Understanding the effect of chlorine substitution in all-polymer solar cells

All-polymer solar cells (all-PSCs) are promising candidates to resolve the critical long-term stability issue in organic photovoltaics. Such an all-polymer combination exhibits features such as enhanced light absorption, high mechanical strength, and excellent morphological stability. In this work, to fully understand the correlation between the molecular structure and photovoltaic performance in all-PSCs, we performed systematical chlorine (Cl) substitution investigation in a benchmark all-PSC blending of the donor PBDB-T and acceptor N2200. The chlorine (Cl) substituted polymers show blue-shifted absorptions and down-shifted energy levels due to the stronger electron-withdrawing ability of the Cl atom. Besides, the chlorinated polymers exhibit noncovalent bonding, resulting in the change of polymer aggregation during the film-forming process, which further tunes the phase separation between the donor and acceptor polymers. When applied to all-PSCs, PBDB-T:N2200 based devices output a maximum efficiency of 8.44% with an appropriate blend morphology and a high fill factor. And the devices based on chlorinated all-PSCs yield higher open-circuit voltages approaching 1.0 V, but with decreased short-circuit current density. We believe that these results would provide more insights into the understanding of the correlation between the molecular structure and device performance in all-PSCs.