Research On Electron Injection Materials For Organic Light-emitting Diodes And The Related Mechanisms

Organic light-emitting diodes (OLEDs) have many attractive advantages as flat panel displays and flat lighting devices. They are current injection devices and the improvement of electron injection is essential for efficient and stable OLEDs. In this dissertation, we have investigated the electron injection mechanisms of two kinds of cathode modification material. Novel cathode structure has been developed to fabricate high efficiency OLEDs. The main contents of the dissertation are as follows:1. Thermodynamic calculation was applied to study the possible decomposing reactions of thermal evaporated cesium carbonate (Cs2CO3), indicating that it is thermodynamically reasonable for Cs2CO3 to decompose into metallic Cs. Thermal decomposing experiment of Cs2CO3 was carried out and the decomposing temperature of Cs2CO3 in the experiment was very close to the calculated temperature at which Cs2CO3 decomposes into metallic Cs. The X-ray photoelectron spectroscopy (XPS) analysis of the thermally deposited Cs2CO3 film and the study of the I-V characteristics of the electron-only devices of Cs2CO3both confirm the release of metallic Cs.2. A novel composite cathode, Cs2CO3:Ag/Ag, was developed for efficient OLEDs. The maximum current efficiency of 4.4 cd/A was achieved with the device structure of ITO/NPB (50 nm) /Alq3 (50 nm) /Cs2CO3:Ag (1:10, 5 nm) /Ag (100 nm), higher than the control devices with the cathode of Cs2CO3 (0.5 nm) /Ag and LiF (0.5 nm) /Al, respectively. The effects of the EIL composition and thickness upon the current density and luminance have been investigated. The XPS analysis was applied to demonstrate that CsAg alloy was formed and responsible for the improved device performance. Transparent OLEDs (TOLEDs) based on the Cs2CO3:Ag/Ag composite cathode were fabricated. The devices are highly transparent, and show total efficiencies comparable with conventional bottom-emitting devices. The electroluminescence (EL) spectrum from the cathode side of the device was much closer to the photoluminescence (PL) spectrum of the light-emitting material than that from the anode side.3. Thermodynamic calculation was applied to study the possible decomposing reactions of thermal evaporated potassium borohydride (KBH4), indicating that it is thermodynamically reasonable for KBH4 to decompose into metallic K. The XPS analysis of the thermally deposited KBH4 film also confirmed the release of low-work-function metallic K.4. KBH4 was applied for the first time as the electron injection material (EIM) into OLEDs. The optimized OLEDs showed a high efficiency of 11.0 cd/A with the device structure of ITO/F4-TCNQ (4.5 wt%) : m-MTDATA (100 nm) /NPB (20 nm) /C545T (1.5 wt%) : Alq3 (30 nm) /Alq3 (30 nm) /KBH4 (5.0 nm) /Al (100 nm), better than the device with LiF (0.5 nm) /Al cathode. Within a wide range of KBH4 thickness (3.0 to 7.0 nm), the devices show high performance. Moreover, with KBH4 as the EIL, the devices using different cathode metals (Ag and Al) both showed high efficiency, indicating that the electron injection behavior of KBH4 is independent of the cathode metal.