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1 March 2016

NREL collaboration boosts maximum potential for CdTe solar cells to above 1V

The US Department of Energy's (DoE's) National Renewable Energy Laboratory (NREL), in collaboration with Washington State University (WSU) and the University of Tennessee, has improved the maximum voltage available from a cadmium telluride (CdTe) solar cell (a key factor in improving energy conversion efficiency (James Burst et al, 'CdTe solar cells with open-circuit voltage breaking the 1V barrier', Nature Energy 16015 (2016); DOI: 10.1038/nenergy.2016.15). The research was funded by the DoE's SunShot Initiative, which aims to make solar cost-competitive with traditional energy sources. It was also supported in part by Oak Ridge National Laboratory's Center for Nanophase Materials Sciences.

Silicon currently represents 90% of the solar cell market, but it is difficult to significantly reduce their manufacturing costs. Offering a low-cost alternative, CdTe solar cells also have the lowest carbon footprint and adapt better than silicon in real-world conditions (including hot, humid weather and low light), says NREL. However, CdTe solar cells have not been as efficient as silicon-based cells until recently. 

A key area where CdTe has underperformed is in the maximum voltage available from the solar cell (the open-circuit voltage). For decades, the quality of CdTe materials has prevented anyone from obtaining an open-circuit voltage of more than 900mV despite billions of CdTe solar cells having been produced; the vast majority have been limited to 750-850mV.

The research team improved cell voltage by shifting away from a standard processing step using cadmium chloride. Instead, they placed a small number of phosphorus atoms on tellurium lattice sites and then carefully formed ideal interfaces between materials with different atomic spacing to complete the solar cell. This approach improved the CdTe conductivity and carrier lifetime each by orders of magnitude, enabling the fabrication of CdTe solar cells with an open-circuit voltage breaking the 1V barrier for the first time.

The NREL researchers treated the crystals and built and characterized the solar cells. WSU researchers (including Santosh Swain and Tursun Ablekim) developed the crystal material used in the cells.

The WSU researchers grow their crystals using melt growth, which allows precise control over purity and composition. Purity is critical to the process, so the researchers mix, prepare and vacuum-seal the materials in an industry-standard cleanroom. They then synthesize the crystal in a furnace above 1100°C and cool it from the bottom up at a rate of about 1mm per hour. The crystal is cut into polished wafers to make the solar cells.

"Others have tried dopants, but they didn't have the control and purity that we have. And the purity matters," says Kelvin Lynn, Regents professor in WSU's School of Mechanical and Materials Engineering and Department of Physics, who led WSU's effort. "WSU is known for growing really high-quality and purity crystals," he adds "You have to control every step."

While researchers have improved silicon-based cells almost to their theoretical limit, there is significant room for efficiency improvements for CdTe, which could be bettered by an additional 30%, reckons Lynn.

Tags: Thin-film photovoltaic CdTe NREL

Visit: www.nature.com/articles/nenergy201615

Visit: http://energy.gov/eere/sunshot/sunshot-initiative

Visit: www.nrel.gov

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