Efficient Sky‐Blue Organic Light‐Emitting Diodes Using a Highly Horizontally Oriented Thermally Activated Delayed Fluorescence Emitter

Organic thermally activated delayed fluorescent (TADF) materials can harvest 100% of the electrically generated excitons as a result of their small singlet–triplet energy difference. However, maximizing the external quantum efficiency (EQE) of a device also requires enhancing the light out‐coupling efficiency. This work presents a new acceptor–donor–acceptor (ADA) emitter employing an indolocarbazole donor and diphenyltriazine acceptors that show nearly‐completely horizontal orientation regardless of the host matrix, leading to a sky‐blue organic light‐emitting diode (λEL = 483 nm, CIE coordinates of 0.17, 0.32) with EQEMAX of 22.1%, a maximum luminance of 7800 cd m−2, and blue emission.


Introduction
Organic thermally activated delayed fluorescent (TADF) materials have become a competitive class of emitters capable of harvesting both singlet and triplet excitons to thus achieve up to 100% internal quantum efficiency (IQE) in organic lightemitting diodes (OLEDs). [1,2] In 2012, Adachi reported the first example of a highly efficient TADF OLED, with an IQE of 93% and a maximum external quantum efficiency, EQE max , of 19.3%, the cathode, can reduce the amount of generated light exiting the device to only about 25% of the light generated by the emitters. Since molecules emit most of the light in a perpendicular direction to their transition dipole moment (TDM), achieving horizontal orientation of the TDM of an emitter in a thin-film layer would maximize the amount of light exiting the device.
The design challenge now becomes a matter of trying to create an emitter that presents preferential horizontal orientation and good TADF properties in terms of a small singlet-triplet energy gap (ΔE ST ) and fast reverse intersystem crossing rates. Yokoyama et al. showed that longer molecules and the introduction of terminal bulky groups lead to a compound with a higher degree of horizontal orientation. [6] Diphenyltriazine has been employed as an acceptor group in many highly oriented TADF emitters (Figure 1). Lyn et al. studied the relationship between structure and the propensity for the emitter to be horizontally oriented in several acridine-triazine derivatives. Doped films of DMAC-TRZ, DPAC-TRZ, and SpiroAC-TRZ in mCPCN [7] matrices show orientations from isotropic to horizontal with anisotropy factor values (see below), a, of 0.28, 0.22, and 0.17, respectively, showing the positive effect that bulkier groups at the terminus of the molecule have on the orientation of the emitter. Herein, the anisotropy factor a is defined as the fraction of power emitted by vertical dipoles, with a value of 0.33 indicating isotropic dipole orientation and 0 denoting perfectly horizontal alignment. [8] Note that different conventions and terminology are used to quantify anisotropy of the TDM orientation in lightemitting devices. The parameter used here a (or synonymously Θ v = <cos²ϑ>) denotes the second moment of the TDM's angular distribution around the surface normal of the film, where ϑ is the angle between the molecule's TDM vector and the said direction.
This strategy was also employed by Beyon, Kaji, et al. where they reported four carbazole-based emitters CzTRZ, BCzTRZ, TCzTRZ, [9] and DACT-II. [10] Orientation becomes progressively more horizontal with increasing number of pendant carbazoles on the donor. CzTRZ (a = 0.34) is isotropically oriented while there is almost complete horizontal orientation in TCzTRZ (a = 0.05); DACT-II is preferentially horizontally aligned (a = 0.14), and the OLED employing this emitter reached an EQE max of 29.6%. Xiang et al. and Maeng et al. incorporated the more planar and rigid ICz (indolocarbazole) donor in Ind-CzpTr1 and IndCzpTr2, [11] and TRZ-TPDICz, [12] respectively, achieving high EQE max values of 14.5%, 30%, and 30.3%, respectively, due in part to the preferential horizontal orientation of these emitters. Indolocarbazole has been previously employed in TADF emitter design as the donor moiety in a small number of reports, although none of these cases showed a di-functionalized ICz unit. [13][14][15][16] Here, we report the first   The ICz donor was selected due to its planar and rigid structure, suitable electron-donating strength and the possibility to functionalize the two nitrogen atoms of the donor to obtain a longer and linear structure with the expectation to achieve a horizontal orientation. [6,17] By adding a second acceptor onto the ICz donor, this should lead to an increased HOMO-LUMO gap and a blue-shifted emission than the previously published mono-functionalized emitters employing the ICz moiety. [11,12,18] Indeed, ICzTRZ shows both very high photoluminescence quantum yield and strongly horizontally oriented transition dipole moments (TDMs) and demonstrates its potential in a highly efficient sky-blue OLED.
The tert-butyl groups were incorporated onto the acceptors to: 1) enhance solubility; 2) extend the effective length and bulkiness of the target emitter in order to reduce the propensity for aggregation-caused quenching; 3) enhance the likelihood that the emitter would be horizontally oriented; and 4) tune the electron-withdrawing strength of the TRZ group. [19,20] The indolocarbazole possesses an excellent charge-transporting ability and its greater conjugation leads to an enhanced donor strength when compared to the more commonly used carbazole. The latter is evidenced by DFT calculations run on CzPh-TRZ (also known as Cz-TRZ) and IndCzpTr-2, [11] where the HOMO energy level is higher for the ICz based molecule (−5.14 vs −5.64 eV for the Cz based one).

Synthesis
The target molecule, ICzTRZ (Figure 2a), was synthesized following a three-step procedure (Scheme 1). First, the triazine acceptor 1 was obtained through the Grignard reaction of two equivalents of tert-butylphenyl magnesium bromide with 2,4,6-trichloro-1,3,5-triazine. Next, a 4-fluorophenyl group was added to 1 using Suzuki-Miyaura cross-coupling to afford 2. The indolocarbazole donor, ICz, was obtained via an acid-promoted cyclization of 3,3-diindolylmethane on a multigram scale. Nucleophilic aromatic substitution, using tripotassium phosphate as the base, of 2 with ICz afforded ICzTRZ in good yield. The emitter was fully characterized by 1 H NMR, 13 C NMR, HRMS, IR, Mp, and EA. The structure of ICzTRZ was determined by single crystal X-ray diffraction (Figure 2b). The molecule shows a 46.5° dihedral angle between the ICz and the adjacent phenyl ring of the TRZ acceptor, similar to conformation found in CzPhTRZ, [21] which has a dihedral angle of 45.1°. There are two independent molecules in the asymmetric unit and these show π-π stacking between the central benzene ring of the ICz donors and the triazine ring of the acceptors of each of the two independent molecules, at centroid···centroid distances of 3.70 and 3.76 Å.

Theoretical Calculations
Density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations at the PBE0/6-031G(d,p) level revealed the promise of ICzTRZ as a TADF emitter (Figure 3). The groundstate geometry optimization was carried out in the gas phase. The nature of the S 1 and T 1 states and their corresponding energies were then obtained using the Tamm-Dancoff approximation (TDA) [22] to TD-DFT. ICzTRZ possesses a ΔE ST of 0.22 eV and an S 1 energy of 2.92 eV. The high oscillator strength (f) of 0.72 is helpful for favoring radiative decay. There are additionally two intermediate triplet excited states residing between S 1 and T 1 , which is beneficial as it has previously been shown that a density of triplet states can mediate a vibrationally assisted reverse internal conversion that leads to an enhanced efficiency RISC process. [23][24][25][26][27] The T 3 state is locally excited in nature (see Table S1, Supporting Information) and thus RISC from T 3 to S 1 will be facile according to El-Sayed's rule. [28] The HOMO and LUMO of the molecule are mainly localized, respectively, on the donor and the acceptor moieties. ICzTRZ presents a small permanent dipole moment (PDM) of 0.3 Debye, but a much larger transition dipole moment (TDM) of 7.9 Debye.

Optoelectronic Properties
We first studied the electrochemical properties of ICzTRZ in degassed DCM with tetra-n-butylammonium hexafluorophosphate as the electrolyte and Fc/Fc + as the internal reference ( Figure 4a). Unlike what is typically observed for carbazolebased oxidations, which are known to be irreversible, [21,[29][30][31][32] we observed a reversible oxidation wave with E ox at 0.96 V measured from the DPV. By contrast, the reduction wave at −1.83 V is irreversible. The corresponding HOMO and LUMO levels inferred from the DPV are −5.66 eV and −3.17 eV, respectively. The UV-vis absorption spectrum in toluene coincides broadly with the absorption spectra of other indolocarbazolebased compounds (Figure 4b). [11] There are a number of high-intensity absorption bands from 280 to 350 nm that are assigned both to charge-transfer (CT) transitions (for the bands at 321 nm and 331 nm) and locally excited (LE) transitions on the indolocarbazole (for the band at 340 nm) (confirmed by DFT calculations, see Table S3, Supporting Information). There are low intensity bands around 400 nm that are assigned to a charge-transfer (CT) transition between the ICz and the TRZ acceptor moieties. In the ground state, there is a small dipole, supported by DFT calculations (see Figure 3), reflective of the lack of solvatochromism present in the absorption spectra ( Figure 4d). This can be understood by the cancellation of the large CT dipoles that originate from the transitions from the ICz to the TRZ. The photoluminescence (PL) spectrum in toluene is broad and unstructured, indicating an emission from a CT state, which was expected from the large TDM calculated from TDA-DFT (see Figure 3). The emission maximum, λ PL , is centered at 462 nm in toluene. The PL spectra are sensitive to solvent polarity. With increasing solvent polarity from toluene to DCM, there is a characteristic red-shifting of the emission coupled with a broadening of the spectrum. However, in methylcyclohexane (least polar solvent), the emission is structured, reflecting a radiative decay from a LE state on the ICz moiety, also predicted from DFT calculations (see Figure S10, Supporting Information).
Transient PL measurements in degassed toluene showed monoexponential decay kinetics with τ p of 9.0 ns for the prompt fluorescence and τ d of 229.2 µs for the delayed fluorescence. Upon exposure to oxygen, the delayed emission could no longer be detected. The Φ PL in degassed toluene was high at 72%, which decreased to 56% after the solution was exposed to air ( Table 1).   We next investigated the solid-state photophysical behavior in a number of different host matrices (Table 1). We started by spin-coating 10 wt% doped film in PMMA to study ICzTRZ in a non-polar matrix. The film presented emission in the blue region at λ PL of 470 nm and a Φ PL of 31% under N 2 . Transient PL measurements were then carried out (Figure 5). A prompt fluorescence (see Figure S13, Supporting Information) with a biexponential decay kinetics and an average τ p of 11.5 ns [τ 1 = 7.6 ns (62%), τ 2 = 17.6 ns (38%)], and a delayed component with a biexponential decay with an average τ d of 252.8 µs [τ 1 = 85.5 µs (51%), τ 2 = 427.3 µs (49%)] were observed. A study involving a range of hosts and concentrations then revealed that the Φ PL was highest at 5 wt% emitter doping in mCBP (70%, a value comparable to that obtained in degassed toluene); this decreased to 59% when the film was exposed to air. In mCBP, ICzTRZ shows sky-blue emission at λ PL of 479 nm, a maximum only slightly red-shifted compared to measurements in toluene and PMMA. Transient PL measurements in mCBP (Figure 5a) showed prompt fluorescence with a biexponential decay kinetics and an average τ p of 8.7 ns [τ 1 = 4.5 ns (34%), τ 2 = 10.9 ns (66%)], and a delayed fluorescence with biexponential decay kinetics and an average τ d of 121.09 µs [τ 1 = 19.1 µs (26%), τ 2 = 157.0 µs (74%)]. The multiexponential decay kinetics in both PMMA and mCBP reflect a heterogeneity of environments experienced by the ICzTRZ emitter. The TADF nature of the material was corroborated by measuring the temperature dependence of the delayed emission, which shows the characteristic increased contribution with increasing temperature (Figure 5b). The ΔE ST was obtained from the onset of the prompt emission and phosphorescence spectra, measured at 77 K, with a value of 0.23 eV (Figure 5c). This value is in excellent agreement with the TDA-DFT calculations of 0.22 eV.

Orientation Measurements
The anisotropy factor (a) for ICzTRZ was measured in several OLED-relevant host materials ( Table 2). [33] The doped films (10 wt% ICzTRZ in the host) were obtained via vacuum-deposition and the orientation of the emitter molecule was then measured using polarization-and angle-dependent luminescence spectroscopy, after which the data were analyzed by optical simulation (see Supporting Information for details). ICzTRZ presents a nearly completely horizontal orientation in all the host materials tested (see Figure S14, Supporting Information) with the best results obtained from the DPEPO doped film, with an anisotropy factor of 0.06. In the host mCBP (Figure 6), used for OLEDs, the orientation is a little less horizontal but the anisotropy value of 0.09-that is, 91% horizontal dipole contribution-is still among the best values reported in literature. Comparing the anisotropy factor for ICzTRZ to those of other carbazole-and triazine-based materials such as TCzTRZ [9] (a = 0.05), CC2TA [34] (a = 0.08), and 4CzTPN-Ph [35] (a = 0) we can observe the positive trend that large and flat molecules have on the anisotropy factor. Also, the longer shape of ICzTRZ led to a significant improvement in anisotropy values over the previously reported indolocarbazole emitters, IndCzpTr-1, and IndCzpTr-2 (a = 0.27 and 0.16, respectively). [11]

OLED Devices
We next fabricated OLEDs using the following optimized device structure: ITO/TAPC (40 nm)/TCTA (10 nm)/5 wt% ICzTRZ:mCBP (20 nm)/bis [2-(diphenylphosphino)phenyl] ether oxide (DPEPO) (10 nm)/TmPyPb (50 nm)/LiF (0.6 nm)/Al (100 nm). Indium tin oxide (ITO) is the semitransparent anode, 4,4′-cyclohexylidenebis[N,N-bis(4-methylphenyl)benzenamine] (TAPC) and tris(4-carbazoyl-9-ylphenyl)amine (TCTA) act as hole transport layers, 3,3′-di(9H-carbazol-9-yl)-1,1′-biphenyl (mCBP) Measured at room temperature; b) λ exc = 340 nm; c) Obtained via the optically dilute method (see Supporting Information), λ exc = 360 nm; d) τ p (prompt lifetime) and τ d (delayed lifetime) were obtained from the transient PL decay of degassed solution/doped film, λ exc = 378 nm; e) S 1 was obtained from the onset of the prompt emission measured at room temperature; f) T 1 was obtained from the onset of the phosphorescence spectrum measured at 77 K; g) ΔE ST = S 1-T 1 .  and Figure S15, Supporting information). The electroluminescence (EL) spectrum shows sky-blue emission with a λ EL at 483 nm and associated CIE chromaticity coordinates of (0.17, 0.32) (Figure 7b). Figure 7c shows the EQE versus luminance curve. The OLED shows an EQE max of 22.1% at 16 cd m −2 . At 100 cd m −2 , the EQE 100 value is 17.3%, a decrease of 22% from its maximum value. At 1000 cd m −2 , the EQE 1000 values decrease to 9.1%. The device shows a low turn-on voltage (≈3.5 eV) and brightness level reaching 7805 cd m −2 (EQE = 2.5%) ( Table 3). The out-coupling efficiency was inferred to be 31% based on the EQE max value, the high Φ PL of 70%, and considering the charge balance and exciton utilization efficiency are each 100% (see Device and Device Simulation in Supporting Information). This is confirmed by optical simulations of the device EQE as discussed in detail in the Supporting Information ( Figure S16, Supporting Information). As shown in Figure 8, using the above measured input parameters (PLQY and a) together with the optical constants of the given OLED layer stack, we predict an EQE of 21% in excellent agreement with the experimental result. Moreover, the simulation clearly demonstrates that at the given PLQY of 70%, a device EQE in excess of 20% is only possible due to the strong horizontal orientation of the emitter TDMs. If they were randomly oriented in the film, the device EQE would just be around 15%. Thus, we can infer that the molecules are strongly horizontally oriented in the device, in line with our thin-film orientation measurements.  where the PL quantum yield (vertical axis) and the anisotropy factor (horizontal axis) are varied to visualize the influence of both parameters. The blue star marks the predicted EQE for the measured PLQY and the a parameter, which is in excellent agreement with the experimental result. V on , turn on voltage; b) EQE, external quantum efficiency; c) Subscript 100 and 1000 refer to values taken at 100 and 1000 cd m -2 ; d) L, luminance; e) CIE, in Commission Internationale de L'Éclairage coordinates; f) EL, electroluminescence.