Unraveling film transformations and device performance of planar perovskite solar cells

High performance (>15%) organometaltrihalide based solar cells have been demonstrated in recent years to be a promising candidate for low cost photovoltaics and have attracted significant attention in the photovoltaic community. Planar thin film perovskite solar cells, which are more easily fabricated, provide a great platform to investigate the perovskite film properties. Until now, many of the properties of perovskite thin films remain unexplored and the link between film properties and device performances is in need of investigation to further boost the efficiencies of these devices. Here, film transformation of perovskite materials is demonstrated as a critical factor to reach high performance in planar heterojunction CH₃NH₃PbI_3-xCl_x solar cells. Reaction induced secondary phases can be observed and carefully controlled by tuning the processing conditions during film formation. The properties of CH₃NH₃PbI_3-xCl_x films are investigated and a possible formation pathway is proposed. It is shown that the high performance devices are attainable with a small portion of secondary phases coexisting with CH₃NH₃PbI₃ film and power conversion efficiencies of up to 14% are achieved. The correlations between the phases present, device performance and physical properties are discussed to identify the role of the secondary phases in CH₃NH₃PbI_3-xCl_x material.