Interfaces in All-Perovskite Tandem Solar Cells

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All-perovskite tandem solar cells promise high efficiencies and low costs. Hybrid perovskites stand out because it is possible to produce films of only a few hundred nanometers and low defect density with relatively simple processes. The tandem geometry enables efficiencies beyond the radiative limit of single junction solar cells. Despite rapid progress, perovskite-perovskite tandem solar cells still face several challenges: (i) The stability of the absorbers. The high bromide content necessary for high bandgaps of top cell absorber favors phase, while Sn2+ in the low-bandgap absorber might oxidize to Sn4+. (ii) Interface recombination at the interface between absorber and the electron transport layers (ETL) or at one of the various interconnection layers, e.g. caused by lateral heterogeneity and leakage. Furthermore, the loss of volatile components, ion migration, or chemical reaction might lead to recombination-active trap states or extraction barriers. (iii) Optical losses, such as parasitic absorption losses in charge transport and interconnection layers, reflection losses and incomplete absorption, which limit the current especially of the bottom low-bandgap solar cell. The key to solving these challenges lies in the many interfaces in a tandem solar cell. Strategies include additives, interface passivation, new contact materials and thinning of charge transport and interconnection layers as well as structuring for increased absorption. For a target-oriented optimization of tandem cells, a thorough understanding of the associated interfaces is of crucial importance for the further technology development. Therefore, the aim of this project is to investigate the physical and chemical effects that lead to performance losses at the interfaces of perovskite-perovskite tandem solar cells. To this end, we will manufacture perovskite-perovskite tandem device, and will develop advanced spatially resolved characterization tools complemented by theoretical modeling all the way from nanoscale interface effects to the device level.