Understanding the role of organic cations on the electronic structure of lead iodide perovskite from their UV photoemission spectra and their electronic structures calculated by DFT method

Abstract

Organometal halide perovskites are very attractive as they can be used as efficient light sensitizing materials for solar cells with high power conversion efficiency. In this paper, we report the results of a study on various lead iodide perovskite crystals, which were prepared with different organic cations, and their properties in relation to their electronic structures, particularly their valence bands. For this purpose, we have carried out the XRD and UV Photoemission Spectroscopy (UPS) measurements along with the electronic structure computations. Among the organic cations studied here, it is found that not all organic cations can favorably form a perovskite crystal, though it meets the Goldschmidt tolerance factor. The organic cation, which shows the favourability of the formation of perovskite crystal structure, exhibits a band-edge shifting in the UPS spectrum of its perovskite crystal structure toward to smaller binding energy in comparison to the UPS spectrum of its organic salt. Such band-edge shifting is corresponding to the formation of the valence band in its perovskite crystal structure. The electronic structure calculations using Density Functional Theory (DFT) method, confirmed the formation of an energy gap that is in good agreement with the experimental results. However, the valence band is mainly constructed by the I− orbitals, while the conduction band is mainly constructed by the Pb2+ orbitals, as reported in some literatures. The present study results then suggests that the organic cations play only a minor role in their photovoltaic processes, though they substantially contribute to the valence band formation by transferring their valence electrons to the iodide elements.