Origin of photoluminescence and experimental determination of exciton binding energy, exciton-phonon interaction, and urbach energy in γ-CsPbI3 nanoparticles

Recently inorganic perovskites have had a deep impact in perovskites-based devices allowing the fabrication of high efficiency and high stability devices. To continue with the development of these technologies a detailed study of the optoelectronic properties of these materials is necessary since this is a fundamental tool for the design of new and more efficient devices. In the present work, the optical properties of γ-CsPbI3 nanoparticles are studied using temperature dependent spectroscopic techniques of transmittance and photoluminescence. The absorption spectrum shows a marked excitonic behavior especially at low temperatures. Analyzing this spectrum with the Elliot function the evolution with temperature of the bandgap energy and the exciton binding energy is studied. From the evolution of the shape of the excitonic peak, both in the absorption and photoluminescence, we determined the excitonic recombination nature of photoluminescence and exciton-phonon interaction energy. Once proven the excitonic nature of photoluminescence we prove that the Stokes Shift has its origin in excitonic thermalization. Finally, analyzing the low energy region of the absorption spectrum the Urbach energy was determined. The obtained values, which are very low for solution processed nanoparticles, indicates that the nanoparticles present excellent crystallinity.