MOCVD of Cd(1−x)Zn(x)S/CdTe PV cells using an ultra-thin absorber layer

Ultra-thin Cd(1−x)Zn(x)S/CdTe devices were produced by atmospheric pressure metal organic chemical vapour deposition (AP-MOCVD) with varying CdTe absorber thicknesses ranging from 1.0 to 0.2 μm and compared to baseline cells with total CdTe thickness of 2.25 μm. The ultra-thin CdTe layers (≤1 μm) were intentionally doped with As to induce p-type conductivity in the absorber. Cell performance reduced with CdTe thickness, with the magnitude of photo-current generation loss becoming more significant for the very thin CdTe layers. The decline in cell performance was lower than the optically limited performance relating to a decrease in shunt resistance, Rsh, especially for the thinnest cells due to areas of incomplete CdTe coverage and large presence of pin-holes leading to micro-shorts. Incorporation of Zn into the CdS window layer improved cell performance for all devices except when 0.2 μm thick CdTe was used. This improvement was markedly in the blue region owing to enhanced optical transparency of the window layer. External quantum efficiency (EQE) measurements showed a red-shift of the window layer absorption edge due to leaching out of Zn during the CdCl2 treatment. Reduction of the CdCl2 deposition time was demonstrated to recover the blue response of the ultra-thin cells.

► The wider band gap CdZnS window layer improved blue response of the ultra-thin CdTe cells resulting in better performance.
► EQE of CdZnS/CdTe cells showed a shift in window layer absorption edge towards longer wavelengths as CdTe thickness reduced.
► EQE and SIMS revealed that excessive post-growth CdCl2 treatment caused Zn to leach out from the window layer.
► Reduction of the post-growth CdCl2 treatment process recovered the blue response in the CdZnS/CdTe cells.