Tuning the optoelectronic properties for high-efficiency (>7.5%) all small molecule and fullerene-free solar cells

Abstract

A new non-fullerene small molecule acceptor, MPU2, that incorporates dicyano-rhodanine moieties on the edges of thiophene–diketopyrrolopyrrole was designed and prepared. In films MPU2 showed a broad absorption in the 550–850 nm region with an optical energy gap of 1.59 eV. MPU1 and MPU2 were used as small molecule acceptors to construct solar cells. DTS(QxHTh₂)₂, which has a D₁–A–D₂–A–D₁ structure and complementary absorption in the 470–700 nm region of the solar spectrum, was selected as the electron-donor. All small molecule solution-processed bulk heterojunction organic solar cells were prepared using these small molecules as acceptors along with a small molecule donor. In these devices, MPU2 showed higher power conversion efficiency than MPU1. The morphology of the active layer was improved by applying thermal annealing and vacuum drying methodologies. The devices for which the latter methodology was used showed 6.28% (MPU1) and 7.76% (MPU2) power conversion efficiencies (PCEs), with the latter value being one of the highest PCEs for all small molecule fullerene-free solar cells. The results of this study show that vacuum drying treatment of the active layer is better than thermal annealing to achieve high PCE values.