Hole Mobilities of 2,7- and 2,2′-Disubstituted 9,9′-Spirobifluorene-Based Triaryldiamines and Their Application as Hole Transport Materials in OLEDs

Hole mobilities of up to 2 × 10 −3 cm 2 V −1 s −1 of 2,7- and 2,2′-disubstituted spirobifluorene-based triaryldiamine derivatives have been measured using time-of-flight (TOF) techniques. Among these derivatives, 27DPSF (possessing diphenylamino groups attached onto the same biphenyl branch of spirobifluorene) and 22DPASF (possessing extended π-conjugation of the chromophore) exhibited the highest hole mobilities because of their greater intermolecular interactions. The charge transport parameters were extracted, using a Gaussian disorder model (GDM), from detailed temperature- and field-dependent hole mobility measurements. For 2,2′-disubstituted systems possessing different diarylamino groups, the hole mobilities decreased in the order 22DPSF > 22DTSF > 22DBPSF, indicating that spatial hindrance had a crucial effect on the hole transport behavior. To verify these disubstituted spirobifluorene-based amines can be used as HTLs in OLED devices. A standard device was fabricated with the configuration ITO/ m -MTDATA (20 nm)/triaryldiamine derivative (40 nm)/Alq 3 (60 nm)/LiF (0.5 nm)/Al (150 nm), which gave a relatively high current density (ca. 5200 mA/cm 2 ), impressive maximum brightness (1.3 × 10 5 cd/m 2 ), and high external quantum efficiency (1.8%). The device performances are strongly correlated to the charge transporting behavior of such disubstituted spirobifluorene-based amines.