Density of gap states in CHNHPbIsingle crystals probed with ultrahigh-sensitivity ultraviolet photoelectron spectroscopy.

The advantages of methylammonium triiodideplumbate (CHNHPbI)-based organic-inorganic hybrid halide perovskite have led to devices with power conversion efficiencies of >20%. The CHNHPbIstructure is prone to be more sensitive towards external effects due to its higher flexibility than inorganic counterparts. Nevertheless, a direct photoemission spectroscopy study is still lacking on the density of gap states (DOGS) influenced by air exposure and synchrotron light-induced degradation. In this paper, we investigate the evolution of electronic structure in CHNHPbIsingle crystals after air exposure and intense synchrotron light irradiation to reveal the effects on its density of states distribution below and above the valence band maximum (VBM) by using ultrahigh-sensitivity photoelectron spectroscopy. We find that the PbIcompounds, decomposed from CHNHPbIafter air exposure, could not affect the DOGS distribution but only give the VBM shift in the high binding energy region, which is dramatically different from the impacts of an impurity found for other organic or inorganic counterparts. A further study using intense synchrotron irradiation confirms the decomposed processes for CHNHPbI: (i) the initial degradation would induce the formation of PbI, which gives a negligible impact on the DOGS above the VBM; (ii) the continuous intense light irradiation could further degrade PbIto metallic Pb, in which DOGS appears in the energy bandgap.