Effect of oxide thin films in back contact on the optical absorption efficiency of thin crystalline Si solar cells

Abstract

The optical absorption efficiency (OAE) of thin crystalline Si (c-Si) solar cells was examined by using a direct absorption calculation algorithm based on the three-dimensional modeling and ray-tracing technique. The back contact was assumed to be made of ZnO based oxide layers with different optical constants and metallic electrode, and the front surface was assumed to be Lambertian. Simulation results showed that the insertion of non-absorptive oxide layer between c-Si and Al improved OAE, and that the relative amount of enhancement increased with decreasing refractive index, manifesting the reduction in absorption loss in Al electrode due to the increased total internal reflection. In the case of absorptive oxide layer, although increase in OAE was still attainable when compared with the back contact without oxide, the OAE was subdued significantly due to large absorption loss in oxide layer. The optimal oxide layer thickness was around 200 nm for non-absorptive oxide, and that of absorptive oxide decreased with increasing extinction coefficient. In the case of Ag metal contact, the enhancement of OAE due to the use of oxide layer was much less than the case of Al because of inherent high reflectivity and low absorption loss in Ag layer.