In spite of the unprecedented advance of organohalide lead perovskites in the photovoltaics scenario, many of the characteristics of this class of materials, including their slow photoconductivity response, solar cell hysteresis, and switchable photocurrent, remain poorly understood. Many experimental hints point to defect migration as a plausible mechanism underlying these anomalous properties. By means of state-of-the-art first-principles computational analyses carried out on the tetragonal MAPbI₃ (MA = methylammonium) perovskite and on its interface with TiO₂, we demonstrate that iodine vacancies and interstitials may easily diffuse across the perovskite crystal, with migration activation energies as low as ∼0.1 eV. Under working conditions, iodine-related defects are predicted to migrate at the electrodes on very short time scales (<1 μs). MA and Pb vacancies, with calculated activation barriers of ∼0.5 and 0.8 eV, respectively, could be responsible for the slow response inherent to perovskites, with typical calculated migration times of the order of tens of ms to minutes. By investigating realistic models of the perovskite/TiO₂ interface we show that negatively charged defects, e.g. MA vacancies, close to the electron transport layer (TiO₂ in our case) modify the perovskite electronic state landscape, hampering charge extraction at selective contacts, thus possibly contributing to the observed solar cell hysteresis. We further demonstrate the role of the electron transport layer in affecting the initial concentration of defects close to the selective contacts, highlighting how charge separation at the perovskite/TiO₂ interface may further change the defect distribution. We believe that this work, identifying the mobile species in perovskite solar cells, their migration across the perovskite material, and their effect on the operational mechanism of the device, may pave the way for the development of new materials and solar cell architectures with improved and stabilized efficiencies.

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.

Lead halide perovskites show marked defect tolerance responsible for their excellent optoelectronic properties. These properties might be explained by the formation of large polarons, but how they are formed and whether organic cations are essential remain open questions. We provide a direct time domain view of large polaron formation in single-crystal lead bromide perovskites CH₃NH₃PbBr₃ and CsPbBr₃. We found that large polaron forms predominantly from the deformation of the PbBr₃⁻ frameworks, irrespective of the cation type. The difference lies in the polaron formation time, which, in CH₃NH₃PbBr₃ (0.3 ps), is less than half of that in CsPbBr₃ (0.7 ps). First-principles calculations confirm large polaron formation, identify the Pb-Br-Pb deformation modes as responsible, and explain quantitatively the rate difference between CH₃NH₃PbBr₃ and CsPbBr₃. The findings reveal the general advantage of the soft [PbX₃]⁻ sublattice in charge carrier protection and suggest that there is likely no mechanistic limitations in using all-inorganic or mixed-cation lead halide perovskites to overcome instability problems and to tune the balance between charge carrier protection and mobility.

In spite of the unprecedented advances of organohalide lead perovskites in optoelectronic devices, many of the characteristics of this class of materials remain poorly understood. Several experimental hints point to defect migration as a plausible mechanism underlying such anomalous properties. Here we present an experimental and theoretical investigation combining measurements of PL rise dynamics at varying temperatures with first principles computational modeling of defect migration under light irradiation. We propose a model in which light irradiation promotes the annihilation of V_I ⁺/I_i ⁻ Frenkel pairs, which we show to be relatively abundant in polycrystalline MAPbI₃. This partly restores a non-defective crystalline environment and eliminates the trapping centers associated with such defect pairs. The PL rise time dynamics at varying temperature provide an activation energy consistent with that calculated for migration of iodine defects, supporting the proposed model. We further illustrate a synergistic effect of ion/defect migration and lattice dynamics, with local reorientation of the methylammonium cations assisting the migration of charged defects. Our results provide the interpretative basis for further investigating the unusual light-induced modifications characterizing organohalide perovskites.