Influence of oxygen concentration in the CdCl2 treatment process on the photovoltaic properties of CdTe/CdS solar cells

The effect of oxygen during the CdCl2 treatment on the optoelectronic properties of CdTe/CdS devices was studied. Higher efficiencies and greater uniformity in device performance were obtained for samples processed with higher amount of oxygen. Oxygen in the activation ambient caused an increase in the net carrier concentration near the back contact of CdTe, which ultimately seems to saturate around 5.5 × 1015 cm− 3 when the O2 concentration is 50%. However, the carrier concentration reduces by two orders of magnitude from the back contact to bulk CdTe. The important role of oxygen in CdCl2 treatment is that it controls the diffusion of Cu. The disappearance of dark and light current-voltage crossover for devices annealed at higher concentration of O2 is due to the oxidation of grain boundaries thereby blocking the diffusion of Cu towards the CdS buffer. The capacitance-voltage profiles suggest that the CdTe/CdS cell behaves more like a p-i-n than p-n device.