Transparent and conducting Zn1−xGaxO thin films with different nominal Ga content (x = 0–4%) on flexible polyimide (PI) substrates were successfully synthesized by conventional sol–gel spin coating method. All the films retained a hexagonal wurtzite structure of ZnO and showed preferential c-axis orientation after air and vacuum annealing and good adhesion to flexible PI substrates. Generally, the particles in the films were small and the connectivity between the particles was good, irrespective of doping concentration and treatment ambient. The average transmittances reached 70% for all the films above a cut-off wavelength of ∼500 nm originated from PI substrates. The average sheet resistances decreased suddenly after Ga doping, while increased slightly from x = 3–4%. According to Hall measurement, the broad minimum resistivity of 1–3% Ga doping concentration might be due to the competition of dopant atoms at lattice sites and the segregated ones at the grain boundaries, which could be demonstrated by decreased Hall mobility and increased carrier concentration with increasing Ga doping content.

•Flexible Al-doped ZnO films were fabricated by conventional sol–gel method.•All the films showed preferential c-axis orientation on flexible PI substrates.•Transmittances reached 70% above a cut-off wavelength of ∼500 nm.•Electrical conductivity could be enhanced by further vacuum post-annealing.1% Al-doped ZnO transparent and conducting thin films on flexible polyimide (PI) substrates were successfully synthesized by the conventional sol–gel spin coating method. All the films with different number of layers (9, 10 and 11) retained a hexagonal wurtzite structure of ZnO and showed preferential c-axis orientation. Good adhesion was evidenced by dense microstructure and homogeneously distributed grains of the films on flexible PI substrates. The average transmittances reached 70% for all the films above a cut-off wavelength of ∼500 nm originated from PI substrates. Comparing to the resistance of films annealed in air, the electrical conductivity could be enhanced by further vacuum (P∼10−2 Pa) post-annealing, and lowest resistivity of 2.52×10−1 Ωcm was obtained in the film with 10 layers.

•(Al,Mg) codoped ZnO powders prepared by sol–gel method.•Mg worked effectively on ZnO band gap engineering.•Al doping efficiency was decreased by higher Mg codoping.•Substitution of Mg ions at Zn sites enhanced the photocatalytic activity of ZnO.Structural and optical properties of 1 at % Al-doped Zn1−xMgxO (x=0–8%) powders prepared by sol–gel method were systematically investigated by means of X-ray diffraction, scanning electron microscopy, ultraviolet–visible absorbance measurement, photoluminescence and Raman scattering spectra. All the powders retained the hexagonal wurtzite structure of ZnO. The band gap and near band emission energies determined from absorbance and photoluminescence spectra increased linearly with increasing Mg content, respectively, which implied that the Mg worked effectively on ZnO band gap engineering, irrespective of Al codoping. However, according to the PL and Raman scattering studies, for the sample of x=8%, the Al doping efficiency was decreased by higher Mg codoping. On the other hand, the effect of Mg codoping on photocatalytic degradation of methylene orange was explored experimentally. The substitution of Mg ions at Zn sites shifted the conduction band toward higher energies and then enhanced the photocatalytic activity, while the incorporation of interstitial Mg ions and decreased Al doping efficiency for higher Mg doping sample (x=8%) reduced the photocatalytic activity.

1 at.% Al-doped Zn1−xCdxO (x = 0–8 at.%) thin films were prepared on glass substrates by sol–gel method. The codoping films retained the hexagonal wurtzite structure of ZnO, and showed preferential c-axis orientation. The effect of annealing ambient (in vacuum and nitrogen) on the optical and electrical properties of (Cd,Al)-codoped ZnO films were investigated using transmission spectra and electrical measurements. The transmittances of the codoping films were obviously degraded by vacuum annealing to 50–60 %, but enhanced to 70–80 % after nitrogen annealing. The carrier concentration and Hall mobility both increased, and resistivity decreased with narrowing band gap of Al-doped Zn1−xCdxO, below different critical concentrations x = 4 % (in vacuum) and x = 6 % (in nitrogen). It is revealed that the conductivity is also improved by Cd doping along with band gap modification. The variations in optical and electrical properties are ascribed to both the changes of the crystallinity and concentration of oxygen vacancies under different ambient. In view of transmittance and conductivity, nitrogen annealing might be a more effective post-annealing way than vacuum annealing for our (Cd,Al)-codoped ZnO films to meet the requirements of transparent conducting oxide (TCO).

Al-doped Zn1−xMgxO and Zn1−yCdyO thin films were prepared on glass substrates by sol–gel method. The codoping thin films showed preferential c-axis orientation, and the lattice constant c evaluated from the shift of the position of (002) peak displayed an increasing evolution from x = 8 at.% to y = 8 at.%, indicating a roughly statistical substitution of Mg2+ and Cd2+ for Zn2+ in their solid solution. The effects of narrowing and widening band gap (Eg) on conductivity of (Cd, Al) and (Mg, Al) codoped ZnO thin films were simultaneously investigated using transmission spectra and electrical measurements. The transmittances of these films are obviously decreased by vacuum annealing to 50–60%. However, the carrier concentration and Hall mobility both increase, and resistivity decreases with narrowing band gap in 1 at.% Al-doped Zn1−xMgxO and Zn1−yCdyO thin films from x = 8 at.% to y = 8 at.%. It is revealed that the conductivity of Al-doped ZnO thin films could be enhanced by this simple band gap modification.