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

Recent research spearheaded by Pei, Lin, Zhang, and colleagues has brought to light a fundamental bottleneck in the reliability of defect passivation strategies applied to wide-bandgap perovskites. Passivation—crucial for suppressing non-radiative recombination and enhancing photovoltaic efficiency—often falters under operational stresses combining illumination and elevated temperatures. The root cause identified in this comprehensive study is the thermal desorption of conventional passivating agents from the perovskite surface, which leads to the resurgence of detrimental defects and accelerated device degradation. This revelation challenges the current paradigm and underscores the necessity of rethinking molecular designs of passivators to withstand real-world stresses in solar device operation.

To confront this challenge, the researchers developed a novel, robust passivator with meticulously engineered functional groups. These groups provide anchoring interactions strong enough to remain affixed to the perovskite surface irrespective of its termination chemistry, a critical feature given the diverse surface compositions encountered during device fabrication. This strategic molecular design effectively prevents passivator desorption, even under combined thermal and illumination stresses that typically induce deterioration in other systems. The result is a dramatic improvement in the durability and efficiency of wide-bandgap perovskite solar cells, marking a significant step forward in tandem solar technology.

The implications of this robust passivation extend beyond mere stability. The researchers observed substantial suppression of phase segregation within the perovskite layer—a common phenomenon where halide ions redistribute unevenly under illumination and heat, forming iodide-rich and bromide-rich domains that degrade device performance. By stabilizing the composition and structure of the perovskite, the newly designed passivator not only prolongs the operational lifetime but also maintains optimal energy band alignment and charge transport properties essential for high-efficiency energy conversion.

Experimentally, wide-bandgap perovskite solar cells treated with the new passivation technique achieved a champion power conversion efficiency of 23.5%. More impressively, these devices exhibited negligible efficiency loss after enduring 1,000 hours of continuous 1-sun illumination at around 50 °C—conditions that closely mimic real-world operational environments. This remarkable stability benchmark addresses one of the principal impediments in transitioning perovskite solar technology from laboratory-scale prototypes to commercial modules capable of durable performance.

Building upon these advancements, the team incorporated such optimized perovskite cells into monolithic tandem architectures with Cu(In,Ga)Se₂ bottom cells. Tandem cells harness the synergistic capture of a broader solar spectrum, effectively surpassing the Shockley-Queisser limit for single junction cells. With the integrated approach, the tandem devices realized an outstanding steady-state power conversion efficiency of 27.93%, which was certified at 27.35%, positioning them among the highest-efficiency tandem cells incorporating CIGS reported to date.

Beyond their efficiency milestones, these tandem devices demonstrated impressive operational stability, maintaining consistent performance over 420 hours at approximately 38 °C in ambient air without encapsulation. This operational longevity under realistic environmental conditions hints at the tangible potential for commercial deployment, as stability has historically been the Achilles’ heel of perovskite-based photovoltaics. Such durability coupled with high efficiency could ultimately accelerate the market adoption of perovskite/CIGS tandem technology for applications demanding lightweight and flexible photovoltaics.

The success of this study is not only a technical feat but also provides critical insight into the fundamental chemistry governing perovskite stability. By elucidating the mechanisms behind passivator desorption and its impact on defect dynamics and phase stability, the work offers a new roadmap for molecular engineering in perovskite research. This approach paves the way for future developments wherein passivator molecules can be systematically optimized based on the underlying surface chemistry and operational stress profiles.

An interesting facet of this research is its practical relevance. Many passivation strategies that have shown promise under idealized conditions fail to translate into durable performance when tested under simultaneous illumination and thermal stress. The new material directly addresses this gap, validating the importance of testing under realistic accelerated aging conditions. It suggests that future standards for perovskite passivation must incorporate such rigorous stress tests to ensure genuine improvements in device stability.

Furthermore, the integration of wide-bandgap perovskites with Cu(In,Ga)Se₂ thin films leverages two well-established photovoltaic technologies, combining the flexibility and tunability of perovskites with the proven stability and manufacturability of CIGS. This tandem configuration exploits complementary absorption edges, thereby maximizing the utilization of incident solar energy. The demonstrated efficiencies bring this hybrid tandem design close to the commercial viability threshold, bridging the longstanding gap between academia and industry for tandem solar applications.

There remain challenges and avenues for further research. Although the newly developed passivator significantly enhances stability, long-term outdoor testing and scaling up device sizes will be essential to fully validate commercial prospects. Additionally, the cost-effectiveness and synthesis scalability of such specialized passivators will need assessment to determine the feasibility of mass production. Nonetheless, this breakthrough sets a new precedent in material design principles that will likely inspire parallel innovations across the photovoltaic community.

In conclusion, the study presented by Pei and colleagues represents a pivotal advancement in tandem solar cell technology. By ingeniously circumventing the limitations of passivation under operational stresses, it not only improves the power output and lifespan of wide-bandgap perovskite cells but also enables record efficiencies in perovskite/CIGS tandems. This breakthrough substantiates the claim that carefully engineered molecular interactions at the perovskite interface are the keys to unlocking robust, high-performance tandem solar cells capable of revolutionizing the renewable energy landscape.

As the world urgently seeks sustainable and scalable energy solutions, such technological innovations provide hope and direction. The convergence of molecular-level chemistry, materials engineering, and device physics embodied in this work exemplifies the multidisciplinary effort necessary to propel solar energy into a new era. The successful certification of a 27.35% efficient perovskite/Cu(In,Ga)Se₂ tandem cell heralds a future where solar energy is not only more efficient but also more resilient and accessible globally.

Looking ahead, the principles elucidated here could well translate into improvements across various perovskite-based optoelectronic devices, including light-emitting diodes and photodetectors, broadening the impact of this research. More immediately, the demonstrated combination of stability and efficiency underscores the readiness of tandem perovskite/CIGS cells for near-term industrial consideration and scale-up, further energizing the race towards sustainable energy transition.

This research invites the scientific community to rethink stability paradigms and to prioritize molecular design that harmonizes with operational realities. It serves as a compelling reminder that breakthroughs often stem from detailed attention to interfacial chemistry, which governs the delicate balance between performance and durability. As a result, the future of photovoltaic innovation shines brighter than ever, reaffirming the central role of perovskite tandem technologies in the global renewable energy portfolio.

Subject of Research: Development of a robust defect passivation strategy for wide-bandgap perovskite solar cells integrated with Cu(In,Ga)Se₂ in monolithic tandem architectures.

Article Title: Inhibiting defect passivation failure in perovskite for perovskite/Cu(In,Ga)Se₂ monolithic tandem solar cells with certified efficiency 27.35%.

Article References:
Pei, F., Lin, S., Zhang, Z. et al. Inhibiting defect passivation failure in perovskite for perovskite/Cu(In,Ga)Se₂ monolithic tandem solar cells with certified efficiency 27.35%. Nat Energy (2025). https://doi.org/10.1038/s41560-025-01761-5

Image Credits: AI Generated