Central to optimizing tandem solar cells is the seamless spectral matching between the top and bottom absorber layers. Achieving this requires the perovskite top cell to possess a wide bandgap, which is effectively attained through a mixed-halide composition rich in bromide. Bromide-rich perovskites tune the absorption onset to higher energies, thus complementing the narrower bandgap CIGS bottom cell and allowing for better utilization of the solar spectrum. Nonetheless, incorporating high bromide content has historically been fraught with difficulties, particularly related to the inhomogeneous distribution of halides during the film formation process, which severely undermines the electronic quality and reproducibility of the resultant perovskite films.

One of the most critical obstacles in scalable manufacturing of these bromide-rich perovskite layers is the tendency for halide segregation and phase heterogeneity, especially when using commonly adopted film deposition techniques. These phenomena cause the formation of regions with varying halide compositions, which adversely affect the perovskite’s optoelectronic properties and, by extension, the overall performance of the tandem solar cells. Addressing this issue demands innovative approaches to control the crystallization dynamics and intermediate phases during film growth to preserve halide homogeneity throughout.

Drawing inspiration from coordination chemistry, Zhang and colleagues employed 2-pyrrolidinone as a coordinating solvent in the perovskite precursor solution. This strategic solvent choice acts to complex with halide and lead ions, thereby suppressing premature crystallization of halide intermediates—a key step that often triggers heterogeneity. By moderating the kinetics of precursor crystallization under ambient blade-coating conditions, this approach ensures a uniform halide distribution and smooth film morphology over large areas, marking a pivotal step towards industrially relevant fabrication methods.

Blade coating, a scalable and cost-effective technique compatible with roll-to-roll processing, was utilized to deposit the perovskite layers. Conventionally challenging for bromide-rich compositions due to rapid crystallization and non-uniformity, the introduction of 2-pyrrolidinone dramatically enhanced film quality by prolonging intermediate phase stability. This allowed for controlled nucleation and growth, resulting in dense and pinhole-free films exhibiting exceptional compositional homogeneity, critical for high-performance tandem devices.

The fabricated flexible monolithic two-terminal perovskite/CIGS tandem solar cells demonstrated a remarkable power conversion efficiency (PCE) of 27.3%. This level of efficiency not only surpasses previous benchmarks for similar devices but also positions these tandem cells among the top contenders for commercial flexible photovoltaics. The monolithic design further minimizes mechanical and electrical losses, enhancing the device integration possibilities for lightweight and deformable photovoltaic systems suitable for portable and wearable applications.

Beyond the impressive efficiency gains, the device stability exhibited minimal degradation over 500 hours of continuous operation. Long-term operational stability has often limited the practical deployment of perovskite and tandem solar cells, so this durability milestone suggests a promising path forward for real-world application. The effective suppression of phase segregation and improved film uniformity underpin this robust operational lifetime, aligning with industry demand for reliable, sustainable solar technologies.

This study not only exemplifies the profound impact of solvent engineering on perovskite film formation but also highlights the importance of precursor solution chemistry in addressing compositional challenges associated with mixed-halide systems. The ability to finely tune crystallization behavior is paramount to unlocking high-efficiency devices with reproducibility across large areas—a fundamental requirement for commercial-scale solar module production.

The combination of perovskite layers with the well-established CIGS technology leverages the best of both worlds: the superior optoelectronic properties and tunability of perovskites alongside the mature and reliable thin-film chalcogenide bottom cell. This hybrid tandem architecture maximizes photovoltaic performance by capturing the complementary parts of the solar spectrum, providing a powerful route to push efficiencies well beyond single-junction limits.

This technological advance also paves the way for expanded investigations into flexible energy harvesting devices, where the intrinsic flexibility and light weight of perovskite/CIGS tandems can usher in new application realms—from building-integrated photovoltaics to wearable electronics and beyond. The suppression of halide crystallization intermediate phases thus stands as a paradigm-shifting approach that may influence broad perovskite fabrication strategies in the years to come.

Moreover, the demonstrated stability under ambient blade-coating conditions signifies a major stride towards environmental compatibility and manufacturing scalability. Processing under ambient conditions without the need for inert atmospheres reduces production costs and complexity, strengthening the economic viability of perovskite-based tandem solar modules.

The research underscores the critical role of molecular-level design in the precursor solution stage to circumvent fundamental material challenges. Through deep understanding and precise control of intermediate species in film formation, the path toward commercially relevant mixed-halide perovskite films becomes clearer, bridging the gap between laboratory breakthroughs and market-ready solar solutions.

In conclusion, the work led by Zhang and colleagues represents a formidable leap in perovskite/CIGS tandem solar cell technology, overcoming long-standing bottlenecks in halide distribution and device efficiency. With a record power conversion efficiency of 27.3% achieved on flexible, large-area tandem cells, alongside robust operational stability, this development signifies a new horizon for flexible photovoltaics with broad societal and environmental benefits.

As the push towards sustainable energy intensifies globally, innovations like these serve as critical enablers for next-generation solar technologies that can be seamlessly integrated into diverse environments. This research vividly illustrates the confluence of chemical engineering, materials science, and device physics driving the future of clean energy, positioning perovskite/CIGS tandem solar cells at the forefront of the photovoltaic revolution.

Subject of Research:
Perovskite/Cu(In,Ga)Se2 (CIGS) tandem solar cells; mixed-halide perovskite film fabrication; halide crystallization suppression; scalable blade-coating technique; flexible and lightweight photovoltaics.

Article Title:
Crystallization suppression of mixed-halide intermediates for perovskite/Cu(In,Ga)Se2 tandem solar cells with improved efficiency.

Article References:
Zhang, S., Bi, E., Lei, B. et al. Crystallization suppression of mixed-halide intermediates for perovskite/Cu(In,Ga)Se2 tandem solar cells with improved efficiency. Nat Energy (2026). https://doi.org/10.1038/s41560-026-01975-1

Image Credits: AI Generated

DOI: https://doi.org/10.1038/s41560-026-01975-1

The challenge with perovskite solar cells (PSCs) lies not only in achieving high power conversion efficiencies but also in addressing long-term operational stability and scalability for practical applications. Traditional hybrid perovskites—comprising mixed organic and inorganic components—are plagued by issues such as phase segregation, ion migration, and poor crystallinity, which accelerate degradation and reduce device longevity. The all-inorganic CsPbI₃ perovskite emerges as a superior alternative due to its enhanced thermal stability and resistance to phase segregation, offering a more robust material platform for tandem solar cell architectures.

Yet, developing scalable, semi-transparent CsPbI₃ devices with competitive efficiencies has remained elusive, particularly for mechanistically stacked tandem modules at practical device sizes. One crucial bottleneck is the damage inflicted during the deposition of transparent conductive oxides (TCOs) via magnetron sputtering onto organic charge transport layers. This process can compromise the underlying layers, limiting device lifespan and performance. Conventional buffer layers like MoOx provide some protection but suffer from limited charge transport capabilities, increased parasitic absorption, and present challenges when scaled up.

Addressing these obstacles, the research team engineered a sandwich-like MAM buffer structure whereby a thin silver (Ag) layer is encapsulated between two MoOx layers. This design not only safeguards the fragile organic layers beneath from sputtering damage but also enhances charge carrier transport and optical transparency. A key discovery was the in-situ formation of Ag₂MoO₄ within the MAM layer during fabrication, which acts as an efficient carrier transport facilitator while maintaining high visible light transmission between 400 and 800 nm. This fine-tuned balance of electrical and optical properties is critical for optimizing semi-transparent solar cells.

The improved MAM buffer layer facilitated semi-transparent CsPbI₃ solar cells to achieve a remarkable power conversion efficiency (PCE) of 18.86% on small active areas (0.50 cm²). More impressively, when integrated into a four-terminal (4-T) mechanically stacked tandem cell with a TOPCon silicon bottom cell, the devices exhibited a combined PCE of 26.55%. Such efficiencies represent a significant milestone, underlining the potential of this sandwich structure in merging perovskite and silicon technologies effectively.

Beyond small devices, scalability was demonstrated by fabricating larger-area minimodules with aperture sizes of 6.62 cm². These modules maintained impressive efficiencies of 16.67% for the semi-transparent CsPbI₃ perovskite top cells and 26.41% for the complete 4-T tandem minimodules. Notably, this marks the first reported instance of minimodule demonstrations for this particular device architecture, a critical step towards commercial viability and real-world application of perovskite/silicon tandems.

Stability is a paramount concern for perovskite technologies, often restraining their commercial adoption. The new MAM buffer layer also provides a significant advancement here. Mini-modules retained over 93% of their initial performance after more than 1,000 hours of storage, indicating robust long-term environmental resilience. Such stability ensures that devices can withstand practical operating conditions, including temperature fluctuations and light exposure, fundamental for deployment.

The structural design of the MAM buffer layer not only protects the perovskite and adjacent layers but also optimizes optical management. By enhancing visible transmittance without compromising electrical properties, the buffer layer allows for effective light harvesting in both sub-cells of the tandem device. This synergy between structural design and optical-electrical functionality is essential to push the frontier of tandem solar cell efficiencies further.

Looking ahead, the research signals future directions in transparent and photostable interfacial materials aimed at directly integrating the top and bottom cells electrically in series configurations. This would simplify tandem architectures and potentially reduce fabrication complexity and costs. Additionally, alternative fabrication techniques such as doctor blading and slot-die coating are envisioned to produce higher-quality large-area CsPbI₃ films suitable for scalable production.

Scientific inquiries will also focus on the development of new functional buffer layers that minimize efficiency losses related to interfacial defects and parasitic absorption. The pursuit of Ag-free buffer designs is especially pertinent, given the cost and scarcity considerations of precious metals. Finding cheaper, earth-abundant alternatives while retaining the unique benefits of the MAM sandwich configuration could revolutionize the buffer layer’s role in perovskite tandem solar cells.

The realization of mechanistically stacked 4-T tandem mini-modules with record efficiencies and advanced stability demonstrates the feasibility of translating laboratory-scale innovations into practical, scalable photovoltaic devices. This breakthrough paves the way for next-generation perovskite-based tandem solar cells to achieve widespread adoption in the renewable energy landscape, offering a highly efficient, cost-effective, and durable alternative to conventional photovoltaics.

Published in the international journal Materials Futures, this research sets a new benchmark for the design of buffer layers in perovskite photovoltaic technology. It underscores the critical interplay of material science, device engineering, and scalable fabrication technologies necessary for commercializing high-performance solar cells. The insights from this study can expedite the integration of perovskite/silicon tandem photovoltaics into diverse applications, from building-integrated photovoltaics to large-scale solar power plants.

In summary, the MoOx/Ag/MoOx sandwich buffer layer stands as a transformative innovation in the quest for high-efficiency, scalable, and stable semi-transparent perovskite solar cells and tandem modules. By combining protective, electrical, and optical functionalities in a single tailored layer, this technology addresses long-standing challenges in perovskite solar cell fabrication and opens new avenues for the practical realization of next-generation photovoltaics.

Subject of Research: MoOx/Ag/MoOx sandwich structured buffer layers for high efficiency semi-transparent CsPbI₃-based perovskite solar cells and four-terminal tandem minimodules.

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

News Publication Date: 16-Oct-2025

Web References:
http://dx.doi.org/10.1088/2752-5724/ae0c76

References:
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 CsPbI_3/TOPCon tandem minimodules. DOI: 10.1088/2752-5724/ae0c76

Image Credits: Rui Zhang, Dongmei Li and Qingbo Meng from Institute of Physics, Chinese Academy of Sciences, and Bobo Ma and Jianhui Chen from Hebei University.

Keywords

Hybrid solar cells, Perovskites, Semi-transparent tandem solar cells, CsPbI₃ perovskite, TOPCon tandem minimodules, MoOx/Ag/MoOx buffer layer, Power conversion efficiency, Charge carrier transport, Scalable perovskite photovoltaics, Four-terminal tandem solar cells, Photovoltaic stability, Transparent conductive oxides