Yb-doped zinc tin oxide thin film and its application to Cu(InGa)Se2 solar cells

Abstract : 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.