Pathways To Reduced-Recombination in Fullerene and Non-Fullerene Acceptor Solar Cells
Safa Shoaee a
a University of Potsdam, Optoelectronics of Disordered Semiconductors, Institute of Physics and Astronomy, Germany
Online Conference
Proceedings of NFA-Based Organic Solar Cells: Materials, Morphology and Fundamentals (NFASC)
Online, Spain, 2021 February 3rd - 4th
Organizers: Natalie Banerji and Feng Gao
Invited Speaker, Safa Shoaee, presentation 005
Publication date: 25th January 2021

As an important photovoltaic parameter, the fill factor (FF) of OSCs stands for the effectiveness of charge generation and collection, which plays a critical role in the power conversion efficiency. However, the FF values still lie below their predicted limit. In-depth understanding of charge carrier photogeneration and recombination mechanism in organic solar cells, in particular, the NFA systems, is still an ongoing effort. In donor:acceptor (bulk) heterojunction organic solar cells charge photogeneration and recombination are inter-related via the kinetics of charge transfer states – being singlet or triplet states. While high charge photogeneration quantum yields have been achieved in many donor:acceptor systems, yet only very few systems, all fullerene-based, have shown significantly reduced bimolecular recombination relative to the rate of free carrier encounter. This is a serious limitation for industrialization of organic solar cells, in particular when aiming at thick active layers. In order to obtain a long charge carrier lifetime and a high FF, realizing “reduced- Langevin” recombination is of paramount importance. Herein, we present a meta-analysis of the device performance for numerous bulk heterojunction organic solar cells. From this analysis we introduce a ‘spin-related factor’ which is dependent on the ratio of back electron transfer of the triplet CT states to the decay rate of the singlet CT states. We show that this factor links the recombination reduction factor to the charge generation efficiency. We further find that improving the (energetic and structural) order, slows down the recombination.  

Alexander von Humboldt Foundation (Sofja Kovalevskaja award)

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