Scanning Photoelectron Spectroscopy of Tapered Cross Sections of Perovskite Solar Cells: Element Distribution, Band Alignment, Space Charge Layers and Photo-Voltage Distributions

Abstract

Photoelectron spectroscopy is used for the characterization of chemical and electronic properties of semiconductor interfaces. As the information depth is extremely low, inner interfaces of device stacks are analyzed by depositing sequentially the respective contact material onto the base material in vacuum chambers integrated with the PES system. Thus elemental distribution including ionic state of the materials in contact as well as doping and electronic potential distribution induced by interface charge transfer can be monitored. For devices with non planar architecture or deposition processes not compatible with vacuum as both are given for perovskite solar cells, this procedure is not applicable. In this paper we demonstrate the photoelectron spectroscopic analysis of a complete working perovskite solar cell ranging from crystalline TiO2 on FTO front contact over perovskite intermixed in meso-TiO2 and the perovskite layer to Spiro-MeoTAD and Au back contact by preparing a tapered cross section of the cell stack. A wet chemically prepared mixed FAPbI3 MAPbBr3 solar cell of 18% efficiency is analyzed. As the analyzed spot size is in the range of the layer thicknesses a small angle tapered edge is prepared by polishing in order to drastically increase the local resolution across the cell stack. The tapered cross section is measured in the dark and under open circuit operando conditions. Using core level photoelectron spectroscopy the elemental distributions which forms in the wet chemical deposition process of the precursor solution are derived from the measured core level intensities. In addition, the electronic contact potentials are mapped, which allows conclusions on the band diagram of the full stack. We find that the perovskite absorber is n-type and a strong band bending in the p-type spiro-MeOTAD hole transport contact layer. Shifts of the core level emission binding energies under in situ illumination indicate that the cell photo-potential is induced to a large part at this back contact. We conclude that the diffusion voltage of the cells is not formed across the TiO2-perovskite front contact but mostly across the perovskite/SpiroMeOTAD/Au back contact. This example demonstrates that the wealth of information that can be obtained on full devices applying the method of scanning photoelectron spectroscopy on device cross section using a tapered edge approach.