Difluorochloromethane treated thin CdS buffer layers for improved CdTe solar cells

One of the major improvements for CdTe solar cells is the increase of the current density by enhancing the transparency of the buffer layer. CdS, with its 2.4 eV band gap, results in being opaque at low wavelength regions. For this reason, in order to gain in blue and UV light, extremely thin CdS has to be applied, if the buffer is not substituted with other materials. On the other hand, by thinning the CdS layer we create pinholes also due to the wide intermixing of CdTe and CdS layers and subsequently consumption of CdS. For this reason we have applied and compared different CdS post-deposition treatments in order to stabilize the layer and avoid excess intermixing. Treatments at temperatures above 500 °C in an argon and chlorine atmosphere on 80 nm thick CdS have led to the fabrication of more stable samples compared to untreated CdS, resulting in improved performance with a 10% increase in current density and a 5% increase in open circuit voltage and fill factor. Moreover, light transmission of the CdS treated layers from 300 to 450 nm is increased by about 10%. In this paper, a study of treated thin CdS with recrystallization treatments is presented; the layers have been analysed by means of X-ray photoelectron spectroscopy, X-ray diffraction and atomic force microscopy. Finished CdTe devices made on thin CdS are characterized in terms of current-voltage and quantum efficiency.