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Design and Synthesis of Fluorinated Conjugated Polymers Based on 3,3-Difluoro-2,2-bithiophene for High Performance Polymer Solar Cells
고성능 고분자 태양전지를 위한 3,3-difluoro-2,2-bithiophene에 기반한 불소화 전도성 고분자의 설계 및 합성

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dc.contributor.advisor조원호-
dc.contributor.author조제웅-
dc.date.accessioned2017-07-13T05:43:42Z-
dc.date.available2017-07-13T05:43:42Z-
dc.date.issued2015-02-
dc.identifier.other000000024739-
dc.identifier.urihttps://hdl.handle.net/10371/117975-
dc.description학위논문 (박사)-- 서울대학교 대학원 : 재료공학부, 2015. 2. 조원호.-
dc.description.abstractElectronic energy level engineering of conjugated polymers including low-bandgap for harvesting a wide range of solar spectrum, deep highest occupied molecular orbital (HOMO) energy level for high open circuit voltage (VOC) and sufficient offset of lowest unoccupied molecular orbital (LUMO) energy levels between polymer donor and fullerene acceptor is a key strategy to achieve high performance polymer solar cells (PSCs) and several modification methods of chemical structure have been proposed to control energy levels of conjugated polymers.
Among these methods, introduction of fluorine atom has attracted much attention for the past few years because high power conversion efficiencies (PCEs) over 7% have been reported with fluorine substituted polymer-based solar cells. However, the effects of fluorination on optoelectrical and photovoltaic properties of polymers have not been revealed clearly and have been studied in only few polymer systems. For further investigation, more fluorinated monomers and polymers for PSCs should be designed and synthesized.
In this work, we synthesized difluoro-bithiophene as a new fluorinated building block in conjugated polymers for PSCs and various fluorinated copolymers based on difluoro-bithiophene are successfully designed and synthesized in order to clarify the effect of fluorination on the properties of polymers and its device performance of PSCs.
First, fluorinated poly(3,4-dialkylterthiophenes) (PDATs) composed of difluoro-bithiophene and 3,4-dialkylterthiophene are synthesized. Fluorination on polymer backbone changes its electronic structure, leading to deeper HOMO energy level and enhances molecular packing of the polymers as evidenced by strong vibronic shoulder in UV-Vis absorption spectrum and pi-pi stacking pattern in GIWAXS. When bulky side chain (ethylhexyl) are introduced as a solubilizing group, fluorinated polythiophenes develop finer fibril structure and exhibit a high PCE of 5.12% with a VOC of 0.87 V and a short circuit current density (JSC) of 9.82 mA/cm2.
Fluorinated DA type polymer, copolymerized by diketopyrrolo[3,4-c]-pyrrole (DPP) as an A unit and difluoro-bithiophene as a D unit, is also synthesized for investigating the fluorination effect on D-A type polymer. After introduction of fluorine atoms on DPP-based polymer, deeper HOMO energy level of polymer is observed without significant change of optical properties. Although fluorination increases fibril size and lowers JSC of DPP-based polymer, fluorinated DPP-based polymer exhibits higher PCE of 6.39% than non-fluorinated DPP-based polymer (PCE = 5.47%) due to large improvement of VOC.
Furthermore, for investigating the effect of fluorination position on the properties of D−A type polymer, two types of fluorinated polymers are synthesized, HF with fluorination on D unit and FH with fluorination on A unit. Compared to non-fluorinated polymer, fluorinated polymers exhibit deeper HOMO energy levels without change of bandgap and stronger vibronic shoulder in UV−Vis absorption spectra, indicating that fluorination enhances intermolecular interaction. HF exhibits a high PCE of 7.10%, which is higher than the PCE (6.41%) of FH, with well-developed fibril network, low bimolecular recombination and high hole mobility.
Finally, D−A polymers with different degree of fluorination are synthesized by using 2,1,3-benzothiadiazole (BT) unit substituted by different number of fluorine atoms. 3F with mono-fluorinated BT and 4F with di-fluorinated BT have deeper HOMO energy levels, stronger vibronic shoulder in UV-Vis absorption spectra, and narrower size of fibril in blend film with PCBM than 2F with non-fluorinated BT. Among fluorinated polymers, 3F exhibits the highest PCE of 7.92% with low bimolecular recombination, high hole mobility and well-developed interconnected network with nanoscale fibril.
From these results, it can be concluded that the optoelectrical and photovoltaic properties of conjugated polymer are significantly influenced by introduction of fluorine atoms and fluorinated conjugated polymers are promising materials for achieving high performance PSCs.
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dc.description.tableofcontentsChapter 1 Introduction 1
1.1 Polymer solar cells 1
1.1.1 Background 3
1.1.2 Operating principles and device structure 4
1.1.3 Active layer materials for polymer solar cells 7
1.2 Energy level control of conjugated polymer in active layer of PSC 11
1.2.1 Energy levels of conjugated polymers 11
1.2.2 Factors influencing energy levels of conjugated polymers
14
1.2.3 Alternating polymer copolymerized with electron-donating and electron-accepting units 16
1.2.4 Introduction of heteroatom in backbone 20
1.4 Objectives of this study 23

Chapter 2 Experimental Section 26
2.1 Synthesis and characterization 26
2.1.1 Materials 26
2.1.2 Synthesis of monomers and conjugated polymers 26
2.1.2.1 Synthesis of 3,3-difluoro-2,2-bithiophene and poly(3,4-dialkylterthiophene) polymers 26
2.1.2.2 Synthesis of polymer composed of 3,3-difluoro-2,2-bithiophene and diketopyrrolopyrrole 33
2.1.2.3 Synthesis of fluorinated or non-fluorinated alternating copolymers composed of 2,2-bithiophene and 2,1,3-benzothiadiazole 37
2.1.2.4 Synthesis of alternating copolymers composed of 3,3-difluoro-2,2-bithiophene and fluorinated 2,1,3-benzothiadiazole 41
2.1.3 Characterization methods 46
2.2 Device fabrication and measurements 48
2.2.1 Materials 48
2.2.2 Solar cell device fabrication 48
2.2.3 Solar cell performance measurements 49

Chapter 3 Results and Discussion 50
3.1 Fluorination of polythiophene derivatives for high performance organic photovoltaics 50
3.1.1 Synthesis and characterization 50
3.1.2 Optical and electrochemical properties 60
3.1.3 Computational simulation 64
3.1.4 Photovoltaic properties 68
3.1.5 Molecular orientation 69
3.1.6 Morphologies of active layers 75
3.1.7 Summary 78
3.2 Effect of fluorination in D-A type polymer based on diketopyrrolopyrrole for high performance solar cells 79
3.2.1 Synthesis and characterization 79
3.2.2 Optical and electrochemical properties 83
3.2.3 Computational simulation 87
3.2.4 Photovoltaic properties 91
3.2.5 Molecular ordering 95
3.2.6 Morphologies of active layers 95
3.2.7 Summary 98
3.3 Comparison of two D-A type polymers with each being fluorinated on D and A unit for high performance PSCs 99
3.3.1 Synthesis and characterization 99
3.3.2 Optical and electrochemical properties 106
3.3.3 Computational simulation 112
3.3.4 Photovoltaic properties 116
3.3.5 Molecular orientation 122
3.3.6 Morphologies of active layers 122
3.3.7 Summary 126
3.4 Influence of the degree of fluorination on DA type polymers for organic solar cells 127
3.4.1 Synthesis and characterization 127
3.4.2 Optical and electrochemical properties 136
3.4.3 Computational simulation 140
3.4.4 Photovoltaic properties 143
3.4.5 Molecular ordering 147
3.4.6 Morphologies of active layers 147
3.4.7 Summary 151

Chapter 4 Conclusions 154

Bibliography 157

Korean Abstract 172
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dc.formatapplication/pdf-
dc.format.extent6069012 bytes-
dc.format.mediumapplication/pdf-
dc.language.isoen-
dc.publisher서울대학교 대학원-
dc.subjectpolymer solar cells-
dc.subjectbulk-heterojunction-
dc.subjectconjugated polymer-
dc.subjectlow-bandgap-
dc.subjectfluorination-
dc.subjectorganic electronics-
dc.subject.ddc620-
dc.titleDesign and Synthesis of Fluorinated Conjugated Polymers Based on 3,3-Difluoro-2,2-bithiophene for High Performance Polymer Solar Cells-
dc.title.alternative고성능 고분자 태양전지를 위한 3,3-difluoro-2,2-bithiophene에 기반한 불소화 전도성 고분자의 설계 및 합성-
dc.typeThesis-
dc.contributor.AlternativeAuthorJea Woong Jo-
dc.description.degreeDoctor-
dc.citation.pagesxii-
dc.contributor.affiliation공과대학 재료공학부-
dc.date.awarded2015-02-
Appears in Collections:
College of Engineering/Engineering Practice School (공과대학/대학원)Dept. of Materials Science and Engineering (재료공학부)Theses (Ph.D. / Sc.D._재료공학부)
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