Impact of Minimal Silver Incorporation on Chalcopyrite Absorbers—Origins for Improved Open-Circuit Voltages in (Ag,Cu)(In,Ga)Se2 Solar Cells

The influence of minimal amounts of Ag (0.5–1.4 at%) on elemental distribution and crystalline quality of (Ag,Cu)(In,Ga)Se₂ (ACIGSe) absorbers grown by the three-stage coevaporation without added alkali elements is reported. The elemental ratios affect the amount of Ag to be uniformly incorporated into the chalcopyrite absorber and the open-circuit voltage (V_OC) of the ACIGSe solar cell devices. Ag-containing absorbers deposited at 530 °C achieve a best photoconversion efficiency of 18.2%. Due to an increased V_OC, ACIGSe absorbers perform better than their Ag-free variants at low deposition temperatures. The factors contributing to this increased V_OC of low-temperature devices are: 1) enhanced elemental Ga and In interdiffusion and hence their spatial distribution across the absorber thickness, leading to an increase in the minimum bandgap, 2) an improved absorber crystalline quality with larger grains resulting in high quasi-Fermi-level splitting and lower nonradiative losses. The photoluminescence data obtained on the ACIGSe absorbers reveal the corresponding variations in their bandgap and photoluminescence quantum yield. These material-level insights into Ag incorporation in chalcopyrite help to advance the development of chalcopyrite-based tandem solar cells, which—so far—is limited by the requirement of high deposition temperatures.