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

Abstract

In this study, we examine the effects of changing organic cation concentrations on the efficiency and photophysical implications of exciton trapping in two-dimensional hybrid lead iodide self-assembled quantum wells (SAQWs). We show that 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 that materials formed under these cation concentrations possess morphologies consistent with inhibited crystallization kinetics but exhibit qualitatively similar bulk chemical bonding to nonluminescent materials stabilized in the same structure from precursor solutions containing lower cation concentrations. Temperature-and power-dependent PL spectra suggest that the broadband subgap light emission stems from excitons self-trapped at defect sites, which we assign as edge-like, collective I vacancies using a simple model of the chemical equilibrium driving material self-assembly. These results suggest that 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.

Defect-Induced Narrowband Light Emission from a 2D Hybrid Lead Iodide Perovskite

“…The right panel of Figure compares the subgap light emission spectra of a defective HA 2 PbI 4 sample we measure at 168 and 88 K. While the spectrum we observe at 88 K possesses a prominent, broad peak similar to those reported by several researchers including our group, − the spectrum we measure at 168 K indicates the intensity of this broad feature diminishes significantly and we observe two additional, more narrow peaks at energies above and below that of the broad peak. In addition, we observe the intensities and energies of these narrow features change as a function of sample temperature.…”

“…The top left panel of Figure shows the bulk structure of our HA 2 PbI 4 samples, which we expect produce the X-ray diffraction (XRD) patterns shown in Figure S1. We showed previously by close comparison that the XRD patterns of those samples possessing substantial room temperature subgap light emission differ from similar samples from which we do not measure any subgap light emission above 200 K . However, we proposed these differences stem from morphological differences between these samples and do not indicate the presence of crystallographic structures besides the monoclinic phase.…”

“…To form and characterize the structure and chemical bonding of defective HA 2 PbI 4 samples, we use the same methods we reported previously. , As shown in Figure S1 of the Supporting Information, we find no evidence of the presence of any phase other than the P 2 1 / c monoclinic structure of this material. This material possesses an established bandgap near 2.7 eV, which is consistent with its absorption spectrum, as shown in Figure S2 of the Supporting Information.…”

“…The self-assembly of ammonium cations with main group metal-halide polyhedra in solutions leads to the formation of hybrid organic–inorganic perovskite and perovskite-like materials whose properties make them promising candidates in optoelectronic applications as both sources and detectors of light . When one incorporates sufficiently large molecular cations into precursor solutions, one can drive the formation of crystalline superlattices of very weakly interacting two-dimensional (2D) sheets of metal-halide octahedra spaced by organic sublattices, sometimes referred to as self-assembled quantum wells (SAQWs). , Recently, several groups have shown the appearance of light emission spectra below the energy of the “free” exciton correlates with the presence of defective regions in these materials. − Despite the interest in these color centers for single photon emission, the strong coupling of electron and lattice phonon degrees of freedom in hybrid SAQW samples stringently limits the coherence time of any single photons emitted by these centers. To enable hybrid SAQWs as emitters of single photons possessing properties useful for quantum information processing applications, researchers need chemical methods to control the defect densities well enough to cause the appearance of narrowband light emission features below the peak stemming from “free” exciton recombination in these materials’ luminescence spectra.…”

Morphology-Dependent Ambient-Condition Growth of Perovskite Nanocrystals for Enhanced Stability in Photoconversion Device

“…Lead-halide perovskite nanocrystals (PNCs) have been identified as promising optoelectronic materials due to their excellent photophysical properties and low-cost production. − The first reported three-dimensional (3D) perovskite nanocubes have exceptional electrical and optical properties, resulting in an excellent power conversion efficiency of over 22% for solar cells and/or rapid photoresponses in photodetector/phototransistor applications. − However, the instability of 3D PNCs under operating and storage conditions has prevented their use in commercial applications. − Recently, efforts have been made to improve the stability of PNCs for practical applications by lowering their dimensionalityspecifically, by creating two-dimensional (2D) PNCs. − It has been reported that 2D PNCs exhibit a much higher stability than their 3D counterparts in optoelectronic devices owing to their high environmental stability. − However, the origin of the morphology-dependent stability of the PNCs has not been clarified. Theoretical studies on 2D methylammonium lead triiodide PNCs have shown that two factorshigh formation energy of 2D PNCs and relaxation of hydrogen bonds at the 2D PNC surfacemay contribute to their enhanced stability.…”