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Improving Efficiency of Blue Organic Light-Emitting Diode with Sulfobutylated Lignin Doped PEDOT as Anode Buffer Layer Yuan Li, Ying Wu, Weimei Zeng , Yuda Li, Xueqing Qiu, Lijia Xu, RunFeng Chen, and Wei Huang ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.5b01311 • Publication Date (Web): 09 Mar 2016 Downloaded from http://pubs.acs.org on March 12, 2016

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ACS Sustainable Chemistry & Engineering

Improving Efficiency of Blue Organic Light-Emitting Diode with Sulfobutylated Lignin Doped PEDOT as Anode Buffer Layer Yuan Li,*,‡,∥ Ying Wu,‡,∥ Weimei Zeng,‡,∥ Yuda Li,‡,∥ Lijia Xu,§ Xueqing Qiu,*,‡,∥ Runfeng Chen,*,§ Wei Huang§ ‡

School of Chemistry and Chemical Engineering, South China University of Technology,

Guangzhou, China ∥

State Key Laboratory of Pulp and Paper Engineering, South China University of

Technology, Guangzhou, China §Key

Laboratory for Organic Electronics and Information Displays & Institute of Advanced

Materials, Jiangsu National Synergistic Innovation Center for Advanced Materials, Nanjing University of Posts & Telecommunications, 9 Wenyuan Road, Nanjing 210023, China Abstract: Water-soluble alkyl chain sulfobutylated lignosulfonate (ASLS) doped PEDOT was prepared with lignin as raw material. Water processable PEDOT: ASLS was applied as hole injection layer (HIL) to modify ITO. As blue phosphorescent organic light-emitting diodes play as key role for full color display and are very challenging. With PEDOT: ASLS as HIL, a highly enhanced current efficiency of 37.65 cd/A was achieved. Considering our device structure the result is even better than that of control device using PEDOT: PSS as HIL. Comparing with PSS with regular structure, strong aggregation and oxidation behavior of ASLS contribute to the hole injection capability of PEDOT: ASLS. Our results showed that ASLS is an environment-friendly dopant of PEDOT. Considering that ASLS is of disordered and amorphous structure, which is very different from poly (styrene sulfonic acid), it is exciting that ASLS might be of promising potential as sustainable dopant of 1

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PEDOT. More importantly, this work will guide the design of dopant of PEDOT. Keywords: Dopant, hole transport material, PEDOT: PSS, phenol radical, organic electronic, solar cell, interface engineering Introduction Organic electronic devices including organic light emitting diodes (OLEDs), organic field effect transistors (OFETs), and bulk heterojunction polymer solar cells (BHJPSCs) have attracted worldwide attention considering their advantages such as light-weight, flexibility, and low-cost manufacturability.1-5 With the combination of new interface materials,6-7 conjugated polymers8-10 and application of new device structures,11-13 the power conversion efficiency (PCE) of PSCs have been significantly boosted,14-15 especially for perovskite solar cell. 16-20 Among all of the organic electronic devices, large area, wearable and flexible OLED is the most important and prospective research topic in both academic and industrial fields as it will change our life in future. Indium tin oxide (ITO) is widely used as anode in the device structure of OLEDs, however, it is required to modify its surface due to its unmatched energy level with hole transport layer and active layer in devices.21,22 Therefore, it is important to modify ITO with hole injection materials (HIMs). HIM plays indispensable role in organic electronic devices. One of the most widely employed HIM is poly (3, 4-ethylene dioxythiophene): poly (styrene sulfonic acid) (PEDOT:PSS) which modify indium-tin oxide (ITO) anode with good performances,23,24 however, HIMs are still in demand and chemistry scientists have made great effort to explore new materials.25-27 PEDOT: PSS has showed great potential for application in various of fields including PEDOT: PSS-based organic photovoltaic cells (OPVs) and hybrid solar cells.28-33 2


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The motivation of this report is that PEDOT dispersed with newly developed sulfobutylated lignin might show promising hole injection properties. Firstly, based on our previous work, 28 ASLS with phenol derivative has intrinsic hole transport property. This concept was proposed and shown in Scheme 1. It is well known that poly-N-vinylcarbazole (PVK) is a HTM studied in numerous previous work. It should be noted that the highest occupied molecular orbital (HOMO) level of PVK is -5.6 eV. ASLS showed an oxidation potential of 0.8 V with Ag/AgCl electrode and its HOMO energy level is around -5.2 eV. Secondly, ASLS has much lower sulfonic acid group content comparing with that of PEDOT: PSS and ASLS has relatively weak acidity. Thirdly, our previous work reported that ASLS exhibited good dispersion properties34 and it will be a promising dispersant for PEDOT, a cationic polymer. Furthermore, as one of most abundant plant resource, lignin is a sustainable chemical source, including alkali lignin (AL) from black liquor, which is mainly produced from pulp and paper making industry. AL accounts for more than 90% of industrial lignin. Lignin is not soluble in water and common organic solvents. A lot of chemical modification approaches have been reported to overcome this obstacles. Sulfonation is the most modification for AL and 1,4-butanesultone (1,4-BS) is an efficient way to enhance the water solubility of AL.34,35 Based on the ideas above, water dispersed PEDOT: ASLS was prepared. The oxidation behavior and electrochemical property of ASLS and PEDOT: ASLS were studied carefully and discussed in details. As we know, blue phosphorescent organic light-emitting diodes (PhOLED) play as key role for full color display and is very challenging to achieve performance as high as green 3



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PhOLEDs. For blue PhOLED, it is difficult to inject hole from ITO anode to emission layer due to the large energy barrier between ITO and emission layer. In order to test the hole injection of PEDOT:ASLS, it was applied as anode buffer layer to enhance the hole injection in blue PhOLED. The control devices with none of HTM and with PEDOT: PSS were also fabricated, respectively. The performance of PEDOT: ASLS-1:1 as HIM is better than both of the two control devices. This result provides a new concept for the design of dopant of PEDOT. Experimental section and methods Materials The alkali lignin (AL) was purified from pine wood pulping black liquor using acid treatment and the black liquor was supplied by Shuntai Co. Ltd (Hunan Province, China). 1,4-butanesultone (1,4-BS, 98%) was supplied by Energy Chemical Co. Ltd. (Shanghai, China). 3,4-Ethylenedioxy thiophene (EDOT) was purchased from Bayer AG, and was preserved at -4 ˚C. Ammonium persulphate ((NH4)2S2O8, APS) was obtained from Sigma. PEDOT: PSS (Baytron PVPAI 4083, 1.0%) was used for control experiment and device test. 1,3,5-tri[(3- pyridyl)-phen-3-yl]benzene (TmPyPb), 4,4'-cyclohexylidenebis[N,N-bis(4methyl- phenyl)aniline] (TAPC), 3-bis(N-carbazolyl)benzene (mCP), bis[2-(4,6difluorophenyl)- pyridinato-C2,N](picolinato)iridium(Ⅲ) (FIrPic) and all other chemicals are of analytical grade, including sodium hydroxide (NaOH) and sulfuric acid (H2SO4, 98%). Synthesis and characterization of ASLS 20 g of AL was added in 100 mL of aqueous NaOH solution at pH 12. When AL was dissolved, 1,4-BS (5 g, 41 mmol) and NaOH (1.64 g, 41 mmol) were fed at 70 oC. The 4


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reaction was stopped after stirring for 7 hours. The reaction mixture was filtered and extracted with organic solvent to remove unreacted 1,4-BS. ASLS samples were diluted to 0.1 g/mL and desalted by ion-exchange resin for 4 days to remove the excessive starting materials and salt. Finally, ASLS was freeze-dried to solid powder sample. The FTIR of sample were recorded using potassium bromide pressed-disk technique with Auto system XL/I-series/Spectrum 2000 spectrometry (Thermo Nicolet Co., Madison, WI, USA). The 1H-NMR spectra were recorded with 30 mg of sample dissolved in 0.5 mL of deuterium DMSO (d6-DMSO) at room temperature by DRX-400 spectrometer (400 MHz 1HNMR frequency, Bruker Co., Ettlingen, Germany). The phenolic hydroxyl group (Ph-OH) content of ASLS was measured by Folin-Ciocalteu reagent method (FC method). The sulfonic group (-SO3H) content of ASLSs samples was detected by automatic potentiometric titrator (Type 809 Titrando. Metrohm Corp., Switzerland). Cyclic voltammetry measurement was conducted using CH760D Electrochemical Workstation, CH Instruments (Austin, Texas, USA). All of the detailed test methods were similar with our previous work.34 Preparation and characterization of PEDOT: ASLS A detailed preparation and purification procedure of PEDOT: ASLS are presented as following steps. ASLS samples were dissolved in 50 mL of distilled water and different amounts of EDOT monomer was added, respectively. The mixture solution was conducted with pH value of about 2 under stirring for 10 minutes. The mass ratios of EDOT and ASLS were controlled as 1:1, 1:2, 1:3 and 1:4, respectively. Then 50 mL of 4wt% oxidant ammonium persulfate (APS) solution was dropped slowly under high speed stirring. The molar ratio of EDOT and APS was controlled as 1:1.4. After 5



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stirring at room temperature for 12 hours, blue-black PEDOT: ASLS complex solution was obtained, and the products were named as PEDOT:ASLS-1:1, PEDOT:ASLS-1:2, PEDOT:ASLS-1:3 and PEDOT:ASLS-1:4, respectively. The products were dialyzed by a dialysis membrane (Special products laboratory, USA, MWCO of 1,000 Da) to remove inorganic salt. The UV-vis absorption spectra of PEDOT: PSS and PEDOT: ASLS aqueous dispersion were measured using Shimadzu UV-3600 (Japan). Atomic force microscope (AFM) images of PEDOT:PSS and PEDOT:ASLS were recorded using Park XE-100 instrument by tapping mode. The film of PEDOT:ASLS was prepared as following procedure. ITO-coated glass substrates of area 1.5×1.5 cm2 were cleaned by sonication in acetone, detergent of ITO, deionized water, and isopropyl alcohol then dried in a nitrogen stream, followed by an oxygen plasma treatment. PEDOT:PSS and PEDOT:ASLS filtered through a 0.22 μm syringe filter were spin-cast on the pre-cleaned ITO-coated glass substrates at 1000 rpm for 60 s. The films of thickness with 80~90 nm were obtained and heated at 120 ℃ for 20 min under ambient conditions. Fabrication and characterization of OLEDs Device structure of the OLEDs was ITO/HTM (x nm)/TAPC (20 nm)/MCP(8 nm)/ mCP:FIrpic (10 wt%, 25 nm)/TmPyPb (35 nm)/LiF (1 nm)/Al (100 nm). ITO-coated glass substrates were ultrasonically cleaned with detergent, acetone, ethanol, and deionized water for 15 min and subsequently dried in an oven. The sheet resistance value of ITO used for OLED device fabrication is 15 Ω/□. The as-prepared substrate was treated by UV ozone for 5 minutes. A PEDOT:ASLS solution (~1%) was spin-coated onto the 6


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ITO glass substrate, followed by a 120 °C bake on a hot plate for 20 min before the subsequent depositions of organic layers and metal electrode. In order to thermally evaporate the organic and metal layers on the ITO surface, we set the substrates in a vacuum evaporator. Under a background pressure of 5×10-4 Pa, organic layers and cathode materials were sequentially deposited on the substrates without breaking vacuum. A shadow mask was applied to define the cathode and to make 9 mm2 devices on each substrate. Results and discussion Preparation and characterization of ASLS ASLS was synthesized via direct sulfonation of AL with 1,4-BS and the proposed synthesis route and structure of ASLS were shown in Figure 1(a). Gel-permeation chromatography (GPC) test was conducted to determine the molecular weight (Mw). The results of Mw distribution of AL and ASLS were listed in Table 1. The Mw of ASL (Mw=8900 Da, Mw/Mn=1.93) achieved a factor of 5.89-fold of AL (Mw=1900 Da, Mw/Mn=1.88). The increase of Mw has been made through oxidative coupling reaction between AL molecule radical species and induced further polymerization. Functional groups including phenolic hydroxyl groups and sulfonic groups were listed in Table 1. Mw of ASLS increased significantly and a remarkable decrease of phenolic hydroxyl group contents from 2.30mmol/g to 1.32mmol/g was detected. It is important that partial phenolic -OH group still existed in ASLS and the remained phenol structure provides its potential to act as semiconductor as reported in our recent report. 28 The content of sulfonic group of ASLS increased to 1.87 mmol/g, which is moderate value compared 7



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with lignosulfonate reported previously.28 The efficient sulfonation provided the excellent solubility of ASLS and enhanced Mw made it a good dispersant and dopant for PEDOT. Fourier transform infrared spectroscopy (FTIR) and 1H-NMR were also provided, respectively, (see Figure S1 and Figure S2) and they also confirmed the successful preparation of ASLS.34 Cyclic voltammetry test of ASLS and PEDOT:ASLS In order to study the difference of work functions between PEDOT:PSS and PEDOT:ASLS, the electrochemical behavior of them were investigated. Figure 2 presents the cyclic voltammogram of the films of four samples. It is noteworthy that there was an obvious oxidation peak at 0.76 V, which is ascribed to the oxidation of PEDOT:ASLS. Compared with the CV results of PEDOT:PSS, that one more obvious oxidation peak was observed in PEDOT:ASLS. This is very important information on the difference between them and it indicates that the oxidation process came from the contribution of the phenol structure in ASLS. It will provide great potential for the application PEDOT:ASLS as efficient hole injection material for its work function of 5.1 eV. It should be pointed that the oxidation behaviour in CV spectrum of PEDOT: ASLS samples was very different with each other and changed with the ratio of ASLS dopant. PEDOT: ASLS-1:1 showed the most quasi-reversible peak in all of the CV spectrum of all samples which indicated its potential as a good anode buffer layer. UV-vis absorption, FTIR and dispersion stability of PEDOT: ASLS ASLS was applied to disperse PEDOT and prepare PEDOT:ASLS, and the synthesis route was shown in Figure 1(b). 8


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The UV-vis absorption spectra of PEDOT:ASLS and PEDOT:PSS in aqueous solution were shown in Figure 3 (a). Comparing with PEDOT:PSS, PEDOT:ASLS aqueous dispersion showed a broad absorption from 600 to 900 nm and it was very similar with that of PEDOT:PSS. Moreover, the absorption in of PEDOT: ASLS in UV-vis region (less than 600 nm) was lower than that of PEDOT:PSS. This indicates that the content of EDOT of PEDOT: ASLS is higher than that of PEDOT: PSS. This will induce weak absorption of UV-vis light (less than 600 nm) of OLED and benefit the output of light. To characterize PEDOT:ASLS, FTIR spectra of EDOT monomer, PEDOT:ASLS and PEDOT:PSS, were provided in Figure 3 (b). It is very different from adsorption peaks (between 750 and 1500 cm-1) of EDOT monomer, in the spectrum of PEDOT:ASLS the adsorption signals between 750 and 1500 cm-1 totally changed, demonstrating the successful polymerization of EDOT. The peaks around 1199 cm-1 and 619 cm-1 came from the sulfonic groups in PSS or ASLS. Based on our previously reported PEDOT:SL showed strong aggregation in aqueous solution,28 we used dynamic light scattering (DLS) to study the particle size of PEDOT:ASLS (see Figure 4). With the increase of mass ratio of ASLS/EDOT, the particle size of PEDOT:ASLS decreased. It can be concluded that PEDOT:ASLS existed as nano-particle and ASLS showed good dispersion performance for PEDOT comparing with our reported lignosulfonate.28 Performance of PEDOT: ASLS as anode modifier in OLEDs

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To the most of our knowledgement and with respect of our previous work,28 it is rarely reported to use lignin-derivative doped PEDOT for application in OLED. PEDOT:PSS is widely used in PLEDs, meanwhile, PEDOT:PSS is also widely used in small-molecule OLED, especially in phosphorescence OLEDs.36-38 Moreover, in order to explore its potential as HIL in OLED, we chose blue PhOLED as it is challenging case in organic electronics due to its low HOMO level of host material such 3-bis(Ncarbazolyl)benzene (mCP). Hererin, we tested the application of the new-developed PEDOT:ASLS as HIL in Firpic-based phosphorescent OLEDs. This new HIL material show improved device performance than that of PEDOT:PSS under the same device structure. The detailed performance of OLEDs with PEDOT:ASLS was given in Figure 5 and Table 2. The current and power efficiency of PhOLEDs with PEDOT: ASLS as HIMs are much higher than both of the two control devices. The maximum current and power efficiency of 37.65 cd/A and 12.84 lm/W were achieved. For the bare ITO device without HIL, the hole injection is poor. Meanwhile, there was none of electron blocking layer. The current of device (I-V curve) originated from the leakage current, which was ascribed to excess electron flux from the emission layer. This produced the relatively large operating current and low current efficiency due to the unbalanced carrier recombination. For the devices with HILs including PEDOT:PSS and PEDOT:ASLS, the turn-on voltages (4.5 v and 4.2 V, respectively) were lower than 4.6 V of the bare ITO device without HIL. With the hole injection effect, the hole and electron recombination rate were improved for the devices based on PEDOT:PSS and PEDOT:ASLS, so the current of devices decreased. Consequently, the efficiencies 10


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of devices with HILs were enhanced. The electroluminescence spectra were shown in Figure 6 and it confirmed the emission from sky-blue emitter Firpic. Considering that ASLS is a dopant with so complex, disordered and amorphous structure, it is extremely exciting result that PEDOT:ASLS performed better than PEDOT:PSS. As morphology is a key factor for the charge transport of HIL, we studied the underlying effects of the surface morphology of HILs on the improvement of OLED efficiency, AFM images of the PEDOT:PSS and PEDOT:ASLS films were studied. The results showed that there was significant difference in the film morphology between them, as shown in Figure 7 (a1, b1) and Figure S3 (a1, b1, c1). The AFM image of PEDOT:ASLS-1:1 showed more obvious and larger nano-aggregates than PEDOT:PSS, PEDOT:ASLS-1:2, PEDOT:ASLS-1:3, and PEDOT:ASLS-1:4. The surface of PEDOT: PSS film was relatively smooth before and after annealing at 120 ℃ for 20 min.

The RMSs of PEDOT: PSS film were 1.66 nm and 4.22 nm

before and after they were heated at 120 ℃ for 20 min, respectively. The roughness increased after annealing. In comparison, after annealing at 120 ℃, the AFM image of PEDOT:ASLS-1:1 film (see Figure 7b2) was very unique and different from those of PEDOT:PSS(see Figure 7a2) and other PEDOT:ASLSs with higher mass rations of ASLS dopant (see Figure S3a2, S3b2, S3c2). The AFM images of PEDOT:ASLS-1:2, -1:3 and -1:4 films changed a lot afterannealing at 120 oC for 20 min. However, the nano-aggregate of PEDOT:ASLS-1:1 did not showed obvious change after annealing. The nano-aggregate structure might facilitate the hole transport of PEDOT:ASLS-1:1.

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This is one of the reasons why PEDOT:ASLS-1:1 exhibited the best performances among these four samples. This result is in good agreement with our previous work. 28 Clearly there are a lot of compact nano-structure spheres with size of 100 nm in the film of PEDOT:ASLS-1:1. Comparing with PSS with regular structure, the stronger aggregation characteristic and oxidation behavior of ASLS might contributed the unexpected hole injection performance of PEDOT:ASLS film in OLEDs. All in one word, ASLS showed prospective application potential as dopant for PEODT. Conclusions Water soluble ASLS with flexible alkyl sulfonic acid groups was readily developed. ASLS showed oxidation behaviour in cyclic voltammetry test. Water dispersed PEDOT:ASLS was conveniently synthesized using ASLS as dopant and dispersant. PEDOT:ASLS film was spin-coated on ITO as hole injection layer in blue PhOLED. The maximum current and power efficiency of 37.65 cd/A and 12.84 lm/W were achieved. Our result showed that although ASLS has so complex, disordered and amorphous structure, it is a good candidate as dopant of PEDOT in organic electronic devices including perovskite-based solar cells40-46 and hybrid solar cells46-50. A key point is that PSS is non-conjugated, however, ASLS can act as hole transport material. More importantly, we provide a concept for the design of hole transport materials derived from lignin,51-52 polyaromatic hydrocarbons,53-56 and phenol-based polymers.57-58 In addition, this work will motivate us to understand of PEDOT-based polymer with in-depth perspective.59 ASSOCIATED CONTENT 12


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*Supporting Information

The Supporting Information is available free of charge on the ACS Publications website at DOI:XXX. Parameters of structure characteristics of AL and ASLS, FT-IR and 1H-NMR spectra of AL and ASLS, AFM images of PEDOT: ASLS-1:2 film, PEDOT: ASLS-1:3 film, PEDOT: ASLS-1:4 film, PEDOT: ASLS-1:2 film, PEDOT: ASLS-1:3 film, PEDOT: ASLS-1:4 film after annealing at 120 ℃for 20 min. AUTHOR INFORMATION Corresponding Author * Yuan Li, Email: [email protected], Tel/Fax: +86-20-87114033. * Xueqing Qiu, Email: [email protected] * Runfeng Chen, Email: [email protected] Notes The authors declare no competing financial interest.

Acknowledgements The authors would like to acknowledge the financial support of National Natural Science Foundation of China (21402054, 2143600421274065), National Basic Research Program of China 973 (2012CB215302), International S&T Cooperation Program of China (2013DFA41670), Scientific Research Foundation for Advanced Talents (D614020-3).

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Highly Improved

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Mater. 2015, 27, 3391–3397.

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Captions of figures and tables Scheme 1. The oxidation potential and HOMO level of PVK (a) and ASLS (b) as hole transport material, respectively in organic electronic device. Table 1 Parameters of structure characteristics of AL and ASLS. Table 2. Performances of OLEDs with PEDOT: PSS and PEDOT: ASLS as HTMs, respectively. Figure 1(a) Synthetic route of ASLS, (b) Proposed schematic for the polymerization of EDOT in the presence of ASLS as dopant. Figure 2 Cyclic voltammogram of sample films with 0.1 M Bu4NPF6 in CH2Cl2 solution: (a) PEDOT: PSS, (b) ASLS, (c) PEDOT: ASLS-1:1, (d) PEDOT: ASLS-1:2. Figure 3 (a) UV absorption spectra of PEDOT: PSS and PEDOT: ASLS, (b) FT-IR spectra of PEDOT: PSS and PEDOT: ASLS. Figure4 Dynamic light scattering (DLS) measurement of (a) PEDOT: ASLS-1:1, (b) PEDOT: ASLS-1:2, (c) PEDOT:ASLS-1:3, (d) PEDOT: ASLS-1:4. Figure 5 (a) The current density J versus voltage curve (hollow symbols) and luminescence-voltage curve (solid symbols), (b) The current/power efficiency versus luminance curves of OLEDs with

PEDOT:PSS and PEDOT:ASLS as HTMs,

respectively. Figure 6 The electroluminescence spectra of OLEDs with PEDOT: ASLS as HTMs. Figure 7 AFM images of (a1) PEDOT: PSS film, (b1) PEDOT: ASLS-1:1 film, (a2) PEDOT: PSS film, (b2) PEDOT: ASLS-1:1 film after annealing at 120 ℃ for 20 min. The size of the images is 3 × 3 μm.

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Scheme 1. The oxidation potential and HOMO level of PVK (a) and ASLS (b) as hole transport material, respectively in organic electronic device.

Table 1. Parameters of structure characteristics of AL and ASLS.

Samples

-OH

m(AL): m(1,4-BS)

-SO3

(mmo

(mm

l/g)

ol/g)

Mw

Mn

(Da)

(Da)

PDI

AL

1:0.0

2.30

0.00

1900

1000

1.88

ASLS

1:0.2

1.32

1.87

8900

5800

1.53

Table 2. Performances of OLEDs with none of anode modifier, with PEDOT: PSS and PEDOT: ASLS as anode modifier, respectively.

Anode modifier

Von (V)

CEmax (cd/A)

PEmax (lm/W)

None

4.6

14.5

5.82

PEDOT: PSS

4.5

25.09

8.25

PEDOT: ASLS-1:1

4.2

37.65

12.84

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Figure 1

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Figure 2

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Figure 3

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Figure 4

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Figure 5

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Figure 6

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Figure 7

(a1)

(a2)

(b1)

(b2)

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For Table of Contents Use Only

As an efficient hole injection layer, sulfobutylated lignin doped PEDOT showed highly enhanced performances in organic light-emitting diodes.

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Improving Efficiency of Blue Organic Light-Emitting ... - ACS Publications

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