Hybrid metal-organic perovskite semiconductors such as methylammonium lead iodide CH3NH3PbI3 have emerged as promising new materials for photovoltaic devices and optoelectronics. The power conversion efficiency of solar cells based on perovskite materials exceeded 20% within eight years and a theoretical limit of 30% has been proposed. Such an impressive performance results from favorable material properties such as direct band gap of roughly 1.6 eV, large absorption coefficient and high charge carriers mobility. We measured the surface of CH3NH3PbI3 single crystals by making use of two photon photoemission spectroscopy (2PPE). Shallow traps have been introduced by annealing the samples in atmosferic conditions at 100 °C (MA100) or 200 °C (MA200). The Figure shows that the long timescale evolution of integrated 2PPE signal deviates from the diffusive 1/ √ t decay only in the case of annealed samples. The onset of the trapping strongly depends on the quality of the crystal. In MA100 the localization takes place on the timescale of few picoseconds whereas it falls below the picosecond in highly degraded MA200. Probably the defect density of MA200 is so high that localization takes place as soon as the electrons cool down below the mobility edge of the traps. The picosecond localization of excited electrons in degraded CH3NH3PbI3 samples is consistent with the progressive reduction of photoconversion efficiency in operating devices. Minimizing the density of shallow traps and solving the aging problem may boost the macroscopic efficiency of solar cells to the theoretical limit.
 Time-resolved photoemission spectroscopy of electronic cooling and localization in CH3NH3PbI3 crystals, Z. Chen, M. Lee, Z. Zhang, H. Diab, D. Garrot, F. Lédée, P. Fertey, E. Papalazarou, M. Marsi, C. Ponseca, E. Deleporte, A. Tejeda and L. Perfetti, Physical Review Materials 1, 045402 (2017).