Researchers at Tallinn University of Technology (TalTech) have increased the efficiency of next-generation thin-film solar cells by partially substituting copper with silver in absorber material. A thin-film solar cell consists of several thin layers of light-absorbing semiconductor materials. Silicon, which has traditionally been used in solar cells since the 1950s, requires fairly thick layering due to its low light-absorption. Next-generation lightweight flexible monograin layer solar cell developed by TalTech researchers. Courtesy of Professor Jüri Krustok. TalTech researchers are developing compound semiconductor materials named kesterites, which in addition to high absorption of light, contain earth-abundant and low-cost chemical elements such as copper, zinc, tin, sulfur, and selenium. To produce kesterites, TalTech researchers use a monograin powder technology. “The monograin powder technology we are developing differs from other similar solar cell manufacturing technologies used in the world in terms of its method,” said Marit Kauk-Kuusik of the Department of Material and Environmental Technology’s Laboratory of Photovoltaic Materials. “Compared to vacuum evaporation or sputtering technologies, which are widely used to produce thin-film structures, the monograin powder technology is less expensive.” Powder growth technology is the process of heating chemical components in a special chamber furnace at 750 °C for four days. The product of that heating is then washed and sieved in special machines. The monograin powder consists of unique microcrystals that form parallel connected miniature solar cells in a large module covered with an ultrathin buffer layer. “We have reached the point in our development where partial replacement of copper with silver in kesterite absorber materials can increase efficiency by 2%. This is because copper is highly mobile in nature, causing unstable solar cell efficiency. The replacement of 1% copper with silver improved the efficiency of monograin layer solar cells from 6.6% to 8.7%,” Kauk-Kuusik said. This provides major advantages over the photovoltaic modules of the previous generation, that is, silicon-based solar panels: The photovoltaics cells are lightweight and flexible, and they can be transparent, while being environmentally friendly and significantly less expensive. The researchers intend to increase the efficiency to 15% before the technology is commercialized. The research was published in Journal of Materials Chemistry A (www.doi.org/10.1039/c9ta07768e).