White Polymer Light-Emitting Diodes Based on Exciplex

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White Polymer Light-Emitting Diodes Based on Exciplex Electroluminescence from Polymer Blends and a Single Polymer Junfei Liang, Sen Zhao, Xiao-Fang Jiang, Ting Guo, HinLap Yip, Lei Ying, Fei Huang, Wei Yang, and Yong Cao ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b11926 • Publication Date (Web): 17 Feb 2016 Downloaded from http://pubs.acs.org on February 23, 2016

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ACS Applied Materials & Interfaces

White Polymer Light-Emitting Diodes Based on Exciplex Electroluminescence from Polymer Blends and a Single Polymer

Junfei Liang,†,‡ Sen Zhao,†,‡ Xiao-Fang Jiang,† Ting Guo,† Hin-Lap Yip,† Lei Ying,*,† Fei Huang,*,† Wei Yang,† Yong Cao†

†

State Key Laboratory of Luminescent Materials and Devices, and Institute of Polymer

Optoelectronic Materials and Devices, South China University of Technology, Guangzhou, 510640

1 ACS Paragon Plus Environment


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ABSTRACT In this manuscript, we designed and synthesized a series of polyfluorene derivatives, which consist of the electron-rich 4,4'-(9-alkyl-carbazole-3,6-diyl)bis(N,N-diphenylaniline) (TPA-Cz) in the side chain and the electron-deficient dibenzothiophene-5,5-dioxide (SO) unit in main chain. The resulting copolymer PF-T25 that did not comprise SO unit exhibited blue lightemission with the Commission Internationale de L’Eclairage coordinates of (0.16, 0.10). However, by physically blending PF-T25 with a blue light-emitting SO-based oligomer, a novel low-energy emission correlated to exciplex emerged owing to the appropriate energy level alignment of TPA-Cz and the SO-based oligomers, which showed extended exciton lifetime as confirmed by time-resolved photoluminescent spectroscopy. The low-energy emission was also identified in copolymers consisting of SO unit in main chain, which can effectively compensate for the high-energy emission to produce binary white light-emission. Polymer light-emitting diodes based on the exciplex-type single greenish-white polymer exhibit the peak luminous efficiency of 2.34 cd A-1 and the maximum brightness of 12410 cd m-2, with Commission Internationale de L’Eclairage color coordinates (0.27, 0.39). Device based on such polymer showed much better electroluminescent stability than those based on blending films. These observations indicated that developing single polymer with the generated exciplex emission can be a novel and effective molecular design strategy toward highly stable and efficient white polymer light-emitting diodes.

KEYWORDS: polymer light-emitting diodes, white light-emitting, solution process, exciplex, highly stable electroluminescence


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INTRODUCTION Solution processable white polymer light-emitting diodes (WPLEDs) have attracted tremendous interests due to their great potential for the applications in flat-panel displays, backlighting sources for liquid-crystal and solid-state light sources.1-8 To realize efficient white lightemission, one needs to simultaneously involve certain proportions of three primary colours (blue, green and red), or two complementary colours (blue with orange or yellow) in the emissive layer.9 In this respect, various strategies have been utilized to attain white light-emission, such as physically blending the monochromic emitters into the host polymeric materials, or chemically tethering light-emitting species into polymer chains. 10 - 12 From the perspective of device engineering, multi-layered WPLEDs are highly favourable since they can allow for the independent contribution of each single layer to either charge-injection or light-emission.13,14 However, the intermediate bimolecular excited state (exciplex) may be formed at the interface of a hole-dominant and an electron-dominant layer, for which the electron and hole are confined in the interface of such two layers.15-19 Exciplex is fundamentally unfavourable for the attainment of high colour purity of the monochromic colour due to its broad emission profile.20-22 However, when it comes to white light-emission with multi-component characteristics, exciplex can be potentially utilized as low-energy emissive species to compensate for the high-energy blue emission for the achievement of white light-emission.5,23-30 It is well-established that the fabrication of multi-layered PLEDs remains a great challenge since the employed solvents for the sequential processing of organic layers are detrimental for the prefabricated layer. To address this issue, Cheng et al. used a cross-linked hole transport layer, which allows for the sequential processing to achieve mutilayered devices without destroy the preformed underlayer due to its solvent-resistant property.31 Of particular interest is that this



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hole transport layer can form red light-emitting exciplex with the emissive layer, giving near white light emission with Commission Internationale d’Eclairage (CIE) coordinates of (0.32, 0.42).31 It is also worth mentioning that the color of exciplex can be controlled by tuning the electron affinity of acceptor, which can reach white-emission from a simple mixture of supramolecular acceptors.32 This strategy has a specific advantage of integrating low-energy emission from exciplex to realize white light-emission in a single emissive layer. With respect to WPLEDs based on the physically blended film as the emissive layer, single white light-emitting polymers are of particular interests since the molecularly dispersion of chromophores can lead to more stable electroluminescence as the annihilation at high current densities can be effectively avoided. However, the single white light-emitting polymers typically require comparatively low proportions of low-energy emitters, which is challenging to control the stability among different batches. Regarding to this issue, Wang et al. developed a series of single white light-emitting polymer that can allow for the incorporation of high molar ratio of dopant up to 1%, which can be achieved based on the molecular design strategy based on the mechanism of electron trapping on host.33 In this work, we introduced a novel strategy to attain white light-emission in one single polymer chain by utilizing the low-energy exciplex emission, which can combine with the blue light-emitting polymer chain to present the complementary white light-emission. The synthesized polymers consist of an electron-donating side chain of 4,4'-(9-alkyl-carbazole-3,6diyl)bis(N,N-diphenylaniline)

(TPA-Cz),

and

the

electron-withdrawing

moiety

of

dibenzothiophene-5,5-dioxide (SO) unit in the backbone of polyfluorene derivatives (PF). Lowenergy emission corresponding to the formed exciplex can be realized in both of physically blended films of polymer consisting of TPA-Cz side chain and model compound FFSOFF


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comprising of SO unit, and the prepared polymers covalently tethering TPA-Cz side chain and SO unit in backbone. The formation of exciplex can be ascribed to the fact that the injected holes and electrons can be confined in the TPA-Cz side chain and the PFSO backbone, respectively as a result of their appropriate frontier molecular orbital energy levels. Considering the moderate molecular weight of these resulting copolymers, the formed exciplex emission can be in either intramolecular or intermolecular fashion. The prepared single white light-emitting polymer based on such exciplex emission as the low-energy band exhibited a maximum luminous efficiency of 2.34 cd A-1 and the CIE coordinates of (0.27, 0.39).

EXPERIMENTAL SECTION Materials All reagents were purchased from commercial sources and used without further purification. Toluene, tetrahydrofuran (THF) were purified according to standard procedures and distilled under nitrogen before use. Compounds 2,7-dibromo-9,9-bis(4-((2-ethylhexyl)oxy)phenyl)-9Hfluorene

(M6),

2,2'-(9,9-bis(4-((2-ethylhexyl)oxy)phenyl)-9H-fluorene-2,7-diyl)bis(4,4,5,5-

tetramethyl-1,3,2-dioxaborolane) (M7), 3,7-dibromo-dibenzothiophene-S,S-dioxide (M8) were synthesized

according

to

reported

procedures.

The

oligomer

9,9-dihexylfluorene-

dibenzothiophene-S,S-dioxide (FFSOFF) was synthesized according to literature.34 Synthesis of Monomers and Polymers Synthesis of 2,7-dibromo-9-phenyl-9H-fluoren-9-ol (M1) 2,7-Dibromo-9H-fluoren-9-one (10 g, 9.59 mmol) was dissolved in dry THF (100 mL) and stirred the mixture at –78 oC for 2 h, then phenylmagnesium bromide (5.36 g, 29.59 mmol) was added into reaction vessel, then stirred the mixture at –78 oC for 1 h under Ar. The reaction was



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quenched by adding water, and extracted 3 times by using dichloromethane. After removing the organic phase under reduced pressure, the crude product was purified by column chromatography by using silica gel. Yield: 64 %. 1H NMR (500 MHz, CDCl3) δ (ppm): 7.50 (m, 4 H), 7.44 (t, J = 1.1 Hz, 2 H), 7.36 – 7.27 (m, 5 H), 2.50 (s, 1H). 13C NMR (126 MHz, CDCl3) δ (ppm): 152.12, 141.69, 137.62, 132.59, 128.65, 127.91, 125.35, 122.66, 121.70, 83.42. HRMS (ESI) (m/z): calcd. for (C19H12Br2ONa)+ 436.9153; found, 436.9147. Synthesis of 3-(2,7-dibromo-9-phenyl-9H-fluoren-9-yl)-9H-carbazole (M2) 2,7-Dibromo-9-phenyl-9H-fluoren-9-ol (5 g, 12.02 mmol) and carbazole (6.03 g, 36.05 mmol) were dissolved in anhydrous dichloromethane (DCM, 100 mL) and stirred the mixture at room temperature for 2 h, then boron trifluoride diethyl etherate (BF3·OEt2, 1.22 g, 18.02 mmol) was added into reaction vessel, then the mixture was stirred at room temperature for 16 h under Ar. The reaction was quenched by adding water, and extracted 3 times by using dichloromethane. After removing the organic phase under reduced pressure, the crude product was purified by column chromatography by using silica gel to give the target compound as a white solid. Yield: 70 %. 1H NMR (500 MHz, CDCl3) δ (ppm): 8.04 (s, 1 H), 7.96 – 7.91 (m, 1 H), 7.81 (d, J = 1.9 Hz, 1 H), 7.65 – 7.59 (m, 2 H), 7.57 (d, J = 1.5 Hz, 2 H), 7.50 (dd, J = 8.1, 1.8 Hz, 2 H), 7.44 – 7.36 (m, 2 H), 7.32 (dd, J = 8.5, 0.5 Hz, 1 H), 7.30 – 7.26 (m, 3 H), 7.25 – 7.22 (m, 5 H).

13

C NMR (126 MHz, CDC3) δ (ppm): 153.86, 145.29, 139.97, 138.64, 138.17,

135.59, 130.99, 129.67, 128.69, 128.20, 127.27, 126.37, 126.19, 123.43, 123.20, 121.98, 121.74, 120.59, 110.84, 65.82. HRMS (ESI) (m/z): calcd. for (C31H19Br2NNa)+ 585.9782; found, 585.9776. Synthesis of 4,4'-(9H-carbazole-3,6-diyl)bis(N,N-diphenylaniline) (M3)


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3,6-Dibromo-9H-carbazole (9 g, 27.69 mmol), N,N-diphenyl-4-(4,4,5,5-tetramethyl-1,3,2dioxaborolan-2-yl)aniline (21.59 g, 58.15 mmol), and toluene (300 mL) were added into reaction vessel and stirred the mixture at room temperature, then K2CO3 (30.62 g, 221.54 mmol) in diwater (100 mL) and Pd(PPh3)4 (3.2 g, 2.77 mmol) were added into reaction vessel, heated to 110 o

C in an oil bath for 16 h under Ar. The reaction was quenched by adding water, and extracted 3

times by using dichloromethane. After removing the organic phase under reduced pressure, the crude product was purified by column chromatography by using silica gel to give the product as a white powder. Yield: 78 %. 1H NMR (500 MHz, CDCl3) δ (ppm): 8.32 (s, 2H), 7.67 (dd, J = 8.4, 1.5 Hz, 2H), 7.61 (s, 4H), 7.53 – 7.37 (m, 2 H), 7.29 (dd, J = 16.0, 8.5 Hz, 8 H), 7.19 (t, J = 10.4 Hz, 12 H), 7.05 (t, J = 7.1 Hz, 4 H). 13C NMR (126 MHz, CDCl3) δ (ppm): 147.93, 146.59, 136.46, 129.37, 128.03, 125.35, 124.59, 124.34, 124.15, 122.84, 118.52, 110.08. HRMS (ESI) (m/z): calcd. for (C48H35N3)+ 653.2831; found, 653.2825. Synthesis of 4,4'-(9-(6-bromohexyl)-9H-carbazole-3,6-diyl)bis(N,N-diphenylaniline) (M4) 4,4'-(9H-carbazole-3,6-diyl)bis(N,N-diphenylaniline (5 g, 7.65 mmol), KOH (858.13 mg, 15.29 mmol) and N, N-dimethylformamide (DMF, 100 mL) were added into reaction vessel and stirred the mixture at room temperature for 10 minutes, then 1,6-dibromohexane (7.46 g, 30.59 mmol) was added into reaction vessel, then the mixture was stirred at 80 oC for 16 h under Ar. The reaction was quenched by adding water, and extracted 3 times by using dichloromethane. After removing the organic phase under reduced pressure, the crude product was precipitated in methanol, and purified by column chromatography by using silica gel to give the target compound as a white solid. Yield: 88 %. 1H NMR (500 MHz, CDCl3) δ (ppm): 8.33 (d, J = 1.6 Hz, 2 H), 7.71 (dd, J = 8.5, 1.7 Hz, 2 H), 7.60 (d, J = 8.6 Hz, 4 H), 7.45 (d, J = 8.5 Hz, 2 H), 7.32 – 7.27 (m, 8 H), 7.21 – 7.13 (m, 12 H), 7.03 (t, J = 7.3 Hz, 4 H), 4.35 (t, J = 7.0 Hz, 2 H), 3.38 (t,



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J = 6.7 Hz, 2 H), 1.95 (dt, J = 14.5, 7.1 Hz, 4 H), 1.59 – 1.40 (m, 2 H), 1.33 – 1.20 (m, 2 H). 13C NMR (126 MHz, CDCl3) δ (ppm): 147.96, 146.54, 140.25, 136.48, 132.12, 128.01, 125.14, 124.60, 124.31, 123.64, 122.82, 118.61, 109.11, 43.24, 33.87, 32.69, 29.06, 28.05, 26.62. HRMS (ESI) (m/z): calcd. for (C54H46BrN3Na)+ 838.2773; found, 838.2767. Synthesis of 4,4'-(9-(6-(3-(2,7-dibromo-9-phenyl-9H-fluoren-9-yl)-9H-carbazol-9-yl)hexyl)9H-carbazole-3,6-diyl)bis(N,N-diphenylaniline) (M5) 3-(2,7-Dibromo-9-phenyl-9H-fluoren-9-yl)-9H-carbazole (M2, 1 g, 1.77 mmol), KOH (198.50 mg, 3.54 mmol), and 1,4-dioxane (60 mL) were added into a reaction vessel, and the mixture was stirred at room temperature for 10 min. Then 4,4'-(9-(6-bromohexyl)-9H-carbazole3,6-diyl)bis(N,N-diphenylaniline) (1.59 g, 1.95 mmol) was added into reaction vessel, the mixture was stirred at 80 oC for 16 h under Ar. Then water (100 mL) was added, and the mixture was extracted 3 times by using dichloromethane. After removing the organic phase under reduced pressure, the crude product was purified by column chromatography by using silica gel to give the target compound as a white solid. Yield: 70 %. 1H NMR (500 MHz, CDCl3) δ (ppm): 8.31 (s, 2 H), 7.93 (d, J = 7.7 Hz, 1 H), 7.81 (s, 1 H), 7.65 (d, J = 8.4 Hz, 2 H), 7.59 (d, J = 8.2 Hz, 6 H), 7.55 (d, J = 1.7 Hz, 2 H), 7.49 – 7.44 (m, 2 H), 7.41 – 7.33 (m, 3 H), 7.30 – 7.26 (m, 14 H), 7.25 (t, J = 3.0 Hz, 8 H), 7.24 – 7.12 (m, 4 H), 7.03 (t, J = 7.0 Hz, 4 H), 4.22 (dd, J = 21.4, 14.4 Hz, 4 H), 1.93 – 1.75 (m, 2 H), 1.46 – 1.34 (m, 4 H), 1.34 – 1.21 (m, 2 H). 13C NMR (126 MHz, CDCl3) δ (ppm): 145.18, 140.75, 140.10, 139.46, 138.03, 136.39, 134.72, 131.97, 130.84, 129.53, 129.25, 8.55, 128.07, 127.89, 127.12, 1.9, 125.85, 124.99, 124.50, 24.19, 123.50, 122.73, 122.69, 122.55, 121.83, 121.60, 120.51, 119.3, 118.86, 118.47, 108.96, 108.72, 108.66, 77.28, 77.19, 76.77, 65.77, 43.09, 42.94, 31.60, 28.29, 27.13. HRMS (ESI) (m/z): calcd. for (C83H64Br2N4)+ 1298.3498; found, 1298.3510.


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Synthesis of Polymers General procedures of Suzuki polycondensation (taking PF-T25 as an example). Under an argon atmosphere, to a two-neck flask was added 2,7-dibromo-9,9-bis(4-((2ethylhexyl)oxy)phenyl)-9H-fluorene

(M6,

183.2

mg,

0.25

mmol),

2,2'-(9,9-bis(4-((2-

ethylhexyl)oxy)phenyl)-9H-fluorene-2,7-diyl)bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolane) (M7, 413.5 mg, 0.5 mmol), 4,4'-(9-(6-(3-(2,7-dibromo-9-phenyl-9H-fluoren-9-yl)-9H-carbazol-9yl)hexyl)-9H-carbazole-3,6-diyl)bis(N,N-diphenylaniline) (M5, 325.3 mg, 0.25 mmol) and toluene was added to palladium acetate (3.4 mg, 0.015 mmol), tricyclohexylphosphine (8.4 mg, 0.03 mmol) and toluene (10 mL). The reaction mixture was stirred and heated to 80 oC. Then tetraethyl ammonium hydroxide (Et4NOH, 20% aq, 2 mL) was added. The temperature was kept at about 80–85 oC, and the solution was allowed to stir for 36 h. The reaction was end-capped by adding phenylboronic acid (0.05 g, 0.4 mmol) and was stirred for 12 h. Then bromobenzene (0.125 g, 0.8 mmol) was added followed by stirring for another 12 h. After cooling to room temperature, the mixture was precipitated into methanol (150 mL) and filtered. The collected solids were dried, re-dissolved in chloroform and washed three times by de-ionized water. The organic phase was concentrated under reduced pressure, followed by re-precipitation in methanol. The crude product was further purified by Soxhlet extraction by methanol and acetone successively. The target polymer was collected after drying under vacuum with a yield of 60%. 1

H NMR (500 MHz, CDCl3) δ (ppm): 8.27 (br, ArH), 7.96 (br, ArH), 7.86 (br, ArH), 7.64 (br,

ArH), 7.33 (br, ArH), 7.05 (br, ArH), 6.73 (br, ArH), 4.18 (m, N-CH2), 3.76 (br. N- CH2), 1.81 (br, CH2), 1.65 (br, CH2), 1.25 (br, CH2), 0.84 (t, CH3).



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PF-T5SO15: 1H NMR (500 MHz, CDCl3) δ (ppm): 8.29 (br, ArH), 8.01 (br, ArH), 7.65 (br, ArH), 7.16 (br, ArH), 6.17 (br, ArH), 4.18 (m, N-CH2), 3.77 (s, N-CH2), 1.67 (br, CH2), 1.39 (br, CH2), 0.88 (t, CH3). PF-T25SO15: 1H NMR (500 MHz, CDCl3) δ (ppm): 8.28 (br, ArH), 7.97 (br, ArH), 7.63 (br, ArH), 7.14 (br, ArH), 6.74 (br, ArH), 4.18 (m, N-CH2), 3.76 (br, N-CH2), 1.85 (br, CH2), 1.65 (br, CH2), 1.43 (br, CH2), 0.83 (t, CH3).

Measurement and Characterization. 1

H NMR spectra were recorded on a Bruker DRX 500 spectrometer in deuterated

chloroform solution with teramethysilane as a reference. UV-vis absorption spectra were recorded on HP 8453 spectrophotometer. Photoluminescent spectra were recorded on a PerkinElmer LS 55 spectrofluorometer. The luminous efficiency-current density-luminance (LE–J–L) characteristics were collected by using a Keithley 236 source measurement unit and a calibrated silicon photodiode. The luminance was calibrated by a PR-705 spectra scan spectrophotometer (Photo Research), with simultaneous acquisition of the electroluminescent (EL) spectra and CIE coordinate, driven by Keithley model 2400 voltage-current source. Time resolved photoluminescent spectra were recorded by a Hamamarsu C11367 compact fluorescence lifetime spectrometer.

Fabrication and Characterization of PLEDs The fabrication of devices followed a well-established process. The ITO glass substrates (sheet resistance of 15 - 20 ohm/square) were cleaned in an ultrasonic bath successively in acetone, detergent, deionied water, and isopropanol. The ITO glass were dried in vacuum oven


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overnight at 80 oC. A 40 nm thick PEDOT:PSS layer was spin-coasted directly on the ITO substrate after oxygen plasma treatment. The PEDOT:PSS was treated at 120 oC for 20 min. The active layer was spin-coated in p-xylene solution and then annealed at 100 oC on a hotplate for 20 min. The thickness of active layer was determined by a Tencor Alpha-step 500 Surface Profilometer. The deposition of cathode was carried out by using the equipment set out in a glove-box (MBraun XK20) filled with inert atmosphere. The cathode of CsF (1.5 nm)/Al (100 nm) was deposited in vacuum at a pressure of about 1×10-6 Pa, where the thickness was monitored by a STM-100/MF Sycon quartz crystal. The high vacuum was achieved by a Mechanical pump (EDWARDS: RV12) and a molecular pump (PFEIFFER VACUUM: TC 400 PB, made in Germany in July 2013, see Figure S2 in the SI). A 16 mm2 pixel area was defined through a shadow mask between the cathode and anode. Except for depositing the PEDOT:PSS layer, all other procedures were carried out in nitrogen filled glove-box containing less than 10 ppm oxygen and moisture. The PLEDs were encapsulated with a UV-cured epoxy resin.

RESULTS AND DISCUSSION Synthesis of Monomer and Polymers The synthetic routes of the monomer and polymers are shown in Scheme 1. The intermediate compound 2,7-dibromo-9-phenyl-9H-fluoren-9-ol (M1) was synthesized by treating 2,7-dibromo-9-fluorenone with the Grignard reagent of phenylmagnesium bromide. The reaction of compound M1 with carbazole gave intermediate compound 3-(2,7-dibromo-9-phenyl-9Hfluoren-9-yl)-9H-carbazole (M2) in a yield of 70%. The palladium-catalyzed Suzuki coupling reaction of 3,6-dibromocarbazole with N,N-diphenyl-4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan2-yl)aniline gave the compound 4,4'-(9H-carbazole-3,6-diyl)bis(N,N-diphenylaniline) (M3),



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which can be reacted with 1,6-dibromohexane to give the compound 4,4'-(9-(6-bromohexyl)-9Hcarbazole-3,6-diyl)bis(N,N-diphenylaniline) (M4).

Further reaction of M4 with M2 in the

presence of potassium hydroxide gave the target monomer 4,4'-(9-(6-(3-(2,7-dibromo-9-phenyl9H-fluoren-9-yl)-9H-carbazol-9-yl)hexyl)-9H-carbazole-3,6-diyl)bis(N,N-diphenylaniline) (M5) in a yield of 70%. The polymerization was carried out based on the palladium-catalyzed Suzuki coupling reaction with feed ratios of M7:M6:M8:M5 of 50:25:0:25, 50:30:15:5 and 50:10:15:25, and the corresponding copolymers were denoted as PF-T25, PF-T5SO15 and PF-T25SO15, respectively. All copolymers were end-capped by using phenylboronic acid and bromobenzene sequentially to remove the functional end-groups. These target polymers were purified by Soxhlet extraction by using methanol, acetone and hexane successively to remove the small molecular fractions and catalyst residues. The number average molecular weight (Mn) and polydispersity index (PDI) of copolymers were estimated by gel permeation chromatography (GPC). The experiments were carried by using THF as the eluent and linear polystyrene as the standard.

The Mn was

determined to be 22.1, 26.2 and 20.1 kDa for PF-T25, PF-T5SO15 and PF-T25SO15, respectively with PDI in the range of 1.8−2.4.


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H N MgBr

(a)

Br

Br

Br

KOH

+

OH

THF, -78 C

Br

Br

Br BF3OEt2 in anhydrous DCM for 16 h

O

N H

M2 yield = 70%

M1 yield = 64%

N

N

N O

Br

Br

B

N

O Pd(PPh3)4 / KOH

+ N H

HN

toluene 110 C for 16 h

N

N

1,6-dibromohexane

M2

KOH, DMF 80 C for 16 h

KOH

N

N Br N

M4

M3 yield = 78%

Br

Br

N

M5 yield = 70%

yield = 88%

N

O

O

O

O

N

O O S Br

O B

Br

M6

Br

O B O

O

Br N N

M7

M8

M5 Br

Br

N

N

O

N

O

N O O S

Pd(OAc)2,PCy3 Et4NOH,toluene x

80-85 C for 36 h

y

yield = ~ 60%

C8H17

C8H17

O O S

C8H17

n

PFC6-T25: x = 0, y = 0.25 PFC6-T5SO15: x = 0.15, y = 0.05 PFC6-T25SO15: x = 0.15, y = 0.25

C8H17

(b) C8H17 C8H17

C8H17 C8H17

FFSOFF

Scheme 1. Synthesis of monomers and polymers (a); molecular structure of compound FFSOFF (b).

Theoretical Calculation 13 ACS Paragon Plus Environment


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It is well-established that the typical exciplex is formed by an electron donor layer and an electron accepter layer with appropriate energy levels which can efficiently limit the electron and hole in the interface of the double layers.26 In this work, TPA-Cz unit was covalently tethered by an alkyl chain to the backbone of PF-SO copolymers, which will not affect the conjugation of backbone regarding to the non-conjugated linkage of the TPA-Cz units. Considering that TPACz species have excellent hole transport property and shallow highest occupied molecular orbital (HOMO) level, thereby the injected hole from the anode would primarily located at the TPA-Cz moiety instead of directly injected to the PF-SO backbones. In this regard, the TPA-Cz unit can act as the electron-dominate moiety.35 On the other hand, due to the strong electron-withdrawing properties of SO moiety, the incorporation of SO unit into the polymer backbone can lead to relatively deep lowest unoccupied molecular orbital (LUMO) level, thus the copolymer backbone of PF-SO can act as the electron-deficient moiety.34 It is also worth noting that the monomer M5 comprises an additional carbazole (Cz) unit that covalently connected to the carbon bridge of the fluorine unit. One may surmise that this electron-rich Cz unit can also potentially form the exciplex with the electron deficient FSOF segment. However, this assumption can be ruled out since such non-conjugated Cz unit has analogous properties as the Cz moiety in the hole-transport polymer of poly(vinyl carbazole) (PVK), which has been proved to be the effective hole-transport material for various SO based copolymers. 36 Therefore, the holes and electrons located in the TPA-Cz and FSOF moieties, respectively, can potentially promote the formation of inter-/intramolecular exciplex when the electron orbital is efficiently overlapped. To clarify this issue, the theoretical calculation was utilized to simulate the frontier orbitals of TPA-Cz and SO unit. The calculation was carried out by using the B3LYP/6-31G (d) basis of


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the Gaussian 09 package.37 To simplify the calculation, the alkyl side chain attached to the Natom of carbazole unit was replaced by a methyl group, and the model compound 3,7-bis(9,9dimethyl-9H-fluoren-2-yl)dibenzo[b,d]thiophene 5,5-dioxide (FSOF) was used to replace SO since the existence of conjugation between the SO and adjacent fluorene unit. Figure 1 shows the calculated frontier molecular orbitals and the energy levels of TPA-Cz and FSOF units. It was noted that the TPA-Cz unit has very low HOMO level of –5.05 eV, which can efficiently trap the hole from the anode and block the electron. The FSOF unit shows the LUMO level of – 1.96 eV. It is worth pointing out that there are relatively high barrier for both hole and electron injection between these units. Such a big barrier can efficiently restrict the injected holes and electrons in the interface between the TPA-Cz and FSOF units, which is favourable for the formation of exciplex.

Figure 1. The frontier orbitals of TPA-Cz and FSOF in optimized conformation calculated with DFT at the B3LYP/6-31G(d)* level (a); and the energy levels and emissive mechanism of exciplex (b).



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Optophysical property of blend films To identify the formation of exciplex between the side groups of TPA-Cz and the FFSOFF chromophore (molecular structure shown in Scheme 1 and Figure S1 in the Supporting Information), we initially recorded the UV-vis and photoluminescent (PL) spectra of the blend films of copolymer PF-T25 and model oligomer FFSOFF. Figure 1 showed the UV-vis absorption and PL spectra of the PF-T25 (Figure 2a and b), FFSOFF (Figure 2d and e) and the blend film based on PF-T25:FFSOFF with weight ratio of 50:50 (Figure 2g and h). One notes that UV-vis spectrum of blend film exhibited comparable absorption profiles with the maximal absorption at 386 nm (Figure 2g), indicating no new ground state transitions formed in the blend film. The PL spectra of pure films of PF-T25 and FFSOFF showed blue emission with the maximal emission peaked at 428 and 448 nm, respectively. In contrast, the PL spectrum of the blend film showed broad spectra with dual peaks of 448 nm related to the FFSOFF associated with an emerged low-energy emission signal located at 482 nm, while the characteristics at 428 nm disappeared (Figure 2h). It is worth noting that the emission profile of the blend film is different from the conventional emission of the exciplex formed in the interface of the electron/hole dominant layers, which typically shows a broad emission band with one peak. This observation indicates the existence of incomplete energy transfer from the FFSOFF to the formed exciplex, and thus providing a novel approach to achieve white light-emission by simply blending two wide-bandgap blue emitters in relatively high ratio. Considering that the higher blend ratios of components offer less sensitivity of the emitting spectra to the blend ratios, thus this strategy has a specific advantage relative to the conventional strategy that the delicate control of the small ratio of low-energy emitters can be avoided.

Time-resolved

photoluminescent (PL) spectra were measured to investigate the fluorescent lifetime of such


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films, with relevant curves shown in Figure 2. It was noted that copolymer PF-T25 exhibited a dual exponential decay with average lifetime of 1.2 ns (Figure 2c), and the oligomer FFSOFF exhibited typical mono-exponential decay with lifetime of 2.66 ns (Figure 2f), both of which were the characteristic values corresponding to the fluorescence emission. In contrast, the blend film of PF-T25:FFSOFF exhibited triple-exponential decay with obviously prolonged average lifetime of 19.4 ns (Figure 2i). These observations demonstrated the emergence of new excited state in PF-T25:FFSOFF blend film, while the generated long lifetime (τ3 = 99.7 ns) excitons in the blend film can be attributed to the exciplex state. Detailed results of fluorescent lifetime are summarized in Table 1. Similar observations were also realized for blend films with other ratios (see Figure S3 and Table S1 in the Supporting Information).



(b) 1.2 PF-T25

0.8 0.6 0.4 0.2

300

350

400

450

1

PF-T25

0.8 0.6 0.4 0.2 0 400

500

450

Wavelength (nm)

FFSOFF

0.8 0.6 0.4 0.2

300

350 400 450 Wavelength (nm)

(g) 1.2

FFSOFF

0.8 0.6 0.4 0.2

Normalized PL Intensity (a. u.)

50 : 50

0.8 0.6 0.4 0.2 300


500

-1

10

-2

10

-3

10

-4

0

10

20 Time (ns)

30

40

FFSOFF

10-1

10-2

-3

10

-4

450


10

650

(h) 1.2

1

0 250

1

0 400

500

PF-T25 10

0 (f) 10

Normalized PL intensity (a.u)

1

0 250

650

(e) 1.2 Normalized PL Intensity (a. u.)

Normalized Absorbance (a. u.)

(d) 1.2


0

10

20 Time (ns)

30

40

0 (i) 10

1


0 250

0 (c) 10


1

Normalized PL Intensity (a. u.)


(a) 1.2


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50 : 50

0.8 0.6 0.4 0.2 0 400

450

500

550

600

650

50 : 50 10-1

10-2

10

-3

10

-4

0

200

Wavelength (nm)

400 Time (ns)

600

800

Figure 2. Optophysical performances of films of PF-T25, FFSOFF and the blend film of PFT25:FFSOFF=50:50.

Table 1. The detailed property of fluorescent lifetime for the films of PF-T25, FFSOFF and blend film of PF-T25:FFSOFF = 50:50. Emitter

τ (ns)

A1

τ1 (ns)

A2

τ2 (ns)

A3

τ3 (ns)

PF-T25

1.2

85.3%

0.74

14.7%

4.07

-

-

FFSOFF

2.66

100%

2.66

-

-

-

-


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50 : 50

18.3

63.6%

0.91

4.5%

11.9

15.2%

99.7

Electroluminescence of Blend Films In order to investigate the electroluminescent (EL) performance of the blend films, we fabricated PLEDs with structure of ITO / PEDOT:PSS (40 nm) / emissive layer (80 nm) / CsF (1.5 nm) / Al (100 nm). To investigate the characteristics of the electroluminescence with the constitution of the formed exciplex, we fabricated devices with various PF-T25:FFSOFF blend ratios of 10:90, 30:70, 50:50, 70:30 and 90:10. Figure 3 shows the representative EL spectra (a) and the luminous efficiency – current density (LE – J) (b) characteristics of the fabricated devices. All devices showed relatively low turn-on voltage in the range of 2.8-3.1 V, indicating the excellent charge injection properties of these materials. Devices based on both PF-T25 and FFSOFF exhibited moderate luminous efficiencies of 0.67 and 0.89 cd A-1, associated with the characteristic blue emission with CIE coordinates of (0.16, 0.10) and (0.15, 0.17), respectively. The EL profiles of all blend films are much broader than that of blue light-emitting PF-T25 and FFSOFF owing to the emergence of low-energy emission of exicplex, and the relative intensities of the high-/low-energy emission depend on the ratio of PF-T25:FFSOFF in the blend films. One can also note that the emission peaked at 428 nm completely quenched and can only be observed at a very low content of 10% for FFSOFF, indicating the efficient energy transfer from the high-energy PF-T25 to the low energy FFSOFF and also to the exciplex state. These findings indicated that EL spectra can be tuned by the blend ratios. Relative to the devices solely based on PF-T25 or FFSOFF, slightly improved device performances based on the blend films were observed, which showed a maximal luminous efficiency (LEmax) of 1.71 cd A-1 and a maximal luminance (Lmax) of 6475 cd m-2. Detailed device performances are summarized in Table 2.



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Devices based on the emissive layer PF-T25:FFSOFF with other ratios also exhibited broadened EL profiles with the emergence of a new low-energy emission corresponding to the exciplex emission (see Figure S4 and Table S2 in the Supporting Information).

Figure 3. EL spectra (a); the LE – J characteristics (b) of devices based on the emissive layer of PF-T25, FFSOFF and blend films of PF-T25:FFSOFF (50:50).

Table 2. The detailed results of the blending exciplex devices. Vtha

LEmax

Lmax

CIEb

[V]

[cd A-1]

[cd m-2]

[x, y]

PF-T25

2.8

0.67

2919

(0.16, 0.10)

FFSOFF

3.1

0.89

2126

(0.15, 0.17)

PF-T25:FFSOFF = 50:50

2.9

1.71

6475

(0.22, 0.35)

Emissive layer


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a

Turn-on voltage (Vth) is defined as the voltage at which a luminance of 1 cd m-2 is reached;

b

CIE coordinates are measured at J = 12.5 mA cm-2.

To understand the effects of driving voltages on the emission from the exciplex, the EL spectra at various bias of devices based on blend films were recorded (Figure 4 and Figure S5 in the SI). It was noted that, for all devices based on blend films, the relative intensity at lowenergy region correlated to the exciplex emission decreased with the increased bias. Considering that the PF-T25:FFSOFF blend films are processed from the solution of p-xylene, the distinct polarity of such two components may lead to potential phase separation with relatively large domain size.

This phase separation may result in inefficient migration of excitons and

incomplete energy transfer from bulk excitons to the localized exciplex formed at the heterojunction interface, thus give rise to the apparent voltage dependent EL spectra of such blend films.38,39



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Figure 4. The EL spectra of the blending exciplex devices with the blending ratio of PF-T25 : FFSOFF = 30 : 70 (a), 50 : 50 (b), 70 : 30 (c), 90 : 10 (d) under different voltages.

Optophysical Property of Polymers Consisting of SO and TPA-Cz Moieties It is well-established that phase separation is an inevitable issue for blend films which would seriously influence the performance of the working WPLEDs, thus leading to unstable EL spectra and fast roll-off of efficiency.40,41 An effective strategy to avoid such issue is covalently copolymerizing the chromophores into polymer chain to attain the molecularly dispersion. In 22 ACS Paragon Plus Environment

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this respect, we herein simultaneously copolymerize the electron-withdrawing SO unit into the PF backbone, and covalently incorporate the electron-donating TPA-Cz unit to construct single polymers that can form exciplex. The used molar ratios of the TPA-Cz/SO are 5%/15% and 25%/15%, respectively as shown in Scheme 1. Figure 5 shows the UV-vis absorption and PL spectra of PF-T5SO15 and PF-T25SO15. As can be seen in Figure 5, UV-vis absorption profiles of PF-T5SO15 and PF-T25SO15 are quite comparable with the maximal absorption peaked at 390 nm, implying no new ground state transition formed in these polymers.

In contrast, PL spectra of both PF-T5SO15 and PF-

T25SO15 showed relatively broad profiles with dual characteristics, where the relatively weak shoulder peak located at about 448 nm can be attributed to the characteristic emission of the backbone of PF-SO15, while the low-energy emission peak can be ascribed to the emerged emission of exciplex. It is also worth noting that at high molar ratio of TPA-Cz moiety, the relative intensity of low-energy exciplex is much stronger and red-shifts to 516 nm. The increased intensity can be attributed to the more favourable energy transfer from host matrix to exciplex, or the increased formation probability of exciplex in the emissive layer, or the combination of both factors. Time-resolved PL was conducted to investigate the fluorescent lifetime of PF-T5SO15 and PFT25SO15 film, with relevant curves shown in Figure 5b. It was noted that the fluorescent decay curves of both polymers fitted well with dual-exponential function. Both of these curves consisted of a prolonged lifetime component, which was obviously enhanced with the increased ratio of TPA-Cz unit due to the higher probability to form exciplex state. The average fluorescent lifetime of PF-T5SO15 and PF-T25SO15 is 2.6 ns and 43.5 ns, respectively. Detailed results are summarized in Table 3.



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Figure 5. The UV-vis absorption and PL spectra (a), and PL decay curves (b) of FC-T5SO15 and PF-T25SO15 films.

Table 3. The property of fluorescent lifetime of the films for PF-T5SO15 and PF-T25SO15. Polymer

τ (ns)

A1

τ1 (ns)

A2

τ2 (ns)

PF-T5SO15

2.60

95.5%

1.18

4.5%

32.16

PF-T25SO15

43.50

47.3%

6.47

52.7%

76.80

Electroluminescent Property of Polymers Consisting of SO and TPA-Cz Moieties To investigate the EL performances of the prepared polymer, we fabricated PLEDs with device structure of ITO / PEDOT:PSS (40 nm) / polymer (80 nm) / CsF (1.5 nm) / Al (100 nm). Figure 6 shows the EL spectra and LE – J characteristics of devices. The fabricated devices also exhibited relative low threshold voltage below 3.4 V. In addition, both polymers exhibited quite analogous EL profiles (Figure 6a) to the PL spectra (Figure 5a), indicating that the analogous mechanism is involved. Device based on PF-T5SO15 showed a broad emission profile with dual peaks, exhibiting a LEmax of 2.34 cd A-1 and Lmax of 12410 cd m-2. We note that despite the 24 ACS Paragon Plus Environment

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emission corresponding to the exciplex cannot ideally compensate for the blue emission from the polymer backbone, the greenish white light-emission with CIE coordinates of (0.27, 0.39) lied in the range of white region. Future efforts will focused on developing pure white light-emission by optimizing of the energy levels of components to form exciplex with more appropriate spectrum profile, or by incorporating an additional red-light-emitting species to facilitate the realization of state-of-art white emission.

Nonetheless, to our knowledge, this is the first

prototype of single greenish white light-emitting polymer based on the emission from backbone and the formed intrinsic exciplex. Moreover, although device based on PF-T25SO15 also exhibited broad emission of exciplex, the emission profile dominated with the peak emission located at 542 nm with CIE coordinates of (0.36, 0.51) lied in the yellowish-green region (see Figure S6 in the SI). It is also worth noting that for copolymers (p(F-S)x) consisting of randomly copolymerized fluorene-SO unit, the dual emission corresponding to the local excited state and charge transfer state can be simultaneously realized that can give rise to the broadened greenishwhite light-emission with CIE coordinates of (0.24, 0.41).42 When considering the analogous molecular backbone of the resulting copolymers of PF-T5SO15 and PF-T25SO15 with p(F-S)x42, it is rational to anticipate that the emission from the charge transfer state of the electron-rich fluorene and the electron-deficient SO units can also contribute to the apparently broadened EL spectra. On the other hand, we also recognize that the primary emission at the low-energy band locates at about 520 nm for PF-T5SO15 and 540 nm for PF-T25SO15, which can be attributed to the emission of the emerged exciplex.

However, more sophisticated photophysical

characterizations are required to quantify the exact contribution of the emission from the charge transfer state and from the exciplex. This is beyond the scope of our current work. Furthermore, it is also worth mentioning that both devices showed relatively stable efficiency with slow



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Page 26 of 36

efficiency roll-off with current densities (Figure 6b). Detailed device performances are summarized in Table 4.

Figure 6. EL spectra (a) and LE – J characteristics (b) of devices with PF-T5SO15 and PFT25SO15 as the active layer.

Table 4. The detailed results of PLEDs with the exciplex-type polymer as the active layer. Vth a,

Vth b

LEmax

Lmax

CIE c

[V]

[V]

[cd A-1]

[cd m-2]

[x, y]

PF-T5SO15

3.4

3.8

2.34

12410

(0.27, 0.39)

PF-T25SO15

2.9

3.3

2.69

8811

(0.36, 0.51)

Emitter

a

Vth is defined as the voltage at which a luminance of 1 cd m-2 is reached;

b

Vth is defined as the voltage at which a luminance of 10 cd m-2 is reached;

c

CIE coordinates are measured at J = 12.5 mA cm-2.

To evaluate the colour stability of these polymers, we recorded the EL spectra of both polymers at various driving voltages, with relevant spectra shown in Figure 7. It is noted that the EL spectra exhibited excellent stability, which showed nearly identical profiles with the driving voltage increased from 5 V to 15 V. The increased stability of devices can be attributed to the 26 ACS Paragon Plus Environment

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molecularly dispersed charge trapping species of TPA-Cz and SO in polymer side chain or backbone, enabling the formation of exciplex uniformly in film. These observations indicated that the single polymers covalently tethering with charge trapping species can be an effective strategy to get rid of charge accumulations or non-uniform charge transportation across the entire film that can cause unstable EL spectra.43,39

Figure 7. EL spectra at various driving voltages of PLEDs based on PF-T5SO15 (a) and PFT25SO15 (b) as emissive layer.

CONCLUSION In summary, we developed a series of conjugated polymers based on polyfluorene derivatives as the main chain, and covalently incorporating an electron-rich 4,4'-(9-alkylcarbazole-3,6-diyl)bis(N,N-diphenylaniline) (TPA-Cz) through an alkyl side chain, and an electron-deficient dibenzothiophene-5,5-dioxide (SO) unit into the backbone.

Of particular

interests is that the low-energy emission correlated to exciplex can be realized in both blend films and the single polymer as demonstrated by time-resolved photoluminescent measurements, which led to appreciably broadened emission. Greenish-white polymer light-emitting diodes were fabricated based on the exciplex-type single white polymer, exhibiting the peak luminous 27 ACS Paragon Plus Environment


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efficiency of 2.34 cd A-1 with Commission Internationale de L’Eclairage color coordinates of (0.27, 0.39). The electroluminescent spectra of the single white light-emitting polymer exhibited excellent stability with driving voltage ranged from 5 to 15 V. These observations indicated that the development of exciplex-type single white light-emitting polymer can be a novel and promising strategy to attain stable emission for white polymer light-emitting diodes.

ASSOCIATED CONTENT Supporting Information The molecular structure of FFSOFF, optophysical and EL properties of films. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Authors E-mail: [email protected] (L. Ying); [email protected] (F. Huang). Tel: +86 20 87114346 Fax: +86 20 87110606 Author Contributions ‡

These authors contributed equally.

Notes The authors declare no competing financial interest.


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ACKNOWLEDGMENT The authors are grateful for financial support from the Ministry of Science and TechnologyChina (2015AA033402 and 2015CB655004), the National Natural Science Foundation of China (Grants 51303056, 21125419 and 21490573), Guangdong Natural Science Foundation (Grant No. S2012030006232 and 2015A030313229), and Guangdong Innovative Research Team Program of China (201101C0105067115).

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Graphical Abstract


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White Polymer Light-Emitting Diodes Based on Exciplex

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