Optimization of ZnO/Ag back-reflectors for silicon thin film solar cell applications

Sputtered zinc oxide/silver double layers as a back-reflector for silicon thin film solar cells with nip configuration were prepared on the flexible metal substrates. Surface morphologies of Ag films were controlled by changing the deposition conditions such as substrate temperature and film thickness to improve the total/diffused reflectance and light scattering properties of ZnO/Ag back reflectors. The surface roughness of Ag films deposited on the stainless steel substrates increases from 14.1 nm to 36.1 nm with increasing the substrate temperature from room temperature to 500°C. Although the total reflectance of ZnO/Ag back reflectors shows minor variation, the diffused reflectance is enhanced with increasing the surface roughness of Ag films, meaning the light scattering on the rough surfaces is improved significantly. It is also found that the surface roughness of Ag films increases with the film thickness at fixed substrate temperature, resulting in the enhancement of diffused reflectance of ZnO/Ag back-reflectors. Silicon thin film solar cells are fabricated on the back-reflectors with the different surface roughness of Ag layers by PECVD and their cell performances are investigated systematically.