Research On Cathodes For Organic Light-emitting Diodes And The Related Mechanism

The enhancement of electron injection is crucial for efficient organiclight-emitting diodes（OLEDs）. In this dissertation, we have designed three types ofefficient cathodes for OLEDs. Meanwhile, the related mechanisms were studied. Themain contents of the dissertation are as follows:1. Thermally co-evaporated Li₃N and Al film has been employed as a highly efficientcathode for OLEDs with tris（8-hydroxyquinolato） aluminutthraronusgpho t sclaanyneirn（Eg TeLle）c.t roAn mαi cprhoascseo poyf （LSiE: MA）l, aXll-orya yo np htohteo esluebcsmttrroa（tAne lqshpa3）es c atbsre oetsnhc eoc poeynle（fXicrtmroPSend）and X-ray diffraction （XRD） measurements. The device with optimized Li: Al alloycathode showed the highest current efficiency of4.78cd/A, which is about17%higher thanLiF/Al device. The photovoltaic measurement indicates that this Li: Al alloy cathode has alower electron injection barrier leading to a electron injection enhancement.2. Three phases of Li: Mg alloys were formed by co-evaporating Li₃N and Mg withdifferent doping ratios, and were confirmed by XRD measurements. The XPS result revealedthe charge transfer between Li and Mg. An optimized green OLED with Li: Mg alloy as thecathode shows a current efficiency of3.58cd/A, about15%higher than the control deviceusing Mg: Ag cathode. Quantum calculation was performed to get the surface work functionsof different Li: Mg alloys. The20%Li doped alloy possesses the lowest work function of2.497eV, leading to the lowest electron injection barrier and best device performance.3. An efficient composite Li₃N/Mg: Ag/Ag cathode was designed for chemicallystable electron transpot materials which consist of only two elements of carbon andhydrogen, such as2-phenyl-9,10-di（naphthalen-2-yl）-anthracene（PADN）. Thedevice performance with commonly used cathodes like LiF/Al，Mg: Ag/Ag,Cs₂CO₃/Al were poor, while the Li₃N/Mg: Ag/Ag cathode achieved efficientelectron injection, and the current efficiency is about300times that of the device withLiF/Al cathode. The mechanism for the electron injection enhancement is as follows:the Li atoms diffuses into PADN layer and n-doping is formed, increasing the eletron density in PADN layer; the thermally activated Mg atoms react with Li2O to releasemetallic Li at Li/Mg interface, further enhancing the electron injection.