Perovskites for photovoltaics: a combined review of organic-inorganic halide perovskites and ferroelectric oxide perovskites

Over the past few years, very interestingly, two subclasses of perovskites - organic-inorganic halide perovskites and ferroelectric oxide perovskites, have simultaneously become the hotspots in the research field of photovoltaics. Organic-inorganic halide perovskites have launched a new era of low-cost, high-efficiency solar cells, due to their easy solution processability and superior optical and electrical properties for the photovoltaic effect. More recently, a so-called giant switchable photovoltaic effect has been demonstrated in organic-inorganic halide perovskites, thus promising a new memristive functionality. On the other hand, the recent renaissance of ferroelectric oxide perovskites for photovoltaics is caused by their fundamentally new photovoltaic mechanisms, which can produce a photovoltage far beyond the bandgap and may even lead to a boost of energy conversion efficiency. In addition, the combination of photovoltaic properties with the ferroic orders may create many novel functionalities for ferroelectric oxide perovskites. Toward the common goals of developing high-efficiency photovoltaics and novel opto-electronic functional devices, these two different subclasses of perovskites shall be brought together into a combined review. In this context, we review both organic-inorganic halide perovskites and ferroelectric oxide perovskites for photovoltaics, focusing on the material nature and the photovoltaic mechanisms. We also discuss their respective unresolved issues, along with useful suggestions for future research.