To enhance performance of polymer solar cells, a series of polymers which were based on electron-deficient benzimidazole segments and electron-rich benzodithiophene segments for polymer solar cells were designed and synthesized by the Stille polycondensation. The synthesized polymers which had long alkyl side chain exhibited good solubility in organic solvents. The optical and electrochemical properties of polymers were measured by UV/Vis spectroscopy and cyclic voltammetry (CV). The resulting polymers exhibited low optical bandgaps and electrochemical bandgaps. Furthermore, HOMO and LUMO energy levels of -3.4 ~ 6.0 eV were suitable for photovoltaic applications. These HOMO and LUMO energy levels are appropriate for charge transport to achieve high performance of polymer solar cells. The optical and electrochemical properties were tuned by introducing different substituents and alkyl side chains on benzimidazole segments. The polymer solar cells with benzimidazole based polymer achieved power conversion efficiency close to 1% due to their enhanced light harvesting property and efficient interpenetrating network which were caused by a hydrogen atom on benzimidazole segment and a lack of long alkyl side chain. This work implies that the electron-deficient benzimidazole segments based polymers can achieve high PCE through structural modification of the polymer. ⓒ 2013 DGIST
Table Of Contents
1. INTRODUCTION 1 -- 2. THEORETICAL BACKGROUND 3 -- 2.1. Principle and Characters of Polymer Solar Cells 3 -- 2.2. Structures of Polymer Solar Cells 7 -- 2.3. Donor-Acceptor Polymers for Polymer Solar Cells 10 -- 2.4. Improvement of Performance of Polymer Solar Cells 15 -- 2.4.1. Improvement of Light Absorption 15 -- 2.4.2. New Device Structures 16 -- 3. EXPERIMENTAL 18 -- 3.1. Synthesis of Materials 18 -- 3.1.1. Synthesis of Monomers 18 -- 3.1.2. Synthesis of Polymers 26 -- 3.2. Fabrication of Devices 29 -- 3.3. Measurement and Characterization 30 -- 4. RESULT AND DISCUSSION 31 -- 4.1. Physical Properties of Polymers 31 -- 4.2. Optical Properties of Polymers 34 -- 4.3. Electrochemical Properties of Polymers 37 -- 4.4. Photovoltaic Properties of Polymers 39 -- 5. CONCLUSION 41 -- REFERENCES 43 -- SUMMARY 48 -- ACKNOWLEDGEMENT 49