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 the low-frequency resonant Raman spectrum of methylammonium lead-iodide, a prototypical perovskite for solar cells applications, on mesoporous Al2O3. The measured spectrum assignment is assisted by DFT simulations of the Raman spectra of suitable periodic and model systems. The bands at 62 and 94 cm(-1) are assigned respectively to the bending and to the stretching of the Pb-I bonds, and are thus diagnostic modes of the inorganic cage. We also assign the librations of the organic cations at 119 and 154 cm(-1). The broad, unstructured 200-400 cm(-1) features are assigned to the torsional mode of the methylammonium cations, which we propose as a marker of the orientational disorder of the material. Our study provides the basis to interpret the Raman spectra of organohalide perovskites, which may allow one to further understand the properties of this important class of materials in relation to their full exploitation in solar cells.

Hybrid organic-inorganic semiconductors feature complex lattice dynamics due to the ionic character of the crystal and the softness arising from non-covalent bonds between molecular moieties and the inorganic network. Here we establish that such dynamic structural complexity in a prototypical two-dimensional lead iodide perovskite gives rise to the coexistence of diverse excitonic resonances, each with a distinct degree of polaronic character. By means of high-resolution resonant impulsive stimulated Raman spectroscopy, we identify vibrational wavepacket dynamics that evolve along different configurational coordinates for distinct excitons and photocarriers. Employing density functional theory calculations, we assign the observed coherent vibrational modes to various low-frequency ( 50 cm −1 ) optical phonons involving motion in the lead-iodide layers. We thus conclude that different excitons induce specific lattice reorganizations, which are signatures of polaronic binding. This insight on the energetic/configurational landscape involving globally neutral primary photoexcitations may be relevant to a broader class of emerging hybrid semiconductor materials.

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.

photovoltaic properties of MAPbI 3 perovskites across the tetragonal to cubic transition, due to structural fluctuations on a sub-picosecond timescale that make the instantaneous electronic energy levels and band-gap of the formally cubic, high temperature structure, to differ only slightly from those of the room temperature stable tetragonal phase. disordered position of the Cl anions 45 and the presence of a Raman signal at 66 cm -1 in the high temperature phase of MAPbCl 3 . 47 From a technological perspective, these results help to explain the lack of an observable abrupt change in photovoltaic device performance above room temperature 23 as would be expected to be observable if the light-harvester undergoes a phase transition. This is also further evidence indicating that ferroelectricity contributions to the optoelectronic properties, as in traditional inorganic materials, 54-57 are negligible, since the ferroelectric polarizability is expected to change dramatically across the transition between two different crystalline structures.On the contrary, this view supports other proposed mechanisms, as the spatial charge localization 50, 58 and/or stable band bending effects at the interfaces and grain boundaries, 59 as the basis of the impressive inherent performance of hybrid lead halide perovskites. ASSOCIATED CONTENT Supplementary InformationExperimental methods; theoretical methods and models; EQE measurements on different MAPbI3 devices; corresponding JV measurements of the devices; EQE of a cell with MAPbI 3-xClx ; comparison between theoretical and experimental radial distribution function of MAPbI 3 perovskite; theoretical fluctuation of the band edges.