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

Abstract

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 SnO2 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.

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

“…A similar band bending has been reported by both TCS‐PES and the classical sequential deposition interface experiments. [ 20,27,29 ] At the front contact a sharp Ti2p 3/2 peak can be seen at 459.3 eV, and it does not change from the perovskite toward the TiO 2 layer (Figure 6b) confirming that there is no band bending at the perovskite/TiO 2 interface. Therefore, the TCS‐PES shows that there is a downward band bending at the spiro‐MeOTAD/perovskite interface and that the perovskite/TiO 2 interface is flat.…”

“…Moreover, the Ti2p binding energy does not change from the interface to TiO 2 layer showing that there is no band bending at the perovskite/TiO 2 interface, in agreement with the literature. [ 20,27,29 ] The O1s spectra as measured by SDP‐PES show a single peak at around 530.4 eV at the perovskite/TiO 2 interface whereas two peaks are observed at around 530.4 and 532.5 eV for the TiO 2 layer corresponding to the metal oxide and metal hydroxide, respectively. [ 62 ] In the O1s spectra taken with TCS‐PES, two peaks located at 530.0 and 532.7 eV are visible and are attributed to the TiO bond in TiO 2 and to the OH/CO bonds.…”

“…[ 18 ] PES studies have been mostly done on the single crystal surfaces or at the interface using the sequential deposition of overlayer film. [ 5,6,19,20 ] There are also few studies where a special type of PES, namely, hard X‐ray photoelectron spectroscopy (HAXPES), and sputter depth profiling PES (SDP‐PES), were used to study bulk of the PSCs. HAXPES uses synchrotron radiation, i.e., a tunable X‐ray source, where the photon energy can be adapted to selectively increase or decrease the surface sensitivity of the measurements.…”

Two birds with one stone: dual grain-boundary and interface passivation enables >22% efficient inverted methylammonium-free perovskite solar cells

“…We stress that these are not accompanied by changes in the peak position or shape of the I 3d 5/2 , Cs 3d 5/2 and C 1s core levels, which excludes the possibility of a shift due to electronic doping of the perovskite bulk. 87,95,135,136 We therefore relate the damping of the signal at ~138.7 eV for SP and GBP&SP, together with the appearance of a second Pb component at ~139.1 eV as well as a Cl 2p and PEA signal, to the formation of a thin PEACl-based passivation layer on the surface of the films that has a different chemical environment. The fact that for GBP the damping of the signal at ~138.7 eV is less pronounced and the peak becomes broader fits with our observation from SEM and CL that passivation happens mainly close to the grain boundary regions.…”

Two Birds with One Stone: Dual Grain-Boundary and Interface Passivation Enables > 22% Efficient Inverted Methylammonium-Free Perovskite Solar Cells

“…We stress that these are not accompanied by changes in the peak position or shape of the I 3d 5/2 , Cs 3d 5/2 and C 1s core levels, which excludes the possibility of a shift due to electronic doping of the perovskite bulk. 87,95,135,136 We therefore relate the damping of the signal at B138.7 eV for SP and GBP&SP, together with the appearance of a second Pb component at B139.1 eV as well as a Cl 2p and PEA signal, to the formation of a thin PEACl-based passivation layer on the surface of the films that has a different chemical environment. The fact that for GBP the damping of the signal at B138.7 eV is less pronounced and the peak becomes broader fits with our observation from SEM and CL that passivation happens mainly close to the grain boundary regions.…”