Methylammonium fragmentation in amines as source of localized trap levels and the healing role of Cl in Hybrid Lead-Iodide Perovskites

TL;DR

This study uses ab-initio calculations to explore how methylammonium fragmentation creates trap levels in hybrid lead-iodide perovskites and demonstrates that chloride inclusion can heal these defects, improving photoabsorption.

Contribution

It reveals the atomic-level mechanisms of defect formation and healing in perovskites, highlighting chloride's role in passivating trap states.

Findings

Ammonia and methylamine create localized levels within the band gap.

Radical cation CH₂NH₃⁺ introduces partially-occupied levels in p-type conditions.

Interstitial Cl can remove these localized defect states from the band gap.

Abstract

The resilience to deep traps and localized defect formation is one of the important aspects that qualify a material as suited photo-absorber in solar cell devices. Here we investigate by ab-initio calculations the fundamental physics and chemistry of a number of possible localized defects in hybrid methylammonium lead-iodide perovskites. Our analysis encompasses a number of possible molecular fragments deriving from the dissociation of methylammonium. In particular, we found that in stoichiometric conditions both ammonia and methylamine molecules present lone-pair localized levels well within the perovskite band gap, while the radical cation CH$_2$NH$_3$$^+$ observed by EPR after irradiation injects partially-occupied levels into the band gap but only in $p$-type conditions. These defects are thus potentially capable to significantly alter absorption and recombination properties. Amazingly, we found that additional interstitial Cl is capable to remove these localized states from the band gap. These results are consistent with the observed improvement of photoabsorption properties due to the Cl inclusion in the solution processing.