Heteroatom-Embedded Spiro[Fluorene-9,9′-Xanthene]-Based Electron Transport Materials for Improved Driving Voltage and Efficiency in Phosphorescent Organic Light-Emitting Diodes

The organic material 9,9-spirobifluorene (SBF) is a useful building block for the construction of organic light-emitting diodes owing to its rigid molecular structure, good charge transport properties, and high triplet energy. However, the electron transport properties of the SBF-derived electron transport materials (ETMs) are currently not good enough and should be improved. In this work, two ETMs derived from spiro[fluorene-9,9′-xanthene] (SFX) and spiro[cyclopenta[1,2-b:5,4-b′] dipyridine-5,9′-xanthene] (SPX) is developed to enhance the electron transport properties and triplet energy of the SBF-derived ETMs. The SFX core can increase the triplet energy by disconnecting the conjugation on the xanthene part and SPX can improve the electron transport properties through the incorporation of two nitrogen atoms in the fluorene part of the SFX core. The SFX and SPX cores are substituted with diphenyltriazine to produce high mobility and high triplet energy ETMs. As a result, the SFX and SPX-based ETMs showed high triplet energy of over 2.95 eV compared with 2.53 eV of the SBF-derived ETM; in addition, the SPX-derived ETM demonstrated higher mobility than the SBF-derived ETM. As a result, the SPX-derived ETM achieved lowered driving voltage and increased external quantum efficiency compared to other ETMs derived from SBF and SFX.