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Se 2 (CIGS) solar cells with ZnS(O,OH) buffer layers were fabricated on Mo-coated Ti foils. The CIGS thin films were deposited by the three-stage process at substrate temperatures of 460 -550 C. The influence of the substrate temperature during CIGS deposition on film properties and cell performance has been mainly investigated by scanning electron microscopy (SEM), X-ray diffraction (XRD), and secondary ion mass spectroscopy (SIMS) analyses. A double grading of Ga=ðIn þ GaÞ atomic ratio required to achieve high efficiency devices was formed at a substrate temperature of 500 C. The best ZnS(O,OH)/CIGS solar cell yielded a total area efficiency of 17.9% by optimizing cell fabrication conditions. The importance of surface smoothness of Ti foil substrates is also discussed.

Certified efficiency of 22.3% has been achieved for Cu(In,Ga)(Se,S) 2 solar cell. Compared to our previous record cell with 20.9% efficiency, the major breakthrough is due to the increased V oc , benefited from potassium treatment. A lower reverse saturation current and a longer carrier collection length deduced from electron-beam induced current indicate that the degree of carrier recombination at the heterojunction and depletion region for the 22.3% cell is lower. Further characterizations (capacitance-voltage profiling, temperature-dependent V oc , Suns-V oc ) and analysis indicate that the recombination coefficients at all regions were reduced, especially for the interface and depletion regions. Device simulation was performed assuming varying defect densities to model the current-voltage curve for the 22.3% cell. The best model was also used to estimate the achievable V oc if defect densities were further reduced. Furthermore, by using higher bandgap Cd-free buffer layers, a higher J sc was achieved which gives an in-house solar cell efficiency of 22.8%. Recombination analysis on the 22.8% cell indicates that the interface recombination is further reduced, but the recombination coefficients at the depletion region was higher, pointing out that further improvement on the depletion region recombination could help to achieve a higher V oc and therefore an efficiency beyond 23%.

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