CHICAGO — 'THIN-FILM" solar cells are the next generation of product for the photovoltaic industry, says John Corsi, chairman of Solarex. The Amoco Corporation subsidiary is the largest "PV" manufacturer in the United States and second worldwide.So far thin-film cells convert only 5 percent of the sunlight falling on them to electricity. That's far less than the 13 percent for old-fashioned polysilicon wafers that Solarex pioneered and that are still most of its product line. But Mr. Corsi is confident that Solarex can push that conversion rate for thin-film cells to 22 percent someday. And thin-film cells promise to be cheaper to make than conventional ones because they can be produced in a continuous, cost-efficient process that uses very little raw material and wastes none of it. A silicon-based gas is deposited on glass in a thin film, hence the name. Solarex funds about 75 percent of its own research, with the rest coming from the Department of Energy's Solar Energy Research Institute and the Sandia National Laboratory. The DOE labs have been very active in encouraging the private sector to turn discoveries into marketable products, Corsi says. Under its PV manufacturing technology program, the DOE is funding research to develop that technology to bring production costs down 50 percent by 1995. And that should open new markets, Corsi says. Photovoltaic power is already the technology of choice in a number of remote applications and consumer products such as calculators, bringing the industry $250 million in annual sales. But the greatest potential for growth lies in generating power for electric grids. In certain cases, PV arrays are already attractive to utilities wanting to supplement other power sources. Well into the next decade, Corsi says, PV might even be competitive for base-load generation.