Morphologically and electrochemically stable bipolar host for efficient green electrophosphorescence

A new host tBu-o-CzOXD is facilely synthesized through a simple aromatic nucleophilic substitution reaction between 3,6-di-tert-butyl-9H-carbazole and 2,5-bis(2-fluorophenyl)-1,3,4-oxadiazole. Its thermal, electrochemical, electronic absorption and photoluminescent properties are fully investigated. A high glass transition temperature (T_g) of 149 °C is observed for tBu-o-CzOXD due to the introduction of bulky tert-butyl moiety, significantly higher than 97 °C of o-CzOXD without tert-butyl substituent. Moreover, encapsulation of tert-butyl on the 3- and 6-positions of carbazole greatly enhances the electrochemical stability as compared to o-CzOXD. Green phosphorescent OLEDs hosted by tBu-o-CzOXD show a maximum luminance of 48293 cd m^-2 at 17.1 V, a maximum current efficiency of 38.4 cd A^-1 and a maximum power efficiency of 34.7 lm W^-1. Furthermore, the devices exhibit a slow current efficiency roll-off. The device merits, together with the excellent morphological and electrochemical stability, make the new compound an ideal host material for phosphorescent emitters.