13.76% efficiency nonfullerene solar cells enabled by selenophene integrated dithieno[3,2-b:2′,3′-d]pyrrole asymmetric acceptors†
An asymmetric heptacyclic electron-donating core, namely DTPPSe, was developed by integrating selenophene and dithieno[3,2-b:2′,3′-d]pyrrole (DTP) building blocks. Three DTPPSe-based narrow band gap asymmetric fused-ring acceptors (FRAs) (ca. DTPPSe-IC, DTPPSe-2F and DTPPSe-4F) have been synthesized to specifically explore the effect of fluorinated end groups on their photovoltaic performance. The two fluorinated FRAs (ca. DTPPSe-2F and DTPPSe-4F) exhibit red-shifted absorption and significantly deeper energy levels relative to non-fluorinated DTPPSe-IC. When blending with a PBDB-T donor, the DTPPSe-2F blend films show a better miscibility with fibrous and uniformly ordered phase separation morphology compared with the other two counterparts. As a result, a maximum power conversion efficiency (PCE) of 13.76% was achieved for DTPPSe-2F based organic solar cells (OSCs) with a low energy loss (Eloss) of 0.60 eV. To the best of our knowledge, this PCE is among the highest values for OSCs in the literature using selenophene-containing FRAs. Our results not only indicate the great potential of the asymmetric heptacyclic donating core DTPPSe for developing efficient FRAs, but also demonstrate the fluorination of the end groups of FRAs as key to modulate the energy levels in molecular engineering.