Three-dimensional organic–inorganic lead iodide perovskites are potential photoconductive materials for solar cells. Nowadays, a high power conversion efficiency exceeding 20% can be achieved. However, perovskite solar cells are reported to suffer from a large hysteresis in the current–voltage curves. This may be attributed to the motion of organic cations with a permanent dipole moment in response to the applied electric field. Therefore, in order to suppress the hysteresis, organic cations that have a zero dipole moment and a molecular size well fitted to the lead iodide cavity are required. Using density functional theory calculations, we theoretically studied the thermodynamic stability and electronic properties of lead iodide perovskites with large guanidinium cations having a nearly zero dipole moment and compared the results with those predicted via the Goldschmidt tolerance factor. The properties of formamidinium–guanidinium (FA_1–xGA_xPbI₃) intermediate alloys were also investigated.

After a brief introduction concerning the general properties of the perovskite class of materials, we review the most relevant contributions to studies on organic–inorganic halide perovskites concerning their application in photovoltaics. Moving on from the rise of a new solar cell assembly procedure (Kojima et al., J. Am. Chem. Soc., 2009, 131, 6050–6051), we focus on the main developments that have occurred over the last five years that have led to high photoconversion efficiencies. We particularly refer to the contribution provided by theoreticians, and how a synergy between theoreticians and experimentalists has enabled us to understand many fundamental characteristics of such an extremely appealing class of materials. Particular attention is devoted towards the ambipolar (i.e., both p- and n-conductive) nature of organic–inorganic halide perovskites, and to the role that the methylammonium cation plays in the structural and electronic properties of these compounds.