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Highly efficient ternary polymer-based solution-processable exciplex with over 20% external quantum efficiency in organic light-emitting diode
Organic Electronics 76 (2020) 105449
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Highly efficient ternary polymer-based solution-processable exciplex with over 20% external quantum efficiency in organic light-emitting diode
T
Ping-Li Zhonga,b, Cai-Jun Zhenga,∗, Ming Zhanga,b, Jue-Wen Zhaoa, Hao-Yu Yanga,b, Ze-Yu Hea, Hui Lina, Si-Lu Taoa,∗∗, Xiao-Hong Zhangb a
School of Optoelectronic Science and Engineering, University of Electronic Science and Technology of China (UESTC), Chengdu, 610054, PR China Institute of Functional Nano & Soft Materials (FUNSOM), Jiangsu Key Laboratory for Carbon-Based Functional Materials & Devices, Soochow University, Suzhou, Jiangsu, 215123, PR China
b
A R T I C LE I N FO
A B S T R A C T
Keywords: Solution-process Interfacial exciplexes Thermally activated delayed fluorescence Polymer Organic light-emitting diodes
To develop high-performance solution-processed thermally activated delayed fluorescence (TADF) organic lightemitting diodes (OLEDs) based on exciplex emitters, a novel strategy was present to develop ternary polymerbased exciplex emitter by introducing the third small-molecule constituent material into binary polymer-based exciplex in this work. Compared with common binary polymer-based exciplex emitters, such ternary polymerbased exciplex emitters could not only enhance up-conversion of nonradiative triplet excitons and realize higher exciton utilization, but also maintain the superiorities of polymers in solution processing, like excellent filmforming ability and thermal stability. Based on our strategy, a highly efficient ternary polymer-based exciplex PVK:PO-T2T:mCP was developed. As expected, PVK:PO-T2T:mCP successfully realizes much higher photoluminescence quantum yield of 82.0% and rate constant of RISC process of 2.62 × 105 than PVK:PO-T2T. In the solution-processed OLEDs, interfacial PVK:PO-T2T:mCP exhibits a record-high maximum external quantum efficiency of 21.9% for polymer-based exciplexes, demonstrating the superiority of our strategy for further development of solution-processed TADF-OLEDs.
1. Introduction Thermally activated delayed fluorescent (TADF) emitters were widely developed for organic light-emitting diodes (OLEDs) since the first breakthrough in 2012 [1–12]. With extremely small energy gaps (ΔESTs) between the lowest singlet (S1) and triplet (T1) states, TADF emitters can promote the exciton up-conversion from the non-radiative T1 state to emissive S1 state via thermally assisted reverse intersystem crossing (RISC) process, thereby achieving a theoretical 100% exciton utilization. Up to now, numerous TADF emitters have been reported, and blue, green and red TADF-OLEDs via the vacuum-deposited method have all realized high external quantum efficiencies (EQEs) surpassing or approaching 30% [6,8,13–16], comparable with the best phosphorescent OLEDs. Nevertheless, compared with vacuum-deposited OLEDs, solution-processed OLEDs are more attractive for industry application, because they have many unique advantages, such as simple device structure, convenient high-resolution patterning on large-area substrates, and low-cost manufacturing via spin-coating or ink-jet printing. Thus, developing high-performance solution-processed TADF-OLEDs is
∗
of practical and important significance. In the past years, many efforts have been made to develop efficient single-molecule solution-processable TADF emitters with intramolecular charge transfer (CT) transition. And some solution-processed TADF-OLEDs based on single-molecule TADF emitters have already realized satisfactory EQEs over 20% [9,17–21]. Beyond singlemolecule TADF emitters, exciplexes are another kind of TADF emitters with intermolecular CT transition [2,22–26]. They can achieve extremely small ΔESTs naturally due to the complete frontier molecular orbitals (FMOs) separation. However, the solution-processed TADFOLEDs based on exciplex emitters were rarely reported until now. In 2018, Monkman et al. reported a solution-processed TADF-OLED based on TAPC: DCz-DBTO2 with a maximum EQE of 9.5% by using chloroform: 5 vol% chlorobenzene mixture as the solvent to improve the film quality [27]. Very recently, our group successfully achieved a solutionprocessed TADF-OLED with a remarkable maximum EQE of 24% by using a ternary exciplex TPA-3:9PhFDPhTz:PO-T2T as the emitter [28]. Although these works successfully indicated the capabilities of smallmolecule exciplex systems as the emitters in efficient solution-processed
Corresponding author. Corresponding author. E-mail addresses: [email protected] (C.-J. Zheng), [email protected] (S.-L. Tao).
∗∗
https://doi.org/10.1016/j.orgel.2019.105449 Received 1 August 2019; Received in revised form 5 September 2019; Accepted 10 September 2019 Available online 11 September 2019 1566-1199/ © 2019 Elsevier B.V. All rights reserved.
Organic Electronics 76 (2020) 105449
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Fig. 1. Simplified exciplex formation diagrams of conventional polymer-small-molecule exciplex and ternary polymer-based exciplex.
mCP/PO-T2T exhibits high maximum efficiencies of 64.7 cd A−1 current efficiency (CE), 45.2 lm W−1 power efficiency (PE), and 21.9% EQE, which is 4.7 times comparing with the 4.6% EQE of interfacial PVK/PO-T2T. To the best of our knowledge, this is the first report with an over 20% EQE for the OLEDs using polymer-based exciplexes as the emitters. The outstanding performance of the PVK:PO-T2T:mCP-based OLEDs successfully demonstrates the superiority of our novel ternary exciplex-based strategy for further development of solution-processed TADF-OLEDs.
OLEDs, small-molecule materials are normally inapplicable in solutionprocess and additional efforts were required to realize high-quality films, like selecting suitable solvents [27] and developing specific solution-processable small-molecule constituent materials [28]. Compared with small-molecule materials, polymer materials are more applicable in solution-process due to their excellent film-forming ability and film stability. In 2018, Dias et al. reported a polymer-small-molecule blended exciplex PVK (Poly(9-vinylcarbazole)):PO-T2T ((1,3,5triazine-2,4,6-triyl) tris(benzene-3,1-diyl) tris(diphenylphosphine oxide)) and demonstrated its TADF characteristic, which was the first attempt to develop polymer-based exciplexes. However, the solutionprocessed OLED using PVK:PO-T2T as the emitter only exhibited a low maximum EQE of 4.5% [29]. This result indicates common binary polymer-based exciplex might suffer significant excitons loss. As shown in Fig. 1, the long-chain configurations of polymers will suppress the intermolecular interaction between polymers and other molecules, thus, only limited exciplexes can be formed in the polymer-small-molecule blend, which will result in incomplete up-conversion of triplet excitons. To solve this problem, we present a new strategy to develop ternary polymer-based exciplexes by introducing another small-molecule constituent material, which could form the small-molecule-smallmolecule exciplex with higher excited energy than that of polymersmall-molecule exciplex. Our group has already demonstrated ternary exciplexes with multiple reverse intersystem crossing (RISC) channels could enhance the utilization of triplet excitons [28,30,31]. And in this ternary polymer-based systems, high-energy exciplexes can be easily formed due to the stronger intermolecular interaction between small molecules, and up-conversion of most triplet excitons can proceed via the RISC channel on them. With the energy transfer from high-energy exciplex to low-energy exciplex, polymer-small-molecule exciplexes can harvest all excitons theoretically. Moreover, such ternary polymerbased systems could maintain the superiorities of polymers, like excellent film-forming ability and film stability, without additional requirements on other small-molecule constituent materials. According to our novel strategy, we constructed a ternary polymerbased exciplex emitter, PVK:PO-T2T:mCP (3-bis(9H-carbazol-9-yl) benzene) in this work. Both PVK and mCP can act as the electron-donor to form exciplex with electron-acceptor PO-T2T, and mCP:PO-T2T has a higher energy gap than PVK:PO-T2T. As expected, PVK:PO-T2T:mCP exhibits the emission from PVK:PO-T2T and significant TADF behavior. More importantly, the PVK:PO-T2T:mCP successfully realizes much higher photoluminescence quantum yield (ΦPL, 82.0%) and rate constant of RISC process (kRISC, 2.62 × 105) and lower non-radiative decay T rate constant of triplet excitons (knr , 2.07 × 104) comparing with PVK:PO-T2T. In the OLEDs, interfacial exciplex (25%) PVK: (75%)
2. Experimental section 2.1. General information All reagents were purchased from commercial sources and used as received without further purification. Absorption and PL spectra were measured using a Hitachi UV–vis spectrophotometer U-3010 and a Hitachi fluorescence spectrometer F-4600, respectively. Half-and-half mixed constituting molecules in dichloromethane were coated in quartz tubes and treated at 40 °C under vacuum overnight to form films. Their fluorescence and phosphorescence spectra were measured at 77 K using a Hitachi F-4600 fluorescence spectrometer. The measurement of the phosphorescence spectra was delayed by a chopper with the chopping speed of 40 Hz, corresponding to a delayed time of ≈6.25 ms. The fluorescence quantum yields were measured with 5 nm thick films in N2 atmosphere with an Edinburgh Instruments FLS920 spectrometer. The transient PL decay characterizations were conducted by the Collaborative Innovation Center of Suzhou Nano Science & Technology. The AFM images were measured using chlorobenzene as the solvent to spin-coat the films under the MFP-3D-BIO AFM system (Asylum Research, Oxford Instruments). 2.2. OLEDs fabrication and characterization ITO-coated glasses with a sheet resistance of 15 Ω square−1 were first cleaned with isopropyl alcohol and deionized water, then dried in an oven at 120 °C, treated with UV-ozone, and finally transferred to a deposition system with a base pressure of about 4 × 10−4 Pa. Organic materials were deposited at a rate of 1–2 Å s−1 and the rates were 0.1 and 10 Å s−1 for LiF and Al, respectively. EL luminescence, spectra, and CIE color coordinates were measured with a Spectrascan PR650 photometer and the current-voltage characteristics were measured using a Keithley 2400 SourceMeter under ambient atmosphere. EQE was calculated from the current density, luminance, and EL spectrum, assuming a Lambertian distribution. 2
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Fig. 2. Molecular structures of mCP, PVK and PO-T2T.
3. Results and discussions
shifted with a single-peak at 489 nm compared with that of PVK:POT2T, it is believed the emission of PVK:PO-T2T:mCP should be completely from low-energy exciplex PVK:PO-T2T because they have similar spectral shape and full width at half-maximum. And such blueshift was also observed in other reported tri-component exciplexes [31,34], and should be caused by the intermolecular interaction between PVK and mCP slightly lowering the HOMO energy level of PVK [35]. With no evident overlap between the emission of mCP:PO-T2T and the absorption spectrum of PVK:PO-T2T, Förster energy transfer with the Coulomb mechanism should be hard to happen from mCP:POT2T to PVK:PO-T2T. In this case, singlet-singlet energy transfer can be also allowed via the exchange mechanism (Dexter energy transfer) [36]. In this system, both the energy transfers of singlet and triplet excitons from mCP:PO-T2T to PVK:PO-T2T should be via the exchange mechanism (Dexter energy transfer). Therefore, PVK:PO-T2T:mCP possesses two RISC channels on mCP:PO-T2T and PVK:PO-T2T, and has complete energy transfer from high-energy mCP:PO-T2T to low-energy PVK:PO-T2T, realizing our strategy as shown in Fig. 1. Then, the ΦPLs of solution-processed PVK:PO-T2T and PVK:POT2T:mCP films were measured by using an integrating sphere under oxygen-free condition. And the ΦPLs were estimated to be 36.8% for PVK:PO-T2T with a weight ratio of 1:1, 47% for PVK:PO-T2T:mCP with a weight ratio of 1:1:1 and 82.0% for PVK:PO-T2T:mCP with a weight ratio of 0.5:1:1.5. Obviously, with the assistance of mCP, the ΦPLs of the polymer-based exciplexes are evidently enhanced and realize a quite high value of 82.0%. In PVK:PO-T2T, the long-chain configuration of PVK will suppress the intermolecular interaction between PVK and POT2T, resulting in limited active exciplexes. And insufficient PVK:POT2T will also induce incomplete up-conversion of triplet excitons and then a low ΦPL. While in ternary exciplexes PVK:PO-T2T:mCP, RISC process can be happened on both mCP:PO-T2T and PVK:PO-T2T, and multiple RISC channels in exciplex systems could enhance the utilization of triplet excitons [28,30,31]. Moreover, due to the much stronger intermolecular interaction between mCP and PO-T2T, active mCP:POT2T exciplexes are much easier to be formed than PVK:PO-T2T. Therefore, with increasing doping ratio of mCP in PVK:PO-T2T:mCP, more mCP:PO-T2T exciplexes could be formed, which will further benefit the RISC process and enhance the ΦPLs of ternary exciplexes. To better prove the superiority of the ternary polymer-based exciplex, we further measured the transient PL decay of PVK:PO-T2T:mCP with a weight ratio of 0.5:1:1.5 at room temperature. As shown in Fig. 3d, PVK:PO-T2T:mCP exhibits nearly identical decay curve with mCP:POT2T. This result does not only demonstrate the TADF characteristic of ternary exciplex but also indicates the RISC process mainly happens on mCP:PO-T2T in the system, which is consistent with our prediction. The key kinetic parameters of PVK:PO-T2T with a weight ratio of 1:1 and PVK:PO-T2T:mCP with a weight ratio of 0.5:1:1.5 were then calculated. For blue and green fluorescent emitters, the internal conversion process
PVK, PO-T2T, and mCP were directly purchased from commercial sources, and their molecular structures are shown in Fig. 2. As green PVK:PO-T2T exciplex was reported with a low maximum EQE of 4.5% in the OLEDs [29], we selected them as the target. Besides, the common hole transporting host mCP is chosen as the third small-molecule constituent material, as it was reported to form a blue TADF exciplex with PO-T2T [32]. In the photophysical characterizations of interfacial exciplexes, it is impossible to confine the photogenerated excitons at the active interface, which will induce the undesired emissions of the constituent materials. To clearly demonstrate the photophysical properties of exciplexes in this work, their PL spectra were all measured in bulk heterojunction. Fig. 3a and b shows the absorption and PL spectra of PVK:PO-T2T, mCP:PO-T2T and their constituting molecules. Both two mixtures exhibit nearly identical absorption spectra to those of their constituting molecules, which suggests that the formation of new ground-state transitions does not occur in the mixed films. Meanwhile, PL spectra of two mixtures are significantly red-shifted relative to those of the constituting molecules. The emissions of PVK:PO-T2T and mCP:PO-T2T are peaked at 511 and 450 nm, respectively, while those of PVK, PO-T2T, and mCP are peaked at 386, 327 and 354 nm, respectively. These results indicate the formation of new excited states in the mixed films. Besides, the PL peaks of PVK:PO-T2T and mCP:PO-T2T correspond to the energies of 2.43 and 2.75 eV, which match well to the differences between the lowest unoccupied molecular orbital (LUMO) energy level of PO-T2T (−3.22 eV) [30] and the highest occupied molecular orbital (HOMO) energy levels of PVK (−5.64 eV) [33] and mCP (−5.84 eV) [30]. All of this evidence suggests the formation of exciplexes in two bi-component mixed films, and mCP:PO-T2T has a higher energy gap than PVK:PO-T2T. As shown in Fig. S1, the fluorescence and phosphorescence spectra of PVK:PO-T2T and mCP:PO-T2T mixed films were measured at 77 K. From the onset position of the fluorescence and phosphorescence spectra, the S1 and T1 energy levels can be estimated as 2.981 and 2.973 eV for mCP:PO-T2T and 2.886 and 2.857 eV for PVK:PO-T2T. Thus, the ΔEST of mCP:PO-T2T and PVK:POT2T are estimated to be 0.008, 0.029 eV, respectively. Besides, as shown in Fig. 3d, transient PL decays of two exciplexes at 300 K in the microsecond range further confirm their TADF characteristic, which are consistent with the reported literature [29,30]. Based on two bi-component exciplexes, we constructed the ternary exciplex PVK:POT2T:mCP accordingly. As shown in Fig. 3c, the ternary mixture also exhibits cumulative absorption compared with three constituting molecules, suggesting still no formation of new ground-state transitions. And the PL spectrum of PVK:PO-T2T:mCP is also significantly redshifted compared with those of three constituting materials. These results indicated that exciplex would be formed in the tri-component mixtures. Although the PL spectrum of ternary exciplex is slightly blue3
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Fig. 3. Absorption and PL spectra of a) mCP:PO-T2T with a weight ratio of 1:1 and their pristine materials, b) PVK:PO-T2T with a weight ratio of 1:1 and their pristine materials, and c) PVK:PO-T2T:mCP with a weight ratio of 1:1:1 and their pristine materials in thin solution-processed solid films (PO-T2T in toluene solution); d) Transient fluorescence decays of mCP:PO-T2T film with a weight ratio of 1:1 at 450 nm, PVK:PO-T2T film with a weight ratio of 1:1 at 510 nm and PVK:PO-T2T:mCP film with a weight ratio of 0.5:1:1.5 at 485 nm (Excitation at 300 nm in vacuum).
of singlet excitons could be ignored compared with fluorescence decay and intersystem crossing process of singlet excitons [11,13,37], thus the calculations were carried out via the following equations [11,37],
kp =
1 τp
(1)
kd =
1 τd
(2)
kF = k p ΦF
(3)
kISC = kP (1 − ΦF)
(4)
kRISC =
kRISCs are evidently increased to 2.26 × 105 for PVK:PO-T2T:mCP comparing with the kRISCs of 0.38 × 105 for PVK:PO-T2T. As a result, ΦTADF is also increased from 13.0% for PVK:PO-T2T to 55.0% for PVK:PO-T2T:mCP. Therefore, the improved ΦPL of PVK:PO-T2T:mCP is mainly ascribed to the higher up-conversion of triplet excitons with two RISC channels on PVK:PO-T2T and mCP:PO-T2T. Meanwhile, with T of PVK:PO-T2T:mCP is evidently remore efficient RISC process, knr 4 duced to 2.07 × 10 comparing with that of 3.57 × 104 for PVK:POT2T, indicating less loss of triplet excitons. These results prove our ternary polymer-based exciplexes will evidently improve the exciton utilization and can be expected to realize high efficiencies in the OLEDs. As additional hole-transporting layer (HTL) and electron-blocking layer are hard introduced in solution-processed OLEDs, PO-T2T in blended exciplexes as the emitting layer (EML) may cause serious electron leakage from EML to hole-injection layer (HIL), resulting in poor device performance. To better evaluate the electroluminescence (EL) performance of PVK:PO-T2T:mCP, we constructed the interfacial exciplex-based OLEDs with a general configuration of ITO/PEDOT:PSS (30 nm)/EDL (electron-donor layer, 35 nm)/PO-T2T (45 nm)/LiF (0.8 nm)/Al (80 nm). In the devices, ITO (indium tin oxide) and Al were served as the anode and the cathode, respectively; PEDOT:PSS (poly (ethylenedioxythiophene):poly(styrenesulfonate) was used as HIL; LiF was acted as an electron injection layer; EDL was served as the HTL; POT2T was acted as the electron-transporting layer. The interfacial exciplexes formed between EDL and PO-T2T are the emitters of these devices. For ternary exciplex PVK:PO-T2T:mCP, the mixtures of PVK:mCP with different weight ratios were used as the EDL. And pure
k p k d ΦTADF kISC
(5)
ΦF
kISC ⎞ T knr = kd − ⎛1 − kRISC k + kISC ⎠ F ⎝ ⎜
⎟
(6)
where ΦF and ΦTADF are the quantum efficiencies of prompt and delayed fluorescence; ΦISC is the efficiency of ISC process; kp, kd, kF, kISC are rate constants of prompt fluorescence, delayed fluorescence decay, fluorescence decay, and ISC process from S1 to T1 state, respectively. ΦF and ΦTADF of two exciplex emitters are estimated from the ΦPLs with relative ratios which were calculated from the transient PL results [11]. Consequently, all key kinetic parameters of two exciplexes are listed in Table 1. With naturally separated HOMO and LUMO distributions, both two exciplexes exhibit relatively small kFs and relatively large kISCs, resulting in all ΦFs with small values. However, for the RISC process, 4
Organic Electronics 76 (2020) 105449
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Table 1 Summary of photophysical parameters for PVK:PO-T2T (weight ratio of 1:1) and PVK:PO-T2T:mCP (weight ratio of 0.5:1:1.5). Emitters
ΦPLa [%]
ΦPb [%]
ΦTADFc [%]
τPd (ns)
τTADFe (μs)
kpf ( × 107 s−1)
kFg ( × 107 s−1)
Kdh ( × 104 s−1)
kISCi ( × 107 s−1)
kRISCj ( × 105 s−1)
knrTk ( × 104 s−1)
PVK:PO-T2T PVK:PO-T2T:mCP
36.8 82.0
21.7 27.0
15.1 55.0
28.0 18.5
23.12 10.60
3.57 5.40
0.77 1.46
4.30 9.40
2.80 3.94
0.38 2.62
3.57 2.07
a b c d e f g h i j k
ΦPL is the total photoluminescence fluorescence quantum efficiency value. the prompt fluorescence quantum efficiency. The delayed fluorescence quantum efficiency. prompt fluorescence lifetime. Delayed fluorescence lifetime. the rate constants of prompt fluorescence. the rate constant of fluorescence decay. The rate constants of delayed fluorescence decay. The rate constant of intersystem crossing. the rate constant of reverse intersystem crossing. the rate constant of nonradiative transitions of triplet excitons.
Fig. 4. 3D AFM height images (5 × 5 μm) of 60 nm thick unannealed films: a) pure PVK; b) pure mCP; c) mCP:PVK (weight ratio of 1:1) and films with annealing at 100 °C for 30 min: d) pure PVK; e) pure mCP; f) mCP:PVK with a weight ratio of 1:1.
Fig. 5. a) EQE-current density plots and b) EL spectra of the solution-processed OLEDs based on interfacial exciplex emitters of mCP/PO-T2T; PVK/PO-T2T; 75% PVK:25% mCP/PO-T2T; 50% PVK:50% mCP/PO-T2T; 25% PVK:75% mCP/PO-T2T; 15% PVK:85% mCP/PO-T2T.
optimized solution-process, the root-mean-square (RMS) surface roughness values of PVK, mCP and PVK:mCP spin-coated films were respectively 0.241, 0.281 and 0.213 nm without further annealing. And after annealing at 100 °C for 30 min, the RMS surface roughness values of PVK, mCP and PVK:mCP films became 0.282, 1.010 and 0.220 nm, respectively. Obviously, pure PVK and PVK:mCP films showed no significant change after annealing at 100 °C for 30 min and all exhibited
mCP and pure PVK were also used as the EDL to construct bi-component exciplexes as the comparisons. Prior to device fabrication, the atomic force microscopy (AFM) 3D images were investigated since the surface topography of films could have great influence in OLED performance. As shown in Fig. 4, we measured the morphologies of the spin-coated films of pure PVK, pure mCP and PVK:mCP mixture with a weight ratio of 1:1. With the 5
Organic Electronics 76 (2020) 105449
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Table 2 Performances of the solution-processed OLEDs based on exciplex emitters. Device
λmax [nm]
Vona (V)
CEb (cd/A)
PEb (lm/W)
EQEb (%)
CIE (x, y)
mCP/PO-T2T PVK/PO-T2T 75% PVK:25% 50% PVK:50% 25% PVK:75% 15% PVK:85%
480 534 530 526 514 512
3.5 3.0 4.5 4.2 4.1 3.6
6.3 ± 0.4 (6.7) 14.5 ± 0.3 (14.8) 35.7 ± 1.0 (36.7) 47.2 ± 0.6 (47.8) 63.8 ± 0.9 (64.7) 42.6 ± 0.6 (43.2)
5.0 ± 0.4 (5.4) 9.1 ± 0.2 (9.3) 22.0 ± 0.6 (22.6) 32.3 ± 0.4 (32.7) 44.6 ± 0.6 (45.2) 29.1 ± 0.4 (29.5)
2.8 ± 0.2 (3.0) 4.5 ± 0.1 (4.6) 11.1 ± 0.3 (11.4) 14.9 ± 0.2 (15.1) 21.6 ± 0.3 (21.9) 14.4 ± 0.2 (14.6)
(0.19, (0.32, (0.31, (0.29, (0.26, (0.26,
a b
mCP/PO-T2T mCP/PO-T2T mCP/PO-T2T mCP/PO-T2T
0.32) 0.56) 0.55) 0.55) 0.51) 0.51)
Voltage at 1 cd/m2. The data in parentheses is the maximum.
4. Conclusion
very small RMS surface roughness. These results are consistent with the excellent film-forming ability and film stability of polymer materials, and proves polymer-based mixture films can keep the superior capability of polymers in solution-process. Reversely, the pure mCP film became much rougher after annealing with RMS surface roughness values increased from 0.281 to 1.010 nm, which should be attributed to the crystallization of mCP. And such poor film stability might also harm the performance of the OLEDs using interfacial mCP:PO-T2T as the emitter. As shown in Fig. 5a and listed in Table 2, the solution-processed OLED using interfacial PVK:PO-T2T as the emitter exhibits a turn-on voltage around 3.0 V and maximum CE, PE and EQE of 14.8 cd A−1, 8.9 lm W−1, and 4.6%, respectively, which are consistent with the previous report [29]. And the solution-processed OLED based on interfacial mCP:PO-T2T exhibits a turn-on voltage around 3.5 V and a quite poor EQE of 3.0%, which is much lower than the efficiencies of vacuumdeposited mCP: PO-T2T-based OLEDs [32]. These results might be ascribed to the poor film-stability of pure mCP. To realize the optimal performance of PVK:PO-T2T:mCP, the solution-processed OLEDs were constructed using different PVK:mCP EDLs with weight ratios of 75%:25%, 50%:50%, 25%:75%, and 15%:85%. As shown in Fig. 5b, the solution-processed device based on 75% PVK:25% mCP exhibits almost identical EL spectra with the interfacial PVK:PO-T2T-based device. However, with the increased weight ratio of mCP, the EL spectra of the devices based on tri-component exciplexes exhibit clearly blue-shift, which might be caused by the intermolecular interaction between PVK and mCP slightly lowering the HOMO energy level of PVK [35]. More important, all four devices exhibited maximum EQEs over 11%, much higher than the contrastive OLEDs based on PVK:PO-T2T and mCP:POT2T. Particularly, with the mixing ratio of mCP increased from 25% to 75%, the maximum EQEs of ternary exciplex-based OLEDs are enlarged from 11.4% to 21.9%. In such ternary polymer-based exciplex, upconventions of triplet excitons mainly happen on high-energy mCP:POT2T, thus higher exciton utilization is realized with more mCP in the EDL. The solution-processed device based on 25% PVK:75% mCP exhibits maximum CE of 64.7 cd A−1, PE of 45.2 lm W−1, and EQE of 21.9%, which is significantly improved 4.7 times comparing with PVK:PO-T2T-based device. To the best of our knowledge, this is the first report with an EQE over 20% for the OLEDs based on polymer-based exciplex emitters. According to the ΦPL value of 82.0% for PVK:POT2T:mCP with a weight ratio of 0.5:1:1.5, the out-coupling efficiency of our OLEDs should be above 26.7%, which is located in the reasonable range from 20% to 30%. Relatively, as PVK:PO-T2T with a weight ratio of 1:1 exhibits a ΦPL value of 36.8%, the maximum EQE of PVK:POT2T-based OLED is evidently lower than its theoretical value around 9.9%. As shown in Fig. S4, the current densities of the devices are significantly enhanced with the increasing doping ratio of mCP. Thus, it is speculated that the efficiency of PVK:PO-T2T-based OLED might be further harmed by the unbalanced charge transporting. Particularly, the average values from eight results were also calculated for the efficiencies of all devices. As listed in Table 2, all devices exhibit good reproducibility.
In conclusion, to develop efficient polymer-based exciplex emitters, a novel ternary exciplex PVK:PO-T2T:mCP was constructed by introducing mCP into the binary exciplexes PVK:PO-T2T in this work. In this ternary exciplex system, high-energy exciplexes mCP:PO-T2T can be easily formed due to the stronger intermolecular interaction between small molecules, realizing stronger up-conversion of triplet excitons proceeded via the RISC channel on them. With the energy transfer from mCP:PO-T2T to PVK:PO-T2T, ternary polymer-based exciplex PVK:POT2T:mCP can harvest all excitons theoretically. Thus, PVK:mCP:PO-T2T successfully realizes much higher ΦPL of 82.0% and kRISC of 2.62 × 105 T and lower knr of 2.07 × 104 comparing with PVK:PO-T2T. Meanwhile, PVK:PO-T2T:mCP also maintains excellent film-forming ability and thermal stability of polymer material PVK. By using interfacial exciplex as the emitter, solution-processed OLED based on 25% PVK:75% mCP exhibits high maximum efficiencies of 64.7 cd A−1 CE, 45.2 lm W−1 PE, and 21.9% EQE. To the best of our knowledge, this is the first report for polymer-based exciplex emitter with an over 20% EQE in the OLEDs. These results successfully demonstrate the feasibility of our strategy to develop high-performance polymer-based exciplex emitters of solutionprocessed TADF-OLEDs. Acknowledgements This work was supported by the National Key Research and Development Program of China (Grant No. 2016YFB0401002), the National Natural Science Foundation of China (NSFC Grant Nos. 51773029, 51533005 and 51821002), International Cooperation and Exchange Project of Science and Technology Department of Sichuan Province (Grant Nos. 2019YFH0057, 2019YFH0059 and 18GJHZ0015), the Fundamental Research Funds for the Central Universities (Grant No. ZYGX2016Z010). Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.orgel.2019.105449. References [1] H. Uoyama, K. Goushi, K. Shizu, H. Nomura, C. Adachi, Highly efficient organic light-emitting diodes from delayed fluorescence, Nature 492 (2012) 234–238. [2] K. Goushi, K. Yoshida, K. Sato, C. Adachi, Organic light-emitting diodes employing efficient reverse intersystem crossing for triplet-to-singlet state conversion, Nat. Photonics 6 (2012) 253–258. [3] W. Zeng, T. Zhou, W. Ning, C. Zhong, J. He, S. Gong, G. Xie, C. Yang, Realizing 22.5% external quantum efficiency for solution-processed thermally activated delayed-fluorescence OLEDs with red emission at 622 nm via a synergistic strategy of molecular engineering and host selection, Adv. Mater. 31 (2019) 1901404. [4] K. Wang, C.J. Zheng, W. Liu, K. Liang, Y.Z. Shi, S.L. Tao, C.S. Lee, X.M. Ou, X.H. Zhang, Avoiding energy loss on TADF emitters: controlling the dual conformations of D-A structure molecules based on the pseudoplanar segments, Adv. Mater. 29 (2017) 1701476. [5] M.Y. Wong, E. Zysman-Colman, Purely organic thermally activated delayed fluorescence materials for organic light-emitting diodes, Adv. Mater. 29 (2017) 1605444.
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Organic Electronics journal homepage: www.elsevier.com/locate/orgel
Highly efficient ternary polymer-based solution-processable exciplex with over 20% external quantum efficiency in organic light-emitting diode
T
Ping-Li Zhonga,b, Cai-Jun Zhenga,∗, Ming Zhanga,b, Jue-Wen Zhaoa, Hao-Yu Yanga,b, Ze-Yu Hea, Hui Lina, Si-Lu Taoa,∗∗, Xiao-Hong Zhangb a
School of Optoelectronic Science and Engineering, University of Electronic Science and Technology of China (UESTC), Chengdu, 610054, PR China Institute of Functional Nano & Soft Materials (FUNSOM), Jiangsu Key Laboratory for Carbon-Based Functional Materials & Devices, Soochow University, Suzhou, Jiangsu, 215123, PR China
b
A R T I C LE I N FO
A B S T R A C T
Keywords: Solution-process Interfacial exciplexes Thermally activated delayed fluorescence Polymer Organic light-emitting diodes
To develop high-performance solution-processed thermally activated delayed fluorescence (TADF) organic lightemitting diodes (OLEDs) based on exciplex emitters, a novel strategy was present to develop ternary polymerbased exciplex emitter by introducing the third small-molecule constituent material into binary polymer-based exciplex in this work. Compared with common binary polymer-based exciplex emitters, such ternary polymerbased exciplex emitters could not only enhance up-conversion of nonradiative triplet excitons and realize higher exciton utilization, but also maintain the superiorities of polymers in solution processing, like excellent filmforming ability and thermal stability. Based on our strategy, a highly efficient ternary polymer-based exciplex PVK:PO-T2T:mCP was developed. As expected, PVK:PO-T2T:mCP successfully realizes much higher photoluminescence quantum yield of 82.0% and rate constant of RISC process of 2.62 × 105 than PVK:PO-T2T. In the solution-processed OLEDs, interfacial PVK:PO-T2T:mCP exhibits a record-high maximum external quantum efficiency of 21.9% for polymer-based exciplexes, demonstrating the superiority of our strategy for further development of solution-processed TADF-OLEDs.
1. Introduction Thermally activated delayed fluorescent (TADF) emitters were widely developed for organic light-emitting diodes (OLEDs) since the first breakthrough in 2012 [1–12]. With extremely small energy gaps (ΔESTs) between the lowest singlet (S1) and triplet (T1) states, TADF emitters can promote the exciton up-conversion from the non-radiative T1 state to emissive S1 state via thermally assisted reverse intersystem crossing (RISC) process, thereby achieving a theoretical 100% exciton utilization. Up to now, numerous TADF emitters have been reported, and blue, green and red TADF-OLEDs via the vacuum-deposited method have all realized high external quantum efficiencies (EQEs) surpassing or approaching 30% [6,8,13–16], comparable with the best phosphorescent OLEDs. Nevertheless, compared with vacuum-deposited OLEDs, solution-processed OLEDs are more attractive for industry application, because they have many unique advantages, such as simple device structure, convenient high-resolution patterning on large-area substrates, and low-cost manufacturing via spin-coating or ink-jet printing. Thus, developing high-performance solution-processed TADF-OLEDs is
∗
of practical and important significance. In the past years, many efforts have been made to develop efficient single-molecule solution-processable TADF emitters with intramolecular charge transfer (CT) transition. And some solution-processed TADF-OLEDs based on single-molecule TADF emitters have already realized satisfactory EQEs over 20% [9,17–21]. Beyond singlemolecule TADF emitters, exciplexes are another kind of TADF emitters with intermolecular CT transition [2,22–26]. They can achieve extremely small ΔESTs naturally due to the complete frontier molecular orbitals (FMOs) separation. However, the solution-processed TADFOLEDs based on exciplex emitters were rarely reported until now. In 2018, Monkman et al. reported a solution-processed TADF-OLED based on TAPC: DCz-DBTO2 with a maximum EQE of 9.5% by using chloroform: 5 vol% chlorobenzene mixture as the solvent to improve the film quality [27]. Very recently, our group successfully achieved a solutionprocessed TADF-OLED with a remarkable maximum EQE of 24% by using a ternary exciplex TPA-3:9PhFDPhTz:PO-T2T as the emitter [28]. Although these works successfully indicated the capabilities of smallmolecule exciplex systems as the emitters in efficient solution-processed
Corresponding author. Corresponding author. E-mail addresses: [email protected] (C.-J. Zheng), [email protected] (S.-L. Tao).
∗∗
https://doi.org/10.1016/j.orgel.2019.105449 Received 1 August 2019; Received in revised form 5 September 2019; Accepted 10 September 2019 Available online 11 September 2019 1566-1199/ © 2019 Elsevier B.V. All rights reserved.
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Fig. 1. Simplified exciplex formation diagrams of conventional polymer-small-molecule exciplex and ternary polymer-based exciplex.
mCP/PO-T2T exhibits high maximum efficiencies of 64.7 cd A−1 current efficiency (CE), 45.2 lm W−1 power efficiency (PE), and 21.9% EQE, which is 4.7 times comparing with the 4.6% EQE of interfacial PVK/PO-T2T. To the best of our knowledge, this is the first report with an over 20% EQE for the OLEDs using polymer-based exciplexes as the emitters. The outstanding performance of the PVK:PO-T2T:mCP-based OLEDs successfully demonstrates the superiority of our novel ternary exciplex-based strategy for further development of solution-processed TADF-OLEDs.
OLEDs, small-molecule materials are normally inapplicable in solutionprocess and additional efforts were required to realize high-quality films, like selecting suitable solvents [27] and developing specific solution-processable small-molecule constituent materials [28]. Compared with small-molecule materials, polymer materials are more applicable in solution-process due to their excellent film-forming ability and film stability. In 2018, Dias et al. reported a polymer-small-molecule blended exciplex PVK (Poly(9-vinylcarbazole)):PO-T2T ((1,3,5triazine-2,4,6-triyl) tris(benzene-3,1-diyl) tris(diphenylphosphine oxide)) and demonstrated its TADF characteristic, which was the first attempt to develop polymer-based exciplexes. However, the solutionprocessed OLED using PVK:PO-T2T as the emitter only exhibited a low maximum EQE of 4.5% [29]. This result indicates common binary polymer-based exciplex might suffer significant excitons loss. As shown in Fig. 1, the long-chain configurations of polymers will suppress the intermolecular interaction between polymers and other molecules, thus, only limited exciplexes can be formed in the polymer-small-molecule blend, which will result in incomplete up-conversion of triplet excitons. To solve this problem, we present a new strategy to develop ternary polymer-based exciplexes by introducing another small-molecule constituent material, which could form the small-molecule-smallmolecule exciplex with higher excited energy than that of polymersmall-molecule exciplex. Our group has already demonstrated ternary exciplexes with multiple reverse intersystem crossing (RISC) channels could enhance the utilization of triplet excitons [28,30,31]. And in this ternary polymer-based systems, high-energy exciplexes can be easily formed due to the stronger intermolecular interaction between small molecules, and up-conversion of most triplet excitons can proceed via the RISC channel on them. With the energy transfer from high-energy exciplex to low-energy exciplex, polymer-small-molecule exciplexes can harvest all excitons theoretically. Moreover, such ternary polymerbased systems could maintain the superiorities of polymers, like excellent film-forming ability and film stability, without additional requirements on other small-molecule constituent materials. According to our novel strategy, we constructed a ternary polymerbased exciplex emitter, PVK:PO-T2T:mCP (3-bis(9H-carbazol-9-yl) benzene) in this work. Both PVK and mCP can act as the electron-donor to form exciplex with electron-acceptor PO-T2T, and mCP:PO-T2T has a higher energy gap than PVK:PO-T2T. As expected, PVK:PO-T2T:mCP exhibits the emission from PVK:PO-T2T and significant TADF behavior. More importantly, the PVK:PO-T2T:mCP successfully realizes much higher photoluminescence quantum yield (ΦPL, 82.0%) and rate constant of RISC process (kRISC, 2.62 × 105) and lower non-radiative decay T rate constant of triplet excitons (knr , 2.07 × 104) comparing with PVK:PO-T2T. In the OLEDs, interfacial exciplex (25%) PVK: (75%)
2. Experimental section 2.1. General information All reagents were purchased from commercial sources and used as received without further purification. Absorption and PL spectra were measured using a Hitachi UV–vis spectrophotometer U-3010 and a Hitachi fluorescence spectrometer F-4600, respectively. Half-and-half mixed constituting molecules in dichloromethane were coated in quartz tubes and treated at 40 °C under vacuum overnight to form films. Their fluorescence and phosphorescence spectra were measured at 77 K using a Hitachi F-4600 fluorescence spectrometer. The measurement of the phosphorescence spectra was delayed by a chopper with the chopping speed of 40 Hz, corresponding to a delayed time of ≈6.25 ms. The fluorescence quantum yields were measured with 5 nm thick films in N2 atmosphere with an Edinburgh Instruments FLS920 spectrometer. The transient PL decay characterizations were conducted by the Collaborative Innovation Center of Suzhou Nano Science & Technology. The AFM images were measured using chlorobenzene as the solvent to spin-coat the films under the MFP-3D-BIO AFM system (Asylum Research, Oxford Instruments). 2.2. OLEDs fabrication and characterization ITO-coated glasses with a sheet resistance of 15 Ω square−1 were first cleaned with isopropyl alcohol and deionized water, then dried in an oven at 120 °C, treated with UV-ozone, and finally transferred to a deposition system with a base pressure of about 4 × 10−4 Pa. Organic materials were deposited at a rate of 1–2 Å s−1 and the rates were 0.1 and 10 Å s−1 for LiF and Al, respectively. EL luminescence, spectra, and CIE color coordinates were measured with a Spectrascan PR650 photometer and the current-voltage characteristics were measured using a Keithley 2400 SourceMeter under ambient atmosphere. EQE was calculated from the current density, luminance, and EL spectrum, assuming a Lambertian distribution. 2
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Fig. 2. Molecular structures of mCP, PVK and PO-T2T.
3. Results and discussions
shifted with a single-peak at 489 nm compared with that of PVK:POT2T, it is believed the emission of PVK:PO-T2T:mCP should be completely from low-energy exciplex PVK:PO-T2T because they have similar spectral shape and full width at half-maximum. And such blueshift was also observed in other reported tri-component exciplexes [31,34], and should be caused by the intermolecular interaction between PVK and mCP slightly lowering the HOMO energy level of PVK [35]. With no evident overlap between the emission of mCP:PO-T2T and the absorption spectrum of PVK:PO-T2T, Förster energy transfer with the Coulomb mechanism should be hard to happen from mCP:POT2T to PVK:PO-T2T. In this case, singlet-singlet energy transfer can be also allowed via the exchange mechanism (Dexter energy transfer) [36]. In this system, both the energy transfers of singlet and triplet excitons from mCP:PO-T2T to PVK:PO-T2T should be via the exchange mechanism (Dexter energy transfer). Therefore, PVK:PO-T2T:mCP possesses two RISC channels on mCP:PO-T2T and PVK:PO-T2T, and has complete energy transfer from high-energy mCP:PO-T2T to low-energy PVK:PO-T2T, realizing our strategy as shown in Fig. 1. Then, the ΦPLs of solution-processed PVK:PO-T2T and PVK:POT2T:mCP films were measured by using an integrating sphere under oxygen-free condition. And the ΦPLs were estimated to be 36.8% for PVK:PO-T2T with a weight ratio of 1:1, 47% for PVK:PO-T2T:mCP with a weight ratio of 1:1:1 and 82.0% for PVK:PO-T2T:mCP with a weight ratio of 0.5:1:1.5. Obviously, with the assistance of mCP, the ΦPLs of the polymer-based exciplexes are evidently enhanced and realize a quite high value of 82.0%. In PVK:PO-T2T, the long-chain configuration of PVK will suppress the intermolecular interaction between PVK and POT2T, resulting in limited active exciplexes. And insufficient PVK:POT2T will also induce incomplete up-conversion of triplet excitons and then a low ΦPL. While in ternary exciplexes PVK:PO-T2T:mCP, RISC process can be happened on both mCP:PO-T2T and PVK:PO-T2T, and multiple RISC channels in exciplex systems could enhance the utilization of triplet excitons [28,30,31]. Moreover, due to the much stronger intermolecular interaction between mCP and PO-T2T, active mCP:POT2T exciplexes are much easier to be formed than PVK:PO-T2T. Therefore, with increasing doping ratio of mCP in PVK:PO-T2T:mCP, more mCP:PO-T2T exciplexes could be formed, which will further benefit the RISC process and enhance the ΦPLs of ternary exciplexes. To better prove the superiority of the ternary polymer-based exciplex, we further measured the transient PL decay of PVK:PO-T2T:mCP with a weight ratio of 0.5:1:1.5 at room temperature. As shown in Fig. 3d, PVK:PO-T2T:mCP exhibits nearly identical decay curve with mCP:POT2T. This result does not only demonstrate the TADF characteristic of ternary exciplex but also indicates the RISC process mainly happens on mCP:PO-T2T in the system, which is consistent with our prediction. The key kinetic parameters of PVK:PO-T2T with a weight ratio of 1:1 and PVK:PO-T2T:mCP with a weight ratio of 0.5:1:1.5 were then calculated. For blue and green fluorescent emitters, the internal conversion process
PVK, PO-T2T, and mCP were directly purchased from commercial sources, and their molecular structures are shown in Fig. 2. As green PVK:PO-T2T exciplex was reported with a low maximum EQE of 4.5% in the OLEDs [29], we selected them as the target. Besides, the common hole transporting host mCP is chosen as the third small-molecule constituent material, as it was reported to form a blue TADF exciplex with PO-T2T [32]. In the photophysical characterizations of interfacial exciplexes, it is impossible to confine the photogenerated excitons at the active interface, which will induce the undesired emissions of the constituent materials. To clearly demonstrate the photophysical properties of exciplexes in this work, their PL spectra were all measured in bulk heterojunction. Fig. 3a and b shows the absorption and PL spectra of PVK:PO-T2T, mCP:PO-T2T and their constituting molecules. Both two mixtures exhibit nearly identical absorption spectra to those of their constituting molecules, which suggests that the formation of new ground-state transitions does not occur in the mixed films. Meanwhile, PL spectra of two mixtures are significantly red-shifted relative to those of the constituting molecules. The emissions of PVK:PO-T2T and mCP:PO-T2T are peaked at 511 and 450 nm, respectively, while those of PVK, PO-T2T, and mCP are peaked at 386, 327 and 354 nm, respectively. These results indicate the formation of new excited states in the mixed films. Besides, the PL peaks of PVK:PO-T2T and mCP:PO-T2T correspond to the energies of 2.43 and 2.75 eV, which match well to the differences between the lowest unoccupied molecular orbital (LUMO) energy level of PO-T2T (−3.22 eV) [30] and the highest occupied molecular orbital (HOMO) energy levels of PVK (−5.64 eV) [33] and mCP (−5.84 eV) [30]. All of this evidence suggests the formation of exciplexes in two bi-component mixed films, and mCP:PO-T2T has a higher energy gap than PVK:PO-T2T. As shown in Fig. S1, the fluorescence and phosphorescence spectra of PVK:PO-T2T and mCP:PO-T2T mixed films were measured at 77 K. From the onset position of the fluorescence and phosphorescence spectra, the S1 and T1 energy levels can be estimated as 2.981 and 2.973 eV for mCP:PO-T2T and 2.886 and 2.857 eV for PVK:PO-T2T. Thus, the ΔEST of mCP:PO-T2T and PVK:POT2T are estimated to be 0.008, 0.029 eV, respectively. Besides, as shown in Fig. 3d, transient PL decays of two exciplexes at 300 K in the microsecond range further confirm their TADF characteristic, which are consistent with the reported literature [29,30]. Based on two bi-component exciplexes, we constructed the ternary exciplex PVK:POT2T:mCP accordingly. As shown in Fig. 3c, the ternary mixture also exhibits cumulative absorption compared with three constituting molecules, suggesting still no formation of new ground-state transitions. And the PL spectrum of PVK:PO-T2T:mCP is also significantly redshifted compared with those of three constituting materials. These results indicated that exciplex would be formed in the tri-component mixtures. Although the PL spectrum of ternary exciplex is slightly blue3
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Fig. 3. Absorption and PL spectra of a) mCP:PO-T2T with a weight ratio of 1:1 and their pristine materials, b) PVK:PO-T2T with a weight ratio of 1:1 and their pristine materials, and c) PVK:PO-T2T:mCP with a weight ratio of 1:1:1 and their pristine materials in thin solution-processed solid films (PO-T2T in toluene solution); d) Transient fluorescence decays of mCP:PO-T2T film with a weight ratio of 1:1 at 450 nm, PVK:PO-T2T film with a weight ratio of 1:1 at 510 nm and PVK:PO-T2T:mCP film with a weight ratio of 0.5:1:1.5 at 485 nm (Excitation at 300 nm in vacuum).
of singlet excitons could be ignored compared with fluorescence decay and intersystem crossing process of singlet excitons [11,13,37], thus the calculations were carried out via the following equations [11,37],
kp =
1 τp
(1)
kd =
1 τd
(2)
kF = k p ΦF
(3)
kISC = kP (1 − ΦF)
(4)
kRISC =
kRISCs are evidently increased to 2.26 × 105 for PVK:PO-T2T:mCP comparing with the kRISCs of 0.38 × 105 for PVK:PO-T2T. As a result, ΦTADF is also increased from 13.0% for PVK:PO-T2T to 55.0% for PVK:PO-T2T:mCP. Therefore, the improved ΦPL of PVK:PO-T2T:mCP is mainly ascribed to the higher up-conversion of triplet excitons with two RISC channels on PVK:PO-T2T and mCP:PO-T2T. Meanwhile, with T of PVK:PO-T2T:mCP is evidently remore efficient RISC process, knr 4 duced to 2.07 × 10 comparing with that of 3.57 × 104 for PVK:POT2T, indicating less loss of triplet excitons. These results prove our ternary polymer-based exciplexes will evidently improve the exciton utilization and can be expected to realize high efficiencies in the OLEDs. As additional hole-transporting layer (HTL) and electron-blocking layer are hard introduced in solution-processed OLEDs, PO-T2T in blended exciplexes as the emitting layer (EML) may cause serious electron leakage from EML to hole-injection layer (HIL), resulting in poor device performance. To better evaluate the electroluminescence (EL) performance of PVK:PO-T2T:mCP, we constructed the interfacial exciplex-based OLEDs with a general configuration of ITO/PEDOT:PSS (30 nm)/EDL (electron-donor layer, 35 nm)/PO-T2T (45 nm)/LiF (0.8 nm)/Al (80 nm). In the devices, ITO (indium tin oxide) and Al were served as the anode and the cathode, respectively; PEDOT:PSS (poly (ethylenedioxythiophene):poly(styrenesulfonate) was used as HIL; LiF was acted as an electron injection layer; EDL was served as the HTL; POT2T was acted as the electron-transporting layer. The interfacial exciplexes formed between EDL and PO-T2T are the emitters of these devices. For ternary exciplex PVK:PO-T2T:mCP, the mixtures of PVK:mCP with different weight ratios were used as the EDL. And pure
k p k d ΦTADF kISC
(5)
ΦF
kISC ⎞ T knr = kd − ⎛1 − kRISC k + kISC ⎠ F ⎝ ⎜
⎟
(6)
where ΦF and ΦTADF are the quantum efficiencies of prompt and delayed fluorescence; ΦISC is the efficiency of ISC process; kp, kd, kF, kISC are rate constants of prompt fluorescence, delayed fluorescence decay, fluorescence decay, and ISC process from S1 to T1 state, respectively. ΦF and ΦTADF of two exciplex emitters are estimated from the ΦPLs with relative ratios which were calculated from the transient PL results [11]. Consequently, all key kinetic parameters of two exciplexes are listed in Table 1. With naturally separated HOMO and LUMO distributions, both two exciplexes exhibit relatively small kFs and relatively large kISCs, resulting in all ΦFs with small values. However, for the RISC process, 4
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Table 1 Summary of photophysical parameters for PVK:PO-T2T (weight ratio of 1:1) and PVK:PO-T2T:mCP (weight ratio of 0.5:1:1.5). Emitters
ΦPLa [%]
ΦPb [%]
ΦTADFc [%]
τPd (ns)
τTADFe (μs)
kpf ( × 107 s−1)
kFg ( × 107 s−1)
Kdh ( × 104 s−1)
kISCi ( × 107 s−1)
kRISCj ( × 105 s−1)
knrTk ( × 104 s−1)
PVK:PO-T2T PVK:PO-T2T:mCP
36.8 82.0
21.7 27.0
15.1 55.0
28.0 18.5
23.12 10.60
3.57 5.40
0.77 1.46
4.30 9.40
2.80 3.94
0.38 2.62
3.57 2.07
a b c d e f g h i j k
ΦPL is the total photoluminescence fluorescence quantum efficiency value. the prompt fluorescence quantum efficiency. The delayed fluorescence quantum efficiency. prompt fluorescence lifetime. Delayed fluorescence lifetime. the rate constants of prompt fluorescence. the rate constant of fluorescence decay. The rate constants of delayed fluorescence decay. The rate constant of intersystem crossing. the rate constant of reverse intersystem crossing. the rate constant of nonradiative transitions of triplet excitons.
Fig. 4. 3D AFM height images (5 × 5 μm) of 60 nm thick unannealed films: a) pure PVK; b) pure mCP; c) mCP:PVK (weight ratio of 1:1) and films with annealing at 100 °C for 30 min: d) pure PVK; e) pure mCP; f) mCP:PVK with a weight ratio of 1:1.
Fig. 5. a) EQE-current density plots and b) EL spectra of the solution-processed OLEDs based on interfacial exciplex emitters of mCP/PO-T2T; PVK/PO-T2T; 75% PVK:25% mCP/PO-T2T; 50% PVK:50% mCP/PO-T2T; 25% PVK:75% mCP/PO-T2T; 15% PVK:85% mCP/PO-T2T.
optimized solution-process, the root-mean-square (RMS) surface roughness values of PVK, mCP and PVK:mCP spin-coated films were respectively 0.241, 0.281 and 0.213 nm without further annealing. And after annealing at 100 °C for 30 min, the RMS surface roughness values of PVK, mCP and PVK:mCP films became 0.282, 1.010 and 0.220 nm, respectively. Obviously, pure PVK and PVK:mCP films showed no significant change after annealing at 100 °C for 30 min and all exhibited
mCP and pure PVK were also used as the EDL to construct bi-component exciplexes as the comparisons. Prior to device fabrication, the atomic force microscopy (AFM) 3D images were investigated since the surface topography of films could have great influence in OLED performance. As shown in Fig. 4, we measured the morphologies of the spin-coated films of pure PVK, pure mCP and PVK:mCP mixture with a weight ratio of 1:1. With the 5
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Table 2 Performances of the solution-processed OLEDs based on exciplex emitters. Device
λmax [nm]
Vona (V)
CEb (cd/A)
PEb (lm/W)
EQEb (%)
CIE (x, y)
mCP/PO-T2T PVK/PO-T2T 75% PVK:25% 50% PVK:50% 25% PVK:75% 15% PVK:85%
480 534 530 526 514 512
3.5 3.0 4.5 4.2 4.1 3.6
6.3 ± 0.4 (6.7) 14.5 ± 0.3 (14.8) 35.7 ± 1.0 (36.7) 47.2 ± 0.6 (47.8) 63.8 ± 0.9 (64.7) 42.6 ± 0.6 (43.2)
5.0 ± 0.4 (5.4) 9.1 ± 0.2 (9.3) 22.0 ± 0.6 (22.6) 32.3 ± 0.4 (32.7) 44.6 ± 0.6 (45.2) 29.1 ± 0.4 (29.5)
2.8 ± 0.2 (3.0) 4.5 ± 0.1 (4.6) 11.1 ± 0.3 (11.4) 14.9 ± 0.2 (15.1) 21.6 ± 0.3 (21.9) 14.4 ± 0.2 (14.6)
(0.19, (0.32, (0.31, (0.29, (0.26, (0.26,
a b
mCP/PO-T2T mCP/PO-T2T mCP/PO-T2T mCP/PO-T2T
0.32) 0.56) 0.55) 0.55) 0.51) 0.51)
Voltage at 1 cd/m2. The data in parentheses is the maximum.
4. Conclusion
very small RMS surface roughness. These results are consistent with the excellent film-forming ability and film stability of polymer materials, and proves polymer-based mixture films can keep the superior capability of polymers in solution-process. Reversely, the pure mCP film became much rougher after annealing with RMS surface roughness values increased from 0.281 to 1.010 nm, which should be attributed to the crystallization of mCP. And such poor film stability might also harm the performance of the OLEDs using interfacial mCP:PO-T2T as the emitter. As shown in Fig. 5a and listed in Table 2, the solution-processed OLED using interfacial PVK:PO-T2T as the emitter exhibits a turn-on voltage around 3.0 V and maximum CE, PE and EQE of 14.8 cd A−1, 8.9 lm W−1, and 4.6%, respectively, which are consistent with the previous report [29]. And the solution-processed OLED based on interfacial mCP:PO-T2T exhibits a turn-on voltage around 3.5 V and a quite poor EQE of 3.0%, which is much lower than the efficiencies of vacuumdeposited mCP: PO-T2T-based OLEDs [32]. These results might be ascribed to the poor film-stability of pure mCP. To realize the optimal performance of PVK:PO-T2T:mCP, the solution-processed OLEDs were constructed using different PVK:mCP EDLs with weight ratios of 75%:25%, 50%:50%, 25%:75%, and 15%:85%. As shown in Fig. 5b, the solution-processed device based on 75% PVK:25% mCP exhibits almost identical EL spectra with the interfacial PVK:PO-T2T-based device. However, with the increased weight ratio of mCP, the EL spectra of the devices based on tri-component exciplexes exhibit clearly blue-shift, which might be caused by the intermolecular interaction between PVK and mCP slightly lowering the HOMO energy level of PVK [35]. More important, all four devices exhibited maximum EQEs over 11%, much higher than the contrastive OLEDs based on PVK:PO-T2T and mCP:POT2T. Particularly, with the mixing ratio of mCP increased from 25% to 75%, the maximum EQEs of ternary exciplex-based OLEDs are enlarged from 11.4% to 21.9%. In such ternary polymer-based exciplex, upconventions of triplet excitons mainly happen on high-energy mCP:POT2T, thus higher exciton utilization is realized with more mCP in the EDL. The solution-processed device based on 25% PVK:75% mCP exhibits maximum CE of 64.7 cd A−1, PE of 45.2 lm W−1, and EQE of 21.9%, which is significantly improved 4.7 times comparing with PVK:PO-T2T-based device. To the best of our knowledge, this is the first report with an EQE over 20% for the OLEDs based on polymer-based exciplex emitters. According to the ΦPL value of 82.0% for PVK:POT2T:mCP with a weight ratio of 0.5:1:1.5, the out-coupling efficiency of our OLEDs should be above 26.7%, which is located in the reasonable range from 20% to 30%. Relatively, as PVK:PO-T2T with a weight ratio of 1:1 exhibits a ΦPL value of 36.8%, the maximum EQE of PVK:POT2T-based OLED is evidently lower than its theoretical value around 9.9%. As shown in Fig. S4, the current densities of the devices are significantly enhanced with the increasing doping ratio of mCP. Thus, it is speculated that the efficiency of PVK:PO-T2T-based OLED might be further harmed by the unbalanced charge transporting. Particularly, the average values from eight results were also calculated for the efficiencies of all devices. As listed in Table 2, all devices exhibit good reproducibility.
In conclusion, to develop efficient polymer-based exciplex emitters, a novel ternary exciplex PVK:PO-T2T:mCP was constructed by introducing mCP into the binary exciplexes PVK:PO-T2T in this work. In this ternary exciplex system, high-energy exciplexes mCP:PO-T2T can be easily formed due to the stronger intermolecular interaction between small molecules, realizing stronger up-conversion of triplet excitons proceeded via the RISC channel on them. With the energy transfer from mCP:PO-T2T to PVK:PO-T2T, ternary polymer-based exciplex PVK:POT2T:mCP can harvest all excitons theoretically. Thus, PVK:mCP:PO-T2T successfully realizes much higher ΦPL of 82.0% and kRISC of 2.62 × 105 T and lower knr of 2.07 × 104 comparing with PVK:PO-T2T. Meanwhile, PVK:PO-T2T:mCP also maintains excellent film-forming ability and thermal stability of polymer material PVK. By using interfacial exciplex as the emitter, solution-processed OLED based on 25% PVK:75% mCP exhibits high maximum efficiencies of 64.7 cd A−1 CE, 45.2 lm W−1 PE, and 21.9% EQE. To the best of our knowledge, this is the first report for polymer-based exciplex emitter with an over 20% EQE in the OLEDs. These results successfully demonstrate the feasibility of our strategy to develop high-performance polymer-based exciplex emitters of solutionprocessed TADF-OLEDs. Acknowledgements This work was supported by the National Key Research and Development Program of China (Grant No. 2016YFB0401002), the National Natural Science Foundation of China (NSFC Grant Nos. 51773029, 51533005 and 51821002), International Cooperation and Exchange Project of Science and Technology Department of Sichuan Province (Grant Nos. 2019YFH0057, 2019YFH0059 and 18GJHZ0015), the Fundamental Research Funds for the Central Universities (Grant No. ZYGX2016Z010). Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.orgel.2019.105449. References [1] H. Uoyama, K. Goushi, K. Shizu, H. Nomura, C. Adachi, Highly efficient organic light-emitting diodes from delayed fluorescence, Nature 492 (2012) 234–238. [2] K. Goushi, K. Yoshida, K. Sato, C. Adachi, Organic light-emitting diodes employing efficient reverse intersystem crossing for triplet-to-singlet state conversion, Nat. Photonics 6 (2012) 253–258. [3] W. Zeng, T. Zhou, W. Ning, C. Zhong, J. He, S. Gong, G. Xie, C. Yang, Realizing 22.5% external quantum efficiency for solution-processed thermally activated delayed-fluorescence OLEDs with red emission at 622 nm via a synergistic strategy of molecular engineering and host selection, Adv. Mater. 31 (2019) 1901404. [4] K. Wang, C.J. Zheng, W. Liu, K. Liang, Y.Z. Shi, S.L. Tao, C.S. Lee, X.M. Ou, X.H. Zhang, Avoiding energy loss on TADF emitters: controlling the dual conformations of D-A structure molecules based on the pseudoplanar segments, Adv. Mater. 29 (2017) 1701476. [5] M.Y. Wong, E. Zysman-Colman, Purely organic thermally activated delayed fluorescence materials for organic light-emitting diodes, Adv. Mater. 29 (2017) 1605444.
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