Thin-film photovoltaic cell achieves record 17.7% conversion efficiency
A solar cell fabricated from copper indium gallium diselenide (CuInGaSe2, or CIGS) by researchers at the Department of Energy National Renewable Energy Laboratory (NREL, Golden, CO) converts sunlight to electricity with 17.7% efficiency. The research grou¥has fine-tuned the bandga¥and electronic gradients in the cells by varying the gallium concentration through the film and raised the efficiency 20% over previous CIGS devices. In contrast, the best amorphous-silicon thin-film efficiencies are about 11% and those of conventional polycrystalline-silicon solar cells are 18% to 19%.
Using multiple-source vacuum evaporation, the NREL grou¥deposited 2- to 3-µm-thick polycrystalline films on soda lime glass substrates with molybdenum back contacts to produce 0.5-cm2 test devices; CIGS modules as large as 4 ft2 have already been produced by other methods. According to Ken Zweibel of the NREL, the work shows that efficiency is not an obstacle to realization of practical thin-film photovoltaics. "Fairly radical changes are required to make photovoltaic technology a widespread power alternative; thin films provide that [by being economical to manufacture]. If we combine CIGS efficiency with low-cost design, we get a lot of leverage for achieving cost-competitive photovoltaic electricity," says Zweibel.