Investigation of N-type Doping Effect on Electronic Structures and Interfacial Chemical Reactions in Organic Light-Emitting Devices

• Main Authors: Mei-Hsin Chen, 陳美杏
• Other Authors: Chih-I Wu
• Format: Others
• Language: en_US
• Published: 2009
• Online Access: http://ndltd.ncl.edu.tw/handle/81098934921237739385

博士 === 國立臺灣大學 === 光電工程學研究所 === 97 === In this dissertation, the mechanisms and effects of n-type dopants in organic light-emitting devices (OLEDs) are discussed in detail. The electronic structures and interfacial chemical reactions are studied via ultraviolet and x-ray photoelectron spectroscopy. First of all, the electron injection mechanisms of the effective cathode structures for organic light-emitting devices incorporating cesium carbonate (Cs2CO3) are investigated. The experimental results show that Cs2CO3 is not decomposed during the evaporation. Moreover, the enhanced electron injection is associated with strong n-doping effects after tris-(8-Hydroxyquinoline)-aluminum (Alq3) molecule doped with Cs2CO3. Furthermore, the properties of thermally evaporated Cs2CO3 and its role as electron-injection layers are also examined. Second, the effectiveness of Cs-derivatives (Cs2CO3, CsF, and CsNO3) as dopants of Alq3 are studied systematically. The n-type doping effect resulted from Cs2CO3 in Alq3 is the strongest. From the view of current efficiency, the performance of OLEDs with CsF in cathode structures is similar to that with Cs2CO3, which can function well with both aluminum and silver in cathodes. As for CsNO3, it is effective only with aluminum cathode due to the reaction between aluminum and CsNO3. Finally, 4,7-diphenyl-1, 10-phenanthroline (Bphen) is introduced as electron-transport layers according to the mobility in Bphen being higher than that in Alq3. Therefore, the electronic properties and chemical interactions of cathode structures using Bphen doped with rubidium carbonate (Rb2CO3) as electron injection layers are investigated. Current-voltage characteristics reveal that the devices with Bphen/Rb2CO3/Al as cathode structures possess better electron-injection efficiency than those with cathode structures of Bphen/LiF/Al. On the basis of experimental results, the n-type doping effects caused by Rb2CO3 and the gap states created by aluminum deposition are both keys to enhance carrier-injection efficiency. In parallel, theoretical calculation indicates that the chemical reaction between aluminum and the nitrogen atoms in Bphen is the origin of the gap states.