Electronic Structure and Emission Process of Excited Charge Transfer States in Solids

Moon, Chang‐Ki; Huh, Jin-Suk; Kim, Jae-Min

doi:10.1021/acs.chemmater.8b02011

Cited by 52 publications

(40 citation statements)

References 42 publications

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“…Recently, some researchers focus on the intermolecular interaction between two different molecules in exciplex system which is usually formed by a pair of molecules (a D material and an A material) . The energy levels of exciplex depend on both the chemical structure and the packing mode of materials . According to the existing literature, exciplexes with face‐to‐edge packing mode have not only a forbidden transition between S 1 and S 0 ( f = 0), but also a strong SOC and allowed transition ( f > 0) between T 1 and S 0 .…”

Section: Resultsmentioning

confidence: 99%

Utilizing a Spiro TADF Moiety as a Functional Electron Donor in TADF Molecular Design toward Efficient “Multichannel” Reverse Intersystem Crossing

Gan

Wang

et al. 2019

Adv Funct Materials

121

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Designing thermally activated delayed fluorescence (TADF) materials with an efficient reverse intersystem crossing (RISC) process is regarded as the key to actualize efficient organic light-emitting diodes (OLEDs) with low efficiency roll-off. Herein, a novel molecular design strategy is reported where a typical TADF material 10-phenyl-10H, 10′H-spiro[acridine-9, 9′-anthracen]-10′-one (ACRSA) is utilized as a functional electron donor to design TADF materials of 2,4,6-triphenyl-1,3,5-triazine(TRZ)-p-ACRSA and TRZ-m-ACRSA. It is unique that the intramolecular charge transfer of the ACRSA moiety and the intramolecular and through-space intermolecular charge transfer between the TRZ and ACRSA moieties, provide a "multichannel" effect to enhance the rate of the reverse intersystem crossing process (k risc ) exceeding 10 −6 s −1 . TADF OLEDs based on TRZ-p-ACRSA as an emitter show a maximum external quantum efficiency (EQE) of 28% with reduced efficiency roll-off (EQEs of 27.5% and 22.1% at 100 and 1000 cd m −2 , respectively). Yellow phosphorescent OLEDs utilizing TRZ-p-ACRSA as a host material show record-high EQE of 25.5% and power efficiency of 115 lm W −1 , while phosphorescent OLEDs based on TRZ-m-ACRSA show further lower efficiency roll-off with EQEs of 25.2%, 24.3%, and 21.5% at 100, 1000, and 10 000 cd m −2 , respectively.being consistent with the El-Sayed rule of intersystem crossing (ISC) process, when the natures of 1 CT and 3 CT are different, the SOC between 1 CT and 3 CT might be efficient as well. [2] More importantly, avoiding dual emission phenomenon, which caused by conformational transforming brought by flexible moieties, is considered as the first priority of TADF molecular design strategy. [9] Conformational transforming not only results in energy loss, but also limits the triplet exciton utilization because not all possible conformations have potential to achieve TADF phenomenon. [10] On the basis of this understanding, the key to design TADF

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Section: Resultsmentioning

confidence: 99%

Utilizing a Spiro TADF Moiety as a Functional Electron Donor in TADF Molecular Design toward Efficient “Multichannel” Reverse Intersystem Crossing

Gan

Wang

et al. 2019

Adv Funct Materials

121

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“…Clearly, we can see that decreasing the dipole (D +• →A −• ) distance corresponds to the enhanced Coulombic attraction between D +• and A −• , consequently increasing SOC according to wavelength‐dependent magneto‐PL. Moreover, we can expect that moving the PL detection from short to long wavelength corresponds to the reduced Δ E ST . Simultaneously, moving the PL detection from short to long wavelength largely increases the delayed fluorescence, as shown in Figure .…”

mentioning

confidence: 84%

“…In this situation, the Δ E ST becomes smaller. It has been observed that decreasing the D:A distance leads to a reduced Δ E ST with increased formation of intermolecular charge‐transfer states upon moving PL detection from short to long wavelength.,, Therefore, detecting the PL from short to long wavelength provides a vital method to explore the relationship between spin, energy, and polarization parameters toward harvesting nonradiative triplets into radiative singlets by simultaneously monitoring magneto‐PL and time‐resolved PL. Figure a shows the PL spectrum of the high‐efficiency BCzPh:CN‐T2T exciplex system under 375 nm continuous‐wave (CW) laser excitation.…”

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confidence: 99%

Revealing the Cooperative Relationship between Spin, Energy, and Polarization Parameters toward Developing High‐Efficiency Exciplex Light‐Emitting Diodes

et al. 2019

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Excited states in organic light-emitting diodes (OLEDs) are inevitably formed with both singlets and triplets under electrical excitation. Singlets and triplets are allowed and forbidden to recombine, respectively, due to spin selection rule. It has been shown that the triplets can be almost 100% converted into singlets in thermally activated delayed fluorescence (TADF) molecules based on the design of chemically combining donor and acceptor moieties to enable intramolecular chargetransfer states. [1][2][3][4] Recently, various TADFmolecule-based OLEDs with extremely high external quantum efficiency (EQE) exceeding 35% have been successfully demonstrated. [5,6] Similarly, high EQEs can also be conveniently realized by physically mixing donor and acceptor components to form intermolecular charge-transfer states in exciplex systems, where the nonradiative triplets are also largely converted into radiative singlets. The advantages and versatile applications of exciplex systems for giving high-efficiency OLEDs have been highlighted recently. [7] More significantly, OLEDs with exciplex-forming systems as emitting layer have been reported to achieve EQE higher than 19%, [8][9][10] manifesting their bright and promising prospects in OLED technology based on physically Experimental studies to reveal the cooperative relationship between spin, energy, and polarization through intermolecular charge-transfer dipoles to harvest nonradiative triplets into radiative singlets in exciplex lightemitting diodes are reported. Magneto-photoluminescence studies reveal that the triplet-to-singlet conversion in exciplexes involves an artificially generated spin-orbital coupling (SOC). The photoinduced electron parametric resonance measurements indicate that the intermolecular charge-transfer occurs with forming electric dipoles (D +• →A −• ), providing the ionic polarization to generate SOC in exciplexes. By having different singlet-triplet energy differences (ΔE ST ) in 9,9′-diphenyl-9H,9′H-3,3′-bicarbazole (BCzPh):3′,3′″,3′″″-(1,3,5-triazine-2,4,6-triyl) tris(([1,1′-biphenyl]-3-carbonitrile)) (CN-T2T) (ΔE ST = 30 meV) andBCzPh:bis-4,6-(3,5-di-3-pyridylphenyl)-2-methyl-pyrimidine (B3PYMPM) (ΔE ST = 130 meV) exciplexes, the SOC generated by the intermolecular charge-transfer states shows large and small values (reflected by different internal magnetic parameters: 274 vs 17 mT) with high and low external quantum efficiency maximum, EQE max (21.05% vs 4.89%), respectively. To further explore the cooperative relationship of spin, energy, and polarization parameters, different photoluminescence wavelengths are selected to concurrently change SOC, ΔE ST , and polarization while monitoring delayed fluorescence. When the electron clouds become more deformed at a longer emitting wavelength due to reduced dipole (D +• →A −• ) size, enhanced SOC, increased orbital polarization, and decreased ΔE ST can simultaneously occur to cooperatively operate the triplet-to-singlet conversion.

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“…This is intuitive, as these low energy (short D-A distance) exciplexes will be the first to be disrupted as the host is incorporated into the exciplex films. [19][20][21] The net result is this truncates the shorter end of the D-A distance distribution, and therefore limits the extend of the spectral redshift.…”

Section: Equation 1 ∝ -∆mentioning

confidence: 99%

“…Comparison between the PLQY values and the ΔE ST of the CDBP, CBP and mCP based exciplex phosphorescence value measured for CDBP:UGH-3 evaporated films at[10][11][12][13][14][15][16][17][18][19][20][21][22][23][24][25] …”

mentioning

confidence: 99%

Identifying the Factors That Lead to PLQY Enhancement in Diluted TADF Exciplexes Based on Carbazole Donors

2019

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Here we compare the effects of solid-state dilution on the photophysical and electrical performance of three TADF exciplexes based on carbazole containing donors with a high-performance acceptor PO-T2T. We observe intrinsically different photophysical behaviour between the exciplexes, which is explained by the presence of a heterogeneous distribution of triplet energies in one of the donor molecules arising from a distribution of planar and highly twisted biphenyl bridges. This study of different donors demonstrates that the recently discovered efficiency enhancement of solid-state diluted exciplexes is independent of the separation associated spectral blueshift, with the latter being universal while the former is only found for donors with low structural rigidity. OLEDs produced from these carbazole based exciplexes show decreased electrical performance likely due to the relatively low conductivity of the donors and host, suggesting that functional conductive hosts may, in some cases, be required to translate optical efficiency enhancements into improved electrical performance.

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Electronic Structure and Emission Process of Excited Charge Transfer States in Solids

Cited by 52 publications

References 42 publications

Utilizing a Spiro TADF Moiety as a Functional Electron Donor in TADF Molecular Design toward Efficient “Multichannel” Reverse Intersystem Crossing

Utilizing a Spiro TADF Moiety as a Functional Electron Donor in TADF Molecular Design toward Efficient “Multichannel” Reverse Intersystem Crossing

Revealing the Cooperative Relationship between Spin, Energy, and Polarization Parameters toward Developing High‐Efficiency Exciplex Light‐Emitting Diodes

Identifying the Factors That Lead to PLQY Enhancement in Diluted TADF Exciplexes Based on Carbazole Donors

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