Bi- or multi-functional charge transporting materials for perovskite solar cells

Charge transporting materials mainly including hole-transporting and electron-transporting materials, have played a vital role in charge separation, charge transfer, or charge-blocking in perovskite solar cells (PSCs). Among them, some specific species also possess other functions, such as inducing high-quality perovskite-crystalline formation, passivating defects of perovskite, preventing perovskite from erosion of moist, ultraviolet light or heat, as well as modifying electrodes. These bi-or multi-functional charge transporting materials have effectively improved the performance of PSCs in both efficiencies and stability, therefore it is necessary to summarize and analyze the characteristics of these materials as reference for associated researchers. In this review, we will introduce such materials from two aspects of hole-transporting and electron-transporting materials, which were further classified according to the feature of functions. Finally, future opportunities and challenges of exploration of these types of materials are proposed from the perspective of promoting the practical application of PSCs.