Band Gap Grading of Stacked Cu(In,Ga)S₂ Thin Films

The band gap energy of CIGS, which depends on the composition variation and strain effect, can influence the collection and recombination of photocarriers. The solar cell efficiency is improved by the graded band gap in the absorber layer due to the enhanced carrier collection and the reduced carrier recombination. In our previous study, the photovoltaic performance of solar cells was affected by the stacking combination of layers, where the solar cell with dense-bottom and porous-top layers showed better performance than that with a reversely stacked structure. We studied the stacking effect of CIGS thin films. The stacking did not change E _g of each layer, which led to the double grading of E _g along the depth of the stacked films, mainly due to the difference in E _g between the dense and porous layers. The higher degree of the grading in A+B+A improved J _sc. However, the higher density of the defect states in A+B+A reduced V _oc, which was inferred by the short lifetime of the carriers and the broad bandwidth of photoluminescence. Overall, the efficiency of A+B+A was only slightly improved compared to that of B+A.