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

High power conversion efficiency (PCE) perovskite solar cells (PSCs) rely on optimal alignment of the energy bands between the perovskite absorber and the adjacent charge extraction layers. However, since most of the materials and devices of high performance are prepared by solution-based techniques, a deposition of films with thicknesses of a few nanometers and therefore a detailed analysis of surface and interface properties remains difficult. To identify the respective photoactive interfaces, photoelectron spectroscopy measurements are performed on device stacks of methylammonium-lead-iodide (MAPI)-based PSCs in classical and inverted architectures in the dark and under illumination at open-circuit conditions. The analysis shows that vacuum-deposited MAPI perovskite absorber layers are n-type, independent of the architecture and of the charge transport layer that it is deposited on (n-type SnO(2)or p-type NiOx). It is found that the majority of the photovoltage is formed at the n-MAPI/p-HEL (hole extraction layer) junction for both architectures, highlighting the importance of this interface for further improvement of the photovoltage and therefore also the PCE. Finally, an experimentally derived band diagram of the completed devices for the dark and the illuminated case is presented.