In this paper, we demonstrate device performances with cesium (Cs) doped in oxadiazole (OXD) derivatives as a metal-doped electron transport layer (MD-ETL). Cs is a heavy alkali atom and difficult to diffuse in an organic matrix. The metal quenching effect is therefore reduced in a long-term operation. Three different kinds of OXD-based organic materials were used. However, only one kind of OXDs can effectively improve the device performances. Such a host material exhibits advantages of high glass transition temperature (Tg) of 147 °C. The average roughness of the thin film is small hence the leakage current of the corresponding OLED devices is low. By using a highly reflective and conductive silver cathode, an OLED with a 2.59 V reduction in driving voltage, a 47.3% increase in current efficiency, and a 3.14 times enhancement in operation lifetime was demonstrated.
In this paper, a 1.85V reduction in driving voltage of an OLED is presented by using an alkali-metal doped electron transport layer (ETL) and a silver (Ag) cathode. Such an ETL is composed of 2,9-dimethyl-4,7-diphenyl 1,10- phenanthroline (BCP) doped with Cseium (Cs) to increase the conductivity of the organic material and achieve ohmic contact at the ETL/cathode interface regardless the work function of the cathode material. Ag is used to replace the conventional aluminum as the cathode material due to its lower electrical resistivity and optical absorption in visible range. In our devices, the drive voltage was reduced by 1.45V when using MD technique only. When we change the cathode material from Al to Ag, the driving voltage has a further of 0.4V reduction. In those devices, current efficiency is around 3.3 to 4.1 cd/A at 100 mA/cm2.That corresponds to an improvement of 53.36% in power efficiency at 10 mA/cm2.
In this paper, we demonstrated an organic light emitting device (OLED) of low driving voltage, high current efficiency and long lifetime by using an n-type organic material as the electron transport layer (ETL). Such a layer is composed of a large alkali metal, i.e. Cesium (Cs), doped into the organic material. Cs atom is heavy and hard to diffuse into the emitting layer (EML) material that decreases the metal quenching and increase the operation lifetime. 2,9-dimethyl-4,7-diphenyl 1,10- phenanthrolin (BCP) and 2,5-diaryl-1,3,4-oxadiazoles (OXD) were used as the host organic material. OXD is thermally stable and exhibits a high glass transition temperature (Tg) of 147°C that can further increase the operation lifetime. The resistivity of those doped materials is about 3x105 Ω-cm that is two orders of magnitude lower than the resistivity of the pure organic materials. Compared with the conventional LiF/Al device, about two-volt reduction in driving voltage was observed. Current efficiency is also increased due to better carrier balance. Operation lifetimes of metal dopant devices are longer than that of the conventional device especially by using OXD as the host of the ETL.