A mechanistic understanding of solvent additives’ influence on polymer:fullerene phase separation for organic photovoltaics
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Solution processable conjugated organic materials offer a route to low cost electronic devices which include transistors, LEDs, electrochromics, and photovoltaics. The ability of these materials to be solubilized allows for simple, low temperature, processing on rigid and flexible substrates. This dissertation focuses on processing of conjugated materials for photovoltaic applications. The impact that additives have on solidification, film morphology, and ultimately device performance, is studied, beginning with examination of the solution state, then processing, followed by thin film characterization and device performance. Each chapter focuses on a different conjugated polymeric material and examines the influence that additives have on the solid state film formation. Using in situ measurements, coupled with static thin film measurements, the mechanism of thin film formation is explored when processed with, and without, modifications to the starting ink solution. Chapter 3 focuses on poly(3-hexylthiophene-2,5-diyl) (P3HT) mixed with PC61BM. Blade coating this blend from solvents of varying vapor pressures yield different drying dynamics, and solar cell performance. Chapter 4 explores morphological and performance differences of blade coated, or spin coated, poly[5-(2-hexyldecyl)-1,3-thieno[3,4-c]pyrrole-4,6-dione-alt-5,5-(2,5-bis(3-dodecylthiophen-2-yl)-thiophene)], P(T3-TPD), blended with PC71BM, and processed with, or without, the solvent additive 1,8-diiodooctane (DIO). Results presented in this chapter show processing of aggregated solutions yields similar morphologies and solar cell performance, regardless of processing type. Further, DIO was found to increase the nucleation density, reducing domain size, and improving polymer crystallinity, leading to enhance photovoltaic performance. Chapter 5 examines the impact that co-solvent processing has on the polymer:fullerene blend known as DT-PDPP2T-TT:PC71BM. Blade coating this bulk-heterojunction blend from chloroform leads to liquid-liquid phase separation, whereas, when the co-solvent ortho-dichlorobenzene is introduced the morphology evolution is nucleation and growth dominated, leading to a reduced characteristic length scale, and improved power conversion efficiencies. Lastly, Chapter 6 focuses on the organic photovoltaic market, how organic photovoltaics compare to their Si counterpart, and areas where organic photovoltaics can improve in order for this technology to become viable to the market.