DELOCALIZATION OF CHARGE TRANSFER STATE IN POLYMER SOLAR CELLS By TZUNG HAN LAI A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY UNIVERSITY OF FLORIDA 2015
Â© 2015 Tzung Han Lai
To m y f amily
4 ACKNOWLEDGMENTS First of all, I d like to give my gratitude to my advisor, Prof. Franky So, who oversaw my possibilities and take me into his guidance. A lot of the result comes out of this thesis comes out as a result of his patience and guidance as well as his trust in me. Not only did I learn great science and engineering along the path, I m also prepared to do whatever challenges lies ahead of me, no matter if my life or career. Also, all the thanks go to my lab mates who not only support scientifi cally , but the experience we shared in this particular life period cannot be replaced . Throughout all the meaningful and inspiring discussion, I was able to work myself through all the hardship toward my degrees. First , I d like to thank Dr. Song Chen, who s more like a tutor than a friend who gave me guidance not only on science but also on how to do science. Also, Prof. Sai Wing Stephen Tsang, who taught me with all the patience and also broaden my insight into all the interesting scien tific matters which I was not able to see through alone . To my beloved group members, Jesse Manders, Chao Yu Cas Xiang, Iordania Constantinou and Erik Klump, who always provide inspirational discussion and sometimes not so meaningful chatters that put us through some tough times, you always have my gratitude for the work and laughter we share together. Also, special gratitude is given to Iordania Constantinou who was the co first author on Chapter 4 and Chapter 5. We are a great team in terms of scientific production and wish you the very best. Finally, I would like to give my thanks to my parents and other beloved family members. You put up with me relentlessly no matter spiritually and financially and let me pursue my career without any burdens and put it upon you . A simple thank you would not suffice for all the support and inspiration you provided. In addition to my
5 beloved friend, Chi Ping Liu, who always provide me not only with academic help , but also in inspirational discussions in life itself as well. Also, Po Yuan Wang, who help me go through all the boring times during my PhD years. Finally , to my girlfriend, Yu Shan Tseng, we wor k through all the good and bad in life or in our careers. I m glad you re there with me and hope it continues.
6 TABLE OF CONTENTS page ACKNOWLEDGMENTS ................................ ................................ ................................ .. 4 LIST OF TABLES ................................ ................................ ................................ ............ 9 LIST OF FIGURES ................................ ................................ ................................ ........ 10 LIST OF ABBR EVIATIONS ................................ ................................ ........................... 12 ABSTRACT ................................ ................................ ................................ ................... 15 CHAPTER 1 FUNDAMENTALS OF POLYMER SOLAR CELLS ................................ ................. 17 1.1 Introduction to Organic Semiconductors ................................ ........................... 17 1.2 Electronic Structures of Organic Semiconductors ................................ ............. 17 1.2.1 Molecular Orbital Construction ................................ ................................ 17 1.2.2 Excitons ................................ ................................ ................................ ... 18 1.3 Photo Induced Charge Transfer ................................ ................................ ....... 20 1. 3 .1 Marcus Theory of E lectron T ransfer ................................ ........................ 20 1. 3 . 2 From Excitons to Free Carriers ................................ ............................... 21 1.4 Carrier Transport and Recombination ................................ ............................... 23 1.4.1 Localization ................................ ................................ ............................. 24 1.4.2 Hopping Mechanism ................................ ................................ ................ 24 1.4.3 J V Characteristics ................................ ................................ .................. 25 1. 4.4 Recombination ................................ ................................ ........................ 26 1.4. 4.1 Geminate r ecombination ................................ ................................ 27 22.214.171.124 Bimolecular r ecombination ................................ ............................. 27 126.96.36.199 Shockley Read Hall r ecombination ................................ ................ 28 1.5 Device Architecture ................................ ................................ ........................... 28 1.5.1 Bulk H eterojunction (BHJ) ................................ ................................ ....... 28 1.5.2 Photoactive L ayer ................................ ................................ .................... 29 1.5. 3 Electrode Contacts ................................ ................................ .................. 29 2 C HARACTERIZATION OF POLYMER SOLAR CELLS ................................ .......... 33 2.1 Standard S pectra ................................ ................................ .............................. 33 2.2 Tran sport Measurement ................................ ................................ .................... 33 2.3 Recombination Measurement ................................ ................................ ........... 34 2.3.1 Incident P ower D ependent J V ................................ ................................ 34 2.3.2 Transient Photo V oltage (TPV) ................................ ............................... 35 2.4 Probing the Energy Alignment ................................ ................................ .......... 36 2.4.1 Cyclic Voltammetry (CV) ................................ ................................ ......... 36 2.4.2 Charge Modulated Electroabsorption Spectroscopy (CMEAS) ............... 36
7 2.4.3 Photocurrent Spectral Response ................................ ............................. 38 2.4.4 Photothermal Deflection Spectroscopy ................................ .................... 39 2.5 Probing Exciton Dynamics ................................ ................................ ................ 40 3 DELOCALIZATION OF CT EXCITONS IN ISOINDIGO SYSTEMS WITH DIFFERENT THIOPHENE LENGTH ................................ ................................ ...... 45 3.1 Abstract ................................ ................................ ................................ ............. 45 3.2 Introduction ................................ ................................ ................................ ....... 45 3.3 Experimental ................................ ................................ ................................ ..... 49 3.4 Results and Discussions ................................ ................................ ................... 51 3.4.1 Polymer P ro perties ................................ ................................ .................. 51 3.4. 2 Solar Cell P erf or mance ................................ ................................ ........... 51 3.4.3 Film Morphology ................................ ................................ ...................... 51 3.4.4 Bimolecular Recombination ................................ ................................ ..... 52 3.4.5 CT States Position by CMEAS ................................ ................................ 53 3.4.6 Delocalization of CT excitons ................................ ................................ .. 53 3.4.7 Formation of CT states ................................ ................................ ............ 55 3.4. 8 CT manifolds on V oc ................................ ................................ ................ 56 3. 5 S ummary ................................ ................................ ................................ .......... 57 4 EFFECT OF THERMAL ANNEALING ON CHARGE TRANSFER STATES AND EXCITON DISSOCIATION IN PCDTBT:PC 70 BM BULK HETEROJUNCTION SOLAR CELLS ................................ ................................ ................................ ....... 66 4.1 Abstract ................................ ................................ ................................ ............. 66 4.2 Introduction ................................ ................................ ................................ ....... 67 4.2 Experimental ................................ ................................ ................................ ..... 69 4.3 Results and Discussions ................................ ................................ ................... 70 4.3.1 J V Characteristics ................................ ................................ .................. 70 4. 3.2 Delocalization of Excitons ................................ ................................ ........ 71 4.3.3 Carrier Transport and Recombination ................................ ..................... 73 4.3. 4 Trap Formation ................................ ................................ ........................ 76 4.4 Summary ................................ ................................ ................................ .......... 76 5 EFFECT OF POLYMER SIDE CHAINS ON CHARGE TRASFER STATES , EXCITON DISSOCIATION AND CHARGE TRANSPORT IN PBDT TPD BULK HETEROJUNCTION SOLAR CELLS ................................ ................................ ..... 89 5.1 Abstracts ................................ ................................ ................................ ........... 89 5.2 Introduction ................................ ................................ ................................ ....... 90 5.2 Experime ntal ................................ ................................ ................................ ..... 92 5.3 Results and Discussions ................................ ................................ ................... 93 5.3.1 Film Morphology ................................ ................................ ...................... 93 5.4.2 Device Performance ................................ ................................ ................ 93 5.4. 3 Delocalization of CT States ................................ ................................ ..... 94 5.4.4 Geminate Recombination ................................ ................................ ........ 95
8 5.4.5 Side Chains, Stacking, Recombination and Transport Properties ........... 96 5.4.6 V oc and C arrier C oncentrations ................................ ................................ 99 5.5 Summary ................................ ................................ ................................ ........ 100 6 CONCLUSION AND FUTURE WORKS ................................ ............................... 109 6.1 Concluding R emarks ................................ ................................ ....................... 109 6.2 Future W orks ................................ ................................ ................................ .. 110 APPENDIX A LIST OF MOLECULE STRUCUTRES ................................ ................................ .. 113 B LIST OF PUBLICATIONS AND CONFERENCE PRESENTATION ...................... 115 Peer Reviewed Publications ................................ ................................ ................. 115 Oral Presentation in Conferences ................................ ................................ ......... 116 Poster Presentation in Conferences ................................ ................................ ..... 118 LIST OF REFERENCES ................................ ................................ ............................. 120 BIOGRAP H ICAL SKETCH ................................ ................................ .......................... 127
9 LIST OF TABLES Table page 3 1 Energy levels, solar cell performances and dielectric constants of P (iI T) and P (iI T3) :PC 71 BM ................................ ................................ ................................ .. 65 4 1 Average device characteristics for the OPVs fabricated in this study. ................ 88 4 2 Electron and hole mobilities and energetic disorder for PCDTBT:PC70B M devices with and without thermal annealing ................................ ....................... 88 5 1 Summary of average device characteristics for the PSCs fabricated in this stud y. ................................ ................................ ................................ ................ 108 5 2 Zero field mobility and energetic disorder data for hole transport in the Oct and EtHex devices. ................................ ................................ ........................... 108
10 LIST OF FIGURES Figure page 1 1 Illustration of molecular orbitals ................................ ................................ .......... 30 1 2 Diagram showing different types of excitons in solid materials and their energetic distribution in the bandgap ................................ ................................ .. 30 1 3 The evolution from excitons to free carriers. ................................ ....................... 31 1 4 Free energy diagram showing free energy of different stages of exciton and possible recombination rate ................................ ................................ ................ 32 1 5 Schematics of different recombination mechanisms and where it takes place in an organic photovoltaic cells. ................................ ................................ .......... 32 2 1 Standard solar spectrum ................................ ................................ .................... 41 2 2 Experimental setup fo r transient photo voltage (TPV) ................................ ........ 42 2 3 Stark effect in a two level system ................................ ................................ ....... 42 2 4 Schematic setup for electroabsorption (EA) spectroscopy. ................................ 43 2 5 Working principles for photothermal deflection spectroscopy (PDS). ................. 44 2 6 PL intensity of a pristine polymer and after blending with PCBM.. ...................... 44 3 1 Schematic diagram of e xciton dis sociation mediated by CT states .................... 58 3 2 photocurrent spectral response of PTB7:PC 71 BM with or without 1,8 diodooctane (DIO) as solvent additives.. ................................ ............................ 59 3 3 Chemical structure of the 2 isoindigo system polymers of p(iI T) and p(iI T3). ... 59 3 4 curren t density voltage curve for P(iI T):PC 71 BM and P(iI T3):PC 71 BM .............. 60 3 5 AFM height and phase image of (a) (c) P(iI T1):PC 71 BM and (b) (d) P(iI T3):PC 71 BM blend films. ................................ ................................ ..................... 61 3 6 Transient photovoltage (TPV) result of P(iI T) and P(iI T3) blends with fullerene ................................ ................................ ................................ .............. 62 3 7 C harge modulated electro absorption spectroscopy (CMEAS) of P(iI T1):PC71BM and P(iI T3):PC71BM blend films. ................................ ................. 63 3 8 Photocurrent spectral response (PSR) of P (iI T):PC 71 BM and P (iI T3):PC 71 BM. ................................ ................................ ................................ ....... 64
11 3 9 Transient PL for P(iI T1):PC 71 BM and P(iI T3):PC 71 BM blend and pristine films to realize the exciton dynamics in blend films ................................ ............ 65 4 1 AFM topography images of PCDTBT: PC 70 BM films (a) after thermal anneali ng and (b) as prepared ................................ ................................ ............ 78 4 2 Solar cell performance for PCDTBT:PC 70 BM devices . ................................ ....... 79 4 3 Exciton dynamics of PCDTBT:PC 70 BM results ................................ ................... 81 4 4 Zero field mobilities Vs square of reciprocal temperature for PCDTBT:PC 70 BM annealed and not annealed devices.. ................................ .... 84 4 5 J sc and V oc vs illumination intensity ................................ ................................ ..... 85 4 6 PDS spectra for PCDTBT:PC 70 BM annealed and not ann ealed devices. ........... 87 5 1 Chemical structure of (a) PBDT(EtHex) TPD(Oct) (b) PBDT(EtHex) TPD(EtHex). ................................ ................................ ................................ ..... 102 5 2 AFM topography images for (a) PBDT(EtHex) TPD(Oct):PC 70 BM and (b) PBDT(EtHex) TPD(EtHex):PC 70 BM ................................ ................................ . 102 5 3 Solar cell perfromances for PBDT(EtHex) TPD(Oct): PC70BM and PBDT(EtHex) TPD(EtHex):PC70BM ................................ ................................ 103 5 4 Exciton dyanamics for for the Oct and EtHex devices ................................ ...... 104 5 5 Geminate recombination for for the Oct and EtHex devices . ............................ 105 5 6 Jsc Vs. I fitted for recombination for the EtHex (black) and Oct (red) devices .. 106 5 7 Zero field mobilities Vs square of reciprocal temperature for Et Hex (black) and Oct (red) devices. Energetic disorder can be determined by the slope. .... 106 5 8 Schematic representation of the impa ct of the difference in side chains on the TPD(Oct) and (b) PBDT(EtHex) TPD(EtHex). ................................ ................................ .. 107
12 LIST OF ABBREVIATIONS A Acceptor AC Alternating current AFM Atomic force microscopy Ag Silver AM Air mass BHJ Bulk heterojunction CMEAS Charge modulated electroabsorption spectroscopy CT Charge transfer CTC Charge transfer complex CV Cyclic voltammetry D Donor D/A Donor acceptor DC Direct current r Relative permittivity EA Electroabsorption ETL Electron transporting layer EtHex Ethy hexyl EQE External quantum efficiency F Electric field FF Fill factor FTPS Fourier transform photocurrent spectroscopy GDM Gaussian disorder model HTL Hole transporting layer HOMO Highest occupied molecular orbital
13 ITO Indium tin oxide IQE Internal quantum efficiency J Current density J sc Short circuit current LUMO Lowest unoccupied molecular orbital MoO 3 Molybdenum (VI) oxide OPV Organic photovoltaics OSC Organic solar cell Oct Octyl P3HT P oly(3 hexylthiophene 2, 5 diyl) PBDTTPD Poly[[5 (2 ethylhexyl) 5,6 dihydro 4,6 dioxo 4 H thieno[3,4 c]pyrrole 1,3 diyl][4,8 bis[(2 ethylhexyl)oxy]benzo[1,2 b:4,5 2,6 diyl]] PC 60 BM [ 6,6] phenyl C 6 1 butyric acid methyl ester PC 71 BM [ 6,6] phenyl C71 butyric acid methyl ester PCDTBT Poly[ N heptadecanyl 2,7 carbazole alt 5,5 di 2 thienyl benzothiadiazole)] PCE Power conversion efficiency PDS Photothermal deflection spectrascopy PEDOT:PSS Poly(3,4 ethylenedioxythiophene) : p olystyrene sulfonate PL Photoluminescence PSR Photocurrent spectral response PTB7 Polythieno[3,4 b] thiophene co benzodithiophene PV Photovoltaics RMS Root mean square SCLC Space charge limited current
14 SRH Shockley Read Hall TCSPC Time correlated single photon counting TPV Transient photovoltage V Voltage V oc Open circuit voltage
15 Abstract of Dissertation Presented to the Graduate School of the University of Florida in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy DELOCALIZATIO N OF CHARGE TRANSFER STATE IN POLYMER SOLAR CELLS By Tzung Han Lai May 2015 Chair: Franky So Major: Materials Science and Engineering Organic photovoltaics (OPVs), due to its light weight, mechanical flexibility and solution processability , have been the focus of research for the past decade. Power conversion efficiency (PCE) over 10% have been demonstrated for single junction cells due to advances in photoactive materials, processing techniques and device architecture. However, unlike its inorganic counterparts, excitons instead of free carriers formed upon photo excitation. Therefore, charge transfer (CT) states that formed at donor/ acceptor (D/A) interface are responsible for the majority of the exciton dissociation in BHJ OPVs. While res earches have been fo cused on exciton dynamics at CT states, understanding the factors affecting CT states manifolds would help future design of high efficiency polymers. First, we studied the effect of thiophene length in the polymer backbones of two isoindigo polymers on delocalization of CT manifolds. We found that despite the similarity in energetics, the photovoltaic (PV) performances differ drastically. By looking at difference in CT manifolds, we conclude that the different CT position as well as delocalized state is the reason for the difference in PV performances . We further
16 postulate that the lower manifolds of CT states cause the reduction of V oc while promoting exciton dissociati on. Next, the delocalization of the CT state with thermal annealing of the PCDTBT polymers was being investigated . Despite the similarity in morphology, the annealed samples showed more delocalized CT excitons which results in a higher current generation. We further look into the transport properties of the PCDTBT polymers and found that while the additional states promote exciton dissociation, these states also act as carrier traps which induces a higher degree of Shockley Read Hall Recombination that dete riorate the device performance. Finally, we studied the effect of D/A interactions with side chains at acceptor part of the D A p olymer to control the microscopic interactions. With the linear side chain which promote s D A interaction, more delocalized CT manifolds are observed . A lso, we discovered that the sidechain also affect the transport properties by introducing structural disorder and wider stacking with bulkier branched side chains . The structural disorder resulted in higher energetic disorder a nd higher bimolecular recombination loss.
17 CHAPTER 1 FUNDAMENTALS OF POLYMER SOLAR CELLS 1.1.1 Introduction to Organic Semiconductors Since the discovery of organic conducting polymers, research interests in optoelectronic devices such as organic light emitting diode, organic photovoltaics and organic field effect transistors have been increasing. Due to its possibility for low cost, roll to roll process ability , organic optoelectronic devices have gathered g reat interest. Rapid progress achieved by academia, research laboratories and industries has made the seemingly impossible to make electronics from plastics into real life products. Single junction organic solar cells over 10% have been reported. [ 1 , 2 ] As we progress through the development of organic semiconductors , fundamental physics in order for the realization of cost effective devices with comparable performance for their inorganic counterparts is necessary . 1.2 Electronic Structures of Organic Semiconductors 1.2.1 Molecular Orbital Construction A conjugated system which is the over lapping of p orbitals across bond that intervenes is the reason for their conductive and optical properties of organic semiconductors which allows p orbital electrons to be delocalized. This conjugation is similar to delocalization of carriers in inorganic semiconductors that allows the carriers to be transported. In conjugated organic semiconductors which consist of single and double bonds where bonds overlaps through the conjugation length. Since carbon atoms consist of three sp 2 orbitals, P z orbitals extends above or below the pl ane where bond formed where overlapping of the adjacent P z orbitals form bonds. A simplest conjugating organic material , benzene, is shown in Figure 1 1 . As illustrated, the Pz
18 orbitals extend perpendicularly out of the plane to form bonds. Among the molecular orbitals, ther e exist bonding ( ) and antibonding ( *) orbitals with * having higher energy. When a larger system where more carbon atoms are conjugated, splitting of energy levels as a result of Pauli exclusion principles will occur . [ 3 ] E ventually , as more and more carbon atoms were involved 2 semi continuous bands will form which made up of highest occupied molecular orbitals (HOMO) from orbital bands and lowest unoccupied molecular orbitals (LUMO) from * orbital band . The HOMO and LUMO band in organic semiconductors is analogous to the conduction band and the valence band in inorganic semiconductors although these molecular orbit als contain discrete energy levels instead of continuous ones. However, the localization radius of electrons is limited to the magnitude of molecule geometry since organic materials are bound by van der Waals forces which are weak . 1.2.2 Excitons Excitons are electron hole pairs bound by columbic force which can be viewed as a excited charge less quasi particle. In organic solids, excitons formed either by injection of electron s and holes combined or generated by photo excitation when photons with energy larger than the bandgap of organic materials were absorbed. Three types of excitons exist in organic semiconductor and they are Mott Wannier excitons, Frenkel excitons and charge transfer excitons as depicted in Figure 1 2 . As depicted in Figure 1 2 , different types of excitons are illustrated. Mott Wannier excitons, having separation distance greater than the lattice constant of materials , are most found in inorganic materials where the columbic forces between electrons and holes were small due to h igher permittivity in inorganic semiconductors and usually have large delocalization. Typically, these excitons have a binding energy of about 0.01
19 eV which is small and can be overcome easily by thermal energy (~0.025 eV). [ 4 ] Therefore, Mott Wannier excitons are often easily dissociated into free electrons and holes without the requirement of excess energy. In organic semiconductors, Frenkel excitons are typical due to the low dielectric constants of organic matters, resulting in small radius and la rger binding energy up to ~1 eV. [ 5 , 6 , 7 ] The excitons extend only to one unit cell or one molecule in organics. Due to the large binding energy that cannot be overcome by simply thermal energy for Frenkel excitons, novel engineering design is required for the efficient dissociation of excitons into free carriers. Modern design of organic photovoltaics put electron accepting fullerene s or fullerene derivatives to form excitons that exist between electron do nating organic semiconductors and electron accepting fullerenes that have lower binding energy to increase the exciton dissociation efficiency. These excitons that spanned across two or three molecules are called charge transfer (CT) excitons. CT e xcitons usually have one carrier localized in one molecule and the other one on another molecule. Thus , the energy levels of the CT state are quite complex and are materials dependent. Since in organic semiconductors, the main excitons species are Frenekl exciton s that require the formation of CT excitons for efficient dissociation, understanding of C T excitons and its energy level becomes important in achieving high efficiency OPVs. However, the density of these CT energy levels is low since it only exists in dif ferent material interfaces; techniques to detect it are very limited. In this dissertation, I m going to introduce an additional technique to observe this interface other than the already reported ones.
20 1. 3 P hoto I nduced C harge T ransfer O rganic photovoltaics have attracted a great amount of research interest due to it s solution process ability and modern state of the art cell efficiency has reach ed 10%. [ 2 , 8 ] Such an improvement has been due to the effort for new photoactive materials and advances in device structures. More importantly, understanding of the exciton dynamics from the generation of exciton s to separation toward collection at the interface have led to better design of material absorption and energetics. Here, first look into the process of photovoltaic process in polymer solar cells. 1. 3 . 1 Marcus Theory of E lectro n T ransfer In Polymer solar cells the electron transfer is described by Marcus theory which treats the vibration of the electron as harmonic oscillators to describe the probability of the electron transfer rate from one chemical species to another one. [ 9 ] Fo r outer shell electrons, the potential energy will be affected by the fluctuation of nuclei position . During the vibrations, there ll be probability at which the energy of outer shell electrons overlap s the unoccupied states of the neighboring nuclei. Thes e overlapping of energy results in a probability of electrons transfer through the intersection and reaching the neighboring nuclei which can be described as follows : ( 1 1 ) (r) is the electron transfer coefficient, A 2 is the dimension of collision frequency, is the mean separation distance in the transition state of the reaction, G* is the free energy of activation that is related to the reorganization parameter By considering the quantum effect with the approximati on of the weak electronic coupling ( << 1), the first order transition rate of k can be described as:
21 ( 1 2 ) where H AB is the matrix elements describing the electronic coupling between chemical species, FC is called the Frank Condon term which is the products sum of the integral of the vibrational and solvational wave functions between chemical species. When all coordi nates treated as oscillator and only applied to inner shell coordinates, E q . 1 2 can be rewritten as: ( 1 3 ) r p is the quantum states of the chemical species reacting, the (FC) r ) the Boltzmann probability to find the system in state v r . If only the inner shell coordinates are treated with quantum mechanics while the outer species classically, Eq. 1 3 can be rewritten again as: ( 1 4 ) From E q 1 1 to Eq.1 4 , we can see that the electron coupling between chemical species are affected by molecular structure s , the effective force at the interface, the BHJ morphology which affect the separation between reactants and products. The dielectric constants of the polymer and fullerene derivatives will affect the electron coupling as well. [ 10 ] 1. 3 . 2 From Excitons to Free Carriers Due to low dielectric constant nature of organic photovoltaics, excitons instead of free carriers are formed in organic semiconductors. In order to be collected as carriers, the process from excitons to free carriers goes through an effective interface assisted
22 process which can be illustrated as Fig ure 1 3 . Photo generated excitons in an electron donating molecule are separated at donor acceptor interface, resulting in a hole charged electron donating molecule and a neighboring electron charged electron accepting molecule . [ 11 ] H ole transfer process also applied to this scenario. Sepa rated electrons and holes after this separation process are then transferred toward collecting electrodes through the internal electric field. During the exciton dissociation process, the electron hole pair that is loosely bound across donor and acceptor materials are call ed charge transfer (CT) states or charge transfer complex (CTC) . [ 12 ] It has been discover ed by means of transient absorption study that a sub picosecond proces s took place during the charge tran s fer process. [ 13 15 ] During this charge transfer process, the photo generated excitons first fall into one of the many CT manifolds at the D / A interface. The excitons in higher CT manifolds or hot CT states can go through the c harge separation process easier due to their relatively more delocalized wave function . [ 16 ] On the other hand, the excitons at the lower CT m anifolds (or cold CT states) have less chance to be dissociated as it has less delocalized wave functions . These excitons at cold CT states will have higher probability to decay to ground state. [ 13 ] This process is geminate type of recombination and is detrimental to PV performances as it decreases the number of carriers generated. Figure 1 4 . shows the free energy diagram for the exciton generation to charge transfer process. In OPVs, charge trans fer cannot happen without photo generated excitons due its lack of free carriers in organic semiconductors. P hoto excitation in OPV itself is charge transfer processes that take place within a polymer repeating unit. [ 17 ] Therefore,
23 the binding energy of excitons in OPVs depends on charge dis tribution in the excited states. Usually, the wave functions for electron and holes in OPVs are almost overlapped, leading to a biding energy of 0.3 eV ~ 0.4 eV. In order to dissociate excitons into free carriers, a significantly high internal electric fie ld of ~10 6 V/cm is required. As a result, it s commonly believed that a 0.3 eV of LUMO or HOMO band offsets are required for efficient charge separation. However, it has been shown recently that some polymers without band offsets of 0.3 eV in donor accepto r ( D/A ) type of polymers. [ 18 ] The donor acceptor type polymer contai n s two functional units that have different electron affinity as re peating units. The excitons sit across polymer with electrons in electron accepting units and holes in electron donating units. Since the separation between donor and acceptor units are lar ge compared to between polymer and fullerenes, less energy is required to separate the excitons in D/A polymers which leads to several high efficiency OPVs based on this con cept . [ 19 , 20 ] In conclusion, the organic photovoltaic as an excitonic cell is different from inorganic photovoltaics which generate free carriers. As a result of interface assisted charge transfer, bulk heterojunction are often used to increase interface for reduced exciton diffusion length . [ 21 ] Therefore, the carrier concentration is almost uniform across the photoac tive length . The driving force is therefore weaker th a n inorganic photovoltaics. The u nderstanding of J V characteristics in OPVs has to take insight into exciton dynamic at charge transfer process. 1. 4 Carrier Transport and Recombination In OPVs, t he carrier transport in nano scale networks are based on hopping conduction in disordered systems. [ 22 ] On the other hand, recombination, contrary to
24 prediction from Langevin recombination [ 23 ] , is proportional to mo b ilities of the polymer and fullerene derivatives as discovered through experime nts. [ 24 ] 1.4.1 Localization Due to the lack of periodicity in disorder organic semiconductors, localization of electron wavefunction and thus the bandgap conduction is induced . The disorder ing in organic semiconductors also results in broadening of the density of state (DOS) distribution. [ 25 ] The tail states as a result of broadening of the DOS are more localized than the state which is highly populated. Two models, exponential and Gaussian distribution are often used to describe the band tails state distributions. [ 26 , 27 ] The two DOS distribution models are often adopted in analytical models as fitting parameters despite describing different energy distribution formation mechanism. Due to narrowing of electronic bandwidth as a result of weak van der Waals bonding bet ween organic molecules, the carrier transport in organic semiconductors are more susceptible to energetic disorders near transport band. [ 27 ] Two of the main factors affecting energetic disorder are the structural disorder and polarization. [ 28 ] [ 29 ] 1. 4 .2 Hopping Mechanism Unlike inorganic semiconductors where carriers are delocalized and band transport is dominant , organic semiconductors relied on polaron hopping as their main mechanism for charge transport. Due to the presence of charge carriers in the or ganic mediums which is polarizable, the nearby atoms was polarized by the presence of charge carriers and this induction of polarization will follow the carriers as it move along in organic semiconductors. This phenomenon, charge carriers along with induct ion of polarization, is described as polarons. In this model, the locally induced polarization becomes a potential well as it screens the electric field and reduced its mobility
25 as well as effective mass. In order to move through the materials, p olarons must hops out of the potential well into another one; therefore, the charge carriers are highly delocalized on polymer segments or single molecule which is bond ed only by van der Waal force. Therefore, while single crystal organic solids have mobility up to 1 cm 2 /V, amorphous organics, which is more commonly used, often have motilities below 10 3 cm 2 /V as a result of the hopping transport. 1. 4 . 3 J V Characteristic s Depend on different types of organic molecules or polymer that leads to difference in disorder and carrier localization, the transport mechanisms may vary accordingly. However, Mobility as a classical transport parameter can still correlate the levels of disorder and charge localization with carrier concentration J( , n, F). By adopting the trap free space charge limited current (SCLC) model which assumes that all injected carriers contribute to current flow, indicating that carrier concentration is low i n the materials comparing to injection current and that the current is not injection limited. By deriving the Mott Gurney law from Poisson equation and continuity equation, [ 30 ] we can derive the relation between current density and mobility as: ( 1 5 ) W here J is the current density, is the mobility, is the dielectric permittivity, F is the electric field and d is the thickness between electrodes. [ 31 ] This equation can be used to predict the current density in organic semiconductors with mobility which is affected by different hopping mechanism. While organic semicond uctors are neither trap free nor ordered materials, ther s regions at certain bias condition were J ~ V 2 can be used for estimation of carrier mobility.
26 Some other approach describes J V characteristics with inclusion of temperature and electric field by Poole Frenkel theory. [ 32 ] Under the assumption tha t DOS for HOMO and LUMO as Gaussian disorder model (GDM) with a diagonal disorder , the mobility as: [ 26 ] . ( 1 6 ) w herein C is a fitting parameter . When is smaller than 1.5, is replaced with 1.5. While the mechanism ~ e F to describe mobility , the dissociation of carriers overcoming the columbic potential under the assistance of electric field often has the dependence of e F . Based on Eq 1 6 , we can measure temperature dependent mobility and get the zero field mobility and energetic disorder of the transport band which we will use a lot in this dissertation. 1. 4 . 4 Recombination A s a feature of its excitonic cells, organic solar cells are more prone to different route of recombination that s detrimental to the device performance . As shown in Figure 1 4, routes of recombination have been illustrated and are related to different process of carrier generation. The photovoltaic process can be summa rized in four major steps as follows: (1) light absorption (2) exciton dissociation (3) charge transport and (4) charge extraction. Except the light absorption process, most energy losses during the photovoltaic process is recombination. It can be categori zed into (1 )relaxation of excitons before reaching proper interface for dissociation (2) recombination of geminate pairs before reaching interface or at interface but not fully dissociated and (3) recombination of dissociated carriers. [ 33 ] Schematic of location where these recombination mechanisms take place in a organic photovoltaics was illustrated in
27 Figure 1 .5. The recombination of dissociated carriers also include s few different recombination mechanisms including bimolecular recombination , monomolecular recombination and Shockley Read Hall recombination. [ 34 ] 1. 4 . 4 .1 Geminate r ecombination Geminate recombination denotes the recombination of electron and hole generated from the same photon. This usually happens in low mobility, low conductivity and disordered materials. This process occurs before carriers become free and do not contribute to net current in a solar cell. Even if the excitons diffused away from generated site, the contribution is canceled out by the opposite flow of the opposing carriers. [ 35 ] The geminate recom bination of charge transfer excitons have been regarded as one of the most important process es as it involves electron and hole pairs at donor/acceptor interfaces with a separation distance of few nm s but recombine as a result of electrostatic force. [ 36 ] Through application of external field, geminate recombination of CT excitons can be overcome. Howev er, m echanisms that affect geminate recombination in polymer solar cell are still unclear. While reports adopting pump probe studies give ideas of exciton dynamics at CT states [ 13 15 ] , ways to control and inhibit geminate recombination is still unclear. The purpose of this dissertation is to give insight into factors that affect this type of recombination through processing condition and polymer engineering. 1. 4 . 4 .2 Bimolecular r ecombination Bimolecular recombination is an important topic in organic photovoltaics. Extensive studies have shown that bimolecular recombination affect PCE in all aspect as it affect V oc , J sc and FF. [ 31 , 34 , 37 ] When bimolecular recombination is near zero, the Fermi level splitting will be closed to HOMO, LUMO of donor and acceptor, resulting in
28 loss of Voc nearing zero. [ 38 ] Also, the loss of short circuit current as a result of bimolecular recombination will be reduced as well. The increase of carrier concentration w ill affect the charge generation and extraction efficiency under low internal field condition , which will lead to increase in FF. 1. 4 . 4 .3 Shockley Read Hall r ecombination Shockley Read Hall recombination or mono molecular recombination denotes the process where electrons and holes recombine as a result of trap states or recombination center. Trap states and recombination centers in polymer solar cells originate from impurities or morphological defect such as interfacial states. The trapped electron/hole is stationary and opposite sign of mobile charge can recombine. Therefore, such recombination is proportional to trap density (n e,trap ) an d mobile carrier concentration (n h ) in the case of existence of electron traps and vice versa. [ 34 ] 1. 5 Device Architecture 1. 5 .1 Bulk H eterojunction (BHJ) D ue to the short exciton diff usion length in organic photovoltaics, the BHJ architecture is designed to increase donor acceptor interface area and decrease the path to interface area then conventional bi layer device architecture. [ 39 ] Interpenetrating networks of donor and acceptor phases were formed by blending the donor and acceptor materials in solution. This results in formation of ~10nm domain size of donor and acceptor phase. Th is allows photo generated excitons in polymer domain to be able to reach the heterojunction interface before recombining geminately. Also, carrier transports relies on this connecting network of donor or acceptor phase . The o ver mixed solid solution may res ult in domain size too small for continuous pathway for efficient carrier transport. Therefore, tradeoff between exciton diffusion pathways and transport
29 exist for optimum cell performance. Many processing technique s have been reported achieving ideal morp hology and can be categorized as thermal annealing [ 40 ] , solvent annealing [ 41 , 42 ] and solvent additives. [ 43 ] 1.5.2 Photoactive L ayer As mentioned in the previous paragraph , due to the excitonic nature of OPVs, morphology is the utmost important for making an efficient solar cell. One of the most common ways is thermal annealing of the photoactive films. I s been shown that the annealing help forming interpenetrating nanoscale phase which is crucial for efficiency dissociation and carrier transport. [ 44 ] Solvent additives are another way to control the morphology of the photoactive layers. S olvent additives usually have different boiling point and diff erent solubility for polymer and fullerene. When the film formed after spin coating, the solvent additives dry last and formed favorable phase segregation. [ 45 ] Another technique for forming preferred morphology is through s olvent annealing of the photoactive films. It is often done by placing the freshly spun coated films into a covered petri dish to slow down the drying process. It has been show n that it increases the crystallinity and improves overall PV performance. [ 41 ] 1. 5 . 3 Electrode Contacts In order to reduce carrier extraction barriers and the increment of built in potential, the electrodes need to form Ohmic contacts with the photoactive layer. A common method adopts very large or small work function metals or transparent conducting oxides to achieve good energy alignment with the protective layer. In addition, interlayers such as transition metals, transition metal oxides and polymers were used to further enhance the extraction efficiency.
30 p Fig ure 1 1 . Illustration of molecular orbitals (a) the sp 2 orbitals of a carbon atom and the 1s orbital of H atom (b) the bond of the carbon atom of benzene (c) the orbital in benz e ne Figure 1 2 . Diagram showin g different type s of excitons in solid materials and their energetic distribution in the bandgap
31 Figure 1 3 . The evolution from excitons to free carriers. (a) photo generated excitons in donor domains (b) excitons at interfaces as the charge transfer (CT) states (c) a more loosely bound exciton where electrons in acceptor domain and a hole in the donor domain (d) free carriers
32 Figure 1 4 . Free energy diagram showing free energy of different stages of exciton and possible recombination rate (1) relaxation of excitons before reaching proper interface for dissociation (2) recombination of geminate pairs before reaching interface or at interfa ce but not fully dissociated and non geminate recombination which include (3) bimolecular recombination of free electron and holes and (4)Shockley Read Hall trap assisted recombination Figure 1 5 . Schematics of different recombination mechanisms and wher e it takes place in a n organic photovoltaic cells. (1) relaxation of excitons before reaching proper interface for dissociation (2) recombination of geminate pairs before reaching interface or at interface but not fully dissociated and non geminate recombi nation which include (3) bimolecular recombination of free electron and holes and (4)Shockley Read Hall trap assisted recombination
33 CHAPTER 2 C HARACTERIZATION OF POLYMER SOLAR CELLS 2.1 Standard S pectra I n order to standardize the calculation of external quantum efficiency (EQE) of photovoltaics, it is often chosen as the ratio of output electrical power per 100 mW/cm 2 . The 136 mW/cm 2 constant is defined from the full intensity of sunlight in the clear day condition on earth. The incident of light onto ear th will be affected by air mass, zenith angle as well as incident path length. Therefore, AM 1.5 is often chosen as it represent average conditions for standardizing the calculation of efficiency. Solar spectrums are presented in Figure 2 1 . , under AM 1.5G , the integrated radiative power were 100mW/cm 2 . 2.2 Transport Measurement SCLC model is often used to determine the zero field mobility for electron and hole transport in BHJ films. T rap free SCLC is usually taken as carriers having enough energy to over come energies of most deep traps. In order to correctly extract zero field mobility, the injection of the measured carrier type should be Ohmic while the opposite type of carrier injection should be blocked. Also, the built in potential needs to be subtracted before fitting with SCLC model and voltage drop as a result of contact series resistance has to be subtracted as well. Finally, the Poole Frenkel factor has to be taken into account in order for correct z er o field mobility estimation from SCLC m easurement. Typical structure for hole mobility measurement is ITO/MoO 3 /BHJ/MoO 3 /Ag where ITO/MoO3 provide good injection efficiency for holes and MoO 3 provide good injection barriers for electrons.
34 2.3 Recombination Measurement 2.3.1 Incident P ower D epen dent J V In order to understand the type of recombination in a OPV device, intensity dependent measurements of J sc and V oc were often used. By attenuating the incident light, one would be able to monitor the intensity dependence of J sc and V oc through JV curve at different light intensity. At short circuit current condition, the internal field is at a maximum where recombination is inhibited as carriers having a higher driving force to be collected. In this case, s hort circuit current is correl ated with incident power dependency as follows: ( 2 1 ) W hereas I is the incident light intensity and the scaling exponent. W hen bimolecular recombination is negligible, should be close to 1. [ 46 ] In the case where is not close to one, factors including bimolecular recombination, space charge effect, difference in mobility b etween the two carriers is at play. [ 47 ] Under open circuit condition, the internal field is zero. Therefore, all the carriers are forced to recombine, resulting in current of zero. T his phenomenon provides a perfect situation to study different recombination mechanisms under Voc. The Voc and light intensity relations are described as: ( 2 2 ) W hereas E eff is the effective bandgap, e is the elemental charge, k is Boltzmann constant, T is temperature in Kelvin and n e , n h is the electron and hole density in polymer/fullerene domain at V oc and N c is the effective density of states. Since n e and
35 n h are illumination intensity dependent , the change in Voc as a result of different illumination intensity can be written as : ( 2 3 ) where I is the light intensity. When the device is bimolecular recombination dominant, the slope of V oc vs lnI should be kT/e. This is because n e should be equal to n h and have only one dependency on illumination intensity. When monomolecular recombination is dominant, the n e and n h each has its own dependency on illumination intensity. Therefore, the slope of V oc vs lnI should be 2kT/e. [ 34 ] 2.3.2 Transient Photo V oltage (TPV) Transient photo voltage is one of the common ways to probe bimolecular recombination lifetime with low level excitation under equilibrium condition. This means that the carrier concentration incurred by the low level excitation ( n) should be negligible compared to background concentration (n 0 ) , i.e. n << n 0. [ 31 , 48 ] At V oc , the device is in dynamic equilibrium where generation equals bimolecular recombination as a r esult of the continuity equation . The experiment setup is shown in Figure 2 2 . The device is shined by solar simulator to achieve photo voltage identical to the Voc to achieve dynamic equilibrium . An attenuated laser is used to provide a small perturbation with the increase of carrier concentration of n. When V << V oc , the decay time and n will be the same . Since V can be described as : . When considering the decay rate of photo voltage, it can be written as : . Therefore, we can derive the carrier lifetime through monitoring the decay rate of photo -
36 voltage. This carrier lifetime can be used to quantify bimolecular recombination as the measurement is carried out at V oc which is bimolecular recombination dominant. 2. 4 Probing the E nergy A lignment Due to the excitonic nature of polymer solar cells, energy alignment becomes an important factor in the performance of polymer solar cells. However, understanding of the energy alignments requires careful selection of techniques to realize the energy alignment under working device situation. 2. 4 .1 Cyclic V oltammetry (CV) C yclic voltammetry works by scanning the electrical potential of the working electrode cyclically . Then, the actual value is calibrated by the work function of the reference electrode. It is often used to determine the energy level of the p ristine materials by the onset of charging and discharging currents. However, depending on the material of the reference electrode; the error of the resulting energy level can have errors as large as 0.3 eV . Therefore, the measured result do es not have goo d repeatability and is not acceptable for the study of energy alignment for BHJ. 2. 4 .2 Charge Modulated Electroabsorption Spectroscopy (CMEAS) Conventionally, E lectroabsorption (EA) spectroscopy is used to study the detailed energy band structure in inorg anic semiconductors which cannot be realized by linear absorption measurements. U nder the perturbation of electric field, certain transitions are changed as a result of mixed eigenstates. B ased on Stark effect, the second order perturbation theory predicts that the energy difference as a result of altered transition is proportional to square of external field ~F 2 . The band structure changed when the external field is larger than the perturbation as a result as shown in Figure 2 3 .
37 conju gated polymers, EA signals agree with the stark shift model. [ 49 ] As shown in Figure 2 3 , the optical transi tion in a 2 level dominant system is illustrated. An excitonic transition from 1A g to 1B u and its vibronic features made up their linear absorption spectrum while transitions from 1A g to nA g are forbidden. Under perturbation of electric field, due to mixing of states between 1B u and other higher forbidden states, the original ly allowed transition become less a llowed and those forbidden states become more allowed. Due to perturbation of the electronic wave function under an applied electric field in OPV devices, a shift in the spectrum of the absorption layer occurs and leads to a change in absorption coefficien [ 50 ] For most organic and the square of the electric field ( E ). ( 2 4 ) where is the photon energy, T is the optical transmission and E is the electric field. Given the applied electric field is the sum of the DC and AC bias (V) plus the presence of an internal built in potential ( V bi ), the firs t harmonic EA signal is: ( 2 5 ) where V DC is the dc bias voltage and V AC is the modulation voltage applied to the device. According to Eq . 2 5 , the first harmonic EA signal vanishes when the term is zero at which the applied DC bias cancels out the built in potential of the cell. Using this method, V bi can be determined as the V DC cancels out the first harmonic signals. The schematic setup of an EA measurement setup is shown in F igure 2 4
38 where the fi rst harmonic EA signal is measured with a lock in amplifier as a function of V DC . Recently , charge modulated electro absorption spectroscopy (CMEAS) has been demonstrated that it can be an effected way to probe the lower limit of hot CT manifolds by resolving the changes in optical absorption owing to charges coupled with the modulating electric field. [ 51 ] Upon optical pumping with energies below the bandgap energy of a BHJ device while under reverse bias to inhibit carrier injection, charge modulated signals were observed. This sub bandgap CMEAS features arises from direct pumping of excitons to hot CT states which then dissociate into free charges that alte r the optical absorption. The onset of CMEAS spectrum characterizes the minimum energy for the sub bandgap generated charges which is defined as CT state in a polymer: fullerene heterojunction. This is responsible for the energy level where the charges wil l be mobile in tens of femtoseconds after dissociation process. The setup of CMEAS is essentially the same as EA which is shown in Figure 2 4 . The samples were probed by the incident of a monochromatic parallel beam into the sample through ITO with an inc ident angle of 45Â° and are reflected by the back Al electrode and captured by calibrated silicon and germanium photo detector . The sample internal electric field is modulated by a DC bias superimposed with a small AC voltage at a modulation frequency of 1000 Hz. A current amplifier and a lock in amplifier were connected to the detector to increase the signal to noise ratio. 2. 4 .3 Photocurrent Spectral Response P hotocurrent spectral response (PSR) was often used in plastic solar cells to study the elec trical output from below bandgap regions where charge transfer states is responsible for the electrical output . [ 52 ] [ 53 ] It s essentially a high resolution external
39 quantum efficiency measurement to look at current output per photon inputs. By inserting set s of long pass and short pass filters along with lock in amplifier, one would be able to lower the noise level and detect the relatively small current output efficiency at below bandgap regions of the OSCs. This gives us qualitative analysis of the delocal ization of the charge transfer states by monitoring the EQE from the measurement. The measurement was done by chopping a monochromatic light source with different wavelength and incident on the device active area. The beam should be smaller than the device active area. The device is connected to a pre amplifier and lock in amplifier to amplify the signal and exclude the noise. By comparing with the current output from a standard silicon detector with known responsivity, we can then obtain the external quantum efficiency and thus get the spectral response of the device. 2.4.4 Photothermal Deflection Spectroscopy P hotothermal deflection spectroscopy (PDS) is a high sensitivity technique to pr obe the absorption coefficient ( ) through heating of the surrounding medium by light absorption of certain wavelength [ 54 ] In PDS, the sample is immersed into liquids, favorably with liquids having a large change in index of reflection with low temperature perturbation. A pump beam and a probe beam are incident on the samples to perform such technique. During measurement, a monochromatic pump light is incident onto the sample. If the incident light is absorbed by the samples at certain wavelength s , the relaxation of excited excitons relaxed into the gro und state and transformed part or all of the photon energy into thermal energy. The thermal energy then heats up the surrounding liquids and change the index of reflection. The probe light that shines on the sample will be deflected due to the change of th e index of reflection. The beam is
40 being detected by a position sensitive detector and record the event of absorption. The sensitivity of the PDS technique can be as small as 10 4 and wouldn t be influenced by the scattering event of the incident beam. The working principle of PDS is illustrated in Figure 2 5. 2. 5 Probing Exciton Dynamics Transient Photoluminescence : Due to the ultra fast process of exciton dissociation, understanding of exciton dynamics have not been easy due to the fast dissociation process . Several attempts have been made through transient studies. [ 13 15 ] However, one of the simpler ways to probe exciton dynamics is through transient photoluminescence. [ 55 ] The mechanism can be described by static PL in Figure 2 6 where the PL intensity of the pristine polymer and polymer:fullerene blend were shown. After blending with fullerene, the PL signals strength is less th an 10% of the PL signals of the pristine polymer. This is due to the excited excitons being quenched by the exciton dissociation mechanism provided after blending with fullerene. By monitoring the time resolved photoluminescence signals, we re able to unde rstand exciton quenching mechanism and signal lifetime. A pulsed laser with an average power of 1 mW, operating at 40 MHz, wi th duration of 70 ps was used for excitation . The PL transient was performed using a time correlated single photon counting (TCSPC) spectrometer . With the existence of a different mechanism, the excitons being excited will be quenched through such dissociation mechanism. I n this case, a biexponential decay signal can be discovered after the incident of the laser pulse C ombing with techniques to look into energy alignment at interface, we are able to distinguish the effect of charge transfer state on exciton dissociation pathways and its relative lifetime. [ 56 ]
41 Figure 2 1 . Standard solar spectrum
42 Figure 2 2 . Experimental setup for transient photo voltage (TPV). An attenuated laser is incident onto the device with background light source. Device is connected to oscilloscope to monitor the decay time of photo voltage. Figure 2 3 . Stark effect in a two level system
43 Figure 2 4 . Schematic setup for electroabsorption (EA) spectroscopy. First harmonic EA signal is measured with a lock in amplifier referenced at ac modulation frequency and then measured as a function of dc bias (V DC ) with fixed small ac modu lation (V AC ). The device is constantly incident by monochromatic light.
44 Figure 2 5 . Working principles for photothermal deflection spectroscopy ( PDS). Figure 2 6 . PL intensity of a pristine polymer and after blending with PCBM. The PL signal after blending with PCBM shows less than 10% than that of the pristine polymer. This is due to the excited excitons being quenched by the exciton dissociation mechanism provided after blending with fulle rene.
45 CHAPTER 3 DELOCALIZATION OF CT EXCITONS IN ISOINDIGO SYSTEMS WITH DIFFERENT THIOPHENE LENGTH 3.1 Abstract Delocalization of charge transfer (CT) excitons in two isoindigo polymers are discussed in this report. We found that two isoindigo polymer s, while identical in energetics and chemical structures as well as dielectric constants, different morphology were shown after blending with PC 71 BM, resulting in drastic differences in photovol taic performances . T o identify its effect on CT manifolds and exciton dynamics, charge modulated electroabsorption spectroscopy (CMEAS) was carried out. The result indicates that the position of the CT manifolds is different for the two isoindigo polymers despite identical HOMO/LUMO levels. With the photocurrent spectral response , a higher output of photogenerated carriers was found in the P ( iI T3) polymer devices which may have a more delocalized CT states . The presence of the delocalized CT states is fur ther confirmed by the transient PL which shows an additional dissociation mechanism. We also discovered a higher dielectric constant in the P ( iI T3) polymer blends which could be the reason for higher orders of delocalization in CT states since similar ph enomenon in CT manifolds has not been shown in other systems with identical morphological difference. We hypoth esize that the microscopic D A interaction along with the morphological difference is the reason for the delocalization of the CT states. Also, t he lowering of the CT manifolds as a result of a more delocalized CT could the reason for the reduction of open circuit voltage ( V oc ). 3.2 Introduction In bulk heter o junction organic photovoltaics (OPV), the charge transfer (CT) states at the donor (D) an d acceptor (A) interface have been shown not only to
46 determine the effective bandgap at the heterojunctions, but also to mediate the exciton dissociation process. Various experimental approaches such as electroluminescence, photoluminescence, [ 57 ] Fourier transform photocurrent spectroscopy (FTPS) [ 58 , 59 ] as well as charge modulat ed electro absorption spectroscopy ( CMEAS ) [ 60 , 61 ] have been recently applied to probe the CT manifolds. These experimental results suggested that the CT states serve as the effective bandgap which is directly related to the open circuit voltage ( V oc ) by taking into account the recombination losses at D/A interfaces . [ 58 , 59 , 62 ] Furthermore, the CT states have also been shown to determine the exciton dissociation processes; Bakuli n et al found that femtosecond photo induced electron transfer process occur s at high er CT manifolds while the excitons at low energy CT manifolds have a higher probability of recombination. [ 13 15 ] Upon photo excitation, the majority of excitons was generated in the polymer domains, which in the case of OPVs, are the main absorbers . T he excitons then diffuse to the D/A interfaces where the exciton dis sociation process took place . The CT states at the D/A interfaces serve as an intermediate state where excitons wait for photo induced electron transfer to happen then dissociate into the transport band. Due to the low dielectric constant nature of organic materials ( r ~2 4) , the binding energy which is described as cannot be overcome by thermal energy alone . Owing to the requirement of this additional energy, the CT states at the D/A interfaces plays an important role in mediating the exciton dissociation process as illustrated in Figure 3 1 . [ 13 15 ] The efficiency of this CT mediated dissociation process not only affects the Voc , but also deter mines the carrier generation rate (G). Therefore, the binding energy of the excitons and thus the dielectric constant of the materials should have a direct effect on
47 the efficiency of the CT mediated dissociation and result in different solar cell performa nces. The dielectric effect on the photovoltaic processes in OPV has been discussed extensively. [ 5 , 6 , 7 ] The strong binding energy of excitons in organics 1) reduces the photovoltag e with an energy offset at the D/A interface; [ 5 ] 2) increases bimolecular and ge minate recombination; [ 6 ] 3) increases space charge effect which limit the maximum number of charges that can be accommodate d in the thin film ; [ 7 ] 4) induces V oc loss due to self trapping of the polaron. [ 63 ] However, the investigation of the correlation between the dielectric effect and the CT states is still limited, mainly due to the challenge of direct experimental approac h to account those issues . Recently, b y using charge modulated electroabsorption spectroscopy (CMEAS), we probe d the sub bandgap charge generation from the effective CT manifold and show ed that the loss of Voc has a direct correlation on the dielectric con stant of the polymeric bulk heterojunction blends . [ 61 ] Consequently, understanding the material properties on dielectric effect in the photovoltaic process would bring insight into the future development in organic photovoltaic cells. In an effort to explore the dielectric effect and the charge transfer state in the photovoltaic process with material properties, careful selection of materials is important to have meaningful results. Since a change in the dielectric constant shou ld be a result of D/A interaction for increased polarizability, choosing a material system with significant morphological differences seems straightforward. However, during materials selection, Polythieno[3,4 b] thiophene co benzodithiophene (PTB7) with or without processing additives was selected as one of the high efficiency polymers that showed significant
48 morphological difference. However, such morphology differences did not translate into the difference in dielectric constants. The blend with DIO showe d a dielectric constant of 5.03 Â± 0.22 and the one without DIO have a dielectric constant of 4.87 Â± 0.15 measured with capacitance voltage measurements . The difference in dielectric constant between different morphology is within the measurement error. M or eover, difference in CT state position less than 0.025 eV is observed , indicating a minimal change in D/A interaction ( Figure 3 2 ) . [ 19 ] On the other hand, isoindigo based D/A polymer s as active material s, which have low optical bandgaps and high carrier mobilities, ha ve been demonstrated with excellent performance in OPV s and other organic devices . [ 64 ] In this paper, we used two isoindigo (iI) based polymer s with different number of thiophene units as our model system s to investigate the CT states and photovol taic processes . The P(iI T3) (P [T3(C6) iI(HD)] ) polymer has three thiophene units, while P (iI T) ( P[T iI(HD)] ) has only one thiophene unit. Interestingly, the two polymers have nearly identical optical and electrical properties, but different interactions with fullerene. The two polymer blends exhibit a significant difference in root mean square roughness of the polymer blend morphology indicating a smaller domain size after blending with PC 71 BM which resulted in a dr astic difference in photovoltaic performances . Also, the pristine film of the two polymers had the same dielectric constants, however, after mixing with PC 71 BM , P(iI T3) :PC 71 BM showed a larger dielectric constant than that in P(iI T) :PC 71 BM as well as the pristine fullerene , hinting a favorable D/A interaction should be the interplay between morphology and polymer:fullerene interactions. We found that the better mixed P(iI T3) polymer blend films had a longer tail absorption at the ban d edge by carrying out CMEAS. Also, the photocurrent spectral
49 response (PSR) shows that CT states in P(iI T3) devices are more delocalized and are more effective in generating carriers. Transient PL study results confirm that a more delocalized CT is presented in P(iI T3) devices. T he excitons are indeed more delocalized and show a higher probability of dissociation. W e hypothesize that the more delocalized CT manifolds are a result of increased dielectric constant in P ( iI T3) : PC 71 BM blends, as a result of favorable D/A int eraction through better miscibility. The delocalization of lower CT manifolds may also reduce the effective bandgap, thus reducing the V oc despite having better exciton dissociation . 3.3 Experimental Sample preparation : P(iI T) and P(iI T3) were blended with PC 71 BM in 1:1.5 ratio in 1 , 2 dichlorobenzene with the concentration of 20mg/ml. PEDOT:PSS was spin coated at 8000 RPM for thickness of 30 nm onto a ITO substrate pre cleaned with acetone and isopropanol and annealed at 140 Â° C for 15 minutes in the air . The polymer solutions were then spun coated onto the substrates with a spin speed of 800 RPM in ambient atmosphere and atmospheric pressure for the desirable thickness of 110 nm . The samples were dried under nitrogen atmosphere and put in ther mal evaporator for deposition of LiF and Al. The device structure is ITO/PEDOT:PSS/P(iI T) or P(iI T3):PC 71 BM/LiF/Al. Capacitance measurement : HIOKI 3523 50 LRC meter was used to measure the geometry capacitances of the devices and the device thickness we re measured using atomic force microscopy. To eliminate the parasitic effect, the devices were calibrated with short and open circuit condition. The capacitance of the device was taken at 100k Hz with a small 20 mV ac modulation.
50 Transient photovoltage : T he sample was under the illumination of white light bias for photo biasing which is provided by a 150 W ozone free xenon arc lamp (Newport). The optical perturbation is obtained using a LASER EXPORT DTL 319QT(527nm 5ns, 60 J/pulse) and a series of neutral density filter were used to lower the power of the laser to to ~ 1 nJ. T he photo voltage decay was obtained using a Tektronix TPS2024 oscilloscope. Charge modulation e lectroabsorption spectra measurement: the samples were probed by the incident of a mono chromatic parallel beam into the sample through ITO with an incident angle of 45Â° and are reflected by the back Al electrode and captured by calibrated silicon and germanium photo detector . The sample internal electric field is modulated by a DC bias superimposed with a small AC voltage at a modulation frequency of 1000 Hz. A current amplifier and a lock in amplifier were connected to the detector to increase the signal to noise ratio. Pho tocurrent spectral response: photocurrent spectral response was measured by connecting the device in series with a 120k resistor and an SRS 830 lock in amplifier. Monochromatic light from a monochromator with the light spot smaller than the device area we re chopped and incident onto the device. Current output was measured through the lock in amplifier for the spectrum. Transient photoluminescence : Transient photoluminescence are performed with a time correlated single photon counting (TCSPC) spectrometer (Picoquant, Inc.) with the counting rate of 1.00 e+007(1/s). A pulsed laser with an average power of 1mW with the duration of 70 ps and the frequency of 40 MHz was used to excite the blend films of
51 P(iI T) and P(iI T3) blended with PC 71 BM. A 700 nm long pa ss filter was used to filter out emission from polymers singlet states. 3.4 Results and Discussions 3.4.1 Polymer Properties In order to investigate the CT states and the photovoltaic process , two isoindigo oligothiophene based polymers were being used for this purpose. As seen in F igure 3 3 , the polymer backbone composed of repeating unit of one electron deficient isoindigo unit coupled with varying number of thiophene units with one thiophene unit for P(iI T) and three thiophene units for P(iI T3) as i llustrated in Figure 3 3. These two polymers show very similar electrochemical levels an d optical bandgaps (Table 3 1) which serves as a good model for studying the CT states and the photovoltaic process. 3.4.2 Solar Cell perfor mance Figure 3 4 shows the solar cell performance of the P(I T) and P ( iI T3) : PC 71 BM devices. W hile these two polymers appear similar in energetics as well as chemical structures, their solar cell performances varied dramaticall y . The energetics and its solar cell performa nces are summarized in Table 3 1. For the P(iI T3) devices, the V oc is 0.7 V with a higher J sc of 14 mA/cm 2 . The P(iI T) devices only have a J sc of 2.45 mA/cm 2 , but a relatively high V oc of 0.91 V compared to P(iI T3) devices. While similar solar cell result has been published previously [ 59 ] , the effect of dielectric constant on delocalization of CT states ha s not been considered. 3.4.3 Film Morphology To understand the difference in J sc , we investigate the morphology of these two polymer blends. Due to the excitonic feature of OPVs, morphology with fine domain size is critical as excitons diffusion length is short. Therefore, having fine domain size is
52 es sential for efficient exciton dissociation. [ 44 , 65 ] Atomic force microscopic (AFM) is carried out to identify the morphology of P(iI T) and P(iI T3):PC71BM samples. AFM results of 1 m by 1 m topographical images were shown in Figure 3 5. T he two polymers show significant difference in the surface roughness after blending with PC 71 BM. In P(iI T), a root mean square (RMS) roughness of 5.27 nm is observed, indicat ive of the existence of larger domains of D/A materials after blending with PC 71 BM . For P(iI T3) devices, better mixing with fullerene can be observed with a RMS roughness of 2.59 nm , which is favorable for better exciton dissociation. The better mixing of P(iI T3): PC 71 BM indicated a better exciton dissociation as a result of better D/A interaction and shorter exciton diffusion length. 3.4.4 Bimolecular Recombination To identify that the difference in the Jsc is indeed from increased exciton dissociation as a result of favorable morphology, we first look at the bimolecular recombination of these two isoindigo polymer blends. Bimolecular recombination is another factor causing the reduction of the short circuit current. In order to verify the bimolecular recombination in these devices, transient photo voltage is carried out to study the r ecombination dynamics under open circuit condition. A laser was used as an excitation source while under the illumination of a solar simulator to provide photo bias identical to that of open circuit condition. The decay dynamics of photo carriers are monit ored by probing the decay of perturbation provided by the laser pulse (<5 mV). The decay dynamics can be used to represent the bimolecular recombination rates as the decay mechanism of the photo carriers are the bimolecular recombination. [ 48 , 66 ] As illustrated in Figure 3 6 , we can see that the decay dynamics for both P(iI T) and P(iI T3): PC 71 BM blends have a transient time with in the experimental error for t he photo -
53 carrier perturbation of 1.38 0.5 s and 1.25 0.5 s respectively . [ 31 ] This indicates an identical bimolecular recombination rate despite the difference in morphology. Therefore, we can exclude the loss of short circuit current through the bimolecular recombination and conclude that the difference in J sc is a result of better exciton dissociation and reduced geminate recombination at the D/A interfaces. 3.4.5 CT States Position by CMEAS As mentioned previously, the CT states play an important role in t he exciton dissociation process. In order to investigate the difference in the CT states as a result of the morphological difference, CMEAS is carried out to probe the CT states in P(iI T) and P(iI T3):PC 71 BM blends. Apart from the identical signal at 1.6 eV for both polymers which arises from perturbation of the electric field to absorption due to stark effect , [ 67 ] additional sub bandgap signals extends into the long wavelength region . This signal is a result of direct pumping of the hot CT excitons into CT state s at donor/acceptor interfaces . We have shown that these sub bandgap signals are a result of polaron generation due to direct excitation of excitons into CT states and the signal showed stronger dependen ce on frequency than S tark effect signals. [ 60 ] To exclude signals from carriers injected into the device, samples were measured under negative bias. The result in F i gure 3 7 shows that f or P(iI T):PC 71 BM device and P(iI T3):PC 71 BM , a sub bandgap cutoff around 1. 3 eV and 1.0 eV is observed respectively , despite having identical energetics in pristine materials. 3.4.6 Delocalization of CT excitons Beside the existence of the difference in the CT states manifolds, we further investigate the effectiveness of these CT states by looking into the photocurrent spectral response (PSR). PSR is carried out to monitor the absorption and understand the low
54 energy CT states in lon g wavelength regime for b oth polymer blends ( Figure 3 8 ). By monitoring the low current response below the bandgap of the pristine polymers, we were able to look into the effectiveness of CT states and its energy levels in terms of amount of carriers colle cted. It can be seen that the result s not only compliment the CMEAS result in which the P(iI T3) device have a cutoff energy near 1.3 eV while the P(iI T) device shows a cutoff energy around 1.0 eV. It also shows that more carriers were extracted toward th e lower energy regime. The result indicates that the CT states extend further below the bandgap and that these states are more effective in the dissociation process from excitons to polarons for P(iI T3):PC 71 BM blends which exhibit a higher PSR response af ter 1.6 eV. Also, there is more states below the bandgap that are capable of generating charges where it hit the noise floor at 1.1 eV for P(iI T3) but only 1.3 eV for P(iI T). Since these CT states are essential in mediating the CT exciton dissociation pr ocess, we propose that more CT states in P(iI T3) are available for facilitating the dissociation process and should lead to more delocalized CT excitons . These delocalized CT excitons then result in better charge generation and thus better current as well as solar cell performance. In an effort to identify the order of the delocalization of the CT excitons in P ( iI T3) and P (I T) blends, transient photoluminescence (PL) of the two isoindigo polymers and its fullerene blends are performed to understand the exciton dynamics under the influence of different effective CT state position s as shown in F igure 3 9 ( a ). Previously, it has been shown by Bernardo et al. that hi gher delocalization of excitons at CT states will lead to a fast decay component in transient PL intensity. [ 56 ] For P(iI T):PC 71 BM samples, transient PL showed a slow decay component with a lifetime of 987 ps. On the
55 other hand, an additional fast decay component which consists of 79% of the signal strength is disc overed with a lifetime of 143 picoseconds (ps) for T3 polymer blends. Such a fast decay component in P(iI T3) polymer blends is not observable in pristine polymer transient PL ( Figure 3 9 (b) ), indicating that the geminate recombination of excitons at CT s tates was inhibited through mechanisms only exist ed after blending with fullerene. The fast decay component in P ( iI T3) polymer blends indicated that almost 80% of the excitons at CT states goes through a fast dissociation process and this fast dissociati on process leads to higher generation rate and thus higher current for the P ( iI T3) devices. We can conclud e that CT excitons in P(iI T3) indeed are more delocalized due to the higher density of CT states that are capable of mediating the exciton dissociat ion process. 3.4.7 Formation of CT states While the difference in Jsc can be explained by the morphological difference, the difference of the CT manifolds cannot be simply explained by the morphology. As mentioned previously, we investigated the PTB7:PC 71 BM blends with or without processing additives 1,8 Diiodooctane (DIO). While it s well known that the inclusion of processing additives decreases the domain size, no significant difference can be seen from PSR of the two blend film s with or without DIO com pared to the difference in PSR result for the isoindigo polymers . Since the exciton binding energy is decided by the columbic force, the dielectric constants could be the reason for higher orders of delocalization for the CT manifolds in the P(iI T3) polym er device. In order to verify the difference in the columbic force, the dielectric constant of the blend film is determined by measuring the geometric capacitance of the device and modeled as a parallel plate capacitor. T he result is shown in Table3 1 . Int erestingly, t he pristine dielectric constant
56 of the two polymers is identical before blending with fullerene. After blending with PC 71 BM, however, t he dielectric constant of the blend films shows that the blend film of P(iI T3) indeed has a higher dielectr ic constant after mixing with fullerene. The resulting dielectric constant shows that while the difference in the dielectric constant of the pristine film for the two polymers is less than 0.1, the dielectric constant of the P(iI T3) polymer appears 0.5 hi gher than the P(iI T) after blending with PC 71 BM. Also, the dielectric constant of the blend is higher than the individual pristine material, which cannot be determined by the Effective Media Theory. [ 68 ] I nstead , the blend film has a dielectric constant higher than that of the result of the increased D/A interactions, which formed by blending with fullerene de rivatives , have high er polarizability due to increase D/A interaction. The result is the higher effective dielectric constant of the blend films. [ 69 ] Therefore, the difference in the CT manifolds can not only be the difference in morphology but the diffe rence in dielectric constants as well. Such redshift of the CT manifolds have been discovered as a result of increased dielectric constant as well, which is the case of the P ( iI T3) : PC 71 BM polymer blends . [ 62 , 70 ] 3.4. 8 CT manifolds on V oc While increased recombination has been correlat ed with reduction of the V oc as a result of coarse domain size in morphology, it cannot simply explain the difference in V oc since such drastic morphological difference has been shown in several materials by the addition of solvent additives . [ 19 , 71 ] One such example is the PTB7 that was discussed in the previous paragraph w hich show no significant difference in the CT states position and dielectric constant after adding processing additives ( Figure 3 2 ). On the other hand, the P(iI T3) polymer blend showed significant difference in the CT manifolds
57 along with a change in the dielectric constant. Therefore, we postulate that the difference in the CT manifolds could be the reason for the V oc difference which has been regarded as the effective bandgap and showed a linear relationship with the open circuit voltage. [ 58 , 59 , 62 ] Although the more delocalized CT states observed in the P(iI T3):PC 71 BM device favors exciton dissociation and provide more current output, the delocalization resulted in the formation of the lower CT manifolds. This r edshift of effective CT manifolds causes a decrease of effective bandgap which can be the reason for the reduction of the V oc in the P(iI T3) devices. A s for the P(iI T) devices, the lower CT manifolds are higher compared to the P(iI T3) and thus a higher V oc is observed while it s unfavorable for effective exciton dissociation. 3.5 S ummary In conclusion, photo physical properties of the effect on the morphology, the dielectric constant and the associated delocalization of CT excitons in two isoindigo polymers were studied. Despite having similar energetics and chemical structures, the solar cell performance shows a great difference which is not expected. AFM results showed a favorable morphology in the P(iI T3) devices. Since the order o f bimolecular recombination is identical for the two polymers, geminate recombination should be the factor affecting Jsc. We showed that the position of the effective bandgap is affected and that a more effective generation of carriers through more delocal ized CT excitons is observed in the P(iI T3) device. We hypothesize that the more delocalized CT excitons may be a result of favorable morphology and decreased columbic interaction as higher dielectric constant is observed in P(iI T3):PC 71 BM blends. Also, the lower CT manifolds may be the reason for the reduction of the V oc despite having a favorable exciton dissociation rate.
58 Figure 3 1 . Schematic diagram of exciton dissociation mediated by CT state s . Upon excitation, excitons have to diffuse to donor/acceptor interface. CT states at the interface are more delocalized and can mediate the exciton dissociation process. The effective bandgap here denotes the CT states where excitons can dissociate effectively. The binding energy for exciton is invers ely proportional to dielectric constant ( r ).
59 Figure 3 2 . photocurrent spectral response of PTB7:PC 71 BM with or without 1,8 diodooctane (DIO) as solvent additives. No significant difference in CT cutoff is observed despite the difference in morphology. Figure 3 3 . Chemical structure of the 2 isoindigo system polymers of p(iI T) and p(iI T3). Note that the only difference is in the number of oligothiophene unit per repeating unit.
60 Figure 3 4 . current density voltage curve for P(iI T):PC 71 BM and P(iI T3):PC 71 BM
61 Figure 3 5 . AFM height image of (a) P(iI T1):PC 71 BM and (b) P(iI T3) :PC 71 BM blend films. The phase picture of (c) P(iI T) shows large domain size while (d) P(iI T3) shows better miscibility for PC 71 BM . S urface RMS roughness of the P(iI T):PC 71 BM is 5.27nm while the RMS roughness of the P(iI T3):PC 71 BM is 2.59nm, indicating a smaller domain size for the P(iI T3):PC 71 BM blends then the P(iI T):PC 71 BM blends.
62 Figure 3 6 . Transient photovoltage (TPV) result of P (iI T) and P(iI T3) blends with fullerene. The result showed a carrier lifetime of 1.38 Â± 0.5 and 1.25 Â± 0.5 molecular recombination rate.
63 Figure 3 7 . C harge modulated electro absorption spectroscopy (CMEAS) of P(iI T1):PC71BM and P(iI T3):PC71BM blend films. All measurements were done under reverse bias to prevent carrier injection. Sub bandgap signals indicate a generation of carriers from CT excitons where the onset i s the effective bandgap (E eff ). The result shows that P(iI T1) have a sub bandgap cutoff at ~1.3eV and P(iI T3) have a sub bandgap cutoff at ~ 0.9 eV
64 Figure 3 8 . Photocurr ent spectral response (PSR) of P (iI T):PC 71 BM and P (iI T3):PC 71 BM. The result not only compliment the CMEAS result for sub bandgap cutoff , but also shows that P(iI T3) have more states below bandgap that s capable of dissociation of excitons into polarons.
65 Figure 3 9 . Transient PL for (a) P(iI T1):PC 71 BM and P(iI T3):PC 71 BM blend films to realize the exciton dynamics in blend films. P(iI T1) shows an single exponential decay while for P(iI T3), a bi exponential decay were discovered, indicating a fast exciton dissociation process in P(iI T3) blend films. (b) Transient PL result of pristine p(iI T) and p(iI T3). No fast decay component is observed. Table 3 1 . E nergy level s, solar cell performances and dielectric constants of P ( iI T) and P (iI T3) :PC 71 BM HOMO/LUMO (eV) Eg (eV) V OC (V) J SC (mA/cm2) FF (%) PCE(%) r Blended r P (iI T) 5.57/ 4.02 1.62 0.91 2.45 47.1 1.05 2.37Â±0.076 4.23Â±0.11 P(iI T3) 5.50/ 3.97 1.67 0.70 13.45 57.5 5.65 2.45Â±0.10 4.76Â±0.18
66 CHAPTER 4 EFFECT OF THERMAL ANNEALING ON CHARGE TRANSFER STATES AND EXCITON DISSOCIATION IN PCDTBT:PC 70 BM BULK HETEROJUNCTION SOLAR CELLS 1 4.1 Abstract The effect of thermal annealing on charge transfer (CT) states and exciton dissociation is studied on PCDTBT:PC 70 BM bulk heterojunction solar cells. PCDTBT, unlike one of the early high efficiency semi crystalline polymer poly(3 hexylthiophene 2, 5 diyl) (P3HT), is well known for its amorphous properties. While no significant morphology can be recognized t hrough the thermal annealing process, we observed an increase in the short circuit current (J sc ) follow by a decrease in fill factor (FF). We discovered through high field extraction that the annealed device does have higher current output at higher revers e bias. It was further proved by the photocurrent spectral response (PSR) and transient photoluminescence that a more delocalized CT manifolds is observed in the annealed device , resulting in higher J sc . However, through the illumination intensity dependen t current density voltage (JV) analysis, we found that the annealed device exhibit higher degree of Shockley Read Hall recombination which is detrimental to the devices. T he temperature dependent J V was modeled and found that the mobility of the electron and holes as well as the energetic disorder is not affected by the thermal treatment. This result indicates the trap for trap assisted recombination is not near the transport band but should be mid gap trap states. Photothermal deflection spectroscopy confirms the assumption and showed that higher density of mid gap traps were formed after thermal annealing. 1 The authors Iordania Constantinou and Tzung Han Lai from the Franky So group at the University of Florida contributed equally as co first author to this work.
67 4.2 Introduction Recent researches have been focused on the exciton dyanamics at the charge transfer state located at the D/A interfaces. [ 55 , 58 , 61 , 72 ] The charge transfer states manifolds mediate the exciton dissociation process by photo induced charge transfer process in order to achieve a efficienct polymer solar cells. In the higher CT manifolds, the exciton dissociation process rate is faster than the geminate recombination rate the CT states which resulted in higher probabili ty for exciton dissociation to happen. For the lower CT manifolds, the CT states are more localized and the exciton dissociation rate is slower and have a higher probability to end up in CT ground state and then recombines. [ 14 ] In order to achieve high efficiency solar cells, a better understanding of the formation of these CT manifolds and its delocalization is essential. In Chapter 3 , we showed that the delocalization of the charge transfer (CT) states is independent of the morphology in the polymer/fullerene bl ends and the dielectric constants of the blends indicate a better polymer/fullerene interactions. On e question was then raised that whether the delocalization of the CT states can be observed in polymer/fullerene blends without the significant difference in the morphology . Through these ways, we can have a better understandings of the formation of the charge transfer state independent of the morphology effect in polymer solar cells. R ecently, it has been shown that in F8T2 polymer, the thermal annealing promote the better interface between the F8T2 and the PCBM nanosphere which promotes the charge transfer process. [ 73 ] However, such annealing process has lead to a change the the surface morphology observed under atomic force microscopy which should not be excluded in studying the inte raction between the charge transfer process. On possible ways to achieve this is by looking into thermal annealing effect on some of the
68 amourphos polymers to limit the difference in morphology with thermal treatment while providing free energy fot the pol ymer fullerene to reconstruct for better polymer/fullerene interactions. On e such polymers that serves as a good model systems is the poly [N hepta decanyl 2,7 carbazole alt 5,5 (4Â´,7Â´ di 2 thienyl 2Â´,1Â´,3Â´ benzothiadiazole)] (PCDTBT) polymer s. [ 74 ] Various report s have shown that thermal annealing in the polymer systems has a minimum effect in terms of surface morphology under annealing temperature of 150 C . [ 75 ] Also, a stronger EL from 750 nm to 1100 nm is observed after thermal annealing at around 110 C, indicating higher density of charge transfer state formation for PCDTBT polymers. [ 76 ] We also observe a increase in dielectric constant after thermal annealing from 3.91 0.17 to 4.32 0.04 hinting better polymer/fullerene interaction after thermal annealing process to hav e a higher polarizability of the charge transfer states. Therefore, it served as a good system for us to study the effect CT delocalization with no morphological difference. [ 77 ] By looking into the photocurrent spectral response (PSR) and the transient photoluminescence data, we were able to study the delocalization of the CT state without significant morphological difference being observed after thermal annealing . Therefore, we can focus on the effect of polymer fullerene interaction and its relative c harge transfer process disregarding the effect of morphology. In addition , we discovered that the effect of thermal annealing deteriorate the transport properties of the polymer/fullerene blend as as result of increased trap s formation after thermal anneal ing that enhances the Shockley Read Hall recombination. Throughout this reports, we re
69 going to address the annealed sample annealed and the as prepared sample as as prep 4.2 Experimental Device Fabrication: PCDTBT polymers were blended with PC70BM in 1:4 ratio in a mixed solution of CB and oDCB with a concentration of 20mg/ml. PEDOT:PSS was spin coated at 8000 RPM for a desirable thickness of 30 nm onto a ITO substrate pre cleaned with acetone and isopropanol and annealed at 140 Â° C for 15 minutes in the air . The polymer solutions were then spun coated onto pre cleaned indium tin oxide substrates for a spin speed of 1200 RPM for a desirable thickness of 70 nm . The annealed samples were annealed under 1 5 0 Â° C for 20 minutes in nitrogen atmosphere whi le the as prepared samples were dried under nitrogen atmosphere . These samples were then put in thermal evaporator for deposition of LiF and Al. The device structure is ITO/PEDOT:PSS/ PCDTBT: PC 71 BM/LiF/Al. Single carrier device were fabricated with the stru cture of ITO/MoOx/ annealed or as prepared PCDTBT: PC 71 BM /MoOx/Ag for hole only devices and ITO/ZnO/ annealed or as prepared PCDTBT: PC 71 BM /LiF/Al the electron only devices. Device Characterization : J V characteristics was measured with a keithley 4201 semi conductor parameter analyzer. EQE and PSR measurements were carried out with a xenon lamp and an an ORIEL 74125 monochromator to provide monochromatic lights. K eithley 428 current amplifier were used to amplied the raw signal then the signal go through the SR830 lock in amplifier for better signal to noise ratio. Janis PF 100 N 2 cryostat were used for the temperature dependent measurements along with the keithley 4201 semiconductor analyzer.
70 Transient photolu minescence : Transient photoluminescence are performed with a time correlated single photon counting (TCSPC) spectrometer (Picoquant, Inc.) with the counting rate of 1.00 e+007(1/s). A pulsed laser with an average power of 1mW with the duration of 70 ps and the frequency of 40 MHz was used to excite the annealed or as prepared PCDTBT blended with PC 71 BM. A 700 nm long pass filter was used to filter out emission from polymers singlet states. PDS measurements were carried out by using a 1 kW Xe arc lamp and a 1/4 m grating monochromator (Oriel) as the incident light source . T he monochromated light source were then modulated by a mechanical chopper. The samples were immersed into Carbon tetrachloride and a HeNe laser was used to detec the absorption. SRS 830 lo ck in amplifier were used to filter the noise signal. 4.3 Results and Discussions 4.3.1 J V Characteristics In order to exclude the effect of the morphology in terms of th e formation of charge transfer states, we first look the surface morphology of the PCDTBT:PC70BM blends. A s shown in Figure 4 1 ( a ) and 4 1 ( b ) , w e see that atomic force microscopy (AFM) result of the PCDTBT:PC70BM blend film does not show significant difference in film morphology for the annealed and as prepared samples with the Root mean square morphology of 0.95 nm for the annealed film and 0.87 for the as prepared films. The annealing temperature of 150 Â°C was shown previously to have minimum effect on the PCDTBT:PC 70 BM morphology while difference in the current density voltage (J V) c haracteristics is observed . [ 78 ] Since no significant difference can be observed in the morphology of the PCDTBT:PC 70 BM films for thermal treatments , we can exclude the morphology effect on the formation of the charge transfer states and look at the J V
71 characteristics. As shown in Figure 4 2 ( a ) , we found that the device showed a decrement in power conversion effici ency (PCE) for the annealed device as a result of the lower device fill factor. On the other hand, the short circuit curre nt (Jsc) for the annealed and the as prep device is identical at 10.3 mA/cm 2 and 10.4 mA/cm 2 respectively. In addition, on 20 mV difference in the open circuit voltage (Voc ) is observed between the annealed and the as prep devices, indicating that the effect of thermal annealing is minimum on Jsc and Voc which is in consistent with what previously reported [ 79 ] Ho wever, when we look at the Jsc at reverse applied bias, a higher Jsc can be observed for the annealed device. Under high reverse bias, the recombination due to lower transport properties can be neglected. Therefore, the higher Jsc at the reverse bias could be an indication of a more carrier generation from the annealed device possibly due to higher order of delocalization of the CT states. 4.3.2 Delocalization of Excitons In order to verify if larger amount of carriers were generated from the annealed device, we look at the external quantum efficiency (EQE) under reverse bias with incident light of 540 nm which have identical EQE for both device s with or without thermal treatment under 0 V bias . The su perposition of the external voltage and the bulid in electric field would enhanced the collection efficiency which seems to be worse fo the annealed device. The result was shown in Fi gure 4 2 ( b ) . We can see that the EQE at 5 V does show a higher EQE for the annealed device. Since the collection efficiency should be better under higher internal field, the reverse EQE should exclude the loss due to the difference in transport properties and collec t all the carriers generated in the device . F rom the reverse bias EQE result s, we can conclude that the annealed device does have higher charge generation efficiency.
72 Since higher charge generation efficiency is discovered in the annealed device, we hypot hesize that the CT states in the annealed device should be more delocalized. Photocurrent spectral response (PSR) was carried out to look at the charge generation efficiency at CT states which is below the bandgap of both polymer and fullerene. The result showed in Figure 4 3 ( a ) shows that the overall efficiency for the annealed device below 1.65 eV is higher than that of the as prep device without thermal annealing. Also, a lower sub bandgap cutoff of 1.25 eV is observed for the annealed device than the 1. 3 eV of the device without annealing. These results i ndicates that the device withou t thermal annealing either have higher density of charge transfer states or the e xciton dissociation from these CT states are more efficient . [ 80 ] In both cases, it means that the CT states for the annealed device are more delocalized . Since for the photovoltaic process, the exciton have to first form CT excitons that span across the polymer/fullerene interfaces, the quantum efficiency of these CT state is important for the exciton dissociation as well as carrier generation despite have a external quantum efficiency of 10 3 % to 10 4 %. A higher effeiciency for the annealed device indicates that the CT excitons during the photovoltaic process would have higher probability of dissoci ation, resulting in higher carrier generation . We further look into the transient photoluminescence (PL) to verify if the annealed device has higher degree of delocalization comparing to the device without any thermal annealing process . As seen in Figure 4 3( b ) , we see similar result as in the isoindigo system reported in Chapter 3 where not annealed device showed single exponential decay while biexponential decay is observed in the annealed device. The single exponential decay which has a carrier lifetime of 988 ps in the device without
73 thermal annealing is consistent with the pr istine polymer result shown in F igure 4 3 ( c ) for both annealed and as prep pristine PCDTBT films . This slow decay component is likely the residual signal from the polymer singlet states since samples with or without fullerene have the existence of thsuch slow decay component. On the other hand, a fast decay component that made up 65% of the overall in tensity has a lifetime of 162 ps is observed for the annealed device following an identical slow decay mechanism similar to that of th e not annealed device as well as the pristine polymers. This indicates that a fast dissociation mechanism that takes the e xcitons away after excitation. Since such mechanism only existed after blending with the fullerene and that such fast decay mechanism is not observed in the case of as prep films. W e can assume that such extra dissociation mechanism is a result of the more delocalized CT states observed throught PSR in the annealed device. 4.3. 3 Carrier Transport and Recombination Despite the existence of the more delocalized CT state in the annealed device, t he device PCE is hampered by a 15% decrease in the device FF for the annealed device . S ince FF represents an overall transport properties, we perform single carrier device for both electron and hole to measure the electron and hole mobility as well as its energetic disorder respectively to study the difference in transport properties. Space charge limited current (SCLC) for single carrier were measured under different temperature to resolve the mobility at different temperatures and this JV curves were fitted with Mott Gurney law to calculate the mobility at differ ent temperatures. The mobility at different measuring temperatures was subsequently plotted with respect to verying temperature and Gaussian Disorder Model was fitted to get the energetic disorder of the device. Gaussian Disorder Model describes the transf er between
74 molecule participating in the transport and that that the energy of these charge transporting elements can be described by a Gaussian distribution. Based on this argument we can get the relationship between the carrier mobility and the energetic disorder by the following equation : [ 26 ] ( 4 1(a) ) ( 4 1(b) ) W is carri er mobility at infinity temperature , k is the B oltzmann constant and is the energetic disorder at the transport band . [ 81 ] Through these measurements , we were able to compare the transport properties for the device with or without thermal annealing. The mobility result s for the annealed and not annealed device s were shown in Figure 4 4 and the and energetic disorder for both electron and holes for annealed and as prep device were summarized in Table 4 2. After thermal annealing, minor increase for both the electron and the hole mobility were observed as well as a lowering of the energetic disord er. [ 82 ] While these result compliment the number reported previously in the literature, it is contradicting to the device performance where a decrease in the device FF after thermal annealing were observed. As discussed in Chapter 1, the non geminate recombinatio n can be categorized into the bimolecular recombination and the trap assisted Shockley Read Hall recombination. To resolve the difference in FF, we first look at Jsc dependency as a result of illumination intensity to verify the rate of the bimolecular rec ombination in the
75 annealed and the not annealed devices. As mentioned in Chapter 2, the illumination intensity dependenc e of Jsc should be linear as described in Eq. 4 2 as: ( 4 2 ) Where denotes the slope of J sc vs intensity in log log scale . I f deviate from 1, it indicates that the device is bimolecular recombination dominant since the Jsc decreases as a result of bimolecular re combination. [ 83 ] We measure the Jsc with the illuminaiton intens ity ranging from 0.1~ 100 mW/cm 2 and t he result was showne in Figure 5 ( a ) which shows that both devices with or without thermal annealing have a slope close to one , indicative of bimolecular recombination independent devices . Since bimolecular recombinai ton is not the cause of decrease in the FF, w e further look at illuminaiton dependency of Voc to see if the device with thermal annealing is affect by the SRH recombination. As discussed in Chapter 2, the change of Voc as a result of different illuminatio n intensity can be described as: ( 4 3 ) T he slope of Voc vs ln(I) for bimolecular recombination should be kT/q as both electron and hole concentration have the same dependency on the illumination intensity . [ 84 ] On the other hand, for devices dominated by SRH recombination, slope > 2kT/q should be observed as both electrons and holes has its dependenc y on the illumination intensity. The result in Figure 4 5 ( b ) shows the Voc vs ln(I) plot for both the annealed and the as prep device s . W hile the device without thermal annealing showed a slope less than 2kT/q of 1.87Â±0.18 kT/q , the annealed device does have a slope higher than 2kT/q of 2.47Â±0.35 kT/q , indicating that the SRH recombination is dominant.
76 4.3.4 Trap Formation Although we found that the SRH recombination or the trap assisted recombination process is domin ant for the annealed device, it s not contradicting to the mobility and energetic disorder results since energetic disorder indicate the trap formation near the transport ban d . The only possibility for SRH recombination to be dominant in the annealed device s while having lower energetic disor der would be that the traps for SRH recombination to happens exist in the mid gap which does not affect the transport properties. In order to verify that the traps in mid gap are the reason for the decrease in device FF, we perform the phototherm deflecti on spectroscopy (PDS) to probe the trap formation in the mid gap after thermal annealing . [ 85 ] As shown in Figure 4 6, the o verall absorption increased between 800 2000nm was observed. This indicates a higher density of traps in the mid gap were formed after thermal annealing. O ne indication that these are from trap state absorption rather than noise is from the observation of the absorption near1300 nm coming from the C H vibrational overtone signal. [ 86 ] T he increase in the trap density resulted in higher order of SRH recombination and eventually lower the device FF and the PCE of the device after thermal annealing despite having better charge generation efficiency. 4.4 Summary In conclusion, we discovered the difference in CT state delocalization in PCDTBT polymer system with thermal annealing and as prepared despite having significan morphological difference . The result shows that we do see a different order of delocalization i n polymer/fullerene blends without significant difference in morphology.
77 PSR and transient PL measurement confirms the higher order of delocalization in the annealed PCDTBT:PC 70 BM. However, despite the better charge generation efficiency due to the existen ce of a more delocalized CT, the device performance was hampered by the recombination in the annealed device. Through illumination intensity dependent studies, we see that the SRH recombination is the dominant mechanism in the decrease of the annealed devi ce efficiency. The mobility and the energetic disorder data shows that the traps near the transport band were not affected by the thermal treatment but even increases slightly . PDS result confirms that trap concentration increased after thermal annealing l ocated in the mid gap which does not affect the transport properties of the annealed films. Since we have shown that the morphology is independent of the delocalization of the CT states in polymer solar cells, the next step would be to seek ways to control the formation of CT states.
78 Figure 4 1 . AFM topography images of PCDTBT: PC 70 BM films (a) after thermal annealing and (b) as prepared (as prep) The Root mean square roughness of the not annealed films were 0.87 nm and the film after annealing were 0.95 nm
79 Figure 4 2 . Solar cell performance for PCDTBT:PC 70 BM devices. (a) The current density voltage (JV) characterist ics for PCDTBT:PC 70 BM devices with and without annealing . (b) EQEs were measured at 540 nm as a function of reverse bias .
80 Figure 4 2 . Continued
81 Figure 4 3 . Exciton dynami cs of PCDTBT:PC70BM results (a) PSR spectra confirm that more states below the bandgap is capable of exciton dissociation in the annealed device. (b) PL transients show a single exponential decay for the not annealed device and a bi exponential decay for the annealed d e vice, indicating a fast exciton dissociation process in the annealed PCDTBT : PC 70 BM films.
82 Figure 4 3 . Continued .
83 Figure 4 3 . Continued.
84 Figure 4 4 . Zero field mobilities Vs square of reciprocal temperature for PCDTBT:PC 70 BM annealed and not annealed devices . Energetic disorder can be determined by the slope. Th , can be determined by the y intercepts.
85 Figure 4 5 . J sc and V oc vs illumination intensity (a) J sc V s. I on log log scale fitted for recombination. (b) V oc V s. I fitted for recombination .
86 Figure 4 5 . Continued
87 Figure 4 6 . PDS spectra for PCDTBT:PC 70 BM annealed and not annealed devices .
88 Table 4 1 . A verage device characteristics for the OPV s fabricated in this study. PCDTBT:PC 70 BM Jsc [mA cm 2 ] Voc [V] FF [%] PCE [%] Annealed Not Annealed Table 4 2 . Electron and hole mobilities and energetic disorder for PCDTBT:PC70BM devices with and without thermal annealing PCDTBT:PC 70 BM (cm 2 /Vs) h (meV) (cm 2 /Vs) e (meV) Not Annealed 9 . 92 *10 4 72.83 2.6 4 *10 3 Annealed 3. 03 *10 3 69.20 9 .04 *10 3 56.93
89 CHAPTER 5 EFFECT OF POLYMER SIDE CHAINS ON CHARGE TRASFER STATES , EXCITON DISSOCIATION AND CHARGE TRANSPORT IN PBDT TPD BULK HETEROJUNCTION SOLAR CELLS 2 5.1 Abstracts The charge transfer (CT) states which exist at donor/acceptor (D/A) interfaces in Bulk heterojunction (BHJ) organic photovoltaics (OPVs) with the D A type of polymers ha ve been shown to mediate the exciton dissociation process due to its excitonic feature. Recently, it has been suggested that the side chains at A part of the polymer will affect interactions with fullerene and result in different CT position and its effectiveness due to steric hindrance . In this report, we discuss t he effect of polymer side chains on charge transfer states, exciton dissociation and charge transport in PBDT TPD polymers. Two side chains, n octyl and ethylhexyl, were chosen on TPD moieties and photovoltaic process with these two side chains were assessed. The result shows that the linear side chain n Octyl exhibit more delocalized CT states while the bulkier side chain ethylhexyl shows a more localized CT states accompanying with a higher degree of geminate recombination. We further studied the effect of side chains on transport properties and found that bulkier sidechains resulted in structural disorder and longer stacking. The effects of structural disorder induces lower mobility and energetic disorder in the transport band which is the reason for more bimolecular recombina tion with the bulkier side chain ethyhexyl and deteriorate the device performance . 2 The authors Iordania Constantinou and Tzung Han Lai from the Franky So group at the University of Florida contributed equally a s co first author to this work.
90 5.2 Introduction Charge transfer (CT) state formed at the Donor (D) /Acceptor (A) interface have been shown to be critical in the exciton dissociation process in bulk heterojunction (BHJ) organic photovoltaics (OPVs). Rese a rches have shown that the order of delocalization of CT states increases as the energy of the CT manifold increases. D elocalized CT states at the higher CT manifold undergo faster dissociation process in the femto second regimes via the photo induced charge transfer process which is essential for efficient carrier generation . On the other hand, excitons in the lower CT manifolds experience longer exciton dissociation process and have higher chance to result in CT recombination. [ 14 , 15 ] In Chapter 3 and Chapter 4 , we have shown the delocalization of CT states in different systems and with various treatments as well as morphology. Since CT behavior is a charge transfer process between the polymer and the fullerenes , figuring out ways to control the formation of the CT state is essential for future design of efficiency polymer solar cell. In Chapter 1, we discussed the electron transfer process based on M arcus theory. Since the the CT exciton is a charge transfer process between the polymer and the fullerene, the distance between these two chemical species and its relative charge transfer rate can be describe by Eq. 1 1. In Eq. 1 1, we can see that the ch arge transfer rate is proportional to the square of the mean separation distance between the two chemical species involved in the charge transfer process . Therefore, one factor that might promotes the CT exciton dissociation process is the proximity of t he fullerene and the polymer backbone in space. Recently, the interaction between the polymer and fullerene has been shown to be essential for the formation of more delocalized CT states. [ 87 ] D A conjugated
91 polymers which consists of alternating units of acceptor moiety and donor moiety have been shown to absorb more of the solar spectrum as the orbital mixing of the donor and acceptor moiety lower the bandgap of the polymer. [ 88 ] Upon photoexcitation, the excitons generated in the polymer backbone span across the D A moieties as the hole resides in the donor moiety and the electrons on the acceptor moiety. Therefore , the interaction between the acceptor moiety and the fullerene derivatives is important since the charge transfer process for the electrons happens between these two chemical species. As a result , figuring out ways to control the interaction between the acceptor moiety and the fullerene is important in controlling the charge transfer process between the polymer and the fullerene. In order to study the effect of distance from the polymer backbone to the fullerene and its effect on charge transfer process , we looked into the polymer poly(di(2 ethylhexyloxy)benzo[1,2 b:4,5 b ' ]dithiophene co octylthieno[3,4 c ]pyrrole 4,6 dione) (PBDT TPD ) comparing different side chains on the acceptor benzodithiophene (BDT) moiety as shown in Figure 5 1 . Due to steric hindrance, the linear n octyl side chains on the acceptor moiety should bring the fullerene closer since it takes up less volume in space and we deno te this polymer the OCT polymer . On the other hand, branched ethylhexyl side chains were used to repel the fullerene from the polymer backbone as such branched side chains are bulkier in space, pushing the fullerene away from the polymer backbone and we call this polymer the EtHex polymer . Since the charge transfer rate is proportional to the square of the mean separation distance, slight change in the distance between the polymer backbone and the fullerene will greatly affect the charge transf er rate. In this study, we expect to see the linear side chain
92 polymer with closer proximity to the fullerene to have a better polymer fullerene interaction which should result in better charge transfer process between the polymer and the fullerene . On the other hand, the bulkier ethylheyxl side chain should have a worse polymer fullerene interactions as the fullerene is pushed farther away from the polymer backbones. In this case, we hope to achieve controls of formation of delocalized CT states through si de chain engineering. 5.2 Experimental Device Fabrication : PBDTTPD polymer was blended with PC 70 BM in 1:1.5 ratio in the CB solution with the addition of 1vol% DIO. PEDOT:PSS was spin coated at 8000 RPM for desirable thickness of 30 nm onto ITO substrate pre cleaned with acetone and isopropanol and annealed at 140 Â° C for 15 minutes in the air . The polymer solutions were then spun coated onto pre clea ned indium tin oxide substrates with a spin speed of 800 RPM for a desirable thickness of 100 nm and were dried under nitrogen atmosphere and atmospheric pressure . These samples were then put in thermal evaporator for deposition of LiF and Al. The device structure is ITO/PEDOT:PSS/ PBDTTPD: PC 71 BM/LiF/Al Single carrier device were fabricated with the structure of ITO/MoOx/ PBDTTPD: PC 71 BM /MoOx/Ag Device Characterization : J V characteristics were measured with a keithley 4201 semiconductor parameter analyzer. EQE and PSR measurements were carried out with a xenon lamp and an an ORIEL 74125 monochromator to provid e monochromatic lights. K eithley 428 current amplifier were used to amplied the raw signal then the signal go through the SR830 lock in amplifier for better signal to noise ratio. Janis PF 100 N 2 cryostat were used for the temperature dependent measuremen ts along with the keithley 4201 semiconductor analyzer.
93 Transient photoluminescence : Transient photoluminescence are performed with a time correlated single photon counting (TCSPC) spectrometer (Picoquant , Inc.) with the counting rate of 1.00 e+007(1/s). A pulsed laser with an average power of 1mW with the duration of 70 ps and the frequency of 40 MHz was used to excite the annealed or as prepared PCDTBT blended with PC 71 BM. A 700 nm long pass filter was u sed to filter out emission from polymers singlet states. 5.3 Results and Discussions 5.3.1 Film Morphology As discussed in Chapter 3 and Chapter 4 , morphology is not the only factor affecting the solar cell performances but the polymer/fullerene interacti ons as well. In order to exclude the effect of morphology on the charge transfer states , we look at the film morphology of the EtHex and the Oct polymer blended with fullerene by looking at atomic force microscopy (AFM). As shown in Figure 5 2, the result shows that no significant difference between the EtHex and the Oct is observed. Also, the roughness between these two polymer/fullerene blend films is identical. Therefore, we can exclude the effect of morphology on the device performance between the EtHex and the Oct device and focus on the polymer/fullerene interaction as it translate into delocalization of CT states. 5.4.2 Device Performance First , we look at t he current density voltage (JV) characteristics between the O c t and the EtHex devices. Despite the similarity in the film morphology, significant difference were observed between the Oct and the EtHex devices. The Oct has higher short circuit current (Jsc) of 11.1mA/cm 2 while having slightly lower open circuit voltage (Voc) of 0.89 V . For the EtHex device, much lower Jsc of 3.9 mA/cm 2 while higher Voc
94 of 0.98 V is observed. The device performance of the O c t and the EtHex device were summarized in Table 5 1 and is consistent with the report previously published. [ 89 ] This phenomenon is identical to the situation discussed in Chapter 3 where lower CT manifolds limit the Voc while having more CT state mediating the exciton dissociation process. Therefore, we hypothesize that the Oct could have higher delocalization of CT states and that it could be a result of a linear side chain at the acceptor moiety, enhancing the polymer/fullerene int eractions. Figure 5 3 ( b ) shows the external quantum efficiency of the Oct and the EtHex devices which shows that the Oct device have higher EQE over all the absorption spectrum which is consistent with the JV characteristics. [ 90 ] 5.4.3 Delocalization of CT States In order to verify if the Oct device does have higher order of delocalization of the CT states as hypothesize previously, we look at the photocurrent spectral response to see if the Oct device showed higher carrier generation efficiency. As sho wn in Figure 5 4( a ) higher EQE for the CT states in the Oct device between 1.2 eV to 1.7 eV is observed. Also, a lower sub bandgap cutoff at ~ 1.225 eV is observed for the Oct devices than the sub bandgap cutoff of ~ 1.275 eV for the EtHex devices. This indi cates higher carrier generation efficiency for the Oct device as a result of lower steric hindrance by the linear side chains at the acceptor moiety for better polymer/fullerene interactions w hile the EtHex device with the bulkier side chains is less effec tive in carrier generation. To further verify that the CT states in the Oct device are indeed more delocalized and are more effective in the exciton dissociation process , we performed the transient photoluminescense (PL) measurements. The results was show n in Figure 5 4 ( b ) . For
95 the Oct devices, an fast decay component were observed which made up 92% of the signal strength with a lifetime of 174 ps followed by a slower decay component with a longer lifetime of 733 ps. [ 91 ] On the other hand, the EtHex device showed l ower lifeti me than that of the Oct device while still having a fast decay component. H owever, the lifetime is longer at 215 ps which take up only 80% of the decay intensity and a lifetime of 811 ps for the slow decay component . Despite significant differen ce as observed in Chapter 3 and C hapter 4, we did observed a faster decay time from the Oct device than that of the EtHex device . The small difference in the PL transient is possibly being a result of an identical polymer back bone structure. The transient PL result is a confirmation to the previous results that the Oct does possess a more delocalized CT state than that of the EtHEx device. 5.4.4 Geminate Recombination As a result of less delocalized CT states in the EtHex device presented previously by th e PSR and transient PL data, we expect to see a higher geminate recombination from the EtHex device . In order to determine the difference in geminate recombination, we look at percent increase of EQE comparing the EtHex device and the Oct device as a result of increasing electric field. U nder high electric field, we expect to extract all of the excitons lost due to geminate recombination through field assisted exciton dissociation process. [ 92 ] The result presented in Figure 5 5 ( a ) shows that the EtHex device have higher % in crease of 6% than that of the Oct device of 5%, indicating higher geminate recombination rate in the EtHex devices as a more localized CT states was found. We futher looked at the photo shunt of the device under illumination to verify the geminate recombi nation rate between the EtHex and the Oct devices . [ 93 ] The photo -
96 shunt ident ify the loss of current due to geminate recombination as it derives the shunt resistence under illumination at the short circuit current condition. As the voltage increases toward the open circuit condition, the dominant loss mechanism of the Jsc becomes b imolecular recombination as the internal field reaches zero. [ 94 ] Therefore, we can compare the ratio of geminate recombination and bimolecular recombination base d on the different area presented in Figure 5 5 ( b ) . The results show that the Oct device has higher photo shunt, indicative of lower geminate recombination. On the other hand , the photo shunt for the EtHex device is much lowe r than the Oct device, consistent with previous PSR and percent increase EQE result that higher order of geminate recombination is presented in the EtHex device. [ 95 ] Also, the shaded area also showed that ratio of geminate to nongeminate recombination to each device. We see that both devices contains nongeminate recombination as the internal field reaches zero. In conclusion, t he linear side chain of the Oct device promote the polymer/fullerene interactions and formed a more delocalized CT states while the bulkier side chain of the EtHex device hamper the polymer/fullerene interaction through higher steric hindrance due to its b ulkier properties . 5.4.5 Side Chains, Stacking, Recombination and Transport Properties Despite the difference in the delocalization of the CT states between the EtHex and the Oct devices, such significant difference in overall device performance cannot be attributed to the exciuton dissociation alone. A significant difference in the device FF from 51% to 60% for the EtHex and the Oct device respectively indicates that the transport properties were different between these two polymer with different side cha ins. [ 96 ] To look at the effect of recombination from these two devices, we perform the
97 illumination dependency of the Jsc to see whether the device is bimolecular dependent. Since Jsc can have a correlation to illumination intensity by: ( 5 1 ) W is the slope of Jsc vs. illumination intensity in the log log scale . The device which is dominated by the bimolecular recombination should have a deviating from 1 while the device that is not dominated by the bimolecular recombination should have a close to 1. [ 97 ] The result was shown in Figure 5 6 . For the Oct device, a slope of 0.993 indicates that the bimolecular recombination is not the domina nt factor. On the other hand, the EtHex device has a slope of 0.951 which deviates from 1 , indicating that the bimolecular recombination is the dominant factors in this device . In order to realize difference in the transport properties between these device s which could be the reason for different order of bimolecular recombination, we perform the space charge limited current measurement under varying temperature for the O c t and the EtHex devices. [ 98 ] As discussed previously, the J V characteristic at different temperature were first fitted by the Mott Gurney law to extract the mobility number from each temperature measured. The mobility dependency with different temperature was then fitted with Gaussian Disorder Model as described by the following equatio Gaussian Disorder Model describes the transfer between molecule participating in the transport and that that the energy of these charge transporting elements can be described by a Gaussian distribution. Based on this argument we can get the relationship between the carrier mobility and the energetic disorder by the following equation n : ( 5 2 )
98 W is carri er mobility at infinity temperature , k is the B oltzmann constant and is the energetic disorder . Since the only difference between the Oct and the EtHex device is in the side chains of the polymer and that the polymer domain is responsible for transport ing holes in a bulk heterojunction solar cells, we only look at the hole transport properties of the Oct and the EtHex devices. Interestingly , the Oct devi ce does show a lower mobility of 1.5*10 5 cm 2 /Vs and a higher energetic disorder of 81 meV than the EtHex device of 1.4*10 5 cm 2 /Vs and an energetic disorder of 65 meV. These results are consistent with the illumination intensity dependency of the Jsc whic h shows higher bimolecular recombination for the EtHex device. We have shown that the EtHex device with bulkier side chain shows worse transport properties accompanying higher recombination and the Oct device with the linear side chain with better transpor t properties and lower recombination. Since the only difference between these two polymers was the side chains, we try to correlate the effect of side chains on the transport properties. Previously, it has been shown that the stacking in a polymer/full erene blends can be affected by the side chains of the poly mer . [ 99 ] For the polymer we studied here , 2D grazing incidence X ray scattering (GIXS) for the linear side chains and the branched side chains has been reported previously to have different stacking distance as well as structural disorder reported by Piliego et al. [ 89 ] For the film with EtHex side chains blended with the fullerene, a longer stacking distance of 3.8 Ã… is observed. As for the Oct poly mer blended with fullerene, a shorter packing distance of 3.6 Ã… is observed. Also, full width half maximum which indicate the structural disorder by the distribution of stacking distance shows that the EtHex polymer blended with fullerene also have higher structural disorder than the Oct
99 polymer blended with fullerene. Such difference in stacking distance and structural disorder can be a result of bulkier side chains for the EtHex polymer which not only repels fullerenes but pushes polymer backbones away fr om each other as well. As discussed previously, such difference in the stacking distance could affect the charge transfer process between the polymer backbones as described in Marcus theory. The larger FWHM induced by the EtHex side chain also causes a dif ferent charge transfer rate between the fullerene backbones for the hole transport. T he part where polymer backbones are closer have a higher charger transfer rate while the part where the polymer backbones are farther apart have a slower charge transfer r ate. When considered the mobility as a whole in the polymer domains, the place having slower charge transfer rate act as a trap site. Based on Gaussian Disorder model, it affects the transport and increases the energetic disorder for the hole transport between polymer domains. As the mobility for the ethylhexyl side chain goes downs, so is the recombination rate as well as the device FF for the EtHex device. [ 100 ] To better describe such phenomenon , schematics were shown for the EtHex device and the Oct device which describes the structural disorder and stacking distance which can be translated into energetic disorder which were presented in the previous paragraph . [ 101 ] 5.4.6 V oc and C arrier C oncentrations Finally, we surveyed into the difference in Voc between the Oct and the EtHex devices. The origin of Voc is from the Fermi level splitting as a result of electron and hole concentration s in the device. Therefore, the Voc is pr oportional to the nature log of the electron and the hole concentration and can be described as follows: [ 102 ]
100 ( 5 3 ) Where n and p are the electron and hole concentrations respectively, k is the B oltzmann constant , n i is the intrinsic carrier concentration, T is the temperature in Kelvin and q is the elementary charge. As described in Eq. 5 3, the higher Voc of the EtHe x devices would suggest higher carrier concentration in the devices which cannot be swept out of the device . A lso , the recombination rate in the device can be described as the recombination prefactor times the electron and hole concentration as : [ 103 ] ( 5 4 ) W here k is the Langevin recombination prefactor and np is the electron and hole carrier concentration within the device. Since the recombination prefactor k is dependent on the chemical properties and that the backbones of the EtHex and the Oct devices are the same , we can assume that the recombination k for the EtHex and the Oct device are the same. W e have already shown that the EtHex has higher bimolecular recombination rate R and the k is the same for both devices. We can conclude that the carrier concentration, np, is higher for the EtHex devices. This can be attributing to higher carrier concentration in the device and be attributed to higher Voc as described in F igure 5 3. 5. 5 Summary In this report , we investigated the e ffect of the polymer's side chains on delocalization of CT states as well as transport properties for polymer solar cells with linear and bulkier side chains on the acceptor moiety of the PBDTTPD pollymers . We show ed that the linear side chain version of the polymer have a better interaction with the fullerene and a higher carrier generation from the CT states which was shown in
101 PSR results while the branched side chain showed lower carrier generation due to stronger steric hindrance . Theses phenomenon can be described by the interaction betwe en the polymer and the fullerene with linear or bulkier side chains. Furthermore, we showed that with percent increase EQE with increasing internal field as well as measurement of photo shunt, a higher geminate recombination is observed for the EtHex devic es, consistent with the PSR and transient PL result that a less efficient CT states exist in the EtHex devices. Apart from the effect of side chains on the delocalization of CT states, we also try to correlate the effect of side chains in terms of transpor t properties. We found that bulkier side chains induce longer stacking and higher structural disorder as the bulkier side chains pushes the polymer backbone away. This difference in structural disorder and stacking distance can be translated in to tran sport properties as shown in the SCLC measurements. Such difference in the transport properties than causes a higher bimolecular recombination rate and deteriorate the device FF. Therefore, we conclude that while branched side chains have been commonly use d to increase solubility; its effect on carrier generation as well as transport properties has to be taken into account in order to achieve higher efficiency solar cells.
102 Figure 5 1 . Chemical structure of (a) PBDT(EtHex) TPD(Oct) (b) PBDT(EtHex) TPD( EtHex). Figure 5 2 . AFM topography images for (a) PBDT(EtHex) TPD(Oct):PC 70 BM and (b) PBDT(EtHex) TPD(EtHex):PC 70 BM. The Root mean square roughness for the PBDT(EtHex) TPD(Oct): PC 71 BM is 7 nm and PBDT(EtHex) TPD( EtHex ): PC 71 BM is 8 nm .
103 Figure 5 3 . Solar cell perfromances for PBDT(EtHex) TPD(Oct): PC70BM and PBDT(EtHex) TPD(EtHex):PC70BM (a) Current density voltage (JV) characteristics for PBDT(EtHex) TPD(Oct): PC 70 BM and PBDT(EtHex) TPD(EtHex ):PC 70 BM. (b ) External quantum efficiencies for PBDT(EtHex) T PD (Oct):PC 70 BM and PBDT(EtHex) TPD(EtHex):PC 70 BM devices.
104 Figure 5 4 . Exciton dyanamics for for the Oct and EtHex devices (a) PSR spectra fo r the Oct (red) and EtHex (black) device s. (b) Transient PL decays for Oct (red) and EtHex (black) devices .
105 Figure 5 5 . Geminate recombination for for the Oct and EtHex devices a) Percent increase in EQE under applied electric field measured at 535 nm. (b) J V curves for the EtHex (black) and Oct (red) devices where the v oltage dependent geminate recombination (GR) losses are shown in solid colors and bimolecular recombination losses are represented b y the shaded areas .
106 Figure 5 6 . Jsc Vs. I fitted for recombination for the EtHex (black) and Oct (red) devices Figure 5 7 . Zero field mobilities Vs square of reciprocal temperature for EtHex (black) and Oct (red) devices . Energetic disorder can be determined by the slope.
107 Figure 5 8 . Schematic representation of the impact of the difference in side chains on the transport for (a) PBDT (EtHex) TPD (Oct ) and (b) PBDT(EtHex) TPD(EtHex) .
108 Table 5 1 . Summary of average device characteristics for the PSCs fabricated in this study. Polymer/PC 70 BM Jsc [mA cm 2 ] Voc [V] FF [%] PCE [%] PBDT(EtHex) TPD(Oct) 11.1 0.89 60 5.9 PBDT(EtHex) TPD(EtHex) 3.9 0.98 51 1.95 Table 5 2 . Zero field mobility and energetic disorder data for hole transport in the Oct and EtHex devices . Polymer/PC 70 BM o [cm 2 /Vs] holes [meV] PBDT(EtHex) TPD(Oct) 1.4*10 4 81 PBDT(EtHex) TPD(EtHex) 1.6*10 5 65
109 CHAPTER 6 CONCLUSION AND FUTURE WORKS 6.1 Concluding R emarks I n t his dissertation, the delocalization of charge transfer excitons in polymer: fullerene bulk heterojunction solar cells are discussed from the aspect of molecular structure, process condition as well as side chain engineering. Various experimental techniques were applied to probe the exciton dissociation, charge transfer state formation , cha rge transport, recombination and etc. to explain the solar cell performances in terms of J sc , V oc and FF . Due to the excitonic nature of organic photovoltaics, the excitons dissociation has to rely on external driving force other than just thermal excitati on as in the case of inorganic solar cells. The interface where CT states located became substantial in order to have effective exciton dissociation. Not only did it affect the current output of photovoltaic cells, the lower manifolds of CT states also decide the maximum V oc as it acts as the effective bandgap in BHJ solar cells. While exciton dynamics have been studied thoroughly that the CT states mediate the photo induced charge transfer process to become free carriers, the factors affecting charge transfer state formation is still not clear. As shown throughout this dissertation, we attempt to correlate the delocalization of CT excitons to different methodology such as molecular structure and process conditions. As shown in Chapter 3, we see that the difference in thiophene length in the backbone of the donor polymer resulting in different miscibil ity to the fullerene and result ed in different effective CT states position. The result not only affect s the current output , but show that the V oc indeed is affected by the lower CT manifolds. In Chapter 4, we see that although no significant difference in morphology is observed,
110 the hi gher delocalized CT states are still observed in the annealed case. This result indicates that the macroscopic morphology is not the factor deciding the D/A interaction at the interface but microscopic interaction between D/A. The thermal annealing process provides favorable D/A interaction and resulting in favorable microscopic morphology for exciton dissociation. In Chapter 5, we look further into side chain engineering in PBDTTPD polymers since alteration of side the energetics in the backbone and we can focus on microscopic D/A interactions. Indeed, we saw a higher current output as a result of more delocalized CT states. We showed that a separation in scale of Ã… could make a difference for exciton dissociation efficiency. Delocali zation of CT states i s not the only factors affecting the solar cell performances. The non geminate type of recombination is another important factor detrimental to the photovoltaic processes. In Chapter 4, we see that the Shockley Read Hall recombination reduces the overall power generated after thermal annealing. After thermal annealing, mid gap traps were formed in the mid gap, resulting in higher degrees of energetic disorder. The formation of mid gap traps resulted in more monomolecular recombination a nd lost carriers through the recombination process. In Chapter 5, we further showed that the side chains have an effect on charge transport properties as well. The difference in side chains affects the stacking distribution which alters the energetic d isorder. The increase in energetic disorder results in lower carrier mobility which in turn increases the chances for carriers to recombine. 6.2 Future W orks Despite recent advances in novel organic photovoltaic materials, especially the perovskite solar cells [ 104 ] , the organic p hotovoltaics have a promising future for solar harvesting if higher efficiency and a longer lifetime is achieved. In order to compete with
111 other photovoltaic materials, advanced in chemistry design for high efficiency polymer and understanding of degradati on is required. In order to help chemist in material design, better understanding of factors affecting morphology, exciton dynamics and transport at the same time is necessary as we have shown that one deficiency would be detrimental to the cell performan ce. W e showed that the microscopic interaction between D/A affects the CT states delocalization . In order to further study the D/A interactions, a careful selection of polymer blends with the ability to control D/A separation will be helpful in looking insight into D/A interaction. For example, fatty acid methyl ester as additives has been shown to be able to control the D/A separat ion by using fatty acid methyl ester as additives. [ 105 ] It has one end that is favorably interact ing with fullerene and the other end with polymer. By changing the length of the fatty acid additives, one can control the D/A separation and studied the effect of CT exciton delocalization. Combining with quantum chemistry calculation, o ne should be able to have a clear picture of D/A interaction and CT exciton dynamics at D/A interfaces. Research on the degradation mechanism is equally important in order for OPV to be comparable to other solar harvesting technologies. Despite the notorious degradation of the LiF/Al electrode, the degradation mechanism in optical absorption and carrier transport should be studied . By combining PDS and PSR, we should be able to obtain internal quantum efficiency and get to know whether th e energetics have changed due degradation or the exciton dynamics being affected. Fitting temperature dependent J V with Poole Frenkel law which monitors the zero field mobility and energetic disorder can provide information about the influence of degradat ion to
112 energetics in transport band and the intrinsic mobility which provide s information such as orbital overlapping and chain length in the polymer backbone .
113 APPENDIX A LIST OF MOLECULE STRUCUTRES PC 61 BM PC 71 BM P(iI T) P(iI T3) PBDT(EtHex) TPD(EtHex ) PBDT(EtHex) TPD(Oct) N O N O S S C 6 H 1 3 C 8 H 1 7 C 8 H 1 7 C 6 H 1 3 S n
114 PCDTBT PTB7 P3HT PEDOT:PSS
115 APPENDIX B LIST OF PUBLICATIONS AND CONFERENCE PRESENTATION Peer Reviewed Publications 1. T zung Han Lai, S ai W ing Tsang, J esse R. Manders, S ong Chen, F ranky So , Properties of interlayer for organic photovoltaics Today, 2013, 16, 424 2. I. Constantinou, T zung Han Lai, D ewei Zhao, E rik Klump , J ames J. Deininger, C hi K in Lo, J ohn R. Reynolds , F ranky So High efficiency air processed dithienogermole based polymer solar cell , 2015 3. D o Y oung Kim, T zung Han Lai , J aewoong Lee, J ohn R. Reynolds , F ranky So , Multi Scientific Reports , 2014, 4, 5946 4. J esse R. Manders , T zung Han Lai , Y anbin An, W eikai Xu, J aewoong Lee, D o Y oung Kim, G ijs Bosman, F ranky So Low Noise Multispectral Photodetectors Made from All Solution Processed Inorganic Semiconductors Functional Materials, 2 014 , 24, 7605 5. S. Chen, S ai W ing Tsang , T zung Han Lai, J ohn R. Reynolds , F ranky So , Dielectric Effect on the Photovoltage Loss in Organic Photovoltaic Cells Advanced Materials, 2014, 26, 6125 6. J esse R. Manders , S ai W ing Tsang , M ichael J. Hartel, T zung Han Lai , S ong Chen, C had M. Amb, J ohn R. Reynolds , F ranky So Solution Processed Nickel
116 Oxide Hole Transport Layers in High Efficiency Polymer Photovoltaic Cells Advanced Functional Materials, 2013, 23, 2993 7. S ong Chen, C ephas E. Small, C had M. Amb, J egadesan Su bbiah, T zung Han Lai, S ai W ing Tsang, J esse R. Manders, J ohn R. Reynolds , F ranky So Inverted Polymer Solar Ce lls with Reduced Interface Recombination Materials, 2012, 2, 1333. 8. C ephas E. Small, S ong Chen, J egadesan Subbiah, C had M. Amb, S ai W ing Tsang , T zung Han Lai, J ohn R. Reynolds , F ranky So High efficiency inverted dithienogermole thienopyrrolodione based polymer solar cells Photonics, 2012, 6, 115 Oral Presentation in Conferences 1. T zung Han Lai, C aroline M. Grand, S ujin Baek, I ordania Constantinou, E rik Klump, , S ai W ing Tsang, J ohn R. Reynolds, F ranky So , Direct Measurement of Hot CT States in Isoindigo Polymer Systems for Polymer Bulk Heterojunction Solar Cells by CMEAS . MRS 2014 spring meeting, San Francisco, FF 1.10 2. Erik R. Klump, Tzung Han Lai , Franky So, Using Forster Resonance Energy Transfer to Extend Spectral Harvesting in Polymer Solar Cells . SPIE 2014 Optics and Photonics, San Diego, 9184 14 3. Do Young Kim, Tzung Han Lai, Jaewoong Lee, Jesse R. Manders, Franky So, Recent Progress in Up Conversion Devices , SPIE 2014 Optics and Photonics, San Diego, 9185 214
117 4. T zung Han Lai , Song C he n , S ai W in g Tsang, F ranky So , Direct Measurement of the Effective Bandgap in Polymer Solar Cells . SPIE Optics and Photonics 2013, San Diego, 8830 33 5. Jesse R. Manders, Tzung Han Lai, Jaewoong Lee, Franky So, I nfrared Photodetectors from All Solution Processed Inor ganic Semiconductors . SPIE Optics and Photonics, 2013, San Diego, 8831 220 6. Jesse R. Manders, Sai Wing Tsang, Song Chen, Tzung Han Lai, Michael J. Hartel, Chad M. Amb, Kyukwan Zong, James J. Deininger, John R. Reynolds From Conception to Sol ar Cells: Solution Procssed Nickel Oxide Hole Transport Layers in High Efficnecy Organic Photovoltaics spring meeting, San Francisco, J15.3 7. Sai Wing Tsang, Cephas E. Small, Song Chen, Jegadesan Subbiah, Chad M. Amb, Tzung Han Lai, John R. Reyno 8% Power Conversion Efficiency Polymer Photovoltaic Cells with a Thick Layer of Dithienogemole Thienopyrrolodione:Fullerene 8. Cephas E. Small, Song Chen, Jegadesan Subbiah, Chad M. Amb, Sai Wing Tsa ng, Tzung High Efficiency Inverted Dithienogermole Thienopyrrolodione Based Polymer Solar Cells 2012 spring meeting, Boston V6.8 9. Song Chen, Cephas E. Small, Chad M. Amb, Tzung Han Lai, Kenneth R. Graham, Joh Achieving Higher than 8% Power
118 Conversion Efficiency by Modified Electron Extraction Layer in Polymer Bulk Heterojunction Solar Cells Poster Presentation in Conferences 1. T zung Han Lai, C aroline M. Grand, S ujin Baek, I ordania Constantinou, E rik Klump, , S ai W ing Tsang, J ohn R. Reynolds, F ranky So , Direct Measurement of Hot CT States for Polymer Bulk Heterojunction Solar Cells by CMEAS . SPIE 2014, San Diego , 9184 92 2. Iordania Constantino u, Tzung Han Lai, Jesse R. Manders, Sujin Baek, Jimmy J. Deininger, Caroline Grand, John R. Reynolds, Franky So, Air Processed Polymer Solar Cells with Higher than 5% Efficiency MRS Spring 2014, San Francisco, FF9.40 3. Song Chen, Cephas E. Small, Chad M. Amb, Tzung Han Lai, Kenneth R. Achieving Higher than 8% Power Conversion Efficiency by Modified Electron Extraction Layer in Polymer Bulk Heterojunction Solar Cells 24. 4. Song Chen, Cephas E. Small, Jegadesan Subbiah, Chad M. Amb, Sai Wi ng Tsang, Tzung Han Lai, Jesse R . Manders, John R. Reynolds and Franky So Inverted solar cells with reduced interface recombination meeting, San Francisco, Z7.26. 5. Jese R. Manders , Sai Wing Tsang, Song Chen, Tzung Han Lai, Michael J. Hartel, Chad M. Amb, Kyukwan Zong, James J. Deininger, John R. Reynolds
119 Low Temperature Solution processed Nickel Oxide Hole Transort Layers in High Efficiency Organic Photovoltaics S 2012 spring meeting, San Francisco, W4.26. 6. Tzung Han Lai, Do Young Kim, Jaewoong Lee, Dong Woo Song, Jiho Ryu, Jesse R. Manders, Franky So, Hybrid Infrared to Visbile Up Conversion Device ICSM 2012, Atlanta,
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127 BIOGRAP H ICAL SKETCH Tzung Han Lai was born in a loving family in Taiwan which is a lovely island in the Pacific Ocean in 1986. In 2004, he graduated from Chang Jung High School and attended National Tsing Hua University which is one of the elite universities in Taiwan. There, he majored in materials science and engineering where he acquired interest a nd knowledge in the materials science. In 2008, he graduated with a Bachelor of Science degree from Department of Materials Science and Engineering and spent a year serving his country right after graduation. In order to pursuit his enthusiasm in materials science and especially electronic materials, he decided to join University of Florida for his Ph.D. studies in 2009 in the highly respected Department of Materials Science and Engineering. He then joined Professor Franky So s group and started his studies of organic e lectronics materials and physics . Throughout his Ph.D., he worked on novel organic light emitting devices (OLEDs), inorganic nanocrystal based photodetectors and organic solar cells (OSCs). He focused his research on device physics in OSCs especially in ch arge transfer (CT) states and exciton dynamics at the CT states. During his 5 year studies, several peer reviewed papers were published in three different disciplinary areas including OSCs, photodetectors and IR sensitized OLED. 5 presentations, including 2 oral presentations were given at the conferences such as the Materials Research Society (MRS), SPIE O ptics and Photonics , and International Conference on Science and Technology of Synthetic Metals (ICSM). A U.S. pa tent application is also filed from the invention among his research. In April 2015, Tzung Han was awarded Doctor of Philosophy in the Department of Materials Science and Engineering at the University of Florida.