ADVANCING THE TREATMENT OF RELATIVISTIC THEORY WITHIN THE OLCAO PACKAGE AND WITH APPLICATION TO SOLAR PHOTOVOLTAIC TECHNOLOGIES

Abstract

Methylammonium lead bromide, known as CH3NH3PbBr3, is a promising perovskite material for the absorption layer in solar cells. While its bulk electronic structure is favorable, its long-term stability and interface design remain as primary concerns before it can be used for commercial production. Although experimental methods such as X-ray photoemission spectroscopy can be used to investigate the surface atomic and electronic structures of such materials (e.g., by observing core-level chemical shifts), theoretical support is vital for interpreting the measured data and for guiding the selection of future experiments. Unfortunately, perovskite solar cell materials often contain high Z-number atoms which require relativistic treatment. Here we show the development and implementation of scalar relativistic theory within the density functional theory (DFT) based Orthogonalized Linear Combination of Atomic Orbitals (OLCAO) method for electronic struc- ture calculation. We then demonstrate the application of the method to the calculation of core-level chemical shifts of all elements of CH3NH3PbBr3 and compared the results to those obtained with non-relativistic theory to evaluate the e↵ect of scalar relativistic theory. The results of the relativistic calculations were consistent with experimental expectations, enabling us to accurately calculate the chemical shift resulting from changes in the local en- vironment. In addition, the X-ray absorption near-edge structure (XANES) spectra of CH3NH3 and PbBr2 surface models were studied via depth pro- filing to highlight the influence of the surface e↵ects. The results indicate a shift towards lower energy of the edge onset for Pb-4f and Br-3d atoms in the CH3NH3 surface termination, while the edge onset of Pb-4f and Br-3d does not change for the PbBr2 surface termination. According to the findings, the modifications made on the surface of a material result in alterations in the edge onset.