Their Applications in Organic Light Emitting Diodes
HUEI-JEN SU,1 FANG-IY WU,1 CHING-FONG SHU,1 YUNG-LIANG TUNG,2 YUN CHI,2 GENE-HSIANG LEE3
1Department of Applied Chemistry, National Chiao Tung University, 300, Hsinchu, Taiwan
2Department of Chemistry, National Tsing Hua University, 300, Hsinchu, Taiwan
3Instrumentation Center, College of Science, National Taiwan University, 107, Taipei, Taiwan, Republic of China
Received 1 September 2004; accepted 1 October 2004 DOI: 10.1002/pola.20569
Published online in Wiley InterScience (www.interscience.wiley.com).
ABSTRACT: We present a short, efficient synthetic route for the preparation of a novel polyfluorene copolymer (PF-Q) containing two electron-deficient, 2,4-diphenylquinoline groups functionalized at the C-9 positions of alternate fluorene units that form a three-dimensional cardostructure. The presence of the rigid bulky pendent groups leads to a polyfluorene possessing a high glass-transition temperature (207 °C) and very good thermal stability (5% weight loss observed at 460 °C). A photoluminescence study revealed that the Fo¨rster energy transfer from the excited quinoline groups to the polyfluorene backbone is very efficient; it also demonstrated that the commonly ob-served aggregate/excimer formation in polyfluorenes is suppressed very effectively in this polymer, even after it has been annealed at 150 °C for 20 h. A light emitting diode (LED) device prepared with PF-Q as the emitting layer exhibits a stable blue emission with a maximum brightness of 1121 cd/m2at 12 V and a maximum external quantum
efficiency of 0.80% at 250 cd/m2. We also used PF-Q, which contains diphenylquinoline
units that behave as electron-transporting side chains, as a host material and doped it with 2.4 wt % of a red-emitting phosphorescent dye, Os(fppz), to realize a red electrolu-minescence with CIE color coordinates of (0.66, 0.34). The doped device exhibits a maximum external quantum efficiency of 6.63% (corresponding a luminance efficiency of 8.71 cd/A) at a current density of 47.8 mA/cm2, together with a maximum brightness
of 10457 cd/m2.© 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 859 – 869,
2005
Keywords: charge transport; fluorescence; LED; polyfluorene; quinoline
INTRODUCTION
Since poly(phenylene vinylene) (PPV) was first employed in a polymer-based light-emitting diode (PLED) in 1990,1the synthesis and properties of light-emitting polymers and their applications in PLEDs have been studied intensively.2–5 The
main advantages of organic polymers, when com-pared with inorganic or molecular organic mate-rials, are the ability to fine-tune the luminescence properties of polymers by manipulating their chemical structure and the feasibility of spin-coating and ink-jet printing processes for prepar-ing large-area display devices. Most electrolumi-nescent conjugated polymers, however, such as PPV and polyfluorene (PF), have a -excessive nature, that is, they are typical p-dope-type poly-mers, and are hole-transport-dominated
materi-Correspondence to: C.-F. Shu (E-mail: [email protected])
Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 43, 859 – 869 (2005) © 2005 Wiley Periodicals, Inc.
als.6 – 8Because the electroluminescent (EL) light
in PLEDs arises from the recombination of elec-trons and holes in active polymer layers, any unipolar characteristics of these conjugated poly-mers may result in an imbalance in hole and electron injection and/or transport that would lead to a lowering of the efficiency of the device.
To achieve high electroluminescent efficiency, it is necessary to balance the injection and trans-port of electrons and holes.9 –17In this article, we report the synthesis of a fluorene copolymer con-taining two electron-deficient, 2,4-diphenylquino-line groups functionalized on the C-9 position of the alternating fluorene unit. This polymer de-sign has the advantage of permitting the incorpo-ration of a high concentincorpo-ration of quinoline groups while retaining the conjugation and integrity of the emission spectrum of the PF main chain. We chose PF as the polymer backbone because PFs and their copolymers are very promising candi-dates for blue light-emitting materials because of their high photoluminescence (PL) and EL effi-ciencies and good thermal stability, solubility, and film-forming capability.18 –24In addition, the facile methods for functionalizing the C-9 position of the fluorene unit also offer the ability to tune the optoelectronic properties of PFs through mac-romolecular engineering.14,15,25–31 We chose to
use diphenylquinoline units as the pendent groups because of their electron-deficient na-ture32 and because several polyquinolines and
their copolymers have been demonstrated to be electron injecting/transporting materials in or-ganic light emitting diode (OLED) devices.33–38 The incorporation of electron-deficient quinoline groups into fluorene units will result in an in-creased electron affinity and inin-creased transport-ing properties of the PF polymer, which will re-sult in a more balanced charge recombination in the polymer emissive layer. Moreover, the pres-ence of sterically demanding substituents may prevent the occurrence of -stacking between polymer chains and suppress the formation of excimers in the solid state. Taking all of these features into account, we expected that such a PF copolymer would possess good solubility, excel-lent thermal stability, a low-lying lowest unoccu-pied molecular orbital (LUMO) energy level, and an efficient and stable blue emission that is highly desirable to serve as a blue emitter by itself or even as a host material to generate the high efficiency saturated red phosphorescence.
EXPERIMENTAL
Materials
2,7-Dibromofluorene (1),39 2,7-bis(4,4,5,5-tetra- methyl-1,3,2-dioxaborolane-2-yl)-9,9-dioctylflu-orene (4),40 2,4-diphenylquinoline (DPQ),41 and 1,3,5-tris(N-phenylbenzimidazol-2-yl)benzene (TPBI)42were prepared according to reported pro-cedures. The solvents were dried with standard procedures. All other reagents were used as re-ceived from commercial sources unless otherwise stated.
Characterization
1H and13C NMR spectra were recorded on a
Var-ian Unity 300 MHz or a Bruker-DRX 300 MHz spectrometer. Mass spectra were obtained on a JEOL JMS-HX 110 mass spectrometer. Size ex-clusion chromatography (SEC) was performed with a Waters chromatography unit interfaced to a Waters 410 differential refractometer; three 5-m Waters styragel columns (300 ⫻ 7.8 mm) were connected in series in order of decreasing pore size (104, 103, and 102 A˚ ); tetrahydrofuran
(THF) was the eluent. Standard polystyrene sam-ples were used for calibration. Differential scan-ning calorimetry (DSC) was performed on a Seiko EXSTAR 6000 DSC unit at a heating rate of 20 °C min⫺1and a cooling rate of 40 °C min⫺1. Samples were scanned from 30 to 350 °C, cooled to 0 °C, and then scanned again from 30 to 300 °C. The glass-transition temperatures (Tg) were
deter-mined from the second heating scan. Thermo-gravimetric analysis (TGA) was undertaken on a DuPont TGA 2950 instrument. The thermal sta-bility of the samples under a nitrogen atmosphere was determined by measuring their weight loss while heating at a rate of 20 °C min⫺1. UV–visible (UV–vis) spectra were measured with an HP 8453 diode-array spectrophotometer. PL spectra were obtained on a Hitachi F-4500 luminescence spec-trometer. Cyclic voltammetry (CV) measure-ments were performed with a BAS 100 B/W elec-trochemical analyzer in anhydrous acetonitrile with 0.1 M tetrabutylammonium hexafluorophos-phate (TBAPF6) as the supporting electrolyte at a
scanning rate of 50 mV/s. The potentials were measured against an Ag/Ag⫹(0.01 M AgNO3)
ref-erence electrode with ferrocene as the internal standard. The onset potentials were determined from the intersection of two tangents drawn at the rising and background current of the cyclic
voltammogram. Single-crystal X-ray diffraction data were obtained from a Bruker Smart Apex-CCD diffractometer with (Mo-K␣) radiation ( ⫽ 0.71073 A˚); data collection was executed with the SMART program. Cell refinement and data reduction were undertaken with the SAINT pro-gram. The structure was determined with the SHELXTL/PC program and refined with full-ma-trix least squares methods.
Fabrication of LEDs
We fabricated LED devices with the structure ITO/poly(styrenesulfonate)-doped poly(3,4-ethyl-enedioxythiophene) (PEDOT) (35 nm)/polymer emitting layer (50 –70 nm)/TPBI (30 nm)/Mg:Ag (100 nm)/Ag(100 nm). The PEDOT was spin-coated directly onto the indium tin oxide (ITO) glass and dried at 80 °C for 12 h in vacuo to improve hole injection and the smoothness of the substrate. The light-emitting layer was spin-coated on top of the PEDOT layer with chloroben-zene as the solvent and the sample was then dried for 3 h at 60 °C in vacuo. Before film casting, the polymer solution was filtered through a Teflon filter (0.45m). The TPBI layer, which we used as an electron transporting layer that would also block holes and confine excitons, was grown by thermal sublimation in a vacuum of 3 ⫻ 10⫺6 torr.43 Subsequently, the cathode Mg:Ag (10:1,
100 nm) alloy was deposited by coevaporation onto the TPBI layer; this process was followed by placing an additional layer of Ag (100 nm) onto the alloy as a protection layer. The current–volt-age–luminance relationships were measured un-der ambient conditions with a Keithley 2400 source meter and a Newport 1835C optical meter equipped with an 818ST silicon photodiode.
2,7-Dibromo-9,9-bis(4-acetylphenyl)fluorene (2)
A mixture of 1 (3.47 g, 10.7 mmol), 4-fluoroaceto-phenone (3.40 g, 24.6 mmol), K2CO3 (4.43 g,
32.1 mmol), and dimethylformamide (DMF; 18 mL) was heated under reflux for 27 h. The mixture was then poured into water (200 mL). The precipitated product was purified by column chromatography (hexane/EtOAc, 6:1) to yield 2 (3.94 g, 65.8%). 1H NMR (300 MHz, CDCl 3,␦): 2.55 (s, 6H), 7.20 (dd, J⫽ 6.7, 1.8 Hz, 4H), 7.43 (d, J ⫽ 1.6 Hz, 2H), 7.52 (dd, J⫽ 8.1, 1.8 Hz, 2H), 7.61 (d, J ⫽ 8.1 Hz, 2H), 7.85 (dd, J ⫽ 6.8, 1.8 Hz, 4H). 13C NMR (75 MHz, CDCl3, ␦): 26.6, 65.5, 121.9, 122.1, 128.0, 128.8, 129.1, 131.6, 136.2, 138.1, 149.0, 151.4, 197.4. High-resolution mass spectrometry (HRMS) [M⫹] calcd. for C29H2179Br2O2 558.9908,
found 558.9908. Anal. Calcd. for C29H20Br2O2: C,
62.17; H, 3.60. Found: C, 62.30; H, 3.89.
Synthesis of Monomer 3
A mixture of 2 (1.00 g, 1.79 mmol), 2-aminoben-zophenone (739 mg, 3.75 mmol), diphenyl phos-phate (DPP; 2.23 g, 8.93 mmol), and freshly dis-tilled m-cresol (1.0 mL) was flushed with nitrogen while stirring at 65 °C for about 30 min and then heated under nitrogen at 140 °C for 3 h. After cooling, the reaction mixture was added to a so-lution of 10% (v/v) triethylamine in methanol. The precipitated product was purified by column chro-matography (toluene/EtOAc, 10:1) to yield 3 (530 mg, 67.3%). 1H NMR (300 MHz, CDCl 3, ␦): 7.35 (d, J ⫽ 8.4 Hz, 4H), 7.43–7.54 (m, 14H), 7.58 (d, J ⫽ 1.6 Hz, 2H), 7.61 (d, J⫽ 8.1 Hz, 2H), 7.71 (ddd, J ⫽ 8.4, 7.0, 1.3 Hz, 2H), 7.76 (s, 2H), 7.90 (d, J⫽ 7.6 Hz, 2H), 8.07 (d, J⫽ 8.4 Hz, 4H), 8.23 (d, J ⫽ 8.4 Hz, 2H). 13C NMR (75 MHz, CDCl 3, ␦): 65.4, 119.3, 121.7, 122.0, 125.6, 125.7, 126.4, 127.9, 128.5, 128.6, 129.45, 129.53, 131.2, 138.1, 138.2, 138.8, 145.4, 148.7, 149.3, 152.5, 156.4. HRMS [M⫹] calcd. for C55H35N279Br2 881.1167, found 881.1163. Anal. Calcd. for C55H34N2Br2: C, 74.84; H, 3.88; N, 3.17. Found: C, 74.67; H, 4.02; N, 3.09. Preparation of PF-Q
Aqueous K2CO3(2.0 M, 2.0 mL) and aliquat威 336
(ca. 28 mg) were added to a mixture of monomers
3 (200 mg, 227mol) and 4 (146 mg, 227 mol) in
distilled toluene (3.5 mL). The mixture was de-gassed and tetrakis(triphenylphosphine)palladi-um (6 mg, 2.5 mol %) was added in one portion under N2. The solution was heated at 110 °C for
72 h. The end groups were then capped by heating under reflux for 12 h with benzeneboronic acid (58.9 mg, 480mol) and then for 12 h with bro-mobenzene (76 mg, 480mol). The reaction mix-ture was cooled to room temperamix-ture and precip-itated into a mixture of MeOH and H2O (1:1 v/v,
100 mL). The crude polymer was collected, washed with excess MeOH, dissolved in chloro-form, and then reprecipitated into MeOH. Fi-nally, the polymer was washed with acetone for 48 h with a Soxhlet apparatus and then dried in
1H NMR (300 MHz, CDCl 3, ␦): 0.64–0.74 (m, 10H), 1.03 (br, 20H), 2.03 (br, 4H), 7.42–7.58 (m, 20H), 7.67–7.76 (m, 10H), 7.86 –7.94 (m, 4H), 8.09 (d, J⫽ 8.0 Hz, 4H), 8.20 (d, 2 H, J ⫽ 8.0 Hz).13C NMR (75 MHz, CDCl3, ␦): 13.4, 22.5, 23.8, 29.1, 30.0, 31.6, 40.3, 55.3, 65.6, 119.4, 120.0, 120.6, 120.9, 121.3, 124.9, 125.0, 125.6, 125.7, 126.3, 127.2, 127.4, 127.8, 128.4, 128.5, 128.9, 129.5, 130.1, 138.3, 138.5, 139.0, 140.1, 141.5, 147.1, 148.8, 149.1, 151.5, 151.8, 156.8.
RESULTS AND DISCUSSION
Synthesis
Scheme 1 illustrates the synthetic route we fol-lowed for the preparation of the PF-Q. Starting from 2,7-dibromofluorene (1), which contains an activated methylene group at the C-9 position, our synthetic strategy was based on the nucleo-philic aromatic substitution of 4-fluoroacetophe-none with the fluorenyl anion generated by the action of K2CO3in DMF. This approach is a
sim-ple and direct method for introducing two 4-acetylphenyl groups to the C-9 position of flu-orene, that is, for the preparation of 9,9-bis(4-acetylphenyl)-2,7-dibromofluorene (2); in compar-ison, previously reported methods for the syn-thesis of 9,9-bis(4-acetylphenyl)fluorene require several synthetic steps.44,45 Subsequent
conden-sation of 2 with 2-aminobenzophenone, under the conditions of an acid-catalyzed Friedla¨nder reac-tion, furnished the desired dibromo monomer
3.46,47 The alternating polyfluorene copolymer
PF-Q was synthesized by performing a Suzuki coupling reaction between dibromide 3 and the diboronate 4,40with Pd(PPh
3)4as the catalyst in
a mixture of toluene and aqueous K2CO3(2.0 M)
in the presence of aliquat 336 as a phase transfer reagent. The structure of the polymer we obtained was confirmed by1H and13C NMR spectroscopy.
In the 13C NMR spectrum, two signals that
su-perimpose with the signals of the C-9 carbon atom of monomers 3 and 4 appear at␦ ⫽ 65.6 and 55.3; we ascribed these signals to the C-9 carbon atoms of the two different fluorene units in the copoly-mer.
To examine the steric requirements of the di-phenylquinoline pendent groups, we elucidated the molecular structure of monomer 3 in the solid state by X-ray crystallographic analysis. Single crystals were obtained by slow diffusion of hexane into a chloroform solution of 3; this monomer cocrystallized with a chloroform solvated mole-cule. Figure 1 displays the ORTEP plot of 3 de-termined by X-ray diffraction at 295 K. The aro-matic units of the diphenylquinoline group are aligned in a nonplanar arrangement: the substi-tuted phenyl group at the C-4 position is signifi-cantly twisted out of the plane of the quinoline ring, as evidenced by the large dihedral angle of about 50°. The key feature of this solid-state structure is that the incorporation of the two 2,4-diphenylquinoline groups at the C-9 position of the fluorene ring results in a rigid three-dimen-sional cardostructure that may prevent -stack-ing or excimer formation.
PF-Q is readily soluble in organic solvents such as chloroform and chlorobenzene, but it is soluble
Scheme 1
Figure 1. ORTEP diagram of monomer 3 determined by X-ray crystallography. All hydrogen atoms have been omitted for clarity.
in THF and toluene only on heating. The molec-ular weight of the polymer was determined by SEC analysis, with THF as the eluent and cali-brating against polystyrene standards. The poly-mer possesses a number-average molecular weight (Mn) of about 2.0⫻ 10
4
and a polydisper-sity index of 3.2. The thermal properties of PF-Q were investigated by TGA and DSC. As revealed by a TGA thermogram (Fig. 2), PF-Q exhibits good thermal stability, with 5% weight loss occur-ring at 460 °C. In the DSC measurements, we observed a distinct Tg at 207 °C (Fig. 3); in
trast, poly(9,9-dioctylfluorene) (POF), which con-tains two flexible n-octyl chains at the C-9 posi-tion of each repeating fluorene unit, exhibited its
Tgat 75 °C. 48
These results indicate clearly that the incorporation of the two rigid, nonplanar di-phenylquinoline moieties at the C-9 position of every alternate fluorene unit in the polymer back-bone does enhance the chain rigidity of PF-Q and restricts its segmental mobility, which signifi-cantly increases both the Tgand the thermal
sta-bility. Such a relatively high Tg, which could
pre-vent morphological change and suppress the for-mation of aggregates and excimers upon exposure to heat, is desirable for polymers used as emissive materials for light-emitting applications.49
Photophysical Properties
The optical properties of the quinoline-containing fluorene copolymer were investigated both in di-lute solution and in the solid state (Table 1). Fig-ure 4 presents the absorption and PL spectra of PF-Q, POF, and DPQ, which serves as a model compound in studying the optical properties of the quinoline pendants. In a chloroform solution, PF-Q exhibits its main absorption peak at 391 nm, with an absorption shoulder appearing at 347 nm. In comparison to the absorption spectra of POF and the model compound DPQ, we ascribe the band at 391 nm to a–*transition derived
from the conjugated polyfluorene backbone and the shorter-wavelength shoulder to an absorption originating from the pendent quinoline groups. On excitation of the PF main chain at 390 nm, the emission spectrum displays a vibronic fine struc-ture with two sharp bands appearing at 418 and 441 nm. This PL spectrum is nearly identical to that obtained from POF, which indicates that linking quinoline groups to fluorene units through their C-9 carbon atoms does not cause
Figure 2. TGA thermogram of PF-Q.
Figure 3. DSC data of PF-Q (heating rate, 20 °C/min) under nitrogen.
Figure 4. (a) UV–vis absorption (solid line), excita-tion (dashed line), and PL (solid and dotted line, excited at 390 and 347 nm, respectively) spectra of PF-Q in CHCl3solution. UV–vis absorption and PL spectra of
perturbation of the main chain’s conjugation. In addition, the good spectral overlap between the emission bands of the DPQ pendants (max
⫽ 405 nm) and the absorption band of the conju-gated main chain of PF-Q suggests that most of the excitons formed in the pendent groups by direct photoexcitation are likely to migrate to the PF main chain, from which emission occurs. As a result, the PL spectrum obtained on excitation of the DPQ moieties at 347 nm is the same as that obtained under excitation of the PF backbone at 390 nm; in contrast, no luminescence from the DPQ side chains was detected in the deep-blue region (350 – 400 nm). Furthermore, the excita-tion spectrum of PF-Q, monitored at 450 nm, is perfectly superimposed with its absorption spec-trum. This observation reveals that the Fo¨rster energy transfer from the excited pendant groups to the PF backbone is very efficient. PF-Q exhibits a high quantum yield (⌽f) in a diluted solution.
The PL quantum yield in toluene excited at 365 nm, was 1.0 as measured relative to 9,10-diphenylanthrancene (⌽f⫽ 0.9) as a standard50;
this value is even higher than that measured for POF (⌽f⫽ 0.85).
Figure 5 presents the absorption and PL spec-tra of a PF-Q film spin-coated from a toluene solution onto a quartz plate. In comparison to the dilute solution, the absorption spectrum of the thin film is slightly broadened, whereas the emis-sion spectrum displays a redshift of 8 –10 nm. The PL quantum yield of the film was estimated to be 0.54 by comparing its fluorescence intensity with that of the POF polymer thin film sample excited at 380 nm (⌽f⫽ 0.55).51It has been demonstrated
that a thin film of POF containing flexible alkyl
chains exhibits poor spectral stability upon expo-sure to heat.48 –54 To examine the effect that
in-corporating rigid and bulky DPQ pendent groups has on the thermal stability of PF-Q, the polymer film was heated for 20 h on a hot plate at 150 °C under a nitrogen atmosphere. The absorption and PL spectra were recorded when the films had cooled to room temperature. As indicated in Figure 5, both the absorption and PL spectra re-mained almost unchanged after this thermal treatment. In contrast, previous reports have shown that the annealing of a POF film results not only in a bathochromic shift in the PL wave-length and a significant reduction in emission intensity but also in the appearance of an additional emission band between 500 and 600 nm.28,59 The cause of the undesirable
emis-sive color instability of the POF film has been attributed to the formation of aggregates and in-terchain excimers or to keto defects.52–58 In the
case of PF-Q, because of the presence of the rigid, nonplanar DPQ pendent groups—the steric de-mand of which restricts close packing of the poly-mer chains and reduces the probability of inter-chain interactions—the tendency to form aggre-gates and excimers in the polymer film on thermal treatment is suppressed. The higher value of the Tg of PF-Q also accounts for the
enhanced thermal stability of the polymer film.
Electrochemical Studies
We employed CV to investigate the redox behav-ior of PF-Q and to estimate its highest occupied molecular orbital (HOMO) and LUMO energy lev-els. The electrochemical processes of this polymer film coated on a glassy carbon electrode were monitored in a standard three-electrode electro-chemical cell with ferrocene as the internal stan-dard in an electrolyte of 0.1 M TBAPF6in
aceto-nitrile at a scanning rate of 50 mV/s. By knowing the energy level of the ferrocene/ferrocenium ref-erence, one can calculate the LUMO and HOMO energies with the assumption that the energy level of ferrocene is 4.8 eV below a vacuum.60On
the basis of the onset potentials of the oxidation and reduction, which were 0.93 and ⫺2.52 V, respectively, we estimated the HOMO and LUMO energy levels of PF-Q to be 5.73 and 2.28 eV, respectively. For comparison, the same electro-chemical experiments were conducted with POF. The onset potentials of the oxidation and reduc-tion of POF were 0.96 and⫺2.74 V, respectively, and, in turn, we calculated the HOMO and LUMO
Figure 5. UV–vis absorption and PL spectra of PF-Q film before and after annealing at 150 °C for 20 h under a nitrogen atmosphere.
energy levels of POF to be 5.76 and 2.06 eV, respectively; these values are in good agreement with the data reported previously for POF (Ip
⫽ 5.8 eV; Ea ⫽ 2.12 eV).61 Figure 6 depicts the
relative HOMO/LUMO energy levels of PF-Q and POF. The markedly lower LUMO energy level of PF-Q that originates from the electron-deficient nature of the quinoline substituents, suggests there may be an increase in the electron affinity and an improvement in the electron injection of the polymer.
EL Properties of LED Devices
To evaluate the potential of PF-Q to behave as a blue emissive material in polymer LED applica-tions, we fabricated a polymer LED with the con-figuration ITO/PEDOT/PF-Q/TPBI/Mg:Ag. As in-dicated in the inset of Figure 7, the EL spectrum of PF-Q displays a maximum peak at 428 nm with a shoulder at 451 nm; no undesirable excimer/ aggregate emission is observed at a long wave-length as has been reported in the literature for the spectrum of POF. The EL spectrum is almost identical to the corresponding PL spectrum pre-sented in Figure 5; this finding indicates that both the PL and EL originate from the same ra-diative decay process of the singlet exciton. As displayed in Figure 7, the device based on PF-Q reaches its maximum brightness of 1121 cd/m2at
12 V and its maximum external quantum effi-ciency of 0.80% at 250 cd/m2, with a bias of 9.5 V.
This device demonstrates a much higher bright-ness and efficiency than does the device prepared from the unmodified POF that had a maximum brightness of about 600 cd/m2 and an external
quantum efficiency of 0.2%.51 We attribute the
improved device performance to the better charge injection and transport from PF-Q and to the ef-ficient energy transfer from the quinoline side chains to the PF main chains. In addition, the introduction of an electron injecting and trans-porting TPBI layer that is also used for hole blocking and exciton confinement may contribute to the high performance, even though we have used a high-work-function alloy of Mg:Ag as the cathode, rather than Ca. Moreover, the EL spec-tra of PF-Q exhibited no significant changes as the applied voltage was increased. Thus, the phe-nylquinoline pendent groups in PF-Q do simulta-neously provide both the functions of suppressing the aggregation and improving the charge injec-tion.
Figure 8. Solid-state PL of the PF-Q film and the normalized absorption and PL spectra of Os(fppz) re-corded in a CH2Cl2solution.
Figure 6. Relative HOMO/LUMO energy levels of PF-Q and POF.
Figure 7. Current density–voltage–luminance char-acteristics of ITO/PEDOT/PF-Q/TPBI/Mg:Ag. Inset: the corresponding EL spectra recorded at different ap-plied voltages.
The preparation of electrophosphorescent LEDs that use polymers as the host materials for their emitting layers is an attractive prospect be-cause such devices have the potential for applica-tions, among other things, in large-area devices prepared with simple processes. Efficient EL polymer-based devices have been prepared by
doping PF or poly(N-vinylcarbazole) (PVK) with triplet complexes.62– 64 For charge balance
pur-poses, some electron-transporting materials, such as 2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxa-diazole (PBD) or
3-(4-biphenylyl)-4-phenyl-5-(4-tert-butylphenyl)-1,2,4-triazole (TAZ), have also
been introduced to compensate for the poor electron-transporting ability of the host poly-mers.65,66In this study, we used PF-Q, which is possessed of PF backbones and DPQ as electron-transporting side chains, as the host material and doped it with 2.4 wt % of an efficient red-emitting phosphorescent dye, Os(fppz),67to realize red EL. The Os complex is chelated by two anions of 3-tri-fluoromethyl-5-(2-pyridyl) pyrazole (fppzH) to balance the ⫹2 charge on the metal center and two phosphine donors to complete the coordina-tion requirement. Its lifetime (0.7 s) is shorter than typical red emitting Ir(III) complexes.68This shorter triplet lifetime can help to minimize the exciton quenching by triplet–triplet annihilation at a high current density.69The PL spectrum of
PF-Q and the absorption spectrum of Os(fppz) (Fig. 8) overlap to a moderate extent in the region 400 –500 nm, which meets the requirement for efficient energy transfer. On photoexcitation at the absorption maximum of PF-Q, the PL profile of the blend contains two emission bands: one centered at about 440 nm that originates from the remaining emission of the PF-Q host, whereas the other, at about 624 nm, corresponds to the emis-sion of Os(fppz) (see the inset in Fig. 9). In con-trast, the host emission is quenched completely when stimulated by an electric field; this process resulted in red triplet emission from the Os com-plex that had Commission Internationale de L’Eclairage (CIE) color coordinates of (0.66, 0.34)
Figure 9. Current density–voltage–luminance char-acteristics of ITO/PEDOT/Os(fppz):PF-Q/TPBI/Mg:Ag. Inset: the PL spectrum of Os(fppz):PF-Q and the corre-sponding EL spectra recorded at different applied volt-ages.
Table 1. Photophysical Properties of PF-Q, POF, and DPQ
Solutiona Filmb Abs (nm) PL (nm)c ⌽ f d Abs (nm) PL (nm)c ⌽ f PF-Q (347) 391 418 (441) 1.00 (349) 390 428 (449) 0.54 POF 389 418 (442) 0.85 387 424 (448) 0.55e DPQ 340 405 aEvaluated in chloroform.
bEvaluated in the solid state and prepared from toluene solutions. cExcited at 390 nm.
dQuantum yield (⌽
f) determined in toluene, relative to 9,10-diphenylanthracence in
cyclohex-ane, with excitation at 365 nm.
eThe thin film quantum efficiency of POF, as measured in an integrating sphere, was 0.55. fPeaks that appear as shoulders or weak bands are provided in parentheses.
at 11 V. These results suggest that both Fo¨rster energy transfer and direct charge trapping/re-combination on the Os(fppz) guest are responsible for the observed EL.63,64Table 2 summarizes the
characteristics of the devices based on the doped and undoped polymers. The maximum external quantum efficiency of 6.63% (corresponding a lu-minance efficiency of 8.71 cd/A) is obtained from the doped device at a current density of 47.8 mA/ cm2, together with a brightness of 4163 cd/m2.
The maximum luminance for bright red emis-sion, as depicted in Figure 9, is found to be 10,457 cd/m2, at a bias of 18.5 V and a current
density of 161 mA/cm2. Furthermore, the turn-on
voltage for the host-only device is 7.2 V, whereas that for the doped device increased to 10.7 V. This observation also supports the proposed charge trapping mechanism.63,64
In summary, we have developed a direct and simple method for the synthesis of a PF copoly-mer, PF-Q, that contains two electron-deficient, 2,4-diphenylquinoline groups functionalized on the C-9 position of alternate fluorene units. Be-cause of the 3-D cardostructure, the PF exhibits a high Tgand very good thermal and spectral
sta-bility. The incorporation of pendent quinoline groups also results in improved electron injection and transport in PF-Q, without significantly al-tering the electronic properties of the conjugated backbone. An EL device based on PF-Q exhibited a stable blue emission with color coordinates of (0.16, 0.10), a maximum brightness of 1121 cd/m2,
and a maximum external quantum efficiency of 0.80%. Moreover, with PF-Q as the host material
and doping it with 2.4 wt % of a red-emitting osmium complex, we realized a red electrolumi-nescent device with a maximum brightness of 10,457 cd/m2and CIE coordinates of (0.66, 0.34);
these coordinates are very close to those of the standard red (0.67, 0.33) demanded by the Na-tional Television System Committee (NTSC). We thank the National Science Council for financial support. Our special thanks go to C.-H. Cheng, J.-P. Duan, and H.-T. Shih for their support and cooperation during the preparation and characterization of the light-emitting devices.
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