The effect of interface roughness on the optical properties of amorphous silicon (a-Si:H) solar cells was investigated using rms roughness measurements and computer modeling. We deposited four single junction a-Si:H solar cells on Asahi U type substrate each with a different intrinsic layer thickness. The roughness of the substrate and of the subsequent interfaces of the cells was measured by Atomic Force Microscopy. The relations between the computer input parameters, which describe the diffuse part of reflected and transmitted light at a rough interface, and the rms roughness of the interfaces in a-Si:H solar cell are presented. After obtaining a good matching between the simulated and measured external quantum efficiencies (QE) of the four cells, we investigated the effect of interface roughness on the absorption in all individual layers of the a-Si:H solar cell using the determined scattering parameters.
The rms roughness of the Asahi U-type substrate surface is determined to be 40 nm. Deposition of a 9 nm thick p-type a-SiC:H layer on the substrate has not changed the surface roughness. Due to a high refractive index of n-type a-Si:H the back contact interface acts as a nearly perfect diffuser for the reflected light when the rms roughness of the interface is higher than 25 nm. The rms roughness of the back interface of the four cells is found to be dependent on the intrinsic layer thickness and is larger than 25 nm.