Direct Evidence of the Internal Deterioration Mechanism due to Molecular Chain Ends in Polymer Solar Cells by Operando Spin Detection AITranslate

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DOI：https://doi.org/10.1021/acsapm.1c01543

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Clarification of the internal deterioration mechanism in polymer solar cells is an important issue for further improvement of not only device durability but also initial device performance. In particular, the charge accumulation at polymer-chain ends has been discussed for such an internal deterioration mechanism. However, the direct correlation between polymer-chain-end structures and performance deterioration has not yet been investigated from a microscopic viewpoint. Here, we report direct evidence of the internal deterioration mechanism due to the charge accumulation at polymer-chain ends by operando electron spin resonance (ESR) spectroscopy at the molecular level. Using semiconducting polymers with different molecular chain-end structures, that is, poly[(3,4-ethylenedioxythiophene-2,5-diyl)-(9,9-dioctylfluorene-2,7-diyl)] (PEDOTF) with bromine ends (PEDOTF-Br) and that with hydrogen ends (PEDOTF-H), we demonstrate the correlation between the chain-end structures and solar-cell durability, showing a larger amount of charge accumulation and larger performance deterioration for PEDOTF-Br-based solar cells compared to those for PEDOTF-H-based solar cells, owing to the inhabitation of charge transport and the formation of additional potentials in the cells. The origins of observed charge accumulation are precisely clarified with the fitting analyses of operando light-induced ESR spectra and density functional theory calculation, demonstrating that the main internal deterioration is due to the charge accumulation at polymer-chain ends from a microscopic viewpoint. Because such charge accumulation also affects the initial device performance, the optimization of polymer-chain ends will be important for further improvement of not only device durability but also initial device performance for other polymer solar cells.