ON MICROCAVITY AND TRIPLET EXCITON BEHAVIOUR IN ORGANIC AND HYBRID LIGHT EMITTING DIODES By CHAOYU XIANG A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT OF THE REQUIREMEN TS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY UNIVERSITY OF FLORIDA 2014
201 4 Chaoyu Xiang
To my mother and f ather
4 ACKNOWLEDGMENTS F irst and foremost, I would like give my gratitude to my great parents who are always loving support ing and patient to me. They never lose faith in me and always help me with their wisdom. T hroughout my academic career I own my thanks to Dr Franky So for his full academic guidance and a few living philosophy, both of which beco me my life time resources. I also appreciate the time and help from Dr Michele Manuel, Dr Rajiv Singh, Dr Jing Guo, Dr. Jiangeng Xue, and Dr. Stephen Pearton for agreeing to serve on my supervisory committee. I also want to acknowledge all of my lab mates for their support and collaboration through my PhD academic career. They are Do Young Kim Lei Qian, Kaushik Roy Chouhury, Subbiah Jegadesan, Leo Chuang, Galileo Sarasqueta, Cephas Small, Michael Hartel, Song Chen, Jae Woong Lee, Wooram Youn, Jesse Mander s, Fred Steffy, Tzung Han Lai, Wonhoe Koo, Chieh Chun Chaing, Sai Wing Tsang, Shuyi Liu Zhe Ying, Jiho Ryu Erik Klump, Dania Constantinou, Xiangyu Fu, Sujin Park, David Yu, Rui Liu, Micheal Sexton, Cheng Peng, Samuel Ho, Wesley Hamlin, Jong Hyun Kim, Ale ssandra Pereira, Ying Cheng, and Dewei Zhao Together we made a good team. I also want to thank other colleagues from the department: Mark Davison, Ying Zheng, Weiran Cao, Yixing Yang, Renjia Zhou and Nate Showmon. They also helped me with their excellent abilities and knowledge. Also, there are a lot of collaborators out of campus, who provides materials to support the research used in this thesis. I also appreciate their cooperation.
5 Finally I would like to say thanks to University of Florida and around Gainesville over the past four and half years. They made my life colorful in Gainesville where great deals of memories of my life are
6 TABLE OF CONTENTS page ACKNOWLEDGMENTS ................................ ................................ ................................ .. 4 LIST OF TABLES ................................ ................................ ................................ ............ 8 LIST OF FIGURES ................................ ................................ ................................ .......... 9 LIST OF ABBREVIATIONS ................................ ................................ ........................... 12 ABSTRACT ................................ ................................ ................................ ................... 15 CHAPTER 1 FUNDAMENTALS OF ORGANIC SEMICONDUCTORS ................................ ........ 17 1.1 Physics of Organic Semiconductors ................................ ................................ 17 1. 1.1 Molecular Orbitals ................................ ................................ .................... 17 1.1.2 Photophysics of Organic Molecules ................................ ......................... 18 1.1.3 Exciton and Intermolecular Exciton Energy Transfer ............................... 20 1.1.4 Charge Transport ................................ ................................ .................... 22 1.2 Fundamental of OLED ................................ ................................ ...................... 23 1.3 Terminology of Photometry ................................ ................................ ............... 25 1.4 Optical property in OLEDs ................................ ................................ ................ 27 ................................ .......................... 27 1.4.2 Fabry Perot Interference ................................ ................................ ......... 28 1.4.3 Theoretical Analysis of Strong Microcavity ................................ .............. 29 1.5 Optical Simul ation ................................ ................................ ............................. 31 1.5.1 Transfer Matrix Formalism for Passive Layers ................................ ........ 31 1.5.2 Optics of Emissive Multilayer Structures ................................ ................. 32 1.6 Optical Properties of Colloidal Quantum Dots ................................ ................... 34 1.7 Characterization of Organic Materials and Devices ................................ .......... 35 1.7.1 Terms of Efficiency of OLEDs ................................ ................................ 35 1.7.2 Electroabsorption (EA) Spectroscopy ................................ ...................... 36 2 MANIPULATION OF EXCI TON BY MICROCAVITY IN OLEDS ............................. 45 2.1 Background and Motivation ................................ ................................ .............. 45 2.2 Experiments ................................ ................................ ................................ ...... 46 2.3 Results and Discussion ................................ ................................ ..................... 47 2.3.1 Relation between Resonant Enhancement and Reflectivity of DBR ........ 47 2.3.2 Relatio n between Resonant Enhancement and Cavity Length ................ 49 2.3.3 High Efficiency Green OLEDs with Microcavity ................................ ....... 53 2.4 Conclusions ................................ ................................ ................................ ...... 55
7 3 EXCITON CONFINEMENT IN MULTILAYER CADMIUM FREE BLUE EMITTING QD LEDS ................................ ................................ .............................. 66 3.1 Background and Motivation ................................ ................................ .............. 66 3.2 Experiment ................................ ................................ ................................ ........ 67 3.3 Results and Discussion ................................ ................................ ..................... 68 3.3.1 Characterizaion of Cd free QDs ................................ .............................. 68 3.3.2 Deep Blue QD LEDs ................................ ................................ ............... 68 3.4 Conclusion ................................ ................................ ................................ ........ 71 4 TRIPLET EXCITONS IN HIGH EF FICIENT LOW ROLL OFF BLUE EMITTING PHOLEDS WITH TERCARBAZOLE HOSTS ................................ ......................... 77 4.1 Background and Motivation ................................ ................................ .............. 77 4.2 Experiment ................................ ................................ ................................ ........ 80 4.2.1 Synthesis and Characterization of Ter carbazole Hosts .......................... 80 4.2.2 Device Fabrication and Characterization ................................ ................. 81 4.2.3 Transient Measurement ................................ ................................ ........... 82 4.3 Results and Discussion ................................ ................................ ..................... 83 4.3.1 Designing of Ter carbazole Molecules ................................ .................... 83 4.3.2 Characterization of Ter carbazole Materials ................................ ............ 84 4.3.3 Blue Emitting Phosphorescent OLEDs ................................ .................... 85 4.3.4 Annealing Effects on mCP and ETC ................................ ........................ 87 4.3.5 Hole Mobility and Injection into Hosts ................................ ...................... 88 4.3.5 Triplet Lifetime and TTA of mCP and ETC EMLs ................................ .... 90 4.4 Conclusion ................................ ................................ ................................ ........ 95 5 SUB BANDGAP TURN ON OF ELECTROLUMINESCENCE AT RUBRENE /FULLERENE INTERFACE ................................ ................................ 108 5.1 Background and Motivation ................................ ................................ ............ 108 5.2 Experiment ................................ ................................ ................................ ...... 112 5.3 Results and Discussion ................................ ................................ ................... 113 5.3.1 Unipolar Devices ................................ ................................ ................... 113 5.3.2 TTA in Sub bandgap EL ................................ ................................ ........ 113 5.3.3 CT States at the Rubrene/fullerene Heterojunction ............................... 115 5.3.4 Energy Transfer to Triplets ................................ ................................ .... 115 5.4 Conclusion ................................ ................................ ................................ ...... 117 6 SUMMARY AND FUTURE WORK ................................ ................................ ....... 122 LIST OF REFERENCES ................................ ................................ ............................. 125 BIOGRAPHICAL SKETCH ................................ ................................ .......................... 136
8 LIST OF TABLES Table page 2 1 Enhancement from spectra was percentage change between integrated intensities of non ca vity and microcavity having same organic structure. ........... 56 4 1 1 H NMR, HPLC purity and elemental analysis data of these compounds ........... 96
9 LIST OF FIGURES Figure page 1 1 Molecular orbitals and associated band diagram of benzene ring ...................... 39 1 2 The Jablonski diagram indicates the energy lev el of each excitation state and the possible transitions between them ................................ ................................ 39 1 3 Potential energy diagram and Franck Condon transitions ................................ .. 40 1 4 Frenkel, Wannier Mott and CT excitons in terms of relative bounding distances. ................................ ................................ ................................ ........... 40 1 5 T he scheme of Fster transfer between two singlets ................................ ......... 41 1 6 T he scheme of Dexter transfer between a singlet and a triplet. .......................... 41 1 7 E nergy diagram of a common multilayer OLED device ................................ ...... 42 1 8 E xciton transitions in fluorescent and phosphorescent OLEDs .......................... 42 1 9 C omparison of eye sensitivity functions for the phot opic and scotopic vision regime ................................ ................................ ................................ ................ 43 1 10 CIE 1931 (x, y) chromaticity diagram. White light is located in the center. Also shown are the regions of distinct colors ................................ .............................. 43 1 11 Setup for tran sient PL. and transient PL signal ................................ ................... 44 2 1 R eflectivity of DBR substrates for blue, green, and red ................................ ..... 57 2 2 Device structure of green red and blue OLEDs and normalized EL spectra of green, red and blue devices ................................ ................................ ............... 58 2 3 Normalized EL spectra and current efficiency of green red and blue PhOLEDs ................................ ................................ ................................ ........... 59 2 4 T he C IE coordinates (1932) of noncavity devices (black) and optimized microcavity devices (yellow) EL spectra ................................ ............................. 62 2 5 Perforamnce of double emitting layer OL EDs with and without microcavity ...... 63 3 1 Absorption and PL spectra of ZnSe/ZnS QDs in solution ................................ ... 73 3 2 Current density voltage an d optical power density voltage characteristics of poly TPD and PVK devices ................................ ................................ ............ 74 3 3 EL spectra of poly TPD device and PVK device. ................................ ................ 75
10 3 4 EQE as a function of emitting layer thickness and PVK thickness. ..................... 76 4 1 Synthesis route of PTC, ETC and BETC. ................................ ........................... 97 4 2 Struc tures of host molecules ................................ ................................ .............. 98 4 3 DFT calculated HOMO and LUMO orbitals for PTC (top), ETC (middle) and BETC (bottom). ................................ ................................ ................................ ... 99 4 4 Absorpt ion spectra of PTC, ETC and BETC ................................ ..................... 100 4 5 PL spectra of neat PTC, ETC and BETC as well as the spectra of PTC, ETC and BETC doped with FIrpic. ................................ ................................ ............ 101 4 6 Device performance including EL spectra, J V curves, L V curves and current efficiency ................................ ................................ ................................ .......... 102 4 7 AFM of mCP as deposited and after annealed and ETC as deposited and after annea led ................................ ................................ ................................ ... 103 4 8 J V characteristics of mCP and ETC hole only devices before and after 45 annealing. ................................ ................................ ................................ ......... 103 4 9 M obility of host material s used and J V of hole only devices ............................ 104 4 10 Transient PL decay of mCP and ETC doped with FIrpic ................................ .. 104 4 11 Unipolar devices with mC P and ETC ................................ ................................ 105 4 12 TPQ on the PL intensity of mCP and ETC hole only devices; and relative reduced PL intensity vs. applied voltage of mCP and ETC devices. ................ 106 4 13 TPQ under different current injection of long and short electrical pulse. ........... 107 5 1 T he molecular structures of rubrene and C 60 ; the energy band diagram of OLEDs with different electron injecting layers ................................ .................. 118 5 2 The L V curves of rubrene OLEDs with different electron injecting layers and EL spectra below and above band gap voltage. ................................ ............... 118 5 3 J V characteristics of unipolar devices of rubrene/C 60 heterojunction. ............. 119 5 4 T ransient EL decay of rubrene OLED and rubrene/C 60 OLED ......................... 119 5 5 Log log scale of EL decay before bandgap operation. Blue lines are fitting lines having slope of 2. ................................ ................................ .................... 120 5 6 EA and its quadrature signal of rubrene/C 60 blend. ................................ .......... 120
11 5 7 Transient PL of rubrene and mix rubrene/C 60 blend shorter than 500 ns and over 500 ns. ................................ ................................ ................................ ...... 121
12 LIST OF ABBREVIATIONS 3TPYMB Tris[3 (3 pyridyl)mesityl]borane AFM Atomic Force Microscope Alq3 Tris(8 hydroxy quinolinato)aluminium AMOLED Active matrix OLED B3PyPB tetra 3 pyridyl BETC tetrakis(1,1 dimethylethyl) (ethyl) ( 9CI) Ter 9H Carbazole BPhen Bathophenanthroline CBP bis(carbazol 9 yl)biphenyl CIE International Commission on Illumination CT Charge Transfer DEL Double Emitting Layer DBR Distributed Bragg Reflector EA Elec troabsorption EL Electroluminescence EML Emitting layer EQE External Quantum Efficiency ETC ethyl (9CI) Ter 9H carbazole ETL Electron transporting layer Firpic Iridium (III) bis[(4,6 difluorophenyl)pyridinato]picolinate F WHM F ull width at half maximum HAT CN 1,4,5,8,9,11 hexaazatriphenylene hexacarbonitrile HIL Hole injection layer HTL Hole transporting layer HOMO Highest Occupied Molecular Orbital
13 IC Internal Conversion ISC Intersystem Crossin g IQE Internal quantum efficiency Ir(MDQ)2(acac) Bis(2 methyl dibenzo[f,h]quinoxaline)(acetylacetonate)iridium (III) Ir(ppy)3 Tris(2 phenylpyridine)iridium(III) ITO Indium tin oxide LUMO Lowest Unoccupied Molecular Orbital mCP 1,3 bi s(carbazol 9 yl)benzene MEH PPV Poly[2 methoxy 5 ethyl hexyloxy) 1,4 phenylene vinylene] MoOx Molybdenum Oxide NCs Nanocrystals NPB bis(naphthalen 1 yl) bis(phenyl) benzidine OLED Organic Light Emitting Diode OPV Organi c Photovoltaic PCE Power Conversion Efficiency PEDOT:PSS Poly(3,4 ethylenedioxythiophene):polystyrenesulfonate PhOLED P hosphorescent organic light emitting diode PLED P olymer light emitting diode PL Photoluminescence poly TPD Poly(4 but ylphenyl diphenyl amine) PTC phenyl (9CI) Ter 9H carbazole PVK P oly(N vinylcarbazole) QD Quantum dot SiO 2 S ilicon oxide
14 SOC Spin orbital coupling TAPC Di [4 (N,N ditolyl amino) phenyl]cyclohexane TCTA Tris[4 (carbaz ol 9 yl)phenyl]amine TiO 2 T itanium oxide TPBi (1,3,5 benzinetriyl) tris(1 phenyl 1 H benzemidazole) UGH2 1,4 bis(triphenylsilyl)benzene ZnO Zinc Oxide ZnS Z inc sulfide ZnSe Zinc selenide
15 Abstract of Dissertation Presented t o the Graduate School of the University of Florida in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy ON MICROCAVIT Y AND TRIPLET EXCITON BEHAVIOUR IN ORGANIC AND HYBRID LIGHT EMITTING DIODES By Chaoyu Xiang May 2014 Chair: Franky So Major: Materials Science and Engineering from periodic lattice structures as in inorganic semiconductors, a strong bonded hole and electron, named exciton, is used as the basic quasi particle to characterize the properties of organic semiconductors. With the presence of exciton s more interactions occur such as exciton confinement, exciton exciton annihilation, a nd exciton polaron annihilation which will be discussed in this thesis. First microcavity was used to confine the excitons and modify the electroluminescence (EL) characteristics of phosphorescent OLEDs. A systematic study has been conducted on microcavity OLED s using on green, red and blue phosphorescent emitters to elucidate the microcavity effects for different color emitters While the luminance enhancements of blue and red phosphorescent microcavity devices are small, the current efficienc y as high as 224 cd/A is obtained in green phosph orescent microcavity OLEDs. Quantum dots are other excitonic materials with strong confinement of electron s and hole s We demonstrate d blue/ violet emitting hybrid LEDs based on cadmium free zinc selenide/zinc sulfide core/shell quantum dots. Using poly(N v inylcarbazole) with a
16 low lying highest occupied molecular orbital energy for the hole transporting layer, enhanced hole injection and better exciton confinement were observed, resulting in efficient blue/ violet emitting devices. Due to the long lifetime and high density of triplet excitons, exciton exciton annihilation and exciton polaron quenching become the dominating loss mechanisms which contribute to the phosphorescent OLED efficiency roll off We characterize d the inter actions of triplet excitons an d polarons in blue phosphorescent OLEDs with ter carbazole host materials and found that these interactions could be modif ied and the efficiency roll off is suppress ed by changing the host system Highly efficient blue phosphorescent OLEDs with reduced rol l off at high lumin ance were demonstrated. We also show that exciton interaction can further reduce the operating voltage of fluorescent OLEDs with a lumin ance turn on below the corresponding bandgap voltage of the emitter. With experiment results from ele ctroabsorption, transient E L and PL measurements, we verif ied the energy from the charge transfer state of the donor and acceptor can be acquired by the triplet state, which undergoes an up conversion path and transfers its energy to higher singlet state.
17 CHAPTER 1 FUNDAMENTALS OF ORGANIC SEMICONDUCTORS In this chapter, several key concepts governing the processes in organic electronic materials are presented, from the basic molecular orbitals constructions to electronic states, charge carrier transpor t, and excitons. Basic characterization regarding the performance of OLEDs will follows the introduction of OLEDs. Also theory regarding the microcavity and quantum dots used in this thesis will be discussed. 1.1 Physics of Organic S emiconductors 1.1.1 Mo lecular O rbitals Organic semiconductors are organic c ompounds that can conduct carriers. All of them are sharing the common chemical characteristic: conjugated structure, where are bound is overlapped sp2 hybridized orbitals betwe en two adjacent carbon atoms, which bounds consist of the overlap of the remaining p orbitals of the carbon atoms, which are oriented perpendicularly to the molecular plane. Electrons in p orbitals are delocal ized over the whole molecule. The bonding orbitals. electrons which correspond to the outmost orbitals of molecule. In condense d organic semiconductors, orbitals create band structure like in the inorganic semiconductors, where the filled band edge forms the highest occupied molecular orbital (HOMO) and the unfilled band edge contributes to the lowest unoccupied molecular orbital (LUMO). As the results, the optical and electronic properties of organic semiconductors are determined by
18 properties of HOMO and LUMO as well as the bandgap between them. Fi gure 1 1 shows the molecular orbitals and associated HOMO/LUMO diagram of the most basic conjugated molecule: benzene. 1.1.2 Photophysics of O rganic M olecules External energy (from photons, excitons or injected charges) can excite an organic molecule from ground state to higher energy states. Those excited states have different degrees of freedom: electronic, vibrational, rotational and translational ones which correspond to different molecular orbitals. The relaxation of an excited molecule back to ground state can go via radiative transition which involves the emission of photons or non radiative transition where energy is transferred to heat. The various energy levels involving in the excitation and relaxation of an organic molecule are classically presen ted by a Jablonski energy diagram. Fiugre 1 2 illustrates the Jablonski diagram, the thicker lines represent electronic energy levels, while the thinner lines denote the various vibrational energy states within each vibrational level, a number of rotationa l and translational sub energy levels exist which are ignored here. Every transition between each energy level is characterized by a rate constant, the number of which varies in different kinds of transition processes. Due to Pauli exclusion principle, eac h electronic state forms bonding or anti bonding molecular orbital. They will have a total spin number of 0 or 1, which correspond to singlet or triplet state, respectively. In order to preserve the spin conservation, the transitions are allowed within the same total spin number (singlet to which, more strictly speaking, is due to a significant low transition rate. In the ground state, the outmost orbital of
19 spins which have a total spin number of 1, forming a singlet. Therefore in photon absorption transition, organic molecule is excited form S 0 to S n The common radiative decays could be singlet to singlet emission so called fluorescence or triplet to singlet emission so called phosphorescence while the nonradiative decay are included internal conversion (IC) transitions between states of the same multiplicity and intersystem crossing (ISC) tran sitions between states of a di erent multiplicity. Due to the significant mass difference between electrons and unclear, an electronic transition involves no change in the nuclear coordinates. The transition between each vibrational level follows Franck Condon principle 1 that is the intensity distribution in the vibronic progression is dete rmined by the overlap between the ground state vibrational wave function 0 v and the excited state vibrational wave function n v In photon absorption, the strongest transition occurs from the vibrational ground state into a vibrational level of the excite d state having its maximum wave function over the maximum of the ground state. Also, in the relaxation process, the most probable transition is between vibrational levels of the excited state and ground state that have the maximum overlap of wave functions These transitions are represented by vertical transitions on the potential energy diagram. Transitions to other vibrational levels are also possible but with reduced probabilities. The starting vibrational level of radiative transition is governed by Kas electronic state involve non radiative emission of photon, whose rates are much larger than the radiative transition rate. Therefore the radiative transitions are most likely to start from the lowest vibrational l evel of an electronic state.
20 The radiative decay from singlet to ground state is called as fluorescence, while the radiative decay from triplet to ground state is called as phosphorescence. Fluorescence is the spontaneous radiative emission. Once the mole cule is excited the fluorescent relaxation of the excitation occurs within few nanoseconds. Phosphorescence is a relatively slow spontaneous emission. Due to the spin forbidden between singlet and triplet, the transition rates from excited singlets to trip lets and from triplets to ground state are very low. However, in the presence of additional internal or external force, these transitions may occur. For example, by introducing heavy metal into the molecule, strong spin orbit coupling enables the mixing of singlet and triplet states. 2 Thus, those forbidden transitions become weakly allowed. Thereby, the rate of triplet decay as well as the pr obability for intersystem crossing increases, leading to enhanced phosphorescence. There is also another possibility of triplet energy transfer back to singlet which is so called reverse intersystem crossing (RISC). This new formed singlet undergoes delaye d fluorescent radiative decay. 3 4 1.1.3 Exciton and I ntermolecular E xciton E nergy T ransfer The intermolecul ar energy transfer can be a radiative or non radiative process between molecules, which can be described in term of exciton transfer. The exciton is a bound electron hole pair by Coulomb force. Due to the small dielectric constant of organic materials, ele ctron and hole are strongly bounded, resulting in a small excitonic radius and localized exciton state. Therefore compared with inorganic semiconductors, organic semiconductors tend to describe exciton behaviors to explain the optoelectronic properties. De pending on the degree of delocalization, the excitons are classified as Frenkel, Wannier Mott and Charge Transfer (CT) 5 which are shown in Figure 1 4. In organic materials, strong Coulombic interaction bounds the electron hole pair localizing
21 on a single molecule. With its radius comparable to the size of the molecule, the Frenkel exciton is considered as a neutral particle that can di use from site to site. Wannier Mott excitons have the electron hole distance larger than the lattice constant. Due to the weakly correlated, crystalline semiconductors such as Si, Ge, GaAs, the overlap between neighboring lattice atoms reduces the Coulombic interaction between the electron and the hole. Hence, they are not likely found in organic materials. The CT exciton is an intermediate case between a Frenkel and a Wannie r Mott state, being neither very extended nor tightly bound to a single molecule. It is sometimes regarded as an unrelaxed electron hole pair with both the positive and negative of the charge pair located on separated but adjacent molecules. Yet, this loca lized picture is only true for a molecular crystal with weak intermolecular interactions and a small overlap between the neighboring orbitals, i.e. each molecule forms a deep potential well in which the charges Excitons can diffuse inside orga nic solids and transfer energy without transporting net charge. The radiative energy transfer involves the emission and absorption of photons, which is controlled by the emission quantum yield and spectrum of donor and the absorption intensity and spectrum of acceptor. There are two types of non radiative exciton energy transfer. Frster energy transfer refers to resonance between the dipole dipole interaction. This kind of interaction requires the conservation of spin, thus typically occurs in singlet sing let with very fast rate. The transfer intensity depends on the degree of overlap between the emission spectrum of the donor and the absorption spectrum of the acceptor. The transfer rate is proportional to the r 6 where r is the distance between donor and acceptor. 6 This distance can be in a long range of 10
22 nm. Figure 1 5 shows the scheme of Fster transfer between two singlets. The second intermolecular transfer is Dexter transfer which originates from exchange of electrons between molecules, so electron orbitals o f donor and acceptor must be overlapped. As a result, the active distance of Dexter transfer is within short length of 1nm. In the Dexter transfer process, only the total spin of the donor acceptor system is conserved. Therefore e ective energy transfers b etween triplet states or between singlet and triplet states can occur. Figure 1 6 illustrates the Dexter transfer between a singlet and a triplet. 1.1.4 Charge T ransport Presence of charges inside of organic semiconductors leads to polarization of the adja cent environment due to the weak van der Waals interactions. This polarization is much faster than the transfer time of a charge carrier from site to site. This instantaneous following polarization with the moving charge carrier can be treated as a quasipa rticle called polaron. Those polarons have their unique transporting properties and can interact with excitons and phonons. There are two types of charge transport mechanism in organic materials. 7 If the phonon scattering time is relatively small and the mean free path of a charge carrier is large compared to the lattice constant, band transport like inorganic semiconductors can be observed. 8 For example, the mobility in molecular crystals such as pentacene reaches up to 40 cm 2 /V sec 9 However, most organic semiconductors are amorphous, which fails to meet the requirement of band transport. The charge transport is dominated by the hopping transport model. The carrier hops from one molecule to another with the assistant of energy. This thermally activated process, can be expressed as
23 Here, 0 is the intrinsic mobility at T = 0 and E A is the average activation energy needed to move the charge carrier from one localized state to the next site. The lower mobility for organic semiconductors is due to the fact that in termolecular interactions in organic electronic materials are weak causing electronic states to be localized on individual molecules or polymer chains. This causes narrower transport bands that are easily disrupted by disorder. The variation of local envir onments, including molecular orientation and chemical defects, results in energetically disordered electronic states. In order to fully understand charge conduction, we need to understand that adding or removing a charge from a molecule will result in stru ctural relaxation of the molecule and its surrounding environment including changing the bond lengths. Therefore, as a charge moves through a disordered organic solid, it hops between molecules since it is easily localized by defects and disorder. 10 Charge tran sport in organic semiconductors is a result of hops between localized states. The resulting field dependent mobility follows the universal Poole Frenkel form 0 is the zero field mobility in the materials, V is the applied voltage and d is the thickness of the device, is Poole Frankel factor related to temperature and materials. 1.2 Fundamental of OLED OLED uses multilayer structure where the organic layers are sandwiched between two electrodes. When voltage is applied, electrons and holes are injected from
24 cathode and anode respectively, transported to emitting layer (EML) under electric field and recombine. Usually a hole injectio n layer (HIL) and a hole transporting layer (HTL) are inserted between anode and EML, an electron transporting layer (ETL) and an electron injection layer (EIL) are inserted between cathode and EML, in order to lower the barrier for carrier injection and f acilitate carrier transport. EML is usually doped with emitter to form a guest host system. Emitters serve as trap sites for electron hole recombination, and provide a wide range of emitting color to fulfill the need for full color display or white light i llumination. Figure 1 7 shows a general operation of OLEDs. During recombination, electrons and holes bound with each other by Coulomb force and form a quasi emit photons. Since electron and hole can have a spin of or , four spin configurations can occur. Combination of antiparallel spins results in a singlet (total quantum spin number S=0, M S =0), while combination of parallel spins results in triplet (S=1, M S =0,1). According to spin statistics, the four configurations have equal chance (25%) to occur, therefore singlets and triplets are generated in a ratio of 1:3 under electrical excitation. Excitons transit back to ground state (S 0 ) through radiative or non radiative processes. Depe nding on different radiative pathway, OLEDs are categorized into fluorescent OLEDs and phosphorescent OLEDs. In fluorescent OLEDs, the lifetime of singlets is in the scale of nanoseconds, and the lifetime of triplets are very long, ranging from several mic roseconds to seconds 11 Therefore only singlets can go through efficient radiative decay to the ground s tate (called fluorescence), while most of the triplets go through non radiative decay and generate heat. The internal quantum
25 efficiency (IQE), defined as the ratio of generated photons to the injected carriers, has a maximum value of 25% in fluorescent OL ED, since only singlets are harvested. In phosphorescent OLEDs, phosphorescent materials incorporating spin orbit coupling (SOC) are used as emitter. SOC relaxes the transition spin forbiddeness between singlet state and triplet state, enabling transition between first excited singlet state (S 1 ) and first excited triplet state (T 1 ) as well as the radiative decay between T 1 and S 0 12 The transition between S 1 and T 1 is intersystem crossing (ISC), usually in a time scale of picoseconds 13 which transfers singlets to the T 1 state right upon generation, not to be observed as fluorescent light. Triplets then go through radiative decay (usually in a time scale of microseconds) to S 0 called phosphorescence. In this scenar io both singlets and triplets are harvested, and a 100% maximum IQE can be achieved. 1.3 Terminology of P hotometry Light energy can be characterized by radiometry which measures the radiation of electromagnetic wave in terms of absolute power unit; or by p hotometry which calculates the relative response from human eyes. Since the human eye is a specific detector that only response to the range of visible spectrum (400 700 nm) and its sensitivity varies significantly within the spectral range, it is easier t o use photometric units for daily appliance such as lighting panel and display screen. In this section, a few basic concepts of photometry related to OLED characterization are introduced. Luminosity Function is the normalized sensitivity of human visual pe rception of brightness. There are two luminosity functions. At low brightness such as dark night, scotpic luminosity describes the light response of human eye, while at high brightness such as daylight level, the photopic luminosity function is used as the best approximation of the response of the human eye. The photopic luminosity is the CIE
26 standard curve used in the CIE 1931 color space. The standard photopic luminosity function is normalized to a peak value of unity at 555 nm The luminosity curve is shown in Figure 1 9. Luminous intensity represents the perceived light intensity of a light source, the unit of which is candela (cd). 1 cd is defined as an optical power of 1/683 W of a monochromatic light at 555 nm into a solid a ngle of 1 steradian (sr). Luminous flux represents the total emitted power of a light source over all directions, the unit of which is lumen (lm). 1 lm is defined as the luminous flux of a monochromatic light at 555 nm with a n op tical power of 1/683 W. Therefore, according to the definitions of candela and lumen, 1 cd equals 1 lm/sr. In OLED measurement, the luminance of a device is the ratio of the luminous intensity per the projected area of the device in a certain direction. The unit is cd/m 2 or nit. CIE color coordinates are mathematical color space recommended by International Commission on Illumination (CIE). The very first one was given in 1931. The coordinates in the CIE 1931 color space is based on the three tristimulus values X Y and Z defined as .
27 The tristimulus values of a color are calculated by integrating the radiant power of the light source and the spectral color matching functions over the all visible wavelength. The CIE XYZ c olor space was deliberately designed so that the Y parameter was a measure of the brightness or luminance of a color. Figure 1 10 shows the CIE 1931 color space with different color regions. 1.4 Optical property in OLEDs qua tions When incident light propagates to the interface of two different refractive indexes media, the relationship between the incident angle and the refractive index is described where n i is the refractive index o f medium i, n t is the refractive index of medium t, i is the incident angle of the light ray (with respect to the normal), t is the refracted angle (with respect to the normal). There exits an angle that will then approach 90 for some critical incident angle c and for incident angles greater than the critical angle there will be total internal reflection. The critical angle can be calculated from Snell's law by setting the refraction angle equal to 90. Total internal reflection is important in out coupling efficiency in OLEDs, due to the refractive in dex difference between organic layers and other layers such as ITO and substrate. The fraction of the incident power that is reflected from the interface (reflectance R ) and transmitted through the interface (transmittance T ) can be calculated by Fresnel equation. The calculations of R and T depend on polarization of the incident ray. The s polarized light is defined as its electric field perpendicular to the
28 plane containing incident and reflected light. The reflection coefficient of s polarized light is given by: where t can be derived from i by Snell's law. The p polarized light is defined as its electric field parallels to the plane containing incident and reflected light. The reflection coefficient of p polarized light is given by: The tra nsmission coefficient in each case is given by T s R s and T p R p If the incident light is unpolarized, the reflection coefficient is R = ( R s + R p )/2. In OLEDs, these combinations of refraction and reflection always happen at every boundary of or ganic/ITO, ITO/glass, and glass/air. Hence the Fresnel reflection is fundamental reason of weak microcavity effects on OLEDs. 1.4.2 Fabry Perot I nterference Due to the different refractive indices layers in OLED structure, interference from the Fresnel re flection plays an important role in optical property of OLEDs. This interference is an analogy to Fabry Perot interference where a beam of light propagates through a space between two reflectors. Constructive interference occurs if the transmitted beams ar e in phase, and this corresponds to a high transmission peak while destructive interference corresponding to a transmission minimum occurs when the transmitted beams are out of phase. The phase difference between each succeeding
29 through the etalon the thickness between the two reflectors and the refractive index of the material between the reflecting surfaces n If both surfaces have a reflectan ce R the transmittance function is given by where is the coefficient of finesse Maximum transmission ( T e = 1) occurs when the optical path difference (2 nl wavelength. In the absence of absorption, the reflectance R e is the complement of the transmittance, such that T e + R e = 1. The maximum reflectivity is given by: and this occurs when the path difference is equal to half an odd multiple of the wavelength. 1.4.3 Theoretical A nalysis o f S trong M icrocavity Based on the concept of Fabry Perot etalon, one can create microcaviy by using path meets the requirement of the maximum reflectivity. With the presence of microcavity, the density of optical modes will be modified compared with non cavity devices. Microcavity only allows light with resonance wavelength propagating along its
30 optical axis, which redistributes density of photons from free space int o normal direction of device. Consequently, more light can be extracted from thin film guided mode and substrate mode. The resonance wavelength at normal direction is determined by the optical length of microcavity is the resonance wavelength. is the mode order which usually equals to 1 in OLEDs. is the optical length related to the refractive index of each layer, the thickness of device, phase shif t at the interface and light penetration depth into back metal reflector. In the meantime, in order to form a stable resonance inside cavity, standing wave condition must be satisfied, which requires to place the emitting zone at the middle position of the cavity. At the resonance wavelength, the enhancement of emission intensity is calculated as 14 is t he antinodes enhancement factor, which equals to 2 at standing wave condition in OLEDs. and refer to the reflectivity of back metal mirror and front semitransparent mirror, respectively. The ratio, between radioactive lifetime in cavity device and in noncavity device equals to 1 under first order approximation. Another effect of microcavity on OLEDs comes from the modifying profile of emission spectrum. The spectral width ( ) of cavity resonance can be calculated by the uncertainty relation and the photon lift time in the cavity 14
31 The spectral width is usually much smaller in microcavity than that of free space spectrum. For example, a typical microcavity has =100% and =70%. With the optical cavity length setting up to the a ntinodes, the spectral width is calculated as 35 nm with a resonance wavelength at 540 nm. As comparison, one of the most commonly used green emitter Alq 3 with a spectral peak at 540 nm has a full width at half maximum (FWHM) of 120 nm. Thus light from the spectrum overlapping within the spectral width of cavity resonance can only be enhanced. The emitting spectrum of a microcavity device shows a significantly reduced FWHM. On the other hand, by tuning the cavity length, the cavity resonance wavelength can be shifted. 1.5 Optical S imulation 1.5.1 Transfer M atrix F ormalism for P assive L ayers Since OLEDs are based on multilayer structure, the light propagation through the stack can be calculated by the method of transfer matrix. Transfer matrix is describing p assive optical properties of propagation of the electromagnetic waves (for example light) of a given frequency through a stack. The transfer matrix formalism is an extremely useful method to calculate the reflectivity R and the transmission T of a multilay er structure. The electrical field E and magnetic field F of a light passing through a passive layer with a distance L into the positive z direction are described by the matrix and
32 Such a matrix can represent propagation through a layer if k is the wav e number in the medium and L the thickness of the layer. For a system with N layers, each layer j has a transfer matrix M j where j increases towards higher z values. The system transfer matrix is then Therefore by calculating the system transfer matrix, we can easily get the relation between the incident and transmitted light. 1.5.2 Optics of E missive M ultilayer S tructures The emission characteristic of a single molecule is similar to that of a Hertzian dipole. However, in an infinite medium consisting o f isotropically oriented molecules, the emission from the multitude of molecules is homogeneous in all directions. If, on the contrary, the molecules are embedded in a thin layer, interference effects of electromagnetic waves reflected at layer interfaces dominate the emission characteristics. In general, the emissive layer of an OLED is part of a multilayer system and the device emission is subject to interference effects. Based on a classical theory, the calculation of the emission of a Hertzian dipole an tenna describes the influence of optical interfaces on the radiative dynamics of (fluorescent) molecules. Consider the molecules as driven damped harmonic dipole oscillators. The dynamics of the oscillating dipole moment is descr ibed by the following equation:
33 with as the oscillator frequency in the undamped case, b 0 is the initial dipole intensity, e the elementary charge, m the effective mass of the dipole, and the interface reflected field at dipo le position. The intrinsic power of the dipole is therefore given by the sum of a radiative and a non radiative fraction, respectively. These are not affected by the optical feed back onto the dipoles, therefore the dynamics of the dipole within an optical structure in terms of the intrinsic quantum efficiency q 0 and the optical feedback F can be expressed as F can be expressed by means of a generalized parameterization of the emission angle the total dissipated power from the dipole is proportional to the electromagnetic feed back strength F and therefore, a stepwise integration this equation based on the index limits for total internal reflection allows to determine the fractional radiated power that can be assigned to out coupled, guided (in the organic materials), and evanescent modes. The integration limits are given by: radiative modes waveguided modes evanescent modes n t and n e are the refractive indices of the top and the emissive medium, respectively.
34 1.6 Optical P roperties of C olloidal Q uantum D ots Quantum dots are semiconductor nanocrystals whose sizes reduced into nano scale dimensions. In this dimension scale, unique optical properties due to a combination of their material band gap energy and quantum well phenomena exhibits. According to quantum theory, only discrete values of energy are allowed to exist within a quantum well. Every parti cle has a deBroglie wavelength based upon its mass and energy, and the energy states allowed in a quantum well correspond to the energy levels that cause the deBroglie wavelength to form a standing wave. In a three dimensional cubic well, standing waves, o r modes, can exist in all three dimensions independent of one another. Thus, the expression for allowed energies is modified and now contains three values for n one corresponding to each direction When the electrons and holes are confined in such a limi ted volume and dimension of quantum dot that comparable to the average separation between an electron and a hole, known as exciton Bohr radius, this quantum dot confinement leads to the discrete density of states, which gives unique properties in their opt ical, electronic, magnetic and chemical properties from bulk crystals. The optical band gap of the QDs becomes different from that of bulk materials. While its intrinsic energy properties are still determined by the bulk material, the QD has emerging new a bsorption and emission energies near the band gap related to nanocrystal's quantum confined size As with any crystalline semiconductor, a QD's electronic wave functions extend over the crystal lattice Similar to a molecule, a QD has both a quantized ener gy spectrum and a quantized density of electronic states near the edge
35 of the band gap. As the results, the QDs of the same material, but with different sizes, can emit light of different colors. Due to the quantum confinement, the bandgap energy that dete rmines the energy (and hence color) of the fluorescent light is inversely proportional to the size of the QD. The emission peak is quite narrow due to the small size distribution. Quantum confinement can also be enhanced by introducing larger bandgap semic onductor as the shell outside the core QD. These core/shell QDs have shown improvement to the spectroscopic properties of the particles like stability, lifetime and emission intensity. 1.7 Characterization of O rganic M aterials and D evices 1.7.1 Terms of E f ficiency of OLEDs The most fundamental measurement of OLEDs is L (luminance) J (current density) V (voltage) characteristic which measures the electroluminescence of a device with applied voltage by a photometer and a source meter. With the data from L J V measurement, one can calculate the efficiency of OLEDs including the current efficiency, external quantum efficiency (EQE) and power efficiency. Current efficiency, also called luminous efficiency is the ratio of forward luminance to the injected current to the device. It is measured in units of cd/A. EQE is the percentage ratio of the number of photons emitted out of the device to the number of charge carriers injected into the device. The internal quantum efficiency (IQE) is defined as the ratio of the number of photons generated from the recombination zone inside of the OLED to the number of charge carriers injected into the device. The difference between the IQE and EQE is the out coupling efficiency which characterizes how much of the generated photon s can emit to free space. Therefore the EQE of
36 OLEDs, can be expressed as product of IQE and external out coupling efficiency The IQE is related to the charge carrier balance factor ( ), exciton generation ratio by singlet and triplet decay ( ), and intrinsic quantum efficiency for radioactive decay ( ). The relation between the EQE and current efficiency is given by considering the photopic response of the spectrum and the spatial emission distribution. Finally convert EQE in terms of the energy which is characterized by Power efficiency. The power efficiency is the ratio of the luminous power of the device to the electrical power required to drive the device. It is expressed in units of lm/W. 1.7.2 Electroabsorption (EA) Spectroscopy Electroabsorption (EA) spectroscopy is a non invasive method to probe the internal electric fields in organic semiconductor devices. EA spectroscopy is based on externally applied electric fields influencing the interaction of molecules with light. The change in absorption of a molecule with an applied electric field can be explained by the Star k Effect which is illustrated in Figure 1 3. Under an applied electric field, dipoles will be present in materials which are dependent on its polarization. This cause s a splitting in energy levels and previously forbidden transitions become allowed as well as a red shift in the 1A g 1B u transition. The excitonic Stark shift follows:
3 7 where F material. The polarization is the dipole unit per volume a nd is described as: where is the dielectric constant of the vacuum and is the polarizability of the material. The field dependent susceptibility is nonlinear and for molecules and polymers with inversion symmetry, all even terms of disappear. Since the imaginary part of the susceptibility is directly proportional to the absorption constant where E is the energy of the photon the chance in transmittance can be described as: If the applied electric field F is a superposition of both AC and DC biases (V), as well as an internal electric field V bi the change in transmittance is: Therefore, when the V DC is of equal magnitude but opposite direction as the V bi the EA response vanishes. Experimentally, the V AC amplitude is kept constant while changing the applied V DC until V DC exactl y cancels out V bi 1.7.3 Transient Photoluminescence Analysis In transient PL (setup in Figure 1 11), a pulse generator is connected to both N 2 laser source and storage oscilloscope, to generate periodic laser pulse and receive PL signal. N 2 laser has a pe ak wavelength of 337.1 nm which is within the absorption spectrum of materials. Laser pulse is focused by a focal lens onto a spot of emitting layer. The sample should be placed in vacuum or nitrogen atmosphere to avoid
38 quenching from O 2 The excited PL si gnal will go through a filter to screen out reflected laser pulse, second harmonious generated laser pulse and substrate modes. The filtered signal is then received by a photomultiplier tube (PMT). The amplified signal will then be received by oscilloscope which is tuned by the same pulse generator. During the very short time of optical excitation, excitons will be formed in EML. When the pulse is removed, singlets will go through fast fluorescent decay and cause prompt fluorescence (decay time ~ns), where as triplets go through the slow RISC to phased decays can indicate that triplet harvesting mechanisms exist in a device.
39 Figure 1 1 M olecu lar orbitals and associated band diagram of benzene ring 15 Figure 1 2 T he Jablonski diagram indicates the energy level of each excitation state and the possible transitions between them 16
40 Figure 1 3 P otential energy diagram and Franck Condon transitions (E (F C)a for absorption and E (F C) f emission) between electronic ground S 0 and excited S 1 states. Dashed lines represent the vibrational energy sublevel. 17 Figure 1 4 Frenkel, Wannier Mott and CT excitons in terms of relative bounding distances. 17
41 Figure 1 5 The scheme of Fster transfer between two singlets Figure 1 6 The scheme of Dexter transfer between a singlet and a triplet.
42 Figure 1 7 Energy diagram of a common multilayer OLED device (a) (b) Figure 1 8 E xciton transitions in (a) fluorescent and (b) phosphorescent OLEDs
43 Figure 1 9 C omparison of eye sensitivity functions for the photopic and scotopic vision regime. 18 Figure 1 10 CIE 1931 (x, y) chromaticity diagram. White light is located in the center. Also shown are the regions of dis tinct colors 18
44 (a) (b) Figure 1 11 Transient photoluminescence measurement: (a) Setup and (b) transient PL signal
45 CHAPTER 2 MANIPULATION OF EXCITON BY MICROCAVITY IN OLEDS In this chapter, we modified the exciton emission from PhOLEDs by microcavity. A systematic study has been conducted on microcavity organic light emitting diodes based on red, green and blue phosphorescent emitters to elucidate the microcavity effects for different color emitters We found that the luminance output is determined by the reflectivity of the semitransparent electrode and the photopic response of the red green and blue emitters. While the luminance enhancements of blue and red phosphorescent microcavity devices are small, a current efficiency as high as 224 cd/A is obtained in the green phosphorescent microcavity OLEDs. 2.1 Background and M otivation T he spectral characteristics of an OLED can be manipulated using a microcavity structure to produce saturated colors and enhance the color gamut, and therefore microcavity OLEDs are widely used in active matrix OLED (AMOLED) displays today 19 21 In addition, it has been reported that microcavity structure can be used to enhance the light extraction of an OLED by modifying the light distribution within the device 22 25 However, closely examining the previous reports, there is a significant difference in luminance enhancements in different microcavity OLEDs. While two times enhanc ement in current efficiency has been observed in green microcavity OLEDs, no significant improvements were reported in blue microcavity devices 26 28 Moreover, there is a discrepancy between enhancement in current efficiency and enhancement in quantum efficiency 29 It is apparent that the difference in quantum efficiency and luminance enhancements cannot be due to the difference in photon out coupling. It is
46 therefore important to take into account the photopic luminosit y due to the microcavity effects. In this chapter, we report on a systematic study of the microcavity effects on the light out coupling efficiency and luminance output in red, green and blue phosphorescent OLEDs. Specifically, we investigated two importan t parameters in microcavity OLEDs: the reflectivity of the semitransparent electrode and the cavity length A high reflectivity and low absorption semitransparent electrode is required in microcavity OLEDs so as to control the luminance characteristics. We demonstrated that the enhancement in photon out coupling in the microcavity devices can be greatly affected by the reflectivity of the electrodes. By tuning the cavity length, we found the change of photopic luminosity within the microcavity device leads to a difference in enhancements of current efficiency and quantum efficiency. Finally a green microcavity OLEDs with a current efficiency of 224 cd/A was demonstrated by optimizing both the photon out coupling efficiency and photopic luminosity. 2.2 Experi ments ITO coated distributed Bragg reflector (DBR) substrates compos ing of two pairs of quarter wave stacks of alternating layers of titanium oxide ( TiO 2 ) and silicon oxide ( SiO 2 ) were used for OLED fabrication The reflect ance maxima of the DBR substrates for red, green and blue emitting devices were tuned to 610 550, and 474 nm respectively. After UV ozone treatment for 15 minutes a 25 nm thick poly(3,4 ethylenedioxythiophene) polystyrenesulfonic acid (PEDOT:PSS) (AI 4803) as a hole injection layer was spin coated onto ITO substrates and baked at 180 C for 15 minutes in air. To fabricate the devices, the following layers were sequentially deposited by thermal evaporation: a 20 to 45 nm thick 1,1 bis[(di 4 tolylamino)phenyl]cyclohexane
47 (TAPC) as a hole transport layer (HTL) a 20 to 30 nm thick 4,4 N,N dicarbazole biphenyl ( CBP ) doped with 7 wt% tris(2 phenylpyridine)iridium (Ir(ppy) 3 ) as a green emitter a 15 nm thick N N dicarbazolebenzene (mCP) doped with 15 wt% iridium (III)bis [2 methyldiben zo (f,h) quinoxaline](acetylacetonate) (Ir(MDQ) 2 (acac)) and a 15 nm benzenetriyl)tris (1 phenyl 1H benzimidazole) (TPBi) doped with 15 wt% Ir(MDQ) 2 (acac) as a red emitter, a 20 nm thick mCP doped with 5 wt% iridium(III) bis[(4,6 difluor ophenyl) pyridinato N C a 45 to 60 nm thick tris[3 (3 pyridyl) mesityl]borane (3TPYMB) as an electron transporting layer (ETL) for the green and red emitting devices, a 45 to 55 nm thick tetra 3 pyridyl devices, a 1 nm thick LiF layer as an electron injection layer and a 100 nm thick aluminum as a cathode. To extract the substrate mode, a macrolens was attached to the substrate using an index matching gel. Current luminance voltage characteristics were measured using a Keithley Series 2400 source meter and a Keithley Series 6485 picoammeter with a calibrated Newport silicon photo diode. The luminance was calibrated using a Konica Minolta luminan ce meter (LS 100). The electroluminescence spectra were obtained with an Ocean Optics HR4000 spectrometer 2.3 Results and Discussion 2.3.1 Relation between R esonant E nhancement and R eflectivity of DBR The resonant emission enhancement G e along the normal direction in a microcavity structure is related to the reflectivity of the semitransparent mirro r based on the following relationship: 30
48 and refer to the reflectivities of the front semitransparent mirror and back metal mirror, respectively. In order to enhance th e light output from a microcavity, a front semitransparent electrode with high reflectivity and low absorption loss is preferred. A semitransparent metal electrode such as a thin Ag layer is mostly commonly used due to its simple fabrication steps. 22 29 31 33 However a thin Ag layer does not provide a high enough refle ctivity for a microcavity device. Because of the large extinction coefficient of a metal and of the presence of surface plasmon mode at the organic/metal interface, the use of semitransparent metal electrodes is not favorable for microcavity OLEDs. On the other hand, the use of dielectric DBR has the advantages of high reflectivity and low absorption loss 34 Here we chose SiO 2 /TiO 2 DBRs for our microcavity OLED fabrication. Figure 2 1 shows the measured reflectivity of the DBR substrates for blue, green, and red microcavity devices The peak reflectivities of the blue, green and red D B R substrates were tuned to 474, 550, and 610 nm respectively. Due to the dispersion of refractive indices of SiO 2 and TiO 2 t he maximum reflectivities of the D B R substrates are different. The reflectivities are 62, 75, and 65% for blue, green and red DBR substrates respectively, corresponding to a calculated enhancement ratio of 0.581:1.00:0.685 normalized by green enhancement according to equation 1 Because the higher reflectivity of the semitransparent electrodes, they give rise to a stronger microcavit y effect with higher resonance peak intensity, it is expected that the green microcavity device will give a higher efficiency enhancement than the blue and red devices. We fabricated blue, green and red emitting OLEDs on DBR substrates along with devices o n non DBR substrates which were used as references. Figure 2 2 A shows the structures of the optimized devices with and without microcavity
49 structure. Figure 2 2 B shows the normalized electroluminescent (EL) spectr a of the microcavity devices and the EL sp ectra of the reference devices driven at a constant current density Compared to the non cavity devices, the DBR devices show significantly enhanced luminance output along the normal direction with narrow FWHMs due to the strong microcavity effects. The in tensity enhancements of the blue, green and red devices at optimized wavelength are 3.5, 6.0, and 3.9 respectively. The ratio of those enhancements is 0.583:1.00:0.650 for blue, green and red devices, which match the calculation from the reflectivity of ea ch substrate. 2.3.2 Relation between R esonant E nhancement and C avity L ength In order to study the cavity length effect, devices with three different cavity lengths were fabricated on DBR substrates for red, green and blue OLEDs. The cavity length of these devices is based on the device structures of each emitting color as shown in Figure 2 2A For the green emitting device s, we tuned the thicknesses of the TAPC and emitting layers. Device G1 has a 35 nm thick TAPC layer and a 30 nm thick CBP:Ir(ppy) 3 layer, device G2 has a 30 nm thick TAPC layer and a 25 nm thick CBP:Ir(ppy) 3 layer and device G3 has a 30 nm thick TAPC layer and a 20 nm thick CBP:Ir(ppy) 3 layer. Figure 2 3 A shows the electroluminescent (EL) spectr a of the devices with and without DBR driven a t a current density of 0.1 mA/cm 2 The noncavity devices showed broad EL spectra with FWHMs of about 80 nm and t he intensity of shoulder peak in EL spectra is slightly reduced as the optical length decrease s from device G1 to device G3 due to the weak micr ocavity effect 35 On the other hand, in addition to the significantly enhanced intensity along the normal direction and na rrow FWHMs in the microcavity devices, the peak wavelength of the DBR device decreases from 572 nm to 528 nm with decreasing thicknesses of the TAPC and emitting layers
50 While the main peak emission wavelength of Ir(ppy) 3 is around 515 nm, the G 2 DBR devic e exhibits a higher EL intensity than the G 1 DBR device along the normal direction because the DBR substrate for green devices has the highest reflectivity at 550 nm We calculated the enhancement ratio due to microcavity effects by integrating the EL inte nsity of each device over all wavelength s and considering the ratio of the integrated intensity of the DBR device to that of the noncavity device and the results are shown in Table 2 1. DBR devices G 1, G 2, and G 3 show enhancement ratios of 60.1 101, and 70 .2%, respectively indicating the enhanced out coupling efficiency due to the microcavity effects However, the current efficiency shows different enhancement ratios from the number of outcoupled photons. The current efficienc ies for devices G 1, G 2, and G 3 with and without DBR as shown in Figure 2 3 B are 115 175 and 104 cd/A, and 65 73 and 53 cd/A at 0.1 mA/cm 2 respectively, representing enhancements of 75, 140, and 97% due to microcavity effects. While the G 2 DBR device shows the highest quantum effi ciency and current efficiency, the enhancement ratio of the current efficiency is higher than that of the quantum efficiency along the normal direction. This discrepancy in enhancement ratio is caused by the difference in luminosity. The normalized photopi c luminosity ( ) of each spectrum is calcu lated according to e quation is the normalized electroluminescent spectrum of OLEDs, is the standard photopi c luminosity function with an unity value at 555 nm, is wavelength. The integrating region is over all visible wavelengths. We calculated enhancement ratio of the luminosity in the G 1, G 2, and G 3 DBR devices from their references. The results are shown in Table 2 1. Because the narrow EL spectra of the DBR devices are peaked at
51 around 550 nm where the photopic luminosity has a highest value all DBR devices show higher luminosities than the noncavity devices and the G 2 DBR device hav ing a maximum EL intensity at 550 nm represents the highest enhancement of luminosity. By c onsidering both enhancements from the quantum efficiency along the normal direction and the luminosity, the total enhancement ratios of 78, 146, and 95% can be calcu lated for the G 1, G 2, and G 3 DBR devices, respectively, and they are in agreement with the measured enhancement ratios of the current efficiencies. We also examined the cavity length effects on red emitting phosphorescent microcavity OLEDs. Keeping the re st of the device structure the same as the red device shown in Fig ure 2 2A we fabricated devices with different 3TPYMB thickness: device R1 has a thickness of 52 nm, device R2 has a thickness of 55 nm and device R3 has a thickness of 61 nm Figure 2 3 C sh ows the normalized EL spectr a of the devices with and without DBR driven at a constant current density along the normal direction With a significantly reduced FWMH, the emission peak decreases from 599 nm to 630 nm. The R 2 DBR device having a resonant wav elength at 612 nm, corresponding to the peak reflectivity of red DBR shows the highest enhancement of 66.8% by integrating all wavelengths while the shorter and longer microcavity devices showed an enhancement of 46.6% and 31.4% respectively due to the m ismatch of the DBR reflectivity. Figure 2 3 D show s the current efficienc ies of the devices with and without DBR At a current density of 0.1 mA/cm 2 devices R 2 with and without DBR show the highest current efficiencies of 29 and 55 cd/A, respectively, indi cating a 89 % enhancement due to microcavity effects, while the R 1 DBR device shows a lower current efficiency of 50 cd/A and the R 3 DBR device has the same current efficiency as the R 3 noncavity
52 device. Compared to an enhancement of 66.6% in the EL intensi ty along the normal direction, a larger enhancement of 89 % in current efficiency for the R 2 DBR device is attributed to an increase in luminosity (Table 2 1) The importance of the luminosity is apparent in the R 3 DBR device. While the R 3 DBR device shows an enhancement of 31.4% for the integrated EL intensity along normal direction over the R 3 noncavity device, a decrease of the luminosity by 23.7% in the R 3 DBR device leads to insignificant changes in current efficiency even there is an enhancement in out coupled photons. Finally, we investigated the m icrocavity effects on the blue emitting phosphorescent OLEDs B3PyPB having high triplet energy and high mobility was used as the electron transporting layer 36 Devices B1, B2 and B3 have the thicknesses of B3PyPB layer as 45 nm, 49 nm and 53 nm respectively. Figure 2 3 E shows the normalized EL spectra of the DBR devices with along the normal direction compared with the corresponding d evices without DBR. With the increase of cavity length, the DBR devices showed a peak emission wavelength change from 470 nm to 498 nm. While a ll DBR devices showed enhancements of 40.1, 29.4 and 23.2 % in the integrated intensity, the B 1 DBR device shows t he highest enhancement with a resonant wavelength at 470 nm which corresponds to the peak reflectivity of blue DBR However, device B 3 DBR is the only device which shows an enhancement in current efficiency (Figure 2 3 F ) and devices DBR B 1 and B 2 showed t he same current efficiencies as the non cavity B 1 and B 2 devices. This is due to the significantly decreasing luminosity as the resonant wavelength of the emitting light shifts toward shorter wavelengths away from 550 nm. According to Table 2 1, the lumino sity in the B 1, B 2, and B 3 DBR devices
53 decrease by 28.7, 19.5, and 10.6%, respectively. Even if the B 3 DBR device exhibited 12% enhancement in the current efficiency, the Commission Internationale de (0.0 6, 0.51), indicating that the emitting color is no longer blue. Therefore in order to achieve deeper blue from Firpic, one has to force the microcavity peak to shorter wavelengths, which is demonstrated in device B1 DBR. Consequently, it causes a greater m ismatch between the cavity resonance wavelength and the FIrpic emission spectrum, resulting in a lower enhancement in quantum efficiency. Therefore, it is difficult to enhance the efficiency of the FIrpic based blue emitting OLED device while pushing the e mission peak to shorter wavelengths to generate a deeper blue color. Figure 2 4 shows the C IE coordinates for the R, G, and B EL spectra from the noncavity devices and optimized microcavity devices. Because of the reduction of the EL spectral width in the microcavity devices, the CIE coordinates of the R, G, and B emitters moved towards the boundary of CIE chart, from (0.617, 0.373), (0.321, 0.624), (0.148, 0.345) to (0.636, 0.350), (0.289, 0.692), (0.138, 0.170), respectively, enlarging the color gamut in the microcavity devices. 2.3.3 High E fficiency G reen OLEDs with M icrocavity To further enhance the device efficiency, we demonstrate the microcavity effects through a highly efficient OLED device using double emitting layer (DEL) It has been demonstrated that the DEL structure can confine the emitting zone of OLEDs within the center of the emitting layer 37 38 Figure 2 5 A shows the device structure and the energy level diagram of the DEL OLEDs. The resonant peak wavelength of the DBR device was tuned to 55 5 nm to maximize the luminance enhancement. TCTA was selected as a host due to its energy band edge off set regarding C BP. While TCTA has a hole
54 mobility of 310 4 cm 2 V 1 s 1 and a very low electron mobility of 10 8 cm 2 V 1 s 1 CBP has high electron and hole mobilities of 310 4 and 210 3 cm 2 V 1 s 1 39 respectively. As a result, exciton generation zone is confined at the TCTA/CBP interface preventing the quenching of excitons at the interface between the emitting layer and the electron transport layer Additionally, the DEL structure also facilities carrier injection into the emitting layer due to the low lying highest occupied molecular orbitals (HOMO) of TCTA. Figure 2 5 B shows the current density voltage characteristics of the single and double emitting layer devices with and without DBR It is apparent that DEL devices have an order of magnitude higher in current density than that of the single emitting devices. Figure 2 5 C shows the current efficiency of the DEL OLEDs with and without DBR The device with and without DBR show the maximum current efficienc ies of 93 cd/A and 224 cd/A, respectively, which corresponding to an enhancement of external quantum efficiency from 21% to 27%. While the enhancement of 140% in the DEL OLEDs with DBR was the same as that in the #2 DBR d evice with single emitting layer, the current efficiency increase s to 224 cd/A which, to our knowledge, is the highest current efficiency for single emitting unit devices without light extraction ever reported 25 26 Figure 2 5 D shows the angle dependence of emi tting light for the noncavity device and the DBR device with and without macrolens Both of the DBR devices with and without the macrolens showed strong forward emission distribution and w ith macrolens another 89% enhancement was achieved by integrating intensity at all angles. Thus the total enhancement by a factor of 2.4 (=1.28 1.89), promising an EQE of 50%, can be achieved by the microcavity and macrolens in the DEL device with DBR Figure 2 5 E shows the angle dependence of peak wavelengths for non cavity
55 device and DBR devices with and without macrolens. Both DBR microcavity devices show blue shift with the inc reasing of viewing angle, while the peak wavelengths of non cavity device stay the same. 2.4 Conclusions In summary, through a systematic study, we investigated the effects of the electrode reflectivity and cavity length on the performance of microcavity p hosphorescent green red and blue emitting OLEDs. We found the luminance output is a strong function of the reflectivity of the electrodes as well as the EL spectral luminosity in microcavity OLEDs. The resulting enhancement in current efficiency is a comb ined effect of luminescence efficiency and luminosity. By optimizing both factors, a high current efficiency of 224 cd/A was demonstrated in a double emitting layer green microcavity OLED.
56 Table 2 1 Enhancement from spectra was percentage change between integrated intensities of non cavity and microcavity having same organic structure. Device Enhancement fr om q uantum e fficiency Change of l uminosity C urrent e fficiency e nhancement C alculated Measured Green G1 60 .1% 11.3% 78% 75% G2 101% 22.7% 146% 140% G3 70.2% 14.8% 95% 97% Red R1 46.6% 32.6% 94% 100% R2 66.8% 14.2% 90% 89% R3 31.4% 23.7% 0% 2% Blue B1 40.1% 28.7% 1% 2% B2 29.4% 19.5% 4% 1% B3 23.2% 1 0.6% 11% 12%
57 Figure 2 12 R eflectivity of DBR substrates for blue, green, and red
58 Figure 2 2. Microcavity devices: (a) Device structure of green red and blue OLEDs (b) normalized EL spectra of green, red and bl ue devices, spectra were normalized to the peak intensities of their non cavity devices with same organic structure
59 Figure 2 13 Microcavity device performance: normalized EL spectra (a) and current efficiency (b) of green PhOLEDs normalized EL spectra (c) and current efficiency (d) of red PhOLEDs. normalized EL spectra (e) and current efficiency (f) of blue PhOLEDs
60 Figure 2 3 Continued
61 Figure 2 3 Continued
62 Figure 2 14 The noncavity devices (black) and optimized microcavity devices (yellow) EL spectra
63 Figure 2 15 Double emitting layer device performance: (a), device structure of double emitting layer OLED; (b) current density vs voltage of DEL and SEL devices ; (c) current efficiency with and without microcavity ; (d) angle dependence of integrated intensity; (e) angle dependence of peak wavelength.
64 Figure 2 5. C ontin ued
65 Figure 2 5. C ontinued
66 CHAPTER 3 EXCITON CONFINEMENT IN MULTILAYER CADMIUM FREE BLUE EMITTING QD LEDS In this chapter, we demonstrate blue/ violet emitting devices based on cadmium free zinc selenide/zinc sulfide core/shell quantum dots. Using poly(N vinylcarbazole) with a low lying highest occupied molecular orbital energy for the hole transporting layer, enhanced hole injection was observed, resulting in efficient blue/ violet emitting devices. The device charge balance was enhanced by tuning the thicknesses of the hole transporting layer and quantum dot emitting layer. A n external quantum efficiency of 0.65% for these devices was achieved. 3.1 Background and M otivation Recently quantum dot light emitting diodes ( QD LEDs) are considered as the devices for the next generation displays because of their color tunability and highly saturated emitting colors 40 45 The electroluminescent spectra of typical Q D LEDs have a full width at half maximum (FWHM) of 20 nm and by tuning the size of nanocrystals, they can cover the full color gamut of the CIE chromaticity diagram 40 42 Furthermore, these devices can be fabricated by solution processes which are favorable for low cost and large area manufacturing 45 Thus far the most efficient light emitting QD LEDs are green and red with external quantum efficiencies (EQEs) less than 10% 46 47 While the EQEs of orange and red emitting QD LEDs are comparable with the efficiencies of organic LEDs 48 the EQEs of blue/violet emitt ing QD LEDs are substantially lower 49 51 because of the low photoluminescence (PL) yields of blue/violet emitting cadmium ( Cd ) based QDs Although CdSe/ZnS core/sh ell QDs have high PL quantum yields in the green and red regions, the uniformity of the blue emitting QDs is difficult to control due to the lattice mismatch between CdSe and ZnS 52 resulting in defect for mation and low
67 PL quantum yields in these nanocrystals (NCs). For example, the EL spectra of CdS QD based LED have a relatively wide FWHM of ~ 30 nm 53 along with a broad background emission at longer wavelengths due to emission from the trap states. Moreover, the Cd based QDs are not envi ronmental friendly and heavy metal free QDs are desirable for future applications. Recently, zinc based compound semiconductors have been developed as candidates for efficient blue/violet emitting NCs 54 57 Specifically ZnSe is a large gap semiconductor with a bulk bandgap energy of 2.8 eV and its QDs emit in the range between UV and blue. 58 Although there are several studies on PL of ZnSe NCs, blue/violet emitting ZnSe/ZnS QD LEDs ha ve not been reported. Here, we report on efficient solution processed blue/violet emitting QD LEDs using ZnSe/ZnS core/shell QDs 3.2 Experiment We fabricated QD LEDs on glass substrates coated with ITO with a sheet resistance of ~20 sq 1 The substrates were cleaned with deionized water, acetone and isopropanol, consecutively, for 15 min each, and then treated for 15 min UV ozone cleaner. The first layers of PEDOT:PSS (AI 4803) were spin coated and annealed at 180 C for 15 min in a ir. The coated substrates were then transferred to a N 2 filled glove box for spin coating of the Poly(4 butylphenyl diphenyl amine)(poly TPD) (American Dye Source), PVK (American Dye Source), ZnSe/ZnS QD and ZnO nanoparticle layers. The HTL (20 mg ml 1 in chlorobenzene) was spin coated at 6000 r.p.m. for 30 s, followed by annealing at 120 C for 15 min. Then spin coated ZnSe/ZnS QDs (10 mg ml 1 in toluene) spin speed varied from 500 to 2,000 r.p.m to achieve different layer thickness and annealed at 180 C for 30 min. ZnO nanoparticles (30 mg ml 1 in ethanol)  was spin coated at 8,000 r.p.m. and annealing at 80 C for 30 min. These
68 multilayer samples were then loaded into a custom high vacuum deposition chamber (background pressure, ~3 10 7 torr) to de posit the top Al cathode (100 nm thick) patterned by an in situ shadow mask to form an active device area of 4 mm 2 3.3 Results and Discussion 3.3.1 Characterizaion of Cd free QDs ZnSe/ZnS core/shell QDs were synthesized according to the previous reports 56 59 In our synthesis of Zn based QDs, highly toxic reagents which are required for Cd based QD synthesis are avo ided. The size of the ZnSe core was tuned to violet emission. In order to confine the exciton, additional ZnS layers were grown as a shell to form the core shell structure. Our hydrodynamic size measurement shows our core/shell ZnSe/ZnS QDs have an average size of 9.3 nm. From the PL measurements, ZnSe/ZnS QDs suspended in toluene showed a high quantum yield of 40%, similar to that of the deep blue emitting ZnCdS alloy QDs reported previously 60 Fig ure 3 1 shows the optical absorption and PL spectra of the QDs. While the absorption peak of the QDs is at 406 nm t he PL emission peak is at 420 nm with a FWHM of 16 nm, which is significantly narrower than that for blue emitting Cd based QDs, indicating that the quality of our ZnS QDs is indeed better than most Cd based QDs. 3.3.2 Deep B lue QD LEDs Th ere are two configurations of EML of QD LEDs. One approach is to blend the QDs with a wide gap conjugated polymer matrix 42 61 which is responsible for carrier transport. However, in case of wide gap QDs such as ZnSe or CdS, it is difficult to find a wide gap conjugated polymer capable of both transporting carriers and confining excitons in the QDs. As a result, the poly mer matrix can sometime contribute to EL thus lowering the efficiency of the emitting QDs. Another approach is to fabricate multilayer
69 LEDs, wherein a close packed QD layer is sandwiched between a wide gap HTL and a wide gap ETL. The photo generated e xcito ns can be effectively confined in the QD layer by the HTL and ETL, resulting in light emission from the QD layer 41 5 2 57 62 In this work, we used the multilayer approach to fabricate our ZnSe/ZnS QD LEDs. Figure 3 2A shows the current density and optical pow er density as a function of applied voltage for devices with a 40 nm thick QD layer on the poly TPD and the PVK layers, respectively. Poly TPD has been widely used a HTL material for solution processed organic light emitting devices due to its low process temperature and good chemical resistance to organic solvents used for subsequent QD layer deposition 47 However our device with poly TPD layer shows low current density and low optical power density over a wide voltage range. Based on the energy band diagram shown in Figure 3 2 B ZnSe/ZnS ha s a low lying ionization potential at 6.6 eV and poly TPD has a highest occupied molecular orbital (HOMO) level of 5.2 eV. Hole injecti on from the poly TPD layer into the QDs layer can be significantly suppressed by a hole barrier of 1.4 eV at the poly TPD/QDs layer interface whereas electron injection is more efficient due to a better matching of the conduction bands between the ZnSe QD s ( 3.7eV) and ZnO ( 4.2eV) 63 layers. Notice that there are two turn on voltages shown in the current voltage curve in Figure 3 2 A The first current turn on at 1.2 V correspond s to the onset of electron injection and the second turn on at 4.4 V corresponds to the onset of hole injection and light emission. Such a difference in carrier injection causes a severe charge imbalance in our devices, resulting in low electron hole recom bination efficiency. Moreover, it is reported that an imbalanced charge transport leads to strong non radiative Auger recombination, where an exciton recombines and donates its energy to
70 an unpaired carrier which then relaxes to the ground state via intera ctions with phonons 64 In our poly TPD devices, generated excitons were likely to be quenched by excess electrons. In order to improve hole injection, the poly TPD layer was replaced by PVK. 61 65 With a HOMO level at 5.8 eV, PVK reduces the hole injection barrier from 1.4 eV to 0.8 eV, resulting in an enhancement in charge balance and an increase in hole cur rent as shown in Figure 3 2 A The enhancement in hole injection also leads to a reduction of optical power density turn on voltage from 4.4 V to 3.5 V. At an applied voltage of 8 V, the device with the PVK layer shows an optical power density of 4 6 mW/ c m 2 which is significantly higher than 7.5 10 3 mW/ c m 2 of the device with the poly TPD layer. Figure 3 3 A and B show the EL spectra of the poly TPD and PVK device s, respectively. The poly TPD device shows a strong band edge emission peaked at 425 nm with a broad background emission from 400 nm to 700 nm. Considering the insufficient hole injection from the poly TPD into the QDs layer due to the large energy barrier of 1.4 eV, exciplex formation can lead to the background emission from the poly TPD device o r emission through the surface states of the QDs, which gives rise to the broadband emission in Figure 3 A Moreover, the band gap of poly TPD is not sufficient to effectively confine the excitons in the QDs layer Exciton energy can be transferred from the QD layer to the poly TPD layer resulting in a low quantum efficiency of QD emission. In contrast to the poly TPD device, the PVK device only shows band edge emission from the QDs with no background emission as shown in Figure 3 3 B The deep HOMO level of PVK lower s the hole injection barrier and facilitates hole injection into the QDs, thereby shifting the charge recombination zone f ro m the HTL/QDs
71 interface to the bulk of the QDs layer. Compar ed to poly TPD, PVK also provide s better exciton confinement du e to its higher exciton energy. In order to achieve the best device performance, we fabricated devices with different QD layer and PVK layer thicknesses. Fig ure 3 4 A shows the EQE vs. current density for the devices with different QD layer thickness es of 20 30, and 40 nm. Spinning coating a QD layer with thickness larger than 40 nm leads to non uniform thickness and poor device performance. It is apparent that the EQE increases with the QD layer thickness and the device with the 40 nm thick QD layer has an EQE as high as 0.65% with a low roll off up to a current density of 500 mA/cm 2 Increasing the thickness of the QD layer, the distance between the emitting zone and the HTL/QD interface becomes larger, resulting in a reduction of non radiactive recombinati on of excitons at the interface and enhancement in device efficiency. We also fabricated devices with different PVK thicknesses. Figure 3 4 A shows the EQE vs. current density of devices with different PVK layer thicknesses. With the decrease in PVK layer t hickness from 40 nm to 20 nm, the EQE increases from 0.3% to 0.65%. PVK has a deep HOMO level but with a substantially lower hole mobility (10 6 cm 2 /Vs) compared with the electron mobility in ZnO (2.0 10 3 cm 2 /Vs) 47 Based on the above discussion, QD LED devices are electron dominated and decreasing the HTL thickness leads to improved charge balance and higher efficiency, confirming our discussion in the previous section t hat the charge balance is critical to the device performance. 3.4 Conclusion In summary we demonstrated that efficient cadmium free violet emitting QD LEDs can be fabricated using PVK as an HTL. Because of its deep HOMO energy, hole injection into ZnSe/Z nS core shell structured QDs is enhanced, resulting in an increase
72 in exciton recombination in the active layer. Our device data show that the thickness of QDs layer plays a critical role in determining the charge recombination efficiency. With an optim ize d QDs layer, we reported an EQE of 0.65% suggesting a solution based cadmium free ZnSe/ZnS QD LED is a promising candidate for short wavelength LEDs.
73 Figure 3 16 Absorption and PL spectra of ZnSe/ZnS QDs in solution (tol uene)
74 Figure 3 2. Device performance: (a) Current density voltage (black) and optical power density voltage (blue) characteristics of poly TPD and PVK devices; (b) Energy band diagram of multilayer QD LED devices with poly TPD or PVK as HTL. The unit for the energy levels is in eV.
75 Figure 3 3. EL spectra of (a) poly TPD device and (b) PVK device.
76 Figure 3 4. EQE as a function of (a) emitting layer thickness and (b) PVK thickness.
77 CHAPTER 4 TRIPLET EXCITONS IN HIGH EFFICIENT LOW ROLL OFF BLUE EMITTING PHOLEDS WITH TERCARBAZOLE HOSTS Long lifetime and high density of triplets are big problems in PhOLEDs, which cause problems like large roll off and unstable devices. In this section, we studied the triplet dynamic in blue PhOLEDs with three new ter carbazole based host materials designed and synthesized. These hosts exhibited high triplet energy levels (2.90 3.02 eV) and high glass transition temperatures ( > 147 ), which promised high efficiency and improved thermal stability compared with the commonly used carbazole based host material, mCP. Comparable maximum efficiency and improved thermal stability were achieved in the blue emitting PhOLEDs using ETC as the host material doped with the iridium(III) bis (4,6 difluorophenylpyridinato) pico linate (FIrpic). All OLEDs with ter carbazole hosts showed suppressed roll off of efficiency at high luminance due to the low triplet triplet annihilation and triplet polaron quenching revealed by the device physics study. Our study also indicated that not only the mobility of BETC is nearly 2 order magnitudes lower than that of ETC, but also the BETC device is carrier injecting limited. 4.1 Background and M otivation Phosphorescent emitting systems improved the efficiency of organic light emitting diodes (O LEDs) enormously and promoted their wide applications in display and lighting. 25 66 67 While the green and red emitting phosphorescent OLEDs have achieved high efficiency and long lifetime, the performance of blue emitting counterpart is still a bottleneck. 68 70 In order to realize the high performance of blue phosphorescent OLEDs, several critical issues are noted and partially addressed, such as triplet exciton confinement, charge balance, and injection. 36 71 75
78 In order to prevent the energy back transfer from the high triplet energy (E T ) emitters, both transporting and host materials wi th higher E T are therefore needed. 36 71 72 With a high E T electron transporting material used in the ETL, 100% internal quantum efficiency was reported in blue phosphorescent OLEDs. 73 The second concern comes from the charge balance inside the emitting zone. The imbalanced electron and hole carriers will result in a dropped exaction generation rate which leads to low quantum efficiency and excessive polarons within the devices. Therefore, ETLs with a mobility larger than 10 4 cm 2 /V 1 s 1 such as 1,3,5 Tri(m pyrid 3 ylphenyl)benzene (TmPyPB) were applied. 75 In the aspect of host materials, it is also important to consider the matching of the front molecular orbitals of host materials with respect to transporting material molecu les, so as to ensure a better charge injection into the host materials. 76 In addition to high E T mobility and suitable energy levels, a good host for blue phosph orescent emitters also requires the high glass transition temperature (Tg) and high energy transfer rate which lead to high stability and quantum yield for blue dopant emission. Aryl silanes and phosphine oxides are potential candidates. 77 78 However, due to the large band gap, low PL quantum yield and poor charge transfe r properties, devices showed low efficiency and large roll off. 79 80 Carbazole derivatives are very good candidates as host materials for efficient blue phosphorescent emitters. Due to the high ionized potential of carbazole functional group, the derivatives share high E T conjugated structure promises high h ). For example, 1,3 Bis( N h as high as 510 4 cm 2 /Vs and a triplet energy up to 2.9 eV. 81 Previous studies have reported
79 over 90% PL quantum yield in iridium (III) bis(4,6 (di N,C 2 picolinate (FIrpic) doped mCP. 82 Another advantage arises from the singlet and triplet energy split of carbazole derivatives. Due to the strong electron acceptor character of the carbazole, the energy split is small in their n 76 This energy split can be as small as about 0.3 0.5 eV. 83 As a result, a lower luminance turn on in blue phosphorescent OLED s can be achieved, which promises high power efficiency. 76 However, compared with aryl silanes and phosphor oxides, mCP does not have a proper Tg. Due to the flat molecular shape of mCP, Tg of mCP is only 60 o C. 77 This drawback makes the OLEDs thermally unstable under operation. Suppressing efficiency roll off plays a critica l role in the application of OLEDs on display and lighting which require high luminescence of operation. Due to the long lifetime and high density of triplet excitons, the triplet triplet annihilation and triplet polaron quenching become the dominating los s mechanisms that contribute to the efficiency roll off, 74 which lead to the increase of power consumption and eventually reduce the device operating lifetime. Therefore it is very important to characterize the reactions of triplet excitons and po laron so as to reduce the efficiency roll off in PhOLEDs. Emitting layers (EMLs) of most PhOLEDs are based on host guest system, thus changing the host system could help modify these interactions and suppress efficiency roll off. 83 85 Here, we demonstrated blue phosphorescent devices with three ter carbazole host materials doped with FIrpic. High efficiency, small roll off and low luminance turn on were achieved in PT C device. Device physics studies were conducted in order to characterize and rationalize the different performance rising from different side chains.
80 The result and conclusion of our study could provide more information to the materials engineering for fur ther realizing efficient and stable blue phosphorescent materials. 4.2 Experiment 4.2.1 Synthesis and C haracterization of T er carbazole H osts The reaction scheme for synthesizing ter carbazole hosts are shown in Figure 4 1. By using an optimized Buchwald H artwig reaction condition, the final host materials were synthesized and purified with high yields of 85% 89%. The synthetic procedures were designed according to industrial scale protocols with considerations on procedure simplification and safety issues. tBu 3 P was replaced by a less pyrophoric reagent HP t Bu 3 BF 4 In the workup steps, the solvents were directly distilled out from the flask and the solutions were purified by passing a silica filtration which have reduced a large amount of solvent transfer, e xtraction and filtration steps. These reactions has been scaled up to 200 g batches and believed to be reaching kilogram scale with small modifications. All chemicals and solvents were purchased from Sigma Aldrich Corp. without further purification. A rou nd bottom flask was degassed for 30 mins. Palladium Acetate (0.08 eq.) and tri tert butylphosphonium tetrafluoroborate (0.31 eq.) were added respectively into the flask under nitrogen, followed by the addition of sodium tert butoxide powder (2.8 eq.) into the flask. o xylene (20 eq.) was first pressured and transferred into the addition funnel and later slowly added into the flask resulting in a brown solution. The mixture was stirred at the room temperature for 1 hour. In the meantime, carbazole or 3,6 Di tert butylcarbazole (2.1 eq.) was dissolved in o xylene. This solution was transferred into the addition funnel and slowly added into the previous solution. The color of the reaction slowly turned yellowish orange. 3, 6 dibromo 9
81 phenylcarbazole powder or the 3, 6 dibromo 9 ethylcarbazole powder (1 eq.) was weighed and added into the flask in 5 portions when stirring. The reaction mixture was xylene was distilled off under vacuum. Enough amount of DCM was added into the flask to dissolve all the soluble materials. DCM solution was passed via a silica gel column. The filtrate was collected and methanol was added for a recrystallization. A lot of white solids formed overnight. The white solids were colle cted in a frit funnel after a reduced pressure filtration. The solids were washed by DI water, ethanol and TBME to give the pure product as a white solid. 1 H NMR, HPLC purity and elemental analysis data of these compounds are shown in Table 4 1 The UV spec tra were measured by Agilent 8453 UV Vis spectrophotometer. Fluorescent emission spectra were measured by PerkinElmer LS55 Fluorescence spectrometer. Surface morphology was measured by Veeco diInnova Scanning Probe Microscopy 004 1005 000. 4.2.2 Device F ab rication and C haracterization Blue emitting phosphorescent OLEDs were fabricated by the following procedures. Patterned ITO substrates were under UV ozone treatment for 15 minutes. A 25 nm thick PEDOT:PSS (AI 4803) as a hole injection layer was spin coated baked at 180 C for 15 minutes in air. After that, the following layers were sequentially deposited by thermal evaporation: a 18 to 25 nm thick TAPC as a hole transport layer a 20 to 30 nm thick host doped with 6% to 25% FIrpic as a blue emitting layer, a 35 to 40 nm thick TmPyPB as an electron transporting layer for the green and red devices, a 1 nm thick LiF layer as an electron injection layer and a 100 nm thick aluminum as a cathode.
82 The hole only device used for extracting mobility consisted of a sp incoated 25 nm thick PEDOT:PSS and thermal evaporated 200 nm thick host materials and 4 nm thick MoO x and 100 nm Al as electrode. For hole single carrier devices, 20 nm thick TAPC and 50 nm thick host doped 6% FIrpic were thermal evaporated between 25 nm t hick PEDOT:PSS and 4 nm thick MoOx/100 nm thick Al cathode. While for electron single carrier devices, 15 TmPyPB was first evaporated on top of cleaned ITO but with no UV Ozone treatment serving as hole blocking layer, followed by 50 nm thick host doped 6% FIrpic, 30 nm thick TmPyPB and a 1 nm thick LiF and a 100 nm thick aluminum. Current luminance voltage characteristics were measured using a Keithley Series 2400 sourcemeter and a Keithley Series 6485 picoammeter with a calibrated Newport silicon photo di ode. The luminance was calibrated using a Konica Minolta luminance meter (LS 100). The electroluminescence spectra were obtained with an OceanOptics HR4000 spectrometer 4.2.3 Transient M easurement Transient PL measurement was conducted on 100 nm FIrpic d oped mCP and ETC thin film s on quartz glass substrates were put under low vacuum of 17 mTorr. A nitrogen pulse laser (Stanford Research System NL100) with emission of 337 nm and 3.5 ns pulse width was used to excite the thin films. A Hamamastu PMT was put behind a low pass filter (> 400 nm) to detect the PL signals, which was analyzed and recorded by an oscilloscope from Terktronix. PL quenching measurement was conducted by an UV light from a high power LEDs with a peak of 365 nm and FWHM of 30 nm was focu s on the device pixel. A Hamamastu PMT was put behind a low pass filter (> 400 nm) to detect the PL signals. A
83 function power generator controlled the electrical pulse driving the pixel. Data were analyzed and recorded by an oscilloscope from Terktronix 4. 3 Results and D iscussion 4.3.1 Designing of T er carbazole M olecules In the aspect of molecular design, one way to increase the Tg is to manipulate the steric hindrance of a molecule by adding side chains to it. Side chains will increase the twisted configu ration of the molecules, leading to higher Tg and more stable amorphous thin films. 86 Suppression of the electronic coupling between molecules can be achieved by linking of bulky and large gap moieties into the 3,6 positions of a carbazole, resulting in only a small reduction in the triplet energy. Improved performance of blue phosphorescent OLEDs with this family of host materials have been reported by several gro ups. 86 88 Here, we designed and synthesized three carbazole phenyl (9CI) Ter 9H ethyl (9CI) Ter 9H tetrakis(1,1 dimethylethyl) (ethyl) (9CI) Ter 9H carbazole (BETC). Figure 4 2 shows the molecular structures of them. When keeping the same peripheral carbazole groups, the phenyl group of PTC was replaced wit h an ethyl group, which creates a new molecule ETC the ethyl group is unlikely to have a strong affection on the electron state of bone structure. Moreover, it has been long believed that adding alkyl functional groups can imporve the solubility of organic molecules. 89 90 When keeping the same core as the 9 ethyl carbazole, tert butyl groups were incorporated onto the 3 and 6 position of the peripheral carbzole. Due to the limit of the content the solution processed devices
84 fabricated from these materials will be discussed elsewhere. The HOMO and LUMO energies for PTC, ETC and BETC were calculated using Titan software to the 6 31G* level. The LUMOs of these molecules are localized on the core carbazole and the HOMOs are mostly localized on the two peripheral carbazoles. There is no obvious influence from the 9 position substituent on their HOMO or LUMO orbitals as shown in Figure 4 3. While the amorphous characteristics of alkyl groups could p rovide a better film homogeneity by reducing crystal boundaries and islands which help increase the transportation. 91 With this molecular design strategy, we could easily compare the contribution of alkyl group on either th e core carbazole or the pherpheral to the materials properties and the device performance. 4.3.2 Characterization of T er carbazole M aterials The UV vis absorption spectra of the three materials in Tetrahydrofuran (THF) were measured at room temperature as shown in Figure 4 4. All materials shared similar shape of absorption spectra which can be attributed to the core bone of carbazole. The lowest energy absorptions of PTC and ETC were very similar which locate at about 341 nm. Due to the non conjugated ter t butyl side chains, the lowest energy absorption of BETC had an 8 nm red shift showing a peak at 349 nm suggesting a little smaller optical gap than PTC and ETC. Good phosphorescent host materials should have the higher E T than the guests. Also the effic ient energy transfer to emitters is demanded, which requires a good overlap between the host PL emission and the absorption of the guests. One of the neat FIrpic absorption peak locates at 388 nm. 92 We measured the PL emissions of all three host materials. Figure 4 5 shows the PL emissions of all three neat host materials as
85 well as their emissions doped with the same concentrations of FIrpic used in the device fabrica tion. Because of the same core bone molecular structure, the shapes and peak wavelengths of neat host materials are very close. Therefore, the differences of them indicate the side chain effects. Compared with ETC and BETC, the PL of neat PTC is red shifte d. This is due to the energy of the excited state. Another significant difference comes from the shapes of PL spectra. The number and the electron affinity of the side chains can influence the vibration states. Therefore t he transition rates from vibration states between HOMOs and LUMOs varied, which led to the changes of the PL spectral shapes. Above all, due to the emission overlap with the absorption of FIrpic, all hosts showed good energy transfer to FIrpic. From the PL spectra of the doped layers, we observed clear FIrpic emissions with sharp cutoff at 440 nm. The peak intensities at 470 nm and 500 nm are different with three hosts, which also come from the effect of side chains. 4.3.3 Blue E mitting P hosphorescent OLEDs Blue PhOLEDs were fabricated with these three host materials and compared with OLED with mCP. The device structure is ITO/PEDOT:PSS/TAPC/new host: FIrpic/TmPyPB/Lithium Fluoride (LiF)/Aluminum. PEDOT:PSS is hole injecting layer. TAPC and TmPyPB are hole and electron transporting materials, with mobility of 10 2 cm 2 /Vs and 10 4 cm 2 /Vs respectively. 75 79 The thicknesse s and doping concentrations of each optimized device were: ITO/PEDOT:PSS/TAPC (25 nm) /mCP (30 nm):FIrpic(6%)/ TmPyPB(38 nm) /LiF/Al, ITO/PEDOT:PSS/TAPC(22 nm)/PTC(25 nm):FIrpic(10%)/ TmPyPB(40 nm)/LiF/Al, ITO/PEDOT:PSS/TAPC(22 nm)/ETC(25 nm):FIrpic(7%)/ T mPyPB(37 nm)/LiF/Al, ITO/PEDOT:PSS/TAPC(20 nm)/BETC(20 nm):FIrpic(25%)/TmPyPB(37nm)/LiF/Al.
86 Figure 4 6 A shows the electroluminescent spectra of the devices with different hosts. All of them showed only FIrpic emission. The different intensities of should er peaks at 500 nm, which corresponds to their PL emissions, come from the effect of side chains and weak microcavity of different device thicknesses. The small red shifts of the PTC and mCP spectra compared with others are due to the longer device thickne sses. Figure 4 6 B shows the plots of current density vs. applied voltages of each device. Reference device with mCP shows an electrical turn on at 3.1 V. The lowest turn on, 2.9 V, comes from the ETC device, which indicates a good carrier injection into t he emitting layer. Meanwhile the PTC device has a turn on voltage of 3.1 V. The highest turn on voltage belongs to BETC device, which is 5.1 V. The luminance turn on voltage of each device corresponds to their electrical turn on voltage as showed on Figure 4 6 C ETC device shows a very rapid rise of luminance, from 1 cd/m to 1000 cd/m within 0.8 V, while it takes 1.4 V and 1.3 V for PTC and BETC devices to raise the luminance within the same range respectively. mCP device shows slow rises of both current d ensity and luminance. Figure 4 6 D shows the current efficiency of devices studied. ETC devices exhibit the highest current efficiency which is comparable to mCP device, but their efficiency roll off significantly reduces at higher luminance. And all other ter carbazole host devices also achieve very low roll off. We can define a critical luminance ( L 90 ) where the current efficiency reduces to 90% of its maximum value. The higher L 90 is, the lower roll off the OLED has. In mCP device, the maximum efficiency of 49.4 cd/A is achieved at 113 cd/m 2 with a L 90 of 850 cd/m 2 As comparison, a maximum efficiency of 48.4 cd/A at 260 cd/m 2 with a higher L 90 of 3300 cd/m 2 was obtained from ETC device. The PTC device has a maximum efficiency of 35.8 cd/A at 200 cd/m 2 wi th an
87 improved L 90 at 4500 cd/m 2 Even though the maximum efficiency of BETC device, 30.6 cd/A at 124 cd/m 2 is the lowest among all ter carbazole hosts, its L 90 reaches 1540 cd/m 2 4.3.4 Annealing E ffects on mCP and ETC Due to the low thermal conductivi ty of organic materials and glass, 93 the heat dissipation in an OLED is a problem, especially under continuous operation. Local heat will be accumulated and the device temperature can go over 50 in 30 s at high current density. 94 Considering the T g of mCP is only 60 phase transition could easily happen with the elevation of temperature, which will eventually affect the electrical and optical properties of OLEDs. Therefore increasing the T g of organic materials can improve the stability of OLEDs. First we verified that due to the high T g of ETC, le ss effect comes under thermal stress. We measured the morphology of neat mCP and ETC films as deposited and annealed at 45 for 30 min. Figure 4 7 shows the AFM images of them. Both mCP and ETC showed a smooth surface of as deposited films. The roughness of mCP is 1.4 nm while the roughness of ETC film is 2.0 nm. After annealing at 45 for 30 min, the changes of film morpholo gy were quite different. The roughness of mCP significantly increased to 24 nm after annealing. Within 500 nm 2 scanning area, clear over 100 nm. Even worse, clear pikes wer e observed on annealed mCP film. Those dramatic changes in the morphology of annealed mCP contributed to the unstable voltage fluctuation of elevated temperature measurement. On the other side, due to the high Tg of ETC, the change of the morphology of ann ealed film was not significant. Even though, aggregations were also observed, the maximum vertical distance between
88 valleys and hills was only 19 nm which was comparable to 11 nm of as deposited ETC. Besides, no pike existed within the 500 nm 2 scanning ran ge. As a result, the roughness of annealed ETC was 3.1 nm. We also made hole only devices of mCP and ETC and measured the J V characteristics before and after 45 annealing, which are presented in Figure 4 8. Due to the dramatic change of morphology of mCP during the annealing, the interfaces between organic layers become fuzzy along with the change of compositions. As a result, the current density of mCP hole onl y device reduced. 4.3.5 Hole M obility and I njection into H osts We conducted further experiment to study the device transport property to explain the difference in the electrical properties of those materials. First we extracted the field dependence mobilit y from the hole only devices. These devices had the same structure, ITO/PEDOT:PSS(20 nm)/host materials(200 nm)/MoOx(4 nm)/Al(100 nm). MoOx has low lying electron affinity and can align the work function of Al to the HOMO of host materials so as to prevent electron injection. 95 Hole only devices showed a space charge limited current (SCLC). Accordi ng to Mott 96 J SCLC can be expressed as 0 is the permittivity and relative permittivity which is ~3 in most small molecular semiconducting materials 96 97 0 is the zero field mobility in the materials, V is the applied voltage and d is the thickness of the device, is Poole Frankel factor related to temperature and materials. By fitting the J V characteristics, the 0 and can be extracted. The Poole Frankel field dependence mobility can be calculated according to
89 However, this apparent mobility extracted from SCLC is based on the assumption of ohmic contact. We fabricat ed hole only devices to investigate the hole injection into each host. The hole unipolar devices have the same structure: ITO/PEDOT:PSS(25 nm)/TAPC(20 nm)/hosts(50 nm)/MoOx(4 nm)/Al. Figure 4 9 B hole only current density for each device. mCP, ETC and PTC have very similar turn on while BETC device shows a turn on voltage of 4 V. This indicates a larger injection barrier from TAPC to BETC. In case of host material with deep lying HOMO like BETC, current density of hole only device is limited by injection. T herefore, carrier mobility can be extracted from the fitting of injection limited current (ILC). 97 Based on the solution of drift diffusion equation and considering the equilibrium contributions to the current density for charge carriers recombining with their own image in analogy with Langevin bimolecular recombination, 98 the ILC current density with field dependent mobility is where N 0 is the density of states in the organic film, is the barrier height, k B is the Boltzmann constant, e is the electron charge, T is the temperature, and is a function of reduced electric field and can be expressed as Figure 4 9 A shows the mobility of host materials used in this study. The mobility of BETC is nearly two orders lower than ETC. The low mobility of BETC also increases the
90 voltage drop across itself. Therefore we can conclude that not only the mobility of BETC is low, but also there are large barrier heights for both carriers injection into BETC. 4.3.5 Triplet L ifetime and TTA of mCP and ETC EMLs In order to study the different roll off behaviors in the efficiency of FIrpic OLEDs with these host materials, we measu red and characterized different mechanisms that can determine the efficiency roll off in the OLEDs. We focused on the OLEDs with mCP and ETC hosts, because they share the same highest current efficiency, similar device thickness and doping concentration of the optimal conditions. Due to the long lifetime of phosphorescent emitter, triplet triplet annihilation (TTA) plays an important role in phosphorescent OLEDs. One of the significant influences is on the roll off due to the TTA. First, we study the transi ent PL decay of mCP and ETC doped with FIrpic. The concentrations of FIrpic are the same used in the optimal OLEDs. Figure 4 10 shows the transient PL decay of mCP and ETC doped with FIrpic. The differential equation of triplet density after the excitation is decay with [ T ] and the TTA rate, describes the biexcitonic annihilation where TTA increases non linear ly with increasing triplet densities. The equation is solved as and compare them in
91 an triplet lifetime reported before. 99 By estimating the initial [T 0 ] from excita tion power density and the absorption constant, we calculated the in mCP and ETC hosts as 2.00.6 10 13 cm 3 s 1 and 1.60.5 10 13 cm 3 s 1 respectively. The triplet lifetime is longer and the between mCP and ETC cannot fully explain the dramatic roll off in the mCP OLED device. 4.3.7 Triplet polaron Q uenching in mCP and ETC D evices Another factor contributes to the roll off of an OL ED is the polaron. We measure the unipolar devices of them in order to study the charge balance inside of OLEDs. Due to the different electrical field dependence of hole and electron mobility, charge balance could reduce at high applied electrical field wh ich results in the efficiency drop. The unipolar devices are ITO/PEDOT:PSS/TAPC/host:FIrpic(50 nm)/MoOx/Al and ITO/TmPyPB/host:FIrpic(50 nm)/TmPyPB/LiF/Al which are based on the optimal devices for hole and electron only devices, respectively. The increase d thickness of EML is for the consideration of device stability and reliability. The purpose of TmPyPB between ITO and EML in electron only devices is to block holes. Figure 4 11 shows the current density vs. applied electrical field of mCP and ETC devices It is obvious that hole only devices have much higher current density than electron only devices in both mCP and ETC devices. This is due to the intrinsic hole domination of transport property of carbozle molecules. Comparing between mCP and ETC devices, J E characteristics agree with the devices J V characteristic and hole mobility calculation discussed in the previous section. Even though, hole currents of both device are in the same range, the
92 electron currents of mCP and ETC device behave differently. In mCP electron only device, we observed a slow increasing current which is more than three orders lower than its hole counterpart in the whole measuring range. In ETC electron only device, there is a clear current turn on at 0.3 MV/cm, after which follow s a faster current increase. This indicates a better LUMOs matching between ETC and TmPyPB. As a result, electron current is higher in ETC device than mCP device, especially at high electrical field. Consequently the reduction charge balance at high electr ical field in ETC OLED is less than that in mCP OLED. Moreover, those excessed polarons in mCP will lead to greater quenching of excitons. Triplet polaron quenching (TPQ) is another significant annihilation of excitons in phosphorescent OLEDs due to intera ction between polarons (hole polaron and electron polaron ) and triplet excitons ( ). This annihilation by free or trapped charge carriers is possible via the following processes The annihilation rates for hole and electron are and respectively. Since the hole is the dominated carrier in our OLEDs and the previous study of Iridium based phosphor dye shows that TPQ from electron polarons is smaller than hole polarons 100 our study will focus on the hole polaron quenching. In order to characterize the TPQ, we first measured the steady PL quenching of unipolar devices with increased current injection. We used a short pulse to inject the electrical current into the device so as to eliminate the heating effect. The pulse width is 50 microseconds with a frequency of 20
93 Hz, which corresponds to a duty cy cle of 0.1%. Figure 4 12 A and B show the time resolved PL quenching of mCP and ETC hole only device with different injected current densities. The PL intensity of both mCP and ETC devices reduced with the increasing of injected current. And the quenching from mCP device is stronger than ETC device. Under low PL excitation, only considering the triplet decay, time differential equation of triplet density can be expressed as When time is much larger than the equilibrium exaction density [ T 0 ] is [ T excited ] is the initial triplet density from the excitation of light source. When there is current injection, the time differential equation of triplet exciton density under the PL exc itation is [ P ] is the polaron density. Solving this equation, we can find the relation between the final triplet density [ T final ] and [ T excited ] under steady equilibrium PL excitation is Therefore the r atio of triplet density before and after current injection can be
94 Assuming the SCLC inside the EML, the polaron density [ P ] is proportional to then we find the relative triplet density difference is p roportional to the electrical voltage: We plot the relative reduced PL intensity vs. the voltage in Figure 4 12 C and D Both devices show good lineal dependence. From the slopes of the phosphorescence quenching vs electrical fi eld, we extracted the for mCP and ETC devices are 2.00.1 10 13 cm 3 s 1 and 7.80.6 10 14 cm 3 s 1 respectively. of mCP is within the same range of other Ir based phosphor dye studied before 100 101 ETC shows three times lower than that of mCP. The large of mCP is one of the factors contribute to the great efficiency roll off at high current density. We are also interested in the polaron quenching under continuous driving condition where joule heat becomes an inevitable factor especially at higher curren t density. Here instead using a short pulse to drive the device, we conducted a steady measurement on the PL intensity reduction under a long pulse injected current of 1 s. The hole only device structures are the same in previous discussion. Figure 4 13 sh ows the relative reduced PL intensity under hole current density from 1 mA/cm to 200 mA/cm which is within the common range of operating an OLED device. For comparison, we put the data under short pulse quenching in the same plots. In both mCP and ETC devi ces, the PL reduction before 50 mA/cm 2 is close under long and short electrical pulse. However at higher current, significantly larger PL intensity
95 quenching appears, especially in mCP device. At 200 mA/cm 2 an additional 64% reduction was observed with lo ng pulse quenching in mCP while this further reduction from long pulse quenching in ETC is about 40%. The lower T g of mCP makes device more vulnerable to heating under the device operation. 4.4 Conclusion We designed and synthesized three different ter car bazole based host materials, PTC, ETC and BETC with different side chains for blue phosphorescent OLEDs. All of them exhibit high triplet energy, high glass transition temperature and good energy transfer to phosphorescent emitter FIrpic. Improved thermal stability on the morphology was demonstrated compared with mCP which has low Tg. The PhOLED devices with those hosts showed high efficiency and low roll off. Device physics study confirmed the suppressed roll off comes from the shorter lifetime, low TTA an d TPQ rates with FIrpic doped ETC as the EML. Device transporting property study showed the different side chain effects on the mobility. Due to the alkyl group, BTEC device by thermal evaporation exhibited low mobility and poor carrier injections.
96 Tab le 4 1 1 H NMR, HPLC purity and elemental analysis data of these compounds Compound Yield 1 H NMR (CDCl 3 400Hz) PTC 83% 8.27 8.28 (2H, Ar H), 8.15 8.17 (4H, Ar H), 7.69 7.75 (4H, Ar H), 7.65 7.67 (2H, Ar H), 7.56 7.62 (3H, Ar H), 7.38 7.42 (8H, Ar H), 7.26 7.30 (4H, Ar H). Anal. calcd. for C 42 H 27 N 3 HPLC: 99.3% ETC 87% 8.22 8.23 (2H, Ar H), 8.15 8.17 (4H, Ar H), 7.65 7.70 (4H, Ar H), 7.36 7.41 (8H, Ar H), 7.26 7.29 (4H, Ar H), 4.56 4.61 (2H, Et H), 1.62 1.65 (3H, Et H). Anal. calcd. for C 38 H 27 N 3 HPLC: 99.6% BETC 91% 8.18 8.19 (2H, Ar H), 8.15 8.16 (4H, Ar H), 7.64 7.65 (4H, Ar H), 7.43 7 .46 (4H, Ar H), 7.30 7.32 (4H, Ar H),4.53 4.58 ( 2H, Et H), 1.61 1.64 (3H, Et H), 1.46 (36H, tbu H). Anal. calcd. for C 54 H59N 3 : C 86.47 H 7.93 N 5.60; Found:
97 Figure 4 1 S ynthesis route of PTC, ETC and BETC.
98 Figure 4 17 S tructures of host molecules
99 PTC ETC BETC Figure 4 3. DFT calculated HOMO and LUMO orbitals for PTC (top), ETC (middle) and BETC (bottom).
100 Figure 4 4. Abs orption spectra of PTC, ETC and BETC
101 Figure 4 5. PL spectra of neat PTC, ETC and BETC as well as the spectra of PTC, ETC and BETC doped with FIrpic.
102 Figure 4 6. D evice performance:(a)EL spectra, (b) J V curves, (c) L V curves, (d) current efficiency
103 Figure 4 7. AFM of mCP as deposited (a) and after annealed (b) and ETC as deposited (d) and after annealed (d). Figure 4 8. J V characteristics of mCP (a) and ETC (b) hole only devices before and after 45 annealing.
104 Figure 4 9 Host materials characterization: (a) mobility of host materials used, (b) J V of hole only devices Figure 4 18 0. T ransient PL decay of mCP and ETC doped with FIrpic
105 Figure 4 11. U nipolar devices with mCP (a) and ETC (b).
106 Figure 4 12. TPQ on the PL intensity of mCP (a) and ETC (b) hole only devices; and relative reduced PL intensity vs. applied voltage of mCP (c) and ETC (d) devices.
107 Figure 4 13. TPQ under different current injection of long and short electrica l pulse.
108 CHAPTER 5 SUB BANDGAP TURN ON OF ELECTROLUMINESCENCE AT RUBRENE/FULLERENE INTERFACE The efficiency of fluorescent OLED is limited by the spin statistic, which only allowes 25% of the formed excitons decay through radiative process. One possibl e process to take the advantages of 75% triplets is through TTA. In this section, we are going to show that TTA can further reduce the operating voltage by a luminance turn on below the corresponding bandgap of the emitter. Experiment was conducted to veri fy the energy transfer from the charge transfer state to triplet state. It gives a proof to the fundamental mechanism of the sub bandgap luminance phenomenon. 5.1 Background and M otivation Since the invention of OLEDs, efforts have been made on the improve ment of the efficiency. 25 37 66 68 Besides improving the internal quan tum efficiency by using the phosphorescent emitters, 66 102 103 more light can be out coupled by using photonic structure s 25 67 Also new materials with higher mobility we re synthesized to improve the power efficacy 76 Another way to reduce the power consumption is operating the device at low voltage. However the lowest turn on volt age of a LED is determined by the bandgap of the emitting layer This rule governs most of OLEDs where carriers are directly injected in to HOMOs and LUMOs. Fortunately, several groups reported a few organic systems show an EL turn on lower than the bandg ap of emitting semiconductors. 63 104 It opens a new door to improve the power efficiency in OLEDs. One of those sub bandgap systems is 5,6,11,12 Tetraphenylnaphthacene (rubrene)/fullerene bilayer structure. 104 Both rubrene and fullerene are widely studied semiconductors, which have the highest field effect mobility for holes and electrons, respectively. 105 106 Rubrene is a mate rial of great interest for
109 organic optoelectronic research and practical applications with its exceptional properties mobility and crystal structure. Organic field effect transistors have confirmed that hole mobility of rubrene single crystals can be as la rge as 20 cm 2 /Vs 107 108 It has also been demonstrated that a significant improvement of the OLED effic iency can be achieved by adding a thin layer of rubrene 109 Other work also showes with rubrene doping, the position of the recombination zone can be turned 110 as well as the stability of the devices been impo rved 111 Fullerene was demonstrated with good electron transporting and accepting properties. The electron mobility of C 60 2 /Vs is possible in devices. 112 113 With its excellent electron accepting ability, fullerene derivatives are used as the acceptors in organic photovoltaic solar cells and demonstrated high power converted efficiency. 114 115 By applying the rubrene/fullerene heterostructure to OLEDs, the device behaves like a compound semiconductor device S urprisingly, the EL turn on voltage is about 1 V which is about half the value of the rubrene bandgap (2.2 eV) The EL spectrum of only rubrene emission was observed. ( S hown in Figure 5 2) This sub bandgap EL cannot be explained with models of charge injection into organic semiconductors. Figure 5 1 shows the molecurles structures of rubrene and C 60 as well as the energy band diagram of rubrene/C 60 OLEDs. We fabricated rubrene/C 60 device and compared it to different electron injection layers. The L V curves are shown in Figure 5 2. Device with no electron injecting layer, shows the highest turn on voltage. With the presence of LiF, the luminescence turn on approaches to the corresponding bandgap. However, by using C 60 as the injecting layer, the turn on voltage reduces to 1 V which is only half of the
110 bandgap (2.1V). The EL spectra below and above bandgap voltage ar e identical from the rubrene/C 60 heterojunction, which indicates a up conversion process. A physical interpretation, Auger founta in mechanism, was proposed to explain this sub bandgap phenomenon at this heterojunction. 63 104 During the EL process in heterojunction devices, holes and electrons are injected from the rubrene and C 60 layers, respectively. They then rec ombine near the interface to raise another electron at the LUMO of C 60 which overcomes the energy barrier between the LUMOs of rubrene and C 60 After injected into the LUMO of rubrene, this Auger electron recombine with the hole in the HOMO and emits phot on with the emission characteristic of rubrene. Several researches were conducted in order to support this hypothesis. A dual functional device using rubrene and C 60 was fabricated. 104 116 117 While applying voltage, it started emitting light at a sub bandgap voltage. U nder illumination it also generate s electrical power. The solar power conversion efficiency reaches 3% with a 5.3 mA/cm 2 short circuit current density and almost 1 V open circuit voltage under AM 1.5 illumination. This large V oc corresponds to the EL turn on in OLED device. However, further analysis w as not given on how the large V oc can be related to Auger electrons. Studies also focus on the exciplex emission from this heterojunction. 118 The exciplex formation in rubrene based heterojunctions is important because it can indicator of the recombination process of bifunctional devices, since efficien t exciplex emission implies efficient charge transfer (CT) across the interface and their recombination 119 120 In order to distinguish the exciplex formation dependen ence on the molecular organization in the film 121 bulk heterojunction devices and planar heterojunctions were studied co mparatively. Even
111 though n ear IR emission was observed in the planar heterojunction with a thin mixed layer between the rubrene and C 60 no evidence of Auger electron was reported. Another well known up conversion phenomenon in rubrene is triplet triplet a nnihilation (TTA). 122 123 This TTA reaction can either go throng non radiative decay or transfer the energ y to a singlet which can rad i a ti vely recombine. Due to two triplet excitons being involved, this fusion does not violate the spin statistics. The triplet energy of rubrene is just the half of its singlet state. 124 Therefore this kind of triplet fusion is more efficient. From transient studies th is fusion contributing to a delayed fluorescent emission in rubrene was observed. 122 OLEDs with as much a s half of their EL from annihilation of triplet states generated by recombining charge carriers were demonstrated. 125 126 The magnitude of TTA contribution in combination with the remarkably high total EQE [>11%] indicates that the absolute amount of EL attributing to TTA substantially exceeds the limit imposed by spin statistics, which was independently con firmed by studying magnetic field effects on delayed luminescence. The value of 1.3 for the ratio of the rate constants of singlet and triplet channels of annihilation is indeed substantially higher than the value of 0.33 expected for a purely statistical annihilation process. 125 By considering the evidence of exciplex from the heterojunciton of rubrene and C 60 interface and the strong triplet fusion capability of rubrene, we propose another mechanism based on the TTA of rubrene that leads to the sub bandgap EL emission. Up conversion has been demonstrated in solution of rubrene and a sensitizer who has lower absorption band gap than rubrene but with higher triplet energy. 127 129 The photoexcited sensitizer transferred energy to the triplets of rubrene which undergo
112 fusion process giving energy to the singlet. In case of rubrene/C 60 heterojunciton, the energy source beco mes the exciplex, the charge transfer state formed with LUMO of C 60 and HOMO of rubrene. Due to the large barrier hight at between the LUMOs and HOMOs of rubrene and C 60 electrons and holes are accumulating at the interface, which is bonded and forms char ge transfer exciton. 121 Because of the separation of donor and acceptor, the energy split between singlet and triplets is small. Through Dexter transfer, large amount of triple t formed at rubrene molecules. In this study, we investigated the charge transfer exciton by the electroabsorpton measurement, and showed from the transient EL measurement that delayed fluorescent emission dominating the EL of rubrene/C 60 OLED. Finally, t he increase of triplet density was observed with the present of rubrene/C 60 heterojunction. 5.2 Experiment The heterojunction devices consist of 40 nm thick rubrene and 20 nm thick C 60 layers sandwiched between transparent indium tin oxide (ITO) and 100 nm thick metal electrodes. A 30 nm thick layer of poly (3,4 ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT: PSS) at the ITO/rubrene interface was used as buffer layer. For reference, rubrene only devices were also sandwiched between 30 nm thick PEDOT and 100 nm Al metal cathodes. Transient measurement 100 nm rubrene and rubrene/C 60 (2:1) thin film on glass substrates were put under low vacuum of 17 mTorr. A Nd:YAG pulse laser with emission of 527 nm and 10 ns pulse width was used to excite the thin fi lms. A Hamamastu PMT was put behind a band pass filter (peak at 584 nm, FWHM of 20 nm) to detect the PL signals, which was analyzed and recorded by an oscilloscope from Terktronix. Transient EL measurement
113 was conducted on encapsulated devices in air atmos phere. Lenses were used to focus EL emission onto the PMT, which also was placed behind the band pass filter. 5.3 Results and D iscussion 5.3.1 Unipolar D evices First, we characterized the carrier transporting property using unipolar devices. The thickness es of each layer are ITO/PEDOT(30nm)/Rubrene(60nm)/C 60 (35nm)/Au and ITO/Rubrene(90nm)/C 60 (35nm)/Al for hole and electron only devices respectively. No UV Ozone treated ITO was used to increase the hole injecting barrier in electron only devices while, high work function metal Au was applied to block electron injection in hole only devices. Figure 5 3 shows the J V characteristics of both devices. The current density of hole only device is more than two order s larger than that of electron only device, which indicated in this bilayer OLED, hole is the dominating carrier. Due to the large excessive hole carriers accumulated at the interface, a free electron that could f ountain mechanism is not suitable here. 5.3.2 TTA in S ub bandgap E L Due to the long lifetime of triplet excitons, a delayed fluorescent EL emission is expected in TTA up conversion OLEDs. A detail analysis of the triplet dynamics has been given based on sm all molecule such as rubrene 122 and polymer such as F8BT 124 We also conducted transient EL measurement based on rubrene OLEDs with and without C 60 layer. The device structures are ITO/PEDOT/rubrene/Al and ITO/PEDOT/rubrene/C 60 /Al. The rubrene only device has a luminance turn on at 2V, while the rubrene/C 60 heterojunction device shows a low luminance turn on at 1 V. We first measured the EL transie nt decay after electrical current shut down from different
114 voltages. The decay data are presented in Figure 5 4 A It is clear that there is a fast decay less than 100 ns after current shut down following a long time decay in microsecond. Our data show the same TTA figure print decay curves as previous report 122 The fast decay comes from the short lifetime s inglet excitons forming directly from the charge recombination. Due to the spin statistic, 75% recombination leads to triplets, parts of which undergo TTA process and give energy to singlet. With the increase of the voltage, we observed a decrease of decay time of delayed component. Long living triplet is more easily quenched by injected polarons. When there is current injection, the time differential equation of triplet exciton density can be expressed as [ T ] is the triplet densi ty, is the triplet lifetime, [ P ] is the polaron density, is the TTA rate and is the triplet polaron quenching (TPQ) rate. TPQ is not significant at low current density. Theref ore if we only consider the first two terms in the equation at low current density, we obtained a report 130 In case of rubrene/C 60 hetero junction device, we also measured the EL decay after current shut down from sub band gap region to high voltage. The data were presented in Figure 5 4 B We did not observe obvious fast decay component. Long time delayed fluorescent decay dominates all cur ves. With the increase of voltage, decay time decreases as expected from the increasing of TPQ. At the sub bandgap region, TTA is the only mechanism for the rubrene emission. We only consider the second term in the triplet time resolve equation, the triple t density is given as
115 The EL intensity which is resulted only from TTA is Therefore, we expected the time resolved EL decay has a slope of 2 in logarithm plot. Figure 5 5 shows the fitting of EL decay s driven in the sub bandgap region where TPQ is not significant. Both EL decays driven at 1.2 V and 1.8 V show a slope of 2, which indicates the TTA is the reason for the EL under the excitation of sub bandgap. 5.3.3 CT S tates at the R ubrene/fullerene H e terojunction The energy of triplet contributes to the sub bandgap emission comes from the CT state. In order to prove the existence of CT state at the interface of rubrene and C 60 we measured the electroabsorption (EA) signal of pristine rubrene, pristine C 60 and mix rubrene/C 60 (2:1) blend. The quadrature signal from EA can distinguish the exciton and CT state due to slow response of CT state to the AC electrical field. 131 Figure 5 6 shows the EA signal and its quadrature signal of rubrene/C 60 blend. There is an EA peak at 1220 nm, which corresponds to the charge transfer state energy calculated from the energy band diagram. Its quadrature signal follows the EA si gnal. Before 1150 nm, there is no quadrature signal. When the EA signal turns positive, the quadrature signal also starts and reaches the peak at 1220 nm. After the peak, both signals reduce. The negative signal of quadrature indicates the slow response to the AC electrical field. Therefore, the EA single at NIR region is from the CT states, which also agrees with the energy (1 eV) calculated from band diagram (Figure 5 1). 5.3.4 Energy T ransfer to T riplets We further conducted the transient PL measurement on the rubrene and rubrene/C 60 blend (2:1). We focus on the decay fluorescent component from rubrene
116 emission, which is related to the triplet and charge transfer state. Rubrene also shows an efficient singlet fission process. Under high intensity excitati on, significant amount of triplets can be generated which go through the triplet fusion and give energy back to singlet. As a result, a delayed fluorescent emission is also expected in PL measurement. Here we used a green pulse laser (527 nm) to excite the thin film. The excitation wavelength is overlap one of the absorption peak of rubrene. Figure 5 7 A shows the transient PL at shorter time (< 500 ns). The laser signal shows a good Gaussian shape with a Full width at Half Maximum (FWHM) of 10 ns, which is longer than the decay time of rubrene singlet. Therefore, there is no fast fluorescent component but only the delayed component in in both rubrene and rubrene/C 60 blend. And the intensity of delayed component becomes larger in the rubrene/C 60 blend. We sh ow the EL decay in longer time scale and plot them in log log scale in Figure 5 7 B These two lines are paralleled to each other over 3 order magnitudes. Again as in the sub bandgap EL decay, this delayed fluorescent signal solely comes from the TTA, whos e intensity is proportional to the square of triplet density. Considering the first two terms in the time resolved differential equation, the time dependence of triplet density is We fitted both decays according to and extracted the initial triplet density of them. Our fitting shows good agreement over 3 order of magnitudes in both films. And we found out (233) % times more triplets state in rubrene/C 60 blend than pristine rubrene. Due to the large ban d offsets of HOMOs and LUMOs of rubrene and C 60 non geminated recombination can be ruled out. Thus we attribute this increase of
117 delayed rubrene fluorescent emission coming from the charge transfer states which created more initial triplet under photoexci tation. 5.4 Conclusion We proposed a mechanism based on the TTA property of rubrene to explain the sub bandgap EL phonominoun of rubrene/C 60 heterojunction. The holes are the dominating carriers through this heterojunction, where free electrons are less l ikely to exist. Transient EL decay of sub bandgap emission shows the characteristic of TTA. And charge transfer state is found with EA measurement. Finally transient PL decay links them together, demonstrating the increase of triplet density is coming from the energy of charge transfer state.
118 Figure 5 1. The molecular structures of (a) rubrene and (b) C 60 ; (c) the energy band diagram of OLEDs with different electron injecting layers Figure 5 2. The L V curves of rubrene OLEDs with different elect ron injecting layers (left) and EL spectra below and above band gap voltage (right).
119 Figure 5 19 T he J V characteristics of unipolar devices of rubrene/C 60 heterojunction. Figure 5 4. Transient EL decay of (a) rubrene OLED and (b) rubrene/C 60 OLED
120 Figure 5 5. Log log scale of EL decay before bandgap operation. Blue lines are fitting lines having slope of 2. Figure 5 6. EA and its quadrature signal of rubrene/C 60 blend.
121 Figure 5 7. Transient PL of rubrene an d mix rubrene/C 60 blend shorter than 500 ns (a) and over 500 ns (b).
122 CHAPTER 6 SUMMARY AND FUTURE WORK In this thesis, series studies ha ve been conducted on the triplet exciton behaviors in organic electronic materials and devices. Due to van de Waals bo nding force, small dielectric constants and amorphous structures, excitons are the dominating quasi particles that determine the properties of organic semiconductors. In the chapter one, I gave a brief introduction of exciton and its related concepts used in organic electronics research. From there we focused on the exciton emission manipulation, exciton confinement, exciton exciton annihilation, and exciton polaron annihilation on the microcavity OLEDs, quantum dot LEDs, phosphorescent and fluorescent OLED s. First we turned the exciton emission of green, red and blue phosphorescent emitters with the help of microcavity. After the systematic study we elucidate d the microcavity effects for different color emitters We found that the luminance output is deter mined by the reflectivity of semitransparent electrode and the photopic response of the green, red and blue emitters. While the luminance enhancements of blue and red phosphorescent microcavity devices are small, current efficiencies as high as 224 cd/A is obtained in green phosphorescent microcavity OLEDs. Second we study the exciton confinement in QD LEDs. Enhanced hole injection and better exciton confinement were demonstrated with the low lying HOMO energy for the HTL, PVK. By tuning the thicknesses of the HTL and quantum dot emitting layer, we managed the device charge balance and demonstrated 0.65% EQE blue/ violet emitting hybrid LEDs based on cadmium free zinc selenide/zinc sulfide core/shell QDs. In order to suppress efficiency roll off in phosphore scent OLEDs, we introduced three new ter carbazole hosts. We characterized the triplet lifetime, TTA rates and TPQ
123 rates of them and compared with mCP. With ter carbazole host, phosphorescent emitter FIrpic shows short lifetime, reduced TTA and TPQ interac tion, result ing highly efficient blue PhOLEDs with reduced efficiency roll off at high luminescence. Finally, we proposed the TTA is responsible for the sub bandgap EL of rubrene fluorescent OLEDs. This unique property could reduce the power consumption o f OLEDs. With experiment results from electroabsorption, transient EL and PL measurements, we verify the energy from the charge transfer state of the donor and acceptor can be transferred to triplet state, which undergoes an up conversion path and transfer the energy to higher singlet state. The presence of excitons separates apart the organic semiconductors from their inorganic counterpart s Exciton s bring problems which researchers are trying to overcome. It also opens other possibilities leading to the new frontier s and applications. In the future, microcavity OLEDs will be applied to build lighting panels. The increase of the color expression due to the enlargement of primary color triangle in CIE gives the advantages of larger tunable range of color te mperature. A detailed relationship between color temperature, CRI and intensity of RGB microcavity OLEDs is needed. Another interesting topic regarding the microcavity is to investigate the exciton beha viors in very strong microcavit ies Those very strong cavities provide strong exciton confinement, where coupling between exciton s and other particles are tremendously enhanced More compl icated exciton dynamics will occur within those cavities. Based on the excellent results of blue PhOLEDs with ter carbazo le hosts, we expect good device performance when we adapt them into solution processes. Compared with the thermal evaporation deposition, solution process promises low cost
124 and large scale fabrication. The high T g of those new hosts are suitable for the a nnealing step in solution process. First, we are going to use those materials as the hosts for green phosphorescent emitters then extend their application to other emitting colors. My thesis covers a very small area and generates a few interesting results. It is the end of my PhD carrier, but definitely not the end of my pursuing of knowledge and truth. I, was the boy playing on the seashore, am going to discover the great ocean of truth with the power gained in my PhD study.
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136 BIOGRAPHICAL SKETCH Chaoyu Xiang ( ) was born i n Kunming, Yunnan province, China. After graduating from high school attached to Yunnan Normal University, he attended Fudan University in Shanghai, China for his B.S. degree in optical science and technology. In 2009, he came to United Stat es and started graduate school at the University for Florida. In his PhD program, he focused on the device and physics of organic semiconductors in the materials science and engineering department.
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