Improved hole-injection and external quantum efficiency of organic light-emitting diodes using an ultra-thin K-doped NiO buffer layer

In this paper, we used the thermal evaporation method to deposit K-doped NiO (KNO) films as the inorganic buffer layer in thermally activated delayed fluorescence (TADF)-based organic light emitting diode (OLED) devices, and investigated how it improved device performance. The device configuration was ITO/NiO:K₂CO₃ (X mol%, 1 nm)/NPB (40 nm)/CBP:4CzIPN (5 wt%, 20 nm)/TPBi (50 nm)/LiF (1 nm)/Al (150 nm). We used various concentrations of alkali metal-doped metal oxide on the ITO, which was then subjected to UV-O₃ surface treatment. We studied the effects and mechanisms of the improved hole-injection properties of the OLEDs with different measurement methods. X-ray Photoelectron Spectroscopy (XPS) was used to measure the molecular binding energy and confirm formation of the K-doped NiO films. In comparison with the ITO electrode's work function, the KNO film increased the work function from 4.8 to 5.3 eV. Contact angle measurement showed that the deposition of the buffer layer increased both polarity and surface energy. The K-doped NiO film was found to have a smoother surface than that of the ITO electrode, as determined by Atomic Force Microscopy. Reduction in the sheet resistance of the OLEDs was revealed by Admittance Spectroscopy measurement. The opto-electrical performances of the devices showed an increase in luminance from 18420 cd/m² to 36740 cd/m², a decrease in turn-on voltage from 4.3 V to 3.9 V at 100 cd/m², and an increase in external quantum efficiency (EQE) from 7.1% to 9.76% when a 1-nm-thick KNO film with a doping concentration of 1 mol% was inserted into the OLEDs.