In situ deposition rate monitoring during the three-stage-growth process of Cu(In,Ga)Se₂ absorber films

The controllability of the three-stage-growth process of Cu(In,Ga)Se₂ films was investigated, implementing two different substrate temperature monitoring channels simultaneously: (a) a thermocouple at the rear side of the substrate, and (b) a pyrometer, measuring the emission of heat radiation from the front surface of the growing film. By this setup not only the film composition, i.e. the relative incorporation rate of Cu and group III elements, but as well the absolute growth rate can be monitored on-line during growth. The heat radiation intensity during the first growth stage (deposition of (In,Ga)₂Se₃ precursor films) exhibits characteristic oscillations. The oscillation period is inversely proportional to the growth rate. The oscillations are caused by thickness interference effects of black body radiation within the growing film. By a simulation of the infrared emissivity, the refractive index n_igs = 2.9 and absorption coefficient k_igs = 0.2 of the growing film could be extracted. Employing our process, Mo/CIGS/CdS/ZnO/Al solar cells with an efficiency up to 16.4% could be realized.