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The use of rare earth elements with semiconductor materials has attracted immense interest due to their unique properties. In this study, we investigated the characteristics of an ytterbium (Yb)-doped zinc tin oxide (Yb:ZTO) thin film and its application as a potential down-converter of Cu(InGa)Se 2 (CIGS) thin-film solar cells. Yb:ZTO thin films were deposited by reactive sputtering of Zn and Sn metal with oxygen flow. A few pieces of Yb were embedded in a Zn metal target; thus Yb elements were supplied during Zn sputtering. The relative composition of Zn and Sn was controlled by changing the sputtering power (10–70 W) of Sn, in relation to the fixed sputtering power for Zn (70 W). In addition, the substrate temperature was varied from room temperature to 400 °C. It was confirmed that a smaller amount of Sn with lower sputtering power led to more incorporation of Yb into ZTO. X-ray photoelectron spectroscopy analysis confirmed the incorporation of Yb into ZTO, and photoluminescence measurement demonstrated Yb emission. Grazing incidence X-ray diffraction indicated the shift of ZTO emission peaks induced by the difference in the composition of Zn and Sn. Finally, CIGS solar cells with an Yb:ZTO layer were fabricated. The results suggested that cells with the highest Yb photoluminescence emission showed the highest short-circuit current density and cell efficiency. Yb:ZTO films were prepared by reactive co-sputtering of Yb:Zn and Sn targets, and their crystallographic and optoelectronic properties were thoroughly investigated by changing Sn sputtering power (10–70 W) to determine ZTO composition, and substrate temperature (100–400 °C). The potential photon converting effect of Yb:ZTO film was supported by the observation of increased quantum efficiency without an increase of light absorption. Image 1 • Yb:ZTO films were prepared by reactive co-sputtering of Yb:Zn and Sn targets. • PL analysis confirmed the energy transfer from host materials to Yb ions. • Photon converting effect was supported by the increased quantum efficiency without an increase of light absorption. • Highest Yb PL emission showed the highest short-circuit current density and cell efficiency. [ABSTRACT FROM AUTHOR]