The temperature dependences of photoluminescence (PL) decay rates and the PL spectrum of a thermally activated delayed-fluorescence emitter, 1,2,3,5-tetrakis(carbazol-9-yl)-4,6-dicyanobenzene (4CzIPN), are investigated. It is found that not only the lowest singlet (S1) and triplet (T1) excited states but also an additional triplet excited state (Tn) lying between S1 and T1 play an important role in the exciton decay process, particularly around 100 K. At around this temperature, some of the triplet excitons are thermally activated into Tn but not up to S1, and they then decay into the ground state (S0) with phosphorescence emission. Therefore, two kinds of phosphorescence, originating from Tn and T1, are observed. The temperature dependence of the PL decay rates of 4CzIPN can be explained by a four-level model consisting of S1, T1, Tn, and S0, and its energy gaps between Tn and T1 and between S1 and T1 are determined to be 45±5 meV and 135±10 meV, respectively.
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