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 over ~27% and ~30%, respectively. The next target in photovoltaic energy conversion can possibly be met by developing all-perovskite multi-junction solar cells. These require highly efficient and stable perovskite sub-cells with bandgaps in wide spectral range. Especially for narrow and wide bandgap perovskites several challenges remain in reducing the energy loss between bandgap and open-circuit voltage and in stability. By monolithically stacking multiple perovskite sub cells with complementary bandgap 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. Within this general framework I will focus on recent results.

For narrow bandgap (1.25 eV) tin-lead perovskites we developed a 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 we  established a novel mixed-metal mixed-halide perovskite composition. The optimized perovskite did not show signs of light-induced halide segregation during prolonged illumination. Optimizing the device configuration resulted in a power conversion efficiency of 19% [1], among the highest for perovskites in the 1.3 − 1.4 eV bandgap range.

For very wide-bandgap (2.3 eV) perovskite solar cells 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. Integrated into a coupled photovoltaic-electrochemical  system for continuous solar-driven water splitting we achieve unassisted solar-to-hydrogen conversion efficiency of 6.5% [2], outperforming single-absorber reported to date.

Next to device performance, we studied the shallow defect properties in metal-halide perovskite films by measuring transient photoluminescence. Interestingly, lowering the temperature changes the trap energy landscape, making the shallow defects shallower until they vanish into the conduction or valance bands at cryogenic temperature. This result is corroborated by an increase in PL quantum yield and is explained by noting that the shallow defects are intrinsic to the perovskite and formed in a thermally activated process.  The results provide a new insight into perovskite shallow defects and recombination losses and can help us to better understand the effect of surface treatments on shallow defect properties.