Advanced Understanding of the Electronic Properties of Perovskite Solar Cells:Contact Formation, Band Energy Diagrams and involved Surface Photovoltages

Abstract

Solar cells incorporating organic inorganic metal halide perovskites as the absorber material have achieved power conversion efficiencies of more than 25% after only a decade of research. The extremely rapid improvement in efficiency of perovskite solar cells compared to pre-established absorber materials, such as silicon, cadmium telluride, or gallium arsenide, is mainly due to their cheap and easy low-temperature, solution-processing preparation techniques such as spin coating. To further improve the power conversion efficiency of perovskite solar cells, it is still necessary to develop a fundamental understanding of the device physics. The focus of this work lies, therefore, on investigating the energy band diagram of perovskite solar cells, both in the dark and under illumination at open circuit conditions, predominantly using photoelectron spectroscopy (PES). Two different architectures are investigated and compared: i) the classical architecture where the perovskite absorber is deposited onto the electron extraction layer and ii) the inverted architecture where the perovskite is deposited onto the hole extraction layer. Initial experiments showed that perovskite absorbers are extremely light-sensitive, meaning that even small intensities of background light, like the visible light emitted from the X ray source, can induce a photovoltage which will significantly affect the PES measurements by shifting all spectra to higher or lower binding energies. To shield the sample from the visible light emitted by the X ray source, the setup of the X ray photoelectron spectroscopy (XPS) system used in this work was improved by installing an aluminum window in between the sample and the X ray source. In the next step, a comparative study of several different perovskite absorbers on n type SnO2 (classical) and p type NiOx substrates (inverted architecture) was performed. It was proven that the underlying substrate has no effect on the doping level of the perovskite absorbers, as it has previously been proposed in the literature. The perovskite absorbers are always measured to be n doped. It is suggested that the literature reported substrate effect originates from background light during the measurement. This leads to an unnoticed photovoltage resulting in a binding energy shift of all spectra, which leads to the determination of incorrect doping levels. For both architectures, the majority of the photovoltage and hence the open-circuit voltage of the full device is identified at the n-type perovskite | p-type hole extraction layer interface. The interfaces between the perovskite and the respective hole extraction layer (classical: spiro MeOTAD and inverted: NiOx) were then investigated in detail. For the perovskite | spiro MeOTAD interface a classical step-by-step interface experiment was performed. Since spiro MeOTAD films used in perovskite solar cell devices are usually doped with LiTFSI, at first a vacuum deposition process of LiTFSI doped spiro MeOTAD through co-evaporation of both materials was developed. The interface characterization proved that a band bending occurs in the dark, which changes to a flat band situation under illumination, corresponding to a surface photovoltage. For the inverted architecture, the perovskite | NiOx interface was investigated using the tapered cross-section PES method, which demonstrated the presence of a band bending in the dark as well. Finally, the results from the photovoltage measurements and the detailed interface characterizations were combined to derive complete energy band diagrams for both architectures under dark and illuminated open-circuit conditions.