In the ever-evolving landscape of solar energy technology, perovskite/Cu(In,Ga)Se2 (CIGS) tandem solar cells have emerged as promising candidates for next-generation photovoltaic applications, particularly where flexibility and lightweight features are paramount. However, despite their intrinsic advantages, these tandem solar cells have yet to match the efficiencies achieved by other perovskite-based tandem architectures. The recent breakthrough by Zhang, Bi, Lei, and their team marks a significant advancement in addressing these challenges, yielding substantial improvements in power conversion efficiency and operational stability.
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
