Free Excitons and Exciton-Phonon Coupling in CHNHPbI Single Crystals Revealed by Photocurrent and Photoluminescence Measurements at Low Temperatures.

We investigated the exciton-phonon couplings and exciton binding energy in CHNHPbI (MAPbI) single crystals using temperature-dependent photocurrent (PC) and photoluminescence (PL) spectroscopy. The PC and PL data provide clear evidence of the existence of excitons in orthorhombic-phase MAPbI. The temperature-dependent PC data were found to be less influenced by the bound excitons than the PL data, and thus the PC data reflect the intrinsic scatterings of excitons. We observed that the exciton-phonon couplings were strong in MAPbI and determined the longitudinal optical phonon energy to be 16.1 meV. Moreover, on the basis of the temperature dependences of the PC and PL data, we evaluated the exciton binding energy to be 12.4 meV for orthorhombic-phase MAPbI single crystals. Our findings pave a way for using simultaneous PC and PL measurements to determine precisely fundamental properties of perovskites.