A bromine and chlorine concurrently functionalized end group for benzo[1,2-b:4,5-b′]diselenophene-based non-fluorinated acceptors: a new hybrid strategy to balance the crystallinity and miscibility of blend films for enabling highly efficient polymer solar cells

Dihalogenated 1,1-dicyanomethylene-3-indanone (IC) plays a key role in top-performing fused-ring electron acceptor (FREA)-based polymer solar cells (PSCs). Here, we first synthesized a hybrid dihalogenated IC (IC-BrCl), which simultaneously had one Br atom and one Cl atom grafted onto the same IC skeleton. Three non-fluorinated FREAs (BDSe-4Cl, BDSe-2(BrCl) and BDSe-4Br) are synthesized by employing a benzo[1,2-b:4,5-b ']diselenophene-based core unit and dichlorinated IC, hybrid dihalogenated IC (IC-BrCl), and dibrominated IC for highly efficient PSCs, respectively. These three acceptors exhibit very similar absorption spectra with a 1.39 eV optical band gap but are slightly different in the HOMO/LUMO energy levels in thin films. The crystallinity of the acceptors was progressively enhanced and the miscibility with PM7 was gradually reduced with the increase in Br atoms. The BDSe-2(BrCl):PM7 blend films exhibited the strongest face-on crystallization orientation, the most proper phase separation, the highest and most balanced carrier mobility and the weakest charge recombination owing to the excellent balance of the miscibility and crystallinity of the blend film. Notably, BDSe-2(BrCl):PM7-based PSCs demonstrated an outstanding PCE of 14.5% with an impressive FF of 76.5%, which substantially outperformed its counterparts (13.8% for BDSe-4Cl, 13.2% for BDSe-4Br); also, this is the highest value among hybrid IC-based FREAs for binary PSCs. Our results demonstrated that a hybrid dihalogenated IC with one Br atom and one Cl atom provides a promising strategy to tune the crystallinity and miscibility of FREAs for boosting the FF and PCE of PSCs.