Investigation of charge transport in organic photovoltaic materials using lateral device structures
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Understanding of charge carrier transport and recombination in bulk heterojunction (BHJ) materials is important for continued improvement of organic photovoltaics (OPVs). Solar cell efficiencies now approach 12% and answers to lingering questions create a roadmap for increasing this value. OPVs are made as vertical structures and majority of analyses in literature are directed to this structure. In this dissertation, theoretical and experimental analyses of lateral devices are developed to compliment the knowledge base established with vertical devices. Lateral OPVs offer unique insights into transport and recombination physics in BHJs: they decouple charge extraction from charge photogeneration, allow clear formation of space-charge regions and recombination zone, open the BHJ to probing, and allow comparison of ambipolar to unipolar electron & hole currents. Lateral OPVs are simulated to understand their current-voltage behavior and link it to development of space-charge. Modeling focuses on the intermediate 3um channel length. At this transport length effects of space-charge behavior are clearly present and all photogenerated charge can be extracted. Modeling work is used to support analysis of experimental results. BHJs made of electron-transporter [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) and hole transporting polymer poly[3-hexylthiophene-2,5-diyl] (P3HT) and co-polymer poly[2-(5-(4,4-dioctyl-4H-silolo[3,2-b:4,5-b’]dithiophen-2-yl)-3-tetradecylthiophen-2-yl)-5-(3-tetradecylthiophen-2-yl)thiazolo[5,4-d]thiazole] (PDTSi-TzTz) are studied. Transport in PDTSi-TzTz:PCBM is analyzed by profiling the channel potential. The channel potential and current-voltage measurements are used to obtain carrier mobilities and recombination rates. High charge collection efficiency is found even at transport lengths greater than 1 micron. Photocurrent and extracted unipolar injection currents in P3HT:PCBM blends are studied. These measurements yield intensity-dependent mobilities of both electrons and holes. Extraction of both mobility values in the same BHJ point to electron mobility as the limiting factor in OPV performance.