A paradoxical defect: Hydrogen interstitial in 2D perovskite with in-gap states but inefficient nonradiative recombination

Hydrogen interstitials (Hi) in three-dimensional and all-inorganic perovskites have often been discussed as electronically active defects that can promote nonradiative recombination or exhibit unconventional defect dynamics. In contrast, we show that, in the two-dimensional perovskite (PhDMA)PbI4 [(PhDMA)=C6H4(CH2NH3)22+], a phenyl-ring-adjacent Hi introduces a localized in-gap state but remains kinetically ineffective for overall nonradiative recombination because hole capture is strongly suppressed. Heyd–Scuseria–Ernzerhof 06 and spin–orbit coupling first-principles calculations combined with multiphonon nonradiative carrier-capture theory are employed to investigate the stable configuration, electronic structure, formation energy, thermodynamic charge-state transition level, and carrier-capture behavior of a phenyl-ring-related H defect. The lowest-energy configuration places Hi near the C2 site of the phenyl ring, where it forms a stable local adsorption state. The defect exhibits a +/0 thermodynamic charge-state transition level within the bandgap and can capture electrons efficiently. Throughout the 100–500 K temperature range, Cn exceeds Cp by six to fourteen orders of magnitude. The total capture coefficient is therefore limited by the slow hole capture step, rendering the defect an inefficient nonradiative recombination center. These results demonstrate that an in-gap defect state and a thermodynamic transition level alone do not determine recombination activity, and identify the local molecular environment of organic spacer layers as a potential handle for defect-tolerant two-dimensional perovskite design.