Effects of Ultra-Thin Al₂O₃-Doped ZnO Film as Anode Buffer Layer Grown by Thermal Evaporation for Organic Light-Emitting Diodes

In this study, the efficiency of organic light-emitting diodes (OLEDs) was enhanced by depositing an Al2O3-doped ZnO (AZO) film as a buffer layer by thermal evaporation. The enhancement mechanism was systematically investigated, the X-ray photoelectron spectroscopy and ultraviolet photoelectron spectroscopy results of which revealed the formation of the UV-ozone-treated AZO film. The work function increased from 4.8 eV (standard ITO electrode) to 5.24 eV (AZO with 1 wt% for 1 nm), while the surface roughness was smoother than that of the ITO electrode. UV-ozone-treated AZO film increased the surface energy and polarity of the ITO surface. The transmittance increased in the 325–375 nm when the UV-ozone treated AZO was deposited on the ITO substrate. Further, results from admittance spectroscopy showed that the inserted AZO film increased the capacitance and conductance. It was also found that the carrier injection increased in the space-charge region. Moreover, the turn-on voltage of the devices decreased from 4 V to 3.2 V at 0.5 mA/cm2, the luminance increased from 7588 cd/m2 to 31880 cd/m2, and the current efficiency increased from 3.2 cd/A to 4.7 cd/A when a 1 nm-thick UV-ozone-treated AZO film with 1 wt% was inserted as a buffer layer in the OLEDs.