Low-Cost Fused-Ring Electron Acceptors for Efficient Organic Solar Cells by Fine-Tuning Molecular Crystallization and Film Formation Kinetics.

Achieving high power conversion efficiencies (PCEs) with low-cost active layer materials is of vital importance for organic solar cell (OSC) commercialization. However, the PCEs afforded by currently reported low-cost electron acceptors remain substantially limited. Herein, we report the regulation of molecular crystallization and film formation kinetics of the low-cost electron acceptors with A-DA'D-A-type pentacyclic fused-ring structures and achieved an impressive PCE of 18.04% and a tiptop figure-of-merit (FOM) of 0.363, representing the best-known values for OSCs. Single crystal X-ray diffraction studies indicate that the long and flexible side chain on the central core could hinder the unfavorable dimer formation of electron acceptors and lead to three-dimensional network packing, which is critical for exciton diffusion and charge transport. Moreover, the in situ optical spectroscopy revealed that the side chain elongation of the electron acceptor could slow the film formation kinetics of the small molecular acceptor while accelerating those of the polymer donor, which is conducive to forming bi-continuous interpenetrating fibril networks with better intermixing. This work demonstrates that low-cost, simple-structured fused-ring electron acceptors hold a bright future in the OSC commercialization.