Temperature-Dependent Electroabsorption and Electrophotoluminescence and Exciton Binding Energy in MAPbBr₃ Perovskite Quantum Dots

Organic-inorganic lead halide perovskite nanocrystals have attracted much attention as promising materials for the development of solid-state light-emitting devices, but the existence of free or bound excitons or the formation of trap states remains under debate. We recorded the temperature-dependent electroabsorption (E-A) and electrophotoluminescence (E-PL) spectra, that is, electric-field-induced change in absorption and photoluminescence spectra, for methylammonium lead tribromide (MAPbBr₃) colloidal perovskite nanocrystals, that is, quantum dots (QD), doped in a poly(methyl methacrylate) film in the temperature range of 40-290 K. Based on the results, the binding energy of the exciton (electron-hole pair) was estimated. The exciton binding energy of QD of MAPbBr₃ estimated from the absorption and E-A spectra (∼17 meV) is nearly the same as that of a MAPbBr₃ polycrystalline thin solid film, while the exciton binding energy estimated from the temperature-dependent PL spectra (∼70 meV) is much greater than that estimated from the absorption profile. The frequency dependence of the E-A intensity observed at 40 and 290 K for the modulated applied electric field indicates a slow ion migration in nanocrystals, which follows the modulation of the applied electric field at a frequency less than 500 Hz. The observed E-A spectra were analyzed with an integral method on assuming the Stark effect; the magnitudes of the changes in electric dipole moment and polarizability following photoexcitation were determined at each temperature from 40 to 290 K. E-PL spectra show that the PL of QD of MAPbBr₃ is quenched on the application of an external electric field; the extent of quenching is much greater for trap emission than for exciton emission. Exciton-phonon scattering, which is responsible for the line broadening of the PL spectra, is also discussed based on the temperature-dependent PL spectra.