CzSi, known as 9-(4-tert-butylphenyl)-3,6-ditrityl-9H-carbazole, acts as a thermal activated delayed fluorescence (TADF) host material for blue electrophosphorescence. This material exhibits enhanced morphological and superior electrochemical stability.

Zhang et al report the highly efficient pure blue electroluminescent (EL) device based on bis[(3,5-difluoro-4-cyanophenyl)pyridine]picolinate iridium(III) (FCNIrpic) doped 9-(4-tert-Butylphenyl)-3,6-bis(triphenylsilyl)-9H-carbazole (CzSi) film. The matched energy levels of FCNIrpic and CzSi help facilitate the trapping of carriers. In contrast, the high triplet energy of CzSi can avoid undesired reverse energy transfer. EL performances were significantly enhanced, attributed to a wider recombination zone and a better balance of holes and electrons[1].

Carbazole-based materials adopting the nonconjugated substitution of triphenyl silyl (-SiPh3) and trityl (-CPh3) side groups are studied by Tsai et al. as high-triplet-energy, morphologically, and electrochemically stable host materials with tunable carrier-transport properties for organic blue electrophosphorescence. The developed host materials 9-(4-tert-butylphenyl)-3,6-bis(triphenylsilyl)-9H-carbazole (CzSi), 9-(4-tert-butylphenyl)-3,6-ditrityl-9H-carbazole (CzC), and 9-(4-tert-butylphenyl)-3-(triphenylsilyl)-6-trityl-9H-carbazole (CzCSi) all show high triplet energies of 2.97-3.02 eV, along with high glass transition temperatures of 131-163 °C and superior electrochemical stability. Although three different host materials give similar peak electroluminescence efficiencies at low driving currents, the CzSi host, which has more suitable carrier-transport properties, renders broadened distributions of the triplet excitons in phosphorescent devices, reducing the quenching associated with triplet-triplet annihilation and giving larger resistance against efficiency roll-off at higher brightnesses[2].

References

[1] Yunlong Jiang. “Efficient single light-emitting layer pure blue phosphorescent organic light-emitting devices with wide gap host and matched interlayer.” Journal of Luminescence 168 (2015): Pages 38-42.
[2] Ming-Han Tsai. “Triphenylsilyl- and Trityl-Substituted Carbazole-Based Host Materials for Blue Electrophosphorescence.” ACS Applied Materials & Interfaces 1 3 (2009): 567–574.