Dual Passivation of Point Defects at Perovskite Grain Boundaries with Ammonium Salts Greatly Inhibits Nonradiative Charge Recombination

Abstract

Experiments demonstrate that grain boundaries (GBs) exhibit detrimental effect on carrier lifetimes in MAPbI3 (MA= CH3NH3 +). On the basis of the nonadiabatic (NA) molecular dynamics simulations, we demonstrated that NH4Cl can simultaneously passivate the common point defects that introduce recombination centers at GBs and accelerate electron–hole recombination but shows small effects in the bulk. The MA interstitial (MAi) and the substitutional MA to Pb (MAPb) in pristine MAPbI3 leave the band gap and charge recombination rates largely unchanged but create deep electron traps at GBs by separately either distorting inorganic octahedra or creating an I-dimer. Cl– and NH4 + remove the in-gap states by either restoring the distorted octahedra or destroying the I-dimer. Thus, the band gap recovers to the pristine system, NA coupling decreases, and decoherence accelerates, extending the carrier lifetime even twice longer than MAPbI3. This study shows that the negative role of GBs can be removed by dually passivating with NH4Cl.

Defect formation and healing at grain boundaries in lead-halide perovskites

“…Further annealing of the GB can suppress charge trapping at the center GB, which is a key property to ensure high performance in polycrystalline PSCs. Recently proposed passivation strategies for GB defects include alkali metals, 34,88 chlorine doping, 40,89,90 or Lewis bases. 91,92…”

“…Further annealing of the GB can suppress charge trapping at the center GB, which is a key property to This journal is © The Royal Society of Chemistry 2022 ensure high performance in polycrystalline PSCs. Recently proposed passivation strategies for GB defects include alkali metals, 34,88 chlorine doping, 40,89,90 or Lewis bases. 91,92 We further perform dri-diffusion (DD) simulations on a model PSC, using our previous parametrization of MAPbI 3 , 93 to estimate the impact of the observed shallow trap states on the device performance.…”

Rationalization of passivation strategies toward high-performance perovskite solar cells

“…As illustrated in Fig. 3b, the comprehensive passivation effect of ammonium halide salts comes from the following points: (a) ammonium groups heal the negatively charged defects by electrostatic interactions (ionic bonds); 72 (b) ammonium groups reduce organic cation vacancies by occupying the corresponding sites; 14 (c) ammonium groups passivate substitutional defects like MA Pb defects by attracting one iodine atom to break the I-dimer; 73 (d) iodide and bromide substituents fill the halide vacancies of perovskites; 74 (e) fluoride substituents in the halide site with the strong electronegativity form robust ionic bonds with Pb-related defects and hydrogen bonds with organic cations; 75,76 (f) interstitial chlorine ions restore the tilting octahedra induced by the MA interstitial defect via Coulomb interaction with lattice Pb 2+ ions; 74 and (g) additionally, if the ammonium halide salts are attached to electron-withdrawing groups, they will have the ability to passivate positively charged defects. 72 Among these abilities, (a), (c), (e) and (g) belong to the chemical passivation effect.…”

Grain boundary sliding and distortion on a nanosecond timescale induce trap states in CsPbBr3: ab initio investigation with machine learning force field

“…[36][37][38] Nevertheless, atomistic calculations indicate that both pristine and defective GBs accelerate the nonradiative carrier recombination by introducing localized electronic states at dangling and wrongly formed bonds, and the corresponding passivation strategies are developed. [39][40][41][42][43] Besides, point defects are reported to have lower formation energies in the GB region than in the bulk, which leads to defect accumulation at GBs and further increases the carrier recombination rate. [44][45][46] Furthermore, the GBs are reported to facilitate ion migration, but this effect can result in both beneficial defect healing and detrimental structural degradation.…”