Exciton Binding Energy and the Nature of Emissive States in Organometal Halide Perovskites

Abstract

Characteristics of nanoscale materials are often different from the corresponding bulk properties providing new, sometimes unexpected, opportunities for applications. Here we investigate the properties of 8 nm colloidal nanoparticles of MAPbBr3 perovskites and contrast them to the ones of large microcrystallites representing a bulk. X-ray spectroscopies provide an exciton binding energy of 0.32 ± 0.10 eV in the nanoparticles. This is 5 times higher than the value of bulk crystals (0.084 ± 0.010 eV), and readily explains the high fluorescence quantum yield in nanoparticles. In the bulk, at high excitation concentrations, the fluorescence intensity has quadratic behavior following the Saha-Langmuir model due to the nongeminate recombination of charges forming the emissive exciton states. In the nanoparticles, a linear dependence is observed since the excitation concentration per particle is significantly less than one. Even the bulk shows linear emission intensity dependence at lower excitation concentrations. In this case, the average excitation spacing becomes larger than the carrier diffusion length suppressing the nongeminate recombination. From these considerations we obtain the charge carrier diffusion length in MAPbBr3 of 100 nm.

A stable lead halide perovskite nanocrystals protected by PMMA

“…S11), indicating good color purity. The samples show a Stokes shift of~39 meV between the absorption band edge and the emission line, implying a direct exciton recombination process [38]. A power law dependence (I∝L k ) was observed with k values near 1.3 (Fig.…”

Colloidal lead halide perovskite nanocrystals: synthesis, optical properties and applications

“…However, single-particle measurements are sometimes necessary, in order to obtain accurate correlations between their optical properties and size/morphology. 27,71,[94][95][96][97][98][99] Tian et al 100 provided direct visualization of the photogenerated carrier diffusion in single MAPbBr 3 and MAPbI 3 NWs and nanoplates using time-resolved PL-scanning microscopy (Figures 8a-e). This technique is based on the photoexcitation of a single particle at one specific position and subsequently scanning the substrate at different positions by time-resolved PL as a function of delay time.…”

Unveiling the Nature of Charge Carrier Interactions by Electroabsorption Spectroscopy: An Illustration with Lead-Halide Perovskites

“…We then explain how EAS can help nanoplatelets of various thicknesses. [43][44][45][46][47] The latter are denominated 'quasi-2D' (q-2D), as they exhibit a quasi-two-dimensional structure comprising several (3-5) layers of perovskite unit cells, stabilized by a capping layer of octylammonium cations. Each of these nanostructures exhibits a unique optical signature, observable from their absorbance spectrum as shown in Fig.…”

“…This EA response has been previously assigned to a quadratic Stark effect, thus arising from the presence of confined excitons. [43] However, such an assignment is questionable, notably due to the detailed work by Ziffer et al, in which EAS of thinfilm of methylammonium lead-iodide perovskite was carried out. [36] In this context, it was shown that the obtained EA response at room temperature was better described by the low-field FKA model.…”