Efficient light trapping structure in thin film silicon solar cells

Thin film silicon solar cells are believed to be promising candidates for continuing cost reduction in photovoltaic panels because silicon usage could be greatly reduced. Since silicon is an indirect bandgap semiconductor, its absorption coefficient is low for photons in the wavelength ranges between 600nm and 1100nm. For high efficiency thin film modules, effective light trapping is essential. Traditional schemes such as textured transparent conductive oxide (TCO) and metal reflector have several disadvantages such as enhanced surface recombination, parasitic losses at the TCO/metal interface, and the lack of ability to control and optimize the textured surface. We have previously proposed to employ a light trapping structure, which combines a self-assembled submicron grating and a distributed Bragg reflector (DBR) on the backside of thin film silicon solar cells. The DBR works as a one-dimensional photonic crystal to obtain almost 100% reflectivity. The grating scatters the incident light into oblique angles to significantly enhance the optical path length. Numerical calculations predict that by optimizing the feature sizes of the grating and DBR, up to 31% relative efficiency increase can be obtained, compared to the bare thin film Si. By using self-assembly, the organized grating structure can be formed spontaneously at a much lower cost. Current-voltage relations and quantum efficiency measurements were taken to verify the performance of our designed back structure. In the wavelength range of 600-900nm, photon absorption is greatly enhanced. As a result, more than 20% relative efficiency enhancement is achieved for 1.5um thin film silicon cells. These numerical and experimental results show that a light trapping design can be low-cost and increase efficiencies for high performance thin film Si solar cells.