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

• A series of siloxane terminated polymer PBZ-2Si were developed. • The PBZ-2Si:N2200 based OSC achieved an enhanced efficiency of 7%. • OSCs exhibited a record-high stretchability up to 50% strain. • High-efficiency and mechencaial robustness could be simultaneously achieved. 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 ( M n ) of PBZ-2Si, denoted as PBZ-2Si L , PBZ-2Si M and PBZ-2Si H , blended with the N2200 acceptor, the PBZ-2Si M 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-2Si M is attributed to the enhanced molecule aggregation, balanced charge transport and optimal film morphology induced by siloxane side chains. Notably, the PBZ-2Si M :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 M n enhances polymer chain entanglements and boosts the stretchability up to 50.7 %, known as the highest stretchability reported to date. Moreover, the PBZ-2Si M -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.