Molecular Origins of Defects in Organohalide Perovskites and Their Influence on Charge Carrier Dynamics

The chemical origins of charge recombination centers in lead-based organohalide perovskites were investigated using a combination of quantitative solution chemistry, X-ray diffraction, and time-resolved photoluminescence spectroscopy. We explored the complex, concentration-dependent solution equilibria among iodoplumbate coordination complexes that have been implicated as potential midgap states in organohalide perovskites. High concentrations of PbI₂, PbI₃^–, and PbI₄^2– were found in precursor solutions that match those used to deposit perovskite films for solar cell applications. We found that the concentration of tetraiodoplumbate PbI₄^2– is uniquely correlated with the density of charge recombination centers found in the final perovskite films regardless of the lead precursor used to cast the films. However, mixed-halide perovskites commonly referred to as CH₃NH₃PbI_3–<i>x</i>Cl_<i>x</i> suppressed the formation of PbI₄^2– in comparison to perovskites that included only iodide, which is consistent with the longer charge carrier lifetimes reported in mixed-halide perovskites. These findings bring a molecular-level view to the chemical origins of charge recombination centers that provides a fundamental basis from which to understand the reported improvement in uniformity of perovskite films and devices deposited using sequential methods. These findings also suggest new approaches to control the formation of defect precursors during the deposition of organohalide perovskite absorbers

Approaching Bulk Carrier Dynamics in Organo-Halide Perovskite Nanocrystalline Films by Surface Passivation

The electronic properties of organo-halide perovskite absorbers described in the literature have been closely associated with their morphologies and processing conditions. However, the underlying origins of this dependence remain unclear. A combination of inorganic synthesis, surface chemistry, and time-resolved photoluminescence spectroscopy was used to show that charge recombination centers in organo-halide perovskites are almost exclusively localized on the surfaces of the crystals rather than in the bulk. Passivation of these surface defects causes average charge carrier lifetimes in nanocrystalline thin films to approach the bulk limit reported for single-crystal organo-halide perovskites. These findings indicate that the charge carrier lifetimes of perovskites are correlated with their thin-film processing conditions and morphologies through the influence these have on the surface chemistry of the nanocrystals. Therefore, surface passivation may provide a means to decouple the electronic properties of organo-halide perovskites from their thin-film processing conditions and corresponding morphologies