Enhanced uniformity of sputtered oxygenated cadmium sulfide (CdS:O) films for large area photovoltaic applications

The possible beneficial effects of replacing CdS with CdS:O in photovoltaic applications have been reported and commented by many authors. However, the effective scalability of the CdS oxygenation process by RF reactive sputtering has never been addressed. In this paper, we studied the uniformity of the CdS oxygenation on 100 cm2 samples as well as the effect of a Post-Deposition Thermal Treatment (PDTT) on the film properties. The samples were analyzed by profilometry, X-ray diffraction spectroscopy, X-ray photoemission spectroscopy and atomic force microscopy (AFM). Moreover, an ad-hoc system was developed to map the optical transmission on the sample area, hence the CdS:O band gap (Eg) profile was calculated. The as-grown films presented poor oxygenation uniformity. However, 375 °C was identified as the threshold temperature for oxygen expulsion. Hence, we propose a possible method to improve the film uniformity based on an initial over-oxygenation followed by a thermal treatment at the threshold temperature. A 10 × 10 cm2 CdS:O film with remarkable uniformity and Eg = 2.60 ± 0.05 eV was obtained.