Mar 2 2016
A milestone study in solar cell fabrication was conducted by the U.S. Department of Energy’s National Renewable Energy Laboratory (NREL) in partnership with Washington State University (WSU) and the University of Tennessee. This research will facilitate the use of solar energy to directly contend with electricity generated from traditional energy sources.
Image Credit: Gencho Petkov/Shutterstock.com
The researchers were able to enhance the highest available voltage from a cadmium telluride (CdTe) solar cell, rising above the practical limit that has been sought for about six decades and is crucial to optimize efficiency.
The research findings are published in Nature Energy.
Silicon solar cells make up 90% of the solar cell market, however it is hard to significantly bring down their manufacturing costs. CdTe solar cells offer an economical alternative. In solar technology they possess the lowest carbon footprint and offer better performance than silicon in real working conditions, such as under low light, and hot, humid weather. Until now the CdTe cells have not been as effective as silicon cells. One of the main areas where the CdTe cell did not meet expectations is the highest voltage available from the solar cell, called open-circuit voltage. So far researchers have only been able to get just 900 millivolts out of CdTe materials. This was considered its practical limit.
Moving away from a regular processing step using cadmium chloride, the team were able to enhance the voltage of the cell. They instead placed a few phosphorus atoms on tellurium lattice sites, and then cautiously formed suitable interfaces between materials with varying atomic spacing to form the solar cell.
This process optimized the CdTe conductivity, as well as carrier lifetime by orders of magnitude, enabling CdTe solar cell fabrication with an open-circuit voltage that broke the 1-volt barrier. The breakthrough creates novel research paths for solar cells to develop into more efficient energy sources, and offer electricity at a lower cost in comparison to fossil fuels.
It’s a significant milestone. It’s been below 900 millivolts for decades.
Kelvin Lynn, Regents Professor, School of Mechanical and Materials Engineering and Department of Physics, WSU
The research team at NREL treated the crystals and constructed and characterized the solar cells, while the WSU team, including Santosh Swain and Tursun Ablekim, created the crystal material used in the cells. The WSU team grew their crystals using a method known as melt growth, which facilitates accurate control over composition and purity.
As purity is vital to this process, the researchers performed mixing, preparing and vacuum-sealing the materials in an industry-standard clean room.
The crystal was synthesized in a furnace at temperatures of more than 1100°C. The next step was to cool the crystal from the bottom up, at a rate of approximately 1 mm/h. Finally the crystal was cut into polished wafers in order to fabricate the solar cells.
Others have tried dopants, but they didn’t have the control and purity that we have. And the purity matters. WSU is known for growing really high quality and purity crystals. You have to control every step.
Kelvin Lynn, Regents Professor, School of Mechanical and Materials Engineering and Department of Physics, WSU
Lynn added that although researchers have enhanced silicon-based cells near to their theoretical limit, there is still room to improve the effectiveness of CdTe, potentially upto 30% more.
The Energy Department’s SunShot Initiative funded this research. The research also received partial support from by Oak Ridge National Laboratory’s Center for Nanophase Materials Sciences.
The study is based around WSU’s Grand Challenges initiative, which stimulates research to focus on certain complex issues in the society. It is principally in line with the challenge of “Sustainable Resources for Society” and its theme of finding solutions for energy needs without affecting the environment.