High-Performance, Simplified Fluorescence and Phosphorescence

Subscriber access provided by Northern Illinois University

Article

High Performance, Simplified Fluorescence and Phosphorescence Hybrid White Organic Light-Emitting Devices Allowing Complete Triplet Harvesting Xiao-Ke Liu, Wencheng Chen, Hrisheekesh Thachoth Chandran, Jian Qing, Zhan Chen, Xiao-Hong Zhang, and Chun-Sing Lee ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b07629 • Publication Date (Web): 08 Sep 2016 Downloaded from http://pubs.acs.org on September 9, 2016

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

ACS Applied Materials & Interfaces is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 25

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

ACS Applied Materials & Interfaces

High Performance, Simplified Fluorescence and Phosphorescence Hybrid White Organic LightEmitting Devices Allowing Complete Triplet Harvesting Xiao-Ke Liu†, Wencheng Chen†, Hrisheekesh Thachoth Chandran†, Jian Qing†, Zhan Chen†, Xiao-Hong Zhang*‡, and Chun-Sing Lee*†

†Center of Super-Diamond and Advanced Films (COSDAF) & Department of Physics and Materials Sciences, City University of Hong Kong, Hong Kong SAR (P.R. China). ‡Functional Nano & Soft Materials Laboratory (FUNSOM) and Collaborative Innovation Center of Suzhou Nano Science and Technology, Soochow University, Suzhou, Jiangsu 215123 (P.R. China). KEYWORDS: exciplex, delayed fluorescence, phosphor, hybrid WOLED, triplet harvest

ABSTRACT: Causes of efficiency limitation in common fluorescence and phosphorescence hybrid white organic light-emitting devices (WOLEDs) are discussed, and a new device architecture is proposed to address these issues. This architecture employs a fluorescent emitting layer (EML) of blue exciplex-forming cohost, which shows broad and strong thermally activated

ACS Paragon Plus Environment

1


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 2 of 25

delayed fluorescence (TADF). Hybrid WOLEDs based on this architecture not only allow complete triplet harvesting for light generation but also can achieve white light emission with high color rending indexes (CRI) using only two colors. By using 26DCzPPy:PO-T2T as the blue fluorescent EML and 26DCzPPy:Ir complexes as the phosphorescent EML, we prepared a series of two-color WOLEDs with low turn-on voltages of 2.5 ~ 3.3 V, high forward-viewing EQEs of 12.7 ~ 19.3% and high CRIs of 67 ~ 77. These results suggest this new architecture would be an effective way to achieve high performance WOLEDs with simple structures.

INTRODUCTION White organic light-emitting devices (WOLEDs) have been widely studied in the past decades and show their competitiveness in display and lighting applications.1-4 One promising approach to realize high performance WOLEDs is to combine emissions from blue fluorophore and green/red (orange) phosphors.5-13 A key to achieve high-efficiency in such device is to effectively channel energies of singlet and triplet excitons to respectively the blue fluorophore and the red/green phosphor(s).5 A common approach for achieving such channeling/separation is to incorporate an interlayer between exciton generation zone and phosphorescent emitting layers (pEML).5,6,8,14-22 However, efficiencies of these interlayer-based hybrid WOLEDs are modest, showing forward-viewing EQEs below 16.4%, and the causes are still unrevealed. Recently, Ma and coworkers reported a novel concept of hybrid WOLEDs without using such interlayer.9 They fabricated several highly efficient hybrid WOLEDs with a blue fluorophore doped into mixed hosts containing hole- and electron-transporting materials, showing forwardviewing EQEs of 16.5 ~ 19.0%.9 However, even in these high-performance devices, triplet energy in the blue fluorophore layers cannot be fully utilized because energy would be backtransferred from the green phosphor to the blue fluorophore due to the low triplet energy level


2

Page 3 of 25

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


(T1) of the blue fluorophore. Moreover, complexity of such devices is unfavorable for commercial applications. Thus, it is highly desirable to develop simplified hybrid WOLEDs with complete triplet harvesting. Recent breakthrough on thermally activated delayed fluorescent (TADF) emitters by Adachi and co-workers provides a new efficient way to harvest triplet excitons for light emission.23,24 For a TADF emitter, triplet excitons can be efficiently up-converted into singlet excitons through reverse intersystem crossing (RISC) by absorbing environmental thermal energy due to its extremely small energy gap between the singlet excited state (S1) and T1. There are several works using blue TADF emitters instead of conventional blue fluorophores to prepare hybrid WOLEDs.11,25-27 Although these hybrid WOLEDs have realized high efficiencies, problems including severe efficiency roll-off, poor color stability, and low CRI still exists. In this work, we discussed the causes of efficiency limitation in common hybrid WOLEDs and demonstrated a new architecture for high performance hybrid WOLEDs using a TADF blue exciplex-forming cohost as fluorescent emitting layer (fEML). This architecture has several advantages. Firstly, triplet excitons that cannot reach the phosphors can be up-converted into singlet excitons for blue light generation. Secondly, the blue-emitting cohost would intrinsically have T1 higher than those of the green phosphors due to nearly zero singlet-triplet splitting of exciplexes.24,28,29 T1-T1 back transfer from the phosphors to the blue fluorophore is thus prevented. Thirdly, exciplex emissions generally have broad spectra with full width at half maximum (FWHM) over 70 nm,11,28,30,31 which is favorable for preparing WOLEDs with high color rending index (CRI) using only two colors. Thus, by combining the interlayer-free WOLED concept of Ma et al with a TADF blue exciplex fEML, it should be possible to simultaneously achieve 100% triplet harvesting and good CRIs via a highly simplified


3


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 4 of 25

architecture. Following this design, we prepared a series of high performance two-color WOLEDs using 26DCzPPy:PO-T2T as the blue fEML and 26DCzPPy:Ir complexes as the pEML. These two-color WOLEDs show low turn-on voltages of 2.5 ~ 3.3 V, high forwardviewing EQEs of 12.7 ~ 19.3% and high CRI of 67 ~ 77. Efficiencies of these devices are among the highest hybrid WOLEDs; and in particular, their CRIs are considerably higher than those (38 ~ 68) in previously reported two-color WOLEDs.32-35 RESULTS AND DISCUSSION For interlayer-based hybrid WOLEDs, the exciton generation zone(s) is(are) designed to be located on the other side of fEML with regard to the fEML/pEML interface to effectively separate singlet and triplet excitons.5,6,8 An interlayer-based hybrid WOLED with the simplified structure is shown in Figure 1a to illustrate the device design. In this device, the singlet and triplet excitons generated in the electron-hole recombination zone (gray region) at the right of the fEML can be separated via their different diffusion radius RS and RT respectively. Here we have taken Förster and Dexter energy transfer radii into consideration, which means RS is actually the diffusion radius of singlets plus Förster resonant radius and RT is actually the diffusion radius of triplets plus Dexter radius. Color and efficiency of such device are highly sensitive to the thickness (d) of the fEML. When d < RS, the singlets can also be transferred to the phosphors by Förster resonant energy transfer, resulting in insufficient blue emission for realizing white light; if d > RT, triplets would be totally wasted as they cannot reach the phosphors. Thus, the condition of RS < d < RT has to be satisfied for high efficiency white emission in such device. It is commonly believed that RS is limited to 3 nm and RT can be up to 100 nm.5 However, T1-T1 Dexter transfer is found to be inefficient with interlayer thickness > 10 nm.36 This suggests that the efficiency of such devices would be very sensitive to the fEML thickness, which should be


4

Page 5 of 25

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


slightly larger than RS to guarantee exciton separation and avoid triplet energy loss across the interlayer. As reported by Ma and coworkers, hybrid WOLEDs without using interlayer could realize higher efficiencies.9 In this work, our results further show that WOLEDs with thinner pEMLs can realize higher efficiencies due to less triplet energy loss.

Figure 1. Energy transfer diagrams of hybrid WOLEDs with (a) a fEML interlayer, (b) a fEML of doping a fluorophore into mixed hosts, and (c) a fEML of TADF blue exciplex. T1B, T1G, T1R, and T1H are respectively the triplet energy level of blue fluorophore, green, red phosphors and the host. S1B and S1G are respectively the singlet energy levels of blue fluorophore and green phosphor. S0 is the ground state. RS is the diffusion radius of singlets plus Förster resonant radius. RT is the diffusion radius of triplets plus Dexter energy transfer radius. Gray regions represent exciton generation zone. Figure 1b shows a simplified diagram of an interlayer-free hybrid WOLED with a fEML of a fluorophore doped into bipolar mixed hosts.9 By using an appropriately low doping concentration of the fluorophore, average distance between the fluorophore molecules can be adjusted to be the right value, half of which is larger than Dexter transfer radius (~1 nm) but smaller than Förster transfer radius (3 nm).9 In this case, the singlet excitons generated on the hosts can efficiently transferred to and spatially confined on the fluorophore molecules, whereas


5


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 6 of 25

triplet excitons cannot be efficiently transferred to the fluorophores and majority of them reside in the hosts.9 These triplets can effectively diffuse into the nearby pEML since the fEML is much thinner than RT. Although triplet energy loss in interlayer was avoided in such devices, triplet energy loss still exists as the T1 of the blue fluorophore is lower than that of the green phosphor and energy back transfer from the green phosphor to the blue fluorophore occurs. In addition, as singlet excitons at the interface of pEML and fEML can be transferred to the phosphors, the fEML thickness is required to be larger than the RS to maintain sufficient blue emission for white balance. This would cause triplet energy loss in the fEML since more triplet excitons will be trapped by the fluorophore molecules. Therefore, triplet energy in the blue fluorophore layers cannot be fully utilized. Here, we propose a new architecture for interlayer-free WOLEDs with complete triplet harvesting and simplified structures. As shown in Figure 1c, an exciplex-forming cohost with strong blue TADF emission is used as the fEML in this new design, and the blue fluorescent dopant with high T1 is not needed. Blue exciplexes intrinsically have T1 higher than those of the green phosphors due to their small singlet-triplet energy splitting11, which suggests that T1-T1 back transfer could be effectively prevented in our blue exciplex-red phosphor system. In addition, singlet and triplet excitons in the exciton generation zone are no need to be carefully separated for preventing triplet energy loss because the triplets that do not reach the phosphors can be up-converted into singlets for blue light emission. Thus, the thickness of the fEML in this architecture can be more readily tuned, which could be even much larger than the RS while maintaining high efficiencies. As we will show below, high-efficiency WOLEDs with this new architecture can still be realized even when the thickness of fEML (25 nm) is much larger than RS (~8 nm). This implies that triplet energy loss during diffusion in the TADF fEML is


6

Page 7 of 25

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


effectively prevented since triplet excitons that do not reach the phosphors can be up-converted into singlets for radiative decay. In principle, this simple architecture allows complete singlet and triplet excitons harvesting for generating white light with good CRI. In this study, we used a new blue exciplex 26DCzPPy:PO-T2T as the fEML, which is formed by 1:1 mixing (by weight) of a bipolar host 2,6-bis(3-(9H-carbazol-9-yl)phenyl)pyridine (26DCzPPy)37 with an electron-transporting material PO-T2T11,38. Molecular structures of 26DCzPPy and PO-T2T are shown in Figure 2a. Figure 2b shows absorption and photoluminescent (PL) spectra of 26DCzPPy, PO-T2T and 26DCzPPy:PO-T2T. The 26DCzPPy:PO-T2T mixed film shows absorption peaks corresponding to those of its constituents with no extra new peaks; whereas its PL spectrum is clearly red-shifted. The 26DCzPPy film exhibits a PL spectrum peaking at 390 nm with a FWHM of 47 nm. Although the emission of PO-T2T is very weak in solid film, its PL spectrum peak below 400 nm can be observed in dichloromethane. The 26DCzPPy:PO-T2T mixed film shows a PL spectrum with a peak at 482 nm and a FHWM of 82 nm. Energy of this peak (2.57 eV) is close to the energy difference between the highest occupied molecular orbital (HOMO) of 26DCzPPy (-6.05)39 and the lowest unoccupied molecular orbital (LUMO) of PO-T2T (-3.50 eV)40. This suggests that the broad and red-shifted emission in the 26DCzPPy:PO-T2T mixed film is originated from the exciplex formed between 26DCzPPy and PO-T2T.


7

(b) 1.2 Absorption 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

Page 8 of 25

1.2 PO-T2T 26DCzPPy 1.0 26DCzPPy:PO-T2T

1.0 0.8

0.8

0.6

0.6

0.4

0.4

0.2

0.2

PL intensity (a.u.)


0.0

0.0 300

400

500

600

700

Wavelength (nm)

Figure 2. (a) Molecular structures of 26DCzPPy and PO-T2T. (b) UV-vis absorption of 26DCzPPy, PO-T2T and 26DCzPPy:PO-T2T in solid films, and PL spectra of 26DCzPPy (in film), PO-T2T (in dichloromethane) and 26DCzPPy:PO-T2T (in film). Transient PL decays of the 26DCzPPy, PO-T2T and 26DCzPPy:PO-T2T films were measured at 300 K to further confirm the existence of the exciplex (Figure 3). Analysis of the PL decay of 26DCzPPy (Figure 3a) shows two time constants of 6.3 and 11.2 ns; whereas the PO-T2T film shows two decay components with times of 0.5 and 4.9 ns. However, the 26DCzPPy:PO-T2T mixed film has two longer decay times (Figure 3b), which are 27.1 ns and 3.3 µs. The nanosecond and the microsecond decay time constants are respectively assigned to prompt and delayed fluorescence of the exciplex formed between 26DCzPPy and PO-T2T. To confirm that the delayed component is due to TADF, we carried out temperature-dependent transient PL decay measurements for the 26DCzPPy:PO-T2T mixed film. As shown in Figure 4, the PL intensity of the delayed component of the 26DCzPPy:PO-T2T mixed film increases as temperature rises from 100 to 300 K, confirming the nature of TADF.11,28,41


8

Page 9 of 25

(a)

(b) Excitation: 300 nm Emission: 400 nm

100

PL intensity (a.u.)

Normalized PL intensity (a.u.)

PO-T2T 26DCzPPy

10-1

10-2

26DCzPPy:PO-T2T

Excitation: 300 nm Emission: 480 nm

103

102

10-3 101

0

50

100

150

0

200

1000

2000

3000

4000

5000

Time (ns)

Time (ns)

Figure 3. Transient PL decay characteristics of (a) 26DCzPPy and PO-T2T, and (b) 26DCzPPy:PO-T2T in solid films at 300 K.

10

PL 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


4

T=100.00 K T=150.00 K T=200.00 K T=250.00 K T=300.00 K

103

102

101

100 10

100

1000

Time (µ µs)

Figure 4. Temperature-dependent transient PL decay characteristics of 26DCzPPy:PO-T2T mixed film. Electroluminescent (EL) property of the 26DCzPPy:PO-T2T exciplex as a blue emitter was investigated by fabricating a device with an architecture of ITO/TAPC (30 nm)/TCTA (10 nm)/26DCzPPy:50 wt% PO-T2T (25 nm)/PO-T2T (50 nm)/LiF (1 nm)/Al (100 nm). 4,4'Cyclohexylidenebis[N,N-bis(4-methylphenyl)aniline] (TAPC) was used as the hole transporting material due to its high hole mobility of ~ 6×10-3 cm2 V-1 s-1, and PO-T2T was employed as the electron-transporting layer because of its high electron mobility of ~ 4.4×10-3 cm2 V-1 s-1.38,42


9


Figure 5a and b respectively shows current density-voltage-luminance and EQE-luminance plots of this device. The blue device turned on at 3.1 V and delivered a maximum luminance of 6984 cd m-2. It realized a maximum EQE of 7.8% with blue emission peaked at 488 nm. EL data of this blue OLED is summarized in Table S1 (Supporting Information).

101 103

100 10

-1

10

-2

10

-3

102

101

10-4 10-5 10-6

100 0

1

2

3

4

5

6

7

8

9

10 11

Luminance (cd m-2)

104

102

External quantum efficiency (%)

(b)

3 (a) 10

Current density (mA cm-2)

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 10 of 25

10 8 6 4 2 0 -2 1

10

100

1000

10000

-2

Voltage (V)

Luminance (cd m )

Figure 5. (a) Current density-voltage-luminance and (b) external quantum efficiency-luminance characteristics of the 26DCzPPy:PO-T2T exciplex based blue OLED. On the basis of the above blue device, we prepared several hybrid WOLEDs by inserting a pEML of 26DCzPPy:x wt% Ir(MDQ)2acac (y nm), where Ir(MDQ)2acac is iridium(III)bis(2methyldibenzo-[f,h]quinoxaline)(acetylacetonate), between the TCTA layer and the blue exciplex layer. Energy diagram of the white devices including HOMO and LUMO energy levels of the materials as well as singlet and triplet energy levels of the emitters is shown in Figure S1 (Supporting Information). The doping concentration (x wt%) as well as the thickness (y nm) of the pEML are adjusted to simultaneously achieve white light emission and high efficiency. Figure 6a shows current density-voltage-luminance plots of these WOLEDs. All devices deliver maximum brightness of ~ 20,000 cd m-2, whereas their turn-on voltages are quite different. The device with pEML of 26DCzPPy:4 wt% Ir(MDQ)2acac (10 nm) turned on at 3.3 V. The turn-on voltage decreases as the pEML thickness decreases, which may due to the low hole and electron


10

Page 11 of 25

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


motilities of 26DCzPPy (both values ~ 2×10-5 cm2 V-1 s-1)43. The WOLEDs with pEML thicknesses of 2 and 3 nm have very low turn-on voltages of 2.5 V. EQE-luminance plots of these WOLEDs are displayed in Figure 6b and their EL data are summarized in Table S2 (Supporting Information). For the WOLEDs with pEMLs of 6 wt% dopant and various thickness, the EQE increases with decreasing thickness of the pEML. This finding suggests that reduced diffusion length of triplet excitons could cut down triplet energy loss and therefore enhance efficiency. The WOLED with a 10 nm thick pEML with 4 wt% dopant delivers a maximum EQE of 12.7%, which maintains at 11.0% at 1000 cd m-2. The EQE is further enhanced to 16.9% in the device with a 2 nm pEML with 6 wt% dopant. Figure 7 exhibits the EL spectra, CIE and CRI values of these WOLEDs at various luminance. It is interesting to note that the WOLEDs with pEMLs of 3 and 6 nm show stable EL spectra when the luminance ranges from 100 to 1000 cd m-2, and their efficiencies change little. However, this is slightly different when the pEML is 2 nm. Compare with these devices, the WOLED with 2 nm pEML has a higher EQE but its EL spectra show more obvious changes at different luminances as suggested by evolution of the CIE values. Impressively, the WOLED with a 6 nm thick pEML with 6 wt% dopant has a high CRI of 77, which is much higher than those (38 ~ 68) of the reported two-color WOLEDs32-35. This result suggests that blue exciplex in this new concept is favorable for achieving two-color WOLEDs with high CRIs. Gaussian fitting has been conducted for EL spectra (at 100 cd m-2) of these WOLEDs to investigate energy transfer in these devices. As shown in Figure S2 and summarized in Table S3 (Supporting Information), ratio of photons between blue fluorescence and red phosphorescence decreases with reduced thickness of the pEML in the devices with pEML of 26DCzPPy:6 wt% Ir(MDQ)2acac (x nm), suggesting that more triplets could reach the phosphor molecules with


11


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 12 of 25

thinner pEML. This finding is consistent with the evolution of CIE values of these devices. In addition, the device with a ratio of photon of 0.43 suggests a ratio of 0.43 between singlets and triplets which respectively contribute to blue fluorescence and red phosphorescence. This value is larger than the intrinsic value of 0.33 (singlet:triplet=1:3), suggesting some triplets have been up-converted into singlets for blue light emission. In contrast, in the device with a ratio of photon of 0.18, part of singlets have been transferred to the phosphor for red emission, leading to an orange-like device. To gain an insight into exciton diffusion and energy transfer in these WOLEDs, devices with structures of ITO/TAPC (30 nm)/TCTA (10 nm)/26DCzPPy:2 wt% DCJTB (10 nm)/26DCzPPy (x nm, x=2, 4, 6, 8, 10, 12, 14 and 16)/PO-T2T (50 nm)/LiF (1 nm)/Al (100 nm), where DCJTB is

4-(dicyanomethylene)-2-t-butyl-6-(1,1,7,7-tetramethyljulolidyl-9-enyl)-4H-pyran,

were

designed according to the reported methods44,45. As shown in Figure S3a (Supporting Information), there is no interfacial blue exciplex emission in the device with x=2 nm, suggesting that all singlet excitons generated at the 26DCzPPy/PO-T2T interface are transferred to the DCJTB via Förster resonance transfer. When the thickness of the 26DCzPPy spacer is increased to 4 nm, residual emission from the interfacial blue exciplex can be observed, which indicates that Förster energy transfer radius is larger than 2 nm but smaller than 4 nm. This result is consistent with the value of ca. 3 nm in the reported work5. In addition, it is interesting to note that the EL intensity of red emission from DCJTB decreases with increasing thickness (x) of the 26DCzPPy spacer until x>=8 nm (Figure S3a, Supporting Information). This observation suggests that less singlet excitons from the 26DCzPPy/PO-T2T interface can reach DCJTB with thicker spacer, and almost no singlet reaches DCJTB when the spacer is thicker than 8 nm. This finding suggests that the value of RS in our system is ca. 8 nm. Considering that our WOLEDs


12

Page 13 of 25

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


have a fEML of 25 nm, which is much larger than RS of 8 nm, singlets generated at the right side of the fEML (Figure 1c) could not be transferred to the pEML whereas those generated at the left side of the fEML (within 2-4 nm from the pEML/fEML interface) will be transferred to the pEML via Förster resonance. Devices with structures of ITO/TAPC (30 nm)/TCTA (10 nm)/26DCzPPy:6 wt% Ir(MDQ)2acac (10 nm)/26DCzPPy (x nm, x=2, 4, 6, 8, 10, 12, 14 and 16)/PO-T2T (50 nm)/LiF (1 nm)/Al (100 nm) were fabricated to investigate the diffusion and energy transfer of triplets. As shown in Figure S3b (Supporting Information), residual interfacial blue emission can be observed even in the device with x=2 nm, suggesting that Dexter energy transfer is not complete in this device. This observation is consistent with the reported works which demonstrated that Dexter radius is ca. 1 nm9. In addition, it is observed that EL intensity of the red phosphorescence decreases with increasing thickness of the spacer, whereas the EL intensity of the blue exciplex emission changes little. This observation indicates that triplet energy loss increases with increasing diffusion length. This finding is consistent with the reported work which demonstrated that T1-T1 Dexter transfer is inefficient with interlayer thickness > 10 nm36. This finding could also explain the increasing EQE with reduced thickness of pEML for the WOLEDs with pEMLs of 26DCzPPy:6 wt% Ir(MDQ)2acac (x nm). Note that the devices with the spacer of 14 and 16 nm exhibit similar EL intensity of red emission, which means almost no triplets from the 26DCzPPy/PO-T2T interface can reach Ir(MDQ)2acac with a diffusion length >14 nm. This observation indicates that the value of RT is ca. 14 nm in our system.


13


101

104

100 10-1

103

10-2 10-3 10

4 wt% (10 nm) 6 wt% (6 nm) 6 wt% (3 nm) 6 wt% (2 nm)

-4

10-5 10

102 101

-6

10-7 0

1

2

3

4

5

6

7

8

9

100 10 11 12


10

(b)

105

2

Luminance (cd m-2)


3 (a) 10

10

4 wt% (10 nm) 6 wt% (6 nm) 6 wt% (3 nm) 6 wt% (2 nm)

1 1

10

100

1000

10000

Luminance (cd m-2)

Voltage (V)

Figure 6. (a) Current density-voltage-luminance and (b) external quantum efficiency-luminance characteristics of WOLEDs with device structures of ITO/TAPC (30 nm)/TCTA (10 nm)/26DCzPPy:x wt% Ir(MDQ)2acac (y nm)/26DCzPPy:50 wt% PO-T2T (25 nm)/PO-T2T (50 nm)/LiF (1 nm)/Al (100 nm).

1.4 1.2 1.0

100 cd m-2 500 cd m-2 1000 cd m-2 5000 cd m-2

CIE (0.42, 0.37) (0.42, 0.37) (0.42, 0.37) (0.44, 0.37)

(b) 1.6

CRI 71 71 71 70

Normalized intensity (a.u.)


(a) 1.6

0.8 0.6 0.4 0.2 0.0

1.4 1.2 1.0

100 cd m-2 500 cd m-2 1000 cd m-2 5000 cd m-2

CIE (0.39, 0.38) (0.39, 0.38) (0.39, 0.38) (0.41, 0.37)

CRI 77 77 77 75

0.8 0.6 0.4 0.2 0.0

350 400 450 500 550 600 650 700 750 800

350 400 450 500 550 600 650 700 750 800

Wavelength (nm)

Wavelength (nm)

1.4 1.2 1.0

100 cd m-2 500 cd m-2 1000 cd m-2 -2 5000 cd m

CIE (0.42, 0.38) (0.42, 0.38) (0.42, 0.38) (0.43, 0.38)

(d) 1.6

CRI 76 76 76 75


(c) 1.6


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 14 of 25

0.8 0.6 0.4 0.2 0.0

1.4 1.2 1.0

100 cd m-2 -2 500 cd m 1000 cd m-2 5000 cd m-2

CIE (0.52, 0.38) (0.51, 0.38) (0.50, 0.38) (0.49, 0.38)

CRI 73 73 73 73

0.8 0.6 0.4 0.2 0.0

350 400 450 500 550 600 650 700 750 800

350 400 450 500 550 600 650 700 750 800

Wavelength (nm)

Wavelength (nm)


14

Page 15 of 25

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


Figure 7. EL spectra, CIE and CRI values of WOLEDs with pEMLs of (a) 26DCzPPy:4 wt% Ir(MDQ)2acac (10 nm), (b) 26DCzPPy:6 wt% Ir(MDQ)2acac (6 nm), (c) 26DCzPPy:6 wt% Ir(MDQ)2acac (3 nm), and (d) 26DCzPPy:6 wt% Ir(MDQ)2acac (2 nm). To prove that the present architecture is a generic approach for achieving high performance WOLEDs with simple structures. We fabricated several WOLEDs by replacing the Ir(MDQ)2acac with Ir(2-phq)2acac (bis(2-phenylquinoline)(acetylacetonate)iridium(III)). The pEMLs of these devices were 26DCzPPy:2 or 4 wt% Ir(2-phq)2acac (3 nm). As shown in Figure 8 and listed in Table S2 (Supporting Information), both WOLEDs have low turn-on voltages of 2.6 V. The 2 wt% Ir(2-phq)2acac doped device delivered a maximum EQE of 16.8% and a high CRI of 74. By increasing the dopant concentration to 4 wt%, the device realized a high EQE of 19.3%. Gaussian fitting curves for EL spectra (at 100 cd m-2) of these two WOLEDs are shown in Figure S2e and f (Supporting Information), and related parameters have been summarized in Table S3 (Supporting Information). The device with 2 wt% dopant has a ratio of photon of 0.36, which is very close to the intrinsic singlet-triplet ratio of 0.33. However, the 4 wt% based device has a very low photon ratio of 0.15, which means part of singlets have been transferred to the pEML for red phosphorescence. Since the fEML is less efficient than the pEML, singlet excitons transferred to the pEML could contribute to higher EQE for the device. This explains the higher EQE of the 4 wt% based device in reference to the 2 wt% based one. It is also interesting to note that the device with a photon ratio of 0.36 show warm light emission with CIE values of (0.44, 0.38), suggesting that singlets should be prevented from being transferred to the phosphor to maintain white balance. In addition, the EQEs of these devices are among the best-reported hybrid WOLEDs.8,9,21,22,46


15


104

101 100 10

103

-1

10-2

102

10-3

2 wt% 4 wt%

10-4

101

10-5 10-6

100 0

1

2

3

4

5

6

7

8

9

Luminance (cd m-2)


10

2


(b)

(a) 103

10

2 wt% 4 wt%

1 1

10 11

10

(c) 1.6

1.2 1.0

1000

10000

(d) 1.6 100 cd m-2 500 cd m-2 1000 cd m-2 5000 cd m-2

CIE (0.45, 0.38) (0.44, 0.38) (0.44, 0.38) (0.44, 0.38)

CRI 73 74 74 74


1.4

100

Luminance (cd m-2)

Voltage (V)


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 25

0.8 0.6 0.4 0.2 0.0

1.4 1.2 1.0

100 cd m-2 500 cd m-2 1000 cd m-2 5000 cd m-2

CIE (0.52, 0.38) (0.50, 0.38) (0.50, 0.38) (0.48, 0.38)

CRI 62 66 67 69

0.8 0.6 0.4 0.2 0.0

350 400 450 500 550 600 650 700 750 800

350 400 450 500 550 600 650 700 750 800

Wavelength (nm)

Wavelength (nm)

Figure 8. (a) Current density-voltage-luminance, (b) external quantum efficiency-luminance characteristics of WOLEDs with device structures of ITO/TAPC (30 nm)/TCTA (10 nm)/26DCzPPy:2 or 4 wt% Ir(2-phq)2acac (3 nm)/26DCzPPy:50 wt% PO-T2T (25 nm)/PO-T2T (50 nm)/LiF (1 nm)/Al (100 nm). EL spectra of the Ir(2-phq)2acac based WOLEDs with doping concentration of (c) 2 wt% and (d) 4 wt%. CONCLUSION In conclusion, we discussed the causes of efficiency limitation in common hybrid WOLEDs and demonstrated a new device architecture for high performance hybrid WOLEDs by using a TADF blue exciplex as the fEML. This device design not only allows complete triplet harvesting but also be favorable for preparing WOLEDs with high CRIs using only two colors. Following such a design, we prepared a series of high performance two-color WOLEDs with a simplified


16

Page 17 of 25

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


structure and high CRIs. Of these WOLEDs, one with a pEML of 26DCzPPy:2 wt% Ir(2phq)2acac (3 nm) delivers warm white light emission with CIE values of (0.44, 0.38), and shows a low turn-on voltage of 2.6 V, a forward-viewing EQE of 16.8% and a high CRI of 74. The efficiency could be further improved to 19.3% by increasing the dopant concentration. Performance of these devices are among the best reported hybrid WOLEDs. We believe that our device design is a general way for achieving high performance WOLEDs with simple structures. EXPERIMENTAL SECTION Organic materials in this study were commercially available and used as received. Absorption and PL spectra were respectively measured using a Hitachi UV-Vis spectrophotometer U-3010 and a Hitachi fluorescence spectrometer F-4600. Transient fluorescence decays were measured with an Edinburgh Instruments FLS920 spectrometer. The temperature-dependent transient PL decay characterization was conducted by Analysis and Test Center of Beijing University of Chemical Technology. ITO coated glasses with a sheet resistance of 15 Ω per square were used for device fabrication. They were cleaned with detergent and then deionized water, dried in an oven at 120 °C. After treated with UV-ozone for 15 min, the ITO glasses were finally transferred to deposition system. After the pressure of the deposition system drops below 4 × 10–6 Torr, organic materials were deposited at a rate of 1-2 Å s–1. The deposition rates for LiF and Al were respectively ~ 0.1 and ~ 5.0 Å s–1. EL luminescence, spectra and CIE color coordinates were recorded with a Spectrascan PR650 photometer. The current-voltage characteristics were measured using Keithley 2400 SourceMeter. EQE was calculated from current density, luminance and EL spectrum, assuming a Lambertian distribution. The emitting area of the devices is 0.1 cm2. AUTHOR INFORMATION


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 25

Corresponding Author * E-mail: [email protected]. * E-mail: [email protected] ACKNOWLEDGMENT CSL would like to acknowledge supports from the Research Grants Council of the Hong Kong Special Administrative Region, China (Project No. CityU 11304115). REFERENCES (1)

Reineke, S.; Lindner, F.; Schwartz, G.; Seidler, N.; Walzer, K.; Lussem, B.; Leo, K.

White Organic Light-Emitting Diodes with Fluorescent Tube Efficiency. Nature 2009, 459, 234238. (2)

Xiong, Y.; Xu, W.; Li, C.; Liang, B.; Zhao, L.; Peng, J. B.; Cao, Y.; Wang, J. Utilizing

White OLED for Full Color Reproduction in Flat Panel Display. Org. Electron. 2008, 9, 533538. (3)

He, G. F.; Rothe, C.; Murano, S.; Werner, A.; Zeika, O.; Birnstock, J. White Stacked

OLED with 38 lm/W and 100,000-Hour Lifetime at 1000 cd/m2 for Display and Lighting Applications. J. Soc. Inf. Display 2009, 17, 159-165. (4)

D'Andrade, B. W.; Forrest, S. R. White Organic Light-Emitting Devices for Solid-State

Lighting. Adv. Mater. 2004, 16, 1585-1595. (5)

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.


18

Page 19 of 25

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


(6)

Schwartz, G.; Pfeiffer, M.; Reineke, S.; Walzer, K.; Leo, K. Harvesting Triplet Excitons

From Fluorescent Blue Emitters in White Organic Light-Emitting Diodes. Adv. Mater. 2007, 19, 3672-3676. (7)

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 HighEfficiency Fluorescence/Phosphorescence Hybrid White Organic Light-Emitting Device. Adv. Mater. 2012, 24, 3410-3414. (8)

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 Splitting: Hosts for Highly Efficient Fluorescence and Phosphorescence Hybrid WOLEDs with Simplified Structure. Adv. Mater. 2013, 25, 2205-2211. (9)

Sun, N.; Wang, Q.; Zhao, Y.; Chen, Y.; Yang, D.; Zhao, F.; Chen, J.; Ma, D. High-

Performance Hybrid White Organic Light-Emitting Devices Without Interlayer Between Fluorescent and Phosphorescent Emissive Regions. Adv. Mater. 2014, 26, 1617-1621. (10)

Chen, J.; Zhao, F.; Ma, D. Hybrid White OLEDs with Fluorophors and Phosphors.

Mater. Today 2014, 17, 175-183. (11)

Liu, X. K.; Chen, Z.; Qing, J.; Zhang, W. J.; Wu, B.; Tam, H. L.; Zhu, F. R.; Zhang, X.

H.; Lee, C. S. Remanagement of Singlet and Triplet Excitons in Single-Emissive-Layer Hybrid White Organic Light-Emitting Devices Using Thermally Activated Delayed Fluorescent Blue Exciplex. Adv. Mater. 2015, 27, 7079-7085. (12)

Ouyang, X.; Li, X. L.; Ai, L.; Mi, D.; Ge, Z.; Su, S. J. Novel "Hot Exciton" Blue

Fluorophores for High Performance Fluorescent/Phosphorescent Hybrid White Organic Light-


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

Page 20 of 25

Emitting Diodes with Superhigh Phosphorescent Dopant Concentration and Improved Efficiency Roll-Off. ACS Appl. Mater. Interfaces 2015, 7, 7869-7877. (13)

Li, X. L.; Ouyang, X. H.; Liu, M.; Ge, Z. Y.; Peng, J. B.; Cao, Y.; Su, S. J. Highly

Efficient Single- and Multi-Emission-Layer Fluorescent/Phosphorescent Hybrid White Organic Light-Emitting Diodes with Similar to 20% External Quantum Efficiency. J. Mater. Chem. C 2015, 3, 9233-9239. (14) 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. (15)

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 Fluorescent-Phosphorescent Triplet-Harvesting Hybrid Organic Light-Emitting Diodes. J. Appl. Phys. 2010, 107, 014515. (16)

Divayana, Y.; Liu, S. W.; Kyaw, A. K. K.; Sun, X. W. Efficient Extraction of Singlet-

Triplet Excitons for High-Efficient White Organic Light-Emitting Diode with a Multilayer Emission Region. Org. Electron. 2011, 12, 1-7. (17)

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. (18)

Ho, C. L.; Wong, W. Y.; Wang, Q.; Ma, D. G.; Wang, L. X.; Lin, Z. Y. A

Multifunctional Iridium-Carbazolyl Orange Phosphor for High-Performance Two-Element WOLED Exploiting Exciton-Managed Fluorescence/Phosphorescence. Adv. Funct. Mater. 2008, 18, 928-937.


20

Page 21 of 25

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


(19)

Yan, B. P.; Cheung, C. C. C.; Kui, S. C. F.; Xiang, H. F.; Roy, V. A. L.; Xu, S. J.; Che,

C. M. Efficient White Organic Light-Emitting Devices Based on Phosphorescent Platinum (II)/Fluorescent Dual-Emitting Layers. Adv. Mater. 2007, 19, 3599-3603. (20)

Schwartz, G.; Fehse, K.; Pfeiffer, M.; Walzer, K.; Leo, K. Highly Efficient White

Organic Light Emitting Diodes Comprising an Interlayer to Separate Fluorescent and Phosphorescent Regions. Appl. Phys. Lett. 2006, 89, 083509. (21)

Ye, J.; Chen, Z.; An, F.; Sun, M.; Mo, H.-W.; Zhang, X.; Lee, C.-S. Achieving Highly

Efficient Simple-Emission Layer Fluorescence/Phosphorescence Hybrid White Organic LightEmitting Devices via Effective Confinement of Triplets. ACS Appl. Mater. Interfaces 2014, 6, 8964-8970. (22)

Liu, X.-K.; Zheng, C.-J.; Lo, M.-F.; Xiao, J.; Chen, Z.; Liu, C.-L.; Lee, C.-S.; Fung, M.-

K.; Zhang, X.-H. Novel Blue Fluorophor with High Triplet Energy Level for High Performance Single-Emitting-Layer Fluorescence and Phosphorescence Hybrid White Organic Light-Emitting Diodes. Chem. Mater. 2013, 25, 4454-4459. (23)

Hirata, S.; Sakai, Y.; Masui, K.; Tanaka, H.; Lee, S. Y.; Nomura, H.; Nakamura, N.;

Yasumatsu, M.; Nakanotani, H.; Zhang, Q. Highly Efficient Blue Electroluminescence Based on Thermally Activated Delayed Fluorescence. Nat. Mater. 2015, 14, 330-336. (24)

Goushi, K.; Yoshida, K.; Sato, K.; Adachi, C. Organic Light-Emitting Diodes Employing

Efficient Reverse Intersystem Crossing for Triplet-to-Singlet State Conversion. Nat. Photonics 2012, 6, 253-258. (25)

Zhang, D.; Duan, L.; Li, Y.; Zhang, D.; Qiu, Y. Highly Efficient and Color-Stable Hybrid

Warm White Organic Light-Emitting Diodes Using a Blue Material with Thermally Activated Delayed Fluorescence. J. Mater. Chem. C 2014, 2, 8191-8197.


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

(26)

Page 22 of 25

Cho, Y. J.; Yook, K. S.; Lee, J. Y. Cool and Warm Hybrid White Organic Light-Emitting

Diode with Blue Delayed Fluorescent Emitter Both as Blue Emitter and Triplet Host. Sci. Rep. 2015, 5, 7859. (27)

Zhang, D.; Duan, L.; Zhang, Y.; Cai, M.; Zhang, D.; 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. (28)

Liu, X. K.; Chen, Z.; Zheng, C. J.; Liu, C. L.; Lee, C. S.; Li, F.; Ou, X. M.; Zhang, X. H.

Prediction and Design of Efficient Exciplex Emitters for High-Efficiency, Thermally Activated Delayed-Fluorescence Organic Light-Emitting Diodes. Adv. Mater. 2015, 27, 2378-2383. (29)

Zhang, T.; Chu, B.; Li, W.; Su, Z.; Peng, Q. M.; Zhao, B.; Luo, Y.; Jin, F.; Yan, X.; Gao,

Y.; Wu, H.; Zhang, F.; Fan, D.; Wang, J. Efficient Triplet Application in Exciplex DelayedFluorescence OLEDs Using a Reverse Intersystem Crossing Mechanism Based on a ∆ES–T of Around Zero. ACS Appl. Mater. Interfaces 2014, 6, 11907-11914. (30)

Ban, X. X.; Sun, K. Y.; Sun, Y. M.; Huang, B.; Ye, S. H.; Yang, M.; Jiang, W. High

Power Efficiency Solution-Processed Blue Phosphorescent Organic Light-Emitting Diodes Using Exciplex-Type Host with a Turn-on Voltage Approaching the Theoretical Limit. ACS Appl. Mater. Interfaces 2015, 7, 25129-25138. (31)

Chen, D.; Liu, K.; Gan, L.; Liu, M.; Gao, K.; Xie, G.; Ma, Y.; Cao, Y.; Su, S. J.

Modulation of Exciton Generation in Organic Active Planar pn Heterojunction: Toward Low Driving Voltage and High-Efficiency OLEDs Employing Conventional and Thermally Activated Delayed Fluorescent Emitters. Adv. Mater. 2016, DOI: 10.1002/adma.201600612.


22

Page 23 of 25

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


(32)

Peng, T.; Yang, Y.; Bi, H.; Liu, Y.; Hou, Z. M.; Wang, Y. Highly Efficient White

Organic Electroluminescence Device Based on a Phosphorescent Orange Material Doped in a Blue Host Emitter. J. Mater. Chem. 2011, 21, 3551-3553. (33)

Yang, C. L.; Gong, S. L.; Chen, Y. H.; Zhong, C.; Qin, J. G.; Ma, D. G. De Novo Design

of Silicon-Bridged Molecule Towards a Bipolar Host: All-Phosphor White Organic LightEmitting Devices Exhibiting High Efficiency and Low Efficiency Roll-Off. Adv. Mater. 2010, 22, 5370-5373. (34)

Hofmann, S.; Furno, M.; Lussem, B.; Leo, K.; Gather, M. C. Investigation of Triplet

Harvesting and Outcoupling Efficiency in Highly Efficient Two-Color Hybrid White Organic Light-Emitting Diodes. Phys. Status Solidi A 2013, 210, 1467-1475. (35)

Ho, C. L.; Wong, W. Y.; Wang, Q.; Ma, D. G.; Wang, L. X.; Lin, Z. Y. A

Multifunctional Iridium-Carbazolyl Orange Phosphor for High-Performance Two-Element WOLED Exploiting Exciton-Managed Fluorescence/Phosphorescence. Adv. Funct. Mater. 2008, 18, 928-937. (36)

D'Andrade, B. W.; Thompson, M. E.; Forrest, S. R. Controlling Exciton Diffusion In

Multilayer White Phosphorescent Organic Light Emitting Devices. Adv. Mater. 2002, 14, 147151. (37)

Su, S. J.; Sasabe, H.; Takeda, T.; Kido, J. Pyridine-Containing Bipolar Host Materials for

Highly Efficient Blue Phosphorescent OLEDs. Chem. Mater. 2008, 20, 1691-1693. (38)

Hung, W. Y.; Fang, G. C.; Lin, S. W.; Cheng, S. H.; Wong, K. T.; Kuo, T. Y.; Chou, P.

T. The First Tandem, All-Exciplex-Based WOLED. Sci. Rep. 2014, 4, 5161.


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

(39)

Page 24 of 25

Lee, J.; Lee, J. I.; Lee, J. Y.; Chu, H. Y. Stable Efficiency Roll-Off in Blue

Phosphorescent Organic Light-Emitting Diodes by Host Layer Engineering. Org. Electron. 2009, 10, 1529-1533. (40)

Lee, J.-H.; Cheng, S.-H.; Yoo, S.-J.; Shin, H.; Chang, J.-H.; Wu, C.-I.; Wong, K.-T.;

Kim, J.-J. An Exciplex Forming Host for Highly Efficient Blue Organic Light Emitting Diodes with Low Driving Voltage. Adv. Funct. Mater. 2015, 25, 361-366. (41)

Uoyama, H.; Goushi, K.; Shizu, K.; Nomura, H.; Adachi, C. Highly Efficient Organic

Light-Emitting Diodes from Delayed Fluorescence. Nature 2012, 492, 234-238. (42)

Pu, Y.-J.; Nakata, G.; Satoh, F.; Sasabe, H.; Yokoyama, D.; Kido, J. Optimizing the

Charge Balance of Fluorescent Organic Light-Emitting Devices to Achieve High External Quantum Efficiency Beyond the Conventional Upper Limit. Adv. Mater. 2012, 24, 1765-1770. (43)

Doh, Y. J.; Park, J. S.; Jeon, W. S.; Pode, R.; Kwon, J. H. Soluble Processed Low-

Voltage and High Efficiency Blue Phosphorescent Organic Light-Emitting Devices Using Small Molecule Host Systems. Org. Electron. 2012, 13, 586-592. (44)

Zhou, D. Y.; Siboni, H. Z.; Wang, Q.; Liao, L. S.; Aziz, H. Host to Guest Energy

Transfer Mechanism in Phosphorescent and Fluorescent Organic Light-Emitting Devices Utilizing Exciplex-Forming Hosts. J. Phys. Chem. C 2014, 118, 24006-24012. (45)

Baldo, M. A.; O'Brien, D. F.; Thompson, M. E.; Forrest, S. R. Excitonic Singlet-Triplet

Ratio in a Semiconducting Organic Thin Film. Phys. Rev. B 1999, 60, 14422-14428. (46)

Sun, N.; Wang, Q.; Zhao, Y.; Yang, D.; Zhao, F.; Chen, J.; Ma, D. A Hybrid White

Organic Light-Emitting Diode with Above 20% External Quantum Efficiency and Extremely Low Efficiency Roll-Off. J. Mater. Chem. C 2014, 2, 7494-7504.


24

Page 25 of 25

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


Table of Contents Graphic and Synopsis

A new device architecture using a TADF blue exciplex as fluorescent emitting layer is demonstrated for high performance, simplified hybrid white organic light-emitting devices (WOLEDs). This architecture not only allows complete triplet harvesting but also be favorable for preparing WOLEDs with high color rending indexes using only two colors.


25

High-Performance, Simplified Fluorescence and Phosphorescence

Recommend Documents