1,3-Bis(thieno[3,4-b]thiophen-6-yl)-4H-thieno[3,4-c]pyrrole-4,6(5H)-dione-Based Small-Molecule Donor for Efficient Solution-Processed Solar Cells.

A small molecule TBTT-1 with 5-(2-ethylhexyl)-1,3-bis(2-(2-ethylhexyl)thieno[3,4-b]thiophen-6-yl)-4H-thieno[3,4-c]pyrrole-4,6(5H)-dione (TBTT) as the central moiety was designed and synthesized for solution-processed bulk-heterojunction solar cells. TBTT-1 exhibits a broad absorption with a low optical band gap of approximately 1.53 eV in the thin film. An optimized power conversion efficiency (PCE) of 7.47% with a high short-circuit current of 14.95 mA cm-2 was achieved with diphenyl ether (DPE) as additive, which is the highest PCE for TPD-based small-molecule solar cells. According to the detailed morphology investigations, we found that DPE processing helped to enhance π-π stacking and reduce the scales of phase separation, which led to improved exciton splitting and charge transport in BHJ thin film, and thus enhanced device performance.