devices. Further work could also include patterning of the de vices to form individually addre ssable pixels each containing a separate, miniature, EC-VOA.
60 CHAPTER 3 ELECTROCHROMIC DISPLAYS OF MEHPPV AND CARBAZ OLE-CONTAINING COPOLYMERS All conjugated electroactive polymers are pot entially electrochromic with many polymers exhibiting multiple colored states.22 As was introduced in Chap ter 1, the color of the polymer neutral state is determined by the polymer ba ndgap. Some of the most highly researched electrochromic polymers are those based on the alkylenedioxy-bridged thi ophenes and pyrroles. The alkylenedioxythiophenes ar e low bandgap polymers with a transition in the visible region making them cathodically coloring, switch ing from a highly absorptive to a highly transmissive state on oxidation. The presence of electron-donating oxygens adjacent to the thiophene ring acts to raise the HOMO level, lowering the polymer oxidation potential relative to the unsubstituted thiophene polymer. For example, the onset of oxidation for the dioxythiophene polymers, PEDOT and PProDOT-(CH2OEtHx)2 are -0.2 V and 0.2 V vs. SCE, respectively.48, 168, 169 This allows these polymers to be more stable in the oxidized form under ambient conditions as their HOMO levels are above the threshold for air stability, whic h is defined by the O2 to H2O redox couple at 0.5 V vs. SCE, shown in Figure 31, as they are easily oxidized by molecular oxygen.170 On the other hand, polymers with HOMO le vels lower than 0.5 V vs. SCE, such as MEH-PPV and PFO, are stable in their neutral fo rm under ambient conditi ons yet become more difficult to electrochemically switch as the oxidation potential increases.169, 171 Similarly, the addition of an alkylenedioxy brid ge to pyrrole introduces electron density to the already electron-rich pyrrole unit, acting to further lower th e polymer oxidation potential to the lowest values reported for conjugated polymers.22, 50, 54 By substitution at the nitrogen of the pyrrole ring, a reduction in th e effective conjugation length can be induced by the steric interactions brought about by to rsional twisting and the polymer bandgap can be increased to values greater than 3.0 eV, shifting the ne utral polymer absorbance into the UV region.52 By
61 increasing the polymer bandgap, the HOMO level is lowered. However, the N-substituted PProDOPs still maintain a low oxidation potential allowing these polymers to have the lowest values for anodically coloring poly mers to date. As with the dioxythiophenes, the family of PProDOPs are highly stable in the oxidized state. Figure 3-1. Diagram of oxidati on potentials (vs. SCE) for several electrochromic and electroluminescent polymers re lative to the level required for air st ability along with optical bandgap values.48, 52, 168, 169, 171 While high HOMO values allow for a low oxida tion potential and a stable p-doped state, characteristics desirable in electrochromic app lications, this is not the case for light-emitting display applications. For effec tive hole injection to occur, th e polymer HOMO level must be located at levels close to the work function of the anode. Th e most commonly utilized anode material is ITO with a work function of 4.8 eV or PEDOT:PSS-modified ITO with a work function of 5.2 eV.85, 172, 173 In addition, as the polymer active layer utilized in PLEDs is in the pristine, neutral form and the presence of any dope d sites quenches luminescence, it is necessary
62 for the polymer HOMO to be placed at values lower than 5.2 eV to preclude any possibility of oxidation by oxygen. Several classes of polymers fulfill these requirements and include poly(phenylenevinylenes)s (PPVs) polycarbazoles (PCbzs), pol y(p-phenylene)s (PPPs), and polyfluorenes (PFls), among others There is an ever-growing number of polymers available with HOMO levels below 5.3 eV, along with bandgap values spanning 4 to 1 eV. As such, these polymers have relatively high oxidation potentials and their electrochromic properties have not been extensively studied. However, some of th ese polymers offer interesting electrochromism in addition to electroluminescence and may be utilized in dual purpose electrochromic/electroluminescent displays. To this end, we investigate the electrochromic properties of three conjugated electroactive polymers, the commercially available MEH-PPV, poly[(9,9-bis(methoxyet hoxyethyl)-2,7-fluorene)alt -( N -(methoxyethoxyethyl)-3,6-carbazole)] (Cbz2-Fl2), and poly[(1,4-bis(metho xyethoxyethoxyethoxy)-2,5-phenylene)-alt-( N (methoxyethoxyethyl)-3,6-carbazole)] (Cbz2-Ph3) whose structures are shown in Figure 3-2. Figure 3-2. Structures of polym ers studied in this chapter. Electrochromism of MEH-PPV Solutions of MEH-PPV were prepared by di ssolving the polymer in dry and deoxygenated chloroform in a glovebox to a concentration of 5 mg/mL. The solution was allowed to stir
63 overnight and heated to 40C fo r the last 30 minutes. Films we re prepared by drop-casting the polymer solution onto a pre-cleaned ITO/glass slide and the solvent allowed to evaporate. The slides were then placed on a hotplate at 50 C for 1 hour to further remove residual solvent. Electrochemical switching of the film was performed in a threeelectrode cell with the polymercoated ITO as the working elec trode, Pt-flag as the counter, a nd a Ag wire pseudo reference (calibrated with the Fc/Fc+ redox couple) in an electrolyte so lution of 0.2 M LiOTf in ACN. Electrochemical switching was performed on the ITO/glass electrodes in order to allow the observation of spectral and colo r changes during redox switching. To determine the potential range for spectroelectrochemical switching, cyclic voltammetry was performed as is shown in Figure 3-3. The polymer was switched over the range of -0.9 V and 0.66 V vs. Fc/Fc+ at a scan rate of 30 mV/s and was shown to be stable for over 50 switches. Figure 3-3. Cyclic voltammograms of a drop-cast film of MEH-PPV on ITO/glass at a scan rate of 30 mV/s switched in 0.2 M LiOTf/ACN for 50 cycles. Every tenth scan is shown for clarity. In-situ spectroelectrochemistry of the drop-cas t film on ITO was performed in an Ar-filled glovebox with a fiber-optic spect rophotometer (StellarNet). The wavelength range measured was across the visible region from 350 to 850 nm. Each potential was held for approximately 1
64 minute before the spectra was acquired as this wa s the length of time required for optical changes to equilibrate. Figure 3-4A shows the sp ectra of the as-cast, dry film and the spectroelectrochemistry of the film at lo w (-0.94 to 0.36 V) applie d potentials. The spectroelectrochemistry for higher potentials is shown in Figure 34B for clarity. As seen in Figure 3-4A, the neutral, as-cast, film shows a max of 483 nm with an absorption onset of 562 nm, giving an optical bandgap value of 2.2 eV. The max is blue-shifted by 36 nm from the neutralized film (-0.94 V) as the as-cast spectra was taken on a dr y film. This is because the electrochemically switched film is electronically different fr om the as-cast film due to electrochemical annealing of th e film inducing both a shift in the absorbance to longer wavelengths and broadens the spectra. Figure 3-4. Spectroelectrochemist ry (A) of a drop-cast film of MEH-PPV on ITO/glass switched in 0.2 M LiOTf/ACN. The arrow shows dir ection of spectral growth. Photographs (B) of the same film held at potentials indicate d. All potentials given are referenced vs. Fc/Fc+.
65 When a potential sufficiently positive enough (+ 0.31 V) to cause oxida tion of the polymer is applied to the film the absorbance at the max decreases while a broad absorbance beginning at 600 nm begins to increase. This is due to the introduction of lower energy midgap state and depletion of the valence band as discussed in Chapter 1. A further increase in the potential applied to the film (+0.36 V) causes a shift in the max to shorter wavelengths and a further increase in the long wavelength absorbance. The shift in the max is due to the widening of the bandgap. This is because as the polymer is oxidi zed, electrons are removed from the top of the valence band and the now the minimum energy re quired for the electronic transition from the HOMO to the LUMO is higher. On further oxidation, the absorbance at max continues to decrease and shift to shorter wavelengths while the longer wave length absorbances from 600 to 800 nm also decrease as shown in Figure 3-4B. The polymer goes through several colored states on oxidation as is seen in the photographs in Figu re 3-4C. It should be noted that the unevenness in the films, as seen in the phot ographs, is due to the uneven evaporation of the solvent from the electrode edges during drop-casting. The fully reduced polymer is an orange-red color at -0.9 V and blue at +0.66 V vs. Fc/Fc+ with colored states of brown a nd greenish-blue at intermediate potentials. Electrochromism of Cbz2-Fl2 Solutions of the carbazole-fl uorene copolymer were made by dissolving the polymer in dry and deoxygenated chloroform to a concentration of 5 mg/mL. F ilms were cast onto ITO/glass electrodes by spray-casting from a commercial ai rbrush (Testor Corp., Rockford, IL) operated at a pressure of 20 psi. The films were then drie d at 50C in a vacuum ove n for 1 hour. As this polymer is soluble in many electrochemical solvents (i.e., chloroform, ACN, propylene carbonate, methylene chloride) except water, el ectrochemical switching was performed in an electrolyte of 0.2 M LiOTf in water. Th is, however, precluded electrochemical and
66 spectroelectrochemical characteri zation in the glovebox and all experiments were performed on the bench after purging the electrolyte with Argo n for at least 20 minutes. As with the MEHPPV films, the accessible electrochemical window for in-situ spectroelectrochemistry was determined by switching the polymer film on ITO by CV. While distinct color changes could be seen and an increase in current attributed to the polymer oxidation occurred, there was no discernable redox peak found on cycling to allow for determination of a polymer E1/2 as can be seen in Figure 3-5. The peak oxidation of the polymer occurs roughly at the same potential as the electrolyte oxidation, which is shown by the dashed line This oxidation of the supporting electrolyte interfered with the polymer oxidation, not allowing for switching at higher potentials to observe a peak current. Therefore, polymer oxidation was determined as the onset of anodic current increase. Figure 3-5. Cyclic voltammograms of a spray-cast film of Cbz2-Fl3 on ITO/glass switched at 30 mV/s in 0.2 M LiOTf/water for 50 scans. Every tenth scan is shown for clarity. The background scan (0.2 M LiOTf/water) is shown by the dashed line. Film in situ spectroelectrochemistry was performed with a Cary 500 UV/Vis-NIR spectrophotometer by holding the potential applie d to the film for one mi nute and recording the spectra from 350 to 800 nm. The one minute hold time was determined as with MEH-PPV by
67 monitoring the spectra until it no longer fluctuated. The max of the neutral film was determined by casting the polymer onto a quartz slide as the absorbance extends into the UV and could not accurately be determined on ITO/glass. The abso rbance of the neutral, dry film is shown in Figure 3-6A by the dashed line. The max is 349 nm with an absorption onset of 310 nm giving an optical bandgap of 3.0 eV. Figure 3-6. Spectroelectrochemistry (A and B) of a spray-cast film of Cbz2-Fl2 on ITO/glass in 0.2 M LiOTf/water. The arrow shows dir ection of spectral growth and recession. Photographs (C) of the same film held at various potentials. A ll potentials given are referenced versus Fc/Fc+. As with MEH-PPV, the absorbance spectra of th e electrolyte-soaked film is red-shifted (by 13 nm) due to swelling of the film. The spectroelect rochemical series shows a slight decrease in absorbance at 350 nm on oxidation while an in crease in absorbance at 500 nm and a broad absorbance extending from the NIR as can be seen in Figure 3-6A. The absorbances began to decrease beginning at the applied potential of 0.56 V as shown in Figure 3-6B. The polymer was
68 colorless in the neutral state at -0.8 V and brown when fully oxidized at +0.7 V vs. Fc/Fc+, with the colors of orange and reddi sh-brown at intermediate potentials as can be seen in the photographs of the film at seve ral potentials in Figure 3-6C. Electrochromism of Cbz2-Ph3 As with the Cbz2-Fl2 polymer, solutions we re spray-cast onto ITO/glass from a 5 mg/mL solution of the polymer in chloroform and dried in a vacuum oven at 50C for 1 hour. The films were electrochemically switched in 0.2 M LiOTf/water electrolyte by cyclic voltammetry. On the first scan, there was a slight oxidation peak at 0.4 V and a reduction peak at 0.1 V vs. Fc/Fc+ as seen in Figure 3-7. However, on subsequent scans, the current de creased and both oxidation and reduction peaks disappeared due to possible ov eroxidation of the polymer film. In addition, the film was initially colorless in the neutra l state and became a salmon pink color on oxidation, but on reneutralization, the film did not return to the colorles s state and remained salmon pink during switching. Figure 3-7. Cyclic voltammograms of a spray-cast film of Cbz2-Ph3 on ITO/glass at a scan rate of 30 mV/s in 0.2 M LiOTf/water for 5 scan s. Arrows show direction of current decrease.
69 As this polymer is not stable to repe ated switching, a fresh film was cast for spectroelectrochemical measurements without breaking-in the film by switching before measuring the spectra. Similarly with the Cbz2-Fl2 polymer, the max of the neutral film occurred in the UV, as can be seen in Figure 3-8A for the spectra (dashed line) of the as-cast film on quartz. The max was 334 nm with an absorption onset at 385 nm giving an optical bandgap of 3.2 eV. On oxidation, a peak began to emerge at 450 nm that broadened and shifted to 500 nm on further oxidation. On oxidation at low app lied potentials, the abso rption at 350 nm began to decrease while that at 450 nm increased. At potentials above 0.41 V, the absorbance centered at 500 nm decreased until 0.71 V after whic h no further optical changes occurred. Figure 3-8. Spectroelectrochemist ry of a spray-cast film of Cbz2-Ph3 on ITO/glass in 0.2 M LiOTf/water. A) The as-cast neutral a nd dry film (dashed line) and between the potentials of 0.11 V and 0.36 V. B) Between the potentials of 0.41 and 0.71 V. Arrow shows direction of sp ectral growth. Potentials are referenced vs. Fc/Fc+. All three polymers had relatively high oxida tion potentials with onl y MEH-PPV and Cbz2Fl2 exhibiting reversible electrochromism. Even though, Cbz2-Fl2 did change color on oxidation, the inability for the polymer to be switch ed as a film in solvents besides water and the lack of distinct, large contra st color changes is a limitation. As Cbz2-Ph3 is not stable on
70 oxidation and does not exhibit reversible electr ochromism, its electrochromism will not be studied further in this chapter. Reflective Electrochromic Displays One goal of this dissertation is to provide a new method for using combined electrochromism and electroluminescence for a di splay that would exhib it both properties from the same active material and from the same pixe l. To optimize the electrochromic response of a dual electrochromic/electroluminescent device, it is necessary to separately model the electrochromic display-half of the device. This design of the device woul d need to be modified from the ECD discussed previously in Chapter 2. While the device is a reflective type and would contain a back-hidden counter el ectrode and a front-positioned wo rking electrode, there exists added requirements. One such requirement is th e use of a transparent electrode, such as ITO that, during electroluminescent ( EL) mode, would function as the anode. Another requirement is for a third electrode in the de vice that would function as the cathode and be positioned in the device so that light emission w ould occur between the transparen t anode and the cathode. To this end, two reflective ECDs were designed that model the electrochromic half of the dual device with the intention of combining this de sign with a light-emitting device (as will be described in Chapter 4) to create the dual EC/EL. As was discussed in Chapter 1, the most comm on types of electrochr omic devices are of the absorptive/transmissive type or the absorptive/reflective type. While the absorptive/reflective device discussed thus far u tilizes a reflective porous electrode, there exists the possibility for alternative designs. Two el ectrochromic polymer devices that are based on a new reflective design are shown for the first time in Figures 3-9A and B. Both designs are composed of a polymer-coated transparent worki ng electrode, a porous refl ector that does not
71 function as an electrode, and a polymer-coated co unter electrode sandwiched together with the main difference being the type of refl ector utilized (diffu se or mirror-like). Figure 3-9. Schematic of reflective electrochro mic displays with A) porous white diffuse reflector and B) aluminum-coated porous membrane. The first device (Figure 3-9A) contains a gold-coated Kapton counter electrode, on a glass support, onto which a layer of PEDOT is electrochemically depo sited from a monomer solution containing 10 mM EDOT, and 0.2 M LiOTf in propylene carbonate. The electropolymerization was performed poten tiostatically at 1.6 V until 83 mC/cm2 of charge had passed. This is topped with three pieces of whit e filter paper each coated with a layer of gel electrolyte. The electrolyte was prepared by dissolving 1 g of LiOTf and 2.3 g of PMMA in 5.6 mL of propylene carbonate to form a viscous cl ear gel. The active work ing layer was prepared by casting the electrochromic polymer onto ITO/ glass. The entire device was sandwiched together, electrodes contacted with copper tape a nd the device sealed with epoxy on all four sides to hold the device together.
72 The second device, Figure 3-8B, is fabricated similar to the first device, except with an aluminum-coated porous membrane and two pieces of filter paper sandwiched between the working and counter electrodes. The meta llized membrane was prepared by thermally evaporating aluminum to 60 nm thickness onto po rous polycarbonate membranes. With the first device, device A, the white porous membrane acts as a paper-like diffuse reflector while in the second device, device B, the aluminum membrane is the metallic mirror-like reflector. For both devices, the active polymer layer is coated onto the ITO/glass that is inward facing. This is different from the reflective ECD described in Chapter 2 in which the polymer was coated onto the outward facing metallized por ous membrane that functioned as the working electrode. As the active polymer layer is cast onto a rigid substrate (ITO/glass), this allows for application of the polymer by spray-casting or spin-coating, as is required for the active layer in the dual EC/EL device. The reflective porous electrode de tailed in Chapter 2 does not allow for spincoating of the polymer films given that the memb ranes are very thin (10 m) and deformable and highly porous. This is because the method by whic h substrates (the membranes in this case) are secured to the spin-coater chuck is through vacuum suction. For that reason, electrodes coated onto rigid substrates (glass or ~0.2 mm thick plastic film) are required. To test the device designs, a model elect rochromic polymer, with known reproducible electrochromic behavior was utilized as the acti ve layer. The polymer chosen for this study was the previously mentioned PProDOT-(CH2OEtHx)2 for its ease of processability, high ambient stability, and known electrochromic re sponse in a device configuration.47 The active polymer layer was prepared by spray-casting a solution of 5 mg/mL of PProDOT-(CH2OEtHx)2 in toluene onto 1.5x1.5 cm ITO/glass slides. The spectroelectroche mical series of both devices were measured using a Cary 500 UV/Vis-NIR sp ectrophotometer fitted with an integrating
73 sphere with the spectra of a reference device ta ken as the baseline. The reference device was constructed with the same materials and same co nfiguration as device A and device B, but with no active polymer layer deposited on the working electrode. A potential sufficient to fully neutralize the active polymer layer was applied ac ross the device (-1.2 V) and the potential was then stepped in 200 mV increments and held for 1 minute before each spectrum was taken. This was continued until the fully oxidized state of the polymer was achieved. The spectroelectrochemical series for both devices A and B, containing a spray-cast film of PProDOT-(CH2OEtHx)2 as the active layer are show n in Figures 3-10A and B. Figure 3-10. Reflectance spectroelectr ochemistry of spray-cast PProDOT-(CH2OEtHx)2 in an electrochromic display with a A) porous white reflector and B) aluminum-coated porous membrane. The absorbance of the polymer film was broa d across the visible region with the maximum absorbance centered around 550 nm. As oxidizing potentials are applied across the device, the reflectance in the visible region begi ns to increase as was also seen for the same polymer film in the reflective ECDs in Chapter 2. Similarly to the spectroelectrochemistry for MEH-PPV earlier in this chapter, the max shifts to shorter wavelengths on oxidation of the polymer while a decrease in the reflectance in th e red region occurs. While this sp ectroelectrochemical series is
74 only taken of the visible region out to 800 nm, it is expected that the reflectance at longer wavelengths would decrease on oxidation. It can be seen that the active polymer layer is fully oxidized at +1.2 V for device A, with a maximum contrast of 42% in the visible region. However, even beyond an applied voltage of +1.5 V in device B, there remains a residual ab sorbance in the visible region with a maximum contrast of 26%. This can be attributed to the aluminum porous reflector in this device. As the device is switched over time it can be seen that the aluminum metal takes on a grey haze that decreases the reflectance of this device, po ssibly due to oxidation. This also causes the background correction taken with the reference de vice to no longer be adequate in correctly subtracting contributions to the spectra fr om the device components not involved in the electrochromic switching, such as the aluminum re flector. This could possi bly be due to a side reaction occurring in the electroche mical cell between the lithium triflate electrolyte in contact with the aluminum electrode, causing corrosi on of the aluminum while at the same time reneutralizing the oxidized polymer layer, not allowing for full el ectrochromic switching. To effectively model the electrochromic re sponse of the luminescent conjugated polymers in the dual EC/EL display, devices based on the designs of device A and device B were constructed with the blends of the electroluminescent polymers with solid electrolytes as the active layer. The blends were comprised of either MEH-PPV or Cbz2-Fl2 as the active electrochromic material, LiOTf as the salt, and PEO as the ion solvator (for the MEH-PPV blend only). The Cbz2-Fl2 blend does not contain PE O as this polymer contains ion-solvating oligoethoxy side chains. The MEH-PPV blend solution was made by pr eparing master solutions of the three components separately. The master solutions we re comprised of MEH-PPV, PEO, and LiOTf in
75 concentrations of 10 mg/mL, 13 mg/mL, and 13 mg/mL each in cyclohexanone, respectively. The blend solution was then prepared by adding each component in a weight ratio of 10:3:1 (MEH-PPV:PEO:LiOTf) and diluting with cycl ohexanone such that the final MEH-PPV concentration was 5 mg/mL. Th e films were prepared by spin -casting the blend solution onto ITO/glass slides at 1000 rpm to obtain a final thickness of 170 nm. Devices were constructed with designs of device A and B with the MEH-PPV blend as the active electrochromic layer. Figure 3-11A shows the spectroelectrochemical seri es for the electrochromic device A, with the white paper reflector along with photographs of the device in the neutral (orange) and fully oxidized (blue) states in Figur e 3-11B Figures 3-11C and D s how the spectroelectrochemical series and photographs of the MEH-PPV device with the porous alumi num reflector (device design B). The maximum spectral contrast for the device based on design A was ~15% at 480 nm and 35% at 600 nm while that based on design B was ~60% at 480 nm and near 30% at 600 nm. This difference in maximum contrast between the two devices at the shorter wavelength can be explained by the incomplete oxidation of the pol ymer film in device A as seen in the photograph in Figure 3-11B. As the encircled area of the ph otograph shows, there are portions of the film that do not become oxidized to the bl ue color, but rather remain orange. This can be attributed to the already high oxidation potential of this polymer along with th e unevenness of application of the gel electrolyte layer. In the areas where the polymer is not fully switched, the concentration of gel electrolyte could be very thin, and switching of these areas ar e not as efficient, leading to incomplete oxidation. In the device based on design B, porous alumi num reflector, there are residual absorbances at the high energy end of the spectrum (below 400 nm) and the low energy end of the spectrum
76 (above 650 nm). This can be attributed to th e aforementioned ineffective background correction due to the variations between the reference device (without active polymer layer) and the actual measured device. On initially preparing the me tallized membranes, the al uminum metal layer is highly reflective with a silvery finish. However, after being repeatedly sw itched, the finish of the aluminum layer loses the luster and beco mes a dull grey. This can cause a difference between the reference device (that retains its hi gh reflectance) and the wo rking device in which the aluminum layer is oxidized and the reflecta nce is decreased. It should be noted that on resting for several days (~3 days) the aluminum layer in the reference device also loses luster. Figure 3-11. Spectroelectrochemistry (A) and photographs (B) of ME H-PPV/PEO/LiOTf blend reflective ECD with porous white reflect or. Spectroelectrochemistry (C) and photographs (D) of MEH-PPV/PEO/LiOTf blend reflective ECD with aluminumcoated membrane.
77 To further demonstrate an electroluminescent polymer as the electrochromic layer in a reflective electrochromic device, Cbz2-Fl2 blended with LiOTf was utilized as the active layer in both device designs A and B. A solution of Cb z2-Fl2 (5 mg/mL) blended with LiOTf in a weight ratio of 20:4 in cyclohe xanone was spin-cast onto ITO/glass at a rate of 500 rpm yielding a film thickness of 47 nm. As mentioned previousl y, an ion solvating material, such as PEO, is not needed as the polymer contains ion solvati ng oligoethoxy pendant gro ups. This leads to a decreased solution viscosity compared to the MEH-PPV/PEO blend and makes producing thick films by spin-coating difficult. Any future work involving this polymer would involve investigating blending of the Cbz2-Fl2 with PEO. While the presence of oligoethoxy side-chains is intended to solvate the LiOTf without the ne ed for PEO, the side-chains could allow for improvement of film quality and morphology on blending with PEO and other ion conducting polymers Spectroelectrochemistry of the Cbz2-Fl2/LiOTf blend as the active layer in device A and device B are shown in Figure 3-12A and B, respec tively. Both devices switched from a colorless state to a pale orange color on oxidation, however the optical cont rast was not high as the film was rather thin and the polymer does not exhibit highly colored states as was also seen in the photographs in Figure 3-6C. The device based on the porous white reflector (device A) had an optical contrast of 20-25% at 500 nm, requiring a pot ential of at least 1.4 V to reach the oxidized state. The device based on design B, porous alum inum reflector, exhibited a maximum contrast of 30% at 500 nm. This device had little change in color or spectra until an applied potential of 2.0 V. As with the MEH-PPV device with the alum inum reflector, there is a residual absorbance at longer wavelengths, possibly due to the difference in the refl ectance of the reference device compared to the active device.
78 Figure 3-12. Spectroelectrochemistry of Cbz2-F l2/LiOTf blend reflective ECD with A) porous white reflector and B) aluminum-coated membrane. On comparison of the reflectance spectra of the device containing Cbz2-Fl2 as the active layer and the absorbance spectra of that same polymer switched in a LiOTf/water electrolyte, a peak for max is clearly seen for the film in the devi ces while none could be determined for the films switched in water, while the onset for absorb ance is at the same wavelength in all spectra. This shift in the absorbance peak into the visible region can be attributed to solvent effects as the film in the device is switched in a propylene carbonate-based gel el ectrolyte. It should be noted that after switching both devices to the oxidized state, the polym er film would not completely change color back to the neutral, colorless state. This is due to the high solubility of the polymer in propylene carbonate, although as a gel, in which the polymer dissolves, not allowing it to be further switched effectively. Due to this complic ation as well as the low optical contrast, this polymer will not be further studied as a possible material in the dual EC/EL device and attention will be focused on MEH-PPV as the active material for this specific study. Conclusions A newly designed reflective electrochromi c display based on two different porous reflective layers, white paper and aluminized me mbrane, has been demonstrated for the soluble
79 electrochromic polymer PProDOT-(CH2OEtHx)2 and two electroluminescent polymers MEHPPV and Cbz2-Fl2. These devices model the electr ochromic half of the dual EC/EL display that will be the focus of Chapter 4. While the de vices introduced in this chapter effectively demonstrated the possible utilization of two diffe rent electroluminescent polymers as the active electrochromic layer, several modifications to the device would need to be made in further studies to optimize display performan ce. Various salts, such as LiClO4, TBABF4, and even ionic liquids, could be utilized as well as other ion-solvating materials such as crown ethers, and other ionically conducting polymers. In addition, other re flective metals could be utilized in the place of aluminum, such as gold, that are resistant to oxidation. The contrast of the white reflective layer in device A could be optimized by using a wh ite porous paper that does not darken when wetted or applying a coati ng to the paper to increase the brig htness of the white. This can be achieved by utilizing a porous Teflon membrane or coating the white filter paper with a white laminate that is impervious to the solvents be ing used. Other electroluminescent materials can also be investigated as the active layer such as ruthenium complexes, or other conjugated electroactive polymers and copolymers. As the study to be detailed in Chapter 4 will be the first demonstration of a dual EC/EL device, the focus of the work in this chapter has been placed on demonstration of the device components to achieve the desired result of electrochromism and electroluminescence from a single active layer at the sa me pixel in one device. It is expected that further work to be conducted after this dissertation will focus on optimizing the proper combination of materials and device designs to achieve the best device performance.
80 CHAPTER 4 DUAL ELECTROCHROMIC/ELECTROLUMINESCENT DISPLAYS Conjugated conducting polymers are among a unique class of materials that exhibit a wide variety of useful properties. As was discusse d in Chapter 1, these polymers are electrically conductive in the redox-doped state by either ex posure to chemical oxidants/reductants or by electrochemical oxidatio n/reduction. Some of the applica tions in which doped polymers are used include antistatic coatings,174 electrochromic windows/displays,22 supercapacitors,19 actuators,24 and as transparent electrode materials.40 On the other hand, there have been a large number of applications where the undoped form of conjugated polymers are useful as the polymer backbone allows for charge carrier (elect ron and hole) transport within the valence or conduction band. Some of these applic ations include phot ovoltaic devices,23 field-effect transistors,21 and light-emitting diodes.20 The discovery of electroluminescence in conj ugated polymers has been one of the most important developments in the field as it has increased the interest of these materials for consumer display devices. Since then, there has been much interest in improving device lifetime and performance, and on producing colors that sp an the visible region by structural modification of the polymer backbone, along with a number of techniques that incl ude guest-host blends, multi-layered structures, and others. Another development in the field of polymer light-emitting displays are the light-emitting el ectrochemical cells (LECs), as is also described in Chapter 1, that offer the advantage of device operation w ithout the need for low work function, reactive electrode materials. Devices have even been demonstrated with the same metal as both anode and cathode where the electrodes are in a late ral arrangement with light emission occurring between electrode lines separated by up to 1 mm.129
81 The most extensively utilized display in cons umer electronics today is that based on liquid crystal displays (LCDs).175 Typical LCDs contain either a light-source (transmissive) or a reflective mirror behind the display elements (reflective). The transmissive devices operate by varying the amount of light allowe d to pass through the device wh ile the reflective LCDs contain a fully reflective mirror relying on ambient lighting or a front-light to provide illumination of the image. While the transmissive, back-lit displa ys have excellent image quality in low lighting situations, such as indoors, the quality is marked ly diminished in situations where the brightness of the ambient lighting is higher than that of th e back-light used in the display, such as outdoors in direct sunlight. A display able to be uti lized in both low and high ambient lighting is that based on transmissive-reflective, called transrefle ctive, technology. This device contains both a back-light and a reflective mirror. The mirror is partially reflecting, allo wing some of the light from the back-light to pass when the ambient lighting is low and reflects when the ambient lighting is high. However, the brightness of the LCD indoors is not as high as with the transmissive display as the partially reflectiv e mirror decreases the amount of light transmitted and the image quality is compromised in both modes compared to the fully transmissive or fully reflective displays. In addition, LCDs have th e added disadvantage of reduced viewing angle and inability to fabricate mechanically flexible displays. Another type of display is that based on microelectromechanical (MEMs) technology introduced by QUALCOMM and known as an Inte rferometric Modulator Display, or IMOD. This display is fabricated by placing a thin film mirror at a transparent substrate with a small gap between that is several hundred nanometers with the gap acting as an optically resonant cavity. Ambient light that strikes the display is reflecte d at the transparent substrate and the reflective mirror. Depending on the distance between the mirror and the substrate, light of various
82 wavelengths are produced by constr uctive and destructive interfer ence. The distance between the thin film mirror and the transparent substrate is varied by the application of a voltage causing the deformable mirror to collapse towards the substrate by electrostatic forces.176 As with other reflective displays, an external light source is required to illuminate the device. This is accomplished with either ambien t lighting or a front-light. A display that has been gaini ng interest is the electrophoretic display (EPD), also referred to as electronic paper. Electrophoretic displays were designed to imitate the appearance of ink on paper. Typically, the electronic ink is dark (b lue or black) in color and is viewed against a white background, providing a higher optical contrast than achieved with other displays. The technology is based on a suspension of charged micron-sized white particles, such as titanium dioxide, dispersed in an oil that is dyed with a dark ink. Th e suspension is encapsulated in microparticles that are then placed between two para llel electrodes. When a positive potential is applied to the top electro de, the negatively charged white pa rticles migrate to the top of the microcapsules and the viewer sees the white, high ly reflecting, particles. A positive voltage is then applied to the bottom electrode and the white particles migrate to the bottom of the microcapsules, allowing the dark ink to now be seen by the viewer. This technology was developed by E Ink Corporation, a spin-off from MITs Media Labs and through a partnership with Philips, these displays have appeared in several consumer electronic devices such as Sonys Reader177 and Motorolas Motofone.178 As with the other reflectiv e displays, viewability relies on an external light source and a front-light is necessary to vi ew the display in low ambient lighting. This chapter details the design and operat ion of a device that, for the first time, demonstrates electrochromism and light-emission from a single active layer at the same pixel. In
83 low ambient lighting situations, th e display could be operated in the emissive mode without the need for an additional light source. In high ambient lighting situati ons, as in direct sunlight, the display would be operated in the electrochromic mode. The electrochromic function of the display is modeled after the devi ces described in Chapter 3, while the emissive function of the device is described within this chapter. Befo re description and demonstration of the dual display, an investigation into the applicability of two types of polymer light-emitting displays (the PLED and PLEC) are detailed. PLEDs were fi rst investigated as there is a long established history of these devices with publ ished methods for fabrication a nd characterization data. This allowed for a model device to es tablish a prototype for construc tion and development of methods for device fabrication, analyti cal characterization, and mate rials performance along with protocols for obtaining electrooptical data. Thes e materials and methods were then applied to the construction and characterizatio n of PLECs, that are not as es tablished in the literature and require an added level of complexity in device construction and operation. Polymer LEDs The operating principle of PLEDs is given in Chapter 1 and shown in Figures 1-5 and 1-6 with the schematic of a typical device given in Figure 1-7. The device in this research was constructed using a sandwich-type design with the ITO anode as the first layer. The ITO was patterned and cleaned as detailed in Chapter 5. A thin (40 nm) layer of PEDOT:PSS was then spin-coated onto the patterned IT O substrate and baked at 150C in a vacuum oven to remove residual water. The PEDOT:PSS layer is used as a hole transport layer as it lowers the barrier height for hole injection into the polymer layer.43, 84, 85 The PEDOT:PSS-coated substrates were then transferred to an Argon atmosphere glove box with an oxygen cont ent less than 0.5 ppm and water content less than 0.1 ppm. The active el ectroluminescent polymer layer was then spincoated inside the glove box at va rious spin-rates for 30 seconds from a solution of 5 mg/mL of
84 MEH-PPV in chlorobenzene. The spin-rates used are 800, 1000, and 1200 rpm producing films of approximately 70 nm in thickness as measured by AFM. The invariability in film thickness is attributed to the low viscosity of the spin so lution, and as has been s hown previously, slight increases in spin-rate has less of an affect on film thickness for solutions of lower concentration.179 The films were then loaded into a thermal evaporator and the entire chamber pumped down to 1 x 10-6 mbar for ~3 hours. The cathode material (10 nm calcium) was thermally evaporated onto the f ilms followed by 150 nm of an alum inum capping layer to protect the cathode. Both metals are evaporated through a patterned sh adow mask creating an active pixel area of 7.07 mm2 with each device containing 8 pixels. The electroluminescence of the devices were characterized by applying voltages across the pixel with the ITO connected as the anode and Ca connected as the cathode while light output was measured using the fiber-optic spectrometer, as described in Chapter 5, while inside the glove box. To allow for complete fabrication an d characterization of P LED devices, an MBraum glove box system was established in which optic al measurements are performed using a fiberoptic spectrometer that allows remote measuremen t of light output from in side the glove box. To accurately control the position of the fiber-optic ca ble relative to the device pixel, a X-Y-Z stage was utilized and all device measurements were pe rformed in an enclosed dark box fabricated inhouse. All optical measurements were taken norm al to the device surface with the bare optical fiber having a fiber diameter of 400 m. The dist ance from the fiber-optic tip to the pixel is chosen to allow the entire pixel area to be within the acceptance angle of the fiber-optic with an additional 2.19 mm on each side of the pixel to allow for slight variat ions in positioning the fiber-optic. This is shown schematically in Figure 4-1. The full acceptance angle (2 ) of the fiber-optic is 24.8. By defini ng the radius of the circular area to be measured as 3.69 mm
85 (radius of emitting area of pixel plus the 2.19 mm on each side) and the half angle of acceptance as 12.4 a distance can be determined by taking th e ratio of the defined radius to the tangent of the half angle, d = R/tan and is 16.8 mm. Figure 4-1. Schematic of measurement geometry between fiber-optic probe and PLED pixel. The optical measurements were performed using an Ocean Optics USB4000 spectrometer as detailed in Chapter 5. The intensity of the light emitted from the pixel that is incident on the fiber-optic is measured for the wavelength range of 400 to 800 nm and is recorded as the spectral irradiance, E. Spectral irradiance is defined as th e radiance flux per unit area incident on a surface and per unit wavelength interval at the wavelength and is given by the equation: Em= d m dE = ds0d m d2U (4.1) where is the radiant flux (watts) and s0 is the area, and is gi ven in units of W cm-2 nm-1. As the radiation emitting from the pixel is incident on the fiber-optic tip surface, the area is defined by the fiber-optic tip area and is 1.26x10-3cm2. While the total power emitted from the pixel is constant and uniform in all directions and is distributed over an area th at increases with the
86 distance from the pixel squared, the irradiance drops off in pr oportion to the distance from the pixel squared. Therefore, the di stance between the pixel and fiberoptic tip, as defined in Figure 4-1, is kept constant for all measurements performed to allow comparison of irradiance measurements at different vol tages and different pixels. The spectral irradiance measured for an indi vidual MEH-PPV pixel operated at increasing voltages is shown in Figure 4-2. At as low a voltage as 4 V, the spectral irradiance is 6.8x10-3 W cm-2 nm-1 at 585 nm. As the voltage is increased, the intensity of the light emitted increases to a maximum of 9.5x10-2 W cm-2 nm-1 with a shift in the peak wa velength of 13 nm to 572 nm while the visually observed color is a constant or ange-yellow as shown in the inset to Figure 4-2. Figure 4-2. Spectral irradiance of an ITO/PEDOT:PSS/70 nm ME H-PPV/Ca/Al PLED pixel at different applied voltages. Inset sh ows photograph of pixel operated at 9V. The blue-shift in peak wavelength can be at tributed to the increased contribution to the electroluminescence spectra from shorter conjugatio n-length fractions in the polymer film that emit at shorter wavelengths as higher voltages are applied. This is further supported by the
87 shoulder in the spectra at longer wavelengths that decreases in relation to the peak wavelength at higher voltages. This longer wavelength emi ssion is due to the l onger conjugation-length fractions in the polymer film that are present at a lower concentration and have a lower turn-on voltage for emission. At higher voltages, the co ntribution from the highe r concentration shorter conjugation-length fractions begins to dominate. Spectral irradiance is a unit of measurement of radiation and can include not only the visible region of the spectra, but all electromagnetic radiation.180 Quantities such as spectral irradiance, along with others not considered here, are defined as radiometric quantities. In the display field, it is necessary to consider the re sponse of the human eye to this radiation in the visible region and therefore, a subset of radi ometry has been defined that creates a relation between the radiometric measurements and the hu man eye response and is called photometry. A photometric quantity commonly used is luminance (Lv, units of cd m-2) and is a measure of the subjective perception of brightness of a light source as a function of the area of the detector and is the photometric equivalent to radi ance and given by the following equation: Lv= 683 LmV ( m ) d m380 770# (4.2) The subscript, v is used to indicate a photometric quantity, L is spectral radiance and V( ) is the photopic spectral luminous efficiency as defined by the Internat ional Lighting Commission (CIE) in 1924 and given in the IES Lighting Handbook.181 Spectral radiance is the area and solid angle density of radiant flux per unit wavele ngth interval and has the units of watt m-2 sr-1 nm-1. The solid angle is calculated by ta king the ratio of the area of th e pixel to the square of the distance from the pixel to the fiber-optic tip (16.8 mm). Th is value is 0.025 sr for the measurements detailed in this work. The f actor 683 in the equation for luminance is an additional constant necessary for the conversion of watts (radiometric) to lumens (photometric)
88 as V( ) is a relative spectral respons e function of the eye. The luminous efficacy at 555 nm (the wavelength where the human eye is most sensitive) is 683 lm W-1 and accounts for the absolute magnitude of the conversion. The average luminance for a PLED contai ning a 70 nm thick film of MEH-PPV at increasing voltages is shown in Figure 4-3 by the blue line. The luminance of the pixel is 79 cd/m2 with as little as 4 V applied. The brightness of the pixel increases with increasing voltage until a maximum value of 1565 cd/m2 at 9 V. As a comparison, t ypical computer monitors and color televisions have a luminance of 100 cd/m2, and a typical fluorescent lamp has a luminance of 4x103 cd/m2. The current density also increases with applied voltage with a turn-on voltage of 3 V and a maximum current density of 767 mA/cm2. Beyond an applied voltage of 9 V, the luminance and current decrease unt il the pixel ultimately burns-out and fails. The literature shows great variability in reported luminance va lues and is due to the variability in device fabrication methods, materials used and optical measurement methods as no standard methods for PLEDs have yet to be established.20, 43, 82, 86, 92 The PLED exhibits rectifying diode-like beha vior with current flow and light emission occurring only in the positive voltage direction. As is shown in Figure 1-5, hole injection occurs at the anode to the valence band of the polymer while electron in jection occurs at the cathode into the conduction band of the polymer. To mini mize the barrier to charge carrier injection, the Fermi level of electrodes must be as closely matche d as possible to the relevant energy levels of the luminescent polymer. This requires the use of a high work function anode, such as ITO, and a low work function cathode, such as calcium. De vices have been demonstrated with aluminum as the cathode with no calcium layer, but with low luminance values and a high turn-on voltage of over 12 V.84 As the overall goal is to use electroc hemical systems for the dual device, it is
89 desired not to use low work function metals as the high reactivity of the calcium layer is not well-suited for use in a device that is to function as both an electrochromic and electroluminescent device. In addition, doping of the electroactive poly mer is known to quench electroluminescence in PLEDs and switching of th e active layer between neutral and oxidized states during the electrochromic mode will not allow for further operation of the device in the luminescent mode. A type of electroluminescen t device with conjugated conducting polymers as the active layer that would allow for operation without the need for low work-function metals and could tolerate repeated doping/dedoping of th e polymer layer is the LEC as described in Chapter 1. Figure 4-3. Current density (black) and luminance (blue) versus applied voltage for an average of 4 pixels of an ITO/PEDOT:PSS/70 nm MEH-PPV/Ca/Al PLED. Polymer LECs The operating principle for a polymer LEC is shown in Figure 1-8. The device layout is a sandwich-type of design as with the PLEDs. The ITO anode wa s patterned and cleaned in the
90 same manner as was described for construction of the LED devices. As no alignment of the electrode work function with the polyme r conduction/valence bands are necessary, a PEDOT:PSS layer is not utilized in these devices. The active polym er layer contains a blend of the electroluminescent polymer, MEH-PPV, an ion solvating polymer, PEO, and a salt, LiOTf. Master solutions of each component were made by dissolving the material in cyclohexanone to a concentration of 10 mg/mL, 13 mg/mL, and 13 mg/mL for the MEH-PPV, PEO, and LiOTf, respectively. These solutions were allows to stir for 48 hours on a hotplat e at a temperature of 40C to allow the solutes to completely incorpor ate into the solution. Th e blend solutions were then made by mixing each component in a weig ht ratio of 10:3:1 (MEH-PPV:PEO:LiOTf) and diluted with the appropriate am ount of solvent so that the fi nal MEH-PPV concentration is 5 mg/mL. This give a salt concentration of 3 mM. This weight ratio is one of the most commonly referenced in the literature for the best perf orming devices. Further future studies on the dual EC/EL displays would involve optimization of th e blend components and relative weight ratios for the best performance in both EC and EL mode and will be highlighted in the last section. The polymer films were cast by spin-coati ng at spin-rates of 800, 1000, and 1200 rpm to produce film thicknesses of 240, 170, and 75 nm, re spectively. Given that this solution is relatively viscous, the spin-rate ha s a more pronounced effect on the final film thickness than that for the PLED device films. The most effective method for obtaining a film sufficiently thick for optimum device performance (between 100 and 200 nm thick) by spin-coating is to use a fairly concentrated solution. As the viscosity of this solution is higher than with the LED active layer solution (5 mg/mL MEH-PPV in chlorobenzene) car e must be taken to ensure that there are no air bubbles remaining in the solution when it is dispensed onto the ITO substrate for spin-coating as the air bubbles will cause hol es to be present in the film, creating shorts when the cathode
91 layer is deposited. To remove air bubbles in the solution, th e tip of the pipet was dragged through the solution, across the subs trate as the solution was disp ensed. Lowering the viscosity of the solution by dilution is not a satisfactory option as this decreases the active polymer layer thickness on spin-coating to values that create devices that have a lower light output and a higher incidence of shorts between the anode and cathode. As with the PLED devices, the cathode layer (aluminum alone) was deposited by therma l evaporation through a shadow mask. The devices were characterized in the same ma nner as the PLEDs with J-V measured using a Keithley SMU and the EL spectra and luminan ce measured using the Ocean Optics USB4000. The J-V and L-V curves for an average of 3 pixels for a MEH-PPV:PEO:LiOTf (10:3:1) LEC with an active layer thickness of 170 nm and with ITO as the anode and aluminum as the cathode is shown in Figure 4-4. An unexp ected feature of these devices is that the light emission from the device is very low for positiv e voltages but high for negative vo ltages. While ambipolar light emission is expected, as has been shown in the literature and can be seen here with an onset for increase in current density and luminance at V, better device performance is expected with oxidation (p-doping) occurring at the ITO electro de and reduction (n-doping) occurring at the aluminum.97 To date, little has been reported on both th e positively and negativel y biased J-V and L-V characteristics of MEH-PPV:PEO :LiOTF LECs beyond the first pape r to report these devices in 1995. On inspection of the results reported in that paper, it is actually uncl ear as to whether the device performed better at negativ e voltages relative to positive voltages as the luminance seems to level-off at the highest voltage they report (+ 4 V) while a leveling-off is not yet seen at the highest negative voltage shown (-4 V).109 As seen in Figure 4-4, the maximum brightness in the positive voltage direction is 39 cd/m2 with a current de nsity of 170 mA/cm2 at 5 V. In the
92 negative voltage dire ction, the maximum brightness is 227 cd/m2 with a current density of -255 mA/cm2 at -8 V. Figure 4-4. Current density (black) and lumina nce (blue) versus applied voltage for an ITO/blend/Al PLEC with an active layer of a blend of MEHPPV:PEO:LiOTf in a weight ratio of 10:3:1 at 170 nm thick. On comparison of the current density and lumi nance for low applied vo ltages in both the positive and negative directions, the luminance is higher for positive voltages up to V with lower current densities. From the applied voltage of +5 to +6 V, the luminance decreases and the current density more than doubles. On increasin g the negative voltage, the luminance increases and continues to do so until -8 V after which it decreases, possibly due to degradation of the polymer blend at more negative voltages. A po ssible explanation for th e cross-over in higher luminance for positive voltages below 4 V to highe r luminance for negative voltages greater than -4 V is alteration of the aluminum contact to the polymer at these higher positive voltages. A
93 similar effect was observed as discussed in Chapter 3 for the aluminum-coated membranes used in the reflective ECDs. The eluc idation of the mechanism here is beyond the scope of this work. It is also noticed that the cu rrent density peaks at a lower voltage, -7 V, than the luminance and then begins to decrease. This can also be se en in Figure 4-5 for an average of three pixels in a separate device with a 170 nm th ick active layer. At voltages gr eater than -5 V, the current essentially levels off. This can be attributed to the near-thin layer ce ll behavior of a PLEC device. Between the voltages of -3 and -5 V, the majority of the current is devoted to setting up of the pand n-doped polymer junction. At voltage s more negative than -5 V, the majority of the current is devoted to maintaining the high con centration of doped polymer within the cell and regenerating charge carriers as holes and electrons recombine within the p-i-n junction. Figure 4-5. Current density (black) and luminance (blue) versus applied voltage for an average of 3 pixels of an ITO/blend/Al PLEC w ith an active layer of a blend of MEHPPV:PEO:LiOTf in a weight ratio of 10:3:1 at 170 nm thick. The spectral irradiance of a pixel of a MEHPPV LEC device with an active layer of 170 nm at increasingly negative voltages is shown in Figure 4-6. The spectra ar e very similar to that
94 for the PLED device shown in Figure 4-2; however the peak wavelength does not shift to shorter wavelengths as was observed in the PLEDs. Wh ile applying higher voltages to the PLED device allows access to the energy levels of the shor ter-conjugation length fractions of the polymer, these higher energy levels are not able to be accesse d in the PLECs. This is due to the different mechanism for charge carrier injection of PLEDs vs. PLEC (carrier tunneling vs. oxidation/reduction). While the vo ltages applied across the PLEC device are sufficient to cause p-doping and n-doping of the polymer bulk, the volta ges are not sufficient to dope the shorter chain-length fractions as their oxi dation/reduction potentials are higher than that of the bulk film. As can be seen in the inset phot ograph, the emission color is the same for the PLEC as the PLED based on MEH-PPV as emission is from the same ex cited state with the PLED light intensity ~5 times brighter. Figure 4-6. Spectral irradiance of a ITO/170 nm MEH-PPV:PEO:LiO Tf blend/Al PLEC pixel at different applied voltages. Inset show s photograph of pixel operated at -6 V.
95 In addition to device brightness and current density, relative device stability will be another important factor for th e device performance when inco rporated into the dual EC/EL display. Figure 4-7, shows that after an initial break-in cycle, the device shows little hysteresis when cycling between 0 and -8 V and only an 8% drop in luminance between the 2nd and 3rd cycles. While there will conti nue to be a slow decay, the rate of decay lessens. This is encouraging as these devices were characterized without any form of packaging, allowing the polymer layer and contacts to be fully exposed. Figure 4-7. Luminance (A) and cu rrent density (B) as a function of applied voltage for an ITO/blend/Al PLEC with an active layer of a blend of MEHPPV:PEO:LiOTf in a weight ratio of 10:3:1 at 170 nm thick cycl ed between 0V and -8 V for three cycles.
96 Another indication of device stability is a m easurement of pixel lifetime at a constant voltage as is shown in Figure 4-8. The pixel wa s cycled between 0 V and -8 V five times and then held at -6 V for 13 hours. During this time the spectral irradiance integrated from 400 to 800 nm was monitored. As can be seen in the inset in Figure 4-8, the pixel reached maximum brightness within 7 seconds of initial turn-on af ter having the initial cyc ling break-in. At this time, the luminance was 54.5 cd/m2 and the current dens ity was under -200 mA/cm2. After 4 minutes, the luminance dropped 24% from the initi al value. At 1.5 hours a minimum in light output reaches a value of 1.5 cd/m2. However, the light output be gan to increase again and did so for the next 8.5 hours and again decreased. Figure 4-8. Integrated spectral irradiance over the wavelength range of 400 and 800 nm for a LEC pixel operated for 13 continuous hours at -6V. The inset shows the spectral response in the first 2 minutes of operati on. The device consisted of ITO/blend/Al PLEC with an active layer of a blend of MEH-PPV:PEO:LiOTf in a weight ratio of 10:3:1 at 170 nm thick.
97 Previous researchers have also noticed a sec ond increase in light emi ssion after an initial decline during lifetime measurements and attributed this to self-heating of the device.182 PEO is a semicrystalline polymer at room temperature a nd these crystalline regions (60% of the bulk) present barriers for ion diffusion with the ion tr ansport preferentially occurring in the amorphous phases.183 These crystalline regions begin melting at temperatures above room temperature depending on the water content present in the PE O. Since great care was taken to effectively remove the water from the PEO and handle the poly mer in a dry atmosphere, it is expected that the water content is minimal. The melting of the crystalline regions in PEO with weight fractions of water less than 0.05 begins to occur at 30C.184 As these crystalline regions begin to melt, the ionic conductivity of PEO then increases This could then lead to the higher light output after the initial decrease in device brightne ss as the charge carrier mobility is increased. After several hours of operating at this higher temperature, the de vice inevitably begins to fail with light output and current de nsity decreasing to a minimum as seen in the lifetime experiment. The work described here is the first in whic h the PEO component of the LEC blend is dried to this extent. The technique used was lyophili zation, a method to effectively remove water from biological and pharmaceutical samples without destroying the sample as high temperature techniques can do. With lyophilization, the sample in this case PEO, was dissolved in the low boiling point solvent, acetonitrile, that forms an azeotrope with water. The solution was then quickly frozen in liquid nitrogen and placed under vacuum. As the sample is allowed to slowly warm to room temperature, th e acetonitrile/water azeotrope sub limes and collects at a watercooled condenser. LEC active la yer blends made with this PEO were compared to PEO dried as has been previously done th roughout the literature by heating th e sample in a vacuum oven to
98 40C. Figure 4-9 shows the L-V and J-V curves for the device containing the lyophilized PEO (A) and the non-lyophilized PEO (B). While both devices exhibit the same turnon voltage at 3 V, the non-lyophilized PEO device initially has a higher luminance and highe r current density. At -6 V the non-lyophilized PEO device has a luminance of 113 cd/m2 while the device containi ng the lyophilized PEO has a luminance of 45 cd/m2 at that same voltage. However, the device with the non-lyophilized PEO begins to fail with a drop in luminance and current density at voltage s beyond -6 V while the device containing the dry PEO continues to exhi bit an increase in luminance at increasing negative voltages until -10 V. The degree of crys tallinity is dependent on the amount of water present with PEO containing a higher weight fr action of water exhibiting a lower degree of crystallinity. These results show that while the drier PEO may have a lower ionic mobility (exhibited by the lower luminance) than the PEO with a higher percent of water present, the device with the drier PEO shows a higher stab ility, operating efficien tly to higher negative voltages due to the absen ce of water that can have a detrim ental effect on device performance. It has been demonstrated that blends of MEH-PPV/PEO/LiOTf cast as thin films can be utilized as the active layer in reflective electrochromic displays (Chapter 3) with ITO as the working electrode and containing an aluminum-coa ted porous membrane as the reflective layer. This chapter demonstrates that these blends of MEH-PPV with the solid electrolyte PEO/LiOTf can also be utilized as the active layer in light -emitting electrochemical cells with light emission occurring by n-doping at the ITO electrode and p-doping at the aluminum electrode. A device is then constructed that is a combination of both of these devices, showing electroluminescence and electrochromism from MEH-PPV at the same pixel.
99 Figure 4-9. Comparison of the J-V and L-V response of two LECs constructed with PEO dried to different extent. A) PEO dried by lyophilization, B) PEO dried in a vacuum oven at 40C. Dual EC/EL As was discussed earlier in this chapter, there are currently a large variety of device designs for visual displays, some of which operate in a reflective mode, while others operate in an emissive mode. Reflective displays operate at an advantage in situat ions in which ambient light is sufficient to provide cont rast when the device or pixel is switched, but need an external light source in dark situations while light-emittin g displays operate efficiently in low ambient light situations, yet have poor im age quality in highly lit environments. Many devices that are utilized in displays and consumer electronics commonly operate in e nvironments where the ambient lighting varies signifi cantly. This includes cell phones, personal digital assistants (PDAs), laptop computers, iPods and related en tertainment devices, and many Department of
100 Defense oriented displays (such as those in aircraft cockpits). A display that could effectively function in a variety of lighting conditions by switching between a full color reflective electrochromic operation and a light-emitting operation would be advantageous. Here, it is demonstrated for the first time adva nces on just such a device. A combination of both an ECD and a LEC was utilized, which op erates as a reflective ECD when one set of electrodes are biased, and as a LEC when anothe r set of electrodes are biased. The dual EC-EL effect is generated from a single active film mate rial as desired for ultimate ease of production. As shown schematically in Figure 4-10A a nd by the photograph in Figure 4-10B, the device consisted of the active polymer la yer coated onto a patterned ITO substrate. The next layer was a metallized porous membrane followed by a porous separator and finally with the electroactive polymer-coated counter electrode. Light emission occurred when the front ITO (electrode I) and metallized membrane (electrode II) were conne cted as anode and cathod e. Electrochromism occurred when the ITO (electrode I) was conn ected as the working electrode and the back electrode (electrode III) wa s connected as the counter electrode as in an ECD. In the device construction, ITO-coated glass with the polymer blend was used as both the anode for light-emission and the working electrod e for electrochromism. The ITO for the top anode (electrode I) was patterned as two parallel strips 5 mm wide, 2 mm apart, forming two transparent electrodes. The device was assemble d using a blend solution as the active polymer layer. The blend contained MEH-PPV, PEO, and LiOTf in a weight ratio of 10:3:1, respectively, dissolved in cyclohexanone as with the LEC devices described previously. The polymer blend was cast onto electrode I by spin-coating 400 L of the solution at 1000 rpm. A metallized porous membrane was then pla ced on top with the metal side facing the active polymer layer. This electrode was a porous polycarbonate membrane that has been
101 patterned with a 60 nm layer of aluminum by ther mal evaporation deposited in a pattern of two parallel strips that were 5 mm wide and separated from each other by 2 mm. These strips were patterned such that they lay perp endicular to the ITO-patterned st rips, forming 4 separate pixels that are 25 mm2 in area. This electrode acts as the cathode when the device is operated as a light-emitting electrochemical cell. Figure 4-10. Schematic of dual EC/EL device (A). Photograph of actual device (B). The back electrode (electr ode III) was ITO-coated glass on which PEDOT had been electrochemically deposited as described in Ch apter 3. This electrode acts as the counter electrode when the device is opera ted as an electrochromic display. A separator paper, soaked in electrolyte, was used to isolat e the back of the meta llized membrane from the counter polymer layer as some of the metal can penetrate the memb rane and short with the counter electrode. The electrolyte consisted of 5 mg/mL PEO and 0.9 mg/mL Li OTf dissolved in ACN.
102 Before assembly, the solvent was allowed to ev aporate, leaving behind a solid electrolyte. In addition, a layer of the solid electrolyte was also deposited onto the PEDOT layer and allowed to dry before device construction. This was to ensure a sufficient con centration of ions for electrochromic operation of the device. All elect rodes were contacted with copper tape and the device was sealed with epoxy on the edges. With this design, only the active polymer layer is responsible for the light-emission and electrochromism observed, while the counter electrode polymer is used for charge balanc e during electrochromic switching. As the top ITO anode and the middle metallized porous cathode are connected to a voltage source, light emission occurs from the active pol ymer layer. After the addition of a small amount of ACN to improve ion mobility (~250 L), the front ITO electro de was connected to a potentiostat as the working elec trode and the back PEDOT-coate d ITO electrode was connected as the counter electrode. When the device was biased with positive voltages applied to the working electrode, the active polymer layer el ectrochromically switc hed between a neutral orange-red color to an oxidized blue color. The device was then biased with negative voltages applied to the working electrode and the active polymer layer sw itched back to the orange-red neutral state. It should be not ed that the utilizatio n of a lower viscosity electrolyte should improve ion mobility during electrochromic sw itching, thereby eliminating the need for the addition of small amounts of solv ent during electrochromic switching. As is shown in the photographs in Figure 4-11A (entire device) and D (close-up of a pixel from a different device), when the ITO electrode s were connected as the anode and the porous aluminum electrodes connected as the cathode, the pixels emitted light when -21 V was applied. This high voltage for light emission is expected as there is poor contact between the polymer film and the aluminum electrode. This can be improved in further studie s by evaporation of a
103 thin layer of aluminum directly onto the polymer film. The light emitted was orange, with a peak wavelength centered around 600nm. This light-emission was relatively steady and sustained as long as the volta ge is continuously applied. Figure 4-11. Photographs of a dual electrochromic and light-emitting device with MEH-PPV as the active material. A) shows a device wh en it is operating in the light-emitting mode. B) and C) show a separate device wh en it is operating in the electrochromic mode with the polymer oxidized in B) and re neutralized in C). A close-up of a device operated in light-emitting (D), and electrochromic (E and F) mode. The top ITO electrode was then connected as the working electrode and the back ITO electrode (hidden) connected as the c ounter electrode after addition of 250 L of ACN to the cell. When +3.5 V was applied across the cell, th e color of the active pol ymer layer at the ITO electrode changed color from its neutral orange-red state to its oxidized blue state. In the photograph in Figure 4-11B (Figure 4-11E shows a close-up of the same pixel), the pixel on the left was connected to the potentiostat while the pixe l on the right left at open circuit. Therefore, only the left pixel changed color wh ile the right pixel remained in the neutral state. When -0.8 V was applied to the left pixel, the polymer was neut ralized and returned to its orange-red color.
104 This can be seen in Figure 4-11C (and the closeup of the same pixel in Figure 4-11F), where the pixel on the left is again orange-re d. The pixel on the right is stil l at open circuit. These pixels can be repeatedly switched between the two color states. Overview and Future Directions While the ultimate goal of this chapter to demonstrate a dual electrochromic/ electroluminescent device has been attained, many key concepts and methods were also demonstrated and established. Many device construction and measurement methods were established with the fabrication of standard MEH-PPV PLEDs with complete fabrication in an inert atmosphere glovebox along with optical char acterization using a new fiber-optic system. This allowed for the creation of a protocol for fu rther analytical characte rization of materials and devices of the more complicated PLECs. These PLEC devices, never before demonstrated in our laboratory, were constructed with a MEH-PPV/PEO/LiOTf active bl end layer with the onset for light-emission occurring at V and maximum lumi nance values on par with those reported in the literature. As the operational behavior of this type of device was observed with reproducible results, it allowed the incorporation of the LEC concept into the dual EC/EL device. The reflective electrochromic and light-emitting cell designs were then integrated into a single device where both electrochromism and electroluminescence were shown. While this is the first time it has been demons trated for a display device to exhibit both electrochromism and electroluminescence from the same pixel, there are yet a number of future modifications to the device components or desi gn that could be made to improve device performance, as with any proof-of-concept. A possible alternative would be to investigate the possibility of other solid electr olytes by varying the sa lt and/or the ionically conducting polymer. As was also mentioned, PEO is a semicrystalline polymer and the presence of crystalline regions inhibits ionic mobility within this phase. Ot her ion-solvating polymers that have a higher
105 amorphous content at room temperature could possibly offer an improvement over PEO. However, care must be taken to choose a salt that is easily solvated by the ionically conducting polymer and both the salt and ion-solvating po lymer would need to be soluble in a common solvent with the emissive polymer and this re quirement further limits the choice in solid electrolyte. Several researcher s have shown that either salts complexed with crown ether-based systems or ionic liquids as at tractive choices in LEC devices.115, 120 There also exists the possibil ity of utilizing other electrolum inescent polymers in the dual EC/EL to achieve a wide range of colors not onl y for light emission, but also electrochromism. The requirement for a polymer to be utilized in th is device is that it exhibit emission in the solid state, have a HOMO level high enough to allow oxidation of the polymer during electrochromic switching at readily accessible potentials, and have an accessible reduction potential. In addition, alternative device designs are another possibility to overcome some of the limitations encountered and improve device perf ormance. One such device would be the planar/lateral device. This device would be compri sed of an interdigitated microelectrode (IME) on a substrate that has been coated with the activ e blend layer. This is then followed by a porous separator and a counter electrode coated with a count er polymer layer as is shown in the schematic in Figure 4-12. For light emission, the interdigitated electrode s I and II are contacted as the anode and cathode with light emission occurring between the fingers. For electrochromism, both interdigitated electrodes I and II are shorted together as the working electrode and the polymer-coated back electrode III is connected as the counter electrode. For initial experiments, the IMEs could be pur chased from a commercial source that offers line widths and line spacings from 5 to 20 m w ith electrode materials such as gold, platinum, and ITO on glass. Previous research by Heeger et al., has demonstrated the concept of a lateral
106 LEC with electrode line spacings of 15m;109 however, it has been recently shown that line spacings of up to 1mm are possible while maintaining efficient device operation.185 Macrointerdigitated electrodes could be fabricated in-house of va rious electrode materials using photolithography, shadow masking, or line patter ning. This would also lend well to the patterning of PEDOT-PSS onto glas s or plastic as the aqueous pol ymer solution can be printed allowing for a flexible, all-plastic dual device.186 Figure 4-12. Schematic of propos ed lateral dual EC/EL device. As can be seen, there are a large number of possibilities for future direction for these devices, many of which lie outside the scope of th is research to develop and demonstrate a new type of device. However, throughout this diss ertation, new device designs and applications for these designs have been established and add to the growing list of uses and applications for conjugated conducting polymers.
107 CHAPTER 5 INSTRUMENTATION AND EXPERIMENTAL TECHNIQUES As this dissertation covers various, differi ng, applications of c onjugated electroactive polymers, a large variety of techniques have been utilized in fabrication of devices and their subsequent characterization. This chapter sets forth to present an overv iew of the materials, techniques, and instrumentation used during the course of this research. Chemicals and Materials Reagent grade acetonitrile (ACN), propylene carbonate (PC), and cyclohexanone in Sure Seal bottles were purchased from Aldrich. Chlo roform (Aldrich) and ACN were distilled over CaH2. PC was percolated through activated type 3A molecular sieves, followed by fractional distillation under reduced pressure Cyclohexanone was directly transferred to a Schlenk flask under Argon by cannula, followed by freeze-pump-thaw for three cycles and stored in the glovebox. Solvents used in the glovebox were deoxygenated by freeze-pump-thaw before being transferred into the glovebox. Solvents used on the bench were stored under Argon and purged for a minimum of 10 minutes prior to use. Tetrabutylammonium hexafluorophosphate (TBAPF6Fluka), lithium perchlorate (LiClO4 Alrich), and lithium trifluoromethanesulfonate (LiCF3SO3 Aldrich) were dried under vacuum at 150C for 24 hours prior to use. Poly(methyl methacrylate) (PMMA) (Mw 85,000 g/mol, PDI 2.4) was purchased from Aldrich and used wit hout further purification. Poly(ethylene oxide) (PEO Aldrich) (Mw 1,000,000 g/mol) was dried by lyophiliza tion (as detailed in Chapter 4) from ACN prior to use. Poly(3,4-ethyle nedioxythiophene)/poly(styrene sulfonate) (PEDOT:PSS) (Baytron P VP Al 4083) was purchas ed from H.C. Starck and syringe filtered through a 0.45 micron Nylon filter followed by filt ering through a 0.2 micron Nylon filter prior before use. The monomer, EDOT (Baytron M V2) was purchased from H.C. Starck and distilled
108 under vacuum from CaH2. Poly[2-methoxy-5-(2-ethylhexy loxy)-1,4-phenylenevinylene], MEHPPV (OPA6345) (Mw 175,000 g/mol, PDI 1.3) was purchased from H.W.Sands and used as received. Poly(3,3-bis(2-et hylhexyloxymethyl)-3,4-dihydro-2 H -thieno[3,4b ][1,4]dioxepine) (PProDOT-(CH2OEtHx)2) (Mw 75,000, PDI 1.6), and poly(3, 3-dihexyl-3,4-dihydro-2 H thieno[3,4b ][1,4]dioxepine) (PProDOT-Hx2) (Mw 66,000 g/mol, PDI 1.7) were prepared as previously reported.48 Poly[(9,9-bis(methoxyet hoxyethyl)-2,7-fluorene)alt -( N (methoxyethoxyethyl)-3,6-carbazole )] (Cbz2-Fl2), and poly[(1,4bis(methoxyethoxyethoxyet hoxy)-2,5-phenylene)-alt-( N -(methoxyethoxyethyl)-3,6-carbazole)] (Cbz2-Ph3) ,whose structures ar e shown in Figure 3-2, were synthesized specifically for this study.187 Briefly, the polymers were synthesized by Suzuki coupli ng between the diboronate of the carbazole monomer and the corresponding aroma tic dihalide (phenyl or fluorene) monomer. The polymerizations required equimo lar amounts of 3,6-di(dioxaborolanyl)N methoxyethoxyethyl-carbazole and the ar omatic dihalide (1,4-dibromo-2,5bis(methoxyethoxyethoxyethoxy)benzene or 2,7-dib romo-9,9-bis(methoxyethoxyethyl)fluorene) were added to a solution of Aliquat 336 in toluene and bubbled with Argon. To this, a 2M aqueous solution of sodium carbonate was added with freshly prepared palladium tetrakis(triphenyl phosphin e) and refluxed at ~105oC for three days. The organic layer was collected, concentrated by eva poration and the product isolated by dissolving in dichloromethane followed by precipitation into methanol. The pol ymer was collected by filtration and dried under vacuum. ITO-coated glass slides were purchased fr om Delta Technologies (CG-50IN-CUV and CB50IN-0111). Prior to use, the ITO slides were cleaned by light mechanical rubbing with a cotton swab soaked in a surfactant solution (aqueous sodium dodecylsul fate). This was followed by
109 rinsing in a stream of distilled water, then ace tone and isoproanol, then air dried. Track-etched polycarbonate membranes were purchased from GE Osmonics Inc. Membranes are 10 microns thick with 10 micron diameter cylindrical por es. Gold (99.99% pure) was purchased locally (National Coin Investors, Inc.). Platinum wire and sheets, silver wire, and aluminum slugs were purchased from Alfa Aesar. Platinum button el ectrodes were purchased from BAS. ITO/glass interdigitated microelectrodes (IME 1050.5-M-ITO-U) were purchased from AbTech Scientific. The digit length was 5 mm with 10 micron line width and 10 micron spacing between digits. Gold-coated Kapton (100 nm thick gold layer on 1 mm thick Kapton sheets) was purchased from Astral Technologies and cut to size with sc issors. Copper tape, used to contact ITO and Au-coated membrane electrodes was purch ased from 3M electronics division. Device Construction Electrochromic Displays The gel electrolyte used in the devices was composed of either 1 g of TBAPF6 or 0.4 g of LiOTf dissolved with 2.3 g of PMMA, in 5.6 mL of PC giving a salt c oncentration of 0.46 M. The gel was prepared by dissolving the salt in PC, followed by slowly adding PMMA while stirring and heating to 60C unt il all components were incorpor ated to form a highly viscous clear gel. Goldor aluminum-coated polycar bonate membranes were prepared by thermally evaporating the metal using a high vacuum ther mal evaporator (Denton DV-502A) operated at 1x106 mbar. The membranes were sandwiched be tween an aluminum plate and a patterned mask. The metals were deposited to a final thic kness of 60 nm at a deposition rate of 4 /sec. The reflective ECDs as described in Chapter 2, were fabricated by sandwiching together several layers. The fabrication, from bottom to top, began with an acetate (3M transparency film, PP2500) support onto which the counter electrode was placed. The counter electrode consists of a gold-coated Kapton electrode (contacted with copper tape) with an
110 electropolymerized layer of PEDOT. The PEDOT was polymerized using potentiostatic conditions at 1.6 V vs. Ag wire until a depositi on charge of 200 mC has passed. The monomer solution contained 10 mM EDOT and 0.2 M TBAPF6 in PC. This layer is left in the oxidized form for device construction. The next layer is three pieces of filter paper (Fisherbrand, 09-8035A) that are soaked in the gel electrolyte. This is followed by the outward facing working electrode, which is the gold-c oated polycarbonate membrane. The active electrochromic polymer layer is applie d to this electrode by either spray-casting or electropolymerization. Spra y-casting was performed using a 5 mg/mL solution of the desired polymer in toluene that is sprayed from a co mmercial airbrush (Testors Corp., Aztek TS28) operated at an air pressure 20 psi. Electroch emical polymerization was performed by applying a potential high enough to oxidize the monomer (typically 1.2 to 1.6 V vs. Ag wire) in a solution of 10 mM monomer, 0.2 M TBAPF6. This potential was held until the desired polymer thickness was achieved as determined by passed charge dens ity, with values typically ranging from 3 to 17 mC/cm2. The active polymer layer thickness was t ypically equal to or less than the PEDOT counter layer thickness, which ensured full switc hing of the working electrode. A transparent window (3M transparency film, PP2500) was placed on top and the entire device sealed on all four edges using transparent tape. The device schematic and photographs can be seen in Figure 2-1A and B. The ECDs described in Chapter 3 were fabri cated similarly; differing in the type of reflector and working electrode The counter electrode was the same as described above (PEDOT on Au-Kapton). For the device containing the aluminum reflector, the counter electrode was followed by two pieces of gel electrolyte-soaked filter paper. The next layer was an aluminum-coated polycarbonate membrane that is outward facing (as shown in Figure 3-9B).
111 The working electrode is ITO/glass on which the act ive electrochromic layer was spin-cast. The working and counter electrodes we re contacted with copper tape and the entire device sealed with epoxy on all four edges. Fo r the device containing the white paper reflector (Figure 3-9A), the counter electrode was followe d by three pieces of gel electrol yte-soaked filter paper. The working electrode is again ITO/gl ass coated with the active elec trochromic layer by spin-casting. Polymer Light-Emitting Diodes Polymer LEDs were prepared by first etchi ng the ITO/glass (25 x 25 mm) to create an anode pattern. The pattern for ITO etching ca n be found in the disse rtation by a previous doctoral student in the Reynolds group.188 The ITO was covered w ith clear packing tape (ScotchTM Brand) and the pattern cut out using a razor blade. Th e unmasked areas of ITO were etched by exposure to aqua regia vapors (3:1 HCl:HNO3) for ten minutes. The etched ITO was cleaned as described previously. The electrodes were placed in an oxygen plasma cleaner for 30 minutes. The PEDOT:PSS hole transport layer wa s spin-coated onto the ITO electrodes at 4000 rpm to a final thickness of 40 nm The electrodes were baked in a vacuum oven at 150C for 12 hours. The substrates were transferred to an Argon atmosphere drybox where the remaining device fabrication and characte rization took place. The activ e electroluminescent layer was applied by spin-coating a solution of 5 mg/mL MEHPPV in chlorobenzene at various spin rates for 60 seconds. The devices were transferred to a thermal evaporator and the cathodes were deposited at a pressure of 1 x 10-6 mbar. The first layer was calcium deposited to 10 nm followed by 150 nm of aluminum through a shadow mask giving an active pixel area of ~7.07 mm2. The pattern for the shadow mask is as previously described.188 Polymer Light-Emitting Electrochemical Cells Polymer LECs were fabricated in a simila r manner as the LEDs. The ITO/glass anodes were patterned and cleaned by the same methods. The difference being that there is no
112 PEDOT:PSS hole transport layer. After cleaning, the electrodes we re immediately transferred to the glovebox. The electroluminescent layer was spin-cast onto the ITO anodes at various spin rates for 60 seconds. The LECs master soluti ons contained the elec troluminescent polymer, PEO, and LiOTf each at a concentration of 10 mg/mL, 13 mg/mL, and 13 mg/mL in cyclohexanone, respectively. The MEH-PPV ble nd was prepared by combining each component in a weight ratio of 10:3:1 and diluted to a MEH-PPV concentrati on of 5 mg/mL. The carbazole copolymer blend solutions were prepared by ad ding LiOTf to the polymer master solution to form a weight ratio of 20:4 with a final copolym er concentration of 10 mg/mL. The films were then transferred to the thermal evaporator and an aluminum cat hode was thermally evaporated to a thickness of 150 nm through the same sh adow mask described for the PLEDs. Dual Electrochromic/Electroluminescent Devices The layered dual EC/EL devices (as shown in Figure 4-10) were fabricated based on the combination of the standard LEC device and th e aluminum reflector electrochromic device design. The first layer of this device is comprised of a PEDOT-coated gold/Kapton counter electrode on a glass support. The next layer is two pieces of filter pa per soaked with a LiOTfbased gel electrolyte. On that la yer is the aluminum-coated porous membrane that acts as both a cathode for light-emitting operation and a reflective metal for the electrochromic operation. The deposited aluminum is patterned in two strips that are 5 mm in width and 10 mm apart. The active layer is comprised of ME H-PPV:PEO:LiOTf in a weight ra tio of 10:3:1 in cyclohexanone (as with the LECs) spin-cast on th e ITO anode. The ITO is patterned on the anode in two strips that are 5 mm in width and 10 mm apart (as w ith the cathode). Th e strips are arranged perpendicular to the aluminum strips such that the active light-emitting pixel area is defined by the overlap between the aluminum and ITO electrodes and is 25 mm2. The entire device was sealed with epoxy on all four edge s. Operation of the device is described in detail in Chapter 4.
113 Electrochemical Methods The electrochemical methods used in polymer electrochemistry have been extensively reviewed by previous res earchers in the Reynolds group.189, 190 Therefore, the methods specific to this work will briefly be covered here. A ll electrochemical measurements were performed with an EG&G PAR model 273A potentiostat c ontrolled using CorrWare software (Scribner Associates) Electropolymerization The potential for electropolymerization of a monomer is typically determined by cycling the potential of a working electrode and monitoring the resulting cu rrent. Once the peak current for monomer oxidation is determined, that pote ntial is used for subs equent potentiostatic electropolymerization experiments. The typical electrochemical cell setu p is comprised of a working electrode, reference electrode, and coun ter electrode in a glass cell. The working electrodes used in this work include ITO/gla ss, gold-coated polycarbon ate membranes, and goldcoated Kapton. Electrical contact to these electrode s was made with copper tape. The electrode is immersed in the monomer solution in the glass cell. The setup also includes a counter electrode that is a plati num flag made by spot-welding plat inum wire to platinum foil. The size of the foil is such that the area of the co unter electrode is always larger than the area of the working electrode to be used. The reference electrode is a silv er wire pseudoreference that is frequently (every half-hour) calib rated with a standard ferrocene solution (5 mmM ferrocene, 0.2 M LiClO4, ACN). Monomer solutions are composed of 10 mM monomer in 0.2 M salt (either TBAPF6 or LiOTf) in propylene carbonate and are bubbled with Ar for at least ten minutes. During the potentiostatic electropolymerization, the oxidizing potential is applied to the working electrode. This polymerization is term inated when a specific charge has passed where the charge is determined in previous experiment s in which a calibration plot of charge versus
114 final film thickness is prepared for each monome r. By this method the resulting polymer film thickness can be chosen by setting the potential to be applied until the necessary charge has passed. The polymer films are then rinsed with fresh solvent and electr ochemically switched in monomer-free electrolyte. Polymer Electrochemistry Electrochemistry of polymer films, whethe r prepared by spray-casting, drop-casting, or electropolymerization, is performed in a three-electrode cell. This cell is described as above with the polymer-coated electrode as the working electr ode, a platinum flag as the counter electrode, and a Fc/Fc+-calibrated silver wire as the reference electrode. The film is switched by cycling the electrode potential using cyclic voltammetry (C V) in an electrolyte solution of 0.2 M salt in either PC, ACN, or water that has been bubbled with Argon for ten minutes. An Argon blanket is kept over the solution duri ng the measurements. The potential applied to the working electrode is scanned to a value sufficient to oxidize the polymer a nd cycled back to the beginning while the current is measured. Polymer films typi cally require a break-in of at least five scans before repeated cycling of the pot ential is stable to ion and so lvent diffusion in and out of the film. If an anodic and cathodic peak are clearly visible, the E1/2 of the polymer is then determined by summing the peak cathodic curren t with the peak anodi c current and dividing their sum by 2. If an anodic peak current is not clear, the onset for the current increase on oxidation is determined and reported. Optical Methods Spectroelectrochemistry Benchtop spectroelectrochemical measurements were performed with a Varian Cary 500 UV/Vis-NIR spectrophotometer. Polymer films to be measured on ITO/glass were placed in a standard 1 cm cuvette fitted with a Teflon cap to hold the ITO/glass in place along with the
115 reference and counter electrode wire s. These wires were placed on either side of the cuvette so as to not block the source beam or come into contact with the working electrode. The cuvette was filled with the appropriate electrolyte so lution that has been purged with Argon for 10 minutes. The baseline is taken as an identical cell to the one to be measured minus the polymer film. The electrodes were connected to a potentio stat and the desired potential applied to the working electrode and held while the wavelength region of inte rest was being scanned. An electrochemical break-in scan of th e film is performed before the spectroelectrochemical series is performed, except where noted. Spectroelectrochemical meas urements performed in the glovebox were done using a StellarNet EPP2000 Vi s-NIR fiber-optic spectrophotometer. The instrument contains a SL1 calib rated light source and a photo diode array detector. The fiberoptic cables allowed remote measurement of the polymer spectra from inside the glovebox. Benchtop reflectance spectroelec trochemistry of ECDs was performed using the Cary 500 spectrophotometer with an integrat ing sphere attachment mounted to the instrument. The inside of the sphere is coated with highly reflective BaO and has a sample and reference port. The device is placed at the sample port while a BaO standard is placed at the reference port. The baseline is taken using a refere nce device that is constructed w ith the same components as the sample device, except for the active polymer layer. The reflectance measured is total reflectance and is comprised of both specular and diffuse light The device is connec ted to a potentiostat with the working electrode connection to the active electrode. The reference and counter connections are shorted together and connected to the counter electrode of the device. A potential is applied across the de vice and held while the wavelength re gion of interest is scanned. Reflectance measurements using the fiber-opt ic spectrometer were performed using the setup described in Chapter 2. The device is pl aced (face up) between the two plates of the
116 sample holder and the input and output fibers are attached. The light measured from the device is specular only at a 45 angle. The schematic and photograph of the de vice holder is shown in Figure 2-12. Electroluminescence Measurements Electroluminescence of PLEDs and PLECs was measured using an Ocean Optics USB4000 fiber-optic spectrometer containing linear CCD array dete ctor. The response of the detector is calibrated on a monthly basis using a standard LS-1-CAL light source. This allowed measurements to be performed in the glovebox under an Argon atmosphere. Voltages were applied to the device using a Keithley 2400 sour ce measurement unit (SMU ) with the positive bias applied to the ITO contacts and negative bias applied to the aluminum contacts and resulting current measured. Optical measurements were ma de normal to the device surface using the bare fiber-optic of the USB4000. The position of the fi ber-optic probe was controlled, relative to the device pixel, using a X-Y-Z stage while the device is enclosed in a dark box, to not allow for stray light. The spectrometer was contro lled using the SpectraSuite software. Electroluminescence measurements were made by applying a voltage to the desired pixel and measuring the spectra l irradiance (W/cm2/nm) from 350 to 800 nm as detailed more fully in Chapter 4.
117 APPENDIX A POLYMER STRUCTURES S O O n PEDO T S OO S O O S OO S O O S OO S O O n SO3 SO3H SO3H SO3H SO3H SO3 SO3H n PEDO T :PSSS O O n PP r oDOT n PAcN H O O n PProDOPN O O O O OH n PProDOPN -Gly O O n MEH-PPVS O O n PProDOT-Hx2S O O O O n PProDOT-(CH2OEtHx)2 n PPV n PPPN H n PCbz n PFl O O N O 2 2 2 n PC b z2-F l 2N O O O O O 3 3 2 n PC b z2-Ph3 O CH3 O CH3 n PMMA O n PEO
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129 BIOGRAPHICAL SKETCH Aubrey Lynn Dedrickson was born July 13, 1978 in Bataan, Philippines to Sheila and Randy Dedrickson. Her father was in the U.S. Navy, and therefore her family moved around quite a bit. They lived in Homestead, FL, Hawa ii, Pensacola, FL, and Altamonte Springs, FL. She has one younger brother, Shaun Dedrickson, and one younger sister, Stephanie Dedrickson. She graduated high school in Pensacola, FL afte r which she attended the University of West Florida for her first year of college while working full-time. She and Nathan Dyer married in 1999 in Pensacola, Florida and they both moved to Shippensburg, PA wh ere Aubrey attended Shippensburg University of Pennsylvania full-time. She received her B.S. in Chemistry in the Summer of 2002 and began her gradua te studies at the University of Florida that August. In the Fall of 2002, she joined the Reynolds research group.