27 Using classical molecular dynamics they showed the presence of a PbI 2-like phase made of clusters of Pb 2+ and I − before any nucleation is observed. 18 Further insight into the nucleation mechanism of MAPbI 3 in solution was recently reported by Röthlisberger and coworkers. 16,18–26 Interestingly, a strong correlation between the concentration of PbI 4 2− species in the precursor solution and the density of charge recombination centers in the perovskite thin film has been reported. 16,17 These coordinating solvents serve the function of dissolving the precursors and of forming iodoplumbate complexes (such as PbI 2, PbI 3 − and PbI 4 2− coordinated by various solvent molecules) which facilitate perovskite film formation. Within solution-processing techniques, dimethylformamide (DMF) and dimethyl sulfoxide (DMSO) are commonly employed solvents to control the formation of highly crystalline perovskites. 14,15 Controlling the reaction kinetics is crucial to form a homogeneous, defect-free photoactive layer for efficient solar cells. The formation process of the perovskite phase is, however, far more complex and it evolves through various intermediate phases. methylammonium iodide, hereafter MAI) leads to thin film formation by solvent evaporation. The basic chemical reaction for MHP synthesis is surprisingly simple: mixing a solution of a metal−halide precursor ( e.g. 10,11 Related to this, variations in the local structure in form of grain boundaries or incomplete crystallization 12,13 may give rise to accelerated degradation and loss of efficiency. A drawback of MHPs is their propensity to defect formation due to the weak metal-halide bonds. 8,9 In combination with low-cost and large-scale processing techniques such as spin-coating from solution or vapor phase deposition, MHPs fulfill all requirements to compete with established photovoltaic technologies. MHPs show favorable optoelectronic properties such as a direct band gap, 1–3 large dielectric constants, 4 low carrier effective masses, 5,6 sharp optical absorption with low Urbach energies, 7 and low non-radiative recombination. Introduction Metal halide perovskites (MHPs) have emerged in the last decade as the most promising material class for efficient solar energy conversion in photovoltaic devices. Our results provide insight into the key steps of the perovskite formation on a microscopic scale, providing hitherto inaccessible details on the factors limiting the perovskite growth and on the effect of different halides on the kinetics of crystal formation. The relatively fast rearrangement of 2− n complexes followed by motion of MA cations limits the perovskite growth. Undercoordinated 2− n complexes are initially formed which create the 3D perovskite framework mediated by additional nucleophilic attacks. Our results show clear evidence of halide-driven chemistry: MAI iodine ions attack lead ions in the PbI 2 layers and cause a nucleophilic substitution of Pb–I bonds with a subsequent breaking of the PbI 2 layer. methylammonium iodide (MAI) structure, characteristic of intermediate phases observed in sequential deposition methods.To unveil the electronic and atomistic features of this process we carry out ab initio molecular dynamics simulations on a model system which consists of a stoichiometric layered lead iodide (PbI 2) Despite its relevance, there is a lack of microscopic understanding of the nucleation and crystallization processes during the formation of the perovskite phase from its precursors. Controlling the crystallization mechanism of metal halide perovskites is of utmost importance to grow defect-less perovskite layers for efficient solar cells and optoelectronic devices.
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