Metal halide perovskite solar cells have become a viable option for future renewable energy. Record single- and tandem-junction all-perovskite solar cells already provide power efficiencies of 28% and 30%, respectively. By monolithically stacking multiple perovskite sub cells with complementary bandgaps and using recombination junctions designed to provide near-zero electrical and optical losses, it is possible to fabricate monolithic multi-junction configurations with high power conversion efficiencies [1]. These require highly efficient and stable perovskite sub-cells with bandgaps over a wide spectral range. Especially for narrow- and wide-bandgap perovskites several challenges still remain in reducing the energy losses and enhancing the stability. Within this general framework I will focus on recent results.

For narrow-bandgap (1.25 eV) tin-lead perovskites we developed novel insulating-passivating interfaces for the electron and hole transport layers that enable high-photovoltage in single- and double-junction solar cells. To create an optimal bandgap (1.34 eV) absorber for single-junction solar cells mixed-metal mixed-halide perovskite have been developed that do not show light-induced halide segregation during prolonged illumination and provide a power conversion efficiency of 19% [1],  among the highest for perovskites in the 1.3 − 1.4 eV range. For tandem-cells an efficient 1.77 eV bandgap perovskite has been developed. It will be shown how common bulk or surface passivating agents disturb the perovskite stoichiometry in such a way that the performance is reduced. Interestingly, compensating for the deficiency or excess created by the passivation and restoring the desired stoichiometry of the precursor fully recovers the device performance [2]. For triple-junction solar cells, 2.05 eV bandgap perovskites are needed.  By carefully adjusting passivating strategies that prevent early halide segregation, we have been to reach over 10% efficiency at improved stability. For quadruple-junction cells, 2.3 eV perovskite solar cells are needed for which a dual-passivation strategy has been found for bulk and surface passivation, which, combined with a ternary fullerene blend electron transport layer, increase the open-circuit to 1.60 V [3]. Finally we tune absorption onsets to the 2.3–3.0 eV range with mixed Br-Cl perovskites and reveal how chloride incorporation shapes structural, optoelectronic, and photovoltaic properties.