Structural, optical, and electrical properties of p-type NiO films and composite TiO2/NiO electrodes for solid-state dye-sensitized solar cells

p-Type nickel oxide thin films were prepared by sol–gel method, and their structural, optical and electrical properties were investigated. The Ni(OH)2 sol was formed from nickel (II) acetate tetrahydrate, Ni(CH3COO)2·4H2O, in a mixture of alcohol solution and poly(ethylene glycol), and deposited on an ITO substrate by spin coating followed by different heat treatments in air (50–800 °C). The formation and composition of NiO thin film was justified by EDX analysis. It is found that the thickness of the NiO film calcined at 450 °C for 1 h is about 120 nm with average particle size of 22 nm, and high UV transparency (∼75%) in the visible region is also observed. However, the transmittance is negligible for thin films calcined at 800 °C and below 200 °C due to larger particle size and the amorphous characteristics, respectively. Moreover, the composite electrode comprising n-type TiO2 and p-type NiO is fabricated. The current–voltage (I–V) characteristics of the composite TiO2/NiO electrode demonstrate significant p-type behavior by the shape of the rectifying curve in dark. The effect of calcination temperature on the rectification behavior is also discussed.