Competition between charge collection and non-geminate recombination in bulk heterojunction solar cells

Abstract

This thesis is concerned with quantification of non-geminate recombination losses in organic bulk heterojunction solar cells. After description of the context of this work, the theoretical background and the methodology employed are presented. In this thesis, many different polymer:fullerene systems are investigated. In the next chapter, we show that the study of non-geminate losses using charge extraction/transient photovoltage analysis can be applied to many different systems away from P3HT/P3HS blends. We see to what extent ideality factors can give a more precise description of the exact recombination mechanism. Then, the change of optoelectronic properties of a high performance polymer:fullerene blend upon a blend ratio perturbation is investigated. The resulting shifts in energetics and dynamics of the blends are quantified. A quantitative agreement between two methods (charge extraction and electroluminescence) probing the shifts in the energetics at the heterojunction is presented. In the next two chapters, two limits of the common vision of polymer: fullerene systems are explored by combining experiment and 1D drift-diffusion modelling. First, the impact of the variations of the spatial distribution of carriers on the apparent reaction order is experimentally investigated. The study reconciles the apparent contradictions currently in the literature regarding the meaning of high reaction orders. In the following chapter, the often underestimated effects of unintentional doping in polymer blends are addressed experimentally. In particular, its effect on device optimisation, understanding of carrier collection and recombination dynamics are explored. This study suggests that many donor/acceptor blends are not, contrary to common belief, intrinsic semiconductors. Finally, an analysis of the Langevin and non-Langevin behaviour of some efficient systems is presented. The different interpretations of regular observations disproving non-geminate recombination following the Langevin type mechanism are reviewed. We suggest the ratio ’recombination over collision’ is often overlooked and question the underlying assumption that it should be unity.