"Twisted" Terpolymer Donor Enabling High-Performance Intrinsically Stretchable Organic Solar Cells

Intrinsically stretchable organic solar cells (is-OSCs) hold great potential for next-generation wearable power generators. Currently, high-efficiency polymeric donor materials are based on rigid planar conjugated backbones, which limits their mechanical robustness and creates a trade-off between photovoltaic performance and stretchability. To simultaneously improve the power conversion efficiency (PCE) and mechanical robustness of is-OSCs, we develop a series of ductile PM-BOX% terpolymers by incorporating different amounts (X = 5, 10, and 20) of 3-butyloctyl-thiophene (ThBO) units into PM6 backbone. The BO alkyl side chains appended on Th-BO units induce a more significant twist in the backbone compared to PM6, which increases the disordered contents and facilitates the stress dissipation, hence enhancing the mechanical stretchability. While the conjugated thiophene unit preserves the π-electron delocalization, maintaining efficient charge transport. As a result, the PM-BO10%:BTP-eC9-based devices outperform the PM6-based ones, achieving a higher PCE of 18.3% and a superior crack-onset strain of 23.8%. Additionally, PM-BO10%:BTP-eC9-based is-OSCs reach a PCE of 14.2% with an excellent PCE retention (80% of its initial PCE at 31.0% strain), marking one of the highest values for is-OSCs. This twisted terpolymer design strategy paves a novel way for designing efficient polymer donors with excellent stretchability for the application in is-OSCs.