Allyl-terminated small molecule donors effectively regulate miscibility to improve the performance of organic solar cells

Abstract

The development of organic solar cells (OSCs) utilizing small molecule donor (SMD) materials has lagged behind that of polymer donor counterparts, particularly in SMD:polymer acceptor (SMD:PA) systems. Material innovation is vital to advancing OSC performance. Here, we introduce two new SMDs, H35 and H36, based on a benzodithiophene-rhodanine framework, differentiated by slight side-chain modifications: an allyl group in H35 and an ethyl group in H36. Although these modifications minimally influence the photoelectric and thermodynamic properties, they significantly affect surface energy, enhancing blend morphology via improved miscibility with acceptors. As a result, all small molecule OSCs (ASM-OSCs) employing H35 as a donor blended with Y6 as an acceptor achieved a higher power conversion efficiency (PCE) of 15.21%, while the SMD:PA-based OSCs with PY-IT as the PA attained a PCE of 10.78%. These efficiencies notably surpass those of the H36-based devices, which achieved PCEs of 8.37% (with Y6) and 5.54% (with PY-IT). The superior performance of the H35-based devices is attributed to finer domain structures, continuous phase separation, and improved charge transfer. This work highlights the utility of allyl terminal groups in SMD design, offering a new strategy for optimizing molecular morphology and improving performance in the ASM- and SMD:PA-based OSCs.