Understanding the molecular mechanisms of the differences in the efficiency and stability of all-polymer solar cells

In spite of the great success of all-polymer solar cells (all-PSCs) in terms of device efficiency mainly owing to the vigorous development of polymer donors (P(D)s) and polymer acceptors (P(A)s), the synergistic effects of the molecular structure and molecular weight (M-w) of P-D and P-A materials as well as P-D-P-A pair miscibility on device performance are still unclear and rarely reported. Herein, we introduced PBDB-T and its congener materials (PM6 and PM7) as P(D)s with comparable M(w)s and two PYT batches as P(A)s with different M(w)s (PYT-M and PYT-H) to deeply investigate the effects of molecular mechanisms on the device efficiency and stability in these six systems. Benefiting from proper P-D-P-A miscibility owing to the matched molecular structure and M(w)s, both PBDB-T:PYT-H and PM6:PYT-M systems with suitable phase separation show comparable device efficiencies, which are much better than those of the other four all-polymer systems. Impressively, further investigation demonstrates that the PBDB-T:PYT-H active layer is more stable than the PM6:PYT-M one, resulting from the trade-offs between molecular miscibility and M-w. This work not only employs the synergetic effect of the molecular structure and molecular weight on device efficiency and stability but also provides a promising strategy to simultaneously improve the device performance of all-PSCs.