High-efficiency organic solar cells from low-cost pentacyclic fused-ring electron acceptors via crystal engineering†

Achieving high power conversion efficiencies (PCEs) from low-cost materials is essential for the commercialization of organic solar cells (OSCs). Herein, three A–DA′D–A-type pentacyclic fused-ring electron acceptors (FREAs) featuring low synthetic complexity, namely BT-F, BTA-C4-F, and BTA-C4-Cl, were developed by merging core engineering and end-group halogenation. Single-crystal X-ray diffraction revealed that multiple π–π stacking modes appeared after changing the central core from benzothiadiazole to benzotriazole. When further replacing the fluorinated end group with its chlorinated counterpart, molecular packing evolved into a three-dimensional (3D) network, which is the first report of 3D network packing in A–DA′D–A-type pentacyclic FREAs. The unique 3D network packing endowed BTA-C4-Cl with an extended exciton diffusion length and the highest electron mobility. Consequently, a remarkable PCE of 17.16% was obtained by BTA-C4-Cl in a binary OSC, which represents the highest efficiency achieved by pentacyclic FREAs to date and greatly reduced the PCE gap between the low-cost electron acceptors and prevailing Y-series acceptors. This work provides new insights into realizing 3D network packing from non-Y-series electron acceptors and highlights the bright prospect of low-cost A–DA′D–A-type pentacyclic fused-ring electron acceptors as promising candidates in commercialization of OSCs.