Spectroscopic and electrical characteristics of highly efficient tetraphenylsilane-carbazole organic compound as host material for blue organic light emitting diodes

Thermal, electrical and spectroscopic properties have been studied for bis(3,5-di(9H-carbazol-9-yl) phenyl)diphenylsilane (SimCP2) which has exhibited high external quantum efficiency of 17.7% and power efficiency of 24.2 lm/W when it is used as host material for iridium bis(4,6-difluorophenypyridinato)picolate (FIrpic) blue emitter. They are compared with 1,3-bis (9-carbazolyl) benzene (mCP) and 3,5-bis (9-carbazolyl) tetraphenylsilane (SimCP) which have been also used as host for blue emitters. SimCP2 exhibits a highest glass transition temperature (148 °C) and is morphologically more stable. The electron and hole mobilities are higher (4.8 × 10−4 and 2.7 × 10−4 cm2 V−1 s−1, respectively, at electric field of 9 × 104 V cm−1) than those of mCP and SimCP. The zero-phonon S1 emission band is observed at 344 nm, while the T1 emission band at 412 nm, i.e., this material preserves the characteristics of wide band-gap of 3.56 eV and high T1 triplet energy of 3.01 eV. From the intensity ratio of the T1 emission to the S1 emission, it is suggested that the intersystem crossing rate is smaller for SimCP2 than for mCP and SimCP. From these results, we clarify the reasons why SimCP2 is superior to mCP and SimCP as the host material for blue phosphorescence emitter in organic light emitting diodes.