Role of Lead Vacancies for Optoelectronic Properties of Lead-Halide Perovskites

Abstract

Methylammonium lead iodide perovskite materials have been shown to be efficient in photovoltaic devices. The current fabrication process has not been perfected, leaving defects such as site vacancies, which can trap charge and have a detrimental effect on photogenerated charge carriers. Here focus is placed on the effect a Pb site vacancy has on the charge-carrier dynamics following photoexcitation. The electronic structure of materials with vacancies is often found in open-shell configurations with unpaired electrons in the conduction/valence bands To accurately describe unpaired electrons, spin-polarized and noncollinear spin calculations are performed on both neutral and charged vacancy systems. This work presents spin-polarized and noncollinear spin ground-state electronic structures and nonradiative rates of charge-carrier relaxation and introduces an extension to a novel procedure to compute photoluminescence spectra for open-shell models. This study describes nonadiabatic dynamics of MAPbI3 models within a Redfield formalism focusing on the role of a Pb vacancy defect on electronic relaxation processes. Results show the vacancy of the Pb ion introducing a new energy state within the unblemished material band gap region. This additional unoccupied state is expected to increase the nonradiative relaxation lifetime of the excited electron, allowing for a longer lifetime of the charge carrier and increased opportunity for secondary relaxation mechanisms or collection to take place.

Influence of Defects on Excited-State Dynamics in Lead Halide Perovskites: Time-Domain ab Initio Studies

“…The band gap is almost unaffected by the defect, because the shallow trap state generated by the atomic vacancy merges into the VB. Kilin and co-workers studied the effect of the Pb vacancy on the charge carrier dynamics in MAPbI 3 using a Redfield theory formalism . They also found that the Pb vacancy increases the nonradiative relaxation time of the excited electron, in agreement with the NA-MD simulations …”

Ab initio nonadiabatic molecular dynamics of charge carriers in metal halide perovskites

“…We highlighted that charge recombination is sensitive to the oxidation state of these defects. 163,164,165 Chu et al 88 have investigated the influence of various point defects on nonradiative electron-hole recombination in MAPbI 3 . They focus on five point defects with low formation energy: iodine interstitial (I i ), MA substituted by iodine (MA I ) or Pb (MA Pb ), Pb vacancy (Pb V ), and iodine vacancy (I V ).…”

Comprehensive Understanding and Controlling the Defect Structures: An Effective Approach for Organic-Inorganic Hybrid Perovskite-Based Solar-Cell Application

“…The absence of gap states from intrinsic defects of MAPbI 3 can be ascribed to the ionic bonding from organicinorganic hybridization. Besides, Vogel et al claimed that the role of lead vacancies in OHP causes the spin specific trap state at near valence band ( Figure 6; Vogel et al, 2017).…”