The Role of Metallic Dopants in Improving the Thermal Stability of the Electron Transport Layer in Organic Light‐Emitting Diodes

• Published in: Advanced optical materials 2018-09, Vol.6 (17), p.n/a
• Main Authors: Keum, Chang‐Min, Kronenberg, Nils M., Murawski, Caroline, Yoshida, Kou, Deng, Yali, Berz, Cordelia, Li, Wenbo, Wei, Mengjie, Samuel, Ifor D. W., Gather, Malte C.
• Format: Article
• Language: English
• Subjects: Alkali metals; atomic layer deposition; Atomic layer epitaxy; Cesium; Compatibility; Crystallization; Diodes; Dopants; Doping; Electron mobility; Electron transport; Fluorescence; Glass transition temperature; Materials science; molecular doping; Morphology; Optical properties; Optics; Organic light emitting diodes; Phosphorescence; Thermal stability
• ISSN: 2195-1071; 2195-1071
• DOI: 10.1002/adom.201800496

4,7‐Diphenyl‐1,10‐phenanthroline (BPhen) is widely used to create the electron transport layer (ETL) in organic light‐emitting diodes (OLEDs) because of its high electron mobility and good compatibility with alkali metal n‐dopants. However, the morphology of these ETLs is easily altered by heating due to the relatively low glass transition temperature (Tg) of BPhen and this change often reduces the performance of OLEDs. Here, an enhancement in the thermal stability of OLEDs when doping their BPhen‐based ETLs with cesium (Cs) is reported. To investigate the role of the Cs dopant in the BPhen matrix, the crystallization features of Cs‐doped BPhen films with different doping concentrations are examined. Next, the electrical and optical properties of blue fluorescent and red phosphorescent OLEDs containing Cs‐doped BPhen ETLs are characterized after annealing the OLEDs at temperatures up to 100 °C. Cs plays a critical role in inhibiting the undesired crystallization of BPhen films, which enhances the thermal stability of OLEDs beyond the Tg of neat BPhen. Finally, highly stable BPhen‐based OLEDs encapsulated via atomic layer deposition at 80 °C are demonstrated. This work may lead to a new strategy for enhancing the intrinsic thermal durability of organic devices and their compatibility with thermally demanding processes. Controlling the cesium doping concentration in electron transport layers is found to improve the thermal stability of organic light‐emitting diodes (OLEDs) by preventing crystallization. Crystallization of BPhen thin films annealed at different temperatures is investigated by morphological and photophysical characterization. The improved thermal stability of OLEDs allows the use of high‐temperature processes, such as thin‐film encapsulation by atomic layer deposition.