In this talk, we will discuss about the excitonic properties of CH3NH3PbI3 (MAPI) single crystals at low temperature, and compare them to those of thin polycrystalline films. The main feature is the appearance of a sharp emission line (FWHM ~ 5 meV) at high energy that is attributed to the free exciton signature, and which is completely absent in thin-films. The observation of the free excitonic emission in the low temperature phase allow to monitor the huge thermal broadening due to electron-LO-phonon coupling .
 Hiba Diab, Gaëlle Trippé-Allard, Ferdinand Lédée, Khaoula Jemli, Christèle Vilar, Guillaume Bouchez, Vincent L.R. Jacques, Antonio Tejeda, Jacky Even, Jean-Sébastien Lauret, Emmanuelle Deleporte, and Damien Garrot, The Journal of Physical Chemistry Letters 7, pp 5093–5100 (2016)