Transparent conductive indium tin oxide (ITO) films with thin Ni-doped surface layers were prepared for organic light emitting diode (OLED) application. The top Ni-doped ITO surface layer were synthesized using Ni (RF) and ITO (DC) co-sputtering method at 120°C and annealed at 300°C for 10 minutes in vacuum to form a modulated work function layer in contact with the subsequently deposited light emitting organic layers. OLED devices with an Al/Alq3/NPB/Ni-doped ITO/ITO/glass structure were fabricated to investigate the effect of the Ni-doped ITO layer on the characteristics of the luminescence efficiency. The depositions of the Al/Alq3/NPB stacked films on top of the Ni-doped ITO/ITO/glass sample were conducted using thermal evaporation in a cluster tool without breaking the vacuum. Initial results show that the device turn-on voltage decreases from 10 volts to 6 volts and the luminescence efficiency was improved by 36% due to the existence of the Ni-doped ITO layer. It was also found that the optical transmittance of the ITO film decreased with the Ni concentration, resulting in external quantum efficiency deterioration by 3%. It was suspected that the presence of Ni (Φ~5.2eV compared to that of ITO ~4.2eV) on ITO surface decreases the heterojunction barrier height at the ITO/NPB interface, allowing more effective transportation of hole-carriers and hence an enhancement on the external quantum efficiency. However the optical impurity scattering of the Ni atoms in the ITO matrix caused the deterioration of the optical transparency and negative effect on the external quantum efficiency.
The effect of both sputtering temperature and post-annealing on the resistivity and the optical transparency of ITO films was investigated. Transparent conductive ITO films were deposited onto glass substrates using dc magnetron sputtering process with Ar pressure of 5 × 10-3torr, power density of 5.5W/cm2 and temperature ranging from 25°C to 240°C, and the ITO films were then annealed at 250°C in vacuum for 5 minutes. It was found that the sheet resistivity of the as-deposited ITO films decreased with temperature and was dominated by carrier mobility for temperature below 120°C and by the carrier concentration for temperature above 120°C. However after annealing, the lowest sheet resistivity of 1.6 × 10-4 Ω-cm/ and the largest extent of sheet resistivity drop, presented in [ρs(as-grown) - ρs(annealed)]/ρs(as-grown) of 78 percent for the ITO film fabricated at 90°C were observed. The optical transmittance of as-deposited ITO films in visible light region was 76 percent and rapidly increased with temperature to up to T percent equals 88 percent at 120°C, thereafter the optical transmittance varied little with the maximum value of 90.2 percent at 200°C. However after annealing, the transmittance decreased for ITO films deposited at temperature above 120°C and increased for those above 120°C with the maximum transmittance of 89.8 percent at 90°C. This was believed due to the large reduction of defect density in ITO films for low temperature processing and to the appearance of the strong preferred orientation of (222) for high temperature deposited ITO films. The figure of merit, both Ftc (T%/ρs) and Φtc (T%10/ρs, was found to perform the best value of 14.8×10-3 Ω-1 and 5.5×10-3 Ω-1 respectively at substrate temperature of 90°C after annealing. The results suggested the optimized sputter temperature for preparing ITO films could be conducted in the moderate temperature range of 90°C rather than in high temperature if the post-annealing was introduced.