Development of Al-doped ZnO thin film as a transparent cathode and anode for application in transparent organic light-emitting diodes

Abstract

Aluminum doped zinc oxide thin films have been prepared by sputtering under argon gas pressure of 0.15 Pa at different radio frequency (RF) power densities to optimize the conditions for application in both bottom emitting and transparent OLEDs. The films exhibit a wurtzite-type hexagonal structure with the [0001] preferred orientation and optical transmittance of better than 80% in the visible region, but, varying energy bandgap. The sputtering at a high RF power density of 2.47 W cm⁻² yields Al–ZnO films of minimum resistivity (∼1 × 10⁻³ Ω cm) and high work function (∼4.44 eV), appropriate for the anode in bottom emitting OLEDs. On the other hand, AZO films obtained at the low RF power density of 0.31 W cm⁻² correspond to a low work function (3.90 eV) with slightly higher electrical resistivity (5.60 × 10⁻³ Ω cm), useful for cathodes in transparent OLEDs. The comparable performance observed for OLEDs fabricated with AZO and ITO anodes demonstrates the suitability of AZO as an alternative electrode. The increase in RF power density during sputtering leads to perceptible damage of the organic layer. The introduction of a buffer layer of Alq3/LiF/Al just above the organic layer is shown to suppress the damages significantly and improve the performance of the transparent OLEDs.