이중 전자 주입층을 이용한 유기 발광 소자의 전기적 및 광학적 특성에 관한 연구

Abstract

Organic light-emitting devices (OLEDs) have become particularly interesting because of their promising applications in next-generation full-color displays and lighting sources. Moderate controls of carrier injection and transport for the efficient carrier recombination and for the balance of the electrons and the holes in the emitting layer (EML) are necessary to fabricate high-efficiency OLEDs. Because the hole mobility of almost all of organic layers is larger than their electron mobility, the increase of the electron injection is necessary to enhance the luminance efficiency of the OLEDs. Cesium carbonates (CsCO3) with a low work function have been used as an EIL in OLEDs to enhance their stability of the OLEDs. However, codeposition process of organic materials with a low sublimation temperature in OLEDs and metal derivatives with a high sublimation temperature releases organic materials from the codeposited layer due to high thermal energy during the evaporation, resulting in the contamination of the organic layer due to the generation of carbon oxides. Because a cesium nitrate (CsNO3) EIL containing nitrogen oxides does not contaminate organic layers, the device stability and the luminance efficiency of OLEDs with a CsNO3 can be enhanced. Even though some works on the enhancement of the luminance efficiency in green OLEDs with various EILs have been performed, systematic studies concerning the electron injection mechanisms in OLEDs fabricated utilizing a CsNO3/Liq EIL are very important for improving the luminescence efficiency of the OLEDs. This paper reports data on the luminance efficiency enhancement in green OLED with a CsNO3/Liq EIL fabricated by using organic thermal evaporation. Current density-voltage, luminance-voltage, and luminance efficiency-current density measurements on OLEDs with a CsNO3/Liq EIL were carried out to investigate their electrical and the optical properties. Current density-voltage measurements on electron only devices (EODs) were performed to investigate the enhancement of the electron injection magnitude due to the insertion of the CsNO3 EIL. The electron injection mechanisms related to the luminance efficiency enhancement of the OLEDs with a CsNO3/Liq EIL are described on the basis of the experimental results. The current density and the luminance of devices with a CsNO3/Liq EIL were much larger than those of devices I and III. Because Cs ions or cesium oxides of the CsNO3 EIL reduced the electron affinity of the surface of the cathode electrode, resulting in the decrease of the electron injection barrier for the Liq EIL, the number of the injected electrons in the EML increased, resulting in an enhancement of the luminance efficiency due to an achievement of more balance of electrons and holes. EODs showed that the electron injection magnitude was increased due to the insertion of CsNO3 EIL into OLEDs. The electron injection mechanisms of the OLEDs with a CsNO3/Liq EIL are described on the basis of the experimental results. These results can help improve understanding of the electron injection mechanisms correlated to the luminance efficiency enhancement in OLEDs utilizing a CsNO3/Liq bilayer EIL.