Realization of p-type ZnAgO:N thin films on flexible polyimide substrates through co-sputtering for wearable thermoelectric applications

In this study, we deposited a series of Ag and N co-doped ZnO thin films (ZnAgO:N) with different Ag atomic percentage (at. %) ratios on flexible polyimide (PI) substrates to realize p-type ZnO-based thin films for wearable thermoelectric applications by in situ co-sputtering of ZnO and Ag by RF and DC magnetron sputtering in a mixture of Ar and N2O gases. To evaluate the thermoelectric performance of these ZnAgO:N thin films, we measured the Seebeck coefficient S and electrical conductivity σ of the thin films with various Ag at. % ratios and calculated the power factor S2σ. These measurements confirmed that the co-doping of Ag and N into ZnO is an effective method for fabricating p-type ZnAgO:N thin films with σ in the range of about 105–3.3 × 106 S/m, as a function of Ag at. % ratio on the flexible PI substrates. In addition, the presence of an optimal Ag at. % ratio that leads to a maximum S2σ for the p-type ZnAgO:N thin films was observed. Raman spectroscopy and x-ray photoelectron spectroscopy revealed that the p-type conductivity in ZnAgO:N thin films originates from the acceptors AgZn and NO formed by the co-dopants Ag and N. As a result, the S and σ of the p-type ZnAgO:N thin films were investigated to be affected significantly by the acceptors and defects formed by the Ag and N co-dopants in the thin films. The influence of AgZn and NO on the appearance of the p-type conductivity in ZnAgO:N thin films and S and σ of the thin films was intensively studied.