Optical and electrical properties of laser ablated LiNixCo1-xO2 thin films

Thin films of LiNixCo1-xO2 were prepared by pulsed laser deposition technique. Two important deposition parameters such as substrate temperature and oxygen partial pressure during the thin film deposition were controlled. The optical absorbance of LiNixCo1-xO2 thin films have been studied in the vicinity of the fundamental absorption edge. A gradual increase of the optical transmission from 400 to 1000 nm with the appearance of a prominent dip at 750 nm is observed. This feature is more pronounced for the well-crystallized film grown at 700 degrees C. The variation of the optical absorption near the fundamental edge allows determining the energy gap between d-bands of LiNixCo1-xO2. Band gap energies of 2.39 and 2.15 eV for films grown at 500 and 700 degrees C, respectively, with accuracy +/- 0.02 eV were measured. The electrical conductivity of LiCoO2 varies from 10 (-9) to 10 (-4) S/cm in the temperature range from 180 to 500 K. Doping the oxide network with nickel oxide leads to an increase of the electrical conductivity of one order of magnitude from 6 x 10 (-7) S/cm for LiCoO2 to 9 x 10(-6) S/cm for LiNixCo1-xO2 at room temperature. The increase of electrical conductivity could be connected with either an enhancement in the effective carrier concentration or a decrease of the activation energy with substitution of Ni3+ ions for Co3+ ions. The activation energy decreases from 0.36 eV to 0.28 eV on nickel doping. These results suggest that the open and porous structured LiNixCo1-xO2 films find potential applications as binder free electrode in the fabrication of all solid state microbatteries.