An efficiency record of 18.7 percent for flexible copper indium gallium selenide (CIGS) solar cells on plastics has been reported by Swiss researchers at Empa’s Laboratory for Thin Film and Photovoltaics. "The new record value for flexible CIGS solar cells of 18.7 percent nearly closes the 'efficiency gap' to solar cells based on polycrystalline silicon (Si) wafers or CIGS thin-film cells on glass," said Ayodhya N. Tiwari, who led the research. He is convinced that "flexible and lightweight CIGS solar cells with efficiencies comparable to the 'best in class' will have excellent potential to bring about a paradigm shift and to enable low-cost solar electricity in the near future." Flexible thin-film CIGS solar cell on polymer substrate developed at Empa. (Image: Empa) One major advantage of flexible high-performance CIGS solar cells is the potential to lower manufacturing costs through roll-to-roll processing, while at the same time offering a much higher efficiency than those currently on the market. Such lightweight and flexible solar modules also can offer additional cost benefits in terms of transportation, installation and structural frames for the modules and can significantly reduce the so-called "balance of system" costs. Taken together, the new CIGS polymer cells exhibit numerous advantages for applications such as facades, solar farms and portable electronics. With high-performance devices now within reach, the results suggest that monolithically interconnected flexible CIGS solar modules with efficiencies above 16 percent should be achievable with the recently developed processes and concepts. At the forefront of efficiency improvements In recent years, thin-film photovoltaics technology based on glass substrates has gained sufficient maturity toward industrial production; flexible CIGS technology is, however, still an emerging field. The recent improvements in efficiency in research labs and pilot plants — among others by Tiwari's group, first at ETH Zurich and, for the past few years, at Empa — are contributing to performance improvements and to overcoming manufacturability barriers. Working closely with scientists at Flisom AG, a startup company that is scaling up and commercializing the technology, the team made significant progress in low-temperature growth of CIGS layers, yielding flexible CIGS cells that are ever more efficient, up from a record value of 14.1 percent in 2005 to the "high score" of 18.7 percent for any type of flexible solar cell grown on polymer or metal foil. The latest improvements in cell efficiency were made possible through a reduction in recombination losses by improving the structural properties of the CIGS layer and the proprietary low-temperature deposition process for growing the layers as well as in situ doping with sodium during the final stage. With these results, polymer films have proved superior to metal foils as a carrier substrate for achieving highest efficiency. Record efficiencies of up to 17.5 percent on steel foils covered with impurity diffusion barriers were achieved with CIGS growth processes at temperatures exceeding 550 °C. However, when applied to steel foil without any diffusion barrier, the proprietary low-temperature CIGS deposition process developed by Empa and Flisom for polymer films easily matched the performance achieved with high-temperature procedures, resulting in an efficiency of 17.7 percent. The results suggest that commonly used barrier coatings for detrimental impurities on metal foils would not be required. The continuous improvement in energy conversion efficiencies of flexible CIGS solar cells is no small feat, said Empa Director Gian-Luca Bona. "What we see here is the result of an in-depth understanding of the material properties of layers and interfaces combined with an innovative process development in a systematic manner. Next, we need to transfer these innovations to industry for large-scale production of low-cost solar modules to take off." Empa scientists are working with Flisom to further develop manufacturing processes and to scale up production. For more information, visit: www.empa.ch