A mesogenic unit based low melting point solid additive for efficient and stable organic solar cells†

Abstract

Inspired by the multi roles of liquid crystal molecules, which exhibit both crystalline and liquid characteristics, we report a new solid additive, CB8-Br, by combining a biphenyl mesogenic unit and a bromine alkyl chain. The melting temperature of CB8-Br is 80.1 °C, matching well with the annealing temperature of most active layer systems in organic solar cells (OSCs). Therefore, CB8-Br can display not only a liquid state during the active layer annealing process, but also a solid state during the device operation. Moreover, the unique design enables CB8-Br to effectively optimize the morphology of the active layer while avoiding the drawbacks of liquid additives, such as the poor reproducibility and device stability issues. Upon introducing CB8-Br, the absorption spectrum of the active layer exhibited a significant redshift, which is beneficial for more light harvesting and increasing the short-circuit current density of the devices. Additionally, higher and more balanced charge carrier mobilities, along with suppressed carrier recombination, were observed in OSCs optimized with CB8-Br. As a result, the devices by using CB8-Br achieved a superior power conversion efficiency of 18.12%, significantly higher than 16.59% of the control devices without additive. Furthermore, the stability of OSCs was also probed and the CB8-Br based devices could demonstrate higher durability.