Contribution of TADF and exciplex emission for efficient “warm-white” OLEDs
The bicarbazole derivative 4,4′-(9H,9′H-[3,3′-bicarbazole]-9,9′-diyl)bis(3-(trifluoromethyl)benzonitrile), denoted as pCNBCzoCF3, was synthesized and tested for white OLED applications. pCNBCzoCF3 demonstrated an extremely small value of the singlet–triplet energy gap that caused intensive thermally activated delayed fluorescence (TADF). In addition, this compound is able to form exciplex-type excited states at the interface with star-shaped 4,4′,4′′-tris[phenyl(m-tolyl)amino]triphenylamine (m-MTDATA). Combining the TADF emission of pCNBCzoCF3 with the exciplex emission from the pCNBCzoCF3/m-MTDATA interface, we fabricated a number of highly efficient “warm-white” OLEDs, the electroluminescence of which was close to candle emission. The best device demonstrated a very high brightness of 40 900 Cd m−2 (at 15 V), current efficiency of 53.8 Cd A−1 and power efficiency of 19.3 lm W−1, while the external quantum efficiency reached 18.8%. The fabricated devices demonstrated high emission characteristics even for the standard test at 1000 Cd m−2 (current efficiency of 46.2 Cd A−1, power efficiency of 10.6 lm W−1, EQE of 17.0%).