Unlocking a new tandem solar candidate with world-record antimony chalcogenide cells

As silicon solar cells approach their practical efficiency limits, the global photovoltaic community faces a defining challenge: identifying the optimal “top cell” material for next-generation tandem solar technologies that combine high performance, long-term stability, scalable manufacturing and low cost.

Supported by ACAP, UNSW researchers are advancing antimony chalcogenide as a serious tandem top-cell candidate. The material offers a compelling combination of properties, including strong light absorption, inorganic stability and compatibility with low-temperature processing. However, global progress had stalled, with efficiencies plateauing below 10% for several years.

In a major breakthrough, a UNSW research team led by Scientia Professor Xiaojing Hao, School of Photovoltaic and Renewable Energy Engineering, has overcome this bottleneck, achieving a world-record certified efficiency of 10.7% for antimony chalcogenide solar cells. The result, published in Nature Energy, represents the highest independently verified performance for this material worldwide and secured its first-ever inclusion in the international Solar Cell Efficiency Tables (Version 65).

“There is no single perfect top-cell material yet for tandem solar technologies. What we need is a broader and stronger set of candidates that can partner with silicon, and antimony chalcogenide is showing very positive potential.” – Professor Xiaojing Hao, UNSW

Beyond the headline efficiency, the research delivered a critical scientific insight – uneven distribution of sulfur and selenium during hydrothermal deposition created an internal energy barrier that restricted charge transport. Dr Chen Qian, Research Fellow and first author of the study, demonstrated that introducing a small amount of sodium sulfide stabilises the chemical reactions, producing a more uniform absorber layer and significantly improving device performance.

Antimony chalcogenide solar cells fabricated at UNSW reached 11.02% efficiency, with an independently certified value of 10.7% provided by CSIRO.

Solar cells fabricated at UNSW reached 11.02% efficiency, with an independently certified value of 10.7% provided by CSIRO, one of only nine internationally recognised photovoltaic measurement centres. The broader research team also included Dr Jialiang Huang, contributing to materials optimisation and device analysis.

Antimony chalcogenide’s suitability extends beyond tandems. Its ultrathin structure (≈300 nm), semi-transparency and high bifaciality (0.86) enable applications such as solar windows, while its bandgap is well matched to indoor lighting conditions.

These opportunities are already moving toward commercialisation through UNSW spinout Sydney Solar.

The team is now targeting efficiencies approaching the next milestone efficiency of 15% through strategies such as defect passivation. Their work exemplifies ACAP’s role strengthening Australia’s leadership in advanced photovoltaic materials through world-class research, national measurement capability and clear pathways to impact.

Note: This article is an adaptation of Neil Martin’s UNSW Newsroom article “Engineers set efficiency world record for emerging solar cell material”

References:

Nature Energy reference:

Qian, C., Sun, K., Huang, J. et al. Regulation of hydrothermal reaction kinetics with sodium sulfide for certified 10.7% efficiency Sb₂(S,Se)₃ solar cells. Nat Energy (2026). https://doi.org/10.1038/s41560-025-01952-0

Solar Cell Efficiency Tables reference:

Green, M., Dunlop, E., Yoshita, M., Kopidakis, N., Bothe, K., Siefer, G., Hao, X. and Jiang, J. (2025), Solar Cell Efficiency Tables (Version 65). Prog Photovolt Res Appl, 33: 3-15. https://doi.org/10.1002/pip.3867