Fluorene-based narrow-band-gap copolymers for red light-emitting diodes and bulk heterojunction...

RESEARCH ARTICLE

Fluorene-based narrow-band-gap copolymers for red light-emitting diodes and bulk heterojunction photovoltaic cells

Mingliang SUN, Li WANG, Yangjun XIA, Bin DU, Ransheng LIU, Yong CAO (*)

Institute of Polymer Optoelectronic Materials and Devices, Key Laboratory of Special Functional Materials, South China University of

Technology, Guangzhou 510640, China

E Higher Education Press and Springer-Verlag 2008

Abstract A series of narrow band-gap conjugated copo-

lymers (PFO-DDQ) derived from 9,9-dioctylfluorene

(DOF) and 2,3-dimethyl-5,8-dithien-2-yl-quinoxalines

(DDQ) is prepared by the palladium-catalyzed Suzuki

coupling reaction with the molar feed ratio of DDQ at

around 1%, 5%, 15%, 30% and 50%, respectively. The

obtained polymers are readily soluble in common organic

solvents. The solutions and the thin solid films of the

copolymers absorb light from 300–590 nm with two

absorbance peaks at around 380 and 490 nm. The intens-

ity of 490 nm peak increases with the increasing DDQ

content in the polymers. Efficient energy transfer due to

exciton trapping on narrow-band-gap DDQ sites has been

observed. The PL emission consists exclusively of DDQ

unit emission at around 591–643 nm depending on the

DDQ content in solid film. The EL emission peaks are

red-shifted from 580 nm for PFO-DDQ1 to 635 nm for

PFO-DDQ50. The highest external quantum efficiency

achieved with the device configuration ITO/PEDOT/

PVK/PFO-DDQ15/Ba/Al is 1.33% with a luminous effi-

ciency 1.54 cd/A. Bulk heterojunction photovoltaic cells

fabricated from composite films of PFO-DDQ30 copoly-

mer and [6,6]-phenyl C61 butyric acid methyl ester

(PCBM) as electron donor and electron acceptor, respect-

ively in device configuration: ITO/PEDOT:PSS/PFO-

DDQ30:PCBM/PFPNBr/Al shows power conversion effi-

ciencies of 1.18% with open-circuit voltage (Voc) of 0.90 V

and short-circuit current density (Jsc) of 2.66 mA/cm2

under an AM1.5 solar simulator (100 mW/cm2). The

photocurrent response wavelengths of the PVCs based

on PFO-DDQ30/PCBM blends covers 300–700 nm.

This indicates that these kinds of low band-gap polymers

are promising candidates for polymeric solar cells and red

light-emitting diodes.

Keywords 9,9-dioctylfluorene, 2,3-dimethyl-5,8-dithien-

2-yl-quinoxalines, light-emitting diodes, solar cells

1 Introduction

In the last decades, conjugated polymers have attracted

considerable interests due to their excellent properties in

electronic and optoelectronic aspects [1,2]. Significant

progress has been achieved by utilizing conjugated poly-

mer materials in polymer light-emitting diodes [3], poly-

mer photovoltaic cells [4] and field-effect transistors [5].

Among these, polymer light emitting diodes and polymer

bulk heterojunction solar cell are most extensively inves-

tigated and have good prospective future commercial

applications. Among a wide range of conjugated poly-

mers, polyfluorene and their derivatives have received

considerable attention for their exceptional optoelectronic

properties. Normally, polyfluorene homopolymers have a

large band gap and emit blue light [6,7]. Significant efforts

and great success have been made to tune the light-emit-

ting color of polyfluorenes to a longer wavelength. The

red polyfluorene emitters can be synthesized by copoly-

merizing fluorene with a narrow-band-gap monomer such

as 4,7-dithienyl-2,1,3-benzothiadiazole (DBT) [3], 5,7-

dithien-2-ylthieno[3,4-b]pyrazine [8], 4,7-diselenophen-

29-yl-2,1,3-benzothiadiazole [9], 4,7-diselenophen-29-yl-2,1,3-benzose-lenadiazole [9] and thiophene [10]. The

fluorene-based narrow-band-gap copolymers [4], a small

molecular organic substance [11], can be used as the donor

phase in combination with [6,6]-phenyl C61 butyric acid

methyl ester (PCBM) as the acceptor phase to form bulk

heterojunction solar cells. Polymer bulk heterojunction

solar cells have advantages over multilayer devices, such

as large p-n interface area, low cost, good film-forming

ability and easy processing for large area devices by solu-

tion processing. In the recent research, the energy conver-

sion efficiency of polymer bulk heterojunction solar cell

has achieved 5% via better control of the morphology of

Translated from Acta Polymerica Sinica (China), 2007, 10: 952–958[译自: 高分子学报]

E-mail: [email protected]

Front. Chem. Eng. China 2008, 2(3): 257–264DOI 10.1007/s11705-008-0052-x

the active layer composed of poly(3-hexyl-thiophene)

(P3HT) and PCBM by post-thermal annealing and fine

control of the film forming process [12–15]. Lots of

polyfluorene-based narrow band gap polymers such as

PFO-DBT [4] and PFO-Green2 [16] have been synthe-

sized in order to make the absorption spectra of the

conjugated polymers match better with solar terrestrial

radiations.

We have previously reported that PFO-DDQ polymers

with very low 2,3-dimethyl-5,8-dithien-2-ylquinoxalines

(DDQ) content (% 1%) emitted white light due to incom-

plete energy transfer from the PFO segment to the DDQ

unit [17]. In this paper, we reported the synthesis and

characterization of a series of new low band-gap copoly-

mers with 9,9-dioctylfluorene (DOF) and DDQ in high

DDQ content of up to 50% (molar) in the copolymer.

Red light-emitting devices and photovoltaic devices were

fabricated from the obtained copolymers. The device ITO

(indium tin oxide)/PEDOT/PVK/PFO-DDQ15/Ba/Al

exhibits the highest external quantum efficiency of

1.33% with luminous efficiency of 1.54 cd/A. The pho-

tovoltaic devices based on composite thin films of the

copolymer PFO-DDDQ30 and PCBM as an active layer

show short circuit current density (about 2.66 mA/cm2)

and energy conversion efficiency (about 1.18%) under

an AM1.5 solar simulator (100 mW/cm2).

2 Experimental

2.1 General methods and materials

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.co.). UV-Visible absorption

spectra were measured on a HP 8453 spectrophotometer.

PL spectra in solutions and in thin solid films were taken

by Fluorolog-3 spectrofluorometer (Jobin Yvon) under

325 nm light excitation.

All reagents, unless otherwise specified, were obtained

from Aldrich, Acros, and TCI Chemical Co., and were

used as received. All the solvents were further purified

under a nitrogen flow. 2,3-dimethyl-5,8-di(59-bromo-

thien)-2-ylquinoxalines (1), 2,7-dibromo-9,9-dioctylfluor-

ene (2), 2,7-bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-

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

procedure described in reference [3,17] and were charac-

terized by GC-MS and 1H NMR.

2.2 Device fabrication and measurement

LED was fabricated on pre-patterned indium-tin oxide

(ITO) with a sheet resistance of 10–20 V/%. The substrate

was ultrasonically cleaned with acetone, detergent,

deionized water and 2-propanol. Oxygen plasma treat-

ment was done for 10 min as the final step of substrate

cleaning to improve the contact angle just before film

coating. A 50 nm-thick layer of polyethylenedioxythio-

phene-polystyrene sulfonic acid (PEDOT:PSS) film was

spin-coated from its aqueous dispersion (Baytron P

4083, Bayer AG) unto the ITO glass, aimed at improv-

ing the hole injection and avoiding the possibility of

leakage. PEDOT:PSS film was dried at 80uC 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 emit-

ting copolymers or PFO-DDQ/PCBM blend films were

spin-coated on the top of the ITO/PEDOT:PSS (PVK)

surface inside a dry box. The typical thickness of the

emitting layer or photovoltaic blend films was 70–

80 nm or 100 nm, respectively. A thin layer of barium

as an electron injection cathode and the subsequent

180 nm-thick aluminum capping layers were thermally

deposited by vacuum evaporation through a mask at a

base pressure below 26 1024 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 0.15 cm2. The spin-coating of the EL layer

and device performance tests were carried out within a

glove box (Vacuum Atmosphere Co.) with nitrogen cir-

culation. Current-luminance-voltage (I-L-V) characteris-

tics were measured with a computerized Keithley 236

Source Measure Unit and calibrated Si photodiode.

External quantum efficiency was verified by measure-

ment in the integrating sphere (IS080, Lab sphere) and

luminance was calibrated by a PR705 spectragraph-pho-

tometer after the encapsulation of the devices with UV-

curing epoxy and thin cover glass. EL spectra were

taken by InstaSpecTM IV CCD spectragraph.

Energy conversion efficiencies of solar cells were mea-

sured under an AM1.5 solar simulator (100 mW/cm2).

The energy conversion efficiency (ECE) and fill factor

(FF) were calculated by the following equations:

ECE~FF|Jsc|Voc=Pin, ð1Þ

FF~Jm|Vm= Jsc|Vocð Þ, ð2Þ

where Pin is the incident radiation flux, Jsc and Voc are,

respectively, the short-circuit current density and open-

circuit voltage, and Jm and Vm are, respectively, the cur-

rent density and voltage at the maximum power output.

2.3 Synthesis of polymer

The synthesis route of monomers and the copolymers is

shown in Scheme 1.

258 Mingliang SUN, et al.

Carefully purified 2,7-dibromo-9,9-dioctylfluorene (2),

2,7-bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-9,9-

dioctylfluorene (3), 2,3-dimethyl-5,8-di(59-bromo-thien)-

2-ylquinoxalines (1), (PPh3)4Pd(0) (0.5–2.0 mol%) and

several drops of Aliquat 336 were dissolved in a mixture

of toluene and aqueous 2 M Na2CO3. The solution was

refluxed with vigorous stirring for 36 h in an argon atmo-

sphere. At the end of polymerization, the polymers were

end-capped with 2,7-bis(4,4,5,5-tetramethyl-1,3,2-dioxa-

borolan-2-yl)-9,9-dioctylfluorene and bromobenzene to

remove bromine and boronic ester end groups in order to

avoid a possible quenching effect or excimer formation by

boronic and bromine end groups in LEDs. Themixture was

then poured into methanol and the precipitated material

was recovered by filtration and purified by flash column

chromatography. The resulting polymers were air-dried

overnight, followed by drying under vacuum. In the process

of polymerization, the comonomer feed ratios of (2+ 3) to 1

were 99 : 1, 95 : 5, 85 : 15, 70 : 30 and 50 : 50, and the mole

ratio of 2,7-bis-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-

yl)-9,9-dioctylfluorene to 2,7-dibromo-9,9-dioctylfluorene

and 2,3-dimethyl-5,8-di(59-bromo-thien)-2-ylquinoxalines

always remained as 3 : (1+ 2)5 1 : 1. The corresponding

copolymers were named PFO-DDTQ1, PFO-DDTQ5,

PFO-DDTQ15, PFO-DDTQ30 and PFO-DDTQ50,

respectively. Their spectra data are as follows.

PFO-DDQ1: 0.50 equiv. of 3, 0.49 equiv of 2, and 0.01

equiv. of 1 were used in this polymerization. 1H NMR

(300 MHz, CDCl3): 7.88–7.86, 7.74–7.70, 2.15, 1.28–

1.16, 0.86–0.82.

PFO-DDQ5: 0.50 equiv. of 3, 0.45 equiv. of 2, and 0.05

equiv. of 1 were used in this polymerization. 1H NMR

(300 MHz, CDCl3): 7.88–7.85, 7.74–7.70, 2.93, 2.15,

1.16, 0.86–0.82.

PFO-DDQ15: 0.50 equiv. of 3, 0.35 equiv. of 2, and

0.15 equiv. of 1 were used in this polymerization. 1H

NMR (300 MHz, CDCl3): 8.14, 7.94, 7.88–7.85, 7.74–

7.70, 7.51, 2.93, 2.15, 1.28–1.16, 0.86–0.82.

PFO-DDQ30: 0.50 equiv. of 3, 0.20 equiv. of 2, and

0.30 equiv. of 1 were used in this polymerization. 1H

NMR (300 MHz, CDCl3): 8.13, 7.94, 7.88–7.82, 7.77–

7.70, 7.51, 2.93, 2.15, 1.28–1.16, 0.84–0.82.

PFO-DDQ50: 0.50 equiv. of 3 and 0.50 equiv. of 1 were

used in this polymerization. 1HNMR (300 MHz, CDCl3):

8.11, 7.92, 7.75–7.74, 7.48, 7.37, 2.90, 2.08, 1.43, 1.28–

1.14, 0.81. 13C NMR (100 MHz, CDCl3): 152.35, 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.

3 Results and discussion

3.1 Synthesis and chemical characterization

The general synthetic routes toward the monomers and

copolymers are outlined in Scheme 1. The obtained copo-

lymers are readily soluble in common organic solvents,

such as toluene, THF and chloroform. The number-aver-

age molecular weights of these polymers are determined

Scheme 1 Synthesis route of monomers and copolymers

Fluorene-based narrow-band-gap copolymers for red light-emitting diodes and bulk heterojunction photovoltaic cells 259

by GPC using a polystyrene standard, ranging from 5600

to 46900 with a polydispersity index (Mw/Mn) between

1.36 and 2.34 (Table 1). The alternating polymer shows

relatively low molecular weight, which is attributed to the

poor solubility of the DDQ component. The actual ratios

of substituted DOF to DDQ in the copolymers estimated

by elemental analysis (S and C content) are listed in

Table 1, which is in good agreement with the feed ratios

of the monomers within experimental error. The chemical

shifts of 1H NMR at 8.11–8.14 and 2.9 are attributed to

the DDQ structure which increases intensity gradually

with increasing DDQ content in the polymer backbone.

3.2 UV-Vis absorption properties and electrochemical

characteristics

The UV-Vis absorption properties of the conjugated poly-

mers based on DOF and 2,3-dimethyl-5,8-dithien-2-ylqui-

noxalines (DDQ) are presented in Table 2. Figure 1

shows the normalized UV-Vis absorption spectra of the

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

The absorption spectra of copolymers monitored both in

solutions of THF and in solid thin films consist of two

absorption bands. The absorption peaks around 380 nm

are attributed to fluorene segments, and the absorption

peaks around 490 nm are attributed to DDQ units. The

absorption intensity at 490 nm increases gradually with

increasing content of DDQ in the polymer backbone.

The two distinguishable absorption features of the copo-

lymers demonstrate that DDQ is successfully incorpo-

rated into the polyfluorene backbone. For the UV-Vis

spectra of copolymer PFO-DDQ1, the 490 nm peak from

the DDQ unit is not apparent in the solution and in thin

solid film.

The electrochemical properties of the copolymers were

investigated by cyclic voltammetry (CV). Table 2

summarizes oxidation potentials derived from the onset

in the cyclic voltammograms of the copolymers. We can

record one p-doping process in the copolymers (PFO-

DDQ1, PFO-DDQ5, PFO-DDQ15), and the onset of

oxidation process of these copolymers is around 1.30 V

which are attributed to the oxidation process of the DOF

segment in the copolymers. For the copolymer PFO-

DDQ30, two oxidation peaks were observed at around

1.02 and 1.27 V corresponding to the oxidation process

of the DDQ and DOF segment, respectively. The electro-

chemical properties of PFO-DDQ50 are similar to PFO-

DDQ30. The optical band gap (Eg) is estimated from the

onset wavelength of UV-Vis spectra of the copolymer in

solid film. (Table 2) HOMO and LUMO levels calculated

by empirical formulas EHOMO52e(Eox + 4.4) (eV) and

ELUMO52e(EHOMO +Eg) (eV) are also listed in Table 2

[18].

3.3 Photoluminescence properties

Figure 2(a) shows the normalized PL spectra of the poly-

mers in the solutions of THF at 16 1023 mol/L. The PL

spectrum of PFO-DDQ1 has three peaks at about 422,

444 and 577 nm. Peaks at 422 and 444 nm are attributed

to DOF emission and that at 577 nm can be assigned to

DDQ emission. The PL spectrum of PFO-DDQ5 is similar

to PFO-DDQ1 with increased DDQ emission. At this con-

centration (16 1023 mol/L), no DOF emission can be

observed for PFO-DDQ15, 30 and 50 copolymers in the

solutions. The orange-red peaks at 587, 587 and 590 nm

responsible for DDQ emission are observed for PFO-

DDQ15, 30 and 50, respectively.

Figure 2(b) shows the normalized PL spectra of the

polymers in solid thin films. PL emission consists exclu-

sively of DDQ unit emission at around 591–643 nm

depending on the DDQ content even for copolymer with

Table 1 Molecular weights of copolymers and their composition determined by elemental analysis

Copolymers Mn/6 103 Mw/Mn C content in

copolymers/%S content in

copolymers/%Feed ratios of

DOF/DDQ

DOF/DDQ in

copolymersa)

PFO-DDQ1 21.6 1.99 87.89 0.33 99:1 98.1:1.9

PFO-DDQ5 40.0 1.8 87.49 0.97 95:5 94.1:5.8

PFO-DDQ15 35.8 2.34 85.89 2.66 85:15 84.2:15.8

PFO-DDQ30 46.9 2.21 83.38 5.02 70:30 71.1:28.9

PFO-DDQ50 5.6 1.36 77.64 8.60 50:50 52.2:47.8

a)Molar ratio of DOF/DDQ in copolymers calculated from C and S element contents in copolymers

Table 2 UV-absorption in thin solid film and electrochemical properties of copolymers

Copolymers l(ABS)max/nm le/nm Optical band gapa)/eV Eox/V HOMO/eV LUMOb)/eV

PFO-DDQ1 384 420 2.95 1.33 25.73 22.78

PFO-DDQ 5 384, 488 570 2.18 1.33 25.73 23.55

PFO-DDQ 15 387, 490 576 2.15 1.34 25.74 23.59

PFO-DDQ 30 386, 503 578 2.14 1.02 1.27 25.42 23.28

PFO-DDQ 50 379, 504 591 2.10 0.96 1.28 25.36 23.26

a)Estimated from onset wavelength of optical absorption in solid film. b) Calculated from HOMO level and optical band gap


only 1% DDQ in the copolymer, which is different from

the PL spectra in the THF solution. The shapes of the PL

spectra for copolymers with different DDQ content are

very similar and the PL peaks are significantly red-shifted

with increasing DDQ content in the copolymers, from

591 nm for the copolymer with 1% DDQ content to

643 nm for the alternating copolymer. Since no such large

red shift was observed for the UV-Vis absorption spectra

of the corresponding copolymers, the significant red-shift

in PL emission indicates that the Stokes shift was

increased with the increasing DDQ content in the copoly-

mers. The complete disappearance of fluorene emission

and the appearance of new emissions due to the DDQ unit

indicate that the DDQ unit serves as a powerful trap in the

copolymer chain. As we reported previously, for poly-

fluorene copolymers prepared by Suzuki coupling of

fluorene and narrow band gap sulfur-containing hetero-

cycles, the trapping mechanism is especially favorable

since both HOMO and LUMO levels of the narrow band

gap unit are located within the band gap of the host seg-

ment [9].

3.4 Electroluminescent properties

Electroluminescent devices based on PFO-DDQ copoly-

mers are fabricated with configuration as ITO/PEDOT:

PSS/PFO-DDQ/Ba/Al. Figure 3(a) shows the EL spectra

of the copolymers in such devices. EL emission consists

exclusively of orange-red or red emission peaks at around

580–635 nm depending on the DDQ content. Fluorene

host emission and excimer emission are quenched comple-

tely for the device from the copolymers with only as low as

1% DDQ units. This indicates that the energy trapping

process (from fluorene segment to DDQ unit) must be

very fast and efficient in the copolymers. This indicates

again that the trapping mechanism is important in the

devices made from such copolymers, in agreement with

our previously reported results [3].

Fig. 1 UV-Vis absorption spectra in THF solution (a) and in solid film (b)

Fig. 2 PL spectra in THF solution (a) and in solid film (b)


Since the HOMO for the copolymer is around 5.4–

5.7 eV (Table 2), while the work function of PEDOT is

around 5.0–5.2 eV, it would be possible to expect a better

hole injection once PVK (work function 5.5–5.6 eV) is

used as the hole injection anode. We further fabricated

EL devices with a configuration ITO/PEDOT:PSS/PVK/

PFO-DDQ/Ba/Al, which show similar EL spectra with

the above mentioned devices but enhanced device per-

formance. Figure 3(b) shows the EL spectra of the copo-

lymers in such devices. Table 3 shows the light emitting

performance of the device. The device ITO (indium tin

oxide)/PEDOT/PVK/PFO-DDQ15/Ba/Al exhibits the

highest external quantum efficiency of 1.33% with lumin-

ous efficiency of 1.54 cd/A. I-L-V curve of the PFO-

DDQ15-based device with PVK layer is shown in Fig. 4.

3.5 Characteristics of photovoltaic devices

Bulk heterojunction polymer photovoltaic cells (PPVCs)

are made up of copolymers (PFO-DDQ30) as the donor

phase blending with PCBM as the acceptor phase with a

sandwich configuration of ITO/PEDOT:PSS/PFO-

DDQ30:PCBM/Ba/Al or ITO/PEDOT:PSS/PFO-DDQ30:

PCBM/PFPNBr/Al. Table 4 lists the PFO-DDQ30/PCBM

based PPVCs devices performance. With different weight

ratio of acceptor to donor the best PPVCs device perform-

ance obtained from the device based on PFO-DDQ30 :

PCBM (1 : 3) blend is Jsc 2.02 mA/cm2, Voc 0.80 V, and

ge (ECE) 0.74% under AM1.5 illuminator (100 mW/cm2).

The energy conversion efficiency of PPVCs is sensitive to

the weight ratio of acceptor to donor, such as PFO-DBT/

PCBM [4] and PFO-Green2/PCBM. [16]. When the device

configuration is ITO/PEDOT:PSS/PFO-DDQ:PCBM/

PFPNBr/Al, a better device performance was achieved with

Jsc 2.66 mA/cm2, Voc 0.90 V, and ge (ECE) 1.18% under

AM1.5 illuminator (100 mW/cm2). The I-V characteristics

of the device based on ITO/PEDOT:PSS/PFO-DDQ30:

PCBM/PFPNBr/Al is shown in Fig. 5. The photocurrent

response wavelengths of the PVCs based on PFO-DDQ30/

PCBM blends covers 300–700 nm (Fig. 6).

4 Conclusions

We synthesized a series of new conjugated copolymers

PFO-DDQ, composed of random or alternating 9,

Fig. 3 Normalized EL spectra with device configuration ITO/PEDOT:PSS/PFO-DDQ/Ba/Al (a) or ITO/PEDOT:PSS/PVK/PFO-DDQ/Ba/Al (b)

Table 3 Device performances of copolymers (ITO/hole transport layer/polymer/Ba/Al)

Copolymers Hole transport layer Va)/V Ib)/mA Lc)/cd?m22 LEd)/cd?A21 QEmaxe)/% CIE(x,y)f)

PFO-DDQ1 PEDOT:PSS 7.40 1.68 51 0.51 0.33 0.55,0.44

PEDOT:PSS/PVK 12.2 1.13 44 0.66 0.57 0.56,0.43

PFO-DDQ5 PEDOT:PSS 6.20 1.05 64 1.03 0.76 0.58,0.42

PEDOT:PSS/PVK 9.80 0.81 66 1.39 1.20 0.59,0.42

PFO-DDQ15 PEDOT:PSS 5.50 17.7 1155 1.11 0.96 0.60,0.39

PEDOT:PSS/PVK 8.60 1.18 107 1.54 1.33 0.60,0.39

PFO-DDQ30 PEDOT:PSS 4.10 21.3 1030 0.82 0.89 0.62,0.37

PEDOT:PSS/PVK 5.80 0.30 19 1.07 1.25 0.62,0.37

PFO-DDQ50 PEDOT:PSS 4.65 62.0 98 0.03 0.04 0.63,0.30

PEDOT:PSS/PVK 4.13 38.4 172 0.08 0.10 0.64,0.35

a)Voltage. b) Current. c) Luminance. d) Luminous efficiency. e) Max external quantum efficiency. f) CIE


9-dioctylfluorene and 2,3-dimethyl-5,8-dithien-2-ylquinox-

alines, via a palladium-catalyzed Suzuki coupling reaction.

All the obtained copolymers are soluble in common organic

solvents, such as CHCl3, THF and toluene. Efficient energy

transfer due to exciton trapping on narrow-band-gap DDQ

sites has been observed. The PLED device with these poly-

mers as the emitting layer shows orange-red or saturated

red emission. The highest external quantum efficiency is

1.33% with luminous efficiency of 1.54 cd/A. EL peaks at

around 602 nm is obtained for the device fabricated with

copolymer of 15%DDQ content. The best bulk heterojunc-

tion polymer photovoltaic cells (PPVCs) show power con-

version efficiencies of 1.18% with open-circuit voltage (Voc)

of 0.90 V and short-circuit current density (Jsc) of 2.66 mA/

cm2 under an AM 1.5 solar simulator (100 mW/cm2).

Acknowledgements The authors are deeply grateful to the NationalNatural Science Foundation of China (Grant No. 50433030) and theMinistry of Science and Technology (No. 2002CB613404) for financialsupport.

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Fig. 4 I-L-V characteristics of PLEDs based on PFO-DDQ15 with device configuration of ITO/PEDOT:PSS/PVK/PFO-DDQ/Ba/Al

Table 4 Device characteristics of PFO-DDQ30:PCBM solar cells

Active layer Cathode Jsc/mA?

cm22

Voc/V FF/

%ECE/

%

PFO-DDQ30 : PCBM5 1 : 1 Ba/Al 1.79 0.85 30.3 0.46

PFO-DDQ30 : PCBM5 1 : 2 Ba/Al 1.94 0.80 45.7 0.71

PFO-DDQ30 : PCBM5 1 : 3 Ba/Al 2.02 0.80 45.6 0.74

PFO-DDQ30 : PCBM5 1 : 4 Ba/Al 1.89 0.80 47.6 0.72

PFO-DDQ30 : PCBM5 1 : 3 PFPNBr/Al 2.66 0.90 49.4 1.18

Fig. 5 I–V characteristic of the PVC based on device ITO/PEDOT:PSS/PFO-DDQ30 : PCBM(1 : 3)/PFPNBr/Al

Fig. 6 Photosensitivity of PVCs based on PFO-DDQ/PCBM blend


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