Density of gap states in CH3NH3PbI3 single crystals probed with ultrahigh-sensitivity ultraviolet photoelectron spectroscopy

Abstract

The advantages of methylammonium triiodideplumbate (CH3NH3PbI3)-based organic–inorganic hybrid halide perovskite have led to devices with power conversion efficiencies of >20%. The CH3NH3PbI3 structure 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 CH3NH3PbI3 single 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 PbI2 compounds, decomposed from CH3NH3PbI3 after 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 CH3NH3PbI3: (i) the initial degradation would induce the formation of PbI2, which gives a negligible impact on the DOGS above the VBM; (ii) the continuous intense light irradiation could further degrade PbI2 to metallic Pb, in which DOGS appears in the energy bandgap.