Thia[5]helicene-Based D-pi-A-type Molecular Semiconductors for Stable and Efficient Perovskite Solar Cells: A Theoretical Study

Development of ideal small-molecule hole-transporting materials (HTMs) is one of the most effective means to improve the performance of perovskite solar cells. Meanwhile, the theoretical chemistry method is an efficient pathway to optimize the molecular structure and to probe the structure-property relationship. To further find new HTMs, thia[5]helicene and its counterpart perylothiophene are introduced into the D-pi-A-type molecular scaffold of MPA-BT-CA, and the results show that helicene-type HTMs (SM46 and SM48) exhibit obvious advantages in comparison with that of the planar SM45 and SM47. The matched energy levels with the mixed perovskite provide a precondition for the easy hole transfer at the interface, and the blueshifted adsorption spectra can effectually decrease the optical competition between the perovskite and HTMs. More importantly, the helicene-type HTMs show the higher mobilities than that of the planar counterparts, which is mainly contributed by the enlarged hole transfer integrals via compact molecular stacking. Fluorine addition can also stabilize the highest occupied molecular orbitals and enhance the hole transport ability of HTMs. Moreover, a simplified adsorption model for the MPA-BT-CA is constructed to investigate the interfacial properties of adsorbed systems, and the results demonstrate that bidentate adsorption displays more stable adsorption configuration and a better passivation effect for the iodine-vacant FAPbI3 surface. In summary, this work provides useful clues for the design of new D-pi-A-type HTMs, and two promising helicene-based HTMs are proposed.