Letter Cite This: J. Phys. Chem. Lett. 2017, 8, 4967-4973
pubs.acs.org/JPCL
Highly Efficient Solution-Processed Deep-Red Organic Light-Emitting Diodes Based on an Exciplex Host Composed of a Hole Transporter and a Bipolar Host Manli Huang, Bei Jiang, Guohua Xie,* and Chuluo Yang* Hubei Collaborative Innovation Center for Advanced Organic Chemical Materials, Hubei Key Lab on Organic and Polymeric Optoelectronic Materials, Department of Chemistry, Wuhan University, Wuhan, 430072, People’s Republic of China ABSTRACT: With the aim to achieve highly efficient deep-red emission, we introduced an exciplex forming cohost, 4,4′,4″-tris(3-methylphenylphenylamino)triphenylamine (m-MTDATA): 2,5-bis(2-(9H-carbazol-9-yl)phenyl)-1,3,4-oxadiazole (o-CzOXD) (1:1). Due to the efficient triplet up-conversion processes upon the exciplex forming cohost, excellent performances of the devices were achieved with deep-red emission. Using the heteroleptic iridium complexes as the guest dopants, the solution-processed deep-red phosphorescent organic light-emitting diodes (PhOLEDs) with the iridium(III) bis(6-(4-(tert-butyl)phenyl)phenanthridine)acetylacetonate [(TP-BQ)2Ir(acac)]-based phosphorescent emitter exhibited an electroluminescent peak at 656 nm and a maximum external quantum efficiency (EQE) of 11.9%, which is 6.6 times that of the device based on the guest emitter doped in the polymer-based cohost. The unique exciplex with a typical hole transporter and a bipolar material is ideal and universal for hosting the red PhOLEDs and tremendously improves the device performances.
O
carbazol-9-yl)phenyl)-1,3,4-oxadiazole (o-CzOXD). The molecular structures of the materials used in this investigation are shown in Figure 1. Based on a simple architecture, we achieved deep-red PhOLEDs with an EL peak at 656 nm, the Commission Internationale de I’Eclairage (CIE) coordinates of (0.66, 0.33), and a maximum EQE of 11.9%, which is 6.6 times of that of the device with the previously reported polymer based cohost.1 Moreover, the exciton up-conversion mechanism upon the exciplex was unveiled by analyzing the photophysical properties of the emitting layers. Figure 2a shows the absorption and photoluminescence (PL) spectra of m-MTDATA, o-CzOXD and their mixture in thin solid films, respectively. The absorption spectrum of the mixture was nearly the superposition of its constituting materials with no extra new peaks, which suggested that there is no formation of new ground-state transition in the m-MTDATA:o-CzOXD film.20 However, the PL spectrum of the mixture exhibits an obvious red-shift compared with those of the constituting materials. The PL spectra of m-MTDATA and o-CzOXD showed close peaks at 429 and 424 nm, respectively, while the emission peak of m-MTDATA:o-CzOXD was located at 542 nm with a full width at half-maximum (fwhm) of nearly 100 nm which is respectively 40 and 51 nm boarder than those of mMTDATA and o-CzOXD. Besides, the PL onset of mMTDATA:o-CzOXD was located at 475 nm, i.e. 2.61 eV of the lowest singlet state (S1), which is close to the difference
rganic light-emitting devices (OLEDs) have attracted worldwide attention due to the advantages of flexibility, high efficiency, and low power consumption in full-color flatpanel displays and solid-state lightings.1−3 It is worth noting that using phosphorescent or thermally activated delayed fluorescent (TADF) emitters has dramatically improved the electroluminescent (EL) efficiencies of the devices because of sufficiently high utilization ratio of excitons in these emitters.4−7 Despite the emerging applications in displays, medical and sensing equipment, the deep-red OLEDs lag far behind the development of blue, green, and orange counterparts.4,8−10 Great effort has been devoted to improve the EL efficiencies, especially for choosing proper hosts. You’s group synthesized three bipolar host materials and prepared a deep-red PhOLED with a maximum EQE of 18.6% and a peak wavelength of 621 nm.11 In view of the complicated processes in synthesis and purification of the bipolar host materials, using exciplex as host seems to be a cost-effective choice as exciplex can be simply formed by physical blending. Kim’s group8,9,12−19 prepared a series of PhOLEDs based on exciplex forming cohost, which realized quite high efficiencies. It was the efficient energy transfer between the exciplex and the phosphorescent guests that resulted in low driving voltage, high efficiency and low efficiency roll-off at high current density.8,9 There is no doubt that the introduction of exciplex forming cohosts for PhOLEDs is of great significance. In this contribution, we constructed highly efficient solutionprocessed deep-red PhOLEDs based on a new exciplex forming cohost constituting of a typical hole-transporting material, 4,4′,4″-tris(3-methylphenylphenylamino)triphenylamine (mMTDATA), and a bipolar host material, 2,5-bis(2-(9H© 2017 American Chemical Society
Received: August 31, 2017 Accepted: September 26, 2017 Published: September 26, 2017 4967
DOI: 10.1021/acs.jpclett.7b02326 J. Phys. Chem. Lett. 2017, 8, 4967−4973
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Figure 1. Molecular structures of o-CzOXD, m-MTDATA, (TPA-BQ)2Ir(acac), and (TP-BQ)2Ir(acac).
Figure 2. (a) Absorption and PL spectra of m-MTDATA, o-CzOXD and m-MTDATA:o-CzOXD (1:1) in thin solid films, respectively. (b) Timeresolved PL decay of m-MTDATA:o-CzOXD film at 300 K. (c) Transient decay curves and (d) phosphorescent spectra of m-MTDATA, o-CzOXD and m-MTDATA:o-CzOXD in thin solid films.
recursive S1 → S0 transition via the successive up-conversion of triplet excitons. In order to further reveal the mechanism of the abovementioned emission of the exciplex, the phosphorescent spectra (see Figure 2d) of the exciplex and its constituting materials were investigated. From the onset of the low-temperature phosphorescent curves, the lowest triplet levels (T1s) of the donor and the acceptor referred to 2.65 and 2.78 eV, respectively.26 The full decay pathways available in this system are schematically illustrated in Figure 3. Due to the similar time constants of the transient decay of the exciplex and its constituting materials, it is challenging to designate the mechanisms of the exciton upconversion as either TADF or TTA is possible in the exciplex system.26 Nevertheless, either up-conversion process can accelerate excitons transition from nonradiative T1 to the radiative S1. In the exciplex, over 25% singlet excitons can be formed on the exciplex and then transferred to the guest. As the higher utilization ratio of excitons achieved, it is beneficial to acquire high EL efficiencies. Previously, our group reported two deep-red phosphorescent emitters, (TPA-BQ)2Ir(acac) and (TP-BQ)2Ir(acac), rendering the maximum EQEs of 5.2% and 1.8% in the devices hosted by
between the LUMO level of o-CzOXD (−2.56 eV) and the HOMO level of m-MTDATA (−5.10 eV). All the above findings indicated the formation of an exciplex in the mixed mMTDATA:o-CzOXD film.21 The PL spectra of the film of m-MTDATA:o-CzOXD delayed at 0 ns, 50 ns, and 1 μs after the optical excitation at 300 K are shown in Figure 2b. Obviously, no emission of either mMTDATA or o-CzOXD was observed, which further exemplified the exciplex formation.22 What’s more, as the delayed time increased, a moderate red-shift from 527 to 547 nm can be observed, which might be attributed to that the energy difference between the ground states and excited states would change over the delayed time. This phenomenon has been observed in some other exciplex system, such as TCTA:Tm3PyBPZ and mMTDATA:t-Bu-PBD.3,23−25 The transient decay spectra of the exciplex and its constituting materials in thin solid films were also collected. As shown in Figure 2c, the transient PL spectra of mMTDATA and o-CzOXD exhibited simply monoexponential decays with the time constants of 3.45 and 6.38 ns, respectively. However, the transient PL decay of the exciplex consisted of a prompt component of 0.14 μs and a delayed component of 0.61 μs at 300 K. The significantly increased lifetime is due to the 4968
DOI: 10.1021/acs.jpclett.7b02326 J. Phys. Chem. Lett. 2017, 8, 4967−4973
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Figure 3. Full decay pathways available in m-MTDATA:o-CzOXD exciplex emitting. S1 - singlet, T1 - triplet, S0 - ground state, and ISC - intersystem crossing. The superscript letters indicate donor (D), acceptor (A), exciplex (ex), and guest (G).
TmPyPB and Liq are the electron-transporting layer and the electron-injecting layer, respectively. The guests were doped into the EMLs with different ratios of 4 w.t.%, 6 w.t.% and 10 w.t.%, respectively, for the devices A1-A3 with (TPA-BQ)2Ir(acac), and the devices B1−B3 with (TP-BQ)2Ir(acac). The key parameters of the devices are summarized in Table 2. Figure 5a,b shows the current density−voltage−radiance characteristics of the devices. With a simple device structure, deep-red emission with the CIE coordinates of (0.66, 0.33) and a maximum EQE of 11.9% are achievable for the device B1. This EQE is approximately 6.6 times higher than that of the device based on the guest (TP-BQ)2Ir(acac) doped in the polymerbased cohost.1 For (TPA-BQ)2Ir(acac), a decent EQE of 9.8% was also realized. To the best of our knowledge, the results are ones of the best achieved in the solution-processed deep-red PhOLEDs (see Table 3) and roll-offs are relatively lower than those in analogous literatures. To evidence the advantage of the exciplex based host, the device C with the typical bipolar host oCzOXD, i.e., the architecture of glass/ITO/PEDOT:PSS (50 nm)/ o-CzOXD: 10 w.t.% (TPA-BQ)2Ir(acac) (70 nm)/ TmPyPB (40 nm)/ Liq (1 nm)/ Al (100 nm), was constructed as a reference. As shown in Figure 6, the devices C rendered much lower EL efficiency and a higher turn-on voltage. Therefore, the use of exciplex forming cohost has great potential for improving the performances of the devices. In conclusion, we discovered a unique exciplex constituted of a typical hole transporter and a bipolar material. By sufficient experimental data and careful analysis, the exciplex was confirmed to be an excellent host for red PhOLEDs on account of up-conversion process in this system. Herein, we achieved efficient solution-processed deep red PhOLEDs with a maximum EQE of 11.9% based on a simple device architecture. Compared with the polymer based cohost and a bipolar host, the exciplex forming cohost improved the EL performances dramatically by managing the radiative decays and host−guest energy transfer.
conventional mixture of [poly(N-vinylcarbazole (PVK): 2([1,1′-biphenyl]-4-yl)-5-(4-(tert-butyl)phenyl)-1,3,4-oxadiazole (PBD)], respectively.1 To promote the advantage of the exciplexbased host, the mixture of m-MTDATA:o-CzOXD was employed in the emitting layer of the devices. As we can see from Figure 4a, the PL spectrum of the exciplex forming cohost overlaps well with the absorption spectra of the guests, (TPABQ)2Ir(acac) and (TP-BQ)2Ir(acac). After blending, the doped films with (TPA-BQ)2Ir(acac) and (TP-BQ)2Ir(acac) exhibited the main PL peaks at 662 and 655 nm, respectively. At the same time, the residual emission of the exciplex gradually vanished as the concentration of the guest increased (see Figure 4c,d). When the concentration of the guest reached 10%, the energy from the exciplex forming cohost was completely transferred to the guests. To further study the host−guest energy transfer processes, we measured their transient decay spectra excited at 377 nm. As shown in Figure 4e,f, both guest-doped films exhibit monoexponential decays, and the time constants (see Table 1) reduce as the concentration increases. The time constants for the (TPABQ)2Ir(acac)-doped film are respectively 2.06, 1.97, and 1.78 μs at the concentration of 4 w.t.%, 6 w.t.% and 10 w.t.% as well as 1.17, 1.15, and 1.02 μs for the (TP-BQ)2Ir(acac) doped film. It is faster Dexter energy transfer at a high concentration that results in the shortened time constants. Furthermore, the PL quantum yields are, respectively, 22% and 35% for the (TPA-BQ)2Ir(acac)- and (TP-BQ)2Ir(acac)-doped film at the concentration of 4 w.t.%. In this condition, the radiative rate constants (krs) were calculated to be 1.08 × 105 s−1 and 2.97 × 105 s−1, respectively. The latter indicates faster radiative decay. To evaluate the EL performances, the devices with both phosphorescent emitters were constructed with the architecture of glass/[indium tin oxide (ITO)]/poly(3,4ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) (50 nm)/m-MTDATA:o-CzOXD:guests (70 nm)/ 1,3,5-tri(m-pyridin-3-ylphenyl)benzene (TmPyPB) (40 nm)/8hydroxyquinolatolithium (Liq) (1 nm)/Aluminum (Al) (100 nm), where ITO and Al served as anode and cathode, respectively. PEDOT:PSS is the hole-injecting layer, and 4969
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Figure 4. (a) Absorption spectra of (TPA-BQ)2Ir(acac) and (TP-BQ)2Ir(acac) and the PL spectra of m-MTDATA:o-CzOXD. (b) Schematic diagram of the devices. PL spectra of the guests (c) (TPA-BQ)2Ir(acac)- and (d) (TP-BQ)2Ir(acac)-doped films with different concentrations. Transient decay curves of the exciplex host films without and with (e) (TPA-BQ)2Ir(acac) and (f) (TP-BQ)2Ir(acac) doped with different concentrations, respectively.
Table 1. Summary of Photophysical Data
m-MTDATA o-CzOXD m-MTDATA:oCzOXD 4% (TPABQ)2Ir(acac) 6% (TPABQ)2Ir(acac) 10% (TPABQ)2Ir(acac) 4% (TPBQ)2Ir(acac) 6% (TPBQ)2Ir(acac) 10% (TPBQ)2Ir(acac)
Table 2. EL Performances of the Devices
λems, max [nm]
S1/T1 [eV]
fwhm [nm]
[ns]
429 424 542
3.10/2.65 3.26/2.78 2.61/2.59
49 60 100
3.45 6.38 140
0.61
666
-
67
-
2.06
666
-
67
-
1.97
666
-
67
-
1.78
656
-
68
-
1.17
656
-
68
-
1.15
656
-
68
-
1.02
a
b
τPFc
τDFd
emitters
[μs]
device
ratio of doping [w.t.%]
V0.01a [V]
λMax [nm]
fwhmb [nm]
EQEMaxc [%]
A1
4
3.7
666
100
9.8
A2
6
3.5
666
100
6.1
A3
10
4.5
666
100
4.5
B1
4
3.3
656
98
11.9
B2
6
3.4
656
98
8.7
B3
10
3.6
656
98
6.8
C
10
8.6
668
112
2.3
CIEd (x,y) (0.66, 0.33) (0.68, 0.31) (0.68, 0.31) (0.66, 0.33) (0.68, 0.32) (0.69, 0.31) (0.70, 0.29)
Driving voltage estimated at 0.01 mW sr−1 cm−2. bFull width at halfmaximum. cExternal quantum efficiency. dThe Commission Internationale de I’Eclairage (CIE) coordinates. a
a
Measured by the onset of PL spectra. bFull width at half-maximum of the PL spectra. . cTransient lifetimes of the prompt components. d Transient lifetimes of the delayed components.
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Figure 5. Current density−voltage−radiance characteristics curves of the devices A1−A3 (a) and B1−B3 (b). EQE versus current density curves and EL spectra (inset) of the devices A1−A3 (c) and B1−B3 (d), respectively.
Table 3. Comparison of the EL Performances of Solution-Processed Deep-Red PhOLEDs
a
host
guest
λmax/nm
EQEmax (%)
roll-offa (%)
CIE (x,y)
ref.
PVK:PBD NPB:Bebq2 CBP PNB:CBP Cz-BPDPI PVK:PBD TCTA:TPBi m-MTDATA:o-CzOXD
(TP-BQ)2Ir(acac) Ir(SQ)2(acac) 1 5a PtOEP R2 (dpq)2Ir(mprz) (TP-BQ)2Ir(acac) (TPA-BQ)2Ir(acac)
672 688 634 649 650 648 644 656 666
1.8 11.2 4.25 3.8 2.58 7.6 7.32 11.9 9.8
72 41 61 67 54 52 53
(0.65,0.29) (0.71, 0.27) (0.67, 0.30) (0.69, 0.29) (0.65, 0.33) (0.68, 0.30) (0.70, 0.30) (0.66,0.33) (0.66, 0.33)
1 27 28 29 30 31 32 this work
Estimated from the maximum EQE to that at a current density of 100 mA/cm2.
Figure 6. (a) Current density−voltage−radiance characteristics curves and (b) EQE versus current density curves and EL spectra (inset) of the devices C.
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escence spectra were collected at 77 K by a Hitachi fluorescence spectrophotometer F-4600. The transient PL spectra were measured by an FLS920 spectrophotometer (Edinburgh Instruments). OLED Fabrication. The prepatterned indium tin oxide (ITO) substrates were cleaned by ultrasonic acetone bath, followed by
EXPERIMENTAL SECTION
General Information. All reagents used in this work were purchased from commercial sources without further purification. The absorption and PL spectra were measured by a Shimadzu UV-2700 UV−vis spectrophotometer and a Hitachi F-4600 fluorescence spectrophotometer, respectively. The phosphor4971
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ethanol bath. Afterward, the substrates were dried with N2 and then loaded into a UV-Ozone chamber. After UV-Ozone treatment, The PEDOT:PSS layer was spin-coated on the ITO substrate as the hole-injecting layer, and then annealed at 120 °C for 10 min inside the N2-filled glovebox. The emitter layer was also prepared by spin-coating directly on the hole-injecting layer, and then annealed at 50 °C for 10 min. The electron-transporting material and the cathode material were thermally evaporated onto the emitter layer in a vacuum chamber. Before being taken out of the glovebox, the devices were encapsulated with UVcurable epoxy. The voltage−current−luminance characteristics and the EL spectra were simultaneously measured with PR735 SpectraScan Photometer and Keithley 2400 sourcemeter unit under ambient atmosphere at room temperature.
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AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected] (G.X.). *E-mail
[email protected] (C.Y.). ORCID
Guohua Xie: 0000-0003-0764-7889 Chuluo Yang: 0000-0001-9337-3460 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors appreciatively acknowledge the financial support from the National Key R&D Program of China (2016YFB0401002), the National Natural Science Foundation of China (Nos. 61575146 and 91433201), the National Basic Research Program of China (973 Programs 2015CB655002), and the Innovative Research Group of Hubei Province (No. 2015CFA014).
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