Charge generation and recombination in organic photovoltaics

Abstract

At the most basic level, a photovoltaic needs to convert solar power to electrical power. Therefore, the processes which result in electrons reaching the external circuit must out-compete the losses. I address this balance in organic photovoltaics, using a range of experimental techniques and test systems. In Chapter 4, I consider the fundamental limits to solar cell efficiency using detailed balance. I use the voltage loss due to non-radiative recombination as a metric to quantify how close a set of polymer:fullerene blend devices approach the radiative limit, using an indacenodithiophene-co-benzothiadiazole (C2C6-IDTBT) polymer. In particular, I examine the effect of C2C6-IDTBT fractionation on the non-radiative recombination, and find that devices made from high molecular weight polymer exhibit reduced non-radiative recombination losses relative to the non-fractionated polymer. I also probe the effect of polymer molecular weight in Chapters 5 and 6, using a different polymer, a thienothiophene-diketopyrrolopyrrole (DPP-TT-T). In Chapter 5, I find that devices made from high molecular weight DPP-TT-T out-perform devices made from low molecular weight DPP-TT-T, particularly the short-circuit current. While I find some differences in electronic quality, the most striking difference between the polymer fractions is the light absorption: high molecular weight DPP-TT-T absorbs light more strongly than low molecular weight DPP-TT-T. Furthermore, the extinction coefficient maximum of high molecular weight DPP-TT-T is about 50% greater than any conjugated polymer we had measured previously. In Chapter 6, I explore the causes of this high optical absorption. We find that calculated persistence length can be a good predictor of optical absorption strength: polymers with high calculated persistence length, or high linearity, exhibit stronger absorption. We also find that high molecular weigh DPP-TT-T has a greater proportion of linear chain segments than low molecular weight DPP-TT-T, and that this is the likely cause of the greater extinction coefficient. Chapter 7 treats the mechanism of charge separation by using two modified fullerenes with different electron affinities to probe the energetic offset required between the polymer and the acceptor for efficient free charge generation from polymer excitons. We find that, when there is very little energetic offset, the polymer excitons are unable to dissociate, and subsequent charge generation is inefficient. However, when the energetic offset between the polymer and acceptor LUMO levels is increased, the efficiency of polymer exciton dissociation is increased and there is significant charge generation. Overall therefore, I have examined how charge generation, from light absorption to subsequent free charge generation, and recombination, particularly non-geminate recombination in the bulk, affect solar cell performance.