  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.
