Exploiting p-Type Delayed Fluorescence in Hybrid White OLEDs

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Exploiting p-type delayed fluorescence in hybrid white OLEDs: breaking the trade-off between high device efficiency and long lifetime Dongdong Zhang, Deqiang Zhang, and Lian Duan ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b07107 • Publication Date (Web): 19 Aug 2016 Downloaded from http://pubs.acs.org on August 19, 2016

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ACS Applied Materials & Interfaces

Exploiting p-type delayed fluorescence in hybrid white OLEDs: breaking the trade-off between high device efficiency and long lifetime Dongdong Zhang, 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 Despite that the majority of practical organic light-emitting diodes (OLEDs) still rely on blue fluorophors with low triplet (T1) for creating blue light, hybrid white OLEDs based on low T1 blue fluorophors are still much lagged behind in power efficiency. Here, “ideal” hybrid WOLEDs with recorded efficiency as well as low roll-off, good color-stability and long lifetime were realized by utilizing the bipolar mixed materials as the host of green phosphor as well as the spacer to reduce T1 trap while blue fluorophors with p-type delayed fluorescence to recycle the trapped T1. An electron transport material with both high electron mobility and good exciton confinement ability was used to boost the TTA efficiency. Hybrid WOLEDs with maximum current efficiency, external quantum efficiency and power efficiency of 49.6 cd/A, 19.1 %, and 49.3 lm/W, respectively, together with a high color rendering index of 80 and a half lifetime of over 7000 h at an initial luminescence of 1000 cd/m2 were realized, manifesting the high potential of the strategy.

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KEYWORDS (hybrid white organic light-emitting diodes, p-type delayed fluorescence, high efficiency, low efficiency roll-off, good color-stability)

INTRODUCTION White organic light-emitting diodes (WOLEDs) continue to attract intensive interest due to their applications in full color flat-panel displays and lighting sources.1 Ideally, high efficiency, low efficiency roll-off and good color stability should be achieved simultaneously for WOLEDs, which is quite challenging.2,3 The situation is even tougher when the device lifetimes are taken into consideration, considering the relatively shorter lifetime of the blue component, especially blue phosphorescence.4 Hybrid WOLEDs, composed of a stable fluorophor for blue emission and stable/efficient phosphors for longer-wavelength emission (e.g. green and red, or orange), have been developed to fulfill those requirements.5,6 The key prerequisite for high efficiency is the “triplet harvesting” in hybrid WOLEDs, involving the harvest of the singlet excitons on the fluorophors as well as triplets on the phosphors.7 According to the triplet (T1) of the blue fluorophors, which plays an indispensable role in the management of excitons, hybrid WOLEDs can be divided into two kinds, one using blue fluorophors with lower T1 than phosphors, the other utilizing blue fluorophors with higher T1 than phosphors.8-14 Though hybrid WOLEDs with a high T1 blue fluorophor can achieve high performance, the device stability was seldom studied. It should be emphasized that, despite the decades of work on improving blue phosphors as well as the recent advent of the thermally activated delayed fluorescent (TADF) OLEDs,15-16 the majority of practical display and lighting OLEDs still rely on blue fluorophors with low T1 for creating blue light.17 However, hybrid


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WOLED based on low-T1 blue fluorophors are still much lagged behind in power efficiency.8-11 For instance, though a half lifetime of 30920 h at an initial luminescence of 1000 cd/m2 was reported for hybrid WOLEDs utilizing a low T1 blue fluorophor by Peng and Zou et al., the power efficiency of the device at 1000 cd/m2 was only 16.0 lm/W.11 There seems to be a tradeoff between high efficiency and long lifetime. Considering such practical importance of fluorescent blue OLEDs, the improvement of the hybrid WOLEDs based on low T1 blue fluorophors calls for further enhanced effort. To satisfy the above goals, the trapped T1 by the blue fluorophors should be reduced, which can be realized by wise device structure design. For example, by utilizing bipolar mixed materials as the host of phosphorescent emitting layers (EMLs) as well as the spacer to regulate the distribution of charge carriers, Ma et al have demonstrated hybrid WOLEDs exhibiting an external quantum efficiency of 13.7% and a power efficiency of 27.1 lm/W at 1000 cd/m2.10 However, the device efficiency is moderate since the T1 loss through the nonradiative T1 of blue fluorophors is still significant. To further prevent the T1 loss, the trapped T1 should be recycled. One possible triplet up-conversion mechanism is T1-T1 annihilation (TTA) or p-type delayed fluorescence,17 though high performance hybrid WOLEDs based on such a strategy are rarely reported. In this manuscript, aside from the bipolar mixed materials used as the host of green phosphorescent EML as well as the spacer, which enlarges the recombination zone and reduce the triplet concentration near the blue fluorophors, blue fluorophor with TTA was also adopted to recycle the trapped T1, realizing hybrid WOLEDs with high performances. An electron transport material with both high electron mobility and good exciton confinement ability, namely 9,10bis(4-(2-phenyl-1H-benzo[d]imidazol-1-yl)phenyl)anthracene (BPBiPA),18 was used to boost the


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TTA efficiency. Blue fluorescent devices with BPBiPA achieve almost a doubled EQEmax compared with a control device with conventional Alq3 as the ETM. The optimized hybrid WOLED with BPBiPA showed a maximum current efficiency (CEmax), EQEmax and power efficiency (PEmax) of 49.6 cd/A, 19.1 %, and 49.3 lm/W, respectively, together with a high color rendering index (CRI) of 88 and a long half lifetime of over 10000 hrs at an initial luminescence of 1000 cd/m2. The devices efficiency is among the highest for hybrid WOLEDs, even comparable to all-phosphorescent devices, manifesting the high potential of the strategy. EXPERIMENTAL SECTION Device characterization: The OLED devices were fabricated by thermal evaporation under high vacuum (~7×10-4 Pa) onto clean ITO-coated glass substrates. The forward-viewing electrical characteristics of the devices were measured with a Keithley 2400 source meter while the total ones are measured using an integrating sphere (Bluefly illumia 610). The electroluminescence spectra and luminance of the devices were obtained on a PR650 spectrometer. All the device fabrication and characterization steps were carried out at room temperature under ambient laboratory conditions. 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 10V, 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 Edingburg FL920P transient spectrometer. RESULTS AND DISCUSSION To maximize the efficiency of the TTA delayed fluorescence, balanced charges in the EML as well as good exciton confinement should be achieved simultaneously. Since organic


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semiconductors are often better hole transporters than electron ones, electron-transporting materials (ETMs) with high motilities and good exciton blocking property are thereof highly desired.19 Few ETMs can meet those demands since a wide energy gap (Eg) and a high T1, required to confine the excitons in EML, mean the conjugation of the molecule should be weak, which may adversely lead to a low mobility.20 The situation is more difficult considering the stability of ETMs required to achieve long life-time devices. It was reported by Kondakov et al. that replacing tris(8-hydroxy-quinolinato)aluminium (Alq3) with high mobility ETMs would always lead a shorter lifetime in blue OLEDs.21 Therefore, the performances of ETMs are crucial for the device performances based on TTA and further the hybrid WOLEDs. In our previous work, we have demonstrated a new ETM, BPBiPA, possessing large mobility, excellent stability and good exciton blocking property though low T1, simultaneously, realize highly efficient and stable OLEDs.18 Here, the ETM was utilized to promote the efficiencies of blue fluorescent devices as well as hybrid WOLEDs based on p-type delayed fluorescence. To evaluate the superiority of BPBiPA, we fabricated blue OLEDs with Alq3 or BPBiPA as the ETM using the following structure: ITO/96% 2-TNATA: 4% F4TCNQ (150 nm)/NPB (20 nm)/ 95% α,β-ADN: 5% DACrs (30 nm)/ Alq3 (for device B1), or BPBiPA (for device B2) (15 nm)/LiF/Al. Where 2-TNATA is 4,4’,4”-tris(N-(2-naphthyl)-N-phenylamino)-triphenylamine, F4TCNQ is tetrafluorotetracyano-quinodimethane, NPB is 4,4’-N,N’-bis(N-(1-naphthyl)-N-phenylamino) biphenyl, α,β-ADN is 9-(1-naphthyl)-10-(2- naphthyl)-anthracene and DACrs is 6,12-bis(di(3,4dimethylphenyl)amino)chrysene. 22


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The energy structure of the blue device and the molecule structures of the host, the emitter, and the ETMs are shown in Figure 1. For ETM, large mobility and high T1 are usually conflicting factors. We have demonstrated that owing to the antherance unit in BPBiPA, large mobility can be realized. At an electric field of 3×105 V/cm, the electron mobility of BPBiPA is as high as 1.55×10-3 cm2 /Vs,18 which is more than two orders of magnitude higher than that of the widely used Alq3.23 The use of BPBiPA may leads to more balanced electron and hole recombination, as the electron mobility of BPBiPA is comparable to the hole mobility of NPB. In addition, the energy transfer between triplet excitons is a short range Dexter type energy transfer, which requires the over-lapping of the frontier orbitals between the donor and acceptor and therefore can be hindered by increasing the distance between them. The large steric side groups of BPBiPA can prevent the mutual contact between the emitters and the low triplet anthracene units and consequently hinder the high triplet excitons of the emitters from being quenched by the low triplet energy of the ETM effectively.18 Therefore, high mobility, excellent stability and good exciton blocking property were achieved simultaneously in BPBiPA. Figure 2a depicts the current density-voltage-brightness (I-V-B) characteristics of the devices. Owing to the larger electron-transporting mobility of BPBiPA, device B2 shows much lower driving voltages than device B1 with Alq3. For example, the voltage required to attain 1000 cd/m2 is as high as 6.98 V for device B1, while only 4.57 V for device B2. As can be seen from Figure 2b, the CEmax and EQEmax of device B2 are as high as 9.5 cd/A and 7.9 %, respectively, surpassing the limit imposed by spin statistics for fluorescent emitters. On the other hand, the CEmax and EQEmax of device B1 are only 5.5 cd/A and 4.4%, respectively. To understand the origin of the high EQE in device B2, we studied the transient electroluminescence (EL) responses of the blue devices, as shown in Figure 2c. For device B2


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with BPBiPA, after the rapid component, a delayed EL component on the order of several tens of microseconds can be observed and the initial intensity of the delayed component is around 35% of the total EL. For device B1 with Alq3, the delayed component is almost negligible. During the measurement, after applying a forward pulse voltage of 5 V, a –3 V reverse pulse voltage was applied to sweep out the trapped charges. So the observed delayed component can be ascribed to the emission from the TTA process.17 Therefore, TTA is responsible for approximately 35% of the emission in device B2 with BPBiPA. Considering that the singlet-triplet branching ratio in the TTA process of anthracene follows the spin statistics of 1:3, the maximum fraction of singlet excitons formed via TTA (ηTTA) is 15% and the upper limit of TTA related EL to the total EL should be 37.5%,24 which is very close to 35% achieved in device B2. The above results suggested that the use of BPBiPA helps to attain strong TTA in fluorescent blue OLEDs. The strong TTA efficiency in BPBiPA related devices is due to the large electron transporting mobility and the good exciton confinement of BPBiPA. On one hand, the high electron transporting mobility of BPBiPA not only promote the numbers of the T1 in the EML which promote the chance of TTA but also leads to the balanced charges in EML which will reduce the possibility of TPA process. The good exciton confinement of BPBiPA, on the other hand, also contribute the TTA process, resulting from the large energy gap of BPBiPA as well as its large steric side groups. As can be seen from Figure 2d, the emission spectra of the device based on Alq3 show the emission of Alq3 while no BPBiPA emission is observed, demonstrating the superiority of good exciton confinement of BPBiPA. We also measured the lifetimes of blue OLEDs at an initial brightness of 5000 cd/m2. After 429 hours of continuous working, the brightnesses are 2574 and 2520 cd/m2 for device B1 and B2 (see Figure 2d), respectively. It was reported by Kondakov et al. that replacing Alq3 with


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high mobility ETMs would always lead a shorter lifetime in blue OLEDs.25 However, device B1 and B2 show comparable lifetimes, even though the electron mobility of BPBiPA is much higher than that of Alq3, indicating the excellent stability of BPBiPA. As high efficiency and stability are simultaneously attained in BPBiPA based blue OLEDs, we fabricated hybrid WOLEDs with the following structure: ITO/ 96% 2-TNATA: 4% F4TCNQ (150 nm)/ BPAPF (20 nm)/ 97% BPAPF: 3% Ir(MDQ)2acac (15 nm)/ 42.5% BPAPF: 42.5% SBFK: 15% Irppy3 (10 nm)/ 50% BPAPF: 50% SBFK (5nm)/ 95% α,β-ADN: 5% DACrs (30 nm)/ Alq3 (for device W1), or BPBiPA (for device W2) (15 nm)/LiF/Al, where BPAPF is 9,9-Bis(4-(di-(p-biphenyl)aminophenyl)) fluorene,26 SBFK is bis(9,9’-spirobifluorene-2-yl) ketone,27 Ir(MDQ)2acac is (acetylacetonato)bis(2ethyldibenzo[f,h]quinoxalinato)iridium, and Irppy3 is tris(2-phenylpyridine)iridium. Figure 3a depicts the structure and energy-level diagram of the WOLEDs. As reported by Leo et al, the design of the inter-layer is very important as charge trapping at both interfaces of the interlayer may result in substantial efficiency loss.28 To reduce the charge trapping at the interfaces, a mixed layer of the hole transporting BPAPF and the electron transporting SBFK is utilized as the inter-layer between the fluorescent and phosphorescent regions as well as the host for the green phosphors layer. As can be seen from the energy-level diagram of the device, electrons and holes can be easily injected into the mixed layer, while there is energy gap for electrons into the red zone and the same for holes into the blue zone. Therefore, it can be speculated that the recombination zone is across the whole mixed layer, significantly reducing the triplet concentration near the interface of the blue layer. Consequently, the trapped triplet by the blue fluorophors can be greatly reduced. Besides, while electrons and holes pass


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this layer easily, red and green triplet excitons are confined efficiently in the phosphorescent region so as to avoid Dexter transfer of the phosphorescent excitons to the fluorescent region. What is more, once being trapped, the triplet excitons can also be recycled by the TTA process as can be seen from Figure 3b. Therefore, T1 loss through the low-lying T1 of the blue fluorophors can be greatly reduced and consequently high efficiency can be anticipated. I-V-B curves of the WOLEDs are shown in Figure 3c. The driving voltage of device W2 with BPBiPA is much lower than W1 with Alq3, as is the same for the blue OLEDs, owing to the large electron-transporting mobility of BPBiPA. The voltages required to get 1000 cd/m2 are 5.49 and 3.73 V for W1 and W2, respectively. A most remarkable observation from Figure 3d is that forward-viewing CEmax as high as 49.6 cd/A (corresponding to EQEmax as high as 19.1%) and PEmax of 49.3 lm/W were achieved for W2 while only CEmax of 21.1 cd/A (EQEmax of 9.19%) and PEmax of 12.0 lm/W were obtained for W1. The higher performance of W2 may be attributed to, on one hand, the more efficient blue efficiency when BPBiPA is adopted as we discussed above. On the other hand, owing to the higher electron-transporting mobility of BPBiPA, the recombination zone is more far away from blue region compared with Alq3, deduced from the lower intensity of the blue emission of W2 than W1 as can be seen from the device spectra. Furthermore, the higher electron-transporting mobility of BPBiPA also benefits the device efficiency by leading to more balanced charges in the EML, which also results in low efficiency roll-off. Consequently, the triplet concatenation near the interface of the blue region is much lower for device based on BPBiPA, significantly reducing the T1 trapping and thus leading to higher performances. Assuming that the light extraction efficiency is ~20%, an EQE of 19.1% corresponds to an IQE of over 90% and is almost comparable to all-phosphorescent WOLEDs.3 It is obvious that


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harvesting part of the triplet excitons via TTA is essential for achieving the high efficiency. With extra blue emission via TTA, the IQE of the hybrid WOLED can be expressed as:

ηint = γ × ( χ B × (rst + ηTTA ) × φB + χGφG + χ RφR )

(1)

where γ is charge-balance factor; χ B , χ G , and χ R are the fractions of carriers recombined in the blue, green and red emitting regions, respectively; γ st is the fraction of singlet excitons formed via direct recombination; ηTTA is the fraction of singlet excitons formed via TTA; φB , φG , and φR are the photo-luminescence quantum yields (PLQYs) of the blue, green and red emitters, respectively. Assuming the recombination is well balanced and the PLQYs are all unity, the maximum IQE of the hybrid WOLED would be:

ηint,m = χ B × (rst + ηTTA ) + (1 − χ B )

(2)

χ B can be deduced from the equation as follows: χ B = ( χ B (rst + ηTTA )) / ( χ B (rst + ηTTA ) + (1 − χ B ))

(3)

where is the fraction of photons from the blue emitter, which can be calculated by fitting the white spectrum with the EL spectra of the red, green and blue dopants and accounting for the photon energy in these power spectra. From (2) and (3), we get the relationship between χ B and the maximum IQE:

ηint,m = (rst + ηTTA ) / (rst + ηTTA + χ B (1 − rst −ηTTA ))

(4)


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The maximum IQE of the hybrid WOLED can be estimated by equation (4) according to the EL spectrum, where γ st is 0.25 for fluorescent emitters; ηTTA is ~0.15 for the anthracene based blue emitters used in our experiment and χ B is calculated using equation (3) to be around 6% here. Therefore, the maximum IQE is ~92%, agreeing well with the experimental value, indicating that the energy loss can be very small for hybrid WOLEDs using blue fluorescent emitters with TTA up-conversion properties. In fact, if blue fluorescent emitters without TTA (ηTTA=0) are used here, the maximum IQE of the WOLED with the same EL spectrum would be decreased to ~85%. Aside from the high efficiency, the efficiency roll-off of W2 is extremely small. At a brightness 100 cd/m2, which is the brightness required for displays, W2 has a forward-viewing CE of 48.8 cd/A (EQE of 18.8%) and PE of 48.5 lm/W. While at a brightness of 1000 cd/m2, which is generally required for lighting applications, the forward-viewing CEs (EQEs, PEs) are 21.1 cd/A (9.19%, 12.0 lm/W) and 44.8 cd/A (17.6%, 37.8 lm/W) for W1 and W2, respectively. The small efficiency roll-off of W2 can be attributed to, on one hand, the balanced charges in the EML owing to the large electron-transporting mobility of BPBiPA; on the other hand, the wide and stable charge recombination zone benefit from the device structures adopted here. The utilization of a fluorescent blue emitter with TTA delayed fluorescence also benefit the small roll-off, though its contribution may be small since the blue emission intensity is low. Therefore, the strategy utilized here may be superior considering high luminescence are required for practical application. Table 1 lists the performances of W2 and some representative WOLEDs, including hybrid WOLEDs and all-phosphorescent WOLEDs. As can be seen from Table 1, W2 outperforms most of the best hybrid WOLEDs and even compares well with some full-


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phosphorescent WOLEDs.3,

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Besides, at an practical luminescence of 1000 cd/m2, the

superiority of W2 is more obvious owing to its small efficiency roll-off. Inset of Figure 3d compares the EL spectra of W1 and W2, normalized to the red peak at 600 nm. Both devices show emission peaks at 460, 516, and 600 nm, due to the emission from DACrs, Ir(ppy)3 and Ir(MDQ)2acac, respectively. The emission spectra of W2 are rather stable with only slight change from 100 cd/m2 to 1000 cd/m2, owing to the stable recombination zone. Besides, the emission spectra show direct dependence on the electron transporting properties of the ETMs. Device W1 with Alq3 showed higher density of blue emission, owing to the different recombination zone as we discussed above. At 1000 cd/m2, W1 gives a high CRI of 87, with CIE coordinates of (0.36, 0.40). By using BPBiPA with a higher electron transporting ability than Alq3, the blue emissions are reduced as the recombination zone is more far away from the blue region. At 1000 cd/m2, device W2 has a CRI of 80 and CIE coordinates of (0.43, 0.46), close to that of Illuminant A, which has been taken as the most important reference light source for lighting applications. The color quality of WOLEDs with BPBiPA can be further improved by tuning the optical and electrical properties of the device. An n-type material with high mobility, hexaazatriphenylene-hexacarbonitrile (HATCN), is used as the HIL in W3: ITO/ HATCN (5 nm)/ BPAPF (20 nm)/ 97% BPAPF: 3% Ir(MDQ)2acac (15 nm)/ 42.5% BPAPF: 42.5% SBFK: 15% Irppy3 (10 nm)/ 50% BPAPF: 50% SBFK (5 nm)/ 95% α,β-ADN: 5% DACrs (30 nm)/ BPBiPA (15 nm)/LiF/Al. The driving voltage of W3 is lower than W2, due to improved hole injection, as shown in Figure 4a. The improved hole injection in W3 shift the recombination zone towards the blue


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emitting layer, and the relative intensity of the blue emission is enhanced in W3 compared to that of W2 (Figure 4b). The red peak of W3 is red shifted and broadened due to changed optical structure. Therefore, the CRI of W3 is improved to 88, with CIE coordinates of (0.41, 0.44). As the brightness increases, the blue emission of W3 was slightly increased, probably due to the improved TTA at higher current densities. The CIE coordinates of W3 shift from (0.41, 0.46) at 100 cd/m2 to (0.41, 0.44) at 1000 cd/m2, and to (0.40, 0.43) at 10000 cd/m2, representing stable white emissions. The CE and PE at different luminescence are shown in Figure 4c. At a brightness of 1000 cd/m2, the forward-viewing CE, EQE, and PE for W3 are 38.3 cd/A, 16.5 %, and 32.2 lm/W, respectively. The CIR of W3 is 88 at a luminance of 1000 cd/m2, higher than that of W2, demonstrating that the improved color quality of WOLEDs as we anticipated. As illumination sources are typically characterized by the total emitted power of the devices including light from the substrate edges and from the back of the device, W2 and W3 exhibits total CEs, EQEs and PEs of 76.9 cd/A, 29.4% and 68.1 lm/W as well as 65.7 cd/A, 28.0 %, and 57.3 lm/W at 1000 cd/m2, respectively. Long device lifetimes are also one of the most important goals for hybrid WOLEDs. To evaluate device stability, the lifetime of the BPBiPA based WOLEDs were measured and shown in Figure 4d. At an initial brightness of 1000 cd/m2, around 59% and 63% of the initial brightness are retained after ~3700 hrs of continuous working for W2 and W3, respectively. The half lifetimes (T50s) of the W2 and W3 can be extrapolated to be over 7000 and 10000 hrs, using the stretched exponential decay equation.29 The long lifetime demonstrated in this work by using blue emitters with low triplet energy should be a good starting point for future study. Interestingly, the lifetime of W3 with more blue emission is longer than that of W2, suggesting


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that the lifetime of the hybrid WOLEDs is not limited by the blue component but the red or the green ones, and further improvement in lifetime can be expected by using better red and green phosphorescent emitters. As far as we know, this is the first report that high efficiency with low efficiency roll-off, stable emission as well as long lifetime have been achieved simultaneously for a hybrid WOLEDs, demonstrating the superiority of such strategy utilized here. CONCLUSIONS In conclusion, we have demonstrate an effective concept to achieve high efficiency and stability in hybrid WOLEDs simultaneously by using fluorescent blue emitters capable of harvesting triplet excitons by TTA. To improve the device performances, a ETM with large mobility, long-term stability as well as good exciton confinement but low ET was utilized. WOLED based on this concept shows total CE, EQE and PE of 65.7 cd/A, 28.0 %, and 57.3 lm/W at 1000 cd/m2, respectively, together with a high CRI of 80 and a long lifetime of over 7000 hrs at 1000 cd/m2. It is believed that the present study may offer a new possibility for OLEDs towards future lightings. FIGURES


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2.2 2.5 2.9

2.7

3.0 LiF/Al

BPBiPA

α,β-ADN:DACrs

NPB

2-TNATA:F4TCNQ

N N

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DACrs

BPBiPA

Alq3

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N O

O Al N

5.1

ITO

N

O

5.3

5.5

5.7

5.8

5.7

α,β-ADN

Alq3

Figure 1. The energy diagram of the blue devices and the molecule structures of ETMs, host and dopant.

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2

B2 with BPBiPA

10

2

Current Density (A/m )

B1 with Alq3

External Quantum Efficiency (%)

a

Brightness (cd/m )

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

10

4000

B1 with Alq3 B2 with BPBiPA

0.8 0.6

Alq3 emission

0.4 0.2 0.0

3500

400

3000

450 500 550 Wavelength (nm)

600

B1 with Alq3 B2 with BPBiPA

0

10000

20000 time (ns)

30000

2500 0

100

200

300

400

500

Lifetime (hr)

Figure 2. Characteristics of the blue OLEDs with Alq3 or BPBiPA: (a) I-V (closed) and B-V (open) curves; (b) CEs (closed) and EQEs (open) at different luminances; (c) time-resolved


15


electroluminescence responses; and (d) luminance decay curves at an initial brightness of 5000 cd/m2, inset shows the EL spectra.

2.7

T1

X

Förster S1

T1 Dexter

ISC

upconversion by TTA T1

X

fluorescence & delayed fluorescence

phosphorescence

5.3

5.6

5.6

5.6

6.4

6.4

5.8

5.7

phosphorescent EML interlayer

5.7

d

20.0k

W1 with Alq3

15.0k

10.0k

1

10

0.0

-1

2

4

6

8

2

5.0k

0

10

fluorescent EML

100

W1 W2

10

10 1 .0

1

0 .8 0 .6

1

0 .4 W W W W

0 .2 0 .0 400

440

480

520

1 1 2 2

2

1 0 0 c d /m 2 1 0 0 0 c d /m 2 1 0 0 c d /m 2 1 0 0 0 c d /m

560

600

640

680

720

W a v e le n g t h ( n m )

0.1 0

5000

10000

15000

Power Efficiency (lm/W)

2

W2 with BPBiPA 2

10

100 Current Efficiency (cd/A)

10 Current Density (A/m )

S1

LiF/Al

3

10

S1

energy

3.0

E L in t e n s i t y ( a .u .)

2.3 5.5

c

2.9

2.5 2.9

S0

5.1

ITO

2.9

BPAPF: Ir(MDQ)2acac

NPB

2-TNATA:F4TCNQ

2.7

b Exciton

2.3

Alq3

2.3

BPBiPA

2.3

α,β-ADN:DACrs

2.2

BPAPF: SBFK

2.2

BPAPF: SBFK Ir(ppy)3

a

Brightness (cd/m )

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 16 of 23

760

0.1 20000

2

Voltage (V)

Brightness (cd/m )

Figure 3. (a) Device structure and energy level diagram of the WOLEDs; (b) Exciton energy diagram of the WOLEDs and the possible decay ways of singlet and triplet excitons; (c) I-V (closed) and B-V (open) curves; (d) CEs (closed) and PEs (open) at different brightnesses, inset shows the EL spectra of W1 (open) and W2 (closed) at 100 (square) and 1000 cd/m2 (circle).


16

10

1200 1000

2

Luminance (cd/m )

b

1400 4

10

3

800 10

600

2

400 1

10

0

200

2

10

0 2

4

5

0.6 0.4 0.2 0.0

35

35

30

30

25

25

20

20

15

15

10

10

5

5

0

Wavelength (nm)

d 1000 Power Efficiency (lm/W)

40

W2 W3

900

0 10 100 2 Current Density (A/m )

2

W3 100 cd/m 2 W3 1000 cd/m 2 W3 10000 cd/m

0.8

400 440 480 520 560 600 640 680 720 760

40

1

1.0

6

Voltage (V)

c Current Efficiency (cd/A)

3

Normalized EL intensity (a.u.)

a

Current density (A/m )

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


Luminance (cd/m2)

Page 17 of 23

800 700 600 500 0.0

1000

2.0k

4.0k

6.0k

8.0k

10.0k

Lifetime (hour)

Figure 4. (a) I-V (closed) and B-V (open) curves of W3; (b) EL spectra of W3 at 100, 1000 and 10000 cd/m2; (c) CE-I (closed) and PE-I (open) curves of W3; (d) luminance decay curves at an initial brightness of 1000 cd/m2 for W2 (closed) and W3 (open). Table 1. Summary of the performances of some representative WOLEDs.

Max

Ref.3a Ref.10b

Forward viewing EQE (%) 1000 100 Max cd/m2 cd/m2 20.1 13.4 13.7

Ref.11b

11.5e

Work

11.2e

Total EQE (%) 1000 cd/m2 -

CIE(x,y)

CRI

T50 (hrs)f

41.3 -

100 cd/m2 -

(0.38, 0.45) (0.43, 0.43)

85 89

-

-

-

-

(0.28, 0.35)

73

30920

c

Ref.4 15.6 14.6 12.5 26.6 24.8 21.2 (0.46, 0.44) c Ref.6 12.8 11.3 8.6 21.8 14.6 60 Ref.12c 19.0 17.0 82 Ref.13d 19.6 33.3 22.7 7000 W3 17.2 17.0 16.5 28.7 28.4 28.0 (0.41, 0.44) 88 >10000 Notes: [a] all phosphorescent WOLEDs; [b] Hybrid WOLEDs with low ET; [c] Hybrid WOLEDs with high ET; [d] Hybrid WOLEDs with blue TADF emitters; [e] Current efficiency (cd/A); [f] At an initial luminance of


17


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 23

1000 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. ACKNOWLEDGMENT We would like to thank the National Natural Science Foundation of China (Grant No. 51525304) and the National Key Basic Research and Development Program of China (Grant No. 2015CB655002) for financial support. REFERENCES (1) Wang, Q.; Ma, D. G. Management of Charges and Excitons for High-performance White Organic Light-emitting Diodes. Chem. Soc. Rev. 2010, 39, 2387-2398. (2) Gather, M. C.; Köhnen, A.; Meerholz, K. White Organic Light-emitting Diodes. Adv. Mater. 2011, 23, 233-248. (3) Wang, Q.; Ding, J. Q.; Ma, D. G.; Cheng, Y. X.; Wang, L. X.; Wang, F. S. Manipulating Charges and Excitons Within a Single-host System to Accomplish Efficiency/CRI/Colorstability Trade-off for High-performance OWLEDs. Adv. Mater. 2009, 21, 2397-2401. (4) Ye, J.; Zheng, C. J.; Ou, X. M.; Zhang, X. H.; Fung, M. K.; Lee, C. S. Management of Singlet and Triplet Excitons in A Single Emission Layer: A Simple Approach for A High-


18

Page 19 of 23

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


efficiency Fluorescence/Phosphorescence Hybrid White Organic Light-emitting Device. Adv. Mater. 2012, 24, 3410-3414. (5) Schwartz, G.; Reineke, S.; Rosenow, T.C.; Walzer, K.; Leo, K. Triplet Harvesting in Hybrid White Organic Light-emitting Diodes. Adv. Funct. Mater. 2009, 19, 1319-1333. (6) Zheng, C.J.; Wang, J.; Ye, J.; Lo, M.F.; Liu, X.K.; Fung, M.K.; Zhang, X.H.; Lee. C.S. Novel Efficient Blue Fluorophors With Small Singlet-triplet Spliitings: Hosts for Highly Efficient Fluorescence and Phosphorescence Hybrid WOLEDs with Simplified Structure. Adv. Mater. 2013, 25, 2205-2211. (7) Sun, Y.R.; Giebink, N.C.; Kanno, H.; Ma, B.W.; Thompson, M.E.; Forrest, S.R. Management of Singlet and Triplet Excitons for Efficient White Organic Light-emitting Devices. Nature. 2006, 440, 908-912. (8) Ho, C. L.; Wong, W. Y.; Wang, Q.; Ma, D. G.; Wang, L. X.; Lin, Z. Y. Multifunctional Iridium-Carbazolyl Orange Phosphor for High-Performance Two-Element WOLED Exploiting Exciton-Managed Fluorescence/Phosphorescence. Adv. Funct. Mater. 2008, 18, 928-937. (9) Wang, Q.; Ho, C. L.; Zhao, Y. B.; Ma, D. G.; Wong, W. Y.; Wang, L. X. Reduced Efficiency Roll-off in Highly Efficient and Color-stable Hybrid WOLEDs: The Influence of Triplet Transfer and Charge-transport Behavior on Enhancing Device Performance. Org. Electron. 2010, 11, 238-246.


19


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

(10)

Page 20 of 23

Zhao, F. C.; Zhang, Z. Q.; Liu, Y. P.; Dai, Y. F.; Chen, J. S.; Ma, D. G. A Hybrid White Organic Light-emitting Diode with Stable Color and Reduced Efficiency Roll-off by Using A Bipolar Charge Carrier Switch. Org. Electron. 2012, 13, 1049-1055.

(11)

Liu, B. Q.; Wang, L.; Xu, M.; Tao, H.; Zou, J. H.; Gao, D. Y.; Lan, L. F.; Ning, H. L.; Peng, J. B.; Cao. Y. Efficient Hybrid White Organic Light-emitting Diodes with Extremely Long Lifetime: the Effect of n-type Interlayer. Sci. Rep. 2014, 4, 7198.

(12)

Sun, N.; Wang, Q.; Zhao, Y. B.; Chen, Y. H.; Yang, D. Z.; Zhao, F. C.; Chen, J. S.; Ma, D. G. High-performance Hybrid White Organic Light-emitting Devices without Interlayer between Fluorescent and Phosphorescent Emissive Regions. Adv. Mater. 2014, 26, 16171621.

(13)

Zhang, D.D.; Duan, L.; Zhang, Y.G.; Cai, M. H.; Zhang, D. Q.; Qiu, Y. Highly Efficient Hybrid Warm White Organic Light-emitting Diodes Using A Blue Thermally Activated Delayed Fluorescence Emitter: Exploiting the External Heavy-atom Effect. Light-Sci. Appl. 2015, 4, e232.

(14)

Wu, Z.B.; Luo, J. J.; Sun, N.; Zhu, L. P.; Sun, H. D.; Yu, L.; Yang, D. Z.; Qiao. X. F.; Chen, J. S.; Yang, C. L.; Ma, D. G. High-Performance Hybrid White Organic LightEmitting Diodes with Superior Efficiency/Color Rendering Index/Color Stability and Low Efficiency Roll-Off Based on a Blue Thermally Activated Delayed Fluorescent Emitter. Adv. Funct. Mater. 2016, 26, 3306-3313.


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Page 21 of 23

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


(15)

Nishimoto, T.; Yasuda, T.; Lee, S. Y.; Kondo, R.; Adachi, C. A Six-carbazole-decorated Cyclophosphazene as A Host with High Triplet Energy to Realize Efficient DelayedFluorescence OLEDs. Mater. Horiz. 2014, 1, 264-269.

(16)

Zhang, D. D.; Cai, M. H.; Zhang, Y. G.; Zhang, D. Q.; Duan, L. Sterically Shielded Blue Thermally Activated Delayed Fluorescence Emitters with Improved Efficiency and Stability. Mater. Horiz. 2016, 3, 145-151.

(17)

Kondakov, D. Y. Triplet-triplet Annihilation in Highly Efficient Fluorescent Organic Light-emitting Diodes: Current State and Future Outlook. Phil. Trans. R. Soc. A. 2014, 373, 20140321.

(18)

Zhang, D. D.; Zhang, D. Q.; Duan, L. A Sterically Shielded Electron Transporting Material for nearly 100% Internal Quantum Efficiency and Long Lifetime in Fluorescent and Phosphorescent OLEDs. Submitted.

(19)

Antoniadis, H.; Abkowitz, M.A.; Hsieh, B.R. Carrier Deep-trapping Mobility-lifetime Products in Poly(p-phenylene vinylene). Appl. Phys. Lett. 1994, 65, 2030-2032.

(20)

Su, S.J.; Takahashi, Y.; Chiba, T.; Takeda, T.; Kido, J. Structure-Property Relationship of Pyridine-Containing Triphenyl Benzene Electron-Transport Materials for Highly Efficient Blue Phosphorescent OLEDs. Adv. Funct. Mater. 2009, 19, 1260-1267.

(21)

So, F.; Kondakov, D. Degradation Mechanisms in Small-Molecule and Polymer Organic Light-Emitting Diodes. Adv. Mater. 2010, 22, 3762-3777.


21


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

(22)

Page 22 of 23

Wee, K. R.; Han, W. S.; Kim, J. E.; Kim, A. L.; Kwon, S.; Kang, S. O. Asymmetric Anthracene-Based Blue Host Materials: Synthesis and Electroluminescence Properties of 9(2-naphthyl)-10-arylanthracenes. J. Mater. Chem. 2011, 21, 1115-1123.

(23)

Domercq, B.; Grasso, C.; Maldonado, J. L.; Halik, M.; Barlow, S.; Marder, S.R.; Kippelen, B. Electron-Transport Properties and Use in Organic Light-Emitting Diodes of a Bis(dioxaborine)fluorene Derivative. J. Phys. Chem. B. 2004, 108, 8647-8651.

(24)

Kondakov, D.Y.; Pawlik, T.D.; Hatwar, T.K.; Spindler. J. P. Triplet Annihilation Exceeding Spin Statistical Limit in Highly Efficient Fluorescent Organic Light-emitting Diodes. J. Appl. Phys. 2009, 106, 124510.

(25)

So, F.; Kondakov, D. Degradation Mechanisms in Small-Molecule and Polymer Organic Light-Emitting Diodes. Adv. Mater. 2010, 22, 3762-3777.

(26)

Ko, C. W.; Tao, Y. T. 9,9-bis{4-[di-(p-biphenyl)aminophenyl]fluorene: A High T-g and Efficient Hole-transporting Material for Electroluminescent Devices. Synthetic. Met. 2002, 126, 37-41.

(27)

Kondakova, M. E.; Deaton, J. C.; Pawlik, T. D.; Giesen, D. J.; Kondakov, D. Y.; Young, R. H.; Royster, T. L.; Comfort, D. L.; Shore, J. D. Highly Efficient Fluorescentphosphorescent Triplet-harvesting Hybrid Organic Light-emitting Diodes J. Appl. Phys. 2010, 107, 014515.

(28)

Schober, M.; Anderson, M.; Thomschke, M.; Widmer, J.; Furno, M.; Scholz, R.; Lüssem, B.; Leo, K. Quantitative Description of Charge-carrier Transport in A White Organic Lightemitting Diode. Phys. Rev. B. 2011, 84, 165326.


22

Page 23 of 23

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


(29)

Féry, C.; Racine, B.; Vaufrey, D.; Doyeux, H.; Cina, S. Physical Mechanism Responsible for the Stretched Exponential Decay Behavior of Aging Organic Light-emitting Diodes. Appl. Phys. Lett. 2005, 87, 213502.

(30)

Ouyang, X. H.; Li, X. L.; Ai, L.; Mi, D. B.; Ge. Z. Y.; Su, S. J. Novel “Hot Exciton” Blue Fluorophores for High Performance Fluorescent/Phosphorescent Hybrid White Organic Light-emitting Diodes with Superhigh Phosphorescent Dopant Concentration and Improved Efficiency Roll-off. ACS Appl. Mater. Interfaces. 2015, 7, 7869−7877.

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Exploiting p-Type Delayed Fluorescence in Hybrid White OLEDs

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