Abstract
The development of non-fullerene acceptors (NFAs) has facilitated the realization of efficient organic solar cells (OSCs) with minimal burn-in losses and excellent long-term stability. However, the role of NFA molecular structures on device stability remains unclear, limiting commercialization of NFA-based OSCs. Herein, the photostability of 10 OSC devices, fabricated with various NFAs (O-IDTBR, EH-IDTBR, ITIC, and ITIC-M) blended with donor polymers (PTB7-Th, PffBT4T-2OD, and PBDB-T), is investigated. O-IDTBR and EH-IDTBR form highly stable devices with all three polymers, whereas ITIC and ITIC-M devices suffer from burn-in losses and long-term degradation. Conformational instability is found to be responsible for the poor photostability of ITIC and ITIC-M, resulting in poor device stability. Twisting and potential breakage of the chemical bond that links the end group to the main backbone of ITIC and ITIC-M molecules causes undesirable conformational changes. Potential strategies to overcome such detrimental photo-induced conformational changes in NFAs are proposed.
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Excellent photostability of O-IDTBR and EH-IDTBR results in good device stabilityITIC and ITIC-M have poorer photostability, resulting in reduced device lifetimesRaman spectroscopy reveals twisting and breakage of ITIC upon illumination in N2Vinylene linkage between the core and the end groups is a weak point of ITIC and ITIC-M
Clarke et al. find that ITIC and ITIC-M can twist upon illumination in inert conditions, leading to eventual bond breakage and poor device stability. In contrast, O-IDTBR and EH-IDTBR devices show excellent stability. These findings provide insights that could aid with the design of more photostable acceptors.