Highly Efficient Nondoped Organic Light Emitting Diodes Based on

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Highly Efficient Non-Doped Organic Light Emitting Diodes Based on Thermally Activated Delayed Fluorescence Emitter with Quantum-Well Structure Lingqiang Meng, Hui Wang, Xiaofang Wei, Jianjun Liu, Yongzhen Chen, Xiangbin Kong, Xiaopeng Lv, Pengfei Wang, and Ying Wang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b07563 • Publication Date (Web): 25 Jul 2016 Downloaded from http://pubs.acs.org on July 27, 2016

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ACS Applied Materials & Interfaces

Highly Efficient Non-Doped Organic Light Emitting Diodes Based on Thermally Activated Delayed Fluorescence Emitter with Quantum-Well Structure Lingqiang Meng,1,2, Hui Wang,1,2, Xiaofang Wei,1,2 Jianjun Liu,1,2 Yongzhen Chen,1,2 Xiangbin Kong, 1,2 Xiaopeng Lv,3 Pengfei Wang1 and Ying Wang1,* 1

Key Laboratory of Photochemical Conversion and Optoelectronic Materials, Technical Institute

of Physics and Chemistry, Chinese Academy of Sciences, Beijing, 100190, China 2

University of Chinese Academy of Sciences, Beijing, 100049, China

3

Soochow University, Jiangsu Province, China

KEYWORDS Non-doped, Quantum-well structure, TXO-PhCz, Thermally activated delayed fluorescence, Organic Light Emitting Diodes

ABSTRACT: Highly efficiency non-doped thermally activated delayed fluorescence (TADF) organic light emitting diodes (OLEDs) with multi-quantum wells structure were demonstrated. By using emitting layer with seven quantum wells, the non-doped TADF OLEDs exhibit highly efficiency with (EQE) of 22.6%, a current efficiency of 69 cd/A, and a power efficiency of 50 lm/W, which are higher than those of the conventional doped OLED and among the best of the TADF OLEDs. These high performance of the devices can be ascribed to effectively

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confinement the charges and excitons in the emission layer by the quantum well structure. The emission layer with multi-quantum well structure are demonstrated to be cost-effective for the highly efficient non-doped TADF OLEDs and hold great potential for organic electronics.

INTRODUCTION Thermally activated delayed fluorescence (TADF) based on pure organic aromatic compounds have attracted much attention because their thermally accessible gaps between the lowest singlet and triplet excited states enable the harvesting of both singlet and triplet under electrical excitation via the efficient up-conversion of triplet excitons to singlet excitons.1-6 Organic light emitting diodes (OLEDs) based on TADF are able to achieve high efficiency by converting nearly 100% of the injected carriers into photons via relaxation of singlet excitons. Emitters with TADF have been regarded as the third generation of OLEDs emitters due to high efficiency and promising operating stability of TADF OLEDs, holding great potential for lighting and display applications.7 A concentration quenching effect caused by triplet excitons can significantly decreases the efficiency of the devices. Thus, host-guest doping systems have been wildly used to achieving highly efficient OLEDs based on TADF. However, electroluminescence of TADF (both spectra and efficiency) is generally very sensitive to the atmosphere of emitters, leading to the device reproducibility problems which can be often seen when using doping.8 Furthermore, the doping method is a complicated process, and requires precise control of the concentration of dopant, which also obstacles the large-scale commercial application of TADF OLEDs.9 Thus, TADF OLEDs with undoped device structure is highly desirable and promising. Pioneering work in 2015 by C. Adachi and co-workers reported a green-emitting undoped OLED employing 9,9dimethyl-9,10-dihydroacridine/benzophenone derivatives with a maximum EQE of 18.9%, a power efficiency of 59 lm/W.10 These undoped TADF OLEDs require the emitting materials


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with high photoluminescence (PL) quantum yield (PLQY) in the neat film, the bipolar charge– transport capability, the small energy difference between the lowest singlet and triplet excited states and thus the short excited-state lifetime to avoiding concentration quenching, which present significant barriers to the development of the TADF for the undoped OLEDs. Thus, no other highly efficient undoped TADF OLEDs were reported yet, and the construction of undoped TADF OLEDs with high efficiency is still a challenge. Phosphorescent OLEDs, as the former counterpart of TADF OLEDs, can also achieve 100% internal quantum efficiency by the utilization of both singlet and triplet excitons, and similar concentration quenching effect can be found owing the triplet aggregation at high density. Doping-free OLEDs are one of the main topic of phosphorescent OLEDs.11-19 From the materials engineering point, the functionalized and bulky ligands, such as carbazole dendrons, were introduced to enhance the charge-transporting ability, decrease the decay lifetime, and thus reduce the self-quenching, and the non-doped phosphorescent OLEDs based on bis[3trifluoromethyl-5-(2-pyridyl)-1,2-pyrazolato]platinum(II) with EQE above 31% have been reported by Gnade’s group.11,12 From the view point of device engineering, ultrathin emitting layers and quantum well (QW) structures have been applied to the construction of the high efficiency of non-doped OLEDs. Jabbour et al. reported efficient monochromatic OLED of a platinum-complex emissive layer with the peak EQE of 17.5% and power efficiency of 45 lm/W.13 Ma et al. reported highly efficient monochrome blue, green, orange, and red OLEDs with ultrathin nondoped emissive layers, and the maximum EQE can reach 17.1%, 20.9, 17.3, and 19.2% for blue, green, orange, and red monochrome OLEDs.14 Wang et al. reported highly efficient multi-QWs blue phosphorescent OLEDs with a peak EQE of 20.31%, current efficiency of 40.31 cd/A, and power efficiency of 30.14 lm/W.15 The QW structure can effectively confine


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the charge carriers and excitons, affording the high efficiency and low efficiency roll-off of the devices. These interesting results motivated our exploration of experimental strategies for the highly efficient TADF OLED with ultrathin emitter layer or QW structure. In 2014, our group had reported thioxanthone-based thermally activated delayed fluorescence emitters, and the devices based on the emitters with host-guest doping structures exhibited EQE up to 21.5%.20 In this paper, we first investigated the construction of highly efficient OLED based on non-doped TADF emitter with ultrathin emitter layer and QW structure. It is clearly seen that these nondoped EML based OLEDs with QW structure show high efficiency, with a maximum EQE up to 22%, a current efficiency of 69 cd/A and a power efficiency of 50 lm/W. These results are comparable those of the phosphorescent OLEDs with QW structures, demonstrating the advantage of QW structure on the construction of non-doped OLEDs for TADF emitters. EXPERIMENTAL SECTION TXO-PhCz was synthesized as reported previously.20 The hole-injection materials of poly(styrene sulfonic acid)-doped poly(3,4-ethylenedioxythiphene) (PEDOT:PSS, Baytron PVP A4083) was purchased from H.C. Starck GmbH. The hole transporting material of 1,1-bis[(di-4tolylamino)phenyl] cyclohexane (TAPC), the electron-transportign materials of 1,3-bis[(4-tertbutylphenyl)-benzene (TmPyPB), and the host material of 1,3-Bis(carbazol-9-yl)benzene (mCP) was purchased from Jilin Optical and Electronic Materials Company, which were purified by repeated temperature-gradient vacuum sublimation and received with a purity of more than 99%. LiF was purchased from Sigma-Aldrich. OLEDs were fabricated on patterned ITO-coated glass substrates with a sheet resistance of 15 Ω/□. Before device fabrication, the ITO glass substrates were ultrasonically cleaned with detergent, de-ionized water, acetone, and alcohol. After that, the substrates were dried in an oven at 120°C and then treated with UV-ozone for 10 mins. A layer


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of 30 nm thick PEDOT:PSS was spin-coated onto the pre-cleaned substrate and bake in a glovebox under a nitrogen environment (oxygen and water contents less than 1 ppm) at 120°C for 30 mins to extract residual water. Afterward, the substrates transferred to a vacuum deposition system with a base pressure better than 1×10−6 mbar for organic semiconductor layers and metal deposition. The devices were fabricated by evaporating organic semiconductors onto the PEDOT:PSS layer sequentially with an evaporation rate of 1–2 Å/s. The cathode was completed through thermal deposition of LiF at a deposition rate of 0.1 Å/s, and then capped with Al metal through thermal evaporation at a rate of 10 Å/s. The thickness of evaporated films were monitored by frequency counter, and calibrated by Dektak 6M Profiler. The EL luminescence spectra and CIE color coordinates were measured by a Spectra scan PR655 photometer. The current-voltage-brightness characteristics were measured by using a computercontrolled Keithley source measurement unit (Keithley 2400 Source Meter) with a calibrated optical power meter of Newport company (1936R) under ambient atmosphere. RESULTS AND DISCUSSION To achieve high performance non-doped OLEDs based on TXO-PhCz, we first tried to construct OLEDs based on TXO-PhCz with pure emissive layer (∼30 nm). Sadly, the performance of the devices were lower because of the serious singlet-triplet annihilation and triplet-triplet annihilation in the emissive layer, although the device show yellow light with the EL centered at 570 nm. Thus, we constructed OLEDs based on TXO-PhCz with ultra-thin emissive layer, and the thickness of the emissive layer was optimized. The structure of the devices were shown in Figure S1, and the EL spectra of the devices at 6 V were showed in Figure S2. The device with the EL thickness of 0.5 nm exhibited a broad emission with four peaks at 444 nm, 476 nm, 516 nm and 600 nm. The blue peaks at 444 nm, 476 nm were attributed to the emission from the


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excitons leaked outside of the EML. While, orange peak around 600 nm were demonstrated to be the emission from the exciplex formed by TAPC and TmPyPB or the exciplex emission from TAPC (Figure S3). Increasing the thickness of TXO-PhCz, the EL intensities of the devices at 444 nm, 476 nm and 600 nm decreased, and there are only emissions around 516 nm and 448 nm can be observed once the thickness of TXO-PhCz reaches 2 nm. The emission peak in the green region bathochromic-shifted from 516 nm to 540 nm when the thickness of TXO-PhCz increased from 2 nm to 6 nm. These bathochromic shift of EL can be ascribed to the change of the surrounding TXO-PhCz molecules.8 Although the green OLEDs with ultra-thin emission layer can be fabricated, the performance of the devices are not satisfied due to the exciton leakage.

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PEDOT 5.4 eV 5.9 eV

5.57 eV 5.9 eV 6.68 eV

Figure 1. (a) Molecular structures of the molecules for OLEDs with single quantum well; (b) Device structures of OLEDs with single quantum well; (c) The energy-level diagram of OLEDs with single quantum well.


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Figure 2. Electronic properties of the devices with single quantum well: (a) Current densityvoltage-luminance plots; (b) Current efficiency-luminance-power efficiency plots; (c) EQEluminance plots; (d) EL spectra at 6 V. The device structure is ITO/PEDOT (20 nm)/TAPC (20 nm)/mCP (15 nm)/TXO-PhCz (x nm)/mCP (15 nm)/TmPyPB (50 nm)/LiF (0.9 nm)/Al (200 nm) (x=0, 0.5, 1 and 1.5 nm). (a)

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Figure 3. AFM image of the multilayer films of ITO/PEDOT (20 nm)/TAPC (20 nm)/mCP (15 nm)/TXO-PhCz (x nm): (a) 0 nm; (b) 0.5 nm; (c) 1 nm; and (d) 1.5 nm.

To confined charge carriers and excitons inside the emission layer, OLEDs with multilayer quantum wells were fabricated, which have been regarded as the efficient approach to construct non-doped OLED and widely used in the inorganic LED and OLED to achieve higher efficiency.15 A thin layer of TXO-PhCz was inserted at the center of 30 nm N,N’-dicarbazolyl3,5-benzene (mCP) film and the structure of the devices were shown in Figure 1. mCP was used as the potential barrier layer (PBL) due to its high triplet energy level and appropriate HOMO/LUMO energy level. All the devices showed similar performance, and no prominent dependence of the device performance on the thickness of TXO-PhCz can be observed (as show in Figure 2). The device with x of 0.5 nm showed a current efficiency of 2.9 cd/A, a power efficiency of 1.3 lm/W, and an external quantum efficiency of 1.2%. Increasing the thickness of TXO-PhCz to 1.0 nm, the device exhibited higher performance with a current efficiency of 3.4 cd/A, a power efficiency of 1.6 lm/W, and an external quantum efficiency of 1.4%. The higher performance can be attributed to more TXO-PhCz molecules in the quantum well and then efficient energy transfer from mCP to TXO-PhCz. However, further increasing the thickness of TXO-PhCz to 1.5 nm, the performance of the devices slightly decreased. In the blue region, obvious blue emission can be observed from the EL spectra of the devices, indicating inefficient energy transfer from the mCP host to TXO-PhCz. The blue emission of the devices cannot be suppressed by the increased thickness of TXO-PhCz. Combining the similar performance of the devices with the TXO-PhCz thickness, it can be concluded that energy transfer cannot be effectively enhanced by the increased TXO-PhCz thickness. More interestingly, the EL spectra of OLEDs with different well widths red-shifted and the full wave at half maximum (FWHM) of


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the EL spectra increase as the increasing of the well width. The shift in the emission peak energy and the change in the FWHM of the emission spectra for OLEDs coincide with those of the reported organic single-quantum-well devices, indicating that the quantum well effect works in these devices.21 Figure 3 shows the AFM image of the multilayer films with the different thickness of TXO-PhCz. The surface of mCP film is very smooth with the root-mean-square (RMS) of 0.24 nm. As the deposition of 0.5-nm-TXO-PhCz film, smooth surface of TXO-PhCz with similar roughness was achieved and no obvious molecular aggregation can be observed. The RMS surface roughness of TXO-PhCz film with the thickness of 0.5 nm is about 0.243 nm, similar to that of mCP. With the further increasing of TXO-PhCz, there are some islands formed and the roughness of TXO-PhCz films increase to 0.619 nm for 1.0 nm TXO-PhCz and 0.783 nm for 1.5 nm TXO-PhCz, respectively. Thus, increasing the thickness of the TXO-PhCz film facilitates the self-aggregation of the TXO-PhCz. This indicates that TXO-PhCz film grow on mCP film surface via layer by layer mode at the beginning stage and then grow via island growth mode due to the molecular aggregation, leading to the rough surface of the TXO-PhCz film. The transient PL decay curves of the TXO-PhCz films with different thickness were also investigated (as shown in Figure 4). Similar to the TXO-PhCz:mCP doped films, all these films showed two obvious decay components, the prompt component and the delayed component.20 The lifetime of the delayed component for the TXO-PhCz film with 0.5 nm is about 137 µs. The lifetime of TXO-PhCz films with higher thickness is shorter, 127 and 112 µs, respective. These indicate the severe singlet-triplet and triplet-triplet annihilation will be happen in the thick TXO-PhCz film. The lifetime of the delayed component is inverse proportional to the roughness of the TXO-PhCz film. Thus, the serious singlet/triplet-triplet annihilation can be ascribed to the self-aggregation


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of TXO-PhCz molecules, leading to the lower performance of OLEDs with 1.5 nm TXO-PhCz than that with 1.0 nm TXO-PhCz. So, the thickness of TXO-PhCz is better to be 0.5 nm to avoid the singlet-triplet and triplet-triplet annihilation for high performance OLEDs.

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Figure 4. Transient PL decay curves of the multilayer films with the structure of mCP (15 nm)/TXO-PhCz (x=0.5, 1, and 1.5 nm)/mCP (15 nm).

To enhance the energy transfer from mCP to TXO-PhCz and harness the emission from TXOPhCz, OLEDs with multi-quantum wells were constructed. The thickness of TXO-PhCz was adopted to be 0.5 nm due to the weak singlet/triplet-triplet annihilation. The device structure is similar to that in Figure 1b (See Figure 5). The thickness of the emission layer was fixed as 30 nm, and thin TXO-PhCz layers were symmetrically inserted in the mCP layer. Figure 6 shows the performance of the devices with multi-quantum wells. As shown in Figure 6a, all the devices show similar turn-on voltage (Von) (defined as the voltage at 1 cd/m2). The current density decreases with the QW numbers at the same driving voltage when the QW number is lower than 7, which are similar to that of blue phosphorescent OLEDs with multiple QW structure.15 While, the current density of the devices dramatically increased, further increasing the number of quantum wells. As the increase of the quantum wells, the thickness of TXO-PhCz in the EML


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will increase, and more mCP/TXO-PhCz interface will formed. The defects or traps in TXOPhCz films or at mCP/TXO-PhCz interface will thus reduce the current density of the devices.22 Correspondingly, increase the number of QWs, the thickness of the mCP barrier layer in the EML, mCP layer between two thin TXO-PhCz layers, mCP layer between HTL and TXO-PhCz, and ETL and TXO-PhCz, will relatively decrease, since the thickness of mCP layer was fixed to be 30 nm in these devices. This will facilitate the direct trapping and thus transporting of the carriers in TXO-PhCz.23 The synergistic effect of both the negative and positive results induced by the quantum wells leads to the trend of the current density of the devices with the number of quantum well. The electroluminescent (EL) spectra of the devices with quantum structure are shown in Figure 6d. For the device with two quantum wells, there is one main emission peak around 520 nm and an emission shoulder at around 580 nm. Weak emission can also be observed at the wavelength lower than 450 nm. The emission at 520 nm can be ascribed to be the emission from TXO-PhCz, and the other emissions can be ascribed to the emission from mCP and TAPC due to the inefficient energy transfer between mCP and TXO-PhCz and the leakage of the excitons into TAPC layer. Increasing the number of the quantum wells, the intensity of the emissions at 520 nm and the wavelength lower than 450 nm were suppressed. And these emissions disappeared for the devices with the 5 or 7 quantum wells, indicating that the excitons can be efficiently confined into the emission layer by the quantum well structure. However, the emission at 580 nm and the emission from 400 to 450 nm can be observed again for the device with 9 quantum wells. As TXO-PhCz is a bipolar molecules with both donor and acceptor units, the introduction of TXO-PhCz in EML will enhance the electron transport propreties, leading to the shift of recombination zone to TAPC/EML interface. Thus, TAPC emissions observed from EL spectra of the devices with 9 quantum wells can be ascribed to the leakage of the excitons to


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TAPC layer due to the shift of the recombination zone.24 Figure 7 shows the EL spectra of the nondoped TADF OLEDs with seven quantum wells. All the EL spectra at different voltages overlap and no derivation or new peaks can be observed, indicating the stable light emission and the balanced charge carrier injection and transportation.25 Similar trends for the I-V curves and EL spectra can also be observed for the performance of the devices, and the devices with seven quantum wells exhibited highest performance. A current efficiency of 69 cd/A, a power efficiency of 50 lm/W, and a maximum external quantum efficiency of 22.6% are achieved for the devices with seven quantum wells without any light out-coupling enhancement. The exciting efficiencies are higher than those of the devices with the conventional structure (Figure S4) and comparable to those of the devices based on phosphorescent emitters.11-19 As the difference of the device structure is only the emitting layer, the quantum well structure of the devices leads to the high device performance. Also, the serious efficiency roll-off can be observed for the device with

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annihilation.26,27 These serious annihilation can be attributed to the interaction between the quantum wells due to the thin layer with nine quantum wells in EML, which coincides with the IV results of the devices. Compared the EL spectra of the devices with 7 QWs and conventional doping structure at 9 V, the peak energy of the device with 7 QWs shifts to lower energy than that of the conventional doped device with the same guest molecules in the EML. The FWHM of the EL for the device with 7 QWs are narrower than that of the conventional doped device. These demonstrated that the quantum well effect still works here, which is consistent with the results of OLEDs with the single quantum well structure of different width.22,28 Thus, the carriers can be effectively confined in the quantum well, which lead to the increase of the excitons formation probability from the quantized energy state.18,28 These can also be demonstrated by the high


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current efficiency at the low luminescence, in which higher current efficiency can obtained for the device with 7 QWs.18 To our knowledge, this is the first demonstration of the TADF OLEDs with quantum structure and the performance of the device is among the best of the reported nondoped TADF OLEDs.10 All these results demonstrate that nondoped EML with quantum well structure can be used for the construction of high performance TADF-based OLEDs with different colors. High performance full color white OLEDs can also be realized by incorporating TADF emitters with different colors into these nondoped quantum well structure.14,29 These will establish a useful and cost-effective approach for the high performance TADF-based OLEDs and offer high values in the future practical application of TADF emitters in display and lighting.

Figure 5. Device structures and energy level diagram of the multi-quantum wells. The thickness of TXO-PhCz of the quantum wells is fixed to be 0.5 nm. X is the number of quantum-well and Y is the thickness of the mCP block layer in the emitting layers: X = 2, Y = 10 nm; for X = 3, Y = 7.5 nm; for X = 4, Y = 6 nm; for X = 5, Y = 5 nm; for X = 7, Y = 3.75 nm; for X = 9, Y = 3 nm. CONCLUSION In summary, we have demonstrated high efficiency TADF OLEDs with multi-quantum wells structure. The seven quantum well structure can effectively confine the charges and excitons in


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the emission layer, affording highly efficiency none-doped OLEDs with a EQE of 22.6%, a current efficiency of 69 cd/A, and a power efficiency of 50 lm/W. The performance of the devices are higher than those of OLEDs with conventional doped host-guest structure and among the best of the TADF OLEDs. This study demonstrated that the emission layer with multiquantum well structure is a cost-effective method for the construction of non-doped TADF OLEDs with high efficiency.

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Figure 6. Electronic properties of the devices with multi-quantum well: (a) Current densityvoltage-luminance plots; (b) Current efficiency-luminance-power efficiency plots; (c) EQEluminance plots; (d) EL spectra at 9 V.


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Figure 7. (a) EL spectra of the nondoped TADF OLEDs with seven quantum wells at different voltage; (b) EL spectra of the nondoped TADF OLEDs with 7 QWs, the conventional doped OLEDs with same doping concentration, and the device with pure TXO-PhCz EML at 9 V. ASSOCIATED CONTENT Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org. Experiment detail including the device fabrication and measurement, the molecular structure, energy level, EL spectra of OLEDs with pure emissive layers, for OLED with ultra-thin emissive layer, EL spectrum and electronic properties of the device for comparison. AUTHOR INFORMATION Corresponding Author *Email: [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was financially supported by the National Natural Science Foundation of China (Grant No. 61420106002, No.51373189, No. 61178061, and No.61227008), the “Hundred


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Highly Efficient Nondoped Organic Light Emitting Diodes Based on

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