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Hui Wang, Lingqiang Meng, Xingxing Shen, Xiaofang Wei, Xiuli Zheng, Xiaopeng Lv, Yuanping Yi, Ying Wang,* and Pengfei Wang* Phosphorescent organic light-emitting diodes (PHOLEDs) have attracted much attention owing to their exceptionally high theoretical internal quantum efficiency of 100%, which is desirable to achieve high-performance OLEDs for flat-panel displays and solid-state lighting.[1–5] Over the past decades, enormous efforts have been endeavored, both in academic and industrial institute, to get high-performance OLEDs. Highly efficient PHOLEDs with external quantum efficiency (EQE) of 30% have been achieved, which is considered to be the light-outcoupling efficiency limit of PHOLEDs without special structures of light extraction.[6–8] However, the efficiency of PHOLEDs tends to decrease with increasing brightness, the so-called efficiency roll-off. This will impede their future applications with high brightness.[9] Design strategies for materials and devices have been employed to enhance the high-brightness performance of PHOLEDs by decreasing the exciton lifetime, reducing molecular aggregation, broadening the recombination zone, and reducing the Förster radius, and triplet management.[9] Holmes et al.[10] had reported the wide exciton recombination zone used in graded-emissive layer (G-EML) devices to be an effective strategy to reduce the roll-off at high exciton density. Kim et al.[11] reported red and green PHOLEDs with high efficiency, low driving voltage, and an extremely low efficiency roll-off with an exciplex forming H. Wang, L. Meng, X. Wei, X. Zheng, Prof. Y. Wang, Prof. P. Wang Key Laboratory of Photochemical Conversion and Optoelectronic Materials Technical Institute of Physics and Chemistry Chinese Academy of Sciences Beijing 100190, China E-mail: [email protected]; [email protected] H. Wang, L. Meng, X. Wei, X. Zheng, Prof. Y. Wang, Prof. P. Wang University of Chinese Academy of Sciences Beijing 100049, China X. Shen, Prof. Y. Yi Beijing National Laboratory for Molecular Sciences Key Laboratory of Organic Solids Institute of Chemistry Chinese Academy of Sciences Beijing, 100190, China X. Lv Institute of Functional Nano and Soft Materials Soochow University Suzhou 215123, China
DOI: 10.1002/adma.201501373
Adv. Mater. 2015, 27, 4041–4047
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Highly Efficient Orange and Red Phosphorescent Organic Light-Emitting Diodes with Low Roll-Off of Efficiency using a Novel Thermally Activated Delayed Fluorescence Material as Host
cohost. Ma et al.[12] developed an unsymmetrical oligomer-like host materials of fluorenyl–diphenylphosphine oxide featuring independent energy transfer (ET) and charge transfer (CT) channels, which provide a feasible chemical approach to suppress TTA and TPA. Nevertheless, the critical current density J0, the current density at which the EQE drops to half of its maximum value, is still low. The reduction of the efficiency roll-off remains challenging for high efficient PHOLEDs. The triplet–triplet annihilation (TTA) and triplet–polaron annihilation (TPA) of the host materials are most relevant to the efficiency roll-off in PHOLEDs. Thus, it is promising and efficient to reduce the triplet exciton density and thus improve the roll-off behavior by delicate tuning of the optoelectronic properties of the host materials. Recently, Adachi and Qiu’s group utilized the thermally activated delayed fluorescence (TADF) molecules as assistant dopants, and achieved fluorescence-based OLEDs with EQE over 10%.[13,14] Lee’s groups used the exciplex with TADF properties as the host of the fluorescent emitters to achieve high performance OLEDs with high EQE.[15] The TADF molecules in these devices acted as the main carrier recombination centers. The triplet excitons were upconverted to the singlet state of TADF molecules and then resonantly transferred to the singlet of emitter molecules for light emission via the Förster resonant energy transfer (FRET) process. These interesting results inspired us to use TADF molecules as the host for PHOLEDs. The upconversion from triplet to singlet of the TADF hosts will decrease the triplet exciton density on the hosts and probably reduce the efficiency roll-off of PHOLEDs. In this communication, we report the host material of red and orange PHOLEDs with TADF properties, MTXSFCz (Figure 1a), in which the hole-transporting phenylcarbazole (PhCz) and electron-withdrawing thioxanthone (TX) unit were connected with the unconjugated bridge of spirofluorene. MTXSFCz exhibited a small singlet-triplet energy gap (ΔEST) of 0.19 eV, affording efficient reverse intersystem crossing (ISC), and TADF properties. High efficiency red and orange PHOLEDs based on the MTXSFCz host with a maximum EQE of 15.6% and 11.8% were achieved, and the efficient reverse ISC of MTXSFCz endowed the devices with low efficiency roll-off behavior by reducing the triplet density on the host and thus the diminished TTA and TPA. The overall performances of these devices are among the best results of conventional PHOLEDs based on small molecules, which promises MTXSFCz a bright future as the host of PHOLEDs.
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Figure 1. a) Molecular structure of MTXSFCz; b) MTXSFCz’ HOMO and LUMO calculated by Gaussian 03 at the B3LYP/6-31G(d) level; c) absorption spectra, fluorescence spectra in solutions, and neat film of MTXSFCz at room temperature and phosphorescence spectrum in 2-methyltetrahydrofuran at 77 K; d) fluorescence spectra in solutions with different polarity.
MTXSFCz was synthesized by Suzuki coupling reaction of the TX and spirofluorene-linked-PhCz as outlined in Scheme S1 (for the details of the synthesis, see the Supporting Information). To evaluate the frontier molecular orbitals and predict ΔEST of MTXSFCz, density functional theory (DFT) calculation was performed with the Gaussian 09 package at the B3LYP/ 6-31G** level (Table S1, Supporting Information). Similar to the reported TADF emitters and bipolar phosphorescent hosts,[1,16] the highest occupied molecular orbital (HOMO) of MTXSFCz was mainly dispersed over the electron-donating phenylcarbazole moiety, while its lowest unoccupied molecular orbital (LUMO) was localized on the electron-withdrawing TX moiety. No obvious overlap between the HOMO and LUMO can be observed due to the effective separation of electron densities of HOMO and LUMO by the unconjugated fluorene moiety. This indicates a small energy difference between its singlet and triplet state.[17,18] Based on the ground-state geometries, the small ΔEST value of MTXSFCz was calculated to be 0.06 eV, which is comparable to reported TADF emitters.[6,15,16] The thermal stability of MTXSFCz was investigated by using thermal gravimetric analysis (TGA) and differential scanning calorimetry (DSC) under a nitrogen atmosphere (Figure S3, Supporting Information). MTXSFCz exhibited good thermal stability with a high decomposition temperature (Td) of 456.0 °C and a high glass transition temperature (Tg) of 127.3 °C (Figure S3 and Table S2, Supporting Information), which are higher than those of TX-based TADF emitters and spirofluorene-linked-PhCz host materials.[16,19] Accompanied by the twisted molecular structure of MTXSFCz, such excellent thermal stability is beneficial to
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form homogeneous and amorphous films with high morphological stability, which is very crucial for OLEDs by vacuum deposition.[20] Hole-only and electron-only devices were fabricated to measure the associated carrier mobility, and the hole and electron mobility of MTXSFCz film at zero field can be obtained to be 1.45 × 10−5 and 1.75 × 10−8 cm2 V–1 s–1 (Figure S4, Supporting Information), indicating bipolar charge-transport properties. The photophysical properties of MTXSFCz were examined using UV–vis and photoluminescence (PL) spectrometer (shown in Figure 1c). The absorption peak around 380 nm is attributed to the n–π* transition of the thioxanthone core. The sp3 hybridized carbon in unconjugated fluorene moiety blocks the conjugation and the interaction between donor and acceptor units. There is no prominent charge transfer absorption over 400 nm, although solvatochromism of the emission spectrum can be observed for MTXSFCz in solutions with different polarity. Thus, the CT absorption of MTXSFCz is not very prominent, and may be appeared as a shoulder below the center of the n–π* transition of the thioxanthone core.[21] The energy gap of MTXSFCz can be determined to be 2.92 eV from the onset of the absorption spectrum in the long wavelength. According to the HOMO energy of −5.75 eV from cyclic voltammetry (Figure S3c, Supporting Information), we estimate the LUMO energy level to be −2.83 eV. MTXSFCz in DCM solution emits blue light centered at 420 nm and the full width at half maximum (FWHM) is about 53 nm, smaller than that of CT compounds with strong intramolecular charge transfer properties.[16] These further demonstrate the blocking
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energy transfer between the MTXSFCz and mCP. When the doped film is cooled down to lower temperatures, the emission spectrum of the film shows a blueshift and the relative intensity of the emission peak increases. A similar shift of the PL spectra can also be found for poly(para-phenylene) and polyfluorene. These shifts in the electronic energies can be ascribed to the temperature dependence of the actual relaxation process, whereby the exciton remains more localized with increasing the temperature.[26] Figure 2b shows the photoluminescence decay curves for emission of the MTXSFCz doped film at different temperatures. The curve can be resolved into two components: the prompt component and the delayed component (see Figure S5 in the Supporting Information). For the doped film at 300 K, the transient decay times of prompt component and the delayed component were estimated to be 3.1 ns and 45.3 µs from second-order exponential decay fitting. The photoluminescence spectrum of the delayed component is identical to that of the prompt component (Figure 2d). Thus, the prompt fluorescence can be assigned to the fluorescence of MTXSFCz and the delayed fluorescence can be attributed to the TADF occurring via reverse ISC, which coincides with the small ΔEST value of MTXSFCz. Interestingly, the prompt component decreases as the temperature increases due to the enhancement of nonradiative decay from the S1 state. While the delayed component increases monotonically as the temperature increases, similar to the typical TADF emitters with the rate-determining step of
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of interaction between TX and PhCz units.[16] The emission peak of MTXSFCz in neat film is centered at 451 nm, redshifted about 30 nm compared to that in DCM solution. The PL quantum yield of the neat film is evaluated to be only 5.1% by the integrating sphere. Such a low value is predictable due to the forbidden electronic transition rule arising from the largely separated HOMO and LUMO electron density distribution. The phosphorescence spectrum of MTXSFCz was measured in oxygen-free 2-methyl THF at 77 K and the triplet energy (ET) was estimated to be 2.71 eV from the highest energy vibronic band. Thus, the ΔEST can be estimated to be 0.19 eV, and the small ΔEST coincides with the result from the DFT calculation. The small ΔEST possibly endows MTXSFCz excellent TADF properties with efficient reverse intersystem crossing from triplet to singlet.[22–25] To demonstrate TADF properties of MTXSFCz, the transient PL decay of the doped film of MTXSFCz in 1,3-bis(9H-carbazol9-yl) benzene (mCP) and the temperature dependence were investigated. A doped film instead of a neat film is used to suppress concentration quenching, and the doping concentration is 5 ± 1 wt%. The material mCP is chosen due to its relatively high triplet energy to avoid back energy transfer and thus confine the excitons in MTXSFCz. As shown in Figure 2a, the doped film only emits weak blue light at 300 K with the emission peak centered at 455 nm, similar to that of the neat MTXSFCz film. The fluorescence of mCP is completely quenched by the efficient
Figure 2. Transient PL characteristics of MTXSFCz doped film. a) Temperature dependence of the PL intensity for 5 ± 1 wt% MTXSFCz doped in mCP and b) temperature dependence of the lifetime for 5 ± 1 wt% MTXSFCz doped in mCP. c) Temperature dependence of prompt proportion (red circles), and delayed proportion (blue triangles) for 5 ± 1 wt% MTXSFCz doped in mCP based on lifetime fitting result. d) The photoluminescence spectrum of the same film without delay time and 6 µs delay time.
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reverse ISC (see Table S3 in the Supporting Information).[17] These are analogous to the reported TADF emitters, 4CzIPN,[17] whereas the overall result of both temperature effects is that the PL intensity decreases with the increase of temperature. This is attributed to the much stronger temperature dependence of the prompt component, compared with the delayed component. Considering the low photoluminescence quantum yield and higher singlet/triplet energy levels of MTXSFCz, we are interested in the application of MTXSFCz as the host material of PHOLEDs. The PL emission of MTXSFCz overlaps well with the UV–vis absorption of Ir(piq)2acac and Ir(2-phq)3, indicating efficient Förster energy transfer from MTXSFCz to emitters (Figure S6, Supporting Information). Multilayer OLEDs were fabricated using the doped Ir(2-phq)3 or Ir(piq)2acac:MTXSFCz as the emitting layers, and the molecular structures and energy levels used were shown in Figure 3. The OLED structure was: indium tin oxide (ITO)/PEDOT (30 nm)/TAPC (20 nm)/10 ± 1 wt% Ir(2-phq)3:MTXSFCz (device A) or 10 ± 1 wt% Ir(piq)2acac:MTXSFCz (device B) (35 nm)/TmPyPB (55 nm)/
LiF(0.9 nm)/Al, where poly(3,4-ethylenedioxythiophene) (PEDOT) was used as the hole-injection layer (HIL), 1,1-bis[4[N,N′-di(p-tolyl)amino]phenyl] cyclohexane (TAPC) was used as the hole-transporting layer (HTL), 1,3,5-tri(m-pyrid-3-yl-phenyl) benzene (TmPyPB) was used as electron-transporting layer (ETL) and hole-blocking layer (HBL), and the concentration of emitters was optimized as 10 ± 1 wt% according to the PL quantum yield of the dopant film (as shown in Table S4 in the Supporting Information). Device A was turned on at a voltage of 4.0 V and gave orange electroluminescence centered at 588 nm with color coordinates of CIE (0.56, 0.43). The EL spectra were independent of the applied voltage and there were no derivation or new peaks even at high voltage up to 12 V, indicating balanced charge-carrier injection and transportation into the emissive layer.[27] The device afforded a maximum current efficiency of 26.8 cd A–1, a maximum power efficiency of 18.0 lm W–1, and a maximum external quantum efficiency of 11.8% without any light out-coupling enhancement. Similar excellent performance can also be obtained for device B. The device exhibited
Figure 3. a) Energy level diagrams of device A and device B. b) Related molecular structures. c) Current density–voltage–luminance characteristics. d) The EL spectra operated at different voltages of device A and device B. e) The EQE–luminance characteristics of device A and device B.
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Device
Ir(2-phq)3-based device
Ir(piq)2acacbased device
Voltage [V]
Brightness [cd m–2]
Dopant concentration [wt%]
L.E. [cd A–1]
P.E. [lm W–1]
Max EQE [%]
EQE at 1000 cd m–2 [%]
EQE at 10 000 cd m–2 [%]
Reference
2.4
>10 000
20%
–
16
7.5
6.3
4.1
[28]
4
13 010
2%
23.9
–
12.4
–
–
[29]
3.2
–
3%
23.8
18.8
10.4
8.3
–
[30]
4.0
30 546
10%
26.8
18.0
11.8
11.2
7.6
This work
4.0
33 745
5%
19.6
15.4
8.6
7.9
5.8
This work
2.6
34 973
21%
10.2
9.0
9.6
6.9
–
[31]
3.9
24 978
6%
8.2
2.8
9.2
8.5
–
[32]
4.4
27 031
10%
13.8
8.2
15.6
13.1
8.0
This work
3.8
7608
5%
10.5
8.6
12.0
9.4
–
This work
a turn-on voltage of 4.4 V and color coordinates of CIE (0.68, 0.32). The highest power efficiency, the current efficiency, and the external quantum efficiency of device B can be up to 8.2 lm W–1, 13.8 cd A–1, and 15.6%. To our knowledge, these excellent performances of the devices are among the best of orange and red PHOLEDs (see Table 1).[28–32] More interestingly, both devices showed low efficiency roll-off. For the device B, the EQE can be up to 13.1% with an efficiency roll-off of 16.0% at the luminescence up to 1000 cd m–2 and 8.0% with an efficiency roll-off of 48.7% at the luminescence up to 10 000 cd m–2. The efficiency roll-off of the device B was lower than the red light PHOLEDs based on host materials with both fluorenyl and diphenylphosphine oxide moieties.[12] The EQE–current densities of both devices were fitted according to either the triplet–triplet annihilation (TTA) or triplet–polaron quenching (TPQ) mechanism (Figure S7, Supporting Information).[33] The simulation of triplet–triplet annihilation (TTA) model coincides very well with the experimental curve for both devices, especially under higher current density. Thus, the efficiency roll-off of the devices can be mainly ascribed to the TTA mechanism. The J0 of both devices obtained were 80 mA cm–2 for device A and 174 mA cm–2 for device B, which was much larger than those of reported PHOLEDs.[9] Similar efficiency roll-off of the devices with the doping concentration of 5 ± 1 wt% can also be observed. To investigate the reasons for the efficiency rolloff, a reference device of mCP as the host and Ir(piq)2acac as the emitter (device C) was constructed with similar device configuration since mCP exhibited similar electronic structure to MTXSFCz except much larger ΔEST (above 0.4 eV). Although high EQE up to 12.5% at 1 cd m–2 could also be obtained for the device C, the efficiency roll-off was more severe. The EQE decreased down to 3.6% at a luminance of 10 000 cd m–2 for device C with the efficiency roll-off of 71.2% and J0 value of the device was only 17.9 mA cm–2 (Figure S9 and Table S3, Supporting Information). Thus, we inferred that the fast reverse ISC of MTXSFCz facilitated the reduction of the triplet excitons on host, leading to the reduced efficiency roll-off of the devices. To further confirm that the TADF properties of MTXSFCz alleviated the efficiency roll-off of the PHOLEDs, the decay curves of the Ir(piq)2acac-doped films were investigated. Figure 4
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Table 1. The performance summary of orange and red PhOLEDs based on reported hosts and MTXSFCz.
shows the decays of the emission at 620 nm for 5 ± 1 wt% Ir(piq)2acac:MTXSFCz film and 5 ± 1 wt% Ir(piq)2acac:mCP film in air and vacuum. As the concentration of the doped films was very low, the decay of the excited guest molecules and the Dexter-type energy exchange between guest and host can be neglected. The main emission of the doped film will harvest from the triplet state of the guest via the FRET from the host to guest. The decay time of the Ir(piq)2acac emission in the Ir(piq)2acac:MTXSFCz blend is about 864.44 ns in vacuum, and the decay time can be shortened upon exposure to air, while it only slightly affected the emission properties of the Ir(piq)2acac in mCP (764.35 ns in vacuum and 747.12 ns in air). It is well known that exposure to oxygen reduces the lifetime of the triplet excited states on the phosphorescent and TADF materials. Since the decay of the excited state of the doped mCP film was only slightly affected, the shortening of the lifetime cannot be mainly attributed to the oxygen effect on the phosphorescent guest. The main difference between them is that kinetics of reverse ISC between singlet and triplet of the hosts resulting from the small ΔEST, since the triplet decay rates of both hosts are small due to the missing heavy-atom effect. A similar effect of oxygen on the lifetime of the phosphorescent emitter had been observed for FIrpic in PBD-PVK cohost due to the oxygen induced non-radiative relaxation processes of PBD, in which there is the triplet back transfer of the PBD host to FIrpic.[34] Thus, it can be concluded that the fast decay of the red emission of the Ir(piq)2acac:MTXSFCz blend in the presence of oxygen indicates the storage of the triplet excitons on MTXSFCz. In accordance with these, the steady-state PL intensity of the Ir(piq)2acac:MTXSFCz film is largely affected by the presence of oxygen, while the effect is rather small for the Ir(piq)2acac:mCP film. When all the triplet excitons are confined on the phosphorescent guests by the high triplet energy level of the different hosts, similar decay dynamics of phosphorescent guest can be expected.[35–37] Thus, the lifetime of phosphorescent guest–host system will be similar. Both mCP and MTXSFCz exhibit high triplet energy, at least 0.7 eV higher than that of Ir(piq)2acac. However, the lifetime of 5 ± 1 wt% Ir(piq)2acac:MTXSFCz film in vacuum (864.44 ns) is about 100 ns higher than that of 5 ± 1 wt% Ir(piq)2acac:mCP film in vacuum (764.35 ns). Thus, the
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Figure 4. a) The lifetime of 5 ± 1 wt% Ir(piq)2acac: MTXSFCz film in vacuum and in air. b) The lifetime of 5 ± 1 wt% Ir(piq)2acac: mCP film in vacuum and in air. c) The PL intensity of 5 ± 1 wt% Ir(piq)2acac: MTXSFCz film in vacuum and in air. d) The PL intensity of 5 ± 1 wt% Ir(piq)2acac: mCP film in vacuum and in air.
longer lifetime of 5 ± 1 wt% Ir(piq)2acac:MTXSFCz film can be ascribed to the presence of the process of intersystem crossing and reverse intersystem crossing on photoexcited MTXSFCz. To demonstrate the decrease of the triplet population, and thereby low efficiency roll-off, an important factor is the presence of rapid energy transfer via a Förster process.[38] The rate constant of Förster resonance energy transfer (FRET) from host to guest can be estimated: K ET = (1/τ D ) (R06 /R 6 ) , where τ D is radiative decay time of donor molecular, R0 is the Förster transfer radius, and R is the average distance between donor and acceptor molecules.[39–41] For Ir(piq)2acac:MTXSFCz film, R0 was estimated to be 2.6 nm, larger than the average distance between Ir(piq)2acac guests (2.34 nm for the doping concentration of 5 ± 1 wt% and 1.85 nm for the doping concentration of 10 ± 1 wt%) using a molecular modeling program.[39,40] Thus, the rate constant of FRET is very fast (>1010 s−1), nearly two orders of magnitude faster than those of the intersystem crossing (ISC) and reverse intersystem crossing (RISC) of reported TADF molecules (
Hui Wang, Lingqiang Meng, Xingxing Shen, Xiaofang Wei, Xiuli Zheng, Xiaopeng Lv, Yuanping Yi, Ying Wang,* and Pengfei Wang* Phosphorescent organic light-emitting diodes (PHOLEDs) have attracted much attention owing to their exceptionally high theoretical internal quantum efficiency of 100%, which is desirable to achieve high-performance OLEDs for flat-panel displays and solid-state lighting.[1–5] Over the past decades, enormous efforts have been endeavored, both in academic and industrial institute, to get high-performance OLEDs. Highly efficient PHOLEDs with external quantum efficiency (EQE) of 30% have been achieved, which is considered to be the light-outcoupling efficiency limit of PHOLEDs without special structures of light extraction.[6–8] However, the efficiency of PHOLEDs tends to decrease with increasing brightness, the so-called efficiency roll-off. This will impede their future applications with high brightness.[9] Design strategies for materials and devices have been employed to enhance the high-brightness performance of PHOLEDs by decreasing the exciton lifetime, reducing molecular aggregation, broadening the recombination zone, and reducing the Förster radius, and triplet management.[9] Holmes et al.[10] had reported the wide exciton recombination zone used in graded-emissive layer (G-EML) devices to be an effective strategy to reduce the roll-off at high exciton density. Kim et al.[11] reported red and green PHOLEDs with high efficiency, low driving voltage, and an extremely low efficiency roll-off with an exciplex forming H. Wang, L. Meng, X. Wei, X. Zheng, Prof. Y. Wang, Prof. P. Wang Key Laboratory of Photochemical Conversion and Optoelectronic Materials Technical Institute of Physics and Chemistry Chinese Academy of Sciences Beijing 100190, China E-mail: [email protected]; [email protected] H. Wang, L. Meng, X. Wei, X. Zheng, Prof. Y. Wang, Prof. P. Wang University of Chinese Academy of Sciences Beijing 100049, China X. Shen, Prof. Y. Yi Beijing National Laboratory for Molecular Sciences Key Laboratory of Organic Solids Institute of Chemistry Chinese Academy of Sciences Beijing, 100190, China X. Lv Institute of Functional Nano and Soft Materials Soochow University Suzhou 215123, China
DOI: 10.1002/adma.201501373
Adv. Mater. 2015, 27, 4041–4047
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Highly Efficient Orange and Red Phosphorescent Organic Light-Emitting Diodes with Low Roll-Off of Efficiency using a Novel Thermally Activated Delayed Fluorescence Material as Host
cohost. Ma et al.[12] developed an unsymmetrical oligomer-like host materials of fluorenyl–diphenylphosphine oxide featuring independent energy transfer (ET) and charge transfer (CT) channels, which provide a feasible chemical approach to suppress TTA and TPA. Nevertheless, the critical current density J0, the current density at which the EQE drops to half of its maximum value, is still low. The reduction of the efficiency roll-off remains challenging for high efficient PHOLEDs. The triplet–triplet annihilation (TTA) and triplet–polaron annihilation (TPA) of the host materials are most relevant to the efficiency roll-off in PHOLEDs. Thus, it is promising and efficient to reduce the triplet exciton density and thus improve the roll-off behavior by delicate tuning of the optoelectronic properties of the host materials. Recently, Adachi and Qiu’s group utilized the thermally activated delayed fluorescence (TADF) molecules as assistant dopants, and achieved fluorescence-based OLEDs with EQE over 10%.[13,14] Lee’s groups used the exciplex with TADF properties as the host of the fluorescent emitters to achieve high performance OLEDs with high EQE.[15] The TADF molecules in these devices acted as the main carrier recombination centers. The triplet excitons were upconverted to the singlet state of TADF molecules and then resonantly transferred to the singlet of emitter molecules for light emission via the Förster resonant energy transfer (FRET) process. These interesting results inspired us to use TADF molecules as the host for PHOLEDs. The upconversion from triplet to singlet of the TADF hosts will decrease the triplet exciton density on the hosts and probably reduce the efficiency roll-off of PHOLEDs. In this communication, we report the host material of red and orange PHOLEDs with TADF properties, MTXSFCz (Figure 1a), in which the hole-transporting phenylcarbazole (PhCz) and electron-withdrawing thioxanthone (TX) unit were connected with the unconjugated bridge of spirofluorene. MTXSFCz exhibited a small singlet-triplet energy gap (ΔEST) of 0.19 eV, affording efficient reverse intersystem crossing (ISC), and TADF properties. High efficiency red and orange PHOLEDs based on the MTXSFCz host with a maximum EQE of 15.6% and 11.8% were achieved, and the efficient reverse ISC of MTXSFCz endowed the devices with low efficiency roll-off behavior by reducing the triplet density on the host and thus the diminished TTA and TPA. The overall performances of these devices are among the best results of conventional PHOLEDs based on small molecules, which promises MTXSFCz a bright future as the host of PHOLEDs.
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Figure 1. a) Molecular structure of MTXSFCz; b) MTXSFCz’ HOMO and LUMO calculated by Gaussian 03 at the B3LYP/6-31G(d) level; c) absorption spectra, fluorescence spectra in solutions, and neat film of MTXSFCz at room temperature and phosphorescence spectrum in 2-methyltetrahydrofuran at 77 K; d) fluorescence spectra in solutions with different polarity.
MTXSFCz was synthesized by Suzuki coupling reaction of the TX and spirofluorene-linked-PhCz as outlined in Scheme S1 (for the details of the synthesis, see the Supporting Information). To evaluate the frontier molecular orbitals and predict ΔEST of MTXSFCz, density functional theory (DFT) calculation was performed with the Gaussian 09 package at the B3LYP/ 6-31G** level (Table S1, Supporting Information). Similar to the reported TADF emitters and bipolar phosphorescent hosts,[1,16] the highest occupied molecular orbital (HOMO) of MTXSFCz was mainly dispersed over the electron-donating phenylcarbazole moiety, while its lowest unoccupied molecular orbital (LUMO) was localized on the electron-withdrawing TX moiety. No obvious overlap between the HOMO and LUMO can be observed due to the effective separation of electron densities of HOMO and LUMO by the unconjugated fluorene moiety. This indicates a small energy difference between its singlet and triplet state.[17,18] Based on the ground-state geometries, the small ΔEST value of MTXSFCz was calculated to be 0.06 eV, which is comparable to reported TADF emitters.[6,15,16] The thermal stability of MTXSFCz was investigated by using thermal gravimetric analysis (TGA) and differential scanning calorimetry (DSC) under a nitrogen atmosphere (Figure S3, Supporting Information). MTXSFCz exhibited good thermal stability with a high decomposition temperature (Td) of 456.0 °C and a high glass transition temperature (Tg) of 127.3 °C (Figure S3 and Table S2, Supporting Information), which are higher than those of TX-based TADF emitters and spirofluorene-linked-PhCz host materials.[16,19] Accompanied by the twisted molecular structure of MTXSFCz, such excellent thermal stability is beneficial to
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form homogeneous and amorphous films with high morphological stability, which is very crucial for OLEDs by vacuum deposition.[20] Hole-only and electron-only devices were fabricated to measure the associated carrier mobility, and the hole and electron mobility of MTXSFCz film at zero field can be obtained to be 1.45 × 10−5 and 1.75 × 10−8 cm2 V–1 s–1 (Figure S4, Supporting Information), indicating bipolar charge-transport properties. The photophysical properties of MTXSFCz were examined using UV–vis and photoluminescence (PL) spectrometer (shown in Figure 1c). The absorption peak around 380 nm is attributed to the n–π* transition of the thioxanthone core. The sp3 hybridized carbon in unconjugated fluorene moiety blocks the conjugation and the interaction between donor and acceptor units. There is no prominent charge transfer absorption over 400 nm, although solvatochromism of the emission spectrum can be observed for MTXSFCz in solutions with different polarity. Thus, the CT absorption of MTXSFCz is not very prominent, and may be appeared as a shoulder below the center of the n–π* transition of the thioxanthone core.[21] The energy gap of MTXSFCz can be determined to be 2.92 eV from the onset of the absorption spectrum in the long wavelength. According to the HOMO energy of −5.75 eV from cyclic voltammetry (Figure S3c, Supporting Information), we estimate the LUMO energy level to be −2.83 eV. MTXSFCz in DCM solution emits blue light centered at 420 nm and the full width at half maximum (FWHM) is about 53 nm, smaller than that of CT compounds with strong intramolecular charge transfer properties.[16] These further demonstrate the blocking
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energy transfer between the MTXSFCz and mCP. When the doped film is cooled down to lower temperatures, the emission spectrum of the film shows a blueshift and the relative intensity of the emission peak increases. A similar shift of the PL spectra can also be found for poly(para-phenylene) and polyfluorene. These shifts in the electronic energies can be ascribed to the temperature dependence of the actual relaxation process, whereby the exciton remains more localized with increasing the temperature.[26] Figure 2b shows the photoluminescence decay curves for emission of the MTXSFCz doped film at different temperatures. The curve can be resolved into two components: the prompt component and the delayed component (see Figure S5 in the Supporting Information). For the doped film at 300 K, the transient decay times of prompt component and the delayed component were estimated to be 3.1 ns and 45.3 µs from second-order exponential decay fitting. The photoluminescence spectrum of the delayed component is identical to that of the prompt component (Figure 2d). Thus, the prompt fluorescence can be assigned to the fluorescence of MTXSFCz and the delayed fluorescence can be attributed to the TADF occurring via reverse ISC, which coincides with the small ΔEST value of MTXSFCz. Interestingly, the prompt component decreases as the temperature increases due to the enhancement of nonradiative decay from the S1 state. While the delayed component increases monotonically as the temperature increases, similar to the typical TADF emitters with the rate-determining step of
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of interaction between TX and PhCz units.[16] The emission peak of MTXSFCz in neat film is centered at 451 nm, redshifted about 30 nm compared to that in DCM solution. The PL quantum yield of the neat film is evaluated to be only 5.1% by the integrating sphere. Such a low value is predictable due to the forbidden electronic transition rule arising from the largely separated HOMO and LUMO electron density distribution. The phosphorescence spectrum of MTXSFCz was measured in oxygen-free 2-methyl THF at 77 K and the triplet energy (ET) was estimated to be 2.71 eV from the highest energy vibronic band. Thus, the ΔEST can be estimated to be 0.19 eV, and the small ΔEST coincides with the result from the DFT calculation. The small ΔEST possibly endows MTXSFCz excellent TADF properties with efficient reverse intersystem crossing from triplet to singlet.[22–25] To demonstrate TADF properties of MTXSFCz, the transient PL decay of the doped film of MTXSFCz in 1,3-bis(9H-carbazol9-yl) benzene (mCP) and the temperature dependence were investigated. A doped film instead of a neat film is used to suppress concentration quenching, and the doping concentration is 5 ± 1 wt%. The material mCP is chosen due to its relatively high triplet energy to avoid back energy transfer and thus confine the excitons in MTXSFCz. As shown in Figure 2a, the doped film only emits weak blue light at 300 K with the emission peak centered at 455 nm, similar to that of the neat MTXSFCz film. The fluorescence of mCP is completely quenched by the efficient
Figure 2. Transient PL characteristics of MTXSFCz doped film. a) Temperature dependence of the PL intensity for 5 ± 1 wt% MTXSFCz doped in mCP and b) temperature dependence of the lifetime for 5 ± 1 wt% MTXSFCz doped in mCP. c) Temperature dependence of prompt proportion (red circles), and delayed proportion (blue triangles) for 5 ± 1 wt% MTXSFCz doped in mCP based on lifetime fitting result. d) The photoluminescence spectrum of the same film without delay time and 6 µs delay time.
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reverse ISC (see Table S3 in the Supporting Information).[17] These are analogous to the reported TADF emitters, 4CzIPN,[17] whereas the overall result of both temperature effects is that the PL intensity decreases with the increase of temperature. This is attributed to the much stronger temperature dependence of the prompt component, compared with the delayed component. Considering the low photoluminescence quantum yield and higher singlet/triplet energy levels of MTXSFCz, we are interested in the application of MTXSFCz as the host material of PHOLEDs. The PL emission of MTXSFCz overlaps well with the UV–vis absorption of Ir(piq)2acac and Ir(2-phq)3, indicating efficient Förster energy transfer from MTXSFCz to emitters (Figure S6, Supporting Information). Multilayer OLEDs were fabricated using the doped Ir(2-phq)3 or Ir(piq)2acac:MTXSFCz as the emitting layers, and the molecular structures and energy levels used were shown in Figure 3. The OLED structure was: indium tin oxide (ITO)/PEDOT (30 nm)/TAPC (20 nm)/10 ± 1 wt% Ir(2-phq)3:MTXSFCz (device A) or 10 ± 1 wt% Ir(piq)2acac:MTXSFCz (device B) (35 nm)/TmPyPB (55 nm)/
LiF(0.9 nm)/Al, where poly(3,4-ethylenedioxythiophene) (PEDOT) was used as the hole-injection layer (HIL), 1,1-bis[4[N,N′-di(p-tolyl)amino]phenyl] cyclohexane (TAPC) was used as the hole-transporting layer (HTL), 1,3,5-tri(m-pyrid-3-yl-phenyl) benzene (TmPyPB) was used as electron-transporting layer (ETL) and hole-blocking layer (HBL), and the concentration of emitters was optimized as 10 ± 1 wt% according to the PL quantum yield of the dopant film (as shown in Table S4 in the Supporting Information). Device A was turned on at a voltage of 4.0 V and gave orange electroluminescence centered at 588 nm with color coordinates of CIE (0.56, 0.43). The EL spectra were independent of the applied voltage and there were no derivation or new peaks even at high voltage up to 12 V, indicating balanced charge-carrier injection and transportation into the emissive layer.[27] The device afforded a maximum current efficiency of 26.8 cd A–1, a maximum power efficiency of 18.0 lm W–1, and a maximum external quantum efficiency of 11.8% without any light out-coupling enhancement. Similar excellent performance can also be obtained for device B. The device exhibited
Figure 3. a) Energy level diagrams of device A and device B. b) Related molecular structures. c) Current density–voltage–luminance characteristics. d) The EL spectra operated at different voltages of device A and device B. e) The EQE–luminance characteristics of device A and device B.
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Device
Ir(2-phq)3-based device
Ir(piq)2acacbased device
Voltage [V]
Brightness [cd m–2]
Dopant concentration [wt%]
L.E. [cd A–1]
P.E. [lm W–1]
Max EQE [%]
EQE at 1000 cd m–2 [%]
EQE at 10 000 cd m–2 [%]
Reference
2.4
>10 000
20%
–
16
7.5
6.3
4.1
[28]
4
13 010
2%
23.9
–
12.4
–
–
[29]
3.2
–
3%
23.8
18.8
10.4
8.3
–
[30]
4.0
30 546
10%
26.8
18.0
11.8
11.2
7.6
This work
4.0
33 745
5%
19.6
15.4
8.6
7.9
5.8
This work
2.6
34 973
21%
10.2
9.0
9.6
6.9
–
[31]
3.9
24 978
6%
8.2
2.8
9.2
8.5
–
[32]
4.4
27 031
10%
13.8
8.2
15.6
13.1
8.0
This work
3.8
7608
5%
10.5
8.6
12.0
9.4
–
This work
a turn-on voltage of 4.4 V and color coordinates of CIE (0.68, 0.32). The highest power efficiency, the current efficiency, and the external quantum efficiency of device B can be up to 8.2 lm W–1, 13.8 cd A–1, and 15.6%. To our knowledge, these excellent performances of the devices are among the best of orange and red PHOLEDs (see Table 1).[28–32] More interestingly, both devices showed low efficiency roll-off. For the device B, the EQE can be up to 13.1% with an efficiency roll-off of 16.0% at the luminescence up to 1000 cd m–2 and 8.0% with an efficiency roll-off of 48.7% at the luminescence up to 10 000 cd m–2. The efficiency roll-off of the device B was lower than the red light PHOLEDs based on host materials with both fluorenyl and diphenylphosphine oxide moieties.[12] The EQE–current densities of both devices were fitted according to either the triplet–triplet annihilation (TTA) or triplet–polaron quenching (TPQ) mechanism (Figure S7, Supporting Information).[33] The simulation of triplet–triplet annihilation (TTA) model coincides very well with the experimental curve for both devices, especially under higher current density. Thus, the efficiency roll-off of the devices can be mainly ascribed to the TTA mechanism. The J0 of both devices obtained were 80 mA cm–2 for device A and 174 mA cm–2 for device B, which was much larger than those of reported PHOLEDs.[9] Similar efficiency roll-off of the devices with the doping concentration of 5 ± 1 wt% can also be observed. To investigate the reasons for the efficiency rolloff, a reference device of mCP as the host and Ir(piq)2acac as the emitter (device C) was constructed with similar device configuration since mCP exhibited similar electronic structure to MTXSFCz except much larger ΔEST (above 0.4 eV). Although high EQE up to 12.5% at 1 cd m–2 could also be obtained for the device C, the efficiency roll-off was more severe. The EQE decreased down to 3.6% at a luminance of 10 000 cd m–2 for device C with the efficiency roll-off of 71.2% and J0 value of the device was only 17.9 mA cm–2 (Figure S9 and Table S3, Supporting Information). Thus, we inferred that the fast reverse ISC of MTXSFCz facilitated the reduction of the triplet excitons on host, leading to the reduced efficiency roll-off of the devices. To further confirm that the TADF properties of MTXSFCz alleviated the efficiency roll-off of the PHOLEDs, the decay curves of the Ir(piq)2acac-doped films were investigated. Figure 4
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Table 1. The performance summary of orange and red PhOLEDs based on reported hosts and MTXSFCz.
shows the decays of the emission at 620 nm for 5 ± 1 wt% Ir(piq)2acac:MTXSFCz film and 5 ± 1 wt% Ir(piq)2acac:mCP film in air and vacuum. As the concentration of the doped films was very low, the decay of the excited guest molecules and the Dexter-type energy exchange between guest and host can be neglected. The main emission of the doped film will harvest from the triplet state of the guest via the FRET from the host to guest. The decay time of the Ir(piq)2acac emission in the Ir(piq)2acac:MTXSFCz blend is about 864.44 ns in vacuum, and the decay time can be shortened upon exposure to air, while it only slightly affected the emission properties of the Ir(piq)2acac in mCP (764.35 ns in vacuum and 747.12 ns in air). It is well known that exposure to oxygen reduces the lifetime of the triplet excited states on the phosphorescent and TADF materials. Since the decay of the excited state of the doped mCP film was only slightly affected, the shortening of the lifetime cannot be mainly attributed to the oxygen effect on the phosphorescent guest. The main difference between them is that kinetics of reverse ISC between singlet and triplet of the hosts resulting from the small ΔEST, since the triplet decay rates of both hosts are small due to the missing heavy-atom effect. A similar effect of oxygen on the lifetime of the phosphorescent emitter had been observed for FIrpic in PBD-PVK cohost due to the oxygen induced non-radiative relaxation processes of PBD, in which there is the triplet back transfer of the PBD host to FIrpic.[34] Thus, it can be concluded that the fast decay of the red emission of the Ir(piq)2acac:MTXSFCz blend in the presence of oxygen indicates the storage of the triplet excitons on MTXSFCz. In accordance with these, the steady-state PL intensity of the Ir(piq)2acac:MTXSFCz film is largely affected by the presence of oxygen, while the effect is rather small for the Ir(piq)2acac:mCP film. When all the triplet excitons are confined on the phosphorescent guests by the high triplet energy level of the different hosts, similar decay dynamics of phosphorescent guest can be expected.[35–37] Thus, the lifetime of phosphorescent guest–host system will be similar. Both mCP and MTXSFCz exhibit high triplet energy, at least 0.7 eV higher than that of Ir(piq)2acac. However, the lifetime of 5 ± 1 wt% Ir(piq)2acac:MTXSFCz film in vacuum (864.44 ns) is about 100 ns higher than that of 5 ± 1 wt% Ir(piq)2acac:mCP film in vacuum (764.35 ns). Thus, the
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Figure 4. a) The lifetime of 5 ± 1 wt% Ir(piq)2acac: MTXSFCz film in vacuum and in air. b) The lifetime of 5 ± 1 wt% Ir(piq)2acac: mCP film in vacuum and in air. c) The PL intensity of 5 ± 1 wt% Ir(piq)2acac: MTXSFCz film in vacuum and in air. d) The PL intensity of 5 ± 1 wt% Ir(piq)2acac: mCP film in vacuum and in air.
longer lifetime of 5 ± 1 wt% Ir(piq)2acac:MTXSFCz film can be ascribed to the presence of the process of intersystem crossing and reverse intersystem crossing on photoexcited MTXSFCz. To demonstrate the decrease of the triplet population, and thereby low efficiency roll-off, an important factor is the presence of rapid energy transfer via a Förster process.[38] The rate constant of Förster resonance energy transfer (FRET) from host to guest can be estimated: K ET = (1/τ D ) (R06 /R 6 ) , where τ D is radiative decay time of donor molecular, R0 is the Förster transfer radius, and R is the average distance between donor and acceptor molecules.[39–41] For Ir(piq)2acac:MTXSFCz film, R0 was estimated to be 2.6 nm, larger than the average distance between Ir(piq)2acac guests (2.34 nm for the doping concentration of 5 ± 1 wt% and 1.85 nm for the doping concentration of 10 ± 1 wt%) using a molecular modeling program.[39,40] Thus, the rate constant of FRET is very fast (>1010 s−1), nearly two orders of magnitude faster than those of the intersystem crossing (ISC) and reverse intersystem crossing (RISC) of reported TADF molecules (
