ATLANTA, GE--Researchers have successfully cut in half the timeit takes to make a silicon solar cell without diminishing itsperformance, an achievement that should reduce the cost of solarenergy.
Using rapid thermal processing (RTP), researchers at the GeorgiaInstitute of Technology have produced a solar cell with the sameefficiency rating -- 18 percent -- as one made by conventionalfurnace 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 withscreen-printing, an alternative method for applying the cell'smetal 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'sbeing done out in industry today, that will obviously reduce theuse of chemicals, gases and manpower, and it will increase theproduction capacity and throughput," said Dr. Ajeet Rohatgi, aprofessor in Georgia Tech's School of Electrical & ComputerEngineering. "That can result in the significant reduction ofcosts."
Rohatgi presented a paper on these results, titled "HighestEfficiency RTP Silicon Solar Cells by Rapid Thermal Diffusionand Oxidation," during the 14th European Photovoltaic SolarEnergy Conference and Exhibition in Spain, which ended July 4.The paper was written by Rohatgi, graduate student Parag Doshiand research engineer Sachin Kamra.
The research was done under Georgia Tech's University Center ofExcellence for Photovoltaics Research and Education (UCEP),established by the U.S. Department of Energy and supported bythe Sandia National Laboratories.
Proponents of photovoltaics -- the direct conversion of sunlightinto electricity -- say it offers a clean, sustainablealternative to traditional energy sources that arenon-renewable, create pollution and contribute to globalwarming. Despite major strides made in the last two decades,photovoltaic-generated energy remains about four times moreexpensive than energy produced from fossil fuels. At GeorgiaTech, researchers are exploring several ways to reduce that costwithout sacrificing performance.
"My goal is to reduce the cell processing time to the range of ahalf-hour to one hour," said Rohatgi, who also is UCEP'sdirector. "People have tried this, but when they reduced thetime, the efficiency took a dive."
Rapid thermal processing utilizes incoherent radiation as asource of optical and thermal energy. The interaction of high energy photons with matter leads to thermal andphotophysical effects that significantly decrease the activationenergy for various semiconductor processes like diffusion, thusreducing the processing time and temperature needed to fabricatea solar cell. Conventional furnace processing lacks these highenergy photons and requires greater thermal cycle time andtemperature.
Georgia Tech's rapid thermal process has three steps: athree-minute rapid thermal diffusion that simultaneously formsthe front and back of a silicon solar cell; a five-minute rapidthermal oxidation (RTO) for the front emitter surface; and theapplication of metal contacts by evaporations andphotolithography.
In current industrial production, front and back diffusions aredone separately. Each step takes one to three hours, and thecells must be cleaned between each procedure. The solar cellthen goes back into a high-temperature furnace for a processcalled passivation, in which an oxide is grown on the frontsurface of the cell.
Although passivation improves performance, many manufacturersdelete it to save money and increase output. Georgia Tech's RTOprocess offers a time-saving way to include this step.
Forming the metal contacts by evaporations and photolithographyaccounts 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 asolar cell -- instead. Screen-printing is quicker but producesless efficient cells.
Georgia Tech researchers successfully integrated screen printingwith RTP in 1996 and have since raised cell efficiency from 14.7percent to 16.3 percent. They hope to increase it to 18 percent-- the same level already reached for cells produced by RTP andphotolithography -- with further modifications. These includeadding rapid thermal oxidation, a screen-printed aluminum backsurface field (to prevent the loss of light-generated carriersto recombination on the back), and surface texturing (to reducethe amount of light reflected off the front surface and trapmore light into the cell).
"Today, industrial cells made with crystalline silicon are inthe range of 11 to 15 percent," Rohatgi said. " If we succeed inwhat we're doing, we will end up making cells that are close to18 percent, in less than one-half to one-third of the time."
For the paper presented in Barcelona, researchers also usedmodeling and analysis to show how these same modifications couldimprove cells produced by RTP and photolithography. In lateJune, 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 the26th IEEE Photovoltaics Specialists Conference to be held inAnaheim, Calif.
The key to transferring this technology to industry, Rohatgisaid, is developing a batch or continuous-processing machine tomake multiple cells, perhaps 500 to 1,000 an hour. Researchersare perfecting their techniques on machines that produce onlyone cell at a time, and they also work with cells that are 4square centimeters. The industry standard size is 100 squarecentimeters.
One manufacturer recently developed RTP machines that can handlenine 100-square-centimeter cells at once, but Georgia Techresearchers are collaborating with partners in industry,government and other universities to build a machine that canincrease the output further.
Georgia Institute of TechnologyResearch Communications Office223 Centennial Research BuildingAtlanta, Georgia 30332-0828
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.
The above story is based on materials provided by Georgia Institute Of Technology. Note: Materials may be edited for content and length.
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