Dual-acceptor synergy enables hierarchical morphology for efficient exciton and charge dynamics in as-cast organic solar cells

The contradictory morphological requirements for exciton and charge dynamics constitute the principal impediment that restricts the further development of as-cast organic solar cells (OSCs). Herein, we integrated two small-molecule acceptors, Y6-BO and L8-BO, with a polymer donor, D18, to construct efficient as-cast OSCs. In comparison to the D18:L8-BO blend film, the D18:Y6-BO matrix manifests a prolonged diffusion/nucleation stage but more rapid crystal growth and phase separation processes, which are primarily attributed to the smaller binding energy between D18 and Y6-BO, along with the nonlinear aggregation behaviors of Y6-BO. The combination of two acceptors lengthens the duration of both molecular self-assembly and phase separation, resulting in a hierarchical morphology of the active layer. This prompts the concurrent improvement of exciton dissociation and charge transport in the ternary system. Moreover, an optimized balance between short-circuit current density and open-circuit voltage is achieved in ternary devices via manipulating the proportion of Y6-BO/L8-BO. Based on non-halogenated solvents, the ternary as-cast OSCs with 20 wt% L8-BO achieve a record efficiency of 19.84%, which is significantly higher than that of the two binary devices. This work provides a feasible strategy for fabricating eco-friendly, low-cost, and efficient OSCs.