Star-shaped D-π-D hole-transporting materials regulated by molecular planarity and their application in efficient perovskite solar cells

Abstract During the molecular design of hole-transporting material (HTM) for perovskite solar cells (PSCs), the planarity is the critical factor, which can regulate multiple properties affecting the photovoltaic performance, including the hole transport and morphology uniformity. Herein, a planar carbazole group is applied as the core unit to obtain a star-shaped HTM CS-43. Owning to the planar core unit, CS-43 shows more negative HOMO level, which is beneficial to obtain high photovoltage, with respect to the twisted HTM CS-42. More importantly, compared with the twisted HTM CS-42, the planar core unit of CS-43 strengthens the trend of intermolecular stacking, thus resulting in the significant improvement in hole mobility. Furthermore, with the planar core unit, the morphology of CS-43 film is also improved. As an overall result of high hole mobility and excellent film morphology, the PSC based on CS-43 present a quite promising power conversion efficiency of 15.87% with long-term stability, which is over 40% higher than that of PSC prepared by twisted CS-42. Accordingly, the planarity improvement in core unit can optimize both the hole transporting and morphology performance, providing us a powerful strategy for the further development of small molecule HTM.