With the rapid development of wearable electronics, distributed energy systems, and integrated photovoltaic technologies, lightweight and bendable flexible photovoltaic devices are emerging as a key direction for new energy technologies. Cu₂ZnSn(S,Se)₄ (CZTSSe) is a novel type of inorganic thin-film photovoltaic material composed of earth-abundant and environmentally friendly elements. It combines the potential for low cost with excellent mechanical flexibility, making it a significant candidate for future portable and space energy applications. Currently, achieving ordered evolution and defect co-regulation in the crystallization process of this multi-component system remains a core scientific challenge limiting its further advancement.
Addressing this challenge, the team led by Meng Qingbo at the Institute of Physics, Chinese Academy of Sciences / Beijing National Laboratory for Condensed Matter Physics has conducted continuous in-depth research. They have progressively established a comprehensive understanding of the crystallization kinetics, atomic order regulation, and defect evolution mechanisms in CZTSSe materials. Leveraging long-term technical expertise, the team has achieved multiple cross-scale efficiency breakthroughs in recent years, reaching 14%, 15%, and 16%, thereby reinvigorating the rapid development of this material system.
Their world-record efficiency results for CZTSSe cells and modules have been recognized four consecutive times in the "Best Research-Cell Efficiency Chart" published by the U.S. National Renewable Energy Laboratory (now renamed the National Renewable Energy Laboratory). Furthermore, these results have been included ten times to date in the "Solar Cell Efficiency Tables" edited by renowned photovoltaic expert Professor Martin A. Green.
In their latest research, the team collaborated with Hangzhou Dianzi University to systematically reveal the differential roles of alkali metal elements during the crystal growth of CZTSSe thin films. The study shows that while traditional sodium (Na) regulation can promote grain growth and improve film morphology, it simultaneously induces large-scale segregation of SnSeₓ intermediate phases, which limits the improvement of device voltage.
Building on this, the researchers innovatively proposed a "kinetic competition regulation" strategy. By introducing lithium (Li) to modify the free energy characteristics of copper-related phases, these phases preferentially consume selenium during crystallization, effectively suppressing the disordered growth of SnSeₓ phases and enabling the cooperative and ordered evolution of the multi-phase system.
Based on this breakthrough in mechanistic understanding, the research team successfully fabricated high-quality CZTSSe thin films on flexible substrates. The flexible solar cells achieved a power conversion efficiency of 14.5% (certified 14.2%), setting a new record for this type of flexible device. Furthermore, the team constructed a shingled flexible CZTSSe photovoltaic module, achieving a power conversion efficiency of 12.7% on a 10 cm² area (certified 12.0%). This not only surpasses the module efficiency record held for over a decade by Japan's Solar Frontier but also marks the first time the performance of a flexible CZTSSe module has exceeded that of its rigid counterparts. These related results have been included in "Solar Cell Efficiency Tables (Version 66)".
This research deepens the understanding of crystallization behavior in complex multi-component semiconductors from the perspective of material microstructure regulation, providing a new theoretical framework and technical pathway for the ordered growth of multi-component compound materials. The findings not only advance the development of flexible inorganic thin-film photovoltaic technology but also offer significant support for building future high-performance integrable energy systems.
The related work, titled "Alkali-metal-mediated control of phase segregation for flexible kesterite solar cells and modules with improved efficiency," has been published in *Nature Energy*. This research received support from projects including the National Key R&D Program, the National Natural Science Foundation of China, the Zhejiang Provincial Natural Science Foundation, the China Postdoctoral Innovative Talent Support Program, and the Youth Innovation Promotion Association of the Chinese Academy of Sciences.
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