Review of lead-free halide perovskites as light-absorbers for photovoltaic applications: From materials to solar cells

Metal halide perovskite materials have revolutionized the solution-processed solar cells and become the vanguard of research focus with an unprecedented improvement of power conversion efficiencies up to 23.3%, which pose a remarkable challenge to thin film and multicrystalline silicon photovoltaics. Nevertheless, for conventional perovskite solar cells based on lead, it is ineluctable to take the toxicity of lead and the long-term stability of the devices into consideration when the deployment of this technology in mass production is put on the agenda. To tackle the toxicity concern, extensive research effort has been devoted to the development of environmentally friendly lead-free halide perovskites for photovoltaic applications. Herein, we review the advancements in this emerging research area and discuss potential alternative elements to replace lead within the halide perovskite lattice. Particular attention will be focused on the properties of the corresponding perovskites and the relevant production of high efficiency photovoltaic devices, with the aim of identifying the materials to be competitive with lead-based perovskites. For the moment, tin-based perovskites appear likely to be the most preeminent from the point view of power conversion efficiency, despite the stabilities of the materials and the related devices are far from satisfaction. The introductions of germanium, bismuth, antimony, and certain transitional metals into perovskites brought about promising optoelectronic properties as well. This booming class of compounds constitutes unexplored territory and emerges as an affordable and robust candidate for green solar technology.