Highly stretchable All-polymer solar cells enabled by Siloxane-terminated side chains and molecular weight control

Abstract

Stretchable organic solar cells (OSCs) are promising wearable power generators. Here, we synthesize a series of bithienyl-benzodithiophene-alt-fluorobenzotriazole copolymer (PBZ-2Si) with siloxane-terminated side chains, and compare with J52 analogues to investigate side chain effects on the photovoltaic and mechanical properties. When different molecular weight (Mn) of PBZ-2Si, denoted as PBZ-2SiL, PBZ-2SiM and PBZ-2SiH, blended with the N2200 acceptor, the PBZ-2SiM based all-polymer solar cell (all-PSC) shows an improved power conversion efficiency (PCE) of 7.01 % as compared to that of J52 (5.30 %). The improved PCE of PBZ-2SiM is attributed to the enhanced molecule aggregation, balanced charge transport and optimal film morphology induced by siloxane side chains. Notably, the PBZ-2SiM:N2200 blend film shows a significantly improved stretchability of 38.4 % compared to 19.5 % of J52 reference, because the plasticizing effect that extended soft siloxane side chains enlarge interchain distance of polymer backbones and increase the chain rotational free volume. Further increasing Mn enhances polymer chain entanglements and boosts the stretchability up to 50.7 %, known as the highest stretchability reported to date. Moreover, the PBZ-2SiM-based stretchable OSC showed a high PCE of 6.0 % and maintained over 80 % of its initial PCE at 50 % enormous strain, outperforming the J52 reference. Therefore, our work provides practical guidelines for developing highly efficient and stretchable all-PSC.