Emission properties of thermally activated delayed fluorescence emitters: analysis based on a four-level model considering a higher triplet excited state

Kobayashi, Takashi; Niwa, Akitsugu; Haseyama, Shota; Takaki, Kensho; Nagase, Takashi; Goushi, Kenichi; Adachi, Chihaya; Naito, Hiroyoshi

doi:10.1117/1.jpe.8.032104

Cited by 8 publications

(13 citation statements)

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“…In the case of 4CzIPN, the PL quantum efficiencies at 300 and 10 K are almost the same, although the ratio of the Ph in the measured PL varied greatly with temperature . Therefore, we determine

R = 1

for 4CzIPN at 10 K. In contrast, the PL quantum efficiency of 2CzPN at 10 K is slightly higher than that at 300 K . Under the assumption that 50% of the observed PL at 10 K is Ph, we obtain

R = 0.9 / 0.7 = 1.29

for 2CzPN at 10 K. The calculated k ISC values and important quantities used for the calculation are shown in Table .…”

Section: Resultsmentioning

confidence: 77%

“…In this work, we have derived some relations for the determination of k ISC based on the three‐level model. These relations can be easily expanded to the four‐level system in which T n is considered, and almost the same relations will be obtained. The only difference is that the calculated value is the sum of two ISC rates from S 1 into T 1 and T n in the four‐level system, although that between S 1 and T 1 must be much smaller.…”

Section: Discussionmentioning

confidence: 75%

“…This definition allows us to avoid an overestimation of k p due to an additional, even faster component, which is observed in some TADF emitters and has a lifetime of a few nanoseconds in the case of 4CzIPN. At 10 K, another component is shown in Figure b as a shoulder around 10 −6 s. The component is an evidence of contributions of T n and is regarded as Ph . To measure ΦPF and ΦDF + ΦPh (at 300 K) or ΦPh (at 10 K), sums of several exponential functions are fitted to the decay curves and then time integrated.…”

Section: Resultsmentioning

confidence: 99%

“…In 4CzIPN and 2CzPN,

k_{normalr}^{normalS}

and

k_{nr}^{normalS}

are considered to be temperature independent because the PL quantum efficiency is constant around room temperature, and the PL spectral shape of the prompt component is virtually independent of the temperature . Therefore, the temperature dependence of k p can be attributed solely to the temperature dependence of k ISC , and Equation is applicable to 4CzIPN and 2CzPN.…”

Section: Resultsmentioning

confidence: 99%

“…In such a temperature range, it is impossible to correct the error due to the nonradiative decay paths, even if

k_{nr}^{normalS} / k_{normalr}^{normalS}

and

k_{nr}^{normalT} / k_{normalr}^{normalT}

were known. The only exceptional case is

k_{nr}^{normalS} / k_{normalr}^{normalS} = k_{nr}^{normalT} / k_{normalr}^{normalT}

(see the Supporting Information) but this will never occur over the temperature range because

k_{nr}^{normalT}

usually possesses a temperature dependence that is explained in terms of the thermal quench model …”

Section: Discussionmentioning

confidence: 99%

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Intersystem Crossing Rate in Thermally Activated Delayed Fluorescence Emitters

Kobayashi

Kawate

Niwa

et al. 2019

Physica Status Solidi (a)

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For a better understanding of the exciton decay process in thermally activated delayed fluorescence (TADF) emitters, the intersystem crossing rate, k ISC , is one of the important physical constants that have to be determined. Herein, a method to calculate the k ISC value from photoluminescence (PL) measurements is reconsidered. The k ISC value can be determined at very low temperatures where delayed fluorescence (DF) is completely suppressed, as well as around room temperature where triplet excitons mainly decay into the ground state by emitting DF. However, there is a temperature range where the k ISC value cannot be determined accurately because the influences of nonradiative decay paths can be neither ignored nor corrected. For such a temperature range, an alternative approach, which utilizes the temperature dependence of an observed PL decay rate, is presented. In this way, k ISC values from 300 to 10 K are determined for thin films of two TADF emitters, i.e., 1,2,3,5-tetrakis(carbazol-9-yl)-4,6-dicyanobenzene and 1,2-bis(carbazol-9-yl)-4,5-dicyanobenzene, which are known as 4CzIPN and 2CzPN, respectively.

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“…In the case of 4CzIPN, the PL quantum efficiencies at 300 and 10 K are almost the same, although the ratio of the Ph in the measured PL varied greatly with temperature . Therefore, we determine

R = 1

for 4CzIPN at 10 K. In contrast, the PL quantum efficiency of 2CzPN at 10 K is slightly higher than that at 300 K . Under the assumption that 50% of the observed PL at 10 K is Ph, we obtain

R = 0.9 / 0.7 = 1.29

for 2CzPN at 10 K. The calculated k ISC values and important quantities used for the calculation are shown in Table .…”

Section: Resultsmentioning

confidence: 77%

Section: Discussionmentioning

confidence: 75%

Section: Resultsmentioning

confidence: 99%

“…In 4CzIPN and 2CzPN,

k_{normalr}^{normalS}

and

k_{nr}^{normalS}

Section: Resultsmentioning

confidence: 99%

“…In such a temperature range, it is impossible to correct the error due to the nonradiative decay paths, even if

k_{nr}^{normalS} / k_{normalr}^{normalS}

and

k_{nr}^{normalT} / k_{normalr}^{normalT}

were known. The only exceptional case is

k_{nr}^{normalS} / k_{normalr}^{normalS} = k_{nr}^{normalT} / k_{normalr}^{normalT}

(see the Supporting Information) but this will never occur over the temperature range because

k_{nr}^{normalT}

usually possesses a temperature dependence that is explained in terms of the thermal quench model …”

Section: Discussionmentioning

confidence: 99%

See 3 more Smart Citations

Intersystem Crossing Rate in Thermally Activated Delayed Fluorescence Emitters

Kobayashi

Kawate

Niwa

et al. 2019

Physica Status Solidi (a)

Self Cite

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Facile Generation of Thermally Activated Delayed Fluorescence and Fabrication of Highly Efficient Non‐Doped OLEDs Based on Triazine Derivatives

Chih

Chen

Hsieh

et al. 2019

Chemistry A European J

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As eries of donor-acceptor-donor triazine-based molecules with thermally activated delayedf luorescence (TADF) properties were synthesized to obtain highly efficient blue-emitting OLEDs with non-doped emitting layers (EMLs). The targeted molecules use at riazinec ore as the electron acceptor,a nd ab enzene ring as the conjugated linker with different electrond onors to alternate the energy level of the HOMO to further tune the emission color.T he introduction of long alkyl chains on the triazine core inhibits the unwanted intermolecular D-D/A-A-type p-p interactions, resulting in the intermolecularD -A charget ransfer.T he weak aggregation-causedq uenching (ACQ) effectc aused by the sup-pressed intermolecular D-D/A-A-type p-p interaction further enhancest he emission.T he crowded molecular structure allows the electrond onor and acceptort ob en early orthogonal, thereby reducing the energy gap between triplet and singlet excited states (DE ST ). As ar esult, blue-emitting devices with TH-2DMAC and TH-2DPAC non-doped EMLs showed satisfactory efficiencies of 12.8 %a nd 15.8 %, respectively,w hichi so ne of the highest external quantum efficiency (EQEs) reported for blue TADF emitters( l peak < 475 nm), demonstratingt hat our tailored molecular designsare promising strategies to endow OLEDs with excellent electroluminescent performances.

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