Highly Efficient Blue Electroluminescence Using Delayed

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Highly Efficient Blue Electroluminescence Using Delayed-Fluorescence Emitters with Large Overlap Density between Luminescent and Ground states Katsuyuki Shizu, Hiroki Noda, Hiroyuki Tanaka, Masatsugu Taneda, Motoyuki Uejima, Tohru Sato, Kazuyoshi Tanaka, Hironori Kaji, and Chihaya Adachi J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b07798 • Publication Date (Web): 02 Nov 2015 Downloaded from http://pubs.acs.org on November 4, 2015

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The Journal of Physical Chemistry

Highly Efficient Blue Electroluminescence Using Delayed-Fluorescence Emitters with Large Overlap Density between Luminescent and Ground states Katsuyuki Shizu,† Hiroki Noda,‡ Hiroyuki Tanaka,† Masatsugu Taneda,† Motoyuki Uejima, § Tohru Sato, § Kazuyoshi Tanaka, § Hironori Kaji, ║ and Chihaya Adachi*,† †

Center for Organic Photonics and Electronics Research (OPERA), Kyushu University, 744

Motooka, Nishi, Fukuoka 819-0395, Japan ‡

Department of Applied Chemistry and Biochemistry, Kyushu University, 744 Motooka,

Nishi, Fukuoka 819-0395, Japan §

Department of Molecular Engineering, Graduate School of Engineering, Kyoto University,

Kyoto 615-8510, Japan ║

Institute for Chemical Research, Kyoto University, Uji, Kyoto 611-0011, Japan

ACS Paragon Plus Environment

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Complete list of Affiliations Katsuyuki Shizu,†,‡,§ Hiroki Noda,║ Hiroyuki Tanaka,† Masatsugu Taneda,† Motoyuki Uejima, ┴

†

Tohru Sato,┴,# Kazuyoshi Tanaka,┴ Hironori Kaji,‡,§ and Chihaya Adachi*,†,§,∇

Center for Organic Photonics and Electronics Research (OPERA), Kyushu University, 744

Motooka, Nishi, Fukuoka 819-0395, Japan ‡

Institute for Chemical Research, Kyoto University, Uji, Kyoto 611-0011, Japan

§

Japan Science and Technology Agency (JST), ERATO, Adachi Molecular Exciton

Engineering Project, 744 Motooka, Nishi, Fukuoka 819-0395, Japan ║

Department of Applied Chemistry and Biochemistry, Kyushu University, 744 Motooka,

Nishi, Fukuoka 819-0395, Japan ┴

Department of Molecular Engineering, Graduate School of Engineering, Kyoto University,

Kyoto 615-8510, Japan #

Unit of Elements Strategy Initiative for Catalysts & Batteries, Kyoto University, Kyoto 615-

8510, Japan ∇

International Institute for Carbon Neutral Energy Research (WPI-I2CNER), Kyushu

University, 744 Motooka, Nishi, Fukuoka 819-0395, Japan


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ABSTRACT The use of thermally activated delayed-fluorescence (TADF) allows the realization of highly efficient organic light-emitting diodes (OLEDs), and is a promising alternative to use of conventional fluorescence and phosphorescence. Recent research interest has focused on blue TADF emitters. In this study, we use quantum mechanics to reveal the relationship between the molecular structures and the photophysical properties of TADF emitters and derive a direction for the molecular design of highly efficient blue TADF emitters. Theoretical analyses show that the luminescence efficiency of TADF emitters largely depends on the overlap density (ρ10) between the electronic wave functions of the ground state and the lowest excited singlet state. By increasing ρ10, we develop an efficient sky-blue TADF emitter material,

9-(4-(4,6-diphenyl-1,3,5-triazin-2-yl)phenyl)-9'-phenyl-9H,9'H-3,3'-bicarbazole

(BCzT). When doped into a host layer, BCzT produces a high photoluminescence quantum yield of 95.6%. From the transient photoluminescence decays of the doped film, the efficiency of excited triplet state conversion into light is estimated to be 76.2%. An OLED using BCzT as a sky-blue emitter produces a maximum external quantum efficiency (EQE) of 21.7%, which is much higher than the EQE range of conventional fluorescent OLEDs (5– 7.5%). The high EQE is a result of the high triplet-to-light conversion efficiency of BCzT. Our material design based on ρ10 distribution provides a rational approach to develop TADF emitters for high-efficiency blue OLEDs.


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INTRODUCTION Organic light-emitting diodes (OLEDs) have attracted considerable research attention because they hold great promise for realization of flexible flat-panel displays and nextgeneration solid-state lighting sources.1-4 Phosphorescent emitters have been used as emitting dopants to produce high-efficiency OLEDs.5-7 The strong heavy atom effect that occurs in phosphorescent emitters allows all singlet excitons to be converted into triplet excitons, and this leads to efficient phosphorescence. In fact, use of an iridium complex as a phosphorescent emitter has enabled realization of an OLED with internal quantum efficiency (IQE) of nearly 100%.7 However, while phosphorescent emitters produce high-performance OLEDs, they also have several drawbacks, including the use of noble metals and instability problems, which occur in blue OLEDs in particular. Blue fluorescent emitters have therefore been used as alternative materials to realize long-term stability in white OLEDs.8 However, conventional fluorescent OLEDs have commonly suffered from low IQE because only 25% of the total number of excitons can be extracted as photons; because the IQE is limited to 25%, and assuming that the light outcoupling efficiency is 20–30%, the external quantum efficiency (EQE) can only be 5–7.5% at most. Thermally activated delayed-fluorescence (TADF) is a promising way to realize high IQEs without using noble metals.9 Like phosphorescent emitters, TADF emitters allow all triplet excitons to be converted into singlet excitons, and this leads to efficient fluorescence. Purely organic TADF emitters have been developed with a wide variety of chemical structures, and the maximum IQE of TADF-based OLEDs has reached nearly 100%.9-11 In addition, EQEs of more than 20% have been achieved by optimizing the host materials.12-14 The electroluminescence (EL) efficiency of TADF-based OLEDs thus now competes with that of phosphorescent OLEDs. Recent research interest has focused on TADF emitters that


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make it possible to produce blue OLEDs with both high EL efficiency and long-term operational stability.15 TADF emitters have also raised additional interest by acting as singlet exciton harvesters for fluorescent OLEDs, in which singlet excitons generated on the TADF emitters are transferred to the normal fluorescent emitters. Specifically, by using a TADF emitter that has a wide energy gap between its highest occupied molecular orbital (HOMO) and its lowest unoccupied molecular orbital (LUMO), efficient blue EL has been obtained from a normal fluorescent emitter.16 Wide-gap TADF emitters thus have considerable potential to act as both emitters and singlet-exciton harvesters for blue OLEDs. However, the rational design of such TADF emitters remains challenging. The luminescence efficiency of TADF emitters is related to the overlap density (ρ10) between the electronic wave functions of the ground state (S0) and the lowest excited singlet state (S1).17-18 Like the electron density, ρ10 can be visualized as a density map in threedimensional space. Visualization of the ρ10 distribution provides both a physical insight into the photophysical properties of TADF emitters and guidance for material design.17-18 In our previous study, by optimizing the HOMO-LUMO overlap density and the resulting ρ10 distribution, we have developed an efficient green TADF emitter that shows photoluminescence quantum yield (PLQY) of nearly 100%.17 In this study, we apply this approach to blue TADF. By increasing the ρ10 distribution to promote S1 → S0 radiative decay, we have developed a highly efficient TADF emitter, 9-(4-(4,6-diphenyl-1,3,5-triazin2-yl)phenyl)-9'-phenyl-9H,9'H-3,3'-bicarbazole (BCzT). An OLED using BCzT as an emitter produces sky-blue emission and EQE of more than 20%. In addition, by comparing ρ10 for BCzT with that of a TADF emitter with low PLQY,19 9-(4,6-diphenyl-1,3,5-triazin-2-yl)-9'phenyl-3,3'-bicarbazole (CzT), we reveal the physics behind the relationship between the material molecular structure and the luminescence efficiency. Our materials design concept


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based on ρ10 provides a rational approach to improve the luminescence efficiency of TADF emitters.

RESULTS AND DISCUSSION Figure 1 shows the TADF process, in which the lowest triplet state (T1) is converted into a photon via reverse intersystem crossing (RISC) from T1 to S1 caused by thermal activation. The S1 → S0 radiative decay (fluorescence) competes with the S1 → S0 nonradiative decay. Therefore, to enhance the TADF efficiency, the rate of S1 → S0 radiative decay (kr) should be sufficiently large when compared with that of the S1 → S0 nonradiative decay (knr). Similarly, the rate of S1 ← T1 RISC (kRISC) should be suitably large when compared with that of the T1 → S0 nonradiative decay (knr′). Here, the T1 → S0 radiative decay (phosphorescence) is ignored. kRISC is related to the energy difference between S1 and T1 (∆EST): kRISC ∝ exp(−∆EST/kBT),20 where kB is the Boltzmann constant and T is the temperature. Therefore, ∆EST should be sufficiently small to induce rapid S1 ← T1 RISC. A small ∆EST can be realized by spatial separation of the HOMO and LUMO distributions of a TADF emitter. In fact, when these distributions are almost completely separated, ∆EST is nearly 0 eV.21 kr is proportional to |µ10|2, where µ10 is a three-dimensional vector called the transition dipole moment for the S0-S1 transition, and µ10 can be expressed in terms of ρ10 as: µ10 = x− x x.

(1)

The coordinate origin is set as the center of the atomic charge. From Eq. 1, the elements of µ10 become large when ρ10 has a value at x at which these elements are large, i.e., when ρ10 is distributed in regions that are distant from the coordinate origin. This situation is true for TADF emitters with large HOMO-LUMO overlaps.17-18 Therefore, in contrast to ∆EST, the


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magnitude of µ10 (which is denoted as µ10) becomes large when the HOMO-LUMO overlap is large. Therefore, there is a trade-off between ∆EST and µ10, which indicates that efficient TADF requires careful control of the HOMO and LUMO distributions to allow both moderately large kRISC and kr compatible.

Figure 1. State energy diagram for fluorescent emitters. S0 denotes the ground state. S1 and T1 denote the lowest excited singlet and triplet states, respectively. kr: rate of radiative decay from S1 to S0 (fluorescence); knr: rate of nonradiative decay from S1 to S0; kRISC: rate of RISC from T1 to S1; knr′: rate of nonradiative decay from T1 to S0; kr′: rate of radiative decay from T1 to S0 (phosphorescence). ∆EST is the difference between the S1 and T1 energy levels. The geometrical optimization of S0 for isolated BCzT and CzT (Figure 2a,b) was performed with density functional theory (DFT) and the correlation-consistent polarized valence double-zeta (cc-pVDZ) basis set22 using the Gaussian 09 program package.23 The optimized S0 geometries were confirmed to be stable by frequency analysis at the same level of theory. 12 low-lying excited states (six excited singlet and six excited triplet states) were calculated with the time-dependent DFT method24-25 implemented in the Gaussian 09 program package using the optimized S0 geometries. Four functionals were examined: the


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Becke three-parameter Lee-Yang-Parr (B3LYP) hybrid functional;26 Coulomb-attenuated method based on the B3LYP functional, CAM-B3LYP;27 the long-range corrected hybrid functional, ωB97X-D;28 the hybrid meta-generalized gradient-approximation functional, M06-2X.29 The B3LYP functional has been most widely used to investigate electronic structures of organic dyes. The CAM-B3LYP and ωB97X-D functionals perform well for the calculation of charge transfer (CT) excited-states.27-28, 30 The M06-2X functional has been used to calculate excited states of various TADF emitters.9,

17-18, 31

Calculated excitation

energies and |µ10| for S1 ← S0 excitations of BCzT and CzT are listed in Table S1 in Supporting Information. The excitation energy calculated with the B3LYP functional is small compared with those calculated with CAM-B3LYP, ωB97X-D, and M06-2X functionals. The CAM-B3LYP and M06-2X functionals give the similar excitation energies and |µ10| values. The ωB97X-D functional gives the larger excitation energy and |µ10| value than the CAMB3LYP and M06-2X functionals. Detailed theoretical investigation is performed using the CAM-B3LYP functional. Figure 2a,b shows the chemical structures, HOMOs and LUMOs for BCzT and CzT, which both have donor-acceptor-type structures with good spatial separation between the HOMO and LUMO. For BCzT and CzT, HOMO-LUMO excitation predominantly contributes to S1 in both cases (see Supporting Information, Table S2 and Figure S1): the HOMO and LUMO distributions have significant effects on the TADF efficiencies of these materials. The HOMOs of BCzT and CzT show similar distribution patterns, because BCzT and CzT contain identical electron-donating units. Their HOMOs are predominantly distributed over the electron-donating units. Note that the HOMO distribution range is extended to the electron-accepting units (the green circled regions shown in Figure 2a,b). In contrast to the HOMOs, the LUMOs of BCzT and CzT have different distribution patterns, reflecting the fact that they have different electron-accepting units. For BCzT, the LUMO is


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predominantly distributed on the electron-accepting unit but is also extended to the electrondonating unit (the orange circled region in Figure 2a), while the LUMO for CzT is strongly localized on the electron-accepting unit and does not extend to the electron-donating unit. Therefore, the HOMO-LUMO overlap is larger for BCzT than for CzT. Figure 3a,b shows ρ10 for both BCzT and CzT. Because the HOMO-LUMO overlap is larger for BCzT than for CzT, ρ10 is thus more widely distributed for BCzT. For BCzT, ρ10 is spread over the outermost phenyl rings and extends over regions that are distant from the coordinate origin. Because of this wide ρ10 distribution, BCzT has a sizable µ10 of 2.718 atomic units (6.909 D). In contrast, CzT has a small µ10 of 0.255 atomic units (0.648 D) because the ρ10 distribution range is limited to small regions located around the coordinate origin. For CzT, ρ10 is strongly localized on the electron-accepting unit. This is because the LUMO of CzT is not distributed on the electron-donating unit (Figure 2b) and the resulting HOMO-LUMO overlap is thus small. Figure 3a,b also shows the electron-density variance associated with the S1 ← S0 excitation (∆ρ10) for BCzT and CzT calculated for their optimized S0 geometries. ∆ρ10 decreases in the electron-donating unit and increases in the electron-accepting unit, which indicates that the S1 ← S0 excitation in BCzT and CzT causes electron transfer from their electron-donating units to their electron-accepting units, i.e., S1 for both BCzT and CzT has a charge-transfer character.


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Figure 2. Chemical structure, HOMO, and LUMO of (a) BCzT and (b) CzT. The isosurface value for the HOMOs and the LUMOs is 0.02 atomic units (yellow regions: positive; blue regions: negative).

Figure 3. Overlap density (ρ10) and electron-density variance (∆ρ10) associated with the S1 ← S0 excitation of (a) BCzT and (b) CzT. The isosurface value for ρ10 and ∆ρ10 is 0.0004 atomic units (yellow regions: positive; blue regions: negative).


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Interestingly, simple insertion of a phenyl ring between the electron-donating and electron-accepting units of CzT causes µ10 to be increased markedly, and the S1 → S0 radiative decay is thus significantly enhanced. To investigate the effect of this chemical modification on µ10, we compare the contributions from the fragment structures to µ10 for both BCzT and CzT. BCzT and CzT are divided into four and three fragment structures, respectively (Figure 4a,b). By introducing a three-dimensional function called the transition dipole moment density (τ10), x = x− x ∙ µ ,

(2)

= x x.

(3)

µ

µ10 can then be expressed as

Because the spatial integral of τ10 gives µ10, τ10 can be interpreted as a density form of µ10.17-18 By visualizing the τ10 distributions of BCzT and CzT and then comparing them, we can obtain physical insights into the relationships between the molecular structures and µ10 values for the two materials. Figure 4a,b shows the τ10 distributions and the spatial integrals of τ10 that were calculated for the fragment structures, which we call fragment transition dipole moments. The sum of these fragment transition dipole moments is equal to µ10. Comparison of the τ10 distributions of BCzT and CzT shows that τ10 is much more widely distributed for BCzT than for CzT, especially in the outer fragments. Consequently, the fragment transition dipole moments of the outer fragments of BCzT (0.797 and 1.914 atomic units) are approximately ten times those of CzT (0.061 and 0.208 atomic units). This leads to the µ10 of BCzT (2.718 atomic units) being larger than that of CzT (0.255 atomic units). Interestingly, the fragment transition dipole moment of the inner phenyl ring of BCzT is quite small (−0.060 atomic units) and thus makes little contribution to the µ10 of 2.718 atomic units.


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Insertion of the phenyl ring into CzT has the effect of changing the LUMO distribution pattern (see Figure 2a,b), expanding the τ10 distribution range (Figure 3a,b), and increasing the fragment transition dipole moments of the outer fragments (Figure 4a,b). This effect leads to the marked increase in µ10 for BCzT. Electronic configurations with configuration interaction coefficients higher than 1.0×10−4 were included in the calculations of ρ10, ∆ρ10, τ10, µ10.

Figure 4. Transition dipole moment densities (τ10) and fragment transition dipole moments for (a) BCzT and (b) CzT. The isosurface value for τ10 is 0.001 atomic units (yellow regions: positive; blue regions: negative). The numbers indicate the fragment transition dipole moments in atomic units. The sum of these fragment transition dipole moments gives the magnitude of the transition dipole moment (µ10). The µ10 values for BCzT and CzT are 2.718 and 0.255 atomic units, respectively. Green arrows indicate the direction of the transition dipole moment.


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To evaluate contributions from electronic transitions other than the HOMO-LUMO transition to µ10, we calculated transition dipole moment for each electronic transition by assuming that S1 consists of each electronic transition alone. For example, transition dipole moment for the HOMO-LUMO transition was calculated by assuming that S1 consists of only the HOMO-LUMO transition. For BCzT, the transition dipole moment for the HOMOLUMO transition was calculated to be 0.934 atomic units and is one-third of µ10 of BCzT, suggesting that electronic transitions other than the HOMO-LUMO transition also significantly contribute to µ10. In fact, for the HOMO−1-LUMO and HOMO-LUMO+4 transitions, calculated transition dipole moments were 0.543 and 0.367 atomic units, respectively, and therefore, the two transitions largely contribute to µ10. For CzT, the transition dipole moment for the HOMO-LUMO transition was calculated to be 0.130 atomic units, which is one half of µ10. Thus, molecular orbitals other than the HOMOs and LUMOs make large contributions to µ10 of BCzT and CzT. Importantly, the transition dipole moment for the HOMO-LUMO transition of BCzT is ten times larger than that of CzT. Hence, the difference of the relative magnitude of µ10 between BCzT and CzT can be reasonably explained in terms of their HOMO and LUMO distributions. Like kr, knr is also influenced by the ρ10 distribution. However, knr is less sensitive to ρ10 than kr.17-18 To confirm this point, we calculate the vibronic coupling constants (V10m) for both BCzT and CzT. V10m measures the strength of intramolecular vibronic coupling caused by the mth intramolecular vibrational mode, and knr is generally expected to be written in terms of the electronic part: knr/2π≈Ʃm(V10m)2/E102, where E10 is the S1 ← S0 excitation energy.32 Vibrational modes with large V10m values promote S1 → S0 nonradiative decay, and small V10m values are therefore desirable for TADF emitters. V10m is related to ρ10 by the following equation:


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x = x − ∑

R e · |R |

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x,

(4)

where ZA, MA, and RA are the charge, mass, and position of the Ath nucleus, respectively. eA(m) is a three-dimensional vector and represents the relative displacement of the Ath nucleus for the mth vibrational mode. Figure 5a,b shows the calculated values of V10m for BCzT and CzT. Electronic configurations with configuration interaction coefficients higher than 1.0×10−4 were included in the calculations of V10m. Unlike µ10, the V10m values of BCzT and CzT are of the same order. The resulting Ʃm(V10m)2/E102 values for BCzT and CzT are 3.70×10−5 and 4.73×10−5 atomic units, respectively, and are of the same order. This indicates that knr is comparable for BCzT and CzT. knr also depends on vibrational terms resulting from vibrational wavefunctions: knr/2π=ƩmPm(V10m)2/E102, where Pm denotes the vibrational term and can be written as32-33 ! =

∑0"#/%exp)"#/%*+ /,- ./ ∑0 exp*+ /,- .

.

(5)

Here, v denotes the vibrational state and ωm is the frequency of the mth vibrational mode. Calculated ƩmPm(V10m)2/E102 for BCzT and CzT were almost identical (2.32×10−5 and 2.58×10−5 atomic units for BzT and CzT, respectively). Thus, knr of BCzT and CzT are comparable, even after taking into account the vibrational terms. From the calculated values of µ10 and V10m, the difference between kr and knr is larger for BCzT than for CzT, and radiative decay is thus promoted more strongly in BCzT. BCzT is thus expected to show higher TADF efficiency. Vibrational modes with the largest V10m (indicated by the arrows shown in Figure 5a,b) involve C−N stretching in the triazine rings of the electron-accepting units. This reflects the large ρ10 distribution on the triazine moieties.


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Figure 5. Vibronic coupling constants (V10m) and vibrational modes with the highest V10m values for (a) BCzT and (b) CzT. To evaluate the potential of BCzT for use as an emitter for OLEDs, we investigate the TADF behavior of BCzT in a solid-state host layer. A 6 wt % BCzT-doped bis(2(diphenylphosphino)phenyl)ether oxide (DPEPO) layer was fabricated by vacuum deposition. DPEPO has a high T1 energy level,34 and has been used previously as a host material to confine the T1 excitons of blue TADF emitters.10-11 The doped film shows sky-blue emission and a high PLQY (ΦPL) of 95.6%, which is much higher than the corresponding value for a CzT-doped DPEPO film19 (ΦPL = 39.7%). Figure 6a shows the transient photoluminescence (PL) decays of the BCzT-doped DPEPO film measured over the time range from 0–500 µs at temperatures of 100, 200, and 300 K. The intense peaks and long tails of the decays correspond to prompt fluorescence and delayed fluorescence, respectively. The lifetime of the delayed component measured at 300 K is 33 µs. The lifetime of the prompt component is determined to be 5.5 ns from the transient PL decay of the doped film measured over the time range from 0–100 ns at 300 K. The delayed fluorescence intensity increases with increasing temperature, which suggests that the delayed fluorescence originates from TADF. From the


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photon counts observed at 300 K, the PLQYs of the prompt (Φp) and TADF (Φd) components of the BCzT-doped film are estimated to be 81.5% and 14.1% (where Φp + Φd = ΦPL), respectively, and Φp is much higher than Φd. In contrast, for the CzT-doped DPEPO film, these quantities are almost identical19 (Φp = 19% and Φd = 20%). Thus, BCzT and CzT show different TADF behaviors, despite the similarities between their molecular structures. The inset figure in Figure 6a shows the PL spectrum of the BCzT-doped film measured at 300 K. The excitation wavelength was 387 nm. Emission from the DPEPO host is not observed in this spectrum, and the broad emission band thus originates from the CT characteristics of the S1 of BCzT. The emission spectrum is red shifted when compared with that for a toluene solution

(see

Supporting

Information,

Figure

S2).

From

the

fluorescence

and

phosphorescence spectra of the doped film, ∆EST is estimated to be 0.29–0.33 eV (see Supporting Information, Figure S3). From the experimental and theoretical analyses, BCzT exhibits higher ΦPL and Φp values than CzT because BCzT has a larger µ10. Figure S4a–c shows the PL processes proposed from the ΦPL, Φp, and Φd that were obtained experimentally (see Supporting Information). For BCzT, µ10 is large and most of the S1 states (81.5% of the total number of excitons) are generated by the photo-excitation decay radiatively (prompt fluorescence). The remaining 18.5% of the excitons are converted into T1 states (Figure S4b), and of these, 14.1% are converted into light as TADF. The T1-to-light conversion efficiency is thus calculated to be 76.2% (Figure S4c). For CzT, because µ10 is small, only 19% of the excitons decay radiatively, and most of the S1 states (81% of the total number of excitons) are first converted into T1.19 In addition, of this 81% of T1 excitons, only 20% are converted into light as TADF and thus the T1-to-light conversion efficiency is approximately 25%. Consequently, CzT shows a lower ΦPL than BCzT. Comparison of the TADF behavior of BCzT and CzT


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shows that increasing µ10 by optimization of ρ10 is an effective approach to promote S1 → S0 radiative decay and thus improve the ΦPL of the TADF emitters. Using BCzT as an emitter, a BCzT-based OLED with a structure of indium tin oxide (ITO) (100 nm)/N,N′-diphenyl-N,N′-bis(1-naphthyl)-1,10-biphenyl-4-4′-diamine (α-NPD) (35 nm)/9,9′-biphenyl-3,3′-diyl-bis-9H-carbazole (m-CBP) (10 nm)/6 wt %-BCzT:DPEPO (20 nm)/1,3,5-tris(N-phenylbenzimidizol-2-yl)benzene (TPBi) (35 nm)/LiF (0.5 nm)/Al (100 nm) was fabricated by vacuum deposition. Figure 6b shows the EQE and current density characteristics of this OLED and its EL spectrum at a current density of 100 mA cm−2. The maximum EQE of 21.7% achieved by this OLED is much higher than that of conventional fluorescent OLEDs (5–7.5%). The OLED produces a sky-blue emission with a peak at 492 nm (the inset figure in Figure 6b), which corresponds to the S1 → S0 radiative decay of BCzT. Figure S4d–f shows the proposed exciton generation and decay processes in the OLED (see Supporting Information). First, S1 and T1 are generated with a ratio of 1:3 by charge recombination: S1 and T1 account for 25% and 75% of the total number of excitons, respectively (Figure S4d). Next, of the 25% proportion of S1, 81.5% (20.4% of the total number of excitons) decay radiatively (prompt fluorescence), and the remaining 18.5% (4.6% of the total number of excitons) are converted into T1, resulting in a T1 total of 79.6% (Figure S4e). Finally, of the 79.6% proportion of T1, 76.2% (60.7% of the total number of excitons) are converted into light as TADF. Therefore, the IQE of the OLED is calculated to be 81.1% (= 20.4% + 60.7%; see Figure S4f). Assuming that the charge recombination factor (γ) is 1 and that the light out-coupling efficiency (ηout) is 0.2–0.3, the theoretical maximum EQE of the OLED is estimated to be 16.2–24.3% (= IQE × γ × ηout). The experimental maximum EQE of 21.7% is within this range. Because prompt fluorescence and TADF account for 20.4% and 60.7% of the IQE,


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respectively, their contributions to the EQE are calculated to be 5.5% (= 21.7×20.4/81.1) and 16.2% (= 21.7×60.7/81.1), respectively. The TADF contribution is approximately three times that of the prompt fluorescence, indicating that the efficient TADF from the BCzT-doped layer contributes significantly to the high EQE of the OLED. The calculated and experimental results are listed in Table 1. In addition to BCzT, we have developed another sky-blue

TADF

emitter,

9'-(4-(4,6-diphenyl-1,3,5-triazin-2-yl)phenyl)-9,9''-diphenyl-

9H,9'H,9''H-3,3':6',3''-tercarbazole (TCzT). TCzT has a molecular structure where an additional electron-donating carbazolyl unit has been introduced into BCzT (see Supporting Information, Figure S5a). Figure S5b shows the EQE and current density characteristics of an OLED using TCzT as an emitter. While the maximum EQE of the TCzT-based OLED (17.1%) is lower than that of the BCzT-based OLED (21.7%), TCzT also shows great potential for use as a sky-blue emitter in OLEDs.

Table 1. Calculated µ10 and ƩmPm(V10m)2/E102 values, photoluminescence characteristics of the 6 wt% BCzT:DPEPO and 3 wt% CzT:DPEPO doped films, and EQEs of OLEDs containing the 6 wt% BCzT:DPEPO and 3 wt% CzT:DPEPO doped films as emitting layers.

ΦPL is PLQY. Φp and Φd are contributions from prompt and delayed components to ΦPL. γηout was set as 0.2–0.3.

Compound

BCzT CzT a

µ10 (atomic units)

2.718 0.255

ƩmPm(V10m)2/E102 (atomic units)

ΦPL (%)

−5

95.6

−5

a

2.32×10 2.58×10

39.7

Φp (%)

81.5 a

19

Φd (%)

14.1 a

20

Φ d /(1−Φp) (%)

IQE (%)

EQE (%) Calc.

Exp.

76.2

81.1

16.2–24.3

21.7

25

28

5.6–8.4

6a

Reference 19.


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Figure 6. (a) Transient photoluminescence decays for a 6 wt % BCzT-doped DPEPO thin film measured at temperatures of 100, 200, and 300 K over the time range of 0–500 µs. The onset of prompt fluorescence is set to be at 0 µs. The inset figure shows the photoluminescence spectrum of the doped film measured at 300 K. (b) EQE-current density characteristics of the BCzT-based OLED. The inset figure shows the electroluminescence spectrum of the device measured at a current density of 100 mA cm−2.


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EXPERIMENTAL METHODS The NMR spectra were recorded on a Bruker Biospin Avance-III 500 NMR spectrometer at ambient temperature. Matrix-assisted laser deposition ionization (MALDI) time-of-flight (TOF) mass spectra (MS) were recorded on a Bruker Daltonics Autoflex III spectrometer in positive mode. Ultraviolet (UV)-visible absorption spectra were obtained using a UV-visible spectrophotometer (UV-2550, Shimadzu, Japan). The PL spectra were obtained using a spectrofluorometer (Fluoromax-4, Horiba Jobin Yvon). The PLQYs were measured using an absolute PLQY spectrometer (C11347 Quantaurus-QY, Hamamatsu, Japan). The transient PL decays of the BCzT-doped DPEPO film were measured using a streak camera (C4334, Hamamatsu Photonics, Japan). A N2 gas laser operating at a wavelength of 337 nm (Ken-X, Usho Optical Systems, Japan) was used as the excitation source.


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CONCLUSIONS By increasing the overlap density between the electronic wave functions of the ground state and the lowest excited singlet state, we have successfully developed two highly efficient TADF emitters, BCzT and TCzT. BCzT shows a high PLQY of 95.6% in the DPEPO host layer. A TADF-based OLED using BCzT as a sky-blue emitter exhibits a maximum EQE of 21.7% and outperforms conventional fluorescent OLEDs. Transient PL decay measurements of the BCzT-doped DPEPO layer show that the efficient TADF from BCzT is responsible for the high EQE. The contributions of the prompt fluorescence and the TADF to the maximum EQE of 21.7% are determined to be 5.5% and 16.2%, respectively. Theoretical analyses based on quantum mechanics indicate that an increase in the HOMO-LUMO overlap density leads to increases in the transition dipole moment and the luminescence efficiency. Our material design method based on quantum mechanics offers a rational direction for enhancement of the EL efficiency of TADF-based blue OLEDs. ACKNOWLEDGMENTS This research was funded by the Japan Society for the Promotion of Science (JSPS) through the “Funding Program for World-Leading Innovative R&D on Science and Technology (FIRST Program)”. The computations were partly carried out using the computer facilities at the Research Institute for Information Technology, Kyushu University, and those at the Academic Center for Computing and Media Studies (ACCMS), Kyoto University. The computation time was provided by the Super Computer System, Institute for Chemical Research, Kyoto University.


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ASSOCIATED CONTENT Supporting Information Available: Synthesis and characterization data and computational details for BCzT and CzT, UV-visible absorption and PL spectra for BCzT in toluene solution, PL process for the BCzT-doped DPEPO film, EL process for the BCzT-based OLED, and EQE-current density characteristics for the TCzT-based OLED. This information is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. Tel: +81-92-802-6920. Fax: +81-92-802-6921. Author Contributions K. S. and C. A. designed the research. K. S., M. U., T. S., K. T., and H. K. carried out theoretical calculations. H. N, H. T., and M. T. synthesized the compounds. K. S., H. N., H. T., and M. T. performed the experimental work. K. S. wrote the manuscript. Notes The authors declare no competing financial interest.

ABBREVIATIONS BCzT, 9-(4-(4,6-diphenyl-1,3,5-triazin-2-yl)phenyl)-9'-phenyl-9H,9'H-3,3'-bicarbazole; CzT, 9-(4,6-diphenyl-1,3,5-triazin-2-yl)-9'-phenyl-3,3'-bicarbazole;

TCzT,

9'-(4-(4,6-diphenyl-

1,3,5-triazin-2-yl)phenyl)-9,9''-diphenyl-9H,9'H,9''H-3,3':6',3''-tercarbazole.


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Highly Efficient Blue Electroluminescence Using Delayed

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