Using steady-state and time-resolved photoluminescence (PL) spectroscopy, we have investigated the temperature dependence of PL properties of 1,2,3,5-tetrakis(carbazol-9-yl)-4,6-dicyano-benzene (4CzIPN), which have a small energy gap between its singlet and triplet excited states and thus exhibits efficient thermally activated delayed fluorescence [H. Uoyama et al., Nature 492, 235 (2012)]. Below around 100 K, PL quantum efficiency of 4CzIPN thin films is largely suppressed and strong photoexcitation intensity dependence appears. These features can be explained by using rate equations for the densities of singlet and triplet excited states considering a triplet-triplet annihilation process.
Triplet-triplet annihilation (TTA) will change the ratio between fluorescence and phosphorescence in the photoluminescence spectrum of a thermally activated delayed fluorescence emitter at very low temperature. Using the resultant spectral blueshift, this study investigated the nature of TTA in 1,2,3,5-tetrakis(carbazol-9-yl)-4,6-dicyanobenzene (4CzIPN) doped in a host material. The spectral blueshift is not caused by singlet-triplet annihilation and the emitter saturation effect and is less influenced by the emitter aggregates, particularly for the case of a lower doping concentration. Using these features, it is possible to focus on TTA. For 4CzIPN, the spectral blueshift due to TTA is recognized even in samples with the doping concentration as low as 1 wt. %.
The photoluminescence properties of a thermally activated delayed fluorescence emitter, 1,2-bis(carbazol-9-yl)-4,5-dicyanobenzene (2CzPN), doped in a host matrix consisting of 1,3-bis(9-carbazolyl)benzene and a polar inert molecule, camphoric anhydride (CA), in various concentrations have been investigated. It is found that the addition of CA stabilizes only the lowest singlet excited state (S1) of 2CzPN without changing the energy level of the lowest triplet excited state (T1), leading to a reduction in the energy gap between S1 and T1. The maximum reduction of energy gap achieved in this work has been determined to be around 65 meV from the shift of the fluorescence spectrum and the temperature dependence of the photoluminescence decay rate.
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