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Researchers break the 900mV CdTe solar cell barrier


March 2, 2016
By Anthony Capkun


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WSU’s Kelvin Lynn with crystals made in his lab. Photo courtesy WSU Photo Services.

March 1, 2016 – An effort led by the National Renewable Energy Laboratory (NREL in the U.S.)—in collaboration with the universities of Washington State (WSU) and Tennessee—promises to help pave the way for solar energy to directly compete with electricity generated by conventional energy sources by breaking the CdTe barrier.

Researchers improved the maximum voltage available from a cadmium telluride (CdTe) solar cell, overcoming a practical limit that has been pursued for six decades, says WSU, and is key to improving efficiency.

Silicon solar cells represent 90% of the solar cell market, but it will be difficult to significantly reduce their manufacturing costs, says WSU. CdTe solar cells offer a low-cost alternative, adding they have the lowest carbon footprint in solar technology, and perform better than silicon in real-world conditions, including in hot, humid weather and under low light. Until recently, however, CdTe cells haven’t been as efficient as silicon.

One key area where CdTe has underperformed is the maximum voltage available from the solar cell, called open-circuit voltage. Limited by the quality of CdTe materials, researchers for the past 60 years were not able to get more than 900mV out of the material, which was considered its practical limit.

The research team improved cell voltage by shifting from a standard processing step that uses cadmium chloride. Instead, they placed a small number of phosphorus atoms on tellurium lattice sites then formed interfaces between materials with different atomic spacing to complete the solar cell.

This approach improved both CdTe conductivity and carrier lifetime by orders of magnitude, enabling fabrication of CdTe cells with an open-circuit voltage breaking the 1V barrier for the first time. The innovation establishes new research paths for solar cells to become more efficient and provide electricity at a lower cost than fossil fuels.

“It’s a significant milestone. It’s been below 900mV for decades,” said WSU’s Kelvin Lynn, who led WSU’s effort.

The NREL researchers treated the crystals and built and characterized the solar cells.
The WSU researchers grow their crystals in a technique called 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 1100ºC and cool it from the bottom up at a rate of about 1mm/hr. 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,’’ said Lynn. “WSU is known for growing really high quality and purity crystals. 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 cadmium telluride, which could be bettered by an additional 30%, said Lynn.

— With files from Tina Hilding, Voiland College of Engineering & Architecture.

PHOTO: WSU’s Kelvin Lynn with crystals made in his lab. Photo courtesy WSU Photo Services.