Numerical study of Cu(In,Ga)Se2 solar cell performance toward 23% conversion efficiency

The effects of conduction band grading in a Cu(In,Ga)Se2 (CIGS) thin film with an average bandgap of 1.4 eV on solar cell performance were investigated by changing the minimum bandgap (E gmin) and its position, employing the software wxAMPS. The calculation was carried out, taking CdS/CIGS heterointerface recombination into account, by incorporating a thin defective layer into the interface. For CIGS with a flat conduction band profile, i.e., without conduction band grading, the effects of the valence band offset (ΔE V) between a CdS layer and a CIGS layer with bandgaps from 1.05 to 1.6 eV were investigated. It was found that efficiency was increased by up to 3% by changing the conduction band profile from flat to double-graded, with a deep notch located in the vicinity of the CdS/CIGS interface. On the other hand, efficiency was increased by over 6% and reached 22% by increasing ΔE V up to 0.3 eV in the case of CIGS with a bandgap of 1.35 eV. Finally, an efficiency of 23.4% was achieved by combining a single-graded conduction band profile with a ΔE V of 0.3 eV. This result shows that a single-graded conduction band profile is required for high-efficiency wide-bandgap CIGS solar cells if the recombination at the CdS/CIGS heterointerface can be suppressed.