Designing Graded Band Gap Active Layer Cu<inf>2</inf>HgSn(S<inf>1-x</inf>Se<inf>x</inf>)<inf>4</inf> over Complex Tandem Structure for Efficient Photovoltaic Cells with Efficiency &gt;20%

Stacked absorber layer-based tandem solar cells have shown potential for overcoming the Shockley-Queisser limit for a single junction solar cell. However, achieving current matching conditions in these devices poses design complexity. In contrast, graded band gap absorber-based solar cell devices may be the suitable alternative over tandem cells for better efficiency, providing enhanced carrier generation within the thin absorber layer. Here, we investigate the Cu2HgSn(S/Se)4-based tandem and graded band gap junction devices and highlighted that the bottom sub-cell in tandem cell configuration saturates and does not allow us to achieve efficiency above 17%. On the other hand, with a graded band gap absorber, we observed the importance of large exponential decay-based positive grading with a characteristic length of ∼200 nm. The sharply increasing Se concentration on the back contact enables the device to perform more efficiently with a total absorber thickness of 2 μm. This positive band gap grading structure is explored for different defect concentrations in the bulk and at the interface. The device can perform efficiently until 1015 cm-3 bulk defect density; however, it is highly sensitive to the interface defects, which downgrade the performance even after 1012 cm-2 interface defect concentration. A positive conduction band offset of ∼0.3 eV is favorable for large efficiency, which inherently provides a large built-in field at the junction, assisting the device to extract photocarrier effectively with efficiency >20%.