The secret to a solar cell's stability?

If solar cells are going to power more than hand-held calculators, new photovoltaic materials will have to be efficient and robust as well as cheap. A semiconductor just entering the market, called copper indium gallium diselenide (CIGS), might fit this bill. It converts sunlight into electricity almost as efficiently as the best solar devices. What's more, it seems to maintain that efficiency indefinitely and doesn't break down even after prolonged exposure to sunlight.

Now, David Cahen of the Weizmann Institute of Science in Rehovot, Israel, and his colleagues propose a new explanation for CIGS' amazing stability. Ordinarily, atoms in a stable compound, don't move around. The researchers, however, suggest that copper atoms can diffuse through CIGS to repair any sites damaged by exposure to radiation. In this way, the material essentially heals itself.

Radiation hitting a CIGS solar cell loosens the chemical bonds holding the copper atoms in the compound's crystal lattice. Cahen's team calculates that these liberated atoms wander freely through the material ready to immediately fill in for any newly liberated atoms.

In this equilibrium, the copper atoms act "as an electrical analogue of a mechanical shock absorber," the researchers report in the August ADVANCED MATERIALS. They argue that in CIGS, the mobile copper atoms "allow the material a degree of flexibility that is essential for accommodating externally imposed changes."

Rommel Noufi of the National Renewable Energy Laboratory (NREL) in Golden, Colo., agrees that the mechanism proposed by Cahen's team is plausible but cautions that the new study provides "no experimental proof." He adds, "It's another perspective on why these materials are stable, but not the only one."

Solar cells generally consist of a stack of several layers of materials. The CIGS cells have a glass foundation topped with molybdenum, CIGS, cadmium sulfide, and then zinc oxide.

Noufi and his colleagues have proposed that a type of CIGS with a modified crystal structure forms at the semiconductor's interface with cadmium sulfide. This 10-to 50-nanometer-thick interfacial layer, they argue, is largely responsible for the 'material's high efficiency and possibly its stability. Cahen, however, doesn't accept the idea of an extra layer.

In an upcoming PROGRESS IN PHOTOVOLTAICS, the NREL team will report on CIGS devices that can convert into electric current 18.8 percent of the sunlight energy falling on them. Previous versions had an efficiency of about 16 percent (SN: 12/4/93, p. 374). The researchers achieved the latest improvements by modifying each of the layers.

Regardless of the continuing uncertainty over how they work, the performance of CIGS in solar cells is nearly as good as that of crystalline silicon, "the workhorse of the photovoltaics industry," Noufi says. Crystalline silicon solar cells, however, need thick layers of high-quality material, making them expensive for large-scale applications, such as solar panels for generating electricity. Using a thin film of CIGS in such devices could cost less per watt of energy, Noufi predicts.