Aug. 4, 1997 ATLANTA, GE--Researchers have successfully cut in half the time it takes to make a silicon solar cell without diminishing its performance, an achievement that should reduce the cost of solar energy.
Using rapid thermal processing (RTP), researchers at the Georgia Institute of Technology have produced a solar cell with the same efficiency rating -- 18 percent -- as one made by conventional furnace processing. They created the cells in 8 1/2 hours, compared with the 17 hours needed for a furnace-processed cell.
In a separate process, researchers also integrated RTP with screen-printing, an alternative method for applying the cell's metal contacts, which slashed the processing time even further, to 1 1/2 hours.
"If we can make the solar cells very fast compared to what's being done out in industry today, that will obviously reduce the use of chemicals, gases and manpower, and it will increase the production capacity and throughput," said Dr. Ajeet Rohatgi, a professor in Georgia Tech's School of Electrical & Computer Engineering. "That can result in the significant reduction of costs."
Rohatgi presented a paper on these results, titled "Highest Efficiency RTP Silicon Solar Cells by Rapid Thermal Diffusion and Oxidation," during the 14th European Photovoltaic Solar Energy Conference and Exhibition in Spain, which ended July 4. The paper was written by Rohatgi, graduate student Parag Doshi and research engineer Sachin Kamra.
The research was done under Georgia Tech's University Center of Excellence for Photovoltaics Research and Education (UCEP), established by the U.S. Department of Energy and supported by the Sandia National Laboratories.
Proponents of photovoltaics -- the direct conversion of sunlight into electricity -- say it offers a clean, sustainable alternative to traditional energy sources that are non-renewable, create pollution and contribute to global warming. Despite major strides made in the last two decades, photovoltaic-generated energy remains about four times more expensive than energy produced from fossil fuels. At Georgia Tech, researchers are exploring several ways to reduce that cost without sacrificing performance.
"My goal is to reduce the cell processing time to the range of a half-hour to one hour," said Rohatgi, who also is UCEP's director. "People have tried this, but when they reduced the time, the efficiency took a dive."
Rapid thermal processing utilizes incoherent radiation as a source of optical and thermal energy. The interaction of high energy photons with matter leads to thermal and photophysical effects that significantly decrease the activation energy for various semiconductor processes like diffusion, thus reducing the processing time and temperature needed to fabricate a solar cell. Conventional furnace processing lacks these high energy photons and requires greater thermal cycle time and temperature.
Georgia Tech's rapid thermal process has three steps: a three-minute rapid thermal diffusion that simultaneously forms the front and back of a silicon solar cell; a five-minute rapid thermal oxidation (RTO) for the front emitter surface; and the application of metal contacts by evaporations and photolithography.
In current industrial production, front and back diffusions are done separately. Each step takes one to three hours, and the cells must be cleaned between each procedure. The solar cell then goes back into a high-temperature furnace for a process called passivation, in which an oxide is grown on the front surface of the cell.
Although passivation improves performance, many manufacturers delete it to save money and increase output. Georgia Tech's RTO process offers a time-saving way to include this step.
Forming the metal contacts by evaporations and photolithography accounts for over 80 percent of Georgia Tech's RTP process. Although these procedures give good resolution and conductivity, Rohatgi said commercial manufacturers often use screen printing -- an alternative method for adding the metal contacts to a solar cell -- instead. Screen-printing is quicker but produces less efficient cells.
Georgia Tech researchers successfully integrated screen printing with RTP in 1996 and have since raised cell efficiency from 14.7 percent to 16.3 percent. They hope to increase it to 18 percent -- the same level already reached for cells produced by RTP and photolithography -- with further modifications. These include adding rapid thermal oxidation, a screen-printed aluminum back surface field (to prevent the loss of light-generated carriers to recombination on the back), and surface texturing (to reduce the amount of light reflected off the front surface and trap more light into the cell).
"Today, industrial cells made with crystalline silicon are in the range of 11 to 15 percent," Rohatgi said. " If we succeed in what we're doing, we will end up making cells that are close to 18 percent, in less than one-half to one-third of the time."
For the paper presented in Barcelona, researchers also used modeling and analysis to show how these same modifications could improve cells produced by RTP and photolithography. In late June, they produced a solar cell with 19 percent efficiency, which is the highest rating ever achieved with a low-cost, rapid (non-furnace) process. They'll release details this fall at the 26th IEEE Photovoltaics Specialists Conference to be held in Anaheim, Calif.
The key to transferring this technology to industry, Rohatgi said, is developing a batch or continuous-processing machine to make multiple cells, perhaps 500 to 1,000 an hour. Researchers are perfecting their techniques on machines that produce only one cell at a time, and they also work with cells that are 4 square centimeters. The industry standard size is 100 square centimeters.
One manufacturer recently developed RTP machines that can handle nine 100-square-centimeter cells at once, but Georgia Tech researchers are collaborating with partners in industry, government and other universities to build a machine that can increase the output further.
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MEDIA RELATIONS CONTACTS: Amanda Crowell (404-894-6980) or John Toon (404-894-6986); Fax (404-894-6983); Internet: or
TECHNICAL CONTACTS: Dr. Ajeet Rohatgi(404-894-7692); Internet:
VISUALS: Color slides of Dr. Rohatgi demonstrating the new process and a close-up of the cell produced.
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