Alkali Element (Li, Na, K, and Rb) Doping of Cu2BaGe1-xSnxSe4 Films

Cu2BaGe1-xSnxSe4 (CBGTSe) represents an exemplary system within the I2-II-IV-X4 (I = Ag, Cu; II = Sr, Ba; IV = Ge, Sn; X = S, Se) family, which has been introduced to target suppressing the formation of anti-site defects and associated defect clusters within the analogous kesterite Cu2ZnSn(S,Se)4. Previous studies on CBGTSe films showed relatively low hole carrier densities (<10^13 cm-3), which may limit their corresponding application as active layers within photovoltaic, thermoelectric, and optoelectronic devices. In the current study, we explore the incorporation of alkali elements (Li, Na, K, and Rb) into CBGTSe films as prospective dopants to address the low hole carrier density and to allow for property tunability. First, incorporation of Na-, K-, and Rb-dopants noticeably increases the average grain sizes for CBGTSe films, while the Li-dopant has relatively limited impact. In addition, the alkali-dopants lead to a 1 to 3 orders of magnitude increase in hole carrier density (up to 10^15 cm-3 is achieved using K doping, corresponding to the alkali element yielding the highest doping efficiency). The alkali-doped films show slightly lower minority carrier lifetimes and carrier mobility values than the non-doped samples, and these values are found to follow an approximate universal dependence with carrier density (also considering data derived from other previously explored vacuum-deposited I2-II-IV-X4 chalcogenide films). As alkali-doping can significantly increase carrier densities, alkali elements can be considered useful p-type dopants for CBGTSe, as well as prospectively for other analogous I2-II-IV-X4 systems.