Optimum bandgap profile analysis of Cu(In,Ga)Se₂ solar cells with various defect densities by SCAPS

The bandgap of a Cu(In,Ga)Se₂ (CIGS) absorbing layer is varied from 1.0 to 1.7 eV by changing the composition ratio of gallium (Ga), realizing an optimum design for solar cell absorbers. In this study, the effects of a graded bandgap profile on the cell performance of a CIGS solar cell are investigated using a device simulator. Moreover, optimum bandgap profiles with various defect densities are simulated. In the case of low defect densities, when the lowest bandgap, Eg_min, is inside the space-charge region (SCR), the double-graded structure is effective for achieving high efficiency. However, when Eg_min is outside the SCR, the negative gradient from Eg_min to the CIGS surface acts as a barrier that impedes the collection of photogenerated electrons, thereby increasing the recombination rate and decreasing cell efficiency. In the case of high defect densities, to decrease the recombination current and improve the efficiency, a more positive gradient from the back contact to the surface is needed.