Abstract
Perovskite photovoltaics advance rapidly, but questions remain regarding point defects: while experiments have detected the presence of electrically active defects no experimentally confirmed microscopic identifications have been reported. Here we identify lead monovacancy (VPb) defects in MAPbI3 (MA = CH3NH3+) using positron annihilation lifetime spectroscopy with the aid of density functional theory. Experiments on thin film and single crystal samples all exhibited dominant positron trapping to lead vacancy defects, and a minimum defect density of ~3 × 1015 cm−3 was determined. There was also evidence of trapping at the vacancy complex ({{{{{rm{V}}}}}}_{{{{{rm{Pb}}}}}}{{{{{rm{V}}}}}}_{{{{{rm{I}}}}}})^{-} in a minority of samples, but no trapping to MA-ion vacancies was observed. Our experimental results support the predictions of other first-principles studies that deep level, hole trapping, {{{{{{rm{V}}}}}}}_{{{{{{rm{Pb}}}}}}}^{2-}, point defects are one of the most stable defects in MAPbI3. This direct detection and identification of a deep level native defect in a halide perovskite, at technologically relevant concentrations, will enable further investigation of defect driven mechanisms.
Highlights
Perovskite photovoltaics advance rapidly, but questions remain regarding point defects: while experiments have detected the presence of electrically active defects no experimentally confirmed microscopic identifications have been reported
Since experimental identification of point defects requires the use of spectroscopic methods that provide direct local structural information, or laborious studies to correlate for example electrical measurements across a sequence of chemically controlled sample sets, the vast majority of our present understanding comes from computational research effort[4,5,6,7,8,9]
When combined with appropriate schemes to correct for localized charge within the supercell these approaches enable defect formation energies (DFE) and charge transition levels to be calculated[10,13]
Summary
Perovskite photovoltaics advance rapidly, but questions remain regarding point defects: while experiments have detected the presence of electrically active defects no experimentally confirmed microscopic identifications have been reported. Our experimental results support the predictions of other firstprinciples studies that deep level, hole trapping, V2PÀb , point defects are one of the most stable defects in MAPbI3. This direct detection and identification of a deep level native defect in a halide perovskite, at technologically relevant concentrations, will enable further investigation of defect driven mechanisms. The iodine interstitial was found to be a deep defect and to have one of the lowest DFE values, and so be one of most stable defects[11,13] It exhibits a (+/–) transition level 0.95 eV above the valence band maximum (VBM), the neutral charge state is energetically less favorable than either Iþi or IÀi. Recent firstprinciples calculations conclude that the iodine interstitial is the primary nonradiative recombination center in hybrid perovskites[13]
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