Wohrer Charles, Thibaut; (2023) Bimolecular Recombination in Non-Langevin ZZ115 Based Blend Systems for Organic Photovoltaics. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Organic solar cells are currently positioned as a renewable energy source, potentially solving issues with current solar cells in application, cost and production. However, organic solar cells suffer from low efficiencies compared to their traditional counterparts. Bimolecular recombination of charge carriers before collection serves as a major loss process in organic solar cells and is a significant contributor to these low efficiencies. In this work, potential factors influencing this bimolecular recombination process in a conjugated polymer (ZZ115) blended with fullerenes were studied, where these ZZ115/fullerene blends show a suppressed, non-Langevin behaviour. This study was done via the combination of an experimental section using UV-VIS spectroscopy, photoluminescence spectroscopy and Transient Absorption Spectroscopy, and a computational section using Density Functional Theory and the VOTCA software for large system modelling. Overall results found a polaron feature in ZZ115 pristine and two polaron features in ZZ115 blends. These features were identified as a ZZ115 polaron in a ZZ115 crystalline medium in both pristine and blend, and a ZZ115 polaron in a mixed amorphous region unique to the blend. Blend ratio variation shows no influence of fullerene concentration on the mixed amorphous polaron feature past 10% concentration while a gradual decrease in the recombination kinetics is observed for the crystalline polaron feature. The latter is attributed to the addition of fullerene disrupting the crystalline ordered phase leading to deeper trap states slowing down charge recombination. Computational calculations have highlighted the lack of unusual molecular properties for ZZ115, but large-scale computational studies are still a work in progress to corroborate experimental findings. Nevertheless, important insights into the morphological and electronic impact on bimolecular recombination in ZZ115 were achieved in this study, providing both device optimisation guidelines and a better understanding of the bimolecular recombination process.

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