Minimizing Energy Losses in Perovskite-Perovskite Tandem Solar Cells

Metal halide perovskite solar cells (PSCs) have gained significant development over the past decade due to their promise for cost-competitive photovoltaics. Up to now, the record power conversion efficiency (PCE) is above 26%, which is comparable to silicon-based technology. However, the large thermalization and transmittance losses theoretically limit the efficiency in single-junction devices. To go beyond the detailed balance limit, perovskite-based tandem solar cell configurations have been developed. Among the tandem solar cells, perovskite/perovskite stacks have the advantages of design flexibility, fully solution-based fabrication, potentially high efficiency as well as low-cost. However, substantial energy losses from the bulk and interfaces in individual sub-cells hinder the further enhancement in efficiencies of tandem devices. Accordingly, this thesis focuses on identifying and minimizing energy losses within sub-cells to enhance the efficiency and stability of monolithic perovskite/perovskite tandem devices. Initially, it investigates the interlayers in mixed lead-tin (Pb/Sn) narrow-bandgap perovskite solar cells (NBG-PSCs) through a combination of experimental quasi-Fermi level splitting and drift-diffusion simulations, revealing that inferior open-circuit voltage (VOC) in n-i-p configurations stems from larger interfacial trap densities and mismatched energy levels hindering hole extraction. The fullerene-based interface exhibits lower interfacial trap densities compared to metal oxides. Furthermore, among other polymeric hole-transporting materials, poly(3-hexylthiophen-2,5-diyl) demonstrates the most favorable energy level alignment with perovskite. And it reveals that p-i-n devices exhibit lower energy losses compared to n-i-p configurations. Subsequently, the thesis focusses on improving the quality of perovskite film in p-i-n based NBG-PSC by introducing a Lewis base β-guanidinopropionic acid (GUA) as additive and post surface treating with reductive hydrazinium iodide. This synergistic passivation approach effectively suppresses the formation of defects in perovskite film, which prolongs the carrier lifetime and reduces the nonradiative recombination. Consequently, VOC and fill factor (FF) losses are substantially suppressed, and the long-term operational stability of the devices is enhanced. A systematic investigation of NBG perovskite films utilizing a diverse array of large spacer cations to enhance film quality. The study encompasses long alkyl chain (LAC) electroneutral cations, short alkyl chain (SAC) iodates, and their combinations. Notably, Ruddlesden-Popper based cations could strongly enhance crystalline and generate low dimensional phases. It is found that LAC cations reduce lattice strain, SAC cations increase it, while a mix of LAC/SAC cations balances the strain. Devices incorporating mixed GUA/phenethylammonium iodide (PEAI) demonstrate superior VOC × FF products compared to control devices and those with single cations. This improvement is primarily due to reduced nonradiative recombination losses at both the bulk and interfaces. Additionally, the formation of low-dimensional phases limits interfacial ionic migration, thereby enhancing the long-term stability of NBGPSCs with GUA/PEAI cations. Consequently, this thesis achieved NBG-PSCs with a power conversion efficiency (PCE) exceeding 20% and improved stability, indicating potential for all-perovskite tandem device fabrication. In the final part of this thesis, an effective strategy is proposed to obtain a high-quality wide bandgap (WBG) perovskite by post surface treatment with binary nonpolar guanidinium bromide and polar 4-fluorophenylammonium iodide cations. This treatment enhances defect passivation and optimizes interfacial energy levels, significantly reducing surface recombination velocity and energy level offset between the perovskite and the electron transport layer, thereby boosting VOC in devices. The binary cations also react with excess insulating lead iodide on the perovskite surface, reducing the series resistance and thus enhance FF. Furthermore, this approach improves the hysteresis and stability of WBGPSCs and demonstrates remarkable reproducibility across different labs. By optimizing interconnection layers based on atomic layer deposition, the WBG perovskite was integrated with NBG perovskite to construct a monolithic tandem device, achieving a PCE above 27% with minimal hysteresis and substantial stability. Overall, this thesis underscores the critical role of interface engineering and crystal growth control in defect management and interfacial alternation in NBG and WBG perovskite sub-cells, leading to the development of a highly efficient perovskite-perovskite tandem device. Furthermore, the strategies presented here for minimizing energy losses in perovskite-perovskite two-junction tandem devices are also applicable to multijunction and other types of perovskite-based tandem devices.