Tapered Cross‐Section Photoelectron Spectroscopy of State‐of‐the‐Art Mixed Ion Perovskite Solar Cells: Band Bending Profile in the Dark, Photopotential Profile Under Open Circuit Illumination, and Band Diagram

Abstract

The purpose of this article is twofold. On the one hand the method of spacial resolved photoemission spectroscopy on small angle tapered cross‐sections (TCS) of complete devices is introduced to analyze simultaneously the chemical and electronic structure. On the other hand, a specific working principle of the analyzed cell type is revealed. Solar cells of 18% efficiency are prepared from a single precursor (FAPbI3)0.85(MAPbBr3)0.15 with excess of 15% PbI2. It is shown that TCS‐phototoelectron spectroscopy allows to determine the chemical composition as well as the potential distribution across the full device in the dark and in operation. The energy converting contact is the hole extraction back contact. Interestingly the photopotential in the analyzed cell type is predominantly created within the hole extraction layer and not in the n‐doped perovskite absorber. With the addition of measured core level to valence band maximum positions of the respective layers, TCS line scans lead to the band diagram for the full device. In addition, depth variations of the chemical composition are found: the bromide concentration increases while the iodide concentration is reduced near and within the mesoporous TiO2 layer.

The Electronic Structure of MAPI‐Based Perovskite Solar Cells: Detailed Band Diagram Determination by Photoemission Spectroscopy Comparing Classical and Inverted Device Stacks

“…Considering the high work function difference between MAPI (4.16 eV) and NiO x (5.41 eV) (see Figure S4, Supporting Information), the formation of a built‐in potential in the dark at this interface is expected. In a detailed analysis performed on the NiO x /MAPI interface using a tapered cross‐section approach, [ 33 ] the presence of a band bending inside the MAPI and thus a built‐in potential at this interface is demonstrated (see Figure S6, Supporting Information). Under illumination, this built‐in potential acts as driving force for the extraction of the excited holes out of the perovskite absorber into the underlying NiO x layer which results in a negative SPV and therefore a shift of the core levels and the valence band to lower binding energies.…”

“…The interfaces where the photovoltages appear, are analyzed in more detail, either by classical step‐by‐step interface experiments [ 32 ] or by preparing and measuring a tapered cross‐section. [ 33 ] For both architectures, it is proven that at the interfaces where the SPV forms under illumination, a built‐in potential exists in the dark. As a result, for both architectures the used MAPI films are observed to be n‐type in the dark, independent of the substrate (EEL or HEL) that they are deposited on.…”

The Hole‐Tunneling Heterojunction of Hematite‐Based Photoanodes Accelerates Photosynthetic Reaction

“…To confirm the hole-tunneling transport in the RuO x -Al 2 O 3 -SF heterojunctions,t he open circuit photovoltage (OCP) of the SF,A SF and SMIS-3 photoanodes,w hich represents the amount of the band bending at the time being with respect to that in the dark condition, [21] was measured in the same solution of 0.1 Mphosphate buffer solution (pH 7.0) in dark and under visible light illumination. As shown in Figure 2d,O CP = open circuit voltage under illumination (OCV light )m inus open circuit voltage in dark (OCV dark ).…”

Top‐Down Approach to Study Chemical and Electronic Properties of Perovskite Solar Cells: Sputtered Depth Profiling Versus Tapered Cross‐Sectional Photoelectron Spectroscopies

“…To overcome the substrate degradation caused by sputtering, recently the mechanical process of etching has been tried by various groups to study the chemistry of the entire PSCs. [ 27–29 ]…”