Two fully fused perylenediimide (PDI) tetramers, TFPDI-MFT and TFPDI-FT, were designed and synthesized through an inner-ring-fusion strategy and investigated as nonfullerene acceptors for organic solar cells. The two acceptors contain different arrangements of thiophene- and naphthalene-based fusion motifs, resulting in distinct molecular geometries and torsional distributions. TFPDI-MFT consists of a central thiophene-fused region flanked by two naphthalene-centred fused PDI units, whereas TFPDI-FT contains a central naphthalene-fused segment with laterally extended FPDI-T units. Both tetramers possess rigid, extended π-conjugated frameworks and exhibit strong optical absorption with high molar extinction coefficients. Their fused structures also produce distinct nonplanar molecular conformations, with the major torsional distortion located at different positions within the two tetrameric frameworks. TFPDI-MFT exhibits a relatively less-twisted central region and more-twisted peripheral regions, whereas TFPDI-FT displays the opposite torsional distribution. Spectroscopic, morphological, and charge-transport measurements reveal different intermolecular interactions and film-state behaviors while both acceptors form relatively uniform blend films with PBDB-T. Organic solar cells using PBDB-T as the donor and TFPDI-MFT or TFPDI-FT as the acceptor achieved optimized power-conversion efficiencies (PCEs) of 6.4% and 6.3%, respectively. A more pronounced difference was observed for the as-cast devices, with PCEs of 5.96% and 5.41%, respectively, while the two acceptors also exhibited different responses to processing additives. These results suggest that the spatial position and distribution of local torsion, rather than simply the overall degree of molecular twisting, represent important molecular-design parameters for fully fused PDI-based nonfullerene acceptors.
