Recent progress in hybrid perovskite solar cells based on n-type materials

Over the past several years, thin film organic-inorganic halide perovskite solar cells (PSCs) have rapidly improved with their power conversion efficiency (PCE) reaching over 22%, nearing that of polycrystalline silicon solar cells. This emerging photovoltaic technology has shown great potential for large-scale application owing to its high efficiency, low material cost and facile fabrication process. Electron-selective contacts, n-type materials that favor the charge transport and collection from the perovskite light absorbers to the front electrode, are important and seen as critical to fabricate high-performance PSCs. In this review, recent progress in such n-type materials is overviewed, with the emphasis on the most explored inorganic semiconducting metal oxides (including TiO, SnO, ZnO and ZnSnO). The impact of the structure of the n-type materials on the morphology of the perovskite films, the charge extraction and recombination, as well as device performance is highlighted. Optimization of n-type materials and their application in different device architectures are also summarized. An understanding of the intricate function of the n-type materials in PSC devices will assist in the design of materials to give precise control over the light harvesting and charge collection within PSC devices leading to high-performance