Development of rigidity-controlled terpolymer donors for high-performance and mechanically robust organic solar cells

Organic solar cells (OSCs) are potential power sources for wearable electronic devices. However, the mechanical stretchability of active materials is not yet sufficient; one of the main reasons is the high rigidity of polymer donors (P_Ds) and the resultant excessive crystalline structures, which makes the active layer mechanically-fragile. In this study, we develop a series of PM6-based terpolymers (PM6-BX; X = 10-30, X indicates the mole percentage of the third component) in which a bulky electro-active third component, 7,8-bis(5-hexylthiophen-2-yl)-11H-benzo[4,5] imidazo[2,1-a]isoindol-11-one (BID), is introduced to reduce the tightness of their molecular packing. As a result, the neat P_D film with 10 mol% BID (PM6-B10) exhibits significantly improved mechanical ductility (i.e., average crack onset strain (COS_avg) of 23.8%) compared to that of the neat PM6 film (COS_avg = 14.9%) without a BID unit. In addition, in terms of a blend film, the well-intermixed domains of PM6-B10 and a small molecule acceptor afford OSCs with a high power conversion efficiency (PCE) of 17.2% and mechanical stretchability (COS_avg = 11.4%), outperforming the PM6 counterpart (PCE = 15.8%, COS_avg = 2.0%). This study suggests important guidelines for the design of efficient P_Ds for high-performance, stretchable OSCs.