Control of the Singlet–Triplet Energy Gap in a Thermally Activated Delayed Fluorescence Emitter by Using a Polar Host Matrix
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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.
KeywordsThermally activated delayed fluorescence (TADF) Photoluminescence spectroscopy Electronic permittivity Polar host matrix
External quantum efficiency
Highest occupied molecular orbital
Lowest unoccupied molecular orbital
Organic light-emitting diodes
Thermally activated delayed fluorescence
In recent years, materials that exhibit thermally activated delayed fluorescence (TADF) have received considerable attention because of their strong potential to realize highly efficient and low-cost organic light-emitting diodes (OLEDs) [1, 2]. One of the important factors that determine the efficiency of TADF is the energy gap between the lowest singlet (S1) and triplet (T1) excited states (ΔEST), which should be small enough that the generated triplet excitons overcome it by thermal energy at room temperature to be converted into singlet excitons. It is known that ΔEST can be reduced by spatially separating the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) [3, 4]. In fact, such charge-transfer (CT) molecules have been reported to exhibit efficient TADF [5, 6, 7, 8, 9, 10]. When a CT excited state is created by, for example, photoexcitation, a change in the permanent dipole moment is induced. In a polar solvent, the reorientation of the polar solvent molecules stabilizes the CT excited state, and consequently, the photoluminescence (PL) spectrum shifts with respect to that in a non-polar solvent . Some CT molecules also have a locally excited (LE) state, which is not stabilized even in a polar solvent, and thus, the energy gap between the CT and LE states and even their order may change depending on the polarity of the solvent used . Such solvent polarity effects have been investigated in some TADF emitters [8, 12, 13]. Ishimatsu et al. have determined the ΔEST values of a TADF emitter in several solvents and found that the ΔEST value reduces as the solvent polarity increases . This result suggests that the originally small ΔEST value could be further reduced by doping the TADF emitter in a polar host matrix. Although TADF emitters are used in solid state in OLEDs, Madian et al. have confirmed that a spectral shift of the solid-state PL similar to that observed in a polar solvent can be induced by using a two-component host consisting of an inert polymer and a polar inert molecule, camphoric anhydride (CA) . Therefore, the addition of a polar inert molecule could be another strategy to achieve a small ΔEST value. In this work, to examine the effectiveness of this strategy, we investigated the PL properties of a TADF emitter, 1,2-bis(carbazol-9-yl)-4,5-dicyanobenzene (2CzPN), doped in host matrixes with different electronic permittivities, which are controlled by adding CA in various concentrations.
Results and Discussion
Energy gaps determined with a four-level model including a higher triplet excited state (Tn)
230 ± 10 meV
165 ± 10 meV
150 ± 5 meV
120 ± 5 meV
We have demonstrated that the ΔEST of 2CzPN can be reduced by increasing the permittivity of the host matrix by CA doping. The maximum reduction in ΔEST at a CA concentration of 50 wt% is determined to be 56–65 meV from the analysis of the temperature dependence of PL decay rate as well as the spectral difference in the fluorescence and phosphorescence spectra. The reduction is achieved because only S1 is stabilized in the host with a larger permittivity. This method is, therefore, effective only for TADF emitters having an LE state as T1. Reduction of ΔEST accelerated the thermal activation process from T1 to S1, thereby reducing the triplet exciton density; the latter is expected to contribute to the suppression of the efficiency roll-off.
This work is supported in part by JSPS KAKENHI Grant Number 15H03883 and 15 J12038 and by a Grant-in-Aid for Scientific Research on Innovative Areas “New Polymeric Materials Based on Element-Blocks (No. 2401)” (No. JP24102011).
SH and AN performed all sample preparation steps and measurements. TK and TN contributed to the discussion of the results. SH wrote the manuscript. KG and CA synthesized 2CzPN. HN supervised the project. All the authors have read and approved the final manuscript.
The authors declare that they have no competing interests.
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