Kinetic Molecular Cationic Control of Defect-Induced Broadband Light Emission in 2D Hybrid Lead Iodide Perovskites

In this study we examine the effects of changing organic cation concentrations on the efficiency and photophysical implications of exciton trapping in 2-dimensional hybrid lead iodide self-assembled quantum wells (SAQWs). We show increasing the concentration of alkyl and aryl ammonium cations causes the formation of SAQWs at a liquid-liquid interface to possess intense, broadband subgap photoluminescence (PL) spectra. Electron microscopy, X-ray diffraction, and X-ray photoelectron spectroscopic studies suggest materials formed under these cation concentrations possess morphologies consistent with inhibited crystallization kinetics, but exhibit qualitatively similar bulk chemical bonding to non-luminescent materials stabilized in the same structure from precursor solutions containing lower cation concentrations. Temperature and power-dependent PL spectra suggest the broadband subgap light emission stems from excitons self-trapped at defect sites, which we assign as edge-like, collective iodide vacancies using a simple model of the chemical equilibrium driving material self-assembly. These results suggest changes to the availability of molecular cations can suitably control the light emission properties of self-assembled hybrid organic-inorganic materials in ways central to their applicability in lighting technologies.