Internal reflector improves solar cells

Newark, DE--To make electricity from sunlight, you must capture photons in your solar cell. If photons pass through the cell, or get reflected out of it, cell efficiency suffers. By putting an irregular reflector on the back of a silicon cell, engineers enable a new photovoltaic cell to capture photons that would otherwise be lost. Photons can bounce back and forth between the front surface and the "mirror" until they are absorbed.

James Rand, AstroPower Inc.'s manager of product technology, explains that there are two types of silicon solar cells: Rather expensive, 400-aem-thick, polycrystalline cells, with an efficiency of about 14%; and super-thin amorphous silicon cells with an efficiency that ranges from 3 to 5%.

AstroPower has developed a new solar cell that combines the advantages of both types. It consists of silicon grown 35- to 50-aem-thick on sheets of inexpensive, ceramic-like substrate--which cuts costs. Current cell efficiency reaches 13% in the laboratory, 10 to 12% in the field. Its novel silicon/ceramic substrate combination makes extra light trapping feasible.

Silicon does not stop all the light that enters it. In fact, says Rand, "in 30 to 50 microns of silicon, about 15% of the light is not absorbed. Light passes into the substrate and is lost. So we put a reflector in the back."

The reflector must be randomly irregular. When a photon comes straight down, it passes through a non-reflective top coating. If it were to hit a flat reflector at the back of the cell, the photon could bounce straight back out of the top coating. By making the reflector irregular, the photon bounces off at an angle, and can't get through the top coating. "So we trap it, and the photon bounces about until it's absorbed," adds Robert Hall, Astro-Power's vice president of research and development.

In fact, a reflective layer can also keep electrons from escaping. "When photons strike the silicon, they generate huge clouds of electrons, which diffuse randomly," Rand explains. "When they reach the p-n junction at the front of the cell, they are collected and converted to useful electrical current." But at the back surface, any defects provide "holes" or places where there the cell's lattice structure lacks an electron, and the electrons can attach. So AstroPower engineers design their cell's rear reflective surface to be defect-free. It amounts to a barrier that holds in electrons until they diffuse to the front of the cell.

The natural, unpolished texture of the substrate is randomly irregular. AstroPower grows refractory-like oxides, nitrides, and carbides on the substrate. Next, a thin silicon layer grows in intimate contact with that combination of materials.

Light trapping should enable cells to reach 19% efficiency in the lab and 17% in the field. Prototypes demonstrated that the reflected light's path length is longer than in conventional cells. Engineers now want to make such a solar cell easy to manufacture.

Additional details...James Rand, Manager of Product Technology, AstroPower Inc., Solar Park, Newark, DE 19716-2000.