Poly(9,9-dioctylfluorene) based hyperbranched copolymers with three balanced emission colors for solution-processable hybrid white polymer light-emitting devices

Accepted Manuscript Poly(9,9-dioctylfluorene) based hyperbranched copolymers with three balanced emission colors for solution-processable hybrid white polymer light-emitting devices Tiaomei Zhang, Jing Sun, Xiaoqing Liao, Minna Hou, Weihua Chen, Jie Li, Hua Wang, Lu Li PII:

S0143-7208(16)30658-1

DOI:

10.1016/j.dyepig.2016.12.029

Reference:

DYPI 5654

To appear in:

Dyes and Pigments

Received Date: 5 September 2016 Revised Date:

30 October 2016

Accepted Date: 5 December 2016

Please cite this article as: Zhang T, Sun J, Liao X, Hou M, Chen W, Li J, Wang H, Li L, Poly(9,9dioctylfluorene) based hyperbranched copolymers with three balanced emission colors for solutionprocessable hybrid white polymer light-emitting devices, Dyes and Pigments (2017), doi: 10.1016/ j.dyepig.2016.12.029. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

ACCEPTED MANUSCRIPT Poly(9,9-dioctylfluorene) based hyperbranched copolymers with three balanced emission colors for solution-processable hybrid white polymer light-emitting devices

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Tiaomei Zhang a, b, Jing Sun a, b, Xiaoqing Liao c, Minna Hou a, b, Weihua Chen a, b, Jie Li a, b, Hua

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Wang a, b, *, Lu Li c, **1

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A series of hyperbranched copolymers were designed and synthesized, achieving the white-light

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emission with a high CRI of 87.

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*Corresponding author: Key Laboratory of Interface Science and Engineering in Advanced Materials, Ministry of Education, Taiyuan University of Technology, Taiyuan 030024, PR China. Tel.:+86 03516014852; fax:+86 0351 6010311; E-mail address: [email protected] (Wang H). **Corresponding author: Co-innovation Center for Micro/Nano Optoelectronic Materials and Devices, Research Institute for New Materials and Technology, Chongqing University of Arts and Sciences, Chongqing 402160, PR China. E-mail address: [email protected] (Li L).

ACCEPTED MANUSCRIPT Poly(9,9-dioctylfluorene) based hyperbranched copolymers with three balanced emission colors for solution-processable hybrid white polymer light-emitting devices

a

Key Laboratory of Interface Science and Engineering in Advanced Materials, Ministry of Education, Taiyuan

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University of Technology, Taiyuan 030024, PR China b

Shanxi Research Center of Advanced Materials Science and Technology, Taiyuan University of Technology, Taiyuan

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030024, PR China c

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Tiaomei Zhang a, b, Jing Sun a, b, Xiaoqing Liao c, Minna Hou a, b, Weihua Chen a, b, Jie Li a, b, Hua Wang a, b, *, Lu Li c, **1

Co-innovation Center for Micro/Nano Optoelectronic Materials and Devices, Research Institute for New Materials and

ABSTRACT

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Technology, Chongqing University of Arts and Sciences, Chongqing 402160, PR China

A series of hyperbranched copolymer system (P1, P2, P3 and P4) with tris[1-phenylisoquinolinato-C2,N]iridium(III)

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(Ir(piq)3) as the red emission core, fluorenone (FO) as the green emission unit and poly(9,9-dioctylfluorene) (PFO) as the

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blue emission branches was designed and synthesized, in which the balanced emission of red-, green- and blue-light was

*Corresponding author: Key Laboratory of Interface Science and Engineering in Advanced Materials, Ministry of Education, Taiyuan University of Technology, Taiyuan 030024, PR China. Tel.:+86 03516014852; fax:+86 0351 6010311; E-mail address: [email protected] (Wang H). **Corresponding author: Co-innovation Center for Micro/Nano Optoelectronic Materials and Devices, Research Institute for New Materials and Technology, Chongqing University of Arts and Sciences, Chongqing 402160, PR China. E-mail address: [email protected] (Li L). 1

ACCEPTED MANUSCRIPT realized by adjusting the contents ratio of Ir(piq)3 and FO. These hyperbranched copolymers showed high fluorescence quantum efficiency (28.1% – 47.2% in neat films) and excellent thermal stabilities. Furthermore, triplet-triplet annihilation was effectively suppressed by this hyperbranched structure. As a result, a high color rendering index (CRI)

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of 87 and a Commission International de l’Eclairage (CIE) coordinate of (0.31, 0.33) were achieved for P2 (0.05 mol% FO and 0.1 mol% Ir(piq)3)-based single-active-layer OLED device, which shows a maximum current efficiency of 3.85 cd A-1 and a maximum brightness of 3354 cd m-2 (at 10.9 mA cm-2).

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Key words: white polymer light-emitting device; hyperbranched copolymer; Ir(piq)3; fluorenone; polyfluorene

1. Introduction

Recently, white organic light emitting devices (WOLEDs) have gained enormous attention owing to their wide application prospects in full-color display, back-lighting sources for liquid-crystal displays as well as the next generation

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solid-state lighting sources, and their compelling superiorities such as large area coverage, flexibility and low power consumption to meet with the current stress of commercial materials [1–5]. A mass of strategies have been attempted to obtain WOLEDs with high efficiency [6–14]. Among them, the traditional one is adopting polymer blend systems, which

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realize white emission by incomplete energy transfer from hosts with wide band gap to small amount of dopants with

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narrow band-gap [15–16]. Although high efficiency may be achieved, the phase separation could lead to deteriorate color stability and short device lifetime, which is a serious disadvantage for application in display techniques and lighting sources. For example, the complementary-color-based WOLED and three-color hybrid WOLED reported by our group had achieved much higher CRI, color stability, but the fabrication processes was quite complicated, at the same time, more interfacial issues existed [14,17]. So, to simplify complicated fabrication processes of white OLED, many researchers focus on designing and synthesizing single polymers for WOLED, which can realize fabrication of white OLED with single-active-layer. Till 2

ACCEPTED MANUSCRIPT now, many white polymers with linear structure had reported, which show better luminous properties. But, it also express such drawbacks as rather lower luminous efficiency and remarkable efficiency roll-off, which is owing to robust interaction between adjacent polymer chains [18–20]. In order to solve this problem, many researchers focus on

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synthesizing single hyperbranched copolymers for WOLED [21–24], which suppressed intermolecular interaction and enhanced solid-state luminescence [25]. By controlling the contents ratio of the composition units such as cores, branches or surface groups, white-light polymer could be obtained. For example, Wang et al. [26] reported a sequence of

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white hyperbranched copolymers with an orange emissive core and blue light-emitting branches, realizing partial energy

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transfer from the blue branches to the orange core. A single-active-layer WOLED emits white light with a CIE coordinate of (0.35, 0.39). Jiu et al. [27] reported a series of four-branched hyperbranched copolymers with 4,7-bis(5-(4-(9H-carbazol-9-yl)phenyl)-4-hexylthio-phen-2-yl)benzo[c][1,2,5] thiadiazole (FTBT) as a red emissive core, poly(9,9-dioctylfluorene) (PFO) as the blue emissive branches and 1,3-benzo thiadiazole (BT) as the green emissive

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units, realizing a stable white emission with a current efficiency of 1.59 cd A-1 and a CIE coordinate of (0.31, 0.34). As we know, only 25% internal quantum efficiency can be achieved from fluorescence polymers, so acquiring white-light electrophosphorescent polymers (100% internal quantum efficiency) is a preferable method. Cao et al. [25] reported a

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series of hyperbranched copolymers with a triphenylamine-based iridium(III) dendritic complex as the orange-emitting

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core and poly(9,9-dihexylfluorene) chains as the blue-emitting branches, also realizing partial energy transfer from the blue branches to the orange core. Single-active-layer WOLEDs exhibit a maximum current efficiency of 1.69 cd A-1 and a CIE coordinate of (0.35, 0.33), which offers a potential of white-light emission from hyperbranched copolymers. Meanwhile, our group have also synthesized a series of white hyperbranched copolymers utilizing PFO as the blue emission branches and Ir(piq)3 as the red emission core with the best CIE coordinate of (0.30, 0.23) [28]. However, above white polymer exhibit the electroluminescence (EL) spectra with blue emission peak and orange emission peak, which is not ideal for white light emission with high CRI due to absence of green emission peak. To solve this problem, 3

ACCEPTED MANUSCRIPT green phosphorescent units, for example, bis(2-phenylpyridine)[3-(2-pyridyl)-5-phenyl-1,2,4-triazole]iridium(III) [Ir(ppy)2(pytzph)], are inserted into above-mentioned white hyperbranched copolymers, exhibiting better current efficiency and brightness. But, the costs of green phosphorescent units are rather higher relative to green fluorescent

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units [29]. Therefore, it is necessary to synthesize an economical white hyperbranched copolymer by replacing green phosphorescent units with green fluorescent units.

In this work, a series of triple-color hyperbranched copolymers is introduced, in which the Ir(piq)3 with efficient red

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emission act as the red cores, PFO with insertion of green fluorescent units of fluorenone (FO) act as blue emission

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branches. The white-light emission can be obtained by adjusting the contents ratio of three chromospheres.

2. Experimental Section 2.1 Materials

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All chemicals and reagents, purchased from commercial sources, were used without further purification. Solvents for chemical synthesis were purified according to the standard procedures. 2.2 Instruments and characterization

H NMR spectra were recorded on a Bruker DRX 600 spectrometer with tetramethylsilane as the internal reference in

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1

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deuterated chloroform. Elemental analyses were performed on a Vario EL elemental analyzer. The molecular weights and polydispersity indices (PDI) of the polymers were determined by a Waters GPC 2410 apparatus using THF as the eluent. The thermal gravimetric analyses (TGA) curves were recorded with a Netzsch TG 209 Instrument at a heating rate of 10 °C min-1 under nitrogen. The differential scanning calorimeter (DSC) curves were recorded by a Netzsch DSC 204 apparatus under nitrogen at a heating rate of 5 °C min-1. Ultraviolet-visible (UV-vis) absorption spectra and photoluminescence (PL) spectra (5 × 10-6 M in CHCl3 solution or in solid films) were recorded with a Hitachi U-3900 spectrophotometer and a Horiba Fluoromax-4 spectrophotometer, respectively. The PL quantum efficiencies (ΦPL) of the 4

ACCEPTED MANUSCRIPT polymers were measured using the integrating sphere excited at 365 nm. The PL emission lifetime experiments (5 × 10-6 M in tetrahydrofuran) were done on FLS 980 from Edinburgh Instruments. Cyclic voltammetry (CV) curves were measured on the Autolab/PG STAT302 electrochemical workstation using tetrabutylammonium perchlorate (0.1 M) in

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acetonitrile as the electrolyte at a scan rate of 50 mV s-1 at room temperature under nitrogen atmosphere. A conventional three-electrode configuration was used to measure the data, which containing two platinum plates (working electrode and the counter electrode), and a calomel electrode as the reference electrode at room temperature.

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2.3 Synthesis

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Tri(2-(4-bromophenyl)-isoquinoline)iridium(III) (Ir(piqBr)3) (M4) was prepared following the literature procedure without further purification.

General procedure for the synthesis of the hyperbranched copolymers. To a solution of predetermined amount of the monomers (M1, M2, M3 and M4) in toluene (20 mL), methyl trioctyl ammonium chloride (Aliquant 336) and

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aqueous solution of 2 M K2CO3 (10 mL), Pd(PPh3)4 (69.33 mg) were added under nitrogen atmosphere sequentially. The mixture was stirred with proper rotate speed and refluxed for 48 h at 90 °C. Then, phenylboronic acid (30 mg) and bromobenzene (1 mL) were added for end-capping. After cooling to room temperature, the reaction solution was poured

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into methanol (300 mL) and stirred, and then the precipitate was filtered out. The crude product was washed with

P1:

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acetone for 48 h in a soxhlet apparatus and further purified by column chromatography. 2,7-Bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-9,9-dioctylfluoren

(M1)

(963.86

mg,

1.5

mmol),

9,9-dioctylfuorene-2,7-dibromofluorene (M2) (820.47 mg, 1.496 mmol), 2,7-dibromo-9H-fluoren-9-one (M3) (1.334 mL, 0.0015 mmol, 0.38 mg mL-1 in toluene), M4 (15 mL, 0.0015 mmol, 10-4 g L-1 in toluene). Absinthe-green solid. Yield: 64.17%. 1H NMR: (600 MHz, CDCl3): δ (ppm) = 7.83–7.95 (2H, Ar–H), 7.68–7.71 (4H, Ar–H), 2.12 (2H, CH2), 1.14–1.21 (10H, 5 CH2), 0.80–0.84 (5H, CH2, CH3). Element anal. calcd (%): C, 89.59; H, 10.36. Found (%): C, 89.09; H, 10.13. 5

ACCEPTED MANUSCRIPT P2: M1 (963.86 mg, 1.5 mmol), M2 (821.01 mg, 1.497 mmol), M3 (0.667 mL, 0.00075 mmol, 0.38 mg mL-1 in toluene), M4 (15 mL, 0.0015 mmol, 10-4 g L-1 in toluene). Yellowish solid. Yield: 52.32%. 1H NMR: (600 MHz, CDCl3): δ (ppm) = 7.86–7.94 (2H, Ar–H), 7.70–7.72 (4H, Ar–H), 2.13 (2H, CH2), 1.15–1.22 (10H, 5 CH2), 0.82–0.84 (5H, CH2,

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CH3). Element anal. calcd (%): C, 89.59; H, 10.36. Found (%): C, 89.60; H, 10.27. P3: M1 (963.86 mg, 1.5 mmol), M2 (821.26 mg, 1.49745 mmol), M3 (0.667 mL, 0.00075 mmol, 0.38 mg mL-1 in toluene), M4 (12 mL, 0.0012 mmol, 10-4 g L-1 in toluene). Yellowish-green solid. Yield: 34.45%. 1H NMR: (600 MHz,

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CDCl3): δ (ppm) = 7.83–7.95 (2H, Ar–H), 7.67–7.70 (4H, Ar–H), 2.12 (2H, CH2), 1.14–1.22 (10H, 5 CH2), 0.80–0.83

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(5H, CH2, CH3). Element anal. calcd (%): C, 89.61; H, 10.36. Found (%): C, 83.92; H, 9.92. P4: M1 (963.86 mg, 1.5 mmol), M2 (821.63 mg, 1.498125 mmol), M3 (0.667 mL, 0.00075 mmol, 0.38 mg mL-1 in toluene), M4 (7.5 mL, 0.00075 mmol, 10-4 g L-1 in toluene). Yellowish solid. Yield: 38.60%. 1H NMR: (600 MHz, CDCl3): δ (ppm) = 7.83–7.85 (2H, Ar–H), 7.68–7.71 (4H, Ar–H), 2.12 (2H, CH2), 1.14–1.22 (10H, 5 CH2), 0.80–0.84

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(5H, CH2, CH3). Element anal. calcd (%): C, 89.62; H, 10.37. Found (%): C, 89.06; H, 10.27. 2.4 OLED fabrication and measurement

The configuration of the single-emitting-layer device was ITO / PEDOT:PSS (40 nm) / Polymer (50 nm) / TPBi (35

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nm) / LiF (1 nm) / Al (100 nm). The indium-tin oxide (ITO) glass substrate was ultrasonic circularly cleaned with

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acetone, isopropanol, and deionized water. After oxygen plasma treatment, PEDOT:PSS, a anode hole-injection layer, was spin-coated onto the ITO glass and annealed at 120 °C for 30 min. The solution of the polymer in toluene (12 mg mL-1) was spin-coated onto the PEDOT:PSS layer, and then the solvent residue was removed at 100 °C for 30 min. Next, an electron injection layer TPBi and cathode of LiF / Al were evaporated successively onto the polymer layer at a base vacuum pressure of 1 × 10-4 Pa. The current density (J) – voltage (V) – luminance (L) characteristics of the OLEDs were recorded on a Keithley 2400 source meter and a L-2188 spot brightness meter. Electroluminescent spectra were obtained using a PR655 spectra colorimeter. 6

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3. Results and Discussion 3.1 Synthesis and characterization

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The synthetic procedure of the copolymers is outlined in Scheme 1. Ir(piq)3 was selected as the core due to its efficient red emission and good solubility in common solvents. PFO was utilized as the branches because of its strong blue emission, high fluorescence quantum yields in the solid state, and good charge transport properties [30]. The FO unit was

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employed as the green emission unit owing to the identical conjugation system with PFO. The monomers (M1, M2, M3,

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and M4) were added with the feed ratios of 1000:997.5:1:1 (P1), 1000:9978:0.5:1 (P2), 1000:998.3:0.5:0.8 (P3), and 1000:998.75:0.5:0.5 (P4), respectively, and the hyperbranched copolymers were synthesized via Suzuki condensation reaction and obtained in yields of 34.45% – 64.17% (Table 1). The structures of the polymers were confirmed by 1H NMR and element analysis. The number average molecular weights ranged from 13600 to 22500 with the polydispersity

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indices (PDI) from 2.23 to 2.84. All of the hyperbranched copolymers were well soluble in common organic solvents (trichloromethane, tetrahydrofuran, dichloromethane, methylbenzene, etc.), suggesting the feasibility for their application in solution-processable devices.

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Scheme 1. Synthetic routes for hyperbranched copolymers P1–P4.

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Table 1. Structural and thermal properties of copolymers P1–P4. 3.2 Thermal properties

The thermal properties of the copolymers were determined by TGA and DSC, as shown in Fig. 1 and Table 1. Four kinds of hyperbranched copolymers exhibited high thermal decomposition temperatures (Td, corresponding to 5% weight loss) in the ranges of 403 – 421 °C. The DSC curves of the copolymers revealed endothermic glass transitions temperatures (Tg) above 130 °C for the second heating scan. The multi-branched structure and the intermolecular entangling mutually in these copolymers are responsible for the better thermal properties than the linear polymers 7

ACCEPTED MANUSCRIPT reported to date [31–32]. Fig. 1. The TGA (a) and DSC (b) curves of copolymers P1–P4. 3.3 Photophysical properties

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Fig. 2. The UV-vis absorption spectra of Ir(piq)3 and FO and the PL spectra of PFO and FO in CHCl3 solution (5 × 10-6 M).

Fig. 3. The fluorescence lifetime decays of copolymers P1–P4 in THF solution (5 × 10-6 M and λex = 375 nm, λem = 423

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nm).

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In order to indentify the energy transfer among emission units, the photophysical properties of PFO, Ir(piq)3 and FO were measured. As shown in Fig. 2, there is an overlapped band between the UV-vis absorption spectrum of Ir(piq)3 and the PL spectrum of PFO (409 – 564 nm), indicating possible Förster resonance energy transfer (FRET) from PFO segments to Ir(piq)3 phosphor [25,33]. In addition, there is a larger overlapped band between the UV-vis absorption

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spectrum of FO and the PL spectrum of PFO (402 – 460 nm), indicating more efficient FRET from PFO segments to FO. Because of a puny area of an overlapped band between the UV-vis absorption spectrum of Ir(piq)3 and the PL spectrum of FO (470 – 594 nm), the FRET from FO to Ir(piq)3 could be ignored. The fluorescence decay curves are shown in Fig.

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3 and the fluorescence lifetimes are listed in Table 2. The fluorescence lifetimes corresponding to the blue emission of

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PFO in copolymers are around 410 ps, which are shorter than that of bare PFO (900 ps) [33], indicating the presence of FRET from PFO to other emission units [33–35]. As a result, white-light emission could be anticipated by the combination of blue-, green- and red-light from the energy donor of PFO to the energy acceptors of FO and Ir(piq)3. Fig. 4. (a) The normalized UV-vis absorption spectra and PL spectra of P1–P4 in CHCl3 solution (5 × 10-6 M) and (b) the normalized PL spectra of P1–P4 in neat films. The UV-Vis absorption spectra of hyperbranched copolymers P1–P4 in CHCl3 solution are shown in Fig. 4a. The main absorption peak of polymers locates at about 380 nm, corresponding to the π–π* transition of the conjugated chains 8

ACCEPTED MANUSCRIPT of PFO [25]. The PL spectra of the copolymers in solution and films are shown in Fig. 4. The main emission peaks in solution locate at about 420 nm and 440 nm. Neither red nor green emission is observed due to the low doping concentration of

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Ir(piq)3 and FO. In film (Fig. 4b), owning to increasing of conjugation, the weak green emission and red emission peaks can be detected. Furthermore, with increasing the FO ratio, the emission intensity of FO (λPL = 523 nm) gradually enhanced.

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In addition, the large steric hindrance of hyperbranched structure could effectively suppress the PFO chain aggregation

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in the hyperbranched copolymers which is distinguished with PFO-based white linear conjugated polymers [36–37]. As shown in Table 2, the PL quantum efficiency (ΦPL) values of polymers P1–P4 are 28.12% – 47.18%, which are higher than the linear chain polymers [30] because of the dominant structure, at the same time, which are higher than the previous bicolor hyperbranched copolymers [25–28], attributing to the proper incorporation of a green emission unit

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(FO) that has the stronger charge trapping capability [37] in the hyperbranched copolymers. Hence, efficient OLEDs with the hyperbranched copolymer emitting layers could be anticipated. Table 2. Photophysical properties of copolymers P1–P4.

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3.3 Electrochemical properties

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The electrochemical properties of polymers P1–P4 are investigated by cyclic voltammetry (CV), as shown in Fig. 5 and Table 2. The highest occupied molecular orbital (HOMO) energy level of P1 was measured to be -5.59 eV according to the onset oxidation potential, using ferrocene as an internal standard (4.8 eV relative to vacuum energy level), EHOMO =﹣( Eox + 4.8)(eV) [38–39]. The lowest unoccupied molecular orbital (LUMO) energy level was calculated to be -2.93 eV by subtracting the optical gap from the HOMO level. The optical band gap was obtained from Eg = 1240/λedge(eV) and the edge is the onset value of the absorption spectrum in films in the long-wavelength direction. The HOMO and LUMO energy levels of other proportional polymers P1–P4 are listed blow-by-blow (see Table 2), which possess similar 9

ACCEPTED MANUSCRIPT consequence as the analogical structures. The red and green units have nearly no influence on the energy level due to the small contents. Fig. 5. Cyclic voltammetry curves of copolymers P1–P4.

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3.4 Electroluminescent performance Fig. 6. (a) The molecular structures of PEDOT:PSS and TPBi and (b) the energy level structure of OLED of P2. Based on the energy levels of polymers P1–P4, single-emitting-layer OLEDs were fabricated with the configuration of

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ITO / PEDOT:PSS (40 nm) / polymers (50 nm) / TPBi (35 nm) / LiF (1 nm) / Al (100 nm). Here, take device P2 as a

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representation. As shown in Fig. 6b, the electron injection barrier in TPBi / P2 interface is only 0.24 eV that is favored for electron injection. From the anode side, a bigger hole injection barrier of 0.40 eV in the PEDOT:PSS / P2 interface induces rather inferior hole injection relative to electron injection. Thanks to the better hole mobility than the electron mobility of PFO, electrons and holes could be effectively recombined in the emitting layer, resulting efficient

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electroluminescence (EL) [39].

Fig. 7. (a) Normalized EL spectra and (b) CIE chromaticity diagram of devices P1–P4 on 9 V. In the EL spectra (Fig. 7a), blue emission peak (440 nm / 464 nm) originating from PFO, green emission peak (530

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nm) originating from FO and red emission peak (630 nm) originating from Ir(piq)3 can be observed in all of the OLEDs.

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Moreover, the intensity of the green and red emission peaks are stronger than that in PL spectra (Fig. 4b). This may be owing to double contribution of FRET and direct charge-trapping of FO and Ir(piq)3 [37]. In addition, due to green emission of FO, the EL spectra of the polymers become broader than those of the double-colored hyperbranched copolymers [25–28].

Attributing to a proper feed ratio of FO (0.05 mol%), device P2 has balanced color for blue, green and red emission with CRI of 87 and a CIE coordinate of (0.31, 0.33) at 9.0 V, which is very close to the pure white-light point of (0.33, 0.33). Due to a higher feed ratio of FO (0.1 mol%), there is a stronger green emission for device P1, leading to a CIE 10

ACCEPTED MANUSCRIPT coordinate of (0.27, 0.34). As for device P3 and device P4, by the reason of a lower feed ratio of Ir(piq)3 (0.08 mol% and 0.05 mol%, respectively), the red emission is so feeble that the corresponding CIE coordinates are (0.27, 0.33) and (0.26, 0.32).

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In Fig. 8a, the EL spectra of device P2 at different voltages (9 – 11 V) are presented. Significantly, the spectral shape changes slightly with increasing the bias voltage from 9 to 11 V. The intensity of the green emission increases with increasing the voltage, probably due to the stronger charge trapping capability of FO [37]. As shown in Fig. 8b and Table

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3, device P2 behaves overall best performance within the OLEDs with a turn-on voltage of 6.5 V, a maximum luminance

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of 3354 cd m-2, and a maximum current efficiency of 3.85 cd A-1 (at 10.9 mA cm-2). Additionally, device P2 exhibits only slightly efficiency roll-off with the increase of current density, which could be attributed to the large steric hindrance of the hyperbranched structure that prevented the concentration quenching between the chain segments [25]. Fig. 8. (a) Normalized EL spectra at different voltages and (b) the luminance-current density-current efficiency curves of

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device P2. Table 3. EL performance of devices P1–P4.

Fig. 9. Normalized EL spectra at different voltages of device P0.

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To verify our devices, a reference device P0 that doping PFO, FO and Ir(piq)3 according to the ratio of P2 was

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fabricated. The EL spectra at different voltage (9 – 11 V) are shown in Fig. 9. Compared with device P2, only blue emission peaks (440 nm, 464 nm and 496 nm) originating from PFO can be detected, while the green and red emission peaks are absent. The larger intermolecular distance than the intramolecular one results in less effective FRET from PFO to FO and Ir(piq)3. Thus, compared to the physical doping, chemical synthesis is more efficient, which is conducive to achieving the preferable device properties.

4. Conclusion 11

ACCEPTED MANUSCRIPT In summary, white EL from a hyperbranched copolymer system with simultaneous three emissive species (blue PFO, red Ir(piq)3 phosphorescence cores and green FO) is achieved. The proper ratios of Ir(piq)3 (0.1 mol%) and FO (0.05 mol%) can ensure the balanced of blue, red and green emission, realizing high-quality white EL with simultaneous blue

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(436 nm / 464 nm), green (530 nm), and red (630 nm) emission. As a result, a maximum current efficiency of 3.85 cd A-1, a maximum brightness of 3354 cd m-2 with a high CRI of 87 and a CIE coordinate of (0.31, 0.33) are achieved for device

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P2, which is very close to the values of standard white emission (0.33, 0.33).

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Acknowledgements

This work was financially supported by Program for New Century Excellent Talents in University of Ministry of Education of China (NCET-13-0927); National Natural Scientific Foundation of China (61307030, 61307029, 51503022); Shanxi Provincial Key Innovative Research Team in Science and Technology (201513002-10); Natural

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Science Foundation of Shanxi Province (2015021070, 201601D011031); Open Foundation of State Key Laboratory of Electronic Thin Films and Integrated Devices (KFJJ201507); Basic and Frontier Research Program of Chongqing

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References

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Municipality (cstc2015jcyjA50036); Natural Science Foundation of Yongchuan District (Ycstc, 2015nc4001).

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from

poly(9,9-dioctylfluorene)

with

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core

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ACCEPTED MANUSCRIPT Table 1. Structural and thermal properties of copolymers P1–P4. Feed ratio Polymer

Mn (×103)

Mw (×103)

PDI

Td/°C

Tg/°C

Yield/%

64.17

M1:M2:M3:M4 1000:997.5:1:1

22.5

64.0

2.84

403

142

P2

1000:998:0.5:1

15.5

35.6

2.29

415

136

52.32

P3

1000:998.3:0.5:0.8

13.6

30.4

2.23

416

142

34.45

P4

1000:998.75:0.5:0.5

15.1

35.8

2.37

421

142

38.60

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P1

Table 2. Photophysical properties of copolymers P1–P4. Solution/nm

Film/nm

ΦPL (%)

HOMOa

τ (ps)

Polymer λabs

λPL

λPL

P1

384

416 / 439

P2

384

P3 P4

LUMO Egb/eV

/eV

/eV

Film

422 / 446 / 523 / 621

79.63

28.12

406.3

-5.59

2.67

-2.92

415 / 439

423 / 439 / 517 / 620

83.15

47.18

410.4

-5.60

2.66

-2.94

381

415 / 438

424 / 446 / 521 / 618

84.46

42.10

415.5

-5.62

2.64

-2.98

381

415 / 438

88.54

38.61

412.6

-5.61

2.68

-2.93

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Solution

422 / 443 / 522 / 619

EHOMO = - ( Eox + 4.8)(eV).

b

The optical band gap was obtained from Eg = 1240/λedge(eV) and the edge is the onset value of the absorption

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a

spectrum in films in the long-wavelength direction.

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ACCEPTED MANUSCRIPT Table 3. EL performance of devices P1–P4.

Device

LEmax (cd

m )

A )

EQEa (%)

Voltage

-1

PEa (lm/W)

9

λEL (nm) -2

436 / 464 / 528 / 629

3441

3.20

1.00

1.93

P2

436 / 464 / 528 / 636

3354

3.85

1.30

2.59

P3

436 / 464 / 532 / 634

6790

2.93

1.07

2.82

P4

440 / 464 / 530 / 628

1858

0.63

0.16

0.36

PE and EQE were measured at 100 cd m-2.

81

(0.27, 0.36)

9

87

(0.32, 0.35)

9

82

(0.27, 0.32)

9

67

(0.26, 0.33)

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a

CIE

(V)

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P1

CRI

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Lmax (cd

Scheme 1. Synthetic routes for hyperbranched copolymers P1–P4.

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Fig. 1. The TGA (a) and DSC (b) curves of copolymers P1–P4.

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M).

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Fig. 2. The UV-vis absorption spectra of Ir(piq)3 and FO and the PL spectra of PFO and FO in CHCl3 solution (5 × 10-6

Fig. 3. The fluorescence lifetime decays of copolymers P1–P4 in THF solution (5 × 10-6 M and λex = 375 nm, λem = 423 19

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nm).

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normalized PL spectra of P1–P4 in neat films.

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Fig. 4. (a) The normalized UV-vis absorption spectra and PL spectra of P1–P4 in CHCl3 solution (5 × 10-6 M) and (b) the

Fig. 5. Cyclic voltammetry curves of copolymers P1–P4.

20

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Fig. 6. (a) The molecular structures of PEDOT:PSS and TPBi and (b) the energy level structure of OLED of P2.

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Fig. 7. (a) Normalized EL spectra and (b) CIE chromaticity diagram of devices P1–P4 on 9 V.

21

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Fig. 8. (a) Normalized EL spectra at different voltages and (b) the luminance-current density-current efficiency curves of

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device P2.

Fig. 9. Normalized EL spectra at different voltages of device P0.

22

ACCEPTED MANUSCRIPT 1. A series of hyperbranched copolymers were designed and synthesized 2. These copolymers have shown high fluorescence quantum efficiency. 3. A high color rendering index (CRI) of 87 and a Commission International de l’Eclairage

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coordinate of (0.31, 0.33) were achieved.