Efficiency Improvement of All‐Small‐Molecule Organic Solar Cells Through Fused‐Aromatic‐Ring Side‐Chained Donors

Side‐chain engineering influences the organic solar cell (OSC) devices active layer morphology fundamentally and it is an efficient strategy to improve device performances. A novel small molecular donor with conjugated fused‐aromatic‐ring side chain on a benzo[1,2‐ b :4,5‐ b′ ]dithiophene core is developed. To the best of our knowledge, this is the first example where fused‐aromatic‐ring units are being utilized as conjugated side chains to extend the conjugation system of small molecular donor materials. The material property, OSC device performance, active layer morphology, and photovoltaic mechanism are investigated. This molecular design strategy has proven to be effective in fine‐tuning and enhancing the molecular stacking/orientation and phase separation, which led to the enhanced charge separation and transportation, suppressed charge recombination, and as a result, devices with an improved short‐circuit current density of 24.9 mA cm −2 , fill factor of 69.9%, and power conversion efficiency of 13.6% are achieved. The results reveal that a fused‐aromatic‐ring utilized as conjugated side chain strategy can potentially be an effective way for molecular rational design, aiming to improve the OSC performances.