Lithium-based compounds as an n-type dopant on Alq3-based organic light-emitting devices

• Main Author: 林書逸
• Other Authors: 高柏青
• Format: Others
• Language: zh-TW
• Published: 2011
• Online Access: http://ndltd.ncl.edu.tw/handle/73350127501763687699

碩士 === 國立嘉義大學 === 電子物理學系研究所 === 99 === inverted bottom-emission types (CBOLED and IBOLED). In the NBOLED, LiOH as well as other lithium compounds (Li2CO3 and LiF) are used as a buffer layer inserted between tris-(8-hydroxyquinolin) (Alq3) electron-transport layer and Al cathode, or as an n-type dopant doped with Alq3. In the IBOLED, lithium compound-doped Alq3 are co-evaporated on the indium tin oxide (ITO) substrate. The current density-voltage-luminance (J-V-L) results show that when a proper buffer layer thickness or doping concentration of LiOH was adopted in OLEDs, device properties such as the turn-on voltage, the maximum luminance, and the device efficiency were improved, becoming better than LiF and Li2CO3. In addition, LiOH-doped IBOLED is more stable than the CBOLED and IBOLED with other lithium compounds at the 50% decay lifetime. Second, different analysis tools (or methods) are used to further investigate the mechanism of LiOH enhancement in charge injection and transport in OLEDs. The higher electron mobility of the LiOH-doped electron-only samples has been derived using steady-state space-charge-limited current (SCLC) and admittance spectroscopy (AS) measurements, respectively. The surface morphology of the Alq3 film became smoother after the LiOH layer was deposited by using atomic force microscopy (AFM) analysis. In addition, LiOH doped Alq3 layer has good morphological stability, which is likely to reduce nonemissive dark spot growth. The reaction mechanisms between LiOH and Alq3 were also investigated. X-ray and ultraviolet photoelectron spectroscopy (XPS and UPS) results show that some electrons transfer from LiOH into Alq3, which increases the electron concentration in Alq3 films and moves the Fermi level close to the lowest unoccupied molecular orbital (LUMO) of Alq3. The charge transfer was further confirmed by absorption (UV-Vis) and photoluminescence (PL) spectra. Thus, the electron injection efficiency was enhanced due to a lower electron injection barrier, which improves the charge carrier balance in OLEDs and leads to better device efficiency.