Strategies for Modifying TiO2 Based Electron Transport Layers to Boost Perovskite Solar Cells

The research on solution processed metal halide perovskite solar cells (PSCs) as a new type of solar cells has experienced explosive growth since the first report in 2009. It is impressive that solar energy conversion efficiency has increased to over 23%. Outstanding optoelectronic properties including high absorption coefficient, high mobility, and long diffusion length of charge carriers have been revealed in the family of hybrid organic inorganic halide perovskite materials that are considered the heart of solar cells. A long-anticipated feature for solar cells that the diffusion lengths of charge carriers outstrip the active layer thickness of a device has been demonstrated in PSCs so that the efficiency of extracting photocarriers, particularly electrons at the interfaces becomes a key parameter controlling global device performance. The n-type semiconductor TiO2 with the merits of thermal and chemical stability, low cost, and suitable band edge positions has been regarded an ideal electron transporting layer (ETL) material in PSCs performing the function of selectively extracting photoelectrons and subsequently delivering them toward a current collector. Besides the highly concerning energy conversion efficiency of PSCs, the challenge of the current-voltage hysteresis phenomenon and instability of PSCs are also revealed to be closely related with TiO2, ETLs. In this review, the recent progress on strategies for modifying TiO2 ETLs by controlling morphology, surface modification, doping, and constructing composites to improve global performance of PSCs is reviewed. Moreover, the perspective on future development of TiO2 based ETLs for high performance PSCs is proposed on the basis of the comprehensive and deep understanding of TiO2 from the area of photocatalysis. It is anticipated that finely tailoring the features and properties of TiO2 ETLs will further release large room for exciting enhancements in the global performance of PSCs.