Can we design new solar materials to be more tolerant of the problems we can't eliminate?

ACAP and UNSW researchers have challenged a fundamental rule of semiconductor design in research recognised on the cover of the Journal of Materials Chemistry C, opening a new pathway for improving promising earth-abundant solar materials.

The next generation of solar technology may depend not on finding one perfect material, but on discovering new ways to make abundant, sustainable materials perform better.

Professor Hao’s blue ribbon stable of top tandem solar cell candidates

At UNSW, Scientia Professor Xiaojing Hao and her team are building a portfolio of potential “top cell” materials that could be combined with silicon in tandem solar cells. By stacking two solar cells with complementary light-absorbing properties, tandems can capture more of the sun’s energy than silicon alone. The challenge is finding materials that combine high performance with the stability, sustainability and abundance needed for large-scale deployment.

The team has made significant progress across several candidates, including certified world records for CZTS kesterite, wide-bandgap CIGS, antimony chalcogenide and perovskite solar cells. Most recently, the team achieved a 15.95% certified world record for wide-bandgap CIGS, an important step towards stable chalcogenide top cells for silicon tandems.[1]

Unlocking the potential of kesterite – more than one way

Kesterites such as CZTS are particularly attractive because they can be made from abundant, non-toxic elements. But their complex crystal structure makes them unusually susceptible to disorder and defects, which can trap the electrical charges generated by sunlight and limit efficiency. For years, CZTS solar cells struggled to move beyond the low-teens in efficiency, and Professor Hao has identified around 20% efficiency as an important threshold for CZTS to become a compelling tandem technology.[2]

The ACAP supported team is making progress through deeper understanding of how these materials form and how their defects can be controlled. In recent work across CZTS and antimony chalcogenides, Hao and her collaborators have shown that controlling the chemistry and reaction pathways as these materials form can reduce the defects that limit performance. In CZTS, this approach achieved a certified 12.4% efficiency and a record 847 mV open-circuit voltage.[3] Work on antimony chalcogenide achieved a certified 10.7% efficiency by controlling the chemistry and reaction rate during formation.[4]

The work opens a new pathway for improving promising earth-abundant solar materials, and featured on the cover of Journal of Materials Chemistry C.

The new approach – designing for disorder to increase the flow of electrons

The new Journal of Materials Chemistry C study takes this thinking in a different direction. ACAP supported researchers Alireza Yaghoubi, Robert Patterson and Professor Xiaojing Hao are asking whether, rather than simply trying to eliminate defects, the material itself can be redesigned to make defects less harmful.

Inspired in part by the unusual defect tolerance observed in halide perovskites, the researchers deliberately challenge a conventional rule for designing semiconductor materials. Their calculations show that replacing tin (which normally carries a 4+ charge) with antimony or bismuth (which do not carry the same charge) causes the crystal to redistribute charge in a way that dramatically strengthens its ability to shield electrical charges from defects.[5]

"Sometimes breaking the usual chemistry rules can lead to better materials.

"Our work shows that charge mismatch doesn't have to be a weakness. The crystal can respond collectively, opening new pathways to solar materials that are more tolerant of defects". 

– Alireza Yaghoubi, UNSW

UNSW researchers Alireza Yaghoubi (pictured), Robert Patterson and Professor Xiaojing Hao are asking whether, rather than simply trying to eliminate defects, the material itself can be redesigned to make defects less harmful.

Published in the Journal of Materials Chemistry C and selected for the journal’s cover, the significance of the research is that it expands the possibilities for how earth-abundant materials can be designed.

Taken together, the team’s advances point to a broader shift in the development of sustainable solar materials – from simply trying to make existing materials more perfect to finding new ways to understand, control and engineer around their limitations. That expanding toolkit could help move promising materials such as CZTS towards the higher efficiencies needed for stable, sustainable tandem solar cells – and ultimately towards cheaper, more efficient solar technology.

Explainer: New solar materials being explored for tandem cells

Chalcogenides

A broad family of semiconductor materials. Among these are kesterites, CIGs and Antimony chalcogenides.

•  Kesterites    

↳ CZTS — copper zinc tin sulphide (Cu₂ZnSnS₄)      

Earth-abundant, but performance has been limited by defects and disorder.

• CIGS — copper indium gallium sulphide (Cu(In,Ga)S₂)    

A proven thin-film technology with strong efficiency potential, including for wide-bandgap tandem cells.

• Antimony chalcogenides    

↳ Sb₂(S,Se)₃ — antimony sulphur/selenium      

Earth-abundant and promising, but affected by defects and recombination.

Halide perovskites

A different family of materials with high efficiency potential and unusual tolerance to defects; improving long-term stability remains a key challenge

References

1. M.A. Green, E.D. Dunlop, M. Yoshita, N. Kopidakis, K. Bothe, G. Siefer & J.Y. Jiang, ‘Solar Cell Efficiency Tables: Version 68’, Joule, 10(7), Article 102494 (2026). https://doi.org/10.1016/j.joule.2026.102494

2. UNSW, ‘World-record photovoltaic efficiency achieved for kesterite solar cell’, 2025. Professor Xiaojing Hao identified 20% efficiency as a level at which CZTS could become a compelling technology for sustainable tandem solar.

3. A. Wang, J. Huang, J. Cong et al., ‘Early-stage local chemistry regulation enabling open-circuit voltage of 847 mV in wide-bandgap Cu₂ZnSnS₄ solar cells’, Nature Energy (2026). https://doi.org/10.1038/s41560-026-02111-9

4. C. Qian, K. Sun, J. Huang et al., ‘Regulation of hydrothermal reaction kinetics with sodium sulfide for certified 10.7% efficiency Sb₂(S,Se)₃ solar cells’, Nature Energy, 11, pp. 415–424 (2026). https://doi.org/10.1038/s41560-025-01952-0

5. A. Yaghoubi, R. Patterson & X. Hao, ‘Non-isovalent substitution drives cooperative charge redistribution and enhanced dielectric screening in chalcogenides’, Journal of Materials Chemistry C, 14, pp. 14924–14935 (2026). https://doi.org/10.1039/d6tc02475k

Additional context: Australian Centre for Advanced Photovoltaics, ‘Solving the efficiency challenge in kesterite and other emerging earth-abundant solar cells’, 2026.