Mechanism of the Photodegradation of A-D-A Acceptors for Organic Photovoltaics

In this work, we elucidate the photodegradation pathway of IT-4F, a benchmark A-D-A type semiconductor for organic photovoltaics. The photoproducts were isolated and shown to be isomers of IT-4F formed via a 6-e electrocyclic reaction between the dicyanomethylene unit and the thiophene ring, followed by a 1,5-sigmatropic hydride shift. Importantly, this photoisomerization is accelerated under inert conditions which is explained by DFT calculations that predict the reaction to occur via the excited triplet state (quenchable by oxygen). Adding controlled amounts of the photocyclized product P1 to PM6:IT-4F bulk heterojunction cells shows a progressive decrease of photocurrent and fill factor attributed to its poorer absorption and charge transport properties. The power conversion efficiency drops from 12% for pure IT-4F to 3% for pure P1 acceptor. This cyclization is a general photodegradation pathway for a series of analogous A-D-A molecules with 1,1-dicyanomethylene-3-indanone termini. However, the rate of the reaction varies widely with the nature of the donor moiety.