Study Of The Electron Injection Property Of Alkali Metal Compounds In Organic Light-Emitting Diode

Since Organic Light-Emitting Diode (OLED) is current-driven device, the balance of electrons and holes in OLED is one of the key factors to obtain high efficient electroluminescent lighting. However, for most organic materials, the hole mobility is higher than the electron mobility. Besides, the hole injection barrier has been greatly reduced by the anode modification technology. Thus, to reduce the electron injection barrier at the organic/cathode interface by adding an electron injection layer (EIL), and to improve the electron transporting speed in the electron transport layer (ETL) by n-doping, have been the focus of attention and study for most researchers. So far, most of the materials have been used to enhance the electron injection and transport are inorganic alkali metal compounds, such as LiF, CsF, Li2CO3and CS2CO3. These materials can effectively reduce the energy barrier for electron injection, and act as good n-dopant. But there are also defects of them. For example, the high sublimation temperature will increase the difficulty and cost of the device fabrication, while easily getting damp characteristic will bring great impact on the stability of OLED. If there were some materials with low sublimation temperature could be used to inject electron efficiently, and to achieve a stable electroluminescent lighting, it would be a huge benefit for the OLED development. The aim of this thesis is to solve the problem, and the mainly contents are listed as follow:1. To study the application of cesium pivalate ((CH3)3CCOOCs) as electron injection material in OLED. The complex exhibits low sublimation temperature, high thermal stability, excellent film-forming property, effect electron injection ability, and widely matching range with electron transport materials. In the multilayer C-545T doped OLED, no matter Alq3or MADN was used as ETL, the device with (CH3)3CCOOCs as EIL can achieve high efficient electroluminescent lighting, with a maximum efficiency up to20cd/A, which is more higher than that with LiF. XPS measurement shows that there are electrons transfer from (CH3)3CCOOCs to Alq3, when (CH3)3CCOOCs and Al are deposited onto Alq3successively. However, there is no electrons transport from (CH3)3CCOOCs to MADN when (CH3)3CCOOCs is deposited on MADN, and only when Al is deposited, the electrons transfer occurs. UPS measurement indicates that the electron injection barrier can be effectively reduced after (CH3)3CCOOCs and Al are deposited successively onto Alq3and MADN films. Considering the excellent electron injection ability from (CH3)3CCOOCs to anthracene derivatives, a singer layer blue fluorescent OLED with PCPI as host and (CH3)3CCOOCs as EIL was fabircated, which exhibits a maximum luminance brightness of12940cd/m2and a highest efficiency of3.6cd/A.2. By fixing the (CH3)3CCOO-anionic group and changing the alkali metal ions gradually, we studied the application of (CH3)3CCOOLi,(CH3)3CCOONa,(CH3)3CCOOK,(CH3)3CCOORb and (CH3)3CCOOCs as electron injection material in OLED. Similar with (CH3)3CCOOCs, all these complexes exhibit low sublimation temperature and high thermal stability, In the multilayer C-545T doped OLED, when Alq3is used as ETL, the devices with these alkali metal pivalates as EIL can all achieve high luminous efficiency up to20cd/A. However, when MADN is used as ETL, the electron injection property and device efficiency are determined by the activities of the alkali metal, and only the devices with (CH3)3CCOORb and (CH3)3CCOOCs as EIL can achieve high efficient electroluminescent lighting.3. By fixing the Cs+ion and changing the acid group, we studied the application of (CH3)3CCOOCs,(CH3)2CHCOOCs, CH3CH2COOCs, CH3COOCs, CsF and Cs2CO3as electron injection material in OLED. All these organic complexes exhibit low sublimation temperature and high thermal stability. In the multilayer C-545T doped OLED, no matter Alq3or MADN was used as ETL, the devices with these organic alkali metal complexes as EIL show no significant difference on the electron injection property and device efficiency for the changing of the alkyl chain. However, for the devices with MADN as ETL and CsF or CS2CO3as EIL, the stability of the devices in the air is poor, and there would be dark points appeared in the emitting area soon, which leads to a much lower luminous efficiency. Although these organic alkali metal complexes can act as excellent electron injection materials, they aren’t efficient n-doping materials. The reason is that the interaction between them and Alq3or MADN is weak, which results in the electrons transferred to Alq3or MADN are too few to form n-doping effect.4. To study the application of cesium oxalate ((COOCs)2) as electron injection material in OLED. In the multilayer C-545T doped OLED,(COOCs)2shows excellent electron injection effect no matter Alq3or MADN was used as ETL. Moreover, it is a good n-dopant. Doping Alq3, MADN and TmPyPb with (COOCs)2can decrease the driving voltage significantly. In phosphorescence OLED, the (COOCs)2doping not only can drop the operating voltage, increase the luminous efficiency, but also weaken the efficiency roll off effect. XPS measurement shows that (COOCs)2exhibits strong electron donating ability, thus it can interact with Alq3and MADN, and transfer a mass of electrons to them to form n-doping effect. UPS measurement indicates that the electron injection barrier can be effectively reduced by (COOCs)2, leading to enhanced electron injection.