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A team at the University of New South Wales has developed a fabrication approach that curbs tiny flaws in copper zinc tin sulphide photovoltaic material, yielding a new voltage benchmark for the technology and a certified conversion rate of 12.4%. The findings, appearing in Nature Energy and spearheaded by Scientia Professor Xiaojing Hao from the university’s photovoltaic engineering school, tackle a long-standing defect issue that has capped kesterite performance.
Hao, working with Dr Ao Wang and Dr Kaiwen Sun, observed that copper atoms migrate away from their intended positions during the initial phase of the high-temperature production process, creating impurities and atomic-scale structural irregularities. By reinforcing copper-sulphur bonds during that early thermal step, the group maintained a uniform distribution of the material’s components, markedly cutting down on defects that capture photogenerated charges and diminish cell voltage.
Kesterite, the compound also known as CZTS, consists of copper, zinc, tin, and sulphur—elements that are plentiful and relatively benign compared with certain competing thin-film semiconductors. This has made it a promising option for tandem solar devices, which pair two distinct materials to harness a wider portion of the sunlight spectrum than silicon alone. In contrast to some alternatives for the upper layer of such cells, CZTS avoids reliance on scarce or hazardous substances, even though its efficiency has lagged behind its theoretical ceiling.
Hao has previously outlined a stepwise efficiency plan for bringing kesterite to market, suggesting that hitting roughly 20% efficiency would signal a real opening for commercial adoption, with interim milestones of 15% and 17% necessary to foster confidence. She has also emphasized that cost remains the primary obstacle for any emerging photovoltaic technology, pointing out that silicon has undergone extensive cost-reduction efforts that rivals must match.
Hao has contrasted her team’s strategy for kesterite with the industry’s approach to perovskite, another prominent tandem-cell contender. While much perovskite work has prioritized high efficiencies first and addressed stability later, Hao has characterized her group’s method as beginning with the essential traits of an ideal material—one that is abundant, eco-friendly, and durable—and then building performance from that base. She anticipates silicon will continue to lead the photovoltaic sector, with kesterite’s main opportunity in tandem configurations rather than as a standalone substitute, alongside other candidates like perovskites, which UNSW is also investigating.
The 12.4% achievement follows a series of UNSW studies on kesterite cells. In January 2025, the same team reported a record 13.2% efficiency for high-bandgap kesterite devices by adding hydrogen during the annealing process, a method targeting a separate category of high-bandgap cells designed for tandem top layers, after years of results hovering near 11%. The current work, however, focuses on boosting voltage in conventional CZTS cells by managing defects during fabrication itself, rather than applying post-production treatments.
Hao has stated that the larger goal is to determine what might succeed silicon, which she views as nearing its theoretical efficiency limit, and to devise ways to produce more electricity per unit area in space-constrained settings. The defect-control principle from this study, the researchers note, applies beyond CZTS to other multi-element semiconductors being developed for future solar uses, where similar elemental movement during production can cause analogous flaws. This frames the work as a broader design strategy rather than a single material outcome, potentially relevant to other tandem top-cell options under exploration.
This research contributes to a wider solar materials initiative at UNSW. The institution has also announced the discovery of an atomic-scale self-repair mechanism in silicon cells exposed to sunlight, and has cautioned that the solar industry might deplete global silver reserves within five years absent broader adoption of commercial-scale module recycling, citing related studies on panel component reuse. Additionally, UNSW has obtained AU$6.52 million from the Australian Renewable Energy Agency to examine how inverter-based resources interact with grid protection systems, highlighting the university’s involvement across various facets of Australia’s renewable energy research landscape.
Even with this progress, CZTS efficiency still trails commercial silicon cells considerably, and the technology’s commercial viability hinges on further advancements before it can be used in tandem cell production.
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