Nickel–lithium oxide alloy transparent conducting films deposited by spray pyrolysis technique

In this research, nickel oxide (NiO) transparent semiconducting films are prepared by spray pyrolysis technique on glass substrates. The effect of Ni concentration in initial solution and substrate temperature on the structural, electrical, thermoelectrical, optical and photoconductivity properties of NiO thin films are studied. The results of investigations show that optimum Ni concentration and suitable substrate temperature for preparation of basic undoped NiO thin films with p-type conductivity and high optical transparency is 0.1 M and 450 °C, respectively. Then, by using these optimized deposition parameters, nickel–lithium oxide ((Li:Ni)Ox) alloy films are prepared. The XRD structural analysis indicate the formation of the cubic structure of NiO and (Li:Ni)Ox alloy films. Also, in high Li doping levels, Ni2O3 and NiCl2 phases are observed. The electrical measurements show that the resistance of the films decreases with increasing Li level up to 50 at%. For these films, the optical band gap and carrier concentration are obtained to be 3.6 eV and 1015–1018 cm−3, respectively.

Research highlightsIn this paper, we report the preparation and characterization of NiO and NiO:Li (or (Li:Ni)Ox alloy) thin films. These films have been deposited on glass substrates by spray pyrolysis technique. Physical properties of prepared NiO films with different nickel concentrations in precursor solution, different substrate temperatures, and various Li doping levels have been investigated. It is observed that the physical properties of films strongly depend on the deposition conditions and Li-doping levels. The obtained results lead to the following conclusions: ▶ The prepared films exhibit a preferential growth along the (1 1 1) direction with a cubic NiO phase. With increasing the Ni concentration in precursor solution and substrate temperature, the (1 1 1) peak intensity is enhanced and crystallite size increases. Also, at 60 at%, 80 at% and 100 at% doping levels, other phases such as Ni2O3 and NiCl2 are observed. ▶ Electrical measurements of the samples show that the sheet resistance of the films decreases with increasing Ni concentration up to 0.1 M and substrate temperature up to 450 °C (in NiO films). Also, the lowest resistance was obtained to be 4.7 (MΩ/□) for Li doping level of 50 at%. The Hall effect and thermoelectrical measurements have shown p-type conductivity in all films. The highest Seebeck coefficient was 503 μV/K at 400 K for 50 at% Li doping. ▶ The transparency of the films decreases with increasing Ni concentration and increases with increasing the substrate temperature. In case of (Li:Ni)Ox alloy films, transparency decreases from ∼80% to ∼50% when the Li doping level increases from 0 at% to 100 at%.