Solarex Corp. of Rockville, a pioneer in producing panels that convert sunlight into electricity, plans to buy out one of its major competitors, the photovoltaic division of RCA Corp., for a reported $3 million.

The purchase will open up a new avenue of solar electric power development for Solarex, which until now had insisted that its own method of making solar power cells was superior to the rival process used by RCA.

With two distinct technologies to develop, Solarex will have two shots at what is expected to become a $4-billion- to $5-billion-a-year market by 1990.

As recently as last November, when Solarex dedicated its "solar breeder"--a plant that uses solar power panels to produce more solar power panels--Solarex founder Dr. Joseph Lindmayer discounted the prospects for the RCA technology.

But in January, Lindmayer gave up the job of chief executive to become chairman of the board, and John Corsi, a Solarex board member, was named president.

Although Solarex did not disclose how much it is paying for the RCA division or where it will get the funds for the purchase, industry sources suggest the cash is coming from Standard Oil Co. of Indiana, better known as Amoco. Amoco owns about 29 percent of Solarex and is the largest single investor in the closely held firm.

Solarex for many years was the world's biggest producer of photovoltaic cells--panels that convert sunlight into electricity. But last year, Arco Solar, a division of Atlantic Richfield Co., surpassed Solarex, and the Rockville firm lost a reported $10 million.

Corsi said that Solarex is pursuing "a whole host of options" for finding additional capital and that "one is a merger with Standard" of Indiana.

In announcing plans to purchase the RCA division, Solarex described its own method of producing solar electric power as "the leading technology in photovoltaic applications during the 1980s."

The process used by RCA is "the most promising third-generation technology for inexpensive solar cells in the 1990s," the announcement said.

The first generation of solar power technology--still widely used--consists of growing salami-shaped crystals of silicon, which then are sliced into round wafers that give off electricity when sunlight strikes them.

Solarex's second-generation process involves casting square blocks of semicrystalline silcon, which also are sliced up and arranged into panels. The square panels fit together better than round ones, producing more electricity per square foot.