Oxygenated CdS window layers for thin film CdTe photovoltaics by pulsed DC magnetron sputtering

Abstract

Absorption in the CdS window layer limits the photocurrent obtained from thin film CdTe solar cells. These absorption losses can be avoided by tailoring the band gap of the CdS material to improve the light transmission at shorter wavelengths. This can be achieved by adding oxygen to the CdS to create Oxygenated Cadmium Sulphide films (CdS:O). In this paper we report on a new process in which oxygen is incorporated into the thin film CdS during deposition by pulsed DC magnetron sputtering. Spectrophotometric measurements of transmission show that the absorption edge is shifted from 500nm to 350nm by incorporating the oxygen into the deposited films. In this way, light absorption in the window layer of the solar spectrum utilised by the CdTe solar cells was significantly reduced. Ellipsometric measurements showed that the refractive index of the CdS decreased from 2.45 to 2.1 at λ=632.8nm and the band gap shifted from 2.38eV to 3.1eV as a result of incorporating the oxygen into the film. The refractive index dispersion profiles suggest that the oxygenated films are amorphous/semi-amorphous and this was confirmed by X-ray Diffraction measurements and TEM cross-section images.