Light-induced degradation of a push–pull copolymer for ITO-free organic solar cell application

A push–pull copolymer-P1 was successfully designed and synthesized by coupling ter-thiophene donor (D) and isoindigo acceptor (A) units by a direct arylation polycondensation method. The optical bandgap of the copolymer was 1.51 eV. The copolymer showed a weak intramolecular charge transfer due to steric hindrance between the D and A units. An ITO-free organic solar cell was fabricated based on P1 donor and PC71BM acceptor. An almost complete bleaching of absorption in less than 50 h of light exposure was found in pristine P1 film. This fast light-induced degradation is due to chain scissioning and intramolecular bond breaking within the single copolymer unit. This degradation is suppressed with the addition of PC71BM. However, the addition of a 3% v/v 1,8-diiodooctane solvent additive in the processing solvent reduces the life time of the P1:PC71BM-based active layer that the remaining absorbance decreases from 30 to 10% after 80 h of light exposure.