THIN FILM SILICON SOLAR CELLS WITH OPTICAL CONFINEMENT AND THEIR FABRICATION BY SOLUTION GROWTH

A new thin film silicon solar cell is designed and fabricated. The design incorporates optical confinement (due to light trapping in the thin silicon layers) and surface passivation. These features can lead to high conversion efficiency in unconventionally thin structures. A novel fabrication process based on metallic solution growth of silicon is used to produce experimental devices. A silicon substrate is textured and coated with an oxide film. Microscopic holes are etched in the oxide film which serve as sites of selective nucleation upon contact with a super-saturated silicon/metal liquid solution. Subsequent lateral overgrowth results in a continuous single crystal film of silicon formed over the oxide. A second silicon film is deposited over the first to form a p-n junction solar cell. The oxide interface acts as a (Lambertian) diffuse back reflector of photons for optical confinement and also provides a degree of back surface passivation. Because the substrate is not an active part of the device, it can be of low quality for reduced cost. This approach thus combines the high efficiency and stability inherent in crystalline silicon devices with the low cost associated with thin film deposition on inexpensive substrates. A detailed analysis and computer-based model of device performance is given. The model predicts that the design has a potential conversion efficiency (with assumed losses) in excess of 20%, provided optical confinement and surface passivation are sufficiently utilized. An optical confinement factor Z is defined as the effective number of passes light makes in the silicon film, or equivalently, Z is the factor by which the effective optical thickness is greater than the actual thickness of the film. Optical confinement and surface passivation in experimental devices are evaluated by spectral response measurements. Z factors up to 7 demonstrated, indicating significant optical confinement. Surface recombination is not severe. Photovoltaic performance of unoptimized solution-grown solar cells is encouraging with open-circuit voltages as high as 570 mV. This new design and fabrication process represents a viable alternative to conventional solar cell technology.