Controlling Solution-Phase Polymer Aggregation with Molecular Weight and Solvent Additives to Optimize Polymer-Fullerene Bulk Heterojunction Solar Cells

The bulk heterojunction (BHJ) solar cell performance of many polymers depends on the polymer molecular weight (M-n) and the solvent additive(s) used for solution processing. However, the mechanism that causes these dependencies is not well understood. This work determines how M-n and solvent additives affect the performance of BHJ solar cells made with the polymer poly(di(2-ethylhexyloxy)benzo[1,2-b:4,5-b']dithiophene-co-octylthieno[3,4-c]pyrrole-4,6-dione) (PBDTTPD). Low M-n PBDTTPD devices have exceedingly large fullerene-rich domains, which cause extensive charge-carrier recombination. Increasing the M-n of PBDTTPD decreases the size of these domains and significantly improves device performance. PBDTTPD aggregation in solution affects the size of the fullerene-rich domains and this effect is linked to the dependency of PBDTTPD solubility on M-n. Due to its poor solubility high M-n PBDTTPD quickly forms a fibrillar polymer network during spin-casting and this network acts as a template that prevents large-scale phase separation. Furthermore, processing low M-n PBDTTPD devices with a solvent additive improves device performance by inducing polymer aggregation in solution and preventing large fullerene-rich domains from forming. These findings highlight that polymer aggregation in solution plays a significant role in determining the morphology and performance of BHJ solar cells.