Liquid CrystallineBenzodithiophene Photovoltaic DonorsThermal Properties Modulation by Side-Chain/Terminal-Alkyl-Group Engineering

Liquid crystalline small molecules have been proven to possess certain advantages in forming a balanced active-layer morphology. In this research, two liquid crystalline small-molecule donors (LC-SMDs) were synthesized. It is found that more flexible groups are beneficial to lowering the liquid crystal (LC) phase transition temperature. LCS6S forms liquid crystals in a relatively low-temperature range (145 °C–154 °C). LCS8S shows an even lower LC temperature of 130 °C–150 °C due to its longer terminal alkyl groups. Lowering the LC temperature allows the active layer to be annealed at the LC state and is more compatible with the device fabrication requirements. Photovoltaic performance is significantly improved after the thermal annealing treatment. The device based on LCS8S:Y6 exhibits balanced crystallization and phase separation, and a higher power conversion efficiency of 10.08% is achieved. This work demonstrates that annealing at the LC state is an effective strategy for optimizing device morphology and provides new insights for controlling the LC temperature of LC-SMDs.