Researchers Experiment with Faster Ways to Make Stable Thin-film Solar Cells

Solar cells made from hydrogenated amorphous silicon (a-Si:H), a material whose chemical bonds lack a regular crystalline structure, have many advantages over their more orderly crystalline silicon counterparts. Amorphous solar cells require less expensive technology to manufacture and can be made much thinner than crystalline silicon solar cells, allowing them to be rolled up or folded over different structures. Unfortunately, the sunlight-to-electricity conversion efficiency of a-Si:H solar cells, which ordinarily lags behind that of crystalline cells, decreases the longer the amorphous silicon cells sit out in the sun. Now researchers from the University of Toledo, in Ohio, have designed a new manufacturing method for thin-film a-Si:H solar cells that not only deposits the silicon at a high rate when compared to other methods, but also produces cells whose efficiency decreases only slightly after absorbing light for more than a thousand hours. They publish their results in the Nov/Dec issue of the Journal of Vacuum Science and Technology A.

Industrial producers often use radio waves to manufacture thin-film a-Si:H solar cells. The process begins when a gaseous silicon-hydrogen compound is fed into a chamber along with hydrogen gas. Radio frequency power applied across two electrode plates then generates plasma that decomposes the gas. The resultant particles settle onto a surface and grow together to form the solar cell.

Varying the parameters of the manufacturing process affects both how fast the solar cells can be made and their material properties. The Ohio researchers sought a method that could speed up the process while still producing quality solar cells whose efficiency did not drop significantly after initial exposure to the sun. "Improving the deposition rate of a-Si:H solar cells is highly desired," the researchers write, "in order to reduce process time, and consequently, production costs." At first the research team experimented with constant flow rates of a gaseous silicon-hydrogen compound called disilane (Si2H6). The deposition rate was more rapid for faster disilane flow rates, but the speedier flow rate also resulted in larger amounts of efficiency degradation after the solar cells were exposed to light.

The result seemed to present a trade-off between speed and quality, but the researchers found a way around the limitation by changing the flow rate of the disilane during the manufacturing process. By slowly increasing the flow rate as the amorphous silicon built up on the solar cell sheet, the research team manufactured high-rate deposited solar cells that retained the same levels of efficiency as medium-rate deposited solar cells. The researchers believe that changing the flow rate during the manufacturing process helps keep the silicon grains at nanoscale dimensions. The smoother surface that results from the ultra-fine grains may help the a-Si:H solar cells retain a larger percentage of their efficiency over time, although the researchers aren't sure of the exact atomic-level mechanisms at work. Their new process might be readily transferred to mass production facilities to help increase the efficiency and decrease the costs of a common photovoltaic product.