Enhance external quantum efficiency of organic light-emitting devices using thin transparent electrodes

• Published in: Organic electronics 2021-02, Vol.89, p.106057, Article 106057
• Main Authors: Lee, Wei-Kai, Wang, Bo-Kai, Lin, Kai-Chen, Wu, Chung-Chih
• Format: Article
• Language: English
• Subjects: External quantum efficiency; Light extraction; OLEDs; Transparent electrodes
• ISSN: 1566-1199; 1878-5530
• DOI: 10.1016/j.orgel.2020.106057

High-index transparent electrodes have been one major origin of light trapping and lower light extraction in organic light emitting diodes (OLEDs). In this work, influences of the bottom transparent electrode thickness on emission properties of OLEDs are systematically studied by both simulation and experiments. Simulation shows that with substantially decreasing the thickness of the high-index indium tin oxide (ITO) electrode, waveguided modes, that otherwise would be significantly induced in regular/thicker ITO devices, can be effectively eliminated. Consequently, the overall coupling efficiencies of OLED emission into substrates can be much enhanced. Through further effective light extraction from the substrate, green phosphorescent OLEDs with a high external quantum efficiency (EQE) of up to ≈57.5% were experimentally demonstrated by adopting the very thin (20 nm) ITO electrode and preferentially horizontal dipole emitters (with a horizontal dipole ratio of 76%). The simulation further predicts that very high optical coupling efficiencies into substrates and EQEs approaching 80% are possible with further adopting purely horizontal dipole emitters and/or low-index electron transport layer (ETL) to suppress surface plasmon modes. Overall, this study clearly reveals the potential of using thin transparent electrodes for highly efficient OLEDs. [Display omitted] •Conducted simulation and experimental studies of influences of ITO electrode thickness on OLED light extraction efficiency.•Reducing ITO thickness effectively suppress waveguided modes in OLEDs and enhance coupling efficiency into substrates.•Demonstrated high OLED EQE of up to 57.5% using thin ITO electrodes and external extraction lens.