Design of Mechanically-Robust Naphthalenediimide-Based Polymer Additives for High-Performance, Intrinsically-Stretchable Organic Solar Cells

High-molecular-weight electro-active polymer acceptor (P_A) is effective in simultaneously increasing photovoltaic performance and mechanical integrity of organic solar cells (OSCs) based on a polymer donor (P_D) and a small molecule acceptor (SMA). In this work, we develop a new naphthalene diimide (NDI)-based P_A, named P(NDI2OD-TCVT), and employ it as a P_A additive in a P_D:SMA blend to fabricate high-performance and mechanically robust OSCs. Copolymerization of NDI, bithiophene, and cyano-vinylene units ensures the n-type characteristics of the P(NDI2OD-TCVT). Noticeably, the cyano-vinylene group alleviates the crystalline nature of the NDI and bithiophene units, providing inter-domain bridges by being a tie molecule. As we vary the weight-averaged molecular weight (M_w) of the polymer, we find that the backbone structure enables a significant reduction of the critical molecular weight that ensures mechanical robustness. High crack-onset strain (COS) of 30.1% is achieved in the P(NDI2OD-TCVT) film with a relatively low M_w of 109 kg mol^-1, which is a stark contrast to the COS value (1.8%) of the reference P(NDI2OD-T2) film with similar M_w of 126 kg mol^-1. In addition, we find that incorporation of P(NDI2OD-TCVT) enhances the photovoltaic performance of the P_D:SMA-based OSCs, achieving a high power conversion efficiency (PCE) of 16.8%. Benefitted from the significantly enhanced mechanical properties of P(NDI2OD-TCVT), we also demonstrate the highly efficient and intrinsically stretchable organic solar cells (IS-OSCs). The IS-OSCs with 10 wt% of P(NDI2OD-TCVT) featured a PCE of 12.6% and retained 85% of the initial PCE after 100 cycles of stretching at 20% strain and releasing, outperforming those of P(NDI2OD-T2)-based IS-OSCs.