Structural softness in photovoltaic perovskites

Perovskite photovoltaics (PVs) are a compelling candidate as a next-generation energy harvesting technology owing to the outstanding optoelectronic properties, low cost, and facile fabrication of perovskite materials. In the meantime, unsolved issues in perovskite stability still challenge the prospect of final commercialization. Recently, many studies have demonstrated that most of these advantages and issues are closely related to the soft nature of perovskites, making the understanding of these properties increasingly important. Here, we summarize and assess the interrelated origins and impacts of this unique property as well as its mechanistic interpretations, ranging from the constituent ions to the perovskite lattice and electronic structures. We also highlight recent advances in making use of this property for improving the perovskite materials. Finally, the remaining challenges in fully understanding the soft nature of perovskites are critically discussed. We hope this effort will provide new insights for making perovskite PVs more reliable and help them reach their full potential.