Hole-transporting material based on spirobifluorene unit with perfect amorphous and high stability for efficient OLEDs

Hole-transporting materials (HTMs) capable of forming thermally stable amorphous film and possessing high electrochemical stability play an important role in organic light-emitting diodes (OLEDs), which can enhance the device performance. Herein, spirobifluorene-based HTM N-([1,1′-biphenyl]-2-yl)-N-(9,9-dimethyl-9H-fluoren-2-yl)-9,9′-spirobifluoren-2-amine (SFAF), employing fluorene-contained arylamine as the hole transporting unit, was synthesized, and the comprehensive properties of material were systematically investigated. Owing to the highly twisted spiro conformation, the SFAF exhibited a high glass transition temperature of 140 °C and a high thermal decomposition temperature of 402 °C. Moreover, a stably and homogeneously amorphous SFAF film was formed by increasing the treatment temperature up to 140 °C for 12 h and a smooth surface film was characterized by the atomic force microscope. In addition, the compound presented excellent electrochemical stability with nearly identical cyclic voltammetry curves during the 100 cycles. Green fluorescent OLED based on SFAF as the hole-transporting layer showed a satisfactory device performance. The SFAF with high stability was also applied as HTM in phosphorescence device, which exhibited a turn-on voltage of 2.2 V, a maximum brightness of 191127 cd m⁻² and the maximum current efficiency of 63.80 cd A⁻¹. This results indicate that SFAF is a great promising and potential hole-transporting material for highly efficient optoelectronic devices.