Designing MoOX/Ag/MoOX sandwich structured buffer layer for four-terminal CsPbI3/TOPCon tandem minimodules

A collaborative research team led by the Institute of Physics at the Chinese Academy of Sciences has developed a new “sandwiched” MoOx/Ag/MoOx (MAM) buffer layer to improve the performance and scalability of semi-transparent CsPbI₃/TOPCon tandem solar cells. The MAM buffer layer enhances light transmittance and charge carrier transport while effectively protecting underlying layers from sputtering damage. This innovation enabled semi-transparent CsPbI₃ solar cells to achieve a power conversion efficiency (PCE) of 18.86% (0.50 cm²) and corresponding 4-T CsPbI₃/TOPCon tandem cells to reach 26.55% PCE. Significantly, the technology was successfully scaled to larger-area minimodules, achieving 16.67% and 26.41% PCE for CsPbI₃ and 4-T tandem minimodules (6.62 cm²), respectively—marking the first reported minimodule demonstration for this architecture. This work provides a scalable and efficient buffer layer strategy, paving the way for next-generation, high-efficiency perovskite-based photovoltaic systems.

Perovskite solar cells (PSCs) have rapidly emerged as promising photovoltaic technology due to their tunable bandgaps, high power conversion efficiencies, and low-cost fabrication. However, despite fast progress, the commercialization of perovskite-based tandem solar cells (PSTSCs) faces critical challenges related to its long-term operational stability, scalable architectures, and effective light management in semi-transparent configurations. One of the major challenges can be located at the top-cell material. While hybrid perovskites with mixed organic and inorganic components are commonly used, they often suffer from poor crystallinity, phase segregation, ion migration, and environmental instability. These issues lead to non-radiative recombination and fast degradation, especially in large-area devices. In contrast, all-inorganic CsPbI₃ perovskites offer better thermal stability and fewer phase segregation problems, making them an ideal candidate for top cells in tandem architectures. Yet, scalable and stable semi-transparent CsPbI₃ devices with high efficiency remain underdeveloped, and mechanically stacked tandem modules with practical sizes have rarely been demonstrated. Another critical bottleneck is the damage caused by magnetron sputtering during the deposition of transparent conductive oxides (TCOs) onto organic charge transport layers. Traditional buffer layers like MoOₓ offer limited protection and often suffer from poor charge transport, parasitic absorption, and fabrication challenges at large scales.

To address these limitations, a team of researchers from different Chinese institutions introduced a novel MoOX/Ag/MoOX (MAM) sandwich-structured buffer layer designed to enhance both performance and stability of semi-transparent CsPbI₃-based perovskite solar cells (ST-PSCs) and four-terminal (4-T) tandem solar cells. The results of the study suggest that in-situ formed Ag₂MoO₄ in the MAM structure can simultaneously enhance carrier transportation/collection and maintain highly visible transmittance in the range of 400 800 nm. This strategy enables semitransparent CsPbI₃ devices to achieve 18.86% efficiency (0.50 cm²) while corresponding 4-T CsPbI₃/TOPCon system presents 26.55% efficiency. As expected, the MAM buffer layer demonstrated exceptional scalability, yielding CsPbI₃ perovskite minimodules with 16.67% PCE and 4-T CsPbI₃/TOPCon tandem minimodules with 26.41% PCE (aperture area: 6.62 cm²) for the first time. Beyond efficiency, the new design also improves long-term stability, with transparent CsPbI₃ perovskite mini-modules retaining over 93% of their initial performance after more than 1,000 h storage -- an encouraging sign for practical deployment. This kind of scalable sandwich-like architecture buffer layers not only expands the design flexibility of functional layers in PSCs and tandem solar cells, but also offers a promising pathway for further efficiency enhancement. 

The Future: Future research will explore more suitable transparent and excellent photostability materials to directly draw the two devices together in series, especially for large-size tandem modules by considering more practical application scenarios while controlling costs.

The semi-transparent CsPbI₃ and 4-T CsPbI₃/TOPCon tandem minimodules are fabricated for the first time, which present 16.67% and 26.41% efficiencies (aperture area: 6.62 cm²), respectively. Future researches will concentrate on the following aspects. One is seeking suitable transparent and excellent photostability materials to draw the two devices together in series. The second is an important scientific issue on developing new functional layers to further redude efficiency loss. The third is how to improve the cell performance of semi-transparent CsPbI₃ solar cells and minimodules, including high quality CsPbI₃ films by doctor blade or slot-die methods. The fourth is focusing on the stabilities of 4-T tandem minimodules. Finally, considering further application, seeking alternative cheap materials without Ag metal will be a positive way to further improve this sandwich-like buffer strategy.

The Impact: This work offers a promising pathway to achieving scalable sandwich-like structured buffer layers for CsPbI₃ perovskite/TOPCon tandem solar cells and mini-modules.

The research has been recently published in the online edition of Materials Futures, a prominent international journal in the field of interdisciplinary materials science research.

Reference: Rui Zhang, Bobo Ma, Yuqi Cui, Chengyu Tan, Bingbing Chen, Yiming Li, Jiangjian Shi, Huijue Wu, Yanhong Luo, Dongmei Li*, Jianhui Chen*, and Qingbo Meng*. Designing MoO_X/Ag/MoO_X sandwich structured buffer layer for four-terminal CsPbI₃/TOPCon tandem minimodules. DOI: 10.1088/2752-5724/ae0c76