Effects of Shortened Alkyl Chains on Solution-Processable Small Molecules with Oxo-Alkylated Nitrile End-Capped Acceptors for High-Performance Organic Solar Cells

Solution-processable small molecules are significant for producing high-performance bulk heterojunction organic solar cells (OSCs). Shortening alkyl chains, while ensuring proper miscibility with fullerene, enables modulation of molecular stacking, which is an effective method for improving device performance. Here, the design and synthesis of two solution-processable small molecules based on a conjugated backbone with a novel end-capped acceptor (oxo-alkylated nitrile) using octyl and hexyl chains attached to pi-bridge, and octyl and pentyl chains attached to the acceptor is reported. Shortening the length of the widely used octyl chains improves self-assembly and device performance. Differential scanning calorimetry and grazing incidence X-ray diffraction results demonstrated that the molecule substituted by shorter chains shows tighter molecular stacking and higher crystallinity in the mixture with 6,6-phenyl-C-71-butyric acid methyl ester (PC71BM) and that the power conversion efficiency (PCE) of the OSC is as high as 5.6% with an open circuit voltage (V-oc) of 0.87 V, a current density (J(sc)) of 9.94 mA cm(-2), and an impressive filled factor (FF) of 65% in optimized devices. These findings provide valuable insights into the production of highly efficient solution-processable small molecules for OSCs.