Li-doped ZnO Sol-Gel Thin Films: Correlation between Structural Morphological and Optical Properties

Transparent Conducting Oxides (TCOs) have important device applications used in solar cells as transparent electrodes. In particular, ZnO material has been investigated for many years due to its excellent physical properties and potential applications in transparent conductive contacts when it is doped by several types of doping. If n-type doping is easily achievable in ZnO, the realization of stable and reproducible p-type ZnO is still difficult. The group I (Li, Na) atoms could be potential candidates to result in p-type ZnO. The choice of lithium as a dopant is motivated firstly by its abundance in nature, its low cost compared to other alkali metals and its wide application. Undoped and Lithium-doped zinc oxide thin films (ZnO:Li) at different percentages (5, 15 and 25%) were prepared on glass substrates, using the sol–gel spin-coating method, and the influence of the Li concentration on the structural, morphological and optical properties of the ZnO thin films was investigated. X-ray diffraction (XRD) analysis shows that the un-doped and 5 at % Li-doped films have a hexagonal wurtzite structure and are preferentially oriented along the c-axis from the substrate. SEM analysis shows a compact surface with mainly hexagonal grains. Optical transmittance measurements show that all samples have average 88% transparency in the visible light, with a sharp fall of the absorption at a wavelength (~376 nm) close to the ZnO band gap. The results suggest that the optical properties of the ZnO:Li films can be further improved for solar cell applications.