Organohalide lead perovskites have revolutionized the scenario of emerging photovoltaic technologies. The prototype MAPbI₃ perovskite (MA = CH₃NH₃⁺) has dominated the field, despite only harvesting photons above 750 nm (∼1.6 eV). Intensive research efforts are being devoted to find new perovskites with red-shifted absorption onset, along with good charge transport properties. Recently, a new perovskite based on the formamidinium cation ((NH₂)₂CH⁺ = FA) has shown potentially superior properties in terms of band gap and charge transport compared to MAPbI₃. The results have been interpreted in terms of the cation size, with the larger FA cation expectedly delivering reduced band-gaps in Pb-based perovskites. To provide a full understanding of the interplay among size, structure, and organic/inorganic interactions in determining the properties of APbI₃ perovskites, in view of designing new materials and fully exploiting them for solar cells applications, we report a fully first-principles investigation on APbI₃ perovskites with A = Cs⁺, MA, and FA. Our results evidence that the tetragonal-to-quasi cubic structural evolution observed when moving from MA to FA is due to the interplay of size effects and enhanced hydrogen bonding between the FA cations and the inorganic matrix altering the covalent/ionic character of Pb–I bonds. Most notably, the observed cation-induced structural variability promotes markedly different electronic and optical properties in the MAPbI₃ and FAPbI₃ perovskites, mediated by the different spin–orbit coupling, leading to improved charge transport and red-shifted absorption in FAPbI₃ and in general in pseudocubic structures. Our theoretical model constitutes the basis for the rationale design of new and more efficient organohalide perovskites for solar cells applications.

We report optical measurements on MAPbI₃solar cells, together withab initiosimulations, to investigate the material property changes across the tetragonal to cubic phase transition.

We report on ab initio electronic structure and Car–Parrinello molecular dynamics simulations for several structural models of the prototype MAPbI₃ perovskite for solar cells applications. We considered both configurations having a preferred orientation of the MA cations, giving rise to a net dipole alignment, and configurations with an isotropic distribution of the MA cations, respectively representative of polar (ferroelectric) and apolar (antiferroelectric) structures. Our calculations demonstrate the preferred stability of a set of polar structures over apolar ones, with an energy difference within 0.1 eV and a conversion barrier within 0.2 eV per unit cell (four MAPbI₃), thus possibly accessible at room temperature. Ferroelectric-like orientations lead to a quasi I4cm structure for the inorganic component, characterized by lack of inversion symmetry, while the antiferroelectric-like orientations are associated to a quasi I4/mcm structure. Ab initio molecular dynamics simulations on the polar structures show no molecular rotations in the investigated time-scale, while several MA rotations are observed in the same time scale for the considered apolar structure, which is thus characterized by a higher disorder. The I4cm and I4/mcm types of structure have markedly different band structures, despite showing a relatively small band gap variation. Simulations carried out on finite surface slabs demonstrate that a net orientation of the MA cations gives rise to a strong bending in the valence and conduction bands, which could definitely assist charge separation and reduce carrier recombination, provided one is able to effectively stabilize polar compared to apolar structures. We believe our results could contribute an important step toward an in-depth comprehension of the basic properties of organohalide perovskites, assisting a further optimization of their photovoltaic response.