Preparation of high performance and stable hole transport layer by coevaporation method

The introduction of benzoxazole or diphenylstylyl group into a triphenyldiamine derivative (TPD), which has excellent hole transporting properties and therefore is often used as a hole transport layer in organic light-emitting diodes (LEDs) (BOS–TPD or DPS–TPD), was shown to suppress the polycrystallization of hole transport layer but could not always improve electroluminescence (EL) properties. Thus, we tried to improve the low EL properties by keeping a stable film structure with a coevaporation method. The polycrystallization was not observed in the coevaporated TPD:DPS–TPD=1:1 thin film after a 3-month standing. The polycrystallization rate of the coevaporated thin film was much slower than that of TPD thin film. The coevaporated hole transport layer of TPD:DPS–TPD=1:1 also has a better and effective luminance efficiency than the DPS–TPD hole transport layer.