by Prof. Marcus Bär, HZB & HI ERN, Berlin, Germany.
The complexity of thin film solar cell structures, which are often comprised of a multitude of layers, interfaces, surfaces, elements, impurities, etc., make it both difficult and crucial to characterize and understand the chemical and electronic structure of each component and their interactions. It is demonstrated that the combination of different soft and hard x-ray as well as electron spectroscopies is an extraordinary method for illuminating the chemical and electronic material characteristics from many different perspectives, ultimately resulting in a comprehensive picture of these properties.
This presentation will first focus on our studies on the impact of different post-deposition treatments (PDT) on the chemical and electronic surface structure of high-efficiency Cu(In,Ga)Se2 (CIGSe) chalcopyrite – based thin film solar cell absorbers to demonstrate the capabilities of the analysis approach outlined above. Alkali-free CIGSe, NaF-PDT CIGSe, and NaF/KF-PDT CIGSe absorbers grown by low-temperature coevaporation were compared and contrasted. Using a combination of depth-sensitive soft and hard x-ray photoelectron spectroscopy as well as soft x-ray absorption and emission spectroscopy, a detailed insight into the chemical structure of the CIGSe surface and how it changes upon different PDTs has been gained. The NaF/KF-PDT leads to the formation of bilayer structure in which a K-In-Se – type species covers the CIGSe compound that, in composition, is identical to the chalcopyrite structure of the alkali-free and NaF-PDT absorber. Furthermore, for all samples the position of the valence band maximum is found to shift away from the Fermi level with increasing surface sensitivity, with this effect being most pronounced for the NaF/KF-PDT CIGSe sample. Additional inverse photoemission measurements suggest the surface band gap for the NaF/KF-PDT absorber (2.52 eV) to be significantly enlarged compared to the CIGSe bulk band gap (1.15-1.20 eV) and to the surface band gap found for an absorber treated with NaF only. The observed electronic structure changes are in agreement with the revealed chemical surface properties and can be linked to the recent breakthroughs in CIGSe device efficiencies.
The second part of the presentation will focus on revealing the relationship between structure – chemical and electronic – and function of mixed-halide perovskites (HaPs) in thin film solar cells. In particular, the presentation will concentrate on the role of SnF2 on the material properties of inorganic HaPs. Using a spectroscopic “toolchest” that is extended by spatially resolved photoemission electron microscopy and ambient pressure hard x-ray photoelectron spectroscopy, it is further revealed why pin holes in HaPs are not as detrimental as in other thin film solar cells and light is shed on the HaP decomposition mechanism when exposed to humidity and light. Ultimately, we used resonant inelastic soft x-ray scattering to examine the projected partial density of states of and molecular dynamic effects in HaPs.
Marcus Bär is professor for “X-ray Spectroscopies” at the Friedrich Alexander Universität Erlangen-Nürnberg and heads the department of “Interface Design” at the Helmholtz-Zentrum Berlin für Materialien und Energie GmbH. Simultaneously, Prof. Bär also heads the research department „X-ray spectroscopy at interfaces of thin films“ at the Helmholtz-Institute Erlangen-Nürnberg for Renewable Energy (HI ERN).
Bär studied Physics at Potsdam University and Environmental Engineering/Renewable Energies at the Hochschule für Technik und Wirtschaft Berlin. In 2004, he received his PhD in Electrical Engineering from the Technische Universität Berlin. In 2005, Bär moved – as an Emmy-Noether Fellow – to the University of Nevada, Las Vegas, where he later became a research assistant professor. In 2008, he moved back to Berlin and started to establish his young investigator group at the Helmholtz-Zentrum Berlin für Materialien und Energie GmbH. 2011 – 2017, he was a W1 Professor for “Photovoltaics” at the Brandenburgische Technische Universität Cottbus-Senftenberg.
The Bär group is renowned for combining different lab- and synchrotron-based photon-in – electron-out and photon-in – photon-out spectroscopic methods to interrogate the chemical and electronic structure of energy conversion materials and thin-film layer stacks, with a particular focus on photovoltaics. The core expertise of the group is to non-destructively reveal composition and electronic structure profiles by deliberately employing different spectroscopies excited with soft and/or hard x-rays.