Star-shaped oligotriarylamines with planarized triphenylamine core: Solution-processable, high-T_g hole-injecting and hole-transporting materials for organic light-emitting devices

Two novel star-shaped oligotriarylamines with planar triphenylamine core and peripheral triarylamine groups, namely FATPA-T and FATPA-Cz, were synthesized by Suzuki cross-coupling reaction. The molecular design imparts the materials with the following features: (i) excellent thermal stabilities with quite high glass transition temperatures (237 °C for FATPA-T and 272 °C for FATPA-Cz); (ii) good solution-processability; (iii) good hole mobility, efficient hole injection, and electron-blocking functions. Furthermore, their optoelectronic properties can be modulated by the peripheral triarylamine groups. For example, FATPA-T with triphenylamine peripheries shows the significantly redshifted absorption and emission, as well as the small band gap as compared to FATPA-Cz with carbazole peripheries. Double-layer Alq ₃-emitting OLEDs using FATPA-T or FATPA-Cz as holetransport layer by spin-coating method were fabricated, and the FATPA-Cz-based devices show greatly improved performance as compared to standard NPB-based device by vacuum-evaporation of NPB. The optimized three-layer Alq₃-emitting OLEDs by using FATPA-Cz and NPB as double hole-transport layers exhibit the maximum current efficiency of 6.83 cd/A, which is the highest for the Alq ₃-based green emission under the similar device structures. The advantages of solutionprocessablity and very high T_g make the star-shaped oligotriarylamines ideal substitutes for conventional arylamines as hole-inject and hole-transport materials.