Crystalline-Size Dependence of Dual Emission Peak on Hybrid Organic Lead-Iodide Perovskite Films at Low Temperatures

In this work, we have investigated the crystalline-size dependence of optical absorption and photoluminescence emission of CHNHPbI films, which is necessary to identify the potential practical applications of the gadgets based on perovskite films. This study was carried out at low temperatures to minimize the extra complexity induced by thermal effects. The purpose was to clarify the origin of the dual emission peak previously reported in the literature. We found that the grain size is responsible for the appearance or disappearance of this dual emission on CHNHPbI at low temperatures, whereas we have inferred that the thickness of the perovskite layer is a much more important factor than the size of the grains in the location of the energy of the band gap. Moreover, the increase in the grain size allows slowing down the phase transition. Additionally, we evidence a decrease in the effective Rydberg energy of the exciton in several samples, from 23-25 meV at 7 K to 12-13 meV at 165 K, by fitting to Elliott-Toyozawa theory. We have extracted other important physical parameters of perovskites from the photoluminescence-data deconvolution, such as the band gap, exciton-phonon interaction, and exciton binding energy. A new phase transition at 45.5 K was determined by the temperature dependence of full width at half-maximum and the integrated intensity of the photoluminescence, and it was confirmed by the radiative lifetime obtained from the time-resolved photoluminescence emission by means of time-correlated single-photon counting at different temperatures, excitation fluencies, and emission energies.