Photovoltaics (PVs), devices that can convert sunlight into electrical power, are becoming increasingly widespread and more people worldwide are now relying on them to generate electricity. Renewable energy engineers worldwide are working to identify materials and processes that could help to further reduce the costs of solar technologies, while further boosting their power-conversion-efficiencies (PCEs).
A promising material for the development of PVs is wide-bandgap kesterite Cu2ZnSnS4 (CZTS), a semiconductor that exhibits a large energy gap and could thus absorb light more efficiently. In contrast with silicon, which is currently the primary material used to fabricate PV technology, CZTS is non-toxic and made of elements that are abundant on Earth. Thus, it could be used to create more sustainable and affordable solar cells.
Despite their advantages, CZTS solar cells have so far exhibited significantly lower efficiencies than their silicon counterparts, reaching a maximum of 11%. Their limited performance is in great part due to a process known as carrier recombination, which entails the recombination of photo-generated electrons and holes before they can be captured to generate electricity.
Researchers at University of New South Wales in Sydney explored the possibility of mitigating the effects of carrier recombination in wide-bandgap kesterite solar cells using a technique known as hydrogen annealing.
Their paper, published in Nature Energy, shows that this technique could help to improve these solar technologies’ carrier collection, by redistributing oxygen and sodium in CZTS layers.
“Our work was inspired by the need to identify a sustainable, low-cost, and environmentally friendly material for next-generation solar technologies,” Kaiwen Sun, senior author of the paper, told Tech Xplore.
“CZTS is a particularly promising candidate as the top cell in tandem solar cell architectures due to its tunable bandgap, stability, and use of earth-abundant, non-toxic elements. However, a key challenge has been improving this material’s carrier collection efficiency.”
The main objective of this recent study was to show that hydrogen annealing, a technique that entails heating up devices in a hydrogen-containing atmosphere, could help to boost the efficiencies of CZTS. To achieve this, the researchers devised a simple and scalable method to anneal CZTS in a hydrogen-containing environment.
“Hydrogen plays a crucial role in our method, by redistributing sodium within the material and passivating defects, particularly near the absorber surface,” explained Sun.
“This process significantly enhances carrier transport and collection, key factors for achieving high-performance devices. By improving these properties, our approach strengthens CZTS’s position as a practical and cost-effective top cell material in tandem solar cells, capable of efficiently pairing with silicon for broader solar spectrum utilization.”
As part of their study, Sun and his colleagues applied their proposed hydrogen annealing method to a cadmium-free CZTS solar cell. Notably, they found that this approach boosted the solar cell’s performance, yielding a record efficiency of 11.4%.
“Our proposed technique is not only specific to CZTS but has also shown promising results in other thin-film solar cell materials, such as CIGS,” said Sun. “Practically, it demonstrates how wide-bandgap CZTS, with its low cost, stability, and environmental friendliness, could serve as an excellent top cell candidate in tandem architectures, paving the way for more efficient and sustainable solar energy solutions.”
The recent paper by Sun and his colleagues introduces a simple and effective technique to regulate the distribution of sodium in CZTS, which can in turn enhance the carrier collection efficiency of CZTS-based solar cells. In the future, their proposed approach could be applied to other wide-bandgap kesterite PVs, potentially contributing to their future deployment in real-world settings.
“Our future research aims to push the efficiency of wide-bandgap CZTS solar cells beyond the 15% benchmark while maintaining their environmental and economic advantages,” added Sun. “This includes refining the hydrogen annealing process and exploring other techniques to further optimize the material’s optoelectronic properties.”
