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

  • Stewart R
  • Grieco C
  • Larsen A
 et al. 
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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. O ne of the central narratives in the organo-halide perovskite field has been that material and device performance depend largely on morphology and processing conditions. 1−4 This narrative is exemplified by the correlation of device performance with cuboid size; 5 by the disparity of perovskite crystallization kinetics when PbCl 2 versus PbI 2 are used as Pb 2+ sources; 6,7 and by the influence of PbCl 2 on nucleation, growth, and electronic structure of perovskite films. 1,8−10 While there is little doubt that material morphology influences overall photovoltaic device performance, the under-lying mechanisms by which morphology or processing conditions affect fundamental properties such as photo-luminescence lifetime or charge recombination kinetics have not been fully clarified. Kinetic models have been proposed to predict how photoluminescence lifetime and recombination kinetics depend on the density of charge carriers and charge traps 11,12 present in the forbidden bandgap of the materials, but the chemical origins of these traps remain unclear. Recent experimental and computational studies suggest that charge recombination centers in organo-halide perovskites are related to the presence of iodide-rich chemical species 13−15 and that chemical passivation of these species upon coordination with small molecules can improve the performance of perovskite solar cells. 16−18 However, the role that charge recombination centers at the surfaces versus the bulk may have in influencing the electronic properties of perovskites remains unclear. Identifying the chemical origins of defects may provide insight about how to increase the long-term stability of organo-halide perovskite electronic materials and solar cells because many degradation pathways likely originate at the surfaces of perovskite crystals.

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  • John AsburyThe Pennsylvania State University, University Park, Pennsylvania

  • Robert J. Stewart

  • Christopher Grieco

  • Alec V. Larsen

  • Joshua J. Maier

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