Screening of alkali elements in Cu2ZnSn(S,Se)4

Group-I dopants are known to benefit Cu(In,Ga)Se₂. Although the exact mechanism is still debated, Na has been shown to improve a variety of device parameters. Due to the similarities between chalcopyrites and kesterites, it is believed that group-I dopants may have a similar effect on kesterites. However, due to the different defect chemistries of these two materials, the optimum concentration and the way in which the dopants interact with native point defects will not be the same. We will present the results of a high-throughput experiment where we have systematically investigated the effect of Li, Na, K, and Rb from 0 to 2 at% in CZTSSe films. We use a full spectrum photoluminescence (PL) technique to extract the quasi-Fermi level splitting, the magnitude of electrostatic potential fluctuations, and the bandgap as a function of dopant concentration. From this we are able to hypothesize about the changes in the quantity of charged point defects and determine the optimum concentration (if any) of the group-I elements. We find that in specific quantities certain group-I elements can improve the quasi-Fermi level splitting. Devices made on Mo/SLG substrates with select group-I dopants have higher power conversion efficiencies than undoped samples. Consistent with the PL data, the doped samples have higher Voc on average but the efficiency gains primarily stem from improved fill factor.