Design and Synthesis of Isoindigo-Based Conjugated Polymers for High Performance Polymer Solar Cells

학위논문 (박사)-- 서울대학교 대학원 : 재료공학부, 2016. 2. 조원호.For high-performance polymer solar cells (PSCs), the molecular engineering of conjugated polymer is significantly important, because the chemical structure of the polymer directly influences on the optoelectronic properties including optical bandgap, extinction coefficient, frontier energy levels, molecular dipole moment, and charge carrier mobility. Optical properties such as bandgap and absorptivity are key factors in conversion process of solar energy to electrical energy. And electronical properties such as energy level, dipole moment, and mobility are related to generation and collection of the free charge carriers, which are separated to electrons or holes from photons. Moreover, physical properties such as crystallinity, solubility and miscibility with fullerene derivatives are conclusively determined by chemical structure of the polymer. To achieve the suitable properties of conjugated polymers, the most efficient strategy is the design of push-pull copolymer, which is copolymerized with alternatively electron-rich monomer and electron-deficient monomer. This method induces intramolecular charge transfer between two units in polymer backbone, resulting in redistribution of molecular orbitals (MO) of each unit. This hybridization of MOs generates new occupied and unoccupied MOs with a narrow bandgap of the polymer. Furthermore, the appropriate combination of electron-rich moiety and electron-deficient moiety can fine-tune other properties for achieving highly efficient PSCs. In this thesis, we studied the conjugated polymers containing isoindigo dye as electron-deficient moiety in push-pull copolymer system. For high-performance PSCs, we designed and synthesized three kinds of novel isoindigo-based copolymers by modifying bithiophene as electron-rich moiety and investigated the photovoltaic performance of the PSCs. First, a conjugated polymer consisting of isoindigo and thieno[3,2-b]thiophene (TT) was synthesized via the Stille coupling reaction in order to demonstrate the effect of molecular fusion from bithiophene to TT on the properties of polymers and those photovoltaic performance of PSCs. Incorporation of TT as electron-rich moiety on conjugated polymer manipulates its photophysical and electrochemical properties, including to lowered optical band gap (1.55 eV) in UV-visible absorption spectra and deeper HOMO energy level (5.44 eV) determined by cyclic voltammetry because of the larger resonance stabilization energy of the fused ring. Furthermore, the polymer with TT shows highly ordered crystallinity along stacking direction, leading to high charge carrier mobility in photoactive layer. PiITT consisting of isoindigo and TT exhibits a power conversion efficiency (PCE) of 6.96% with a short-circuit current density (JSC) of 12.42 mA cm2, an open-circuit voltage (VOC) of 0.91 V and a fill factor (FF) of 0.67 when the PSCs were fabricated from the blend of the polymer and PC71BM. Secondly, highly -extended isoindigo-based copolymer consisting of isoindigo and thienylvinylene (TVT) was designed and synthesized for strong interaction with large overlapping area between chain backbones due to rotational freedom between consecutive aromatic units. Although introduction of TVT in isoindigo-based polymer does not effect on optical band gap and electronic energy levels, the polymer PiITVT composed of isoindigo and TVT shows higher absorption coefficient and highly ordered crystallinity in photoactive layer. Moreover, in optimized condition for PSCs, PiITVT has preferential molecular orientation (face-on orientation) when it was demonstrated by grazing incident wide angle X-ray scattering measurement and the well-developed morphology with finer fibril of PiITVT when it was observed by transmission electron microscopy. PiITVT-based PSCs shows a high PCE of 7.09% with a JSC of 13.2 mA cm2, a VOC of 0.91 V and FF of 0.59. It is higher than that of PiI2T containing bithiophene (5.55%), because of the better coplanar structure of the TVT unit than the bithiophene unit. Third, the fluorinated isoindigo-based polymer was designed and synthesized by combining isoindigo with bulkier alkyl side chain and fluorinated bithiophene for high-performance PSCs. And the polymer are soluble in non-halogenated solvent such as o-xylene. The use of non-halogenated solvents such as o-xylene for fabrication of high performance PSCs has recently attracted much attention from academia and industry, because the halogenated solvents and additives cause serious environmental, health and safety problems. When the fluorinated polymer-based PSC is fabricated with o-xylene as processing solvent and diphenyl ether as additive, the cell exhibits a superior PCE of 8.80% with a VOC of 1.06 V, which are one of the highest values among PSCs processed with non-halogenated solvents. Our work successfully demonstrates that the combination of introduction of bulky alkyl side chain and substitution of fluorine atom on the conjugated polymer backbone is a promising strategy for eco-friendly device fabrication and highly efficient PSCs with high VOC. We revealed that three kinds of modification methods which are introduced in this thesis are promising design strategies for high-performance isoindgo-based PSCs and how each method achieves improved performance.Chapter 1. Introduction 1 1.1 Polymer solar cells 1 1.1.1 Motivation of research 1 1.1.2 Operating principles and device architectures 4 1.1.3 Characterization of the polymer solar cells 9 1.1.4 Determinant factors for efficient polymer solar cell 10 1.2 Design principles of the conjugated polymers 14 1.2.1 Band gap engineering 14 1.2.2 Tuning of frontier energy levels 17 1.2.3 Polymer backbone coplanarity 20 1.2.4 Alternative push-pull copolymers 21 1.3 Isoindigo 25 1.3.1 Characterizations of isoindigo 25 1.3.2 Isoindigo-based polymers for polymer field-effect transistors 27 1.3.3 Isoindigo-based polymers for polymer solar cells 30 1.4 Objectives of this study 35 Chapter 2. Experimental Section 39 2.1 Synthesis and characterization 39 2.1.1 Materials 39 2.1.2 Synthesis of monomers and conjugated polymers 39 2.1.2.1 Synthesis of PiI2T-OD and PiITT 39 2.1.2.2 Synthesis of PiITVT 45 2.1.2.3 Synthesis of PiI2T-DT and PiI2fT 49 2.1.3 Characterization methods 53 2.2 Device fabrication and measurements 55 2.2.1 Materials 55 2.2.2 Solar cell device fabrication 55 2.2.3 Solar cell performance measurements 56 Chapter 3. Results and Discussion 58 3.1 1H nuclear magnetic resonance spectra of the monomers 58 3.2 The polymer composed of thieno[3,2-b]thiophene and isoindigo for high-performance polymer solar cells 66 3.2.1 Background 66 3.2.2 Optical and electrochemical properties 69 3.2.3 The optimized chemical geometry and crtstallinity 72 3.2.4 Photovoltaic properties 75 3.2.5 Summary 80 3.3 The polymer composed of thienylvinylene and isoindigo for high-performance polymer solar cells 81 3.3.1 Background 81 3.3.2 Optical and electrochemical properties 84 3.3.3 Photovoltaic properties 88 3.3.4 Crystallinity and molecular orientation 93 3.3.5 Morphology of active layers 94 3.3.6 Summary 98 3.4 The polymer composed of 3,3-difluoro-2,2-bithiophene and isoindigo for high-performance polymer solar cells 100 3.4.1 Background 100 3.4.2 Optical and electrochemical properties 104 3.4.3 Photovoltaic properties 109 3.4.4 Morphology of active layers 112 3.4.5 Crystallinity and molecular orientation 113 3.4.6 Summary 115 Chapter 4. Conclusion 117 Bibliography 120 Korean Abstract 141Docto

The π-extended isoindigo-based low bandgap polymer for high-performance polymer solar cells

Recently, low bandgap conjugated polymers have attracted considerable attention for polymer solar cells (PSCs). The desirable design directions for the polymers are that those should have deep HOMO energy levels, broad absorption and balanced crystallinity. Isoindigo, (E)-1H,1H-[3,3]biindolylidene-2,2-dione, which is one of the indigoid natural organic dyes is a symmetrical molecule containing two indolin-2-one units. Those contribute towards strong electron-withdrawing characteristic and planar π-conjugated structure of the molecule. Furthermore, isoindigo-based organic materials have broad absorption spectra, high extinction coefficients, and very deep HOMO energy levels which are closely related to open circuit voltage for PSCs. In this work, we present highly π-extended isoindigo-based alternating conjugated copolymer with 6.5% PCE. To the best of our knowledge, its photovoltaic performance is the best value reported to date for PSCs using isoindigo-based polymers.OAIID:oai:osos.snu.ac.kr:snu2013-01/104/0000001236/14SEQ:14PERF_CD:SNU2013-01EVAL_ITEM_CD:104USER_ID:0000001236ADJUST_YN:NEMP_ID:A004558DEPT_CD:445CITE_RATE:0FILENAME:0411 12_정의혁.pdfDEPT_NM:재료공학부EMAIL:[email protected]:

Effect of Different Charcogenophenes in Isoindigo-based Conjugated Copolymer on Photovoltaic Properties

New low bandgap conjugated copolymers composed of isoindigo as electron-deficient unit and various chalcogenophenes (thiophene, selenophene and tellurophene) as electron-rich unit were synthesized (as denoted by PIT, PISe and PITe, respectively) to investigate the effect of different chalcogenophene on the photovoltaic properties of isoindigo-based copolymers. The copolymers (PISe and PITe) show bathochromic shift in UV-Vis absorption and lower LUMO energy level as compared to its thiophene analogue (PIT). The solar cell device based on PISe blended with PC61BM exhibits a promising power conversion efficiency (PCE) of 5.72% with a JSC of 10.21 mA cm?2, which is higher than those of PIT (PCE of 3.98% and JSC of 8.34 mA cm?2), while the device based on PITe shows lower JSC and PCE than those of PIT and PISe because of its coarse morphology of the blend.OAIID:oai:osos.snu.ac.kr:snu2014-01/104/0000001236/2SEQ:2PERF_CD:SNU2014-01EVAL_ITEM_CD:104USER_ID:0000001236ADJUST_YN:NEMP_ID:A004558DEPT_CD:445CITE_RATE:0FILENAME:2014춘계고분자학회_배승환.pdfDEPT_NM:재료공학부CONFIRM:

High Performance n-Channel Organic Field-Effect Transistor of Conjugated Polymers with Fluorine-Substituted Phenylene Unit

Numerous organic semiconductors have been developed to replace conventional inorganic ones. Particularly, conjugated organic polymers have attracted much interest because of their unique properties including solution processability, flexibility and low-cost mass production with fast and large area printing. In the past decade, semiconducting conjugated polymers have contributed to significant progress in organic electronics, especially field effect transistors, light-emitting diodes and organic photovoltaics. Recently, semiconducting polymers based on diketopyrrolopyrrole (DPP) units have emerged as promising active materials for the optoelectronic applications such as organic field-effect transistors (OFETs) and organic solar cells due to its high molar absorptivity and high hole mobility. Although various DPP-based conjugated polymers have recently been reported to show high hole mobilities over 1 cm2V?1s?1 in OFETs, conjugated polymers with n-channel charge carrier transport are scarce because most conjugated polymers usually exhibit p-channel dominant transport characteristics. Herein, we present novel conjugated polymers composed of DPP and fluorinated phenylene for n-channel dominant transport in OFETs. For the purpose, we synthesized a series of alternating copolymers (PDPPnFP) based on thiophene end-capped diketopyrrolopyrrole (DPP) and fluorine-substituted phenylenes (nFT where n=1, 2, 4). Here, n represents the number of fluorine substitution on phenylene unit. Substitution of fluorine has frequently been used to modify the electronic properties of conjugated polymers because the fluorine atom has the highest Pauling electronegativity and its small atomic size hardly induces the steric hindrance for configuration and molecular packing of the polymer. All PDPPnFP show similar optical absorption spectra with nearly identical band gaps of about 1.5 eV, and their cyclic voltammograms indicate that molecular orbital energy levels (both LUMO and HOMO) are lowered stepwise as the number of fluorine substitution on phenylene unit increases. All the fluorinated polymers (PDPPnFP) exhibit ambipolar properties for OFETs with much enhanced charge carrier mobilities compared to non-fluorinated polymers (PDPP0FP). In particular, PDPP4FP which has tetra-fluorophenylene unit exhibits an outstanding electron mobility, 1.7 cm2V-1s-1, which is among the highest electron mobility in OFETs using conjugated polymers to the best of knowledge. The optoelectronic, electrochemical, and charge carrier transport characteristics of the PDPPnFP are also discussed in terms of the number of fluorine substitution.OAIID:oai:osos.snu.ac.kr:snu2012-01/104/0000001236/27SEQ:27PERF_CD:SNU2012-01EVAL_ITEM_CD:104USER_ID:0000001236ADJUST_YN:NEMP_ID:A004558DEPT_CD:445CITE_RATE:0FILENAME:MRS 정의혁.docxDEPT_NM:재료공학부EMAIL:[email protected]:

Enhanced Performance of Polymer Solar Cells with PSSA-g-PANI/Graphene Oxide as Hole Transport Layer

Hole transport layer (HTL) has always been used for high performance polymer solar cells (PSCs) because of efficient hole extraction from active layer to anode and lowering the contact resistance between active layer and anode. Although PEDOT:PSS has commonly been used as a HTL, it has detrimental problems with low electrical conductivity, acidic nature and large aggregates. As an alternative to PEDOT:PSS, we previously synthesized poly(styrenesulfonic acid)-graft-polyaniline (PSSA-g-PANI) which is a self-doped conducting copolymer, and reported significant enhancement of the power conversion efficiency (PCE) of PSCs when it was used as a HTL of PSCs. In this work, we prepared PSSA-g-PANI/graphene oxide (GO) composite by adding GO into PSSA-g-PANI and used the composite as HTL of PSCs for the purpose to further improve the PCE. When the PSSA-g-PANI/GO (95/5) composite was used as HTL, the PCE of PSCs was improved by about 10% as compared to that without GO.OAIID:oai:osos.snu.ac.kr:snu2013-01/104/0000001236/12SEQ:12PERF_CD:SNU2013-01EVAL_ITEM_CD:104USER_ID:0000001236ADJUST_YN:NEMP_ID:A004558DEPT_CD:445CITE_RATE:0FILENAME:0411 12_박흥수.pdfDEPT_NM:재료공학부EMAIL:[email protected]: