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 Tg value of TPD(BTPA)(4) can be up to 125.5 degrees C, which is higher than most of the reported small molecular HTMs (Tg: 54-116 degrees C). The TPD(BTPA)(4) spincoated film showed an outstanding thermal stability which remained amorphous even when annealed at 110 degrees 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 (Alq(3)) 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.