The unidirectional extension of a smaller fused-ring system into a larger one in a single direction will increase the conjugation length, allowing a fine-tuning of electronic properties. Here, we designed and synthesized a unidirectionally extended fused-8-ring-based nonfullerene acceptor, AOIC, and a bidirec- tionally extended fused-11-ring electron acceptor, IUIC2, and compared these with the parent fused-5-ring electron acceptor, FSIC. They share the same electron-accepting groups and alkylphenyl side chains but have different fused-ring electrondonating units. Core extension from 5 to 11 rings up-shifts the energy levels, red shifts the absorption spectra, and reduces bandgaps. The unidirectionally extended AOIC has the highest mobility (2.1 X 10(-3) cm(2) V-1 s(-1)) relative to the parent F5IC (1.0 x 10(-3) cm(2) V-1 s(-1)) and the bidirectionally extended IUIC2 (4.7 x 10(-4) cm(2) V-1 s(-1)). Upon blending with the donor PTB7-Th, AOIC-based organic photovoltaic cells show an efficiency of 13.7%, much better than that of FSIC-based cells (5.61%) and IUIC2-based cells (4.48%).