Fluorene-Based Single-Chain Copolymers for Color-Stable White Light-Emitting Diodes

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Fluorene-Based Single-ChainCopolymers for Color-Stable WhiteLight-Emitting Diodes

Mingliang Sun, Qiaoli Niu, Bin Du, Junbiao Peng, Wei Yang, Yong Cao*

Fluorene-based single-chain copolymers with a white light emitter consisting of a blue and anorange chromophore have been synthesized and their photophysical and electroluminescentproperties are investigated. The experimental results suggest that only a relatively smallfraction of the orange-emitting units incorporated into the fluorene is needed to achieveefficient white light emission by controlled incom-plete energy transfer. A device from a copolymerwith 0.02%DDQ content showed the highest externalquantum efficiency of 2.64% with a luminance effi-ciency of 4.06 cd �A�1 with CIE coordinates (0.28,0.24). The EL emissions are extremely stable over awide range of current densities.

Introduction

White polymeric light-emitting diodes (WPLEDs) have

received great attention due to their potential applications

in full-color displays and next-generation lighting

sources.[1] There are several methods to fabricate WPLEDs,

such as dye/polymer,[2] polymer/polymer blend,[3] or a

single-chain polymer with multiple chromophores on the

backbone.[4] Polymer blend systems are extensively

adopted for the realization of white light-emission but

suffer from problems such as voltage dependence of

emitted color, phase separation, and low efficiency.[5] It is a

great challenge to build white light-emitting multilayer

PLEDs by solution-processing due to the possible inter-

mixing of different layers. Therefore, it is interesting to

develop a new structure of a single polymer with

multichromophores on the backbone to generate highly

M. Sun, Q. Niu, B. Du, J. Peng, W. Yang, Y. CaoInstitute of Polymer Optoelectronic Materials and Devices, SouthChina University of Technology, Key Lab of Specially FunctionalMaterials, Ministry of Education, Guangzhou 510640, ChinaE-mail: [email protected]

Macromol. Chem. Phys. 2007, 208, 988–993

� 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

efficient, stable, and pure white light-emitting diodes.[6]

Recently, some efforts have been made to prepare a single

component white polymeric emitter based on insufficient

energy transfer, because phase segregation can be im-

proved by incorporating RGB chromophores into a single

polymer chain. Lee et al. first reported single fluorene-

based copolymers with RGB units showing a maximum

brightness of 820 cd �m�2 with CIE coordinates of (0.33,

0.35).[7] Almost at the same time, Liu et al.[8] adopted a

slightly different synthetic strategy by attaching a green-

emissive unit to the pendant chain and incorporating a

red-emissive component into the blue-emissive polyfluor-

ene (PFO) backbone. The resulting EL device exhibited a

luminance efficiency of 1.59 cd �A�1 and CIE coordinates of

(0.31, 0.34). A similar strategy with two chromophores for

producing white light-emitting polymers has been repor-

ted respectively, with a luminance efficiency of 3.8 cd �A�1

and CIE coordinates of (0.32, 0.36)[9] and a luminous

efficiency of 7.3 cd �A�1 with CIE coordinates of (0.35,

0.32).[10] Despite these efforts, the external quantum

efficiency of WPLEDs remains much lower than that of

the small-molecule WOLED fabricated by thermal deposi-

tion. Recently, we also reported a single component white

DOI: 10.1002/macp.200700016

Fluorene-Based Single-Chain Copolymers . . .

polymeric emitter by RGB method with external quantum

efficiency of 3.84%, luminance efficiency of 6.20 cd �A�1,

and CIE coordinate of (0.35, 0.34).

In this paper, we report a single chain white light-

emitting polymer consisted of a narrow band gap

chromophore, an orange 2,3-dimethyl-5,8-dithien-2-ylqui-

noxaline (DDQ) chromophore incorporated into the PFO

backbone. By controlling the feed ratio precisely, a small

amount of DDQ units was incorporated into the polymer

backbone, leading to balanced blue and orange-red emis-

sion from the PFO and DDQ units, respectively. A maxi-

mum external quantum efficiency of 2.64% with a

luminance efficiency of 4.06 cd �A�1 has been achieved.

A particular advantage of the PFO-DDQ polymer is that

color coordinates are extremely stable and remain

constant in a wide range of operating current densities.

This feature is very important and desirable for display

and lighting applications.

Experimental Part

Materials

All the reagents were obtained from Aldrich, Acros, and TCI

Chemical Co., and used as received. DDQ, 2,7-dibromo-9,9-

dioctylfluorene (2), and 2,7-bis(4,4,5,5-tetramethyl-1,3,2-dioxa-

borolan-2-yl)-9,9-dioctylfluorene (3) were prepared following the

procedure described in ref.,[11] and characterized by the GC-MS and1H NMR spectra.

General Characterization Methods

1H NMR spectra were recorded on a Bruker DRX 300 spectrometer

operating at 300 MHz and was referred to tetramethylsilane.

GC-MS were obtained on GC-MS (TRANCE2000, Finnigan).

UV-visible absorption spectra were measured on an HP 8453

spectrophotometer. PL spectra in thin solid films were taken by

Fluorolog-3 spectrofluorometer (Jobin Yvon) under 325 nm light

excitation.

Device Fabrication and Measurement

The light-emitting diode was fabricated on pre-patterned indium

tin oxide (ITO) with a sheet resistance of 10–20 V �&�1. The

substrate was ultrasonically cleaned with acetone, detergent,

deionized water, and finally 2-propanol. Oxygen plasma treat-

ment was made for 10 min as the final step of substrate cleaning

to improve the contact angle just before film coating. Onto the ITO

glass a 50 nm-thick layer of polyethylenedioxythiophene (PEDOT)/

polystyrene sulfonic acid (PEDOT:PSS) film was spin-coated from

its aqueous dispersion (Baytron P 4083, Bayer AG), aiming at

improving the hole injection and avoiding the possibility of

leakage. PEDOT:PSS film was dried at 80 8C for 2 h in a vacuum

oven. Then a 40 nm-thick PVK layer was spin-coated on the top of

the ITO/PEDOT:PSS surface from solution of the PVK in



chlorobenzene. The emitting copolymers were spin-coated on

the top of the ITO/PEDOT:PSS PVK surface inside dry box. The

typical thickness of the emitting layer was 70–80 nm. A thin layer

of barium as an electron injection cathode and the subsequent

200 nm-thick aluminum capping layers were thermally deposited

by vacuum evaporation through a mask at a base pressure below

2�10�4 Pa. The deposition speed and thickness of the barium and

aluminum layers were monitored by a thickness/rate meter

(model STM-100, Sycon). The cathode area defined the active area

of the device. The typical active area of the devices in this study

was 0.17 cm2. The spin coating of the EL layer and device

performance tests were carried out within a glove box (Vacuum

Atmosphere Co.) with nitrogen circulation. Current-luminance-

voltage (I-L-V) characteristics weremeasured with a computerized

Keithley 236 Source Measure Unit and calibrated Si photodiode.

External quantum efficiency was verified by measurement in the

integrating sphere (IS080, Lab sphere) and luminance was

calibrated by PR705 spectrograph-photometer after the encapsu-

lation of devices with UV-curing epoxy and thin cover glass. EL

spectra were taken by InstaSpecTM IV CCD spectrograph.

Synthesis

2,3-Dimethyl-5,8-bis(5’-bromothien)-2-ylquinoxaline (1)

2,3-Dimethyl-5,8-dithien-2-ylquinoxaline (1.61 g, 5 mmol) was

dissolved in THF in nitrogen atmosphere, and N-bromosuc-

cinimide (1.78 g, 10 mmol) was immediately added. After the

reaction mixture being stirred at room temperature for 2 h, the

solvent was removed at a reduced pressure and the residue was

dissolved in CH2Cl2 and purified by column chromatography

(silica gel CH2Cl2) (yield: 90%).1H NMR (300 MHz, CDCl3): d¼2.83 (s, 6H), 7.13 (d, J¼ 4.05, 2H),

7.54 (d, J¼4.02, 2H), 7.99(s, 2H).13C NMR (100 MHz, CDCl3): d¼22.54, 22.99, 117.40, 125.14,

125.70, 129.44, 130.59, 140.28, 153.08.

GC-MS: m/z¼480 (Mþ).

Synthesis of Copolymers

1 (1.0�10�5 mol � L�1 solution in toluene), 2, 3, (PPh3)4Pd(0) (0.5–1

mol-%) and Aliquat 336were dissolved in amixture of toluene and

an aqueous solution of 2 M Na2CO3. The solution was refluxed for

24 h with vigorous stirring in an argon atmosphere. Then, 3 and

bromobenzene were added subsequently to end-cap the polymer

chain. The whole mixture was poured into methanol. The

precipitate was filtered off, washed with acetone, and purified

by column chromatography (silica gel, toluene) (yield: 80–85%).

Polymers with 0.01, 0.02, and 1% DDQ content and alternating

copolymer (50% DDQ) in PFO backbone were named PFO-DDQ001,

PFO-DDQ002, PFO-DDQ1, and PFO-DDQ50, respectively.

PFO-DDQ0011H NMR (300 MHz, CDCl3): d¼ 7.88–7.86(m), 7.74–7.70(m), 2.15(s),

1.28–1.16(m), 0.86–0.82(m).

www.mcp-journal.de 989

M. Sun, Q. Niu, B. Du, J. Peng, W. Yang, Y. Cao

Scheme 1. Synthesis of monomers and copolymers.

990

13C NMR (100 MHz, CDCl3): d¼151.83, 140.51, 140.04, 126.18,

121.51, 119.98, 55.36, 40.42, 31.82, 30.07, 29.25, 23.95, 22.62, 14.10.


1.28–1.16(m), 0.86–0.82(m).13C NMR (100 MHz, CDCl3): d¼151.83, 140.50, 140.03, 126.18,

121.51, 119.98, 55.35, 40.42, 31.81, 30.07, 29.25, 23.95, 22.62, 14.10.


1.28–1.16(m), 0.86–0.82(m).13C NMR (100 MHz, CDCl3): d¼151.84, 140.50, 140.02, 126.18,

121.51, 119.98, 55.37, 40.42, 31.81, 30.07, 29.25, 23.95, 22.64, 14.10.

PFO-DDQ501H NMR (300 MHz, CDCl3): d¼8.11(s, 2H), 7.92(s, 2H), 7.75–7.74(d,

J¼ 5.4, 5H), 7.48–7.37(m, 2H), 7.13(s, 1H), 2.90(s, 6H), 2.08(s, 4H),

1.28–1.14(m, 24H), 0.81(s, 6H).13C NMR (100 MHz, CDCl3): d¼152.35, 151.80, 147.49, 144.28,

140.36, 138.48, 137.69, 133.62, 130.72, 127.66, 125.78, 122.96,

120.14, 55.52, 40.52, 31.82, 30.13, 29.28, 22.88, 22.56, 14.08.

Figure 1. UV-Vis absorption spectra in THF solution and thin solidfilms: (a) PFO-DDQ001 in THF, (b) PFO-DDQ002 in THF, (c)PFO-DDQ001 in film, (d) PFO-DDQ002 in film, (e) PFO-DDQ50in THF.

Results and Discussion

Synthesis and Chemical Characterization

The general synthetic routes toward the monomers and

copolymers are outlined in Scheme 1. The obtained copoly-

mers are readily soluble in common organic solvents, such

as toluene, THF, and chloroform. The number-average

molecular weights of these polymers determined by

GPC using a polystyrene standard are 34 700 and 31 900

with a polydispersity index (Mw=Mn) 1.96 and 1.81 for



PFO-DDQ001 and 002. The content of

DDQ units in the copolymers is too low

to be detected by 1H NMR and 13C NMR.

Chemical shifts of PFO-DDQ001 and

PFO-DDQ002 are roughly identical with

PFO homopolymer. In order to identify

DDQ structure in the copolymers we also

synthesized alternating copolymer con-

sisting of 9,9-dioctylfluorene and DDQ

segments where the 1H chemical shifts

at ca. d¼ 8.11, 7.13, and 2.90 ppm are

assigned to DDQ structure.

The orange emission in the PL and EL

spectra which is attributed to DDQ

segment increases with increasing DDQ

feed ratio in synthesis, so the actual

ratios of DDQ in the conjugated back-

bone can be assumed to be equal (or at

least proportional) to the feed ratios.

Absorption Properties andPhotoluminescence Characteristics

Figure 1 shows the normalized UV-vis absorption spectra

of the polymers in the solutions of THF and thin solid films.

It can be seen that the absorption spectra of copolymers

are practically identical to those of the PFO homopoly-

mer[12] due to the very low content of DDQ units. The full

width at half maximum of the absorption spectra in thin

solid film is 10 nm wider than those in THF solution. The

absorption peak of DDQ in PFO backbone is located at

around 490 nm which can be observed for copolymer

PFO-DDQ50 (see Figure 1).

DOI: 10.1002/macp.200700016


Figure 2. PL spectra in thin solid films: (a) PF homopolymer, (b)PFO-DDQ001, (c) PFO-DDQ002, (d) PFO-DDQ1.

Figure 3. Normalized EL spectra for device configuration (a)ITO/PEDOT:PSS/PFO-DDQ001/Ba/Al, (b) ITO/PEDOT:PSS/PFO-DDQ002(b)/ Ba/Al, (c) ITO/PEDOT:PSS/PVK/PFO-DDQ001/Ba/Al,and (d) ITO/PEDOT:PSS/PVK/PFO-DDQ002/Ba/Al.

Figure 2 shows the normalized PL spectra of the poly-

mers in thin solid films. As can be seen in Figure 2, two

emission peaks at 424 and 561 nm are responsible for PFO

and DDQ units in the copolymers. The emission intensity

increases with increasing DDQ content at 561 nm. For the

PL spectrum of PFO-DDQ1 in thin solid film an orange-red

emission dominated and the PFO emission was quenched

completely indicating almost complete energy transfer

with 1% DDQ content in the copolymer backbone.

Electroluminescent Properties

In order to investigate the electroluminescent properties of

the copolymers, single-layer light-emitting diodes made

up of device structure: ITO/PEDOT (50 nm)/PFO-DDQ

(75 nm)/Ba/Al were fabricated. The EL spectra of the

devices from copolymers of different DDQ contents are

Table 1. Device performances of the copolymers (at the maximum exlayer/polymer/Ba/Al).

Copolymers Hole transport layer Va) Jb)

V mA � cmS2

PFO-DDQ001 PEDOT:PSS 4.4 9.37

PEDOT:PSS/PVK 6.6 1.64

PFO-DDQ002 PEDOT:PSS 4.40 3.48

PEDOT:PSS/PVK 6 1.08

a)Voltage; b)Current density; c)Luminance; d)Luminance efficiency; e)P

mission Internationale de l(Eclairage (CIE) Coordinates.



shown in Figure 3. It can be seen that the EL spectra

of copolymers show two balanced blue- and orange-

emissions peaked at 436/572 and 434/568 nm, respec-

tively, for the devices based on PFO-DDQ001 and

PFO-DDQ002. CIE coordinates of (0.34, 0.30) and (0.37,

0.34) of these EL emissions are close to those of pure white

light (0.33, 0.33). We further fabricated devices with

the structure ITO/PEDOT (50 nm)/PVK (40 nm)/PFO-DDQ

(75 nm)/Ba/Al, where PVK was used as hole-injection and

electron-blocking layer. The EL spectra of the devices from

different copolymers in this device configuration are also

shown in Figure 3. The device performances are listed

in Table 1. The highest external quantum efficiency is

2.64% with a luminance efficiency of 4.06 cd �A�1 in a

PFO-DDQ002-based device. As the I-L-V curve of the

PFO-DDQ002-based device with a PVK layer (Figure 4)

shows, the maximum luminance of 4 300 cd �m�2 is

obtained at 11.7 V. Comparing the EL spectra in Figure 3

ternal quantum efficiency; device configuration ITO/hole transport

Lc) LEd) PEe) QEmaxf) CIE coord.g)

cd �mS2 cd �AS1 lm �wS1 %

255.4 2.73 1.94 1.77 0.34, 0.30

58.05 3.53 1.68 2.30 0.31, 0.28

67 1.92 1.37 1.25 0.37, 0.34

43.84 4.06 2.12 2.64 0.28, 0.24

ower efficiency; f)Maximum external quantum efficiency; g)Com-


M. Sun, Q. Niu, B. Du, J. Peng, W. Yang, Y. Cao

Figure 4. I-L-V characteristics of LEDs of PFO-DDQ002 with PVKlayer.

992

with PL spectra in a thin film of the same composition

(Figure 2) we note that the EL emission (Figure 3) shows an

enhanced intensity of the orange emission. The increased

energy transfer in the EL process can be attributed to the

direct trapping mechanism in the EL process since the

LUMO and HOMO levels of DDQ of about�3.3 and�5.4 eV

are located within the gap of the host PFO with an LUMO

and HOMO level of about �2.9 and �5.7 eV, which is most

favorable for the trapping mechanism.[13] By precise

control of the feed ratio, we optimized incomplete energy

transfer from PF segment to DDQ unit to achieve balanced

emission from both PF and DDQ.

White light-emitting devices from the PFO-DDQ copoly-

mers show very good current stability which is very

important and desirable for display and lighting applica-

tions. Figure 5 compares EL spectra of the device from the

Figure 5. Normalized EL spectra with device configuration ITO/PEDOT:PSS/PFO-DDQ002/Ba/Al under different voltages.



PFO-DDQ002 copolymer under different operating vol-

tages. The CIE coordinates show little change in the wide

range of operating voltages: (0.37, 0.36), (0.37, 0.36), (0.36,

0.34), (0.35, 0.34), (0.35, 0.34), (0.36, 0.34), (0.36, 0.35), (0.36,

0.34), (0.36, 0.35), (0.36, 0.35), and (0.37, 0.36) at driving

voltages of 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, and 16 V,

respectively.

Conclusion

In conclusion, we have obtained efficient white light-

emission from a single copolymer with two emissive units

(PFO, DDQ) in conjugated polymer backbone. Incomplete

energy transfer from the blue PFO backbone to the orange

DDQ units led to two distinguished primary emissions,

thereby leading to a broad white light-emission with

satisfying CIE coordinates. The EL device fabricated with

structure ITO/PEDOT/PVK/PFO-DDQ002/Ba/Al exhibited

the best performance with highest external quantum

efficiency of 2.64% and a luminance efficiency of 4.06

cd �A�1. Moreover, the color coordinates of the resulted

white-light emission remained extremely stable over a

wide range of the driving voltages.

Acknowledgements: The authors are deeply grateful to theMOSTNational Research Project (no. 2002CB613402) and the NationalNatural Science Foundation of China (project no. 50433030,U0634003) for financial support.

Received: January 11, 2007; Revised: February 14, 2007; Accepted:February 19, 2007; DOI: 10.1002/macp.200700016

Keywords: 2,3-dimethyl-5,8-dithien-2-ylquinoxaline; 9,9-dioctyl-fluorene; light-emitting diodes (LED); structure; white light-emitting diodes

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Fluorene-Based Single-Chain Copolymers for Color-Stable White Light-Emitting Diodes

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