Correlating acceptor structure and blend nanostructure with the photostability of nonfullerene organic solar cells

The formation of photo-induced traps resulting in the loss of electron mobility deteriorates the performance of organic solar cells under continuous light soaking. The genesis of these loss mechanisms is elucidated by examining the structural stability of halogenated ITIC derivative films and the phase behavior of the respective binary systems by blending with the donor polymer PBDBT-2F. Under constant illumination, ITIC-4Cl is found to maintain its structural integrity, whereas fluorine on the peripheral moieties of ITIC-4F undergo chemical substitution to form a mixture of ITIC and ITIC-4F. Thermal analysis of the light soaked binary films reveals that ITIC-4Cl loses its crystalline phase while the crystallinity of ITIC-4F does not undergo changes. Further, we show that the addition of a small amount of ITIC-4F as a third component hinders the loss of ITIC-4Cl crystalline phase in bulk-heterojunction blends through the formation of co-crystals. These results suggest that long-range ordering of NFAs does not necessarily improve the photo-stability of organic solar cells and that the addition of a third component, irrespective of the crystalline nature, can prevent changes in bulk-heterojunction blend nanostructure.