Light scattering and trapping in thin film silicon solar cells with an n-i-p configuration

Microcrystalline silicon µc-Si: H thin film solar cells with an n-i-p configuration were set up based on the optimized processes for solar cells with a p-i-n configuration. The deposition processes of window, absorber and front contact layers were optimized. The effects of the thickness and doping ratio of p-type layers, the thickness of the front contact layers and the silane concentration of the absorber layers on the solar cell performance were investigated. For all the optimization and investigation, solar cells were prepared on simple glass/etched- ZnO substrates. To improve the short circuit current density, a highly reflective Ag/ZnO back reflector was added on glass/etched-ZnO substrates. Finally, a good process reproducibility and high cell performance were achieved as a base for the subsequent study of light scattering and trapping in µc-Si: H n-i-p solar cells. The effects of the surface morphology of back reflectors on their light-scattering properties and the light trapping in µc-Si: H n-i-p solar cells were investigated. Firstly, the surfaces of sputtered ZnO layers were textured by etching them in HCl solution. The surface morphology was varied by changing the as-deposited ZnO thickness and etching time and measured by Atomic Force Microscopy AFM. Based on the AFM measurement results, statistically evaluation of the ZnO surface morphology was performed in terms of not only the rms roughness but also the diameter, depth and angle of surface features (craters) on the surfaces. With such evaluation, the relationship between the surface morphology and lightscattering properties of reflectors was analysed and related to different physical mechanisms, such as diffraction and geometrical optics. Finally, etched-ZnO layers with different surface morphologies were covered with a thin Ag and ZnO layer and used together as back reflectors in µc-Si: H n-i-p solar cells. This allows us to analyse the direct link between the surface morphology of back reflectors and light trapping in the solar cells. With this analysis, the most beneficial size and angle of craters for light trapping were estimated. In addition, to better understand the light trapping process in the solar cells, angular intensity distributions AIDs in silicon were simulated by the so-called “phase model” both for the transmission at the ZnO/µc-Si: H interface (as on the front side of solar cells) and for the reflection at the back reflector. The AIDs for the transmission were compared to those for the reflection with the same interface morphologies to estimate the dominant scattering process regarding the light trapping in solar cells.