Researchers at the University of Bristol have developed kesterite solar cells, which, in contrast to their perovskite-based competitor technologies, rely on Earth-abundant, nontoxic elements.
The natural mineral kesterite offers very interesting key material properties, including a band gap that can be tuned between approximately 1.4 and 0.9 eV. “Their optical and electronic properties make these compounds a suitable replacement to Cu(In,Ga)(S,Se)2, commonly known as CIGS,” says David J Fermin, professor of electrochemistry and leader of the Electrochemistry and Solar Research Group at the University. “This means that kesterites offer an indium- and gallium-free option to a technology already exploited at the level 1000 MWp module production.”
According to the professor, though, the challenge facing kesterite cells is the power conversion efficiency, in particular the voltage generated by the cells. State-of-the-art lab-scale CIGS cells display power conversion efficiencies of more than 21%, under standard AM 1.5 G illumination. The best performing kesterite cells under similar conditions deliver just below 13%. Current research efforts are focused on understanding the origin of the voltage deficiency, which limits the device performance.
Growing up in the shadow of perovskite solar cells
The rising stardom of perovskite solar cells begs the question, how relevant can kesterites-based devises really become as an alternative? To illustrate the potential impact of kesterite solar cells, Fermin refers to a diagram taken from the 2017 Photovoltaics Report by the Fraunhofer Institute for Solar Energy:
“The spectacular growth rate of the PV sector worldwide is putting pressure on the thin-film industry, currently dominated by CIGS and CdTe. To be able to maintain a sizeable portion of the sector, technologies should be based on scalable and low-cost materials and manufacturing process,” says the expert. “Kesterite solar cells build upon a device architecture that has been developed for many years. If you solve the material problem, then the market path is already there.”
Advantages of kesterite solar cells
As Fermin mentions, Earth-abundancy, low toxicity and pathways to market are very appealing aspects of kesterite materials. Material stability is also crucial. Perovskite and other emergent systems may fulfill some requirements for scalability, such as low manufacturing costs, however, the professor emphasizes that toxicity and primarily stability remain major challenges to overcome, despite the huge amount of resources devoted to this problem worldwide. “If R&D resources are somehow associated with the number of publications, resources devoted to kesterite over the past three or four years represent a small fraction of those invested in perovskites.”
Current challenges with kesterite-based approaches
The scientists are still trying to better understand the properties and peculiarities of this natural mineral: “In our view, and many others in the field, the key challenge is control the structure of these complex materials down to the atomic scale,” Fermin says. “It comes down to making sure that each atom — and there are at least four different ones in kesterites — occupies the right position in the crystal lattice in the bulk and the film surface.”
The illustration on the left shows an atomic resolution transmission electron micrograph in which Sn atoms (bright spots) are arranged as expected for a pure crystal (as shown in the model). The right-hand side depicts another portion of the film that shows a region in which a row of Sn atoms (dark spots) are displaced. These types of defects strongly affect the local electronic structure and can compromise device performance, as the professor explains.
Fermin tells of efforts worldwide devoted to control the formation of high-quality kesterite thin-films employing either solution-based or physical vapor deposition methods. Scientists are also considering the introduction of additional elements, such as, for example, sodium, germanium and antimony, to minimize atomic disorder.
The manufacture of kesterite solar cells and their path to commercialization at scale
Fermin again stresses that kesterite solar cells would be made in a very similar way like CIGS solar cells. “The path is already there,” he says.
What about his own plans regarding kesterite solar cell research? “Our group focuses on solution-based methods to grow kesterite thin-films,” the professor says. “There is a huge scope for research in terms of optimizing the chemical composition of the precursor, which will enable us to create high-quality layers by low-cost methods, such as spray. However, we are also interested in the fascinating physics of these complex systems and how manipulating the structure at the atomic scale manifest itself in solar power conversion. To reach thousands of MWp module production, we need to tackle atomic disorder.”
Written by Sandra Henderson, Research Editor, Solar Novus Today