High-Performance Organic Light-Emitting Diode with Substitutionally

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High-Performance Organic Light Emitting Diode with Substitutionally Boron-doped Graphene Anode Tien-Lin Wu, Chao-Hui Yeh, Wen-Ting Hsiao, Pei-Yun Huang, Min-Jie Huang, Yen-Hsin Chiang, Chien-Hong Cheng, Rai-Shung Liu, and Po-Wen Chiu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b03597 • Publication Date (Web): 07 Apr 2017 Downloaded from http://pubs.acs.org on April 9, 2017

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ACS Applied Materials & Interfaces

High-Performance Organic Light Emitting Diode with Substitutionally Boron-doped Graphene Anode

Tien-Lin Wu,1,2 Chao-Hui Yeh,1 Wen-Ting Hsiao,1 Pei-Yun Huang,2 Min-Jie Huang,2 Yen-Hsin Chiang,2 Chien-Hong Cheng,2* Rai-Shung Liu,2* Po-Wen Chiu1*

1

Department of Electrical Engineering, National Tsing Hua University, Hsinchu 30013, Taiwan

2

Department of Chemistry, National Tsing Hua University, Hsinchu 30013, Taiwan

ABSTRACT Hole injection barrier between anode and hole injection layer is of critical importance to determine the device performance of organic light-emitting diodes (OLEDs). Here, we report on a record-high external quantum efficiency (24.6 % in green phosphorescence) of OLEDs fabricated on both rigid and flexible substrates, with the performance enhanced by the use of nearly defect-free and high-mobility boron-doped graphene as an effective anode and hexaazatriphenylene hexacarbonitrile as a new type of hole injection layer. This new structure outperforms the existing graphene-based OLEDs, in which

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MoO3, AuCl3, or bis(trifluoromethanesulfonyl)amide (TFSA) are typically used as a doping source for p-type graphene. The improvement of the OLED performance is attributed mainly to the appreciable increase of hole conductivity in nearly defect-free boron-doped monolayer graphene, along with the high work function achieved by the use of newly developed hydrocarbon precursor containing boron in the graphene growth by chemical vapor deposition.

KEYWORDS: organic light-emitting diodes, graphene, boron doping, polycyclic aromatic hydrocarbons, chemical vapor deposition, flexibility

*E-mail: [email protected], [email protected], [email protected]


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INTRODUCTION Graphene has been considered as an ideal candidate for transparent and flexible electrode in electronic devices such as organic light-emitting diodes (OLEDs)1-7 and organic photovoltaic (OPV).8-10 To act as an effective anode in OLEDs, highly p-doped graphene is essential because it reduces the hole transport barrier while in contact with a hole injection layer (HIL).2-3,11-12 Generally, two different approaches have been used to create p-doped graphene films: (1) charge transfer by means of molecular adsorption or oxide deposition; (2) introduction of substitutional dopant atoms in carbon lattice. For molecular adsorption or oxide deposition, the doping can be readily achieved through a dip- or spin-coating of the dopant chemical such as acids,2,11,13 metal chlorides,12,14 metal oxides,15-17 or molecules with strong electronegativity functional groups.4,18-21 Despite of its facile implementation, several disadvantages might have long been overlooked. For example, a non-negligible doping degradation occurs as exposed to the ambient atmosphere over time.9 Increase of surface roughness and decrease of light transmittance are also unavoidable side effects in this approach that negatively impact device performance. Likewise, AuCl3 was reported as a potent p-type doping on graphene, with a remarkable increase of work function of up to 5.1 eV.2,22 However, segregation of gold clusters causes opaque spots or even short circuits. As such, a highly stable means of



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doping in the anode made of graphene becomes an imminent need in the development of high-efficiency OLEDs. Recently, a few OLED devices using p-doped graphene as the anode have been demonstrated. The graphene doping was mostly carried out by molecular adsorption or deposition of transition metal oxides, as discussed above. In this work, we show the growth of boron-doped graphene using a lab-synthesized precursor which allows incorporation of boron atoms into hexagonal carbon lattice without introducing much defects, effectively increasing carrier mobility and work function for its use as a high-quality anode material in OLEDs. HIL, an adjacent layer in direct contact with the graphene anode, is another critical issue in the context of OLED performance. Hexaazatriphenylene hexacarbonitrile (HAT-CN) has been known as an effective anode modifier due to its large electron affinity and shown to reduce hole injection barrier as deposited on graphene.23 Here, we report the fabrication of OLEDs on rigid and flexible substrates, on which high-mobility boron-doped graphene is used as an effective anode and HAT-CN works as a new type of HIL. The resultant OLEDs exhibit the highest external quantum efficiency in green phosphorescence, outperforming the reported graphene-based OLEDs with MoO3,3,15,24 AuCl3,2 or (bis(trifluoromethanesulfonyl)amide, (TFSA)4 as the graphene doping source.


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RESULTS AND DISCUSSION Boron is an element that is adjacent to carbon in the periodic table and known to induce a p-type conduction as substitutionally doped in graphene. To date, numerous boron-containing precursors, including diborane (B2H6),25-27 triethlyborane (BEt3),28-29 phenyboronic acid (C6H7BO2)30 and polycyclic aromactic hydrocarbons (PAHs),31-32 have been used to dope graphene in the growth using chemical vapor deposition (CVD). Theoretical understanding33-35 of the electronic structure and proof-of-concept demonstrations of various applications such as field-effect transistors30 and gas sensors28 have also extensively studied on these boron-doped graphene films. However, doping graphene using the above-mentioned boron-containing precursors have shown to cause high-density defects. To cope with this problem, we use molecules with 9,10-dihydro-9,10-diboraanthracene (DBA) core structures, which have been reported to act as a building block to assemble graphene nanoribbons with boron heteroatoms.36-37 We synthesized 9,10-dimesityl-9,10-diboraanthracene (DBA(Mes)2) using a convenient one-pot reaction, and the fresh DBA(Mes)2 was dissolved in toluene and used as a boron-containing carbon precursor for graphene growth on copper foils in a hot-walled ambient-pressure CVD system. Pristine graphene sheets grown using only pure toluene as



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a carbon precursor in the same CVD growth conditions were used as the control. Details of DBA(Mes)2 synthesis and graphene growth are described in Supporting Information and Experiment Section. Raman spectroscopy is a powerful tool to inspect structural properties of CVD graphene, including the number of layers, stacking order, structural disorders, and doping.4,22,27-28 Figure 1a shows the Raman spectra of pristine graphene and boron-doped graphene transferred onto a SiO2/Si substrate. Two prominent peaks, located at ~1590 cm-1 (G peak) and ~2700 cm-1 (2D peak), appear in the spectra. Of particular unique feature here is a negligible D peak induced by defects in the boron-doped graphene, indicating that DBA(Mes)2 functions as an excellent boron source for incorporating boron atoms into the sp2-hybridized carbon network without introducing much structural disorders. This result is in sharp contrast to the previous works in which commercial available B2H625-27 and BEt328-29 were used as the boron sources and caused appreciable amount of lattice defects. The hole doping in our graphene sheets is evident in the upshift of both the G and 2D peaks, consistent with previous experimental and theoretical works where the p-doping is achieved by different approaches.38-40 The doping level can be calculated from the shift of the G peak or the 2D peak.41 A p-doping level as high as 3.6×1012 cm-1 can be achieved in our boron-doped graphene using an empirical formula


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derived in Ref. 32. Figure 1b-c display the X-ray photoelectron spectroscopy (XPS) of the boron-doped graphene. The peak near 190 eV is correlated with the B 1s core levels, corroborating the existence of graphitic borons.28,42 This peak can be decomposed into two subpeaks, assigned to the ideal BC3 at 190.6 eV and the BC4 at 187.5 eV. Figure 1c shows the C 1s spectrum which is also deconvoluted into two subpeaks. The main peak at ~284.1 eV is assigned to sp2 carbon bonding and the shoulder peak at ~285.6 eV could be assigned to C-B or C-O species.28 The boron-doping level in the graphene by using the area ratio of the two peaks after considering the relative sensitivity factor (RSF), was estimated to be 4.6 atom %. The work function, light transmittance, chemical stability, and sheet resistance are the three most relevant parameters in the context of using graphene as a transparent anode in OLEDs. Figure 1d shows the sheet resistance ( ) as a function of aging time for four different graphene sheets prepared on glass. Pristine graphene, TFSA-coated graphene, AuCl3-coated graphene, and boron-doped graphene are compared. For pristine graphene, the ~350 Ω/

is the highest and stays constant over time. Similarly, the of

TFSA-coated graphene is stable throughout the measurements. In contrast, the AuCl3-coated graphene exhibits a lowest right after the coating, but rising up progressively from 100 Ω/ to 270 Ω/ after 300 h. For the boron-doped graphene, the



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doping state is rather stable over time and shows the lowest value (240 Ω/ ) after 300 h of aging in ambient conditions. Figure 1e shows the transmittance as a function of light wavelength for the four different samples. Pristine and boron-doped graphene adsorb 2.3 % and 2.5 % of the incident light at 550 nm with respect to the glass substrate, respectively, while the TFSA- and AuCl3-coated graphene take up 5.6 % and 5.9 % of the light at the same conditions, respectively. The light transmittance of boron-doped graphene is superior than that of graphene doped by molecular or AuCl3 adsorption. Reduction of the hole-injection barrier from the p-type graphene anode to HIL is crucial in OLED performance. To evaluate this energy barrier, ultraviolet photoelectron spectroscopy was used to measure the work function of graphene sheets doped with different approaches (Figure 1f). It is found that the work function of pristine, boron-, TFSA-, and AuCl3-doped graphene sheets are 4.7, 5.0, 5.1, and 5.2 eV, respectively. The hole density in the boron-doped graphene is compatible with both the TFSA- and AuCl3-coated graphene, consistent with the sheet resistance measurements where we see very close for the boron-, TFSA-, and AuCl3-doped graphene sheets. Electrical transport provides a measure of fundamental physical properties of the boron-doped graphene, in particular the doping level and hole mobility. Back-gated field-effect transistors of pristine and boron-doped graphene were made to extract the


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doping level and hole mobility. Figure 2a shows the optical image of a back-gated graphene field-effect transistor. The detail of the fabrication process is provided in the method section. Transport measurements of the pristine and boron-doped graphene field-effect transistors were carried out at 300 K under ambient conditions. Figure 2b shows the transfer characteristics for both transistors. A p-type conduction, as is usually the case for pristine graphene, can be seen in a slight upshift of the charge neutrality point to a positive gate voltage. For the boron-doped graphene, the charge neutrality point is highly upshift to a gate voltage of 41 V, indicating a stronger hole doping in boron-doped graphene than in pristine graphene caused by moisture adsorption. The carrier concentration can be calculated from the product of the gate capacitance and the voltage shift of the charge neutrality point ∆ through the formula ∆ /, yielding a doping level of ~1.6×1012 cm-1. This doping value is in good agreement with that calculated from the Raman shift. In addition, we also extract the hole mobility of boron-doped graphene from the transfer characteristics using a constant-mobility model. The hole mobility of boron-doped graphene at room temperature is around 1600 cm2/V⋅s. To further examine the mobility, Hall measurements for boron-doped graphene on SiO2/Si substrate was carried out. The Hall mobility is 1440 cm2/V⋅s at room temperature, which is rather close to that measured in a FET configuration. In comparison with other



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reported boron-doped graphene on SiO2/Si substrate,30,42 our results presented here show a relatively higher hole mobility. The higher hole mobility is attributed to the very low defect density generated in the current CVD doping methodology. Graphene-based OLEDs usually use multilayer graphene as an anode due to its lower sheet resistance.1-2,4,11 However, it requires multiple transfers of graphene to the substrate in a wet etching process and has to pay the price of more polymer contaminations and of lower light transmittance. In our OLED devices, we use boron-doped monolayer graphene as an anode to achieve the same performance as multilayer graphene does. Pristine graphene and conventional indium-doped tin oxide (ITO) were used as the control anodes in the same OLED structure and fabrication process. Figure 3 shows the schematics of fabrication procedure for an OLED device with a graphene anode. The graphene film was first grown on a copper foil and then transferred onto a polyethylene terephthalate (PET) substrate. Optical lithography and O2 plasma were employed for graphene pattering. In the last step, thermal evaporation is applied to deposit metal electrodes and the subsequent organic layers. In the current OLED structure, we replace MoO3 with HAT-CN as a HIL to improve the hole injection efficiency. The conventional metal oxide tends to form a non-stoichiometric phase at the high-temperature thermal evaporation process and results in an ineffective hole injection


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in OLEDs.43 Figure 4a-b shows the device structure and energy diagram of graphene-based OLED comprising of HAT-CN (10 nm)/TAPC:HAT-CN (10%) (170 nm)/TCTA (45%):TPBi (45%):Ir(ppy)3 (10%) (30 nm)/TPBi (60 nm)/LiF (1 nm)/Al (100 nm). The combination of graphene and HAT-CN is first used in the graphene-based OLEDs. All devices showed a low activation voltage of 2.5 V and OLED performance is summarized in Table 1. The external quantum efficiency vs. luminance, current efficiency vs. current density, and power efficiency vs. current density are shown in Figure 4c-d, while the current density vs. voltage vs. luminance is revealed in Figure S6. Compared to the devices in which pristine graphene and UV/ozone ITO were used as an anode, the OLED device with boron-doped graphene exhxibits the highest external quantum efficiency (EQE, 24.6 %), current efficiency (CE, 95.4 cd/A), and power efficiency (PE, 99.7 lm/W) with 520 nm green emission and CIE (x, y) coordinates of (0.31, 0.63). The excellent performance is likely due to the higher work function (5.0 eV) and lower sheet resistance. Although the UV/ozone ITO anode also exhibits the properties of low sheet resistance and high work function (~4.9 eV), its hole injection efficiency is obviously poorer than that made with boron-doped graphene. In contrast, the device with pristine graphene anode shows the lowest efficiency (19.5 %) among the tested OLEDs due to the highest sheet resistance



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and lower work function (4.7 eV). To compare the hole injection efficiency between HAT-CN and MoO3, boron-doped graphene OLEDs with 5 nm MoO3 as a HIL were also made. It is found that HAT-CN provides higher current density and luminance as well as external quantum efficiency. The comparison of the two device properties are summarized in the Supporting Information. These results show that the use of boron-doped monolayer graphene in combination with HAT-CN effectively boosts the OLED performance by an appreciable higher external quantum efficiency, current efficiency, and power efficiency, outperforming the previous works that use pristine graphene as an anode. Besides, we also demonstrate the flexibility of boron-doped graphene on a 125-µm thick PET substrate. Figure 5 displays the change of boron-doped graphene in the bending test with a curvature radius of 0.75 mm and the performance of graphene-based OLED on PET. The slight increase of after 3000 bending cycles, along with the highest EQE value of 19.4 % in the resulting OLED, suggest that boron-doped graphene is highly suited for flexible OLEDs with stable and remarkable performance.

CONCLUSION In summary, we have successfully demonstrated the use of boron-containing PAH


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for boron-doped-graphene growth. The doped graphene shows high transmittance (97.5 %), low sheet resistance (240 Ω/ ) and p-type behavior with hole mobility of 1600 cm2/V⋅s which appear suitable as anode for OLEDs. The boron-doped graphene was synthesized and characterized by Raman spectroscopy and FET devices. Compared to the general doping method, this substitutionally boron-doped graphene is more stable and higher transparent than charge-transfer doped graphene. The boron doping not only decreases the sheet resistance, but also raises the work function of the graphene resulting in appreciable increase of hole transport ability. Furthermore, we have demonstrated the combination of graphene anode and HAT-CN as HIL to achieve a record-high performance (EQE, 24.6 %) among graphene OLEDs without any out-coupling treatment.

EXPERIMENTAL SECTION Pristine and boron-doped graphene growth and transfer: Clean Copper foils (99.8% purity, 25 µm thick) placed on a quartz boat were inserted into a quartz tube of an atmospheric pressure chemical vapor deposition (APCVD) system. The sample area, located at the center of the furnace, was heated up to 1000 °C within 60 min, followed by an annealing at this temperature for 30 min. Ar and



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H2 were introduced into the reaction chamber at a flow rate of 230 sccm and 10 sccm, respectively. During the growth, toluene or toluene solution (DBA(Mfes)2, 5×10-2 M) is used as a carbon source, and carried out by H2 (2 sccm) bubbling. The growth duration took only 6 min. After the growth, cool down the copper foils from 1000 °C to room temperature rapidly by moving the furnace. Pristine and boron-doped graphene grown on copper foils were then spin-coated with poly(bisphenol A carbonate) (PC) as a supporting layer for a wet-transfer process in an HCl/H2O2 etching solution. The remaining PC/graphene membranes were washed with deionized water and finally transferred onto a transparent substrate such as a plastic or glass.

Fabrication of back-gate field-effect transistor: To fabricate graphene FETs, pristine graphene or boron-doped graphene films were transferred onto silicon substrates with 300 nm SiO2 atop. Electron beam lithography and reactive-ion etching were employed for graphene patterning. Metal contact (Cr 1 nm and Au 60 nm) were thermally evaporated onto the restricted patterns by electron beam evaporator. Electrical transport measurements were carried out using a Keithley 4200 semiconductor system.


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Fabrication of an OLED device: The OLED devices with the configuration of HAT-CN (10 nm)/TAPC : HAT-CN (10%) (170 nm)/TCTA (45%) : TPBi (45%) : Ir(ppy)3 (10%) (30 nm)/TPBi (60 nm)/LiF (1 nm)/Al (100 nm) were fabricated by sequential thermal evaporations onto clean glass or plastic substrates with different experimental anodes. The effective area of the light emitting diode is 9.00 mm2. Current, voltage, and light-intensity measurements were recorded simultaneously using a source meter (Keithley 2400) and an optical meter (Newport 1835-C) equipped with a silicon photodiode (Newport 818-ST). EL spectra were measured on a fluorescence spectrophotometer (Hitachi F-4500). All the measurements were performed at 23 °C in air.

Characterization: Raman spectra were taken using a Raman spectrometer (Thermo Scientific DXR microscope) equipped with a 532 nm wavelength laser. The sheet resistance of the graphene samples were measured by using van der Pauw method. The optical transmittance of the graphene samples was measured by UV-Vis spectrometer (HITACHI U-4100 spectrophotometer). The work function of each doped graphene was obtained from ultraviolet photoelectron spectroscopy (ULVAC-PHI 5000 Versaprobe II), using a



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bias of 5 V and light source of HeI (21.22 eV) within beam spot size of ~1 mm diameter on graphene sample at an angle of 45° to the horizontal surface. The photoelectron spectrometer records the data within the resolution of 0.01 eV. XPS analysis was conducted on a spectrophotometer (ULVAC-PHI Quantera SXM/AES 650) with the Al X-ray. UPS/XPS measurement was performed under 5 ×10-8 Torr at room temperature.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Synthetic route of DBA(Mes)2, NMR spectrum, Mass spectra, CVD growth scheme, UPS spectra and OLED performance. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected], [email protected], [email protected] Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT


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The authors appreciate the project support of Taiwan Ministry of Science and Technology: (MOST 105-2633-M-007-003; MOST 103-2628-M-007-004-MY3; MOST 103-2119-M-007-008-MY3) and Instrumentation Center of NTHU for the use of photoelectron spectrometer. T.-L.W. would like to acknowledge Mr. Chien-Min Lu for the support of 3D-drawing object, Mr. Zheng-Yong Liang for the transistor fabrication and Mr. Chiu-Chuan Liao for the construction of CVD system.

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Figure, Caption and Table


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Figure 1. (a) Raman spectra of boron-doped (red) and pristine (black) monolayer graphene. (b) XPS B 1s core-level spectra of heavily boron-doped graphene, with peak deconvolution. (c) XPS C 1s core-level spectra of heavily boron-doped graphene, with peak deconvolution. (d) Variation of sheet resistance over time for pristine, boron-doped, AuCl3-coated, and TFSA-coated graphene under ambient conditions. (e) Light transmittance of pristine, boron-doped, AuCl3-coated, and TFSA-coated graphene. (f) Ultraviolet photoelectron spectroscopy spectra (UPS) of pristine, boron-doped, AuCl3-coated, and TFSA-coated graphene.



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Figure 2. (a) Optical microscopy image of a boron-doped graphene device; scale-bar: 20 µm. The rectangle framed by the dashed lines indicates the patterned graphene channel. (b) Resistivity as a function of back gate voltage.

Figure 3. Schematic illustrations of fabrication processes for graphene-based OLED.


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Figure 4. (a) Device architecture of OLED with boron-doped graphene as an anode. (b) Energy levels of the constituent materials in an OLED. (c) External quantum efficiency vs. luminance of the graphene-based OLED. The inset shows the optical image of the lighting OLED device. (d) Current and power efficiency as a function of current density.



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Figure 5. (a) Sheet resistance ratio v.s. bend cycle of boron-doped graphene on PET; the inset shows the optical image of the test samples. (b) External quantum efficiency vs. voltage characteristic for a boron-doped graphene OLED made on a PET substrate; the inset shows the optical image of a lighting boron-doped graphene OLED device on a bended PET substrate.

Table 1. Comparison of OLED performance and photophysical properties for devices comprising of different anode materials. Doping Graphene

Graphene

HIL

layer

Anode

ηEQE

b

ηCE

c

ηPE

d

[%]

[cd/A]

[lm/W]

Reference

G-HNO3

4

GraHIL

-

98.1

102.7

Nat. Photon.2

G-(C2H5)3OSbCl6

1

MoO3

20

80

-

Nat. Commun.3

G-CF3SO3H

4

DNTPD

-

104.1

80.7

Angew. Chem.11

G-GO

3

MoO3

-

82.0

98.2

Nanoscale24

1

HAT-CN

19.5

75.3

78.5

This Work

1

HAT-CN

24.6

95.4

99.7

This Work

-

HAT-CN

22.8

85.5

89.1

This Work

Pristine

a

Boron-doped ITO a

a

a

Structure of device: anode/HAT-CN (10 nm)/TAPC:HAT-CN (10%) (170 nm)/TCTA (45%) : TPBi

(45%) : Ir(ppy)3 (10%) (30 nm)/TPBi (60 nm)/LiF (1 nm)/Al (100 nm). bThe maximum external quantum efficiency. c The maximum current efficiency. d The maximum power efficiency.


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Table of Content Graphic


High-Performance Organic Light-Emitting Diode with Substitutionally

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