Abstract
With the emergence of nonfullerene electron acceptors resulting in further breakthroughs in the performance of organic solar cells, there is now an urgent need to understand their degradation mechanisms in order to improve their intrinsic stability through better material design. In this study, we present quantitative evidence for a common root cause of light-induced degradation of polymer:nonfullerene and polymer:fullerene organic solar cells in air, namely, a fast photo-oxidation process of the photoactive materials mediated by the formation of superoxide radical ions, whose yield is found to be strongly controlled by the lowest unoccupied molecular orbital (LUMO) levels of the electron acceptors used. Our results elucidate the general relevance of this degradation mechanism to both polymer:fullerene and polymer:nonfullerene blends and highlight the necessity of designing electron acceptor materials with sufficient electron affinities to overcome this challenge, thereby paving the way toward achieving long-term solar cell stability with minimal device encapsulation.
Highlights
With the majority of research effort still dedicated to taking advantage of the flexibility of nonfullerene acceptors (NFAs) to optimize device efficiencies, the implications for the stability of fullerene-free
It has previously been established that two main pathways for oxygen-induced degradation exist in fullerene-based organic solar cells (OSCs): through singlet oxygen (1O2) generation via energy transfer from triplet excited states and superoxide (O2−) electrons from the generation fullerene via photoinduced transfer of to molecular oxygen.[10−12] We have demonstrated that for polymer:PCBM blends, degradation is dominated by the pathway of triplet-induced singlet oxygen generation through either the polymer[13] or the fullerene[14] component, while the pathway of superoxide formation is suppressed due to a deep-lying lowest unoccupied molecular orbital (LUMO) level of PCBM
The J−V characteristics of P3HT:O-IDTBR devices under dark storage in air or under exposure to AM1.5G conditions in nitrogen for 10 min were measured as control experiments (Figure S2, Table S3), both revealing negligible changes in device efficiency, indicating that the rapid degradation of device efficiency in Figure 1a is primarily due to the combined exposure of light and oxygen, rather than light exposure or oxygen exposure alone
Summary
With the majority of research effort still dedicated to taking advantage of the flexibility of NFAs to optimize device efficiencies, the implications for the stability of fullerene-free. Our results highlight the complexity in the material design to simultaneously achieve superior efficiency and stability of OSCs. A redesign of the NFAs with deeper LUMO energy levels, as well as their matching donor polymers with deepened HOMO levels to compensate the Voc loss, might be a promising route toward the development of both efficient and environmentally stable fullerene-free OSCs. In conclusion, we present a range of advanced optical, chemical, and device stability measurements to investigate the degradation mechanisms of benchmark fullerene-based and fullerene-free OSCs. We establish the critical correlation between the LUMO level of the fullerene and nonfullerene electron acceptors and the resulting environmental stability of OSCs and blend films strongly mediated by the yield of superoxide formation.
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