Highly Efficient Full-Color Thermally Activated Delayed Fluorescent

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Highly efficient full-color thermally activated delayed fluorescent organic light-emitting diodes: extremely low efficiency roll-off utilizing a host with small singlet-triplet splitting Dongdong Zhang, Chongguang Zhao, Yunge Zhang, Xiaozeng Song, Pengcheng Wei, Minghan Cai, and Lian Duan ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b15272 • Publication Date (Web): 17 Jan 2017 Downloaded from http://pubs.acs.org on January 19, 2017

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ACS Applied Materials & Interfaces

Highly efficient full-color thermally activated delayed fluorescent organic light-emitting diodes: extremely low efficiency roll-off utilizing a host with small singlet-triplet splitting Dongdong Zhang, Chongguang Zhao, Yunge Zhang, Xiaozeng Song, Pengcheng Wei, Minghan Cai, Lian Duan* Key Lab of Organic Optoelectronics and Molecular Engineering of Ministry of Education, Department of Chemistry, Tsinghua University, Beijing 100084, China.

ABSTRACT Numerous efforts have been devoted to boost the efficiency of thermally activated delayed fluorescence (TADF) devices; however, strategies to suppress the device efficiency rolloff are still in urgent need. Here, a general and effective approach to suppress the efficiency rolloff of TADF devices is proposed, that is utilizing TADF materials as the hosts for TADF emitters. Bearing small singlet-triplet splitting (∆EST) with donor and acceptor units, TADF materials as the hosts possess the potential to achieve matched frontier energy levels with the adjacent transporting layers, facilitating balanced charge injection, as well as bipolar charge transport mobilities, beneficial to the balanced charges transportation. Furthermore, an enhanced Förster energy transfer from the host to the dopant can be anticipated, helpful to reduce the

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exciton concentration. Based on the principles, a new TADF material based on indeno[2,1b]carbazole/1,3,5-triazin derivation is synthesized and used as the universal host for the fullcolor TADF devices. Remarkable low efficiency roll-off was achieved with above 90% of the maximum external quantum efficiencies (EQEmaxs) maintained even at a brightness of 2000 cd/m2, along with EQEmaxs of 23.2%, 21.0% and 19.2% for orange, green and sky-blue TADF devices, respectively. Through computational simulation, we identified the suppressed exciton annihilation rates compared with devices adopting conventional hosts. The state-of-the-art low efficiency roll-off of those TADF devices manifests the great potential of such host design strategy, paving an efficient strategy toward their practical application.

KEYWORDS (organic light-emitting diodes, thermally activated delayed fluorescence, high efficiency, low efficiency roll-off, exciton annihilation)

INTRODUCTION Received concept has it that both the spin-symmetric and anti-symmetric molecular excitons formed under electrical excitation should be harnessed to maximize the luminous efficiencies of organic light-emitting diodes (OLEDs). This is possible if the emitters are phosphors, which, however, suffer from the high cost of the noble metals and the lack of stable blue emitters. Therefore, alternative approaches are intensively explored to replace phosphors, among which thermally activated delayed fluorescence (TADF) is the most promising strategy.1-3 Bearing small singlet-triplet splitting (∆EST), TADF emitters take advantages of the efficient triplet up-conversion process, achieving unity quantum efficiency.4-6


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Albeit their high efficiencies, TADF OLEDs often suffer a significant efficiency roll-off, which is an obstacle for their further applications.7,8 Recently, imbalanced charge recombination, as well as singlet-triplet and triplet-triplet annihilation (STA and TTA), which are also correlated with the distribution of the carrier density in the emitting layers, are reported to be the main causes of the efficiency roll-off in TADF OLEDs, indicating that the control of the carrier balance is vital in suppressing device efficiency roll-off.9,10 By shifting the recombination zone away from the interface of the emitting layer (EML) with the electron transporting layer through tuning the carrier balance of the host, the efficiency roll-off of TADF OLEDs has been reported to be suppressed, evidencing the importance of hosts engineering in promoting the device performances.11 Multifunctional bipolar hosts instead of unipolar ones have been widely adopted to improve device performances.12, 13 It is often stated that balanced charges in the EML can be realized utilizing bipolar hosts exhibiting comparable electron- and hole-transporting mobilities. However, this concept is too simplistic as the crucial role of charge injection balance is neglected. Essentially, high T1s of the conventional hosts, required to fit the excited level of the guest, are always accompanied with even higher S1s due to their large ∆ESTs hindering the charge injection from the adjacent layers and leading to unbalanced charges in the EML consequently.14,15 Therefore, both charge mobilites as well as charge injections should be taken into consideration to fulfill balanced charges in the EML. To promote balanced charge injection, a narrow energy gap, namely a low energy S1 of the host is required, referring to a small ∆EST given that a high T1 energy is necessary. Bearing with donor and acceptor moieties as well as small ∆EST, TADF materials as the hosts show the potential to achieve balanced charge injection and transporting mobilities simultaneously. In our previous work, by tuning the ∆EST of a series of indocarbazole/triazine derivations, balanced


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charge transportation and injection were realized in PHOLEDs, resulting in high efficiency with decreased efficiency roll-off.16 Also, benefiting from the small ∆EST, which facilitates charge injection, significant reduced driving voltage was achieved.17 Additionally, when the ∆EST of the host is small enough to trigger TADF process, a bonus can be anticipated, that is an enhanced long range Förster energy transfer (FET) from the host S1 to the guest S1 or T1, achieving more efficient energy transfer compared with the devices adopting conventional hosts where only short-range Dexter interaction (DET) from host T1 to guest T1 occurs.18 The enhanced FET conduces to reduce the triplet exciton density in the devices, rendering low triplet annihilation, which has been validated in both PHOLEDs and fluorescent OLEDs (FOLEDs).19, 20 By that analogy, it can be well assumed that adopting hosts with TADF emission should also contribute to reduce the efficiency roll-off of TADF OLEDs on account of the balanced charges transportation and injection as well as efficient energy transfer. In this manuscript, a new compound, 1,3-dihydro-1,1-dimethyl-3-(3-(4,6-diphenyl-1,3,5triazin-2-yl)phenyl)indeno[2,1-b]carbazole (DMIC-TRZ), possessing a ∆EST as small as 0.14 eV with efficient triplet up-conversion, was designed and synthesized. Bipolar charge transporting ability with moderate frontier energy levels matching the adjacent transporting layers was realized, facilitating charges recombination on the host. Also, efficient FET from host S1 to an orange TADF emitter was demonstrated. A maximum external quantum efficiency (EQEmax) of 23.2% was realized for orange TADF OLEDs, with remarkable EQEs of 22.5% at 2000 cd/m2 and 18.9% at 5000 cd/m2, respectively. The extremely small efficiency roll-off arise from the reduced STA and TTA rate by the enhanced carrier balance in the EML and efficient energy transfer compared with devices with conventional host. Moreover, sky-blue and green TADF OLEDs with DMIC-TRZ as the host also achieved EQEmax as high as 19.2% and 21.0% with


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small efficiency roll-off. The extremely low efficiency roll-off of those devices stands for the state-of-the-art results by now, evidencing the great potential of DMIC-TRZ as a universal host. EXPERIMENTAL SECTION The TADF host, DMIC-TRZ, was synthesized according to the method in the supplementary information. 2,3,4,5,6-pentakis(3,6-di-tert-butyl-9H-carbazol-9-yl)benzonitrile (5TCzBN) and bis(4-(9,9-dimethylacridin-10(9H)-yl)phenyl)methanone (DMAC-BP) were synthesized according to the method reported.21,22 Dipyrazino[2,3-f:2',3'-h]quinoxaline2,3,6,7,10,11-hexacarbonitrile (HATCN) was purchased from LG Chem. Co. Ltd., while N,N'bis(1-naphthalenyl)-N,N'-diphenyl-[1,1'-biphenyl]-4,4'-diamine (NPB) was purchased is from Beijing Visionox Technology Co. Ltd. N,N,N-Tris(4-(9-carbazolyl)phenyl)amine (TCTA), 4,4’Bis(N-carbazolyl)-1,1’-biphenyl

(CBP),

4,6-Bis(3,5-di-3-pyridylphenyl)-2-methylpyrimidine

(B3PyMPM) and bathophenanthroline (Bphen) were purchased from Jilin Optical and Electronic Materials Co. Ltd. The 3,4,5,6-tetrakis(3,6-di-tert-butyl-9H-carbazol-9-yl)benzene-1,2-dinitrile (4TCzTPN) was purchased from Xi’an Polymer Light Technology Co. Ltd. Theoretical Calculations: Computational Details: The geometrical and electronic properties of hosts were performed with the Gaussian 09 program package. The calculation was optimized by means of B3LYP (Becke three parameters hybrid functional with Lee–Yang–Perdew correlation functionals) with the 6-31G(d, p) atomic basis set. The triplet and singlet energy were calculated using time-dependent density functional theory (TD-DFT) calculations with B3LYP/ 6-31G (d, p). The molecular orbitals were visualized using Gaussview 5.0. PL characterization: UV-vis absorption spectra were recorded using an Agilent 8453 spectrophotometer. The films were spin coated on the quartz substrate using dichloromethane as


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the solvent, which was volatilized before the measurement. The concentration of DMIC-TRZ was 10 mg/ml. The thicknesses of the films have little effect on their PL characters. The PL efficiencies and spectra of the films were measured using an absolute photoluminescence quantum yield measurement system (Hamamatsu C11347). The PL transient decay curves of the films were measured using a transient spectrometer (Edinburg FL920P). Device characterization: ITO substrates with a sheet resistance of 15 Ω/□ were cleaned and treated with oxygen plasma before use. The OLEDs were fabricated through vacuum deposition of the materials at 10-6 torr onto the ITO glass. The strucutres of the sky-blue and the green TADF devices are ITO/ HATCN (5 nm)/ NPB (40 nm)/ TCTA (10 nm)/ EMLs (30 nm)/ B3PyMPM (40 nm)/ LiF (1 nm)/ Al (150 nm). Where EMLs are DMIC-TRZ: 30 wt% 5TCzBN and DMIC-TRZ: 12 wt% DMAC-BP for the sky-blue and the green TADF devices, respectively. Current density/ voltage/ luminance (J-V-L) characteristics were obtained using a Keithley 4200 semiconductor characterization systems with an optical power meter. The electroluminescence spectra of the OLEDs were obtained using a multichannel spectrometer (PR650). For measurement of the transient electroluminescence characteristics, short-pulse excitation with a pulse width of 15 µs was generated using Agilent 8114A. The amplitude of the pulse is 8V, and the baseline is -3V. The period is 50 µs, and delayed time is 25 µs while the duty cycle is 30%. The decay curves of devices were detected using the Edinburg FL920P transient spectrometer. RESULTS AND DISCUSSION The synthesis procedure of the DMIC-TRZ was shown in Figure 1a. A large plane conjugate donor unit, 1,3-dihydro-3,3-dimethylindeno [2,1-b]carbazole (DMIC) was chosen here on account of its large steric effect as well as rigid structure, beneficial to form twisted


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structures. A widely used electron deficient unit, 1, 3, 5-triazine (TRZ) was chosen to be the acceptor unit. Normally, a high triplet is necessary for a universal host to meet the high triplet energy level of the emitters, especially for the blue one. To maintain high triplet energy, rather than directly linked to each other, a benzene unit as a spacer between the donor and the acceptor is introduced, which will reduce the interaction between the donor and the acceptor and keep high triplet energy consequently. The chemical structure of the compound was fully characterized by NMR spectroscopy, mass spectroscopy, and elemental analysis. The geometrical and electronic properties of the ground state (S0), the S1 state as well as the T1 state of DMIC-TRZ were calculated based on the density function theory (DFT) to reveal the structure-property relationship of the compound at the molecular level. As can be seen from Figure 1b, a highly twisted configuration structure of DMIC-TRZ ground state was observed with an intersection angle of 53.63

o

between the 2,4,6-triphenyl-1,3,5-triazine planar and the

DMIC unit resulting from the large steric effect of DMIC moiety. A similar intersection angle of 55.18 o was also observed for the first T1 state of DMIC-TRZ while a relatively higher value of 77 o was shown for S1 state. The calculated distribution of the highest occupied molecule orbital (HOMO) and the lowest unoccupied molecule orbital (LUMO) of the ground states shows that the HOMO is mainly located on the DMIC unit while the LUMO on the TRZ unit, realizing separated frontier energy levels as anticipated, which is in favor of small ∆EST. The triplet and singlet energy of DMIC-TRZ was also calculated based on time-dependent DFT to be 2.59 eV and 2.69 eV, respectively, indicating the ∆EST is 0.10 eV. It has been reported that both the HOMO-LUMO overlap of S1 and T1 states affect the ∆EST of the molecules.23 The calculated HOMO and LUMO distribution of S1 and T1 states show that the HOMO-LUMO overlap is significant for T1 state while there is merely no overlap for the S1 state, indicating that the small


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∆EST of DMIC-TRZ is mainly resulted from the separated HOMO-LUMO distribution of S1. Furthermore, the spin density distribution (SDD) of T1 was also calculated (Figure S1), showing that the SDD of T1 is mainly located on the DMIC unit, demonstrating the decreased mutual interaction between DMIC and TRZ as expected, which is beneficial to maintain high triplet energy. Benefiting from the twisted configuration of DMIC-TRZ, which gives rise to the enhanced molecular rigidity, a high glass transition temperature of 142 oC and a melting point approaching 253 oC were observed from Figure S2a, testifying the strong morphological stability of DMICTRZ. Besides, the decomposition temperature of DMIC-TRZ is as high as 373 oC (Figure S2b), indicating the good thermal stability. The twisted rigid structure and good thermal stability of DMIC-TRZ make it easy to form amorphous thin films, which benefit for the device performances. As is shown in Figure 2a, the UV-Vis absorption spectrum of DMIC-TRZ in dilute solution (10-5 mol/L in toluene) consists of three bands peaked at 265 nm, 307 nm and 365 nm, with peaks of 265 nm and 307 nm attributed to the intrinsic n-π* or π-π* transition of DMIC while the one around 365 nm can be assigned to the charge transfer from DMIC to TRZ unit. The assignment is manifested by the separated HOMO and LUMO distribution, which leads to charge transfer (CT) states. The wide emission spectrum peaked at 466 nm, without any vibronic structure also refer to the emission from the CT states. The singlet energy of the CT state emission should be calculated from the onset of their broad emission band, which is 2.94 eV for DMIC-TRZ.24 On the contrary, the phosphorescence spectra at 77 K with a delay time of 20 µs are well resolved, indicating that their T1 states are 3ππ* states, where the highest energy peak of


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its emission identifies its zero-zero energy.24 Accordingly, the ∆EST of DMIC-TRZ hereby can be calculated to be 0.14 eV, corresponding to the theoretical calculated ones. The frontier energy levels of the host are vital in facilitating balanced charge injection from the adjacent transporting layers directly into the host. The frontier energy levels of DMIC-TRZ were measured using cyclic voltammetry. As can be seen from Figure 2b, reversible oxidation and reduction peaks were observed at 1.29 V and -1.20 V, respectively, indicating that both the radical cations and anions are stable entities. The HOMO and the LUMO energy levels are calculated to be 5.76 eV and 3.08 eV, respectively. As is known to all, the HOMOs of the conventional hole transporting materials are usually around 5.5~ 5.7 eV while the LUMO of the electron transporting materials are near 3.0 eV. Therefore, both the HOMO and LUMO energy levels matched well with the adjacent transporting layers, facilitating charge injection on the host. Besides proper frontier energy levels, balanced charge transporting mobilities are also crucial to improve device performance. To verify the ambipolar nature of DMIC-TRZ, the carrier mobilities of the DMIC-TRZ thin film were characterized by the time-of-flight (TOF) transient-photocurrent technique. The configuration of the device is ITO/ DMIC-TRZ (1.0 µm)/ Ag (150 nm). As is shown in Figure 2c, the electron mobility of DMIC-TRZ is 7.35×10-4 cm2/Vs at an electric field of 1.2×106 V/cm-1 while the hole mobility is 1.03×10-4 cm2/Vs, illustrating the bipolar charge transfer characteristics of DMIC-TRZ. The comparable hole and electron mobilities benefit the balanced charges in the EML, conducive to suppressed efficiency roll-off.


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The photo-luminance (PL) decay curve of DMIC-TRZ was shown in Figure 2d. Clear prompt and delayed part of the decay curve were observed, which are the intrinsic features of TADF emitters. The rates of the decay processes of DMIC-TRZ were calculated and summarized in Table S1. Due to the separated HOMO-LUMO overlap of S1 as we mentioned above, the value of k rS of DMIC-TRZ was only 3.4×106 s-1, much lower than that of the efficient TADF emitters.1 But, the reverse intersystem crossing rate ( k RISC ) is calculated to be as high as 1.8×105 s-1, benefiting from its small ∆EST, which leads to the efficient triplet up-conversion efficiency as high as 71%. The device based on pure DMIC-TRZ film as the EML was fabricated, achieving EQEmax of only 1.93% (Figure S3b). The measured electro-luminance (EL) decay curve of the device also shows clear prompt and delayed part (Figure S3d), further evidencing the TADF character of DMIC-TRZ. High triplet, bipolar transportation as well as moderate frontier energy levels of DMIC-TRZ render it versatile universal host for full-color emitters. Here, a sky-blue TADF emitter, 5TCzBN, a green TADF emitter, DMAC-BP and an orange TADF emitter, 4TCzTPN were chosen as the dopant. As can be seen from Figure 3a, all the dopant absorption spectra show significant overlap with the emission of the DMIC-TRZ film and the spectra of the doped films with 6 wt% dopant only show the emission from the emitters, manifesting efficient host-dopant energy transfer. The absolute PL quantum efficiency of the doped films are as high as 70%, 72% and 75% for 5TCzBN, DMAC-BP and 4TCzTPN, accompanied with delayed fluorescence lifetimes (τDF) of 1.84 µs, 2.67 µs and 4.07 µs, respectively (Figure 3b), which indicates the well confined excitons on the dopants.


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Further investigation on the PL decay curves of the films with increased dopant concentration can dig more into the energy transfer process. As can be seen from Figure 3c and Table S2, the lifetimes of the prompt parts of the decay curves of the DMIC-TRZ: 4TCzTPN films are gradually reduced with the dopant concentration increased, derived from the FET from the S1 of DMIC-TRZ to the S1 or T1 of 4TCzTPN, which introduces an additional relaxation path for host S1 and thus reduce τ P . The lifetime of the prompt part ( τ P0% ) of the pure DMIC-TRZ film can be defined as:

τ P0% =

1 k ISC + krS + knrS

(1)

Where k ISC is the rate of the intersystem crossing process while k nrS stands for the nonradiative decay rate of S1. When an additional energy transfer is introduced, the τ Px % of the doped films can be expressed as

τ Px % =

1 x% k ISC + k + knrS + k FET S r

(2)

x% Therefore, the Föster energy transfer rate ( k FET ) can be calculated to be

x% k FET =

1

τ

x% P

−

1

τ P0%

(3)

x% The calculated k FET increases accompanying with the increased dopant concentration as

observed from Figure 3d, which is in the order of 107 s-1 even at a very low dopant concentration of 0.4 wt%, demonstrating the efficient energy transfer. Considering the sufficient x% triplet up-conversion combined with large k FET , an additional energy transfer from the host


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triplet to the dopant, besides from the direct short-range Dexter interaction between the T1 of the host and the guest, can be anticipated, that is the triplet of the host can be up-converted into its singlet ones from where the energy can be transferred to the dopant through FET. Such energy transfer has also been proved in both FOLEDs and PHOLEDs by our group.18, 20 The enhanced efficient FET benefit to reduce the exciton density in the devices, facilitating to reduce the device efficiency roll-off. Furthermore, orange TADF devices with DMIC-TRZ as the host for 4TCzTPN were fabricated with device structure of ITO/ HATCN (5 nm)/ NPB (40 nm)/ TCTA (10 nm)/ EML (30 nm)/ Bphen (40 nm)/ LiF (1 nm)/ Al (150 nm). For comparison, a widely used conventional host, 4,4’-Bis(N-carbazolyl)-1,1’-biphenyl (CBP), was also chosen. The energy diagrams of the devices show that the energy gap of DMIC-TRZ is much smaller than that of CBP though its triplet energy is even higher, accounting for the smaller energy gap between the HOMO or LUMO levels of DMIC-TRZ and 4TCzTPN than those of CBP and 4TCzTPN (Figure 4a and

4b). The current density of the devices based on DMIC-TRZ observed from Figure 4c is independent on the dopant concentration, suggesting that charges are injected and recombined on the host, referred to as Langevin recombination.25 In contrast, the current density of the device based on CBP gradually increased with the increased dopant concentration (Figure 4d), manifesting the direct charge injection and recombination on the dopant, known as the trapassisted recombination.25 The turn-on voltages (Vons) of the devices based on DMIC-TRZ as the host are similar with different dopant concentration, which are in the range of 2.5 V-2.6V. In contrast, the Von of the devices based on CBP as the host is gradually reduced with the increased dopant concentration with Von of 3.1 V at 2wt% and 2.6 V at 6 wt%, also corresponding to the behavior of the trap-assisted recombination.


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Remarkably, a high EQEmax of 23.2% was realized for the DMIC-TRZ based devices at the dopant concentration of 2 wt%, with EQE of 22.5% at 2000 cd/m2 and 18.9% at 5000 cd/m2, referring to the extremely small efficiency roll-off as is shown in Figure 4e. Maximum power efficiency of 70.1 lm/W was also achieved, attributing to the high EQE as well as low operation voltage. Controversially, EQEmax of only 15.4% was observed as well as significant efficiency roll-off with EQE of 13.4% at 2000 cd/m2 and 9.3% at 5000 cd/m2 for the device based on CBP as the host with the same dopant concentration (Figure 4f). As we said above, imbalanced charge recombination, as well as exciton annihilations are responsible for the device efficiency roll-off. The superior efficiency roll-off of the device based on DMIC-TRZ can be attributed to on one hand, the balanced charge mobilities of the host, on the other hand, the facile and balanced charge injection into the host benefiting from both small energy barriers and similar carries motilities for the holes and electrons, which leads to a wide recombination zone across the whole EML. Normally, in TADF devices, taking both the STA and TTA into considerations, the exciton dynamics can be described as:9 dnS J = −( k ISC + k rS ) ns + k RISC nT − kST nS nT + α kTT nT2 + dt 4ω e

(4)

dnT 3J = kISC nS − (kRISC + knrT )nT − (1 + α )kTT nT2 + dt 4ωe

(5)

Where nS and nT donote the singlet and triplet exciton density while k nrS and k nrT stand for the nonradiative decay rates of the singlet and triplet excitons, respectively. kSTA and kTTA are defined as the rate of STA and TTA, respectively. α represents the singlet exciton production ratio under


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electrical excitation, ω is the width of the exciton recombination zone and e is the electron charge. Besides, J refers to the current density. For steady-state current, where both the dnS dt and dnT dt are zero, the brightness is proportional to the number of singlet excitons, and hence the EQE can be expressed as9:

EQE( J ) = EQE max

nS (t = ∞, J ) nS0

(6)

Where nS0 denotes the singlet exciton density without considering the TTA and STA processes. By fitting equation 6 to the experiment data assuming α and ω to be 0.25 and 30 nm, respectively, Figure 4e shows that the model agrees well with the experiment data when the values of kSTA and kTTA are 1.2×10-11 cm3s-1 and 9.0×10-13 cm3s-1 , respectively, for DMIC-TRZ based device, both lower than those of the devices based on CBP as the host, which is 1.0×10-10 cm3s-1 and 1.0×10-12 cm3s-1, respectively (Figure 4f). The suppressed STA and TTA of the device with DMIC-TRZ can be ascribed to, on one hand, the enhanced Langevin recombination arised from the bipolar mobilities and matched frontier energy levels, which broadens the recombination zone. It has been demonstrated that wide recombination zone can reduce the efficiecny roll-off.

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On the other hand, as we discussed above, under electrical excitation,

besides from direct being transferred to the dopant triplet, the formed triplet excitons of the TADF hosts can be up-converted into the singlet ones, from where the energy can be transferred to the dopant through long-range FET mechanism. Such energy transfer have been demosntrated to reduce the T1 concetration in the EML, thus benificial to reduce the T1-related annihilation process.19 In contrast, the wide energy gap resulted from the large ∆EST tends to promote the trap-assisted recombination in the CBP based devices, which will accumulate charges and


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excitons on the dopant under the high current density, accounting for the higher kTTA and kSTA and finally server efficiency roll-off. Sky-blue and green TADF devices were also fabricated to verify the performance of DMICTRZ as a universal host with the energy diagram of the devices shown in Figure 5a. Both the LUMO and HOMO energy levels of 5TCzBN are slightly shallower than those of DMIC-TRZ while the other way around was observed for DMAC-BP with its HOMO and LUMO a little deeper than the host. Both cases facilitate charge recombination on the host in spite of the different energy levels. The electro-luminance spectra of the devices, observed from Figure 5b, only show guest emission peaked at 486 nm and 513 nm for 5TCzBN and DMAC-BP, respectively, evidencing efficient energy transfer. The current density of the sky-blue TADF device is similar to that of the green one (Figure 5c), suggesting that the dopant show slight influence on the transportation of the device, which also indicates that the charge transportation is mainly through the host rather than the guests. Furthermore, state-of-the-art EQEmax as high as 19.2% and 21.0% were achieved for 5TCzBN and DMAC-BP, respectively as can be seen from

Figure 5d. Equally remarkable, both devices show quite small efficiency roll-off. The EQE remains 18.0% at 2000 cd/m2 and 14.2% even at 5000 cd/m2 for 5TCzBN while 19.5% and 14.2% for DMAC-BP, respectively. The device performances are summarized in Table 1. As far as we know, most of the reported devices based on TADF emitters usually suffer from significant efficiency roll-off, hindering their utilization in practical application, especially in the high brightness required lighting field. The extremely small efficiency roll-off of our devices indicates that through subtle host engineering, the efficiency roll-off problem can be well resolved. Furthermore, to our knowledge, universal hosts for high performance TADF OLEDs were rarely reported,26 manifesting the great potential of DMIC-TRZ as the host. Devices with a


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Page 16 of 28

conventional bipolar host, 2,6-bis[3-(9H-Carbazol-9-yl)phenyl]pyridine (26DCzPPy) were also fabricated for comparison. As can be seen from Table 1 and Figure S5, green and blue devices with 26DCzPPy as the host shows maximum EQEs of 19.0% and 15.2%, much lagged behind those with TADF material as the host. In addition, the efficiency roll-offs of the devices with 26DCzPPy is more significant that the ones with TADF material as the host, further demonstrating the advantages of TADF materials as the hosts.

CONCLUSIONS In conclusion, a new designed host with TADF emission was utilized as the universal host for full-color TADF devices, boosting the device performance meanwhile reducing efficiency roll-off. Bearing bipolar charge transport ability, adequate frontier energy levels as well as efficient triplet up-conversion efficiency, DMIC-TRZ as host for TADF emitters realized balanced charge injection and transporting mobilities as well as efficient energy transfer, which are beneficial to suppress the triplet related annihilation. Remarkable EQEmaxs of 23.2%, 21.0% and 19.2% were achieved for orange, green and sky-blue TADF OLEDs respectively, with extremely small efficiency roll-off. The high performances of those full-color devices manifest the great potential of such host design strategy, paving an efficient strategy toward the practical application of TADF devices. We believe that even lower efficiency roll-off can be obtained by subtle tuning of the compounds ∆EST. FIGURES


16

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Figure 1. (a) The synthesis process of DMIC-TRZ. (b) The optimized molecule structures and the distribution of HOMOs and LUMOs of the ground state, S1 and T1.


17


Intensity (a.u.)

Absorption Emission at RT Emission at 77k DMIC-TRZ

a

1.00

b

0.75

0.50

Oxidation in CH 2Cl2

0.25

Reduction in DMF 0.00 300

400

500

600

700

-1.5

-1.0

0.5

Wavelength (nm)

1.0

1.5

Voltage (V)

0.1

c

d

10000

0.01 1E-3

DMIC-TRZ

Counts

1000

1E-4

2

u/cm /Vs

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 28

1E-5 1E-6

Hole Electron

100

10

1E-7 1E-8 700

800

900

1000 1/2

1100

E /(V/cm)

1200

1300

1 0

2

1/2

4

6

8

10

Time (us)

Figure 2. a) Electronic absorption, fluorescence in dilute solution and phosphorescence spectra of DMIC-TRZ. b) The oxidation and reduction curves of DMIC-TRZ. c) The hole and electron transporting mobilities under different electrical field. d) The photo-luminance transient decay curve of pure DMIC-TRZ film.


18

1.0

a

Intensity (a.u.)

0.8

5TCzBN DMAC-BP 4TCzTPN DMIC-TRZ Emission

5TCzBN DMAC-BP 4TCzTPN Absorption

0.6 0.4

1

b

5TCzBN DMAC-BP 4TCzTPN

0.1

0.01

1E-3

0.2 1E-4

0.0 300

400

500

600

2

700

4

6

8

10

12

14

Time (us)

Wavelength (nm) 9

0

10

10

c

d DMIC-TRZ: 0.0 %4TCzTPN DMIC-TRZ: 0.1 %4TCzTPN DMIC-TRZ: 0.2 %4TCzTPN DMIC-TRZ: 0.3 %4TCzTPN DMIC-TRZ: 0.4 %4TCzTPN

k rS knrS + kISC

kFET Decay rate (s-1)

-1

10

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Intensity (a.u.)

Page 19 of 28

-2

10

-3

8

10

7

10

10

6

-4

10

10

0.0

0.5

1.0

1.5

2.0

2.5

Time (us)

0.10

0.15

0.20

0.25

0.30

0.35

0.40

Dopant concentration (wt%)

Figure 3. a) Electronic absorption spectra of 5TCzBN, DMAC-BP and 4TCzTPN as well as the emission spectra of pure DMIC-TRZ film and the doped films. b) The PL transient decay curves of DMIC-TRZ doped films. c) The PL decay curves of DMIC-TRZ: 4TCzTPN with increasing dopant concentration, measured at the 460 nm. The distortion of the curves was attributed to the decay of the light source. d) The values of krS , knrS + kISC and k FET of the doped DMIC-TRZ film with increased dopant concentration.


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Figure 4. a) The energy diagram of the device with CBP as the host. b) The energy diagram of the device with DMIC-TRZ as the host. c) The current density-voltage-brightness curves of the devices with CBP as the host. d) The current density-voltage-brightness curves of the devices with DMIC-TRZ as the host. e) The EQE-current density-power efficiency with CBP as the host and the stimulation of the EQE-current density curve. f) The EQE-current density-power efficiency with DMIC-TRZ as the host and the stimulation of the EQE-current density curve.


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Figure 5. a) The energy diagram of the sky-blue and green TADF device. b) The electroluminance spectra of the sky-blue and green TADF devices. c) The current densityvoltage-brightness of the sky-blue and green TADF devices. d) The power efficiency-brightness as well as the EQE-brightness curves of the sky-blue and green TADF devices.

Table 1. The summary of the device performances. Voltage (V) Emitter

Host

EQE (%)

Power efficiency (lm/W)

1 2 cd/m

2000 2 cd/m

5000 2 cd/m

Max

2000 2 cd/m

5000 2 cd/m

Max

2000 2 cd/m

5000 2 cd/m

4TCzTPN

DMIC-TRZ

2.56

3.47

3.94

23.2

22.5

18.9

70.1

59.7

44.8

4TCzTPN

CBP

3.12

4.38

5.44

15.4

13.4

9.3

43.7

30.3

16.7

DMAC-BP

DMIC-TRZ

2.76

4.17

5.01

21.0

19.5

14.2

52.9

42.2

25.7

DMAC-BP

26DCzPPy

3.54

6.08

7.65

19.0

14.4

9.8

38.4

21.7

11.6

5TCzBN

DMIC-TRZ

2.76

4.33

5.22

19.2

18.0

14.2

38.8

30.3

19.7

5TCzBN

26DCzPPy

3.54

6.81

9.29

15.2

9.7

5.5

23.0

9.9

4.3


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AUTHOR INFORMATION

Corresponding Author *Correspondence to: E-mail address: [email protected]. Fax: +86 10 62795137; Tel: +86 10 62782197

Notes The authors declare no competing financial interests. ACKNOWLEDGMENT We would like to thank the National Key Basic Research and Development Program of China (Grant No. 2015CB655002), the National Science Fund for Distinguished Young Scholars of China (Grant No. 51525304) and the CAS "Interdisciplinary Innovation Team" for ﬁnancial support. SUPPORTING INFORMATION The general information about the experiments. Molecule synthesis procedure. The spin density distribution of the host triplet. Thermal properties of the host. The performances of the device with DMIC-TRZ as the emitter. The performances of the orange TADF devices. Physical properties of DMIC-TRZ and 4TCzTPN. The physical properties of the DMIC-TRZ: 4TCzTPN films. The performances of the green and blue TADF devices. This material is available free of charge via the Internet at http://pubs.acs.org. REFERENCES (1) Uoyama, H.; Goushi, K.; Shizu, K.; Nomura, H.; Adachi, C. Highly Efficient Organic Lightemitting Diodes from Delayed Fluorescence. Nature. 2012, 492, 234-240.


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Table of Contents graphic


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Highly Efficient Full-Color Thermally Activated Delayed Fluorescent

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