Moving into the solar energy field for the first time, Standard Oil Co. of Indiana, the nation's sixth largest oil company, has purchased a minority interest in Solarex Corp. of Rockville, the world's largest maker of solar electric cells.

Standard of Indiana's investment will enable Solerex to build a new plant that is expected to more than double the production of solar cells that convert the sun's energy directly into electricity.

Solarex and Standard officials signed an agreement last Friday for the oil company to purchase 20,000 shares of the Rockville firm's stock for $43.75 per share, Solarex President Joseph Lindmayer announced yesterday.

The $860,000 investment makes Standard one of three equal minority partners of Solarex, a closely controlled corporation whose stock is largely owned by its founders and a small number of local investors. Earlier Solarex sold similar minority shares to two European firms, Moteurs Leroy-Somer of France and Holec, N.V. of the Netherlands.

Solarex officials stressed that the purchase of stock by Standard of Indiana does not mean the giant oil company is taking over the solar cell maker.

But the company said Standard's investment is expected to be followed by research contracts and solar cell demonstration projects for the oil giant.

Among the joint ventures under discussion is a "solar breeder" --a factory powered by solar cells that produces more solar cells.

The investment in Solarex is the first move into solar energy for Standard of Indiana, a $16-billion-a-year giant that sells gasoline through Amoco stations Standard is involved in several synthetic fuels projects and is a partner in a venture to extract petroleum from oil sands in Alaska.

But Standard is only the latest of several oil companies to enter the solar field. Exxon has an extensive solar reserch program; Atlantic Richfield owns Arco Solar; Mobil Corp.'s Tyco Laboratories is a major solar firm; and Shell Oil is the largest stockholder of Solar Energy Systems Corp.

By acquiring a minority interest in Solarex, Standard of Indiana is in effect buying the research and development know-how of a company that is already the leader in its field.

Solarex's specialty is what scientists call "photovoltaics" -- solar cells made of thin sandwiches of silicon crystals that emit an electric current when the sun strikes their surface. Although costly to manufacture, solar cells cost nothing to run, producing power whenever the sun shines.

Founded in 1973 by Lindmayer and his partner, Dr. Peter Varadi, Solarex is said to be light years ahead of its competition. The five-year-old firm now produces between 40 and 50 percent of the nation's solar cells, with sales of more than $10 million last year.

Lindmayer and Varadi originally worked for Comsat Corp., developing solar power systems for commumications satellites. They left to bring solar power down to earth to run watches and calculators to irrigation pumps and remote weather stations.

In a plant off I-270 in Rockville, the firm annually produces enough solar cells to provide nearly one megawatt million watts -- of electricity

The nation's total annual solar cell production now amounts to about 2 megawatts of power, but Solarex said the investment by Standard of Indiana will enable it to build a new plant that will initially turn out 2.5 megawatts worth of cells a year and will eventually expand to 5 megawatts of cells a year.

"Several states are courting us," Solarex executive Anthony Clifford said yesterday, and a decision is expected to be made within a month on where to build the plant.

Solarex said the new plant, to be operating by next year, will utilize "a totally new approach to solar cell manufacturing."

Details of the manufacturing process are being kept secret, the company said, calling it "a major advancement in high-efficiency, low-cost cell manufacturing."

Solarex agreed to sell a minority interest to Standard of Indiana after rejecting takeover offers from a number of large corporations. CAPTION: Picture, Solarex's Joseph Lindmayer with solar cells, products. By James M. Thresher -- The Washington Post