Solution-processed thermally stable amorphous films of small molecular hole injection/transport bi-functional materials and their application in high efficiency OLEDs

A series of novel triphenylamine-based small molecular hole transport materials (HTMs) are reported for solution processed organic light-emitting devices (OLEDs). The character of this series of HTMs, denoted as TPD(BTPA)_n (n = 1, 2, 4), is connecting the flexible moieties of butadiene bridged triphenylamine (BTPA) to N,N,N′,N′-tetraphenyl-[1,1′-biphenyl]-4,4′-diamine (TPD). The glass transition temperature and crystallization temperature (T_g and T_c) showed a proportional relationship with the number of BTPA moieties. The T_g value of TPD(BTPA)₄ can be up to 125.5 °C, which is higher than most of the reported small molecular HTMs (T_g: 54-116 °C). The TPD(BTPA)₄ spincoated film showed an outstanding thermal stability which remained amorphous even when annealed at 110 °C, for 48 h. This indicated that the breaking of the planar molecular structure with BTPA moieties can suppress intermolecular stacking. The solution processed OLEDs with 8-hydroxyquinoline aluminum (Alq₃) as emission and electron transport layers showed high stability at high operation current (>400 mA cm^-2). The OLED with TPD(BTPA)₄ achieved a maximum current efficiency of 5.83 cd A^-1 (at the operation current density > 400 mA cm^-2), which is higher than the maximum current efficiency of most evaporation and solution processed OLEDs in identical structures.