Charge carriers in both hybrid and all-inorganic lead halide perovskites form large polarons.

Electron/hole traps related to interstitial iodine defects show the typical features of iodine photo-electrochemistry, inducing MAPbI3 defect tolerance.

Metal halide perovskites have become a popular material system for fabricating photovoltaics and various optoelectronic devices. However, long-term reliability must be assured. Instabilities are manifested as light-induced ion migration and segregation, which can lead to material degradation. Discordant reports have shown a beneficial role of ion migration under illumination, leading to defect healing. By combining ab initio simulations with photoluminescence measurements under controlled conditions, we demonstrate that photo-instabilities are related to light-induced formation and annihilation of defects acting as carrier trap states. We show that these phenomena coexist and compete. In particular, long-living carrier traps related to halide defects trigger photoinduced material transformations, driving both processes. Defect formation can be controlled by blocking under-coordinated surface sites, which act as a defect reservoir. By use of a passivation strategy we are thus able to stabilize the perovskite layer, leading to improved optoelectronic material quality and enhanced photostability in solar cells.

Only a selected group of two-dimensional (2D) lead-halide perovskites shows a peculiar broad-band photoluminescence. Here we show that the structural distortions of the perovskite lattice can determine the defectivity of the material by modulating the defect formation energies. By selecting and comparing two archetype systems, namely, (NBT)PbI and (EDBE)PbI perovskites (NBT = n-butylammonium and EDBE = 2,2-(ethylenedioxy)bis(ethylammonium)), we find that only the latter, subject to larger deformation of the Pb-X bond length and X-Pb-X bond angles, sees the formation of V color centers whose radiative decay ultimately leads to broadened PL. These findings highlight the importance of structural engineering to control the optoelectronic properties of this class of soft materials.

The photoinduced removal of trap states due to Frenkel defects is found to enhance the PLQE of perovskite thin films.

Lead-halide perovskites are outstanding materials for photovoltaics, showing long lifetimes of photo-generated carriers which induce high conversion efficiencies in solar cell and light-emitting devices. Native defects can severely limit the efficiency of optoelectronic devices by acting as carrier recombination centers. The study of defects in lead halide perovskites thus assumes a prominent role in further advancing the exploitation of this class of materials. The perovskites defect chemistry has been mainly investigated by computational methods based on Density Functional Theory. The complex electronic structure of perovskites, however, poses challenges to the accuracy of such calculations. In this work we review the state of the art of defects calculations in lead halide perovskites, discussing the major technical issues commonly encountered and what we believe to be the best practices. By keeping as a test case the prototype MAPbI 3 compound, we discuss the impact of exchange-correlation functional on the electronic structure and on the defect

CsPbI3 nanocrystals are still limited in their use because of their phase instability as they degrade into the yellow nonemitting δ-CsPbI3 phase within a few days. We show that alloyed CsPbxMn1–xI3 nanocrystals have essentially the same optical features and crystal structure as the parent α-CsPbI3 system, but they are stable in films and in solution for periods over a month. The stabilization stems from a small decrease in the lattice parameters slightly increasing the Goldsmith tolerance factor, combined with an increase in the cohesive energy. Finally, hybrid density functional calculations confirm that the Mn2+ levels fall within the conduction band, thus not strongly altering the optical properties.

Tin halide perovskites make up the only lead-free material class endowed with optoelectronic properties comparable to those of lead iodide perovskites. Despite significant progress, the device efficiency and stability of tin halide perovskites are still limited by two potentially related phenomena, i.e., self-p-doping and tin oxidation. Both processes are likely related to defects; thus, understanding tin halide defect chemistry is a key step toward exploitation of this class of materials. We investigate the MASnI3 perovskite defect chemistry, as a prototype of the entire materials class, using state-of-the-art density functional theory simulations. We show that the inherently low ionization potential of MASnI3 is solely responsible of the high stability of tin vacancy and interstitial iodine defects, which are in turn at the origin of the material p-doping. Tin vacancies create a locally iodine-rich environment that could promote Sn(II) → Sn(IV)