Studies Of Device Physics Of Organic Light-emitting Diodes By Admittance Spectroscopy

A high efficiency double layer organic light-emitting diode (OLED) was inventedby C. W. Tang in1987. Due to its advantages of high brightness, full color, and lowpower consumption, OLED is considered as an important candidate for next-generationdisplay and lighting technology. In the recent years, researchers have been focusing onnew organic materials, new device structures, and deeper understanding on devicephyiscs in order to achieve high efficiency and long lifetime from OLEDs. Admittancespectroscopy (AS) has been widely served as a simple, nondestructive, and practicalmethod to investigate inorganic semiconductor. In recent years, AS has alsopreliminarily been used for the studies of OLED device physics. In this work, ASmethod was further used to investigate the injection barrier of p/n type dopedphosphorescence devices, the carrier mobility of wide band-gap materials, and the effectof different bilayer cathode on device degradation.The p/n type doping method was proved to be an efficiency way to form a lowmetal/organic injection barrier at the interface. A MoOx-doped CBP(4,4’-bis(carbazol-9-yl)biphenyl) hole transport layer and CsCO3-doped Bphen(4,7-diphyenyl-1,10-phenanthroline) electron transport layer can reduce themetal/organic interface barrier to0.18eV and0.45eV, respectively. This can be derivedfrom the AS test.A p type doped thin buffer layer inserted between ITO and a layer of testedmaterial in a single carrier device can form an ideal ohmic contact, which makes it ispossible to measure the carrier mobility of wide band-gap materials with the AS method.For example, when a MoOx-doped CBP thin buffer layer is added between ITO andCBP layer, can the hole mobility of CBP be obtained by the AS test.Furthermore, we systematically investigated the change of interface conditions of the devices having different bilayer cathode in the degradation processes and found thatAS method is sensitive to probe the degradation causes. Two groups of bilayer cathodes,LiF/Al vs. Liq/Al and LiF/Al vs. LiF/CsF have been investigated.Therefore, AS method is a useful to the studies of device phyiscs of OLEDs. Withthis method, we investigated a few important interfaces in the devices. Our results canbe informative to device structure design and to material selection in OLEDs.