There is an urgent need for renewable energy technologies that are affordable and can be deployed rapidly to mitigate climate change. Global average temperatures surpassed the 1.5 °C threshold for the first time in 2024, underscoring the shortcomings of current mitigation strategies and reinforcing the demand for scalable, high-efficiency energy solutions. Owing to substantial cost reductions over the past decade, photovoltaics (PV) have become the most cost-effective source of electricity in many parts of the world. However, further cost reductions through improvements in device performance and operational lifetime are required to meet global deployment targets. As the performance of single-junction silicon solar cells, which currently dominate the PV market, approaches its theoretical limit, tandem devices that combine silicon with wide-bandgap metal halide perovskite top cells offer a clear path forward. Such devices have already demonstrated small-area efficiencies exceeding the single-junction limit of 33.7 %. With the promise of low-temperature, low-cost processing and a theoretical efficiency exceeding 40 %, perovskite-silicon tandem solar cells have emerged as a leading candidate for the next generation of solar cell technologies aimed at further reducing energy costs. However, challenges related to upscaling, interfacial losses, and device instability continue to slow down large-scale commercialization. This thesis investigates how interfacial energetics, ionic transport, and chemical degradation processes at the interface with the electron-transporting layer (ETL) fullerene C₆₀ affect the efficiency and stability of metal halide perovskite single-junction solar cells with a 1.68 eV bandgap, tailored for tandem integration. A combined approach of optoelectronic characterization and drift-diffusion simulations is used to quantify losses associated with individual functional layers. Photoelectron spectroscopy reveals that the offset between the perovskite conduction band and the C₆₀ lowest unoccupied molecular orbital leads to an increased hole concentration at the interface, resulting in enhanced non-radiative recombination. To mitigate these interfacial losses, molecular surface passivation strategies are explored. Surface treatment with piperazinium iodide (PI) is shown to effectively eliminate voltage losses caused by recombination at the perovskite/C₆₀ interface, enabling a certified power conversion efficiency of 32.5 % in a tandem device. The improvement is attributed to dipole-induced band realignment and enhanced selectivity, analogous to field-effect passivation or heavily doped emitters in silicon solar cells. Despite the performance gains, long-term stability under continuous illumination is reduced. A systematic investigation of diammonium-based surface treatments reveals a trade-off between voltage enhancement and operational stability, governed by induced band bending and ion redistribution. Finally, the chemical reactivity of the molecular treatments is addressed through the design of chemically robust quaternary ammonium analogues, which suppress degradation while retaining passivation functionality. In summary, this work provides a detailed physical and chemical analysis of interface-related losses in wide-bandgap perovskite solar cells and proposes targeted strategies for their mitigation. The insights gained support the development of efficient and stable perovskite-based tandem photovoltaics and inform future interface and material design.
Florian Scheler (Thu,) studied this question.