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NREL, First Solar Collaboration Improves Thin-Film Solar Cells

Aug. 19, 2019

Scientists at the National Renewable Energy Laboratory (NREL) and First Solar unwound 30 years of history to create a new approach for making cadmium telluride (CdTe) solar cells, paving the way for this technology to further undercut the costs of electricity from conventional energy sources.

For decades, CdTe solar cells have been made with the addition of copper and chlorine, because attempts to make them without one or the other have led to reduced performance. But the historic approach also has its difficulties. Key material properties critical to increased performance have reached a ceiling, and copper can move within the solar cell during extended periods of use, causing the efficiency to change and lifespan to shorten.

Cross-section of a solar cell.

Advanced microscopy on solar cell cross sections illustrates arsenic on tellurium atomic sites (green) versus regions where there is no penetration (blue) or material defects caused by the processing (red). The left and right images from the article compare the diffusion and in-situ approaches described in the text, respectively. 

Researchers at NREL and the world’s leading thin-film manufacturer, First Solar, were able to successfully alter the longstanding approach by removing copper and placing—or “doping”—elements from the fifth column of the periodic table (Group V), such as antimony or arsenic, onto tellurium crystal sites at extremely high speeds by low-cost methods required for mass production. 

The research appears in the Nature Energy article, “Exceeding 20% Efficiency with In-Situ Group-V Doping in Polycrystalline CdTe Solar Cells.” Eight of the paper’s authors are from NREL: Wyatt Metzger, Eric Colegrove, John Moseley, Craig Perkins, Chun-Sheng Jiang, Darius Kuciauskas, David Albin, and Mowafak Al-Jassim. Another nine are from First Solar, the Arizona company that is the market leader for the production and sale of CdTe thin-film solar panels.

The new paper reports a cell efficiency of 20.8%, representing a crossing point for the new group V technology relative to historic copper technology.

“We are excited about the performance potential of CdTe solar cells doped by Group V elements, particularly since the tests have demonstrated slower long-term degradation and the opportunity to harvest additional efficiency,” said Gang Xiong, director of the First Solar California Technology Center. “First Solar has a long record of collaborating with NREL and other institutions to develop our understanding of CdTe doping and drive the fundamental shift towards Group V doping.  We are pleased with the results of this collaborative effort.”

The results build on earlier fundamental research with single crystals reported in 2016 by NREL in collaboration with Washington State University, which demonstrated that CdTe solar cells could generate record voltage by using Group V elements. The resulting replacement of copper boosted carrier lifetime and density by several orders of magnitude, and NREL showed the Group V elements were more stable than copper. But single crystals are too expensive and slow to manufacture. The next step was to transform this foundational work to industrially relevant materials and methods.

Initial efforts attempted to diffuse Group V elements into CdTe using high temperature vapors. But if the temperature was too low, the elements didn’t spread through the cell enough, and if it was too hot, the panel glass would start to melt.

“This is where you run into these real-world constraints of manufacturing,” said Metzger, lead author of the paper.

A different method—adding Group V elements as the cells are being grown (in-situ)—worked.  This enables much better material properties to increase efficiency , and reduces long-term degradation, which can significantly enhance the lifetime of solar panels.

The research was funded by the Department of Energy’s Solar Energy Technologies Office and First Solar.