Kelvin Lynn, a professor in the Washington State University School of Mechanical and Materials Engineering and Department of Physics, has helped achieve a milestone in solar cell fabrication. Photo: Washington State University.Researchers have reached a critical milestone in solar cell fabrication, helping pave the way for solar energy to compete directly with conventional, non-renewable energy sources.
Led by the US Department of Energy's National Renewable Energy Laboratory in collaboration with Washington State University and the University of Tennessee, the researchers improved the maximum voltage available from a cadmium telluride (CdTe) solar cell. In doing this, they finally overcame a practical limit on efficiency that has held firm for six decades. The work is published in Nature Energy.
Silicon solar cells currently represent 90% of the solar cell market, but it will be difficult to significantly reduce their manufacturing costs. CdTe solar cells offer a low-cost alternative. They also have the lowest carbon footprint of any solar technology and perform better than silicon in real world conditions, including in hot, humid weather and under low light. However, until recently, CdTe cells haven't been as efficient as silicon-based cells.
One key area where CdTe has underperformed is in the maximum voltage available from the solar cell, known as the open-circuit voltage. For the past 60 years, limited by the quality of CdTe materials, researchers have not been able to get more than 900 millivolts out of the material, which was considered its practical limit.
The research team has now managed to surpass this voltage limit by shifting away from a standard processing step involving 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 spacings to complete the solar cell. This approach improved both the CdTe conductivity and carrier lifetime by orders of magnitude, thereby allowing the fabrication of CdTe solar cells with an open-circuit voltage that broke the 1-volt barrier for the first time. This innovation establishes new research paths for making solar cells more efficient and providing electricity at lower cost than fossil fuels.
"It's a significant milestone. It's been below 900 millivolts for decades," said Kelvin Lynn, a professor in WSU's School of Mechanical and Materials Engineering and Department of Physics, who led WSU's effort.
The NREL researchers treated the crystals, and built and characterized the solar cells, while WSU researchers, including Santosh Swain and Tursun Ablekim, developed the crystal material used in the cells. The WSU researchers grow their crystals using a technique called melt growth, which allows precise control over purity and composition.
Purity is critical for the fabrication process, so the researchers mix, prepare and vacuum-seal the materials in an industry-standard clean room. 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. Finally, the researchers cut the crystal 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 with CdTe cells, which could be bettered by an extra 30%, said Lynn.
This story is adapted from material from Washington State University, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.