In-Situ Growth-Studies of Metal Halide Perovskites for Tandem Solar Cells.

• Applicant: Dr. Tim Kodalle
• Subject Area: Solid State and Surface Chemistry, Material Synthesis ; Synthesis and Properties of Functional Materials ; Physical Chemistry of Solids and Surfaces, Material Characterisation
• Term: from 2020 to 2023
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 455786566

Using a combination of in-situ photoluminescence and UV-Vis measurements together with X-ray diffraction and imaging techniques, I want to investigate the growth of metal halide perovskites depending on the substrate in use. The goal of the project is to understand how different substrates, in particular fully functional bottom solar cells, affect the nucleation, phase development, and crystallization path (the “growth”) of subsequently deposited perovskite layers. Furthermore, the interplay of the properties of the substrate and the composition, preferential orientation, structure, and morphology of the perovskite will be analyzed. This understanding will enable me to propose individually optimized deposition processes for the perovskite layers in tandem solar cells using various bottom cell technologies. In particular, I will compare the properties of the perovskite layer on standard substrates for single-junction solar cells (glass/indium tin oxide (ITO)/hole transport layer (HTL)-stacks) with its properties on bottom cells based on Cu(In,Ga)Se2 (CIGS) absorber layers and rear-junction SI-heterojunction devices respectively. In the different stages of the proposed work program, I will use CIGS absorber layers prepared by three different deposition techniques (molecular beam epitaxy, thermal co-evaporation, sequential sputtering/annealing) as well as on different substrates (rigid glass and flexible polyimide foil). Furthermore, I will vary the HTL used: sputtered NiOx, atomic layer deposited NiOx, and self-assembling monolayers (SAMs). While the different deposition techniques lead to strong variations in the roughness of the CIGS absorber layer, and therefore indirectly affect the morphology of the substrate of the perovskite growth, the use of different HTLs is directly affecting the nucleation of the perovskite layer. Consequently, the main hypothesis to be tested during this project is that the surface roughness of the bottom cell as well as the surface properties of the HTL will alter the initial nucleation as well as the following growth and the overall properties of the perovskite. Combining my experience in the fabrication and characterization of polycrystalline absorber materials for photovoltaic applications with the expertise and equipment for in-situ, multi-modal analysis of perovskite thin-films available at the chosen host organization, I aim to analyze the growth of the perovskite layer with a high time-resolution being able to develop a fundamental understanding of the involved processes and to give detailed suggestions for the top cell fabrication. Finally, I will also utilize the experimental setup at the host organization to investigate the degradation mechanisms of highly efficient CIGS/perovskite and Si/perovskite tandem solar cells. Doing so, I will be able to unravel the interplay of the properties of the bottom cells on the properties of the perovskite and its stability.

• DFG Programme: WBP Fellowship
• International Connection: USA
• Host: Dr. Carolin M. Sutter-Fella