The Role of Subsurface Valence Band Localization in the Passivation of Perovskite Nanocrystals

Abstract Traditional colloidal quantum dots rely on larger‐bandgap semiconductor shells for the localization of excitons in their emissive core. Unshelled quantum dots are typically weakly luminescent due to severe quenching by deep electronic traps. Lead halide perovskite nanocrystals deviate from this convention and require only common monofunctional aliphatic ligands on their surfaces to attain near‐unity luminescence quantum yields. Here, a combination of density functional theory calculations and experimental studies is employed to elucidate the mechanistic role of the ligands. The attachment of a surface ligand leads to a distortion of the surface, which results in the localization of the valence band deeper into the subsurface layers. This creates an energetic profile that is similar to a conventional core–shell quantum dot and renders the valence electrons unavailable for interaction with adventitious chemical species. It is further shown that a phosphonate moiety offers significantly stronger binding and leads to the long‐term preservation of this beneficial subsurface localized band structure. As experimental proof, a red‐emitting mixed‐halide perovskite nanocrystal, which is usually susceptible to degradation and spectral shifts, exhibits remarkable stability under a phosphonate ligand shell. These findings and mechanistic insights may guide the future engineering of robust perovskite nanocrystals for color displays and light‐emitting applications.