(472a) Efficient and Mechanically Robust All-Polymer Solar Cells

Non-fullerene acceptors (NFAs), that are small-molecule acceptors (SMA) and polymer acceptors (PAs), have been extensively explored, which has yielded significant enhancements in the photovoltaic performance of polymer solar cells (PSCs). The mechanical robustness of the PSCs is of vital and equal importance to ensure the long-term stability and enable their use as power-generators in flexible and stretchable electronics. Here, we report a comparative study of the mechanical properties of SMA-based, PA-based, and fullerene-based PSCs. We chose ITIC (SMA), P(NDI2OD-T2) (PA), and PCBM (fullerene) as three representative acceptor materials and blended them with the same polymer donor PTB7-Th. To understand the difference between the mechanical properties of SMA-based and PA-based PSCs, we control the number-average molecular weight (M_n) of P(NDI2OD-T2) from 15 to 163 kg mol^-1 in all-PSCs. The all-PSCs based high M_n PA exhibit significantly higher cohesion energy (4.03 J m^-2) than SMA-PSCs (1.19 J m^-2) and PCBM-PSCs (0.29 J m^-2). Notably, the all-PSCs exhibited a high strain at fracture of 31.1%, which is 9- and 28-fold higher than those of SMA-PSCs and PCBM-PSCs, respectively. The superior mechanical robustness of all-PSCs is attributed to using a PA above the critical molecular weight (M_c), which produces tie molecules and polymer entanglements that dissipate substantial mechanical strain energy with large plastic deformation. This work provides useful design guidelines for photovoltaic active materials in terms of the mechanical properties and highlights the importance of incorporating high M_n PAs above the M_c for producing the PSCs with excellent mechanical robustness and device performance.