Dependence of Se beam pressure on defect states in CIGS-based solar cells

The influence of Se beam pressure on defect properties in Cu(In1–x,Gax)Se2 (CIGS)-based solar cells fabricated by the three-stage process was investigated by admittance spectroscopy. The Se beam pressure was varied from 1.3×10−3 to 4.4×10−3 Pa, at the position of the samples, by varying the Se cell temperature. The spectra for all samples show three distinct peaks denoted as α, β, and ζ with activation energies of 20, 150, and 300 meV, respectively. The trap density of ζ increased from 5×1014 to 4×1015 cm−3 with decrease in Se beam pressure; the trap densities of α and β did not change. These results suggest that the origin of ζ is related to the Se deficiency. The density of ζ seems to correlate with the cell efficiency. Therefore, it is important to control the Se beam pressure in order to obtain highly efficient solar cells.