Study of the optical properties of amorphous silicon solar cells using admittance analysis

A comprehensive optical admittance method is described to study the optical properties of amorphous silicon thin film solar cells. The method is applied to Schottky barrier solar cells of the type TCO/Au/a-Si:H/ rear contact, and p-i-n type solar cells of the type TCO/p/i/n/rear contact, considering them as multilayer systems composed of absorbing and non-absorbing layers. The numerical technique uses experimental results of refractive index, n(λ), and extinction coefficient k(λ), to calculate the optical absorbance and reflectance of such cells. The interference absorbing peaks thus obtained in the absorbance of a multi-layered cell are found to agree well with experimental results. The results reveal that a high reflecting rear metal contact increases the spectral absorbance in the wavelength region 0.06-0.70 μm, and peak positions in the absorption spectrum depend on the cell thickness. The optimised TCO parameters are found to be almost independent of the thickness of a-Si:H layer. The optimisation of the thickness of p layer in a p-i-n type solar cell is also discussed. This method can be applied for calculating the optical properties of any thin film solar cell. It is expected that, using the present method, one can desiesign a solar cell of optimal efficiency. It is found that by maximising the integrated absorbance in an optimal cell structure, one can increase the short circuit current density by 5.3%.