Mechanism suppressing charge recombination at iodine defects in CH₃NH₃PbI₃ by polaron formation

Abstract

Metal-halide perovskites exhibit high efficiencies in photovoltaic applications and low recombination rates, despite the high concentrations of intrinsic defects. We here study the hole trapping at the negative iodine interstitial, which corresponds to the dominating recombination center in CH 3 NH 3 PbI 3 . We calculate the free energy profile for the hole trapping at 300 K using the Blue Moon technique based on hybrid functional molecular dynamics. We find that the hole trapping is energetically unfavorable and requires overcoming an energy barrier. This behavior stems from the position of the vertical (−/0) transition level of the iodine defect and the formation of a polaron. Our simulations show that the polaron does not interact with the iodine interstitial and hops through the lattice on a sub-picosecond scale. Our results highlight a mechanism by which the low mononuclear (trap-assisted) recombination rates in CH 3 NH 3 PbI 3 can be explained.Metal-halide perovskites, such as methylammonium lead iodide (CH 3 NH 3 PbI 3 ), have emerged as promising materials for high efficiency solar cells. 1-3 One of the main reasons for the outstanding performance of devices based on halide perovskites, is the slow recombination of photogenerated carriers 4-6 . It has been shown that the recombination rates measured in CH 3 NH 3 PbI 3 are dominated by the bimolecular recombination process. 7-9 The very low monomolecular (trap-assisted) recombination rate is surprising, considering that perovskites are generally processed at low temperature, 10-12 which would suggest high concentrations of intrinsic defects. Additionally, using advanced hybrid functional calculations including spin-orbit coupling effects, Du 13 showed that iodine-related defects in CH 3 NH 3 PbI 3 , such as iodine interstitials, introduce deep hole and electron trapping levels inside the band gap and should play a role in carrier trapping and recombination.To understand the low recombination rates at iodine interstitials in CH 3 NH 3 PbI 3 , it is necessary to determine their dominating charge state. Experimental studies indicate that single-crystalline samples of CH 3 NH 3 PbI 3 are intrinsic or slightly p-doped, 14 while n-doping is observed in thin films near the surface. 15 According to computational studies, 13,16 the iodine interstitial should be stable in a negative charge state in intrinsic, n-doped and slightly p-doped CH 3 NH 3 PbI 3 samples. Stability of I − i can be expected for a wide range of Fermi levels, since such a defect corresponds to simply adding an iodine anion in its formal −1 charge state to the lattice. Meggiolaro et al. 16 found that the Fermi level is pinned close to the middle of the band gap in stoichiometric sam-ples. These findings indicate that the negative charge state of the iodine interstitial should be the dominant one in CH 3 NH 3 PbI 3 samples. However, the low recombination rate at this defect state still remains to be clarified. One possible explanation for the low recombination rates at the negatively charged iodin...

Nonempirical hybrid functionals for band gaps of inorganic metal-halide perovskites

“…Indeed, the present construction scheme works most effectively when the hybridization of the defect states with the delocalized band states is minimized [39]. We remark that in our scheme the defect levels are obtained without structural relaxation [36,39], thereby explaining their different location in the band gap with respect to previous studies of defects in such perovskite materials [69][70][71]. The defect calculations in this section are carried out with the pseudopotential set PP1 due to the computational cost entailed by the consideration of supercells.…”

Charge Localization, Stabilization, and Hopping in Lead Halide Perovskites: Competition between Polaron Stabilization and Cation Disorder

“…This corresponds to sub-nanosecond dynamics, up to 2-3 orders of magnitude slower than the subpicosecond hopping time-scale observed in MD simulations. 4,38 This discrepancy clearly indicate that the rearrangement of the Pb-I sub-lattice alone cannot explain the dynamics of polaron hopping in CH3NH3PbI3. To get further insight, we recalculate the energy barriers along the same hopping pathway but removing the organic cations from the simulation.…”

“…42 However, hole and electron polarons were found to hop from one localization site to another on a sub-picosecond time-scale in molecular dynamics simulations, a result apparently contrasting with the low mobility usually associated with charge carriers in metal-halide perovskites. 4,38 Overall, while significant progress has been achieved in the study of the optoelectronic properties of this class of materials, there are still fundamental questions that remain unanswered. In particular, while the occurrence of polarons in CH3NH3PbI3 and other lead-halide perovskites has been demonstrated by both theory and experiments, 4,29,43 the distinct role and the complex interplay between the inorganic sub-lattice and the A-site cations, whether organic or not, still eludes a comprehensive characterization.…”

Defect Activity in Lead Halide Perovskites

“…The trapping of holes to form the neutral X 2 − species induces a more limited lattice rearrangement compared to the first case and it is likely that trapped species decay radiatively, resulting in short lifetimes. Notably a small energy barrier to hole trapping at such defects was reported, possibly reducing the trapping activity of interstitial halides . Thus, while both negative and positive halide interstitials feature low formation energies and are active in trapping holes and electrons (respectively), the large lattice reorganization, resulting in long timescales for electron trapping and recombination at positive interstitials, implicates negative interstitials, or analogously lead vacancies, as the key defect species driving undesirable nonradiative recombination processes in both MAPbI 3 and MAPbBr 3 .…”