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A Sterically Shielded Electron Transporting Material for nearly 100% Internal Quantum Efficiency and Long Lifetime in Thermally Activated Delayed Fluorescent and Phosphorescent OLEDs Dongdong Zhang, Pengcheng Wei, Deqiang Zhang, and Lian Duan ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 15 May 2017 Downloaded from http://pubs.acs.org on May 17, 2017

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ACS Applied Materials & Interfaces

A Sterically Shielded Electron Transporting Material for nearly 100% Internal Quantum Efficiency and Long Lifetime in Thermally Activated

Delayed

Fluorescent

and

Phosphorescent OLEDs Dongdong Zhang, Pengcheng Wei, Deqiang Zhang, Lian Duan* Key Lab of Organic Optoelectronics and Molecular Engineering of Ministry of Education, Department of Chemistry, Tsinghua University, Beijing 100084, China.

ABSTRACT High triplet energy (T1) is usually taken as the prerequisite of the good exciton confinement of electron-transporting materials (ETMs), which, however, is usually trade-off parameter with large mobilities and excellent stabilities. Here we demonstrated that good exciton confinement can also be realized utilizing a low-T1 ETM with sterically shielding low-T1 unit. Given the short-range interaction of the Dexter energy transfer, the large steric side groups of the low-T1 ETM can effectively hinder the T1 of the emitters from being quenched by increasing intermolecular distance. Based on this concept, maximum external quantum efficiency (EQEmax) as high as 21.3% was observed in the sky-blue thermally activated delayed fluorescence (TADF)

ACS Paragon Plus Environment

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device using a low-T1 ETM, with EQE remaining 21.2% at 1000 cd/m2 and 17.8% at 5000 cd/m2. Besides, EQEmax as high as 25.5%, a low turn-on voltage of 2.3 V as well as a long T90 of over 400 h at an initial luminance of 5000 cd/m2 were achieved for green phosphorescent devices. This work points out a viable strategy for developing high-performance ETM, paving their way towards practical applications.

KEYWORDS (anthracene derivative; electron transport; exciton confinement; phosphorescent organic light-emitting diodes; thermally activated delayed fluorcence)

INTRODUCTION Organic light-emitting diodes (OLEDs) have attracted increasing research attention due to their potential applications in displays and lightings.1-4 However, their key performance parameters, such as efficiency and lifetime, still need to be improved so as to unlock the full potential of OLEDs. Towards this end, more and more researches have been focused on the development of new organic semiconducting materials.5-7 Currently, one of the remaining challenges is to develop an ideal electron transporting material (ETM),8-14 which should possess a large mobility for reducing the driving voltage, a wide energy gap (Eg) and a high triplet energy level (T1) for confining both carriers and excitions, and excellent molecular stability for a long working lifetime. This is extremely tough, not only because organic semiconductors are often better hole transporters than electron transporters,15 but also because some of the above requirements are conflicting ones.12 For example, a wide Eg and a high T1 mean the conjugation of the molecule should be weak, which may in turn lead to a low mobility. Xiao and Kido et al. reported a compromised solution by using diphenyl-bis(4-


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(pyridine-3-yl)phenyl)silane, a low mobility ETM with a wide Eg and high T1, for blue phosphorescent OLEDs (PHOLEDs) and achieved a maximum external quantum efficiency (EQEmax) of 22%.16 The situation is even tougher when the stability of the OLED is taken into consideration. Only a few of high mobility ETMs containing conventional electron deficient moieties such as oxidazole,17 benzoimidazole,18 pyridine,12 triazine19 and phenanthroline,20 have been reported to exhibit a decent lifetime. In our previous work, we have proposed that this mobility-stability tradeoff can be broken by attaching electron deficient moieties to a highly conjugated and electrochemically stable chromophore of anthracene.9 The strategy is later confirmed by Prof. Kido’s group in demonstrating a series of bipyridyl, pyridylphenyl, and phenylpyridyl substituted anthracenes, all of which showed high electron mobilities and excellent stability when used as ETMs in fluorescent OLEDs.12 However, as the triplet energy levels of anthracene derivatives are inherently low, it seems that anthracene based ETMs cannot be used in the devices with high triplet energy emitters such as fluorophors with thermally activated delayed fluorescence (TADF) and phosphors, where high triplet ETMs are usually required to confine the triplet excitons. Though this issue may be relieved by inserting an exciton blocking layer (EBL) between the emitting layer (EML) and the ETM,21 the additional EBL would complicate the device structure and might deteriorate the operational stability. Is it possible to realize excellent exciton blocking properties using low triplet energy anthracene derivatives? It is noticed that, fortunately, the energy transfer between triplet excitons is a short range Dexter type energy transfer, in which the excitons hop between molecules and require the overlap of molecular orbitals of neighboring molecules. Therefore, such energy transfer can be hindered by increasing the distance between them.22,23 In our previous work, low-T1 ETMs with large steric side groups have been utilized in devices with conventional


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fluorescent dopant, enhancing device performances.24 To our knowledge, such ETMs have not been adopted in PHOLEDs and TADF OLEDs to testify the confinement of the high-energy excitons, which is more challenging yet highly desired and important. Here, we demonstrated that by sterically shielding its low-T1 units, ETMs with low-T1 used in both PHOLEDs and TADF OLEDs can also realize good exciton confinement. Thanks to the short-range interaction of the Dexter energy transfer, the large steric side groups of the low T1 ETM can effectively hinder the T1 of the emitters from being quenched by increasing intermolecular distance. Based on this concept, maximum external quantum efficiency (EQEmax) as high as 21.3% was observed in the sky-blue TADF device utilizing a low T1 ETM with large steric side groups, with EQE remaining 21.2% at 1000 cd/m2 and 17.8% at 5000 cd/m2. Moreover, a long half-lifetime of 475 h at an initial luminance of 500 cd/m2 were achieved. Besides, EQEmax as high as 25.5%, a low turn-on voltage of 2.3 V as well as a long T90 of over 400 h at an initial luminance of 5000 cd/m2 were achieved for green phosphorescent devices. The overall performances of those device are among the state-of-the-art performance reported for sky-blue TADF OLEDs and green PHOLEDs, confirming the high potential of our design strategy. EXPERIMENTAL SECTION Before the measurements, all of the organic materials used were purified by a vacuum sublimation approach. The energy levels of BPBiPA were measured using photoelectron yield spectroscopy (AC-3, Riken Keiki) and low-energy inverse photoemission spectroscopy (LE-1, ALS Technology). The emssion spectra, the PL quantum efficiency and the transient photoluminescence characteristics of the compounds were measured using a transient spectrometer (Edinburgh FL920). For measurement of the transient electroluminescence


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characteristics, short-pulse excitation with a pulse width of 15 µs was generated using Agilent 8114A. The amplitude of the pulse is 7V for green PHOLEDs while 9 V for the sky-blue TADF devices, 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. Time-of-Flight (TOF) Mobility Measurement: BPBiPA was purified by sublimation before use in subsequent analyses and device fabrication. The TOF device was prepared by vacuum deposition using the following structure: ITO/ BPBiPA (2 µm)/ Ag (200 nm). A third harmonic of Nd: YAG laser (355nm, 15ns) is used as the excitation light source. By switching the polarity of the applied dc bias, the transit times Tt, the time that the carriers pass through the organic film to reach the counter electrode, is measured by a digital storage oscilloscope. With the applied bias V and the thickness of the organic layer D, which is much larger than the optical absorption depth of the excitation, the carrier mobilities could be calculated according to the formula: µ = D/ (TtE) = D2/ (VTt). Fabrication of OLEDs: 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. Encapsulated devices were characterized with Keithley

4200

semiconductor

characterization

systems

in

ambient

conditions.

The

electroluminescent spectra were collected with a Photo Research PR705 spectrophotometer. Electroluminescence quantum yield measurement was carried out using an intergrating sphere PL/EL measurement system by Hamamatsu (C9920-03). RESULTS AND DISCUSSION


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To testify the conception, an anthracene derivative with bulky electron deficient phenylbenzoimidazole moieties, 9,10-bis(4-(2-phenyl-1H-benzo[d]imidazol-1-yl)phenyl) anthracene (BPBiPA), was chosen here. The molecule structure of BPBiPA is shown in Figure 1. As we suggested above, to increase the distance between the neighboring molecule and the anthracene core, a bulk steric unit, 2-phenyl-1H-benzo[d]imidazole moiety was chosen. Additionally, the triplet energy of 2-phenyl-1H-benzo[d]imidazole moiety is high and its electron-deficit properties benefits to enhance the electron-transporting ability as well as deep its LUMO level for efficient electron injection. The absorption spectra and photoluminescence (PL) spectra of BPBiPA in CH2Cl2 (10-4 M) are shown in Figure 1a. The onset of the absorption spectra of BPBiPA is around 412 nm, corresponding to an Eg of 3.0 eV. The PL emission of BPBiPA shows a maximum at 417 nm and a shoulder at 431 nm. The T1 energy of BPBiPA was measured to be 1.82 eV at 77 k with a phosphor as a triplet-sensitizer (Figure S1). The electrochemical properties of BPBiPA were studied by cyclic voltammetry. As shown in Figure S2, the oxidation process is reversible with the oxidation potential of 1.37 V vs. an Ag/Ag+ standard, indicating that the radical cations are stable. The highest occupied molecular orbital (HOMO) of BPBiPA is then calculated to be 5.7 eV, using ferrocene as a reference. The lowest unoccupied molecular orbital (LUMO) is calculated by the HOMO value and the Eg to be about 2.7 eV. The electron affinity and ionization energy of the solid BPBiPA film were further measured using inverse photoemission spectroscopy (IPES) and photoelectron yield spectroscopy to be 2.33 eV and 5.95 eV, respectively. The thermal properties of BPBiPA were determined by differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA), as shown in Figure S3 and S4. BPBiPA has a glass-transition temperature (Tg) of 185 oC, a melting temperature (Tm) of 370 oC and a decomposition temperature (Td, corresponding to 5% weight loss) of 453 oC. The thermal


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stability of BPBiPA is better than previously reported ETMs based on anthracene derivatives,9 largely owing to the bulky and rigid substituent of benzoimidazole. High Tg of the electrontransport material is critical to the device lifetime because low Tg materials can be crystallized or deformed during device operation. Therefore, the good thermal stability of BPBiPA would be beneficial for the stability of the OLEDs. The carrier mobilities of BPBiPA were characterized by time-of-flight (TOF) measurement. The configuration of the device is: ITO/ BPBiPA (2 µm)/ Ag (200 nm). Figure 1b shows the field dependent hole and electron mobilities of BPBiPA, all of which follow the Poole-Frenkel (PF) relationship. BPBiPA displays balanced hole and electron mobilities, while the PF slopes for holes and electrons are all negative, possibly due to the large positional disorders in the evaporated film of BPBiPA.25 The large positional disorders may results from the wellperformed amorphous states of BPBiPA, suggesting the outstanding stability of the evaporated film of BPBiPA, which is beneficial for the device stability. At an electric field of 3×105 V/cm, the electron mobility of BPBiPA is as high as 1.55×10-3 cm2 /Vs, which is an order of magnitude higher than that of the widely used 1,3,5-tris(1-phenyl-1H-benzimidazol-2-yl)benzene (TPBi) and even two order of magnitude higher than that of the bis-4,6-(3,5-di-3-pyridylphenyl)-2methylpyrimi-dine (B3PYMPM) (1.5×10-5 cm2 /Vs).8 Large electron-transporting mobilities usually facilitate balanced charges in the EML and low operation voltages. One of the prerequisite to achieve high device efficiency is that the excitons should be confined in the emitting layers, which is one of the basic characteristics for the ETMs ， especially when they are used for the triplet harvesting emitters. To investigate the excitons confinement of BPBiPA, the photo-luminance (PL) transient decay curves of the binary layer


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films as can be seen from Figure 2a and 2b were measured. As is shown in Figure 2a, (3′-(4,6diphenyl-1,3,5-triazin-2-yl)-(1,1′-biphenyl)-3-yl)-9-carbazole (CzTrz) was chosen as the host for a

sky-blue

(5TCzBN),7

emitter,6 while

2,3,4,5,6-pentakis(3,6-di-tert-butyl-9H-carbazol-9-yl)benzonitrile

9,10-bis(3-(pyridin-3-yl)phenyl)anthracene

(DPyPA),9

BPBiPA

and

B3PyMPM were chosen as the electron-transporting materials for comparison. The triplet energy of 5TCzBN is 2.60 eV, lower than that of B3PyMPM (2.75 eV) but higher than those of DPyPA and BPBiPA. Thanks to the higher T1 of B3PyMPM than 5TCzBN, the excitons formed in the emitting layer under PL excitation will be well-confined on the T1 of 5TCzBN without being quenched by the electron-transporting layer and thus the longest excited lifetime of 3.4 µs, corresponding to the intrinsic delayed lifetime of 5TCzBN,7 and the largest ratio of the delayed part were achieved. On the other hand, in virtue of the low triplet energy of ETMs, i.e. BPBiPA and DPyPA, the triplet energy of sky-blue TADF guest will be inevitably transferred to that of the ETMs, inducing an additional triplet relaxation path and consequently leading to a shorter lifetime and a lower ratio of the delayed part of the guest decay curves. Surprisingly, although the triplet energies of the anthrancene derivations are almost identical, the quenching effects are quite different with the exciton lifetime and the ratio of the delayed parts of the binary film with BPBiPA (3.2 µs) higher than that of the film with DPyPA (2.9 µs), manifesting the relatively better exciton confinement of BPBiPA than DPyPA. The similar behaviors can also be observed in phosphorescence system as can be seen from Figure 2b. A commonly used host material, 1,3bis(9-carbazolyl)benzene (mCP), doped with a green phosphor, tris(2-phenylpyridinatoC2,N)iridium(III) (Ir(ppy)3), was selected as the emitting layer while DPyPA, BPBiPA and TPBi were chosen as the electron-transporting materials for comparison. The triplet energy of Ir(ppy)3 is 2.4 eV, lower than that of TPBi (2.6 eV) but higher than those of DPyPA and BPBiPA. It was


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observed that the exciton lifetime of the guest in the films are in the order of TPBi (1.0 µs) >BPBiPA (0.84 µs) >DPyPA (0.56 µs), also referring to the relatively good exciton confinement of BPBiPA compared with DPyPA. To gain direct insight about the different between the exciton confinement of DPyPA and BPBiPA, the HOMO and LUMO orbital distributions of both molecules as well as the spin density distribution (SDD) of T1 states were calculated. As can be seen from Figure 2c, the dihedral angles between the phenyl units and the anthracene unit are about 86o while those between the phenyl units and the 2-phenyl-1H-benzo[d]imidazole moieties are 56o. The highly twisted structure of BPBiPA facilitate to enhance the large steric effect of the 2-phenyl-1Hbenzo[d]imidazole moieties, increasing the intermolecular distances between the anthracene units and the adjacent molecules. For DPyPA, the dihedral angles between the phenyl units and the anthracene unit are about 81o, and those between the phenyl units and the pyridine units are 38 o. Also, as illustrated in Figure 2c, the HOMOs and LUMOs of both materials are distributed on the anthracene core, rendering them excellent transporting materials with a non-donoracceptor structure10. The SDDs of T1 states of both molecules show that the T1 states of both molecules are thoroughly localized on the 9,10-diphenylanthracene units, surrounded by the side groups of the molecules. The quench of the excitons is through the energy transfer from the highenergy T1 of 5TCzBN or Ir(ppy)3 to the low-energy ones of DPyPA or BPBiPA. Such energy transfer is through the Dexter transfer mechanism, in which the exciton hops directly between molecules. As it is a short-range process dependent on the overlap of molecular orbitals of neighboring molecules, the Dexter transfer can be hindered by increasing the mutual molecule distances, which can be accomplished by introducing large side groups to the molecules. As a bulk steric unit, 2-phenyl-1H-benzo[d]imidazole moiety, the side group of BPBiPA, will induce


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large steric effect to hinder the mutual contact between 5TCzBN and Ir(ppy)3 and its anthracene core and thus the quenching effect will be reduced, as is illustrated in Figure 2d using Ir(ppy)3 as an example. To the contrast of BPBiPA, DPyPA only possesses a side group small in volume and hence more significant exciton quenching is observed, evidencing that the low triplet energy can be protected by large steric units. High electron mobilities, well exciton blocking properties as well as good thermal stability render BPBiPA a good electron transporting material. To further demonstrate its performance in OLEDs, sky-blue TADF devices with structures of ITO/ HATCN (5 nm)/ NPB (30 nm)/ TCTA (10 nm)/ CzTrz: 25% 5TCzBN (30 nm)/ ETM (40 nm)/ LiF (0.5 nm)/ Al (150 nm) were fabricated. HATCN, NPB and TCTA are Dipyrazino[2,3-f:2',3'-h]quinoxaline-2,3,6,7,10,11hexacarbonitrile,

N,N'-bis(1-naphthalenyl)-N,N'-diphenyl-[1,1'-biphenyl]-4,4'-diamine

and

N,N,N-Tris(4-(9-carbazolyl)phenyl)amine, respectively. BPBiPA, DPyPA and B3PyMPM were selected as ETMs for comparison. The energy levels of the devices are shown in Figure 3a. All devices only show sky-blue emission from 5TCzBN (Figure 3b), indicating efficient energy transfer from the host to the guest. As can be seen from Figure 3c, low operation voltage was observed with both turn-on voltages around 3.0 V as well as 4.1 V and 3.9 V at 1000 cd/m2 for BPBiPA and DPyPA, respectively. As a comparison, device based on B3PyMPM shows turn-on voltage as high as 3.2 V and 5.2 V at 1000 cd/m2. Meanwhile, the current densities of the devices based on the anthracene derivations are much higher than that of the device based on B3PyMPM. In principle, operating voltage and current density is relevant to the electron injection efficiencies and transporting abilities of the ETMs. Despite the deeper LUMO of B3PyMPM observed from the energy diagram of the devices, the lower operation voltages and higher current densities of the devices based on BPBiPA and DPyPA than the one based on B3PyMPM can be ascribed to


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the higher electron-transporting mobilities of the anthracene derivations. Given the comparable electron-transporting mobilities of BPBiPA and DPyPA, the higher current density of the DPyPA based device may be attributed to its deeper LUMO energy level. A remarkable observation from Figure 3d is that EQEmax of 21.3% were achieved for the skyblue TADF OLED with BPBiPA as the ETM. In the meantime, an extremely small efficiency roll-off was also realized. As illustrated in Table 1, at 1000 cd/m2, the EQE of the device is as high as 21.2% and even at 5000 cd/m2, the EQE still remains 17.8%, corresponding to 83.5% of the maximum one. Considering the notorious significant efficiency roll-off problem of TADF OLEDs, the performances of the devices here are the state-of-the-art results for sky-blue TADF devices as can be seen from Table S1. On the other hand, in spite of the higher triplet energy of B3PyMPM, EQEmax of only 15.6% is achieved for B3PyMPM and the efficiency roll-off is server with EQE of 13.4% at 1000 cd/m2. The main reason for the different device performances may be ascribed to the different electron-transporting mobilities of BPBiPA and B3PyMPM. On one hand, it has been well known that the device performances are closely related to the charge balance in the EML. Virtually, organic semiconductors are often better hole-transporters than electron-transporters as we pointed out above. The high electron-transporting mobility of BPBiPA, which is comparable to the hole-transporting mobility of NPB, may lead to the balanced charges in the EML and consequently more superior device performance than the one based on B3PyMPM, which possesses much lower electron-transporting mobility. On the other hand, different to the PL excitation where only the relative triplet energies determine the exciton quenching effect of the system, under electrical excitation, the recombination zone will also affect the quenching effect of the ETM. The large electron-transporting mobility of BPBiPA may render the recombination zone away from the interface of the EML/ETM, facilitating to reduce


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the exciton quenching and promoting device performances consequently. However, large electron-transporting mobility is not the only reason for the outstanding device performances, exciton confinement of the ETM is also vital, which can be demonstrated by comparing the performance of the device using BPBiPA as the ETM with the one utilizing DPyPA as the ETM. Although the electron-transporting mobility of DPyPA is comparable to that of BPBiPA and the current density of the device with DPyPA is even higher, the performance of the device based on DPyPA was much lagged behind with an EQEmax of only 17.2%. The low device efficiency indicates the low utilization of excitons formed under electrical excitation which are lost through the low triplet energy of DPyPA in this case. In contrast, nearly unity exciton utilization ratio was realized for device based on BPBiPA as the ETM, testifying the good exciton confinement properties of BPBiPA. Direct evidence about the triplet quenching in the devices under electrical excitation can be revealed by analyzing the electro-luminance (EL) transient decay behaviors of the devices. As can be seen from Figure S5, the device with BPBiPA (7.68 µs) showed a relatively long delayed lifetime of the EL decay curves than that of the device based DPyPA (7.0 µs), which conforms to the behavior observed from the PL decay curves, further demonstrating the better exciton confinement of BPBiPA than DPyPA arising from the fact that the exciton quenching can greatly reduce the lifetimes of the EL decay curves. Ideally, for practical application of the devices, not only high efficiencies but also long lifetimes are highly desired. To evaluate the stability of BPBiPA, the lifetime of the sky-blue TADF OLED with 3,3-di(9H-carbazol-9-yl)biphenyl (mCBP) as the host, which has been demonstrated to be a stable host for blue emitters, was measured as can be seen from Figure S6. At an initial luminance of 500 cd/m2, a long lifetime T50 of 475 hours was achieved, proving the high stability of BPBiPA.


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Additionally, triplet excitons confinement is essential to obtain high efficiency not only in TADF OLEDs but also in the PHOLEDs. To demonstrate the general potential of BPBiPA, it was also used as the ETM in green PHOLEDs. Devices with structures of ITO/ HATCN (5 nm)/ NPB (30 nm)/ TCTA (10 nm)/ DIC-TRZ: 10% Ir(ppy)3 (30 nm)/ BPBiPA (40 nm)/ LiF (0.5 nm)/ Al (150 nm) were fabricated, where DIC-TRZ is 2,4-diphenyl-6-bis(12-phenylindolo)[2,3a]carbazole-11-yl)-1,3,5-triazine. TPBi with high triplet energy was selected as ETM for comparison. The energy diagrams of the devices are shown in Figure 4a. Both devices only show emission from Ir(ppy)3 (Figure S7), indicating efficient energy transfer from the host to the guest. As can be seen from Figure 4b, an unprecedented low turn-on voltage of 2.3 V is achieved for the device based on BPBiPA, even lower than the emitting photon energy (hv) of the Ir(ppy)3 emission, 2.4 eV. Even at 1000 cd/m2, the operation voltage is as low as 2.8 V. As comparison, device based on TPBi shows turn-on voltage as high as 2.8 V and 3.6 V at 1000 cd/m2. Given the same LUMO levels of BPBiPA and TPBi, it is evidential that the lower operational voltage as well as the higher current density of the device based on BPBiPA are mainly attributed to the higher carrier mobilities of BPBiPA. As is shown in Figure 4c, an extremely high EQEmax of 25.5% and a power efficiency as high as 109.2 lm/W were achieved for devices with BPBiPA. The performance of the device is among the highest ones based on Ir(ppy)3 as the emitters.26 The power efficiency is even higher than that of the device based on exciplex as the host, ascribed to the low operation voltage.27 The outstanding performances further testify the good exciton confinement properties of BPBiPA. The good exciton confinement of BPBiPA can be further demonstrated by comparing the EL transient decay curves of the devices with BPBiPA and DPyPA as the ETMs. As can be seen from Figure S8, the


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EL decay curves of the devices with BPBiPA (0.70 µs) also show longer lifetime than that with DPyPA (0.63 µs), evidencing the good excitons confinement of BPBiPA. Surprisingly, an EQEmax of only 18.0% and a power efficiency of 51.4 lm/W were obtained though the higher triplet energy of TPBi. The reason for the higher efficiency of the device based on BPBiPA can be deduced to the high electron-transporting mobilities of BPBiPA, which leads to the balanced charges in the EML and thus the small efficiency roll-off with EQE as high as 24.9% even at luminance of 10000 cd/m2. The device lifetimes were further measured as can be seen from Figure 4d. At an initial luminance of 5000 cd/m2, a long lifetime T90 of over 400 hours was achieved. Device based on TPBi show a relatively shorter lifetime T90 of about 140 h, demonstrating the superiority of BPBiPA. Summarily, highly efficient PHOLEDs with extremely low operational voltage as well as long-term stability are achieved simultaneously, confirming the potential application of BPBiPA as an ETM. CONCLUSIONS In conclusion, we demonstrated a novel design strategy for ETMs, that is sterically shielding the low T1 units to realize good exciton confinement. Thanks to the short-range interaction of the Dexter energy transfer, the large steric side groups of the low T1 ETM can hinder the triplet excitons of the emitters from being quenched by the low triplet energy of the ETM effectively. High mobility, excellent stability and good exciton blocking property were achieved simultaneously in an anthracene based ETM, BPBiPA. EQEmax as high as 21.3% was observed in the sky-blue thermally activated delayed fluorescence (TADF) device utilizing BPBiPA as the ETM, with EQE remaining 21.2% at 1000 cd/m2 and 17.8% at 5000 cd/m2. For green phosphorescent organic light-emitting diodes based on BPBiPA, EQEmax as high as 25.5%,


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a low turn-on voltage of 2.3 V as well as a long T90 (time to 90% of the initial luminance) of over 400 h at an initial luminance of 5000 cd/m2 were achieved. This work not only demonstrates the general potential of BPBiPA, but also points out a viable strategy for developing high-performance ETMs. FIGURES 0.0020

a

1.0

b

Emission Absorption BPBiPA

N

0.0015

N

N

N

2

Mobility (cm /Vs)

0.8 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


0.6 0.4

0.0010 Hole Electron

0.2 0.0 250

300

350 400 450 Wavelength (nm)

500

550

600

0.0005 500

550

600

650 700 1/2 E (V/cm)

750

800

850

Figure 1. (a) The absorption and emission spectra of BPBiPA. (b) The hole- and electrontransporting mobilities of BPBiPA versus electrical field.


15


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Page 16 of 25

Figure 2. a) The structure of the binary films as well as the molecules structures. b) The PL transient decay curves of the binary films. c) The optimized molecule structures and the distribution of HOMO and LUMO as well as the SDD of T1 states of BPBiPA and DPyPA. d) The scheme of the hindering effect of large side groups of BPBiPA.


16

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2.2

2.3 2.74

2.7

LiF/Al

0.8

B3PyMPM

DPyPA

BPBiPA

CzTrz: 5TCzBN

NPB

TCTA

5.45

HATCN

5.7

BPBiPA DPyPA B3PyMPM

3.41

ITO 5.5

b

1.0

2.8

3.28

Intensity (a.u.)

a

0.6

0.4

0.2

5.7 5.9

6.08

7.15

0.0 400

500

ETL

600

700

Wavelength (nm) 1000

25


c

10000

d

20 15 EQE

10


2

Brightness (cd/m )

500 100


10

250

1

5 0 45 Power efficiency

2

Current density (A/m )

750 1000

4

5

6

7


30

15

0 3

0 0

8

2000

4000

6000

8000

10000

Brightness (cd/m2)

Voltage (V)

Figure 3. (a) The energy diagram of the sky-blue TADF devices. (b) The EL spectra of the sky-blue TADF devices. (c) The current density-voltage-brightness curves of the sky-blue TADF devices. (d) The EQE-brightness and power efficiency-brightness curves of the sky-blue TADF devices.

b

2.1 eV 2.6 eV

2

2.3 eV 2.7 eV

2.7 eV

NPB

TCTA

BPBiPA TPBi

5.1 eV

5.7 eV

400

10k

1k 200

2

5.4 eV 5.5 eV

ITO

BPBiPA TPBi

Brightness (cd/m )

LiF/Al DIC-TRZ: Ir(ppy)3

Current Density (A/m )

2.2 eV

a

HATCN

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


100 5.7 eV 6.2 eV

2.0

2.5

3.0

3.5

4.0

4.5

5.0

Voltage (V)


17


30

d

c 100

EQE

20 15

50

10 BPBiPA TPBi Power efficiency

BPBiPA TPBi

5 EQE 0

BPBiPA measured TPBi measured TPBi extrapolated

100 I/I0 (%)

25

Power Efficiency (lm/W)

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 25

0

5000 10000 15000 20000 25000 30000 35000

90 0

2

Luminance (cd/m )

100

200 300 Time (h)

400

500

Figure 4. a) The energy diagrams of the PHOLEDs. b) The current density-voltage-brightness characteristics of the PHOLEDs. c) The EQE-luminance-power efficiency characteristics of the PHOLEDs. d) Lifetimes of PHOLEDs measured with an initial brightness of 5000 cd/m2. Table 1. The summary of the device performances. Voltage (V) emitter

Power efficiency (lm/W)

EQE (%)

ETL 1 cd/m2

1000 cd/m2

5000 cd/m2

Max

1000 cd/m2

5000 cd/m2

Max

1000 cd/m2

5000 cd/m2

Lifetime (h)

5TCzBN

BPBiPA

3.0

3.9

5.4

21.3

21.2

17.8

40.2

34.1

19.9

475a

5TCzBN

DPyPA

3.0

4.2

6.0

17.2

17.1

14.8

32.4

29.9

18.5

-

5TCzBN

B3PyMPM

3.2

5.3

-

15.6

13.4

-

25.9

17.2

-

-

Ir(ppy)3

BPBiPA

2.3

2.8

3.4

25.5

25.5

25.4

109.2

96.8

79.6

407b

Ir(ppy)3

TPBi

2.8

3.8

4.3

18.0

16.5

17.8

51.4

48.9

43.8

140b

2

Note: a) T50 at initial luminance of 500 cd/m . And the host of the device is mCBP with its performance described in supporting information. b) T90 at initial luminance of 5000 cd/m2.

AUTHOR INFORMATION Corresponding Author *Correspondence to: E-mail address: [email protected]: +86 10 62795137; Tel: +86 10 62782197 Notes The authors declare no competing financial interests.


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ACKNOWLEDGMENT We would like to thank the National Science Fund for Distinguished Young Scholars of China (Grant No. 51525304), the National Key Basic Research and Development Program of China (973 program, Grant No. 2015CB655002) founded by MOST and the Open Fund of the State Key Laboratory on Integrated Optoelectronics (Grant No. IOSKL2015KF13) for financial support for ﬁnancial support. We’d like to thank Prof. Toshinori Matsushima and Prof. Chihaya Adachi of Kyushu University for the measurement of the HOMO and LUMO levels of BPBiPA using photoelectron yield spectroscopy and low-energy inverse photoemission spectroscopy and valuable suggestions on the manuscript. SUPPORTING INFORMATION The general information about the experiments. The phosphorescent emission of BPBiPA. The cyclic voltammograms for BPBiPA. TGA thermo-grams of BPBiPA. DSC traces of of BPBiPA. EL decay curves of the sky-blue TADF devices. The performance of the stable sky-blue TADF devices. The EL spectra of the green PHOLEDs. The EL decay curves of the green PHOLEDs. Summary of the blue TADF OLEDs reported. REFERENCES (1) Baldo, M. A.; O’Brien, D. F.; You, Y.; Shoustikov, A.; Sibley, S.; Thompson, M. E.; Forrest, S. R. Highly Efficient Phosphorescent Emission from Organic Electroluminescent Devices. Nature. 1998, 395, 151-154. (2) Su, S. J.; Takahashi, Y.; Chiba, T.; Takeda, T.; Kido, J. Structure-Property Relationship of Pyridine Triphenyl Benzene Electron-Transport Materials for Highly Efficient Blue Phosphorescent OLEDs. Adv. Funct. Mater. 2009, 19, 1260–1267.


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Emission Fluorophors: A Novel Concept to Achieve High-Performance Hybrid White Organic Light-Emitting Diodes. Adv. Funct. Mater. 2016, 26, 776-783.


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Sterically Shielded Electron Transporting ... - ACS Publications

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