Excitonic Emission in Organic-Inorganic Lead Iodide Perovskite Single Crystals

Hybrid perovskite thin films have demonstrated impressive performance for solar energy conversion and optoelectronic applications. However, further progress will benefit from a better knowledge of the intrinsic photophysics of materials. Here, the low temperature emission properties of CH3NH3PbI3 single crystals are investigated and compared to those of thin polycrystalline films by means of steady-state and time-resolved photoluminescence spectroscopy. While the emission properties of thin films and crystals appear relatively similar at room temperature, low temperature photoluminescence spectroscopy reveals striking differences between the two materials. Single crystals photoluminescence exhibits a sharp excitonic emission at high energy, with Full Width at Half Maximum of only 5 meV, assigned to free excitonic recombination and a broad band at low energy. We analyzed the thermal evolution of the free excitonic intensity and linewidth. An excitonic binding energy of 28 meV is extracted from the quenching of the photoluminescence. We highlight a strong broadening of the emission due to LO phonon coupling. The free excitonic emission turned to be very short-lived with a sub-nanosecond dynamics, mainly induced by the fast trapping of the excitons. The free excitonic emission is completely absent of the thin films spectra, which are dominated by trap states band. The trap states energies, width and recombination dynamics present important similarities between films and crystals. These results suggest that the trap states are formed at the surface and grain interface of perovskites.