A Solar Hybrid: Crystalline Silicon and Thin Film in One Cell

RoseStreet_PilotWho says a solar company has to choose between conventional silicon and thin-film solar cells? RoseStreet Labs Energy, a Phoenix-based private company, is combining the two in a double-layered cell that it claims can achieve “practical efficiencies” – meaning efficiencies of cells actually available on the market, not just in the lab – of 25 to 30 percent. On Monday, the company announced the world’s first (or as RoseStreet put it, the “first known”) nitride/silicon tandem solar cell, which it plans to produce in the fourth quarter of next year.

The potential efficiency might not sound breathtaking considering that the National Renewable Energy Laboratory and the Defense Advanced Research Projects Agency both announced last year that they had produced cells that achieved more than 40 percent efficiency in the lab. But lab efficiencies and production efficiencies are not the same thing, and the highest efficiencies for mass-produced solar cells hover around 22 percent. SunPower Corp., which introduced 22-percent efficiency monocrystalline cells in 2007, last year announced it had produced a prototype with 23.4 percent efficiency, which it expects to launch commercially next year.

RoseStreet thinks its new hybrid cell – which “marries low-cost nitride thin film with the massive infrastructure of silicon solar cells,” as CEO Bob Forcier said in the press release – could be cheaper than crystalline silicon cells. The nitride film, the same material used in solid-state lighting and blue lasers, enables the cells to make use of more of the light spectrum, Forcier told us.

The company plans to use a fabless model, manufacturing the cells through a partner, and also plans to license some of its technology, Forcier added. RoseStreet hopes to reach costs of less than $1.50 per watt, or 10 cents per kilowatt-hour, by 2014. The company plans to target applications with space constraints, such as industrial rooftops and mobile devices.

RoseStreet will certainly have competition. Other technologies are also racing toward higher efficiencies, with six companies – Suntech Power, Sunovia Energy Technologies, IMEC, Q-Cells, Fraunhofer Institute for Solar Energy Systems and Oerlikonlast week announcing world-record-breaking efficiencies for different types of crystalline silicon, thin film and multijunction solar cells.

And RoseStreet will have its work cut out. Many companies developing new technologies end up running into complications that result in delays. So far, at least, Forcier said he doesn’t foresee any problems with getting or processing the nitride material that it plans to use. “Nitrides are pretty pervasive in other parts of the world . . . so we see it as a scaleable technology on a worldwide basis,” he said. “We have no concerns about sourcing.”

In any case, it’s a good sign that the company expects to reach production next year. If RoseStreet really can reach such high efficiencies at a lower cost than conventional crystalline silicon, and can manufacture the cells in mass quantities, it should have no trouble finding a market.

The company has been developing higher-efficiency solar technology since at least 2004, when it licensed multiband technology from the Lawrence Berkeley National Laboratory. The company has formed partnerships with Los Alamos National Laboratory and Sumitomo Chemical Co., which also invested in RoseStreet.

The company already operates a 500-kW pilot line in Phoenix and last year launched a solar-cell development center in the same city. The company plans to seek more funding – technically its Series A round – in the first quarter of 2010, Forcier said.