Influence of sputtering a ZnMgO window layer on the interface and bulk properties of Cu(In,Ga)Se₂ solar cells

The authors studied the influence of sputtering a ZnMgO window layer for Cu(In,Ga)Se₂ solar cells on bulk and interface electrical properties. Admittance spectroscopy reveals deep levels at the ZnMgO/CdS interface whose activation energy (∼0.4 eV) increases with reverse bias, indicating an unpinned quasi-Fermi level at the interface. The Cu(In,Ga)Se₂ carrier concentration determined by capacitance-voltage measurements decreases to 3×10¹⁴ cm^-3, compared to 1×10¹⁶ cm^-3 in a device with a ZnO window. Scanning Kelvin probe force microscopy verifies the increased depletion region width and indicates that the junction location is unaltered by ZnMgO. Secondary-ion mass spectroscopy shows the presence of Mg near the top and bottom surfaces of the Cu(In,Ga)Se₂ film. They hypothesize that the decrease in carrier concentration is due to compensation doping of the Cu-poor Cu(In,Ga)Se₂ by Mg. Optimizing sputtering conditions to reduce surface damage and Mg migration eliminates the interface states and restores the carrier concentration, resulting in device performance comparable to those with a ZnO window.