the Applications of Polymer Light-Emitting Diodes
PEI-HSUAN WANG, MING-SHOU HO, SHENG-HSIUNG YANG, KUEI-BAI CHEN, CHAIN-SHU HSU
Department of Applied Chemistry, National Chiao Tung University, Hsinchu 30010, Taiwan, Republic of China
Received 22 August 2009; accepted 27 August 2009 DOI: 10.1002/pola.23712
Published online in Wiley InterScience (www.interscience.wiley.com).
ABSTRACT:Three polyfluorene derivatives which have oxetane-containing phenyl group at C-9 position were synthesized via the palladium-catalyzed Suzuki-coupling reaction. The synthe-sized polymers PFB, PFG, and PFR emit blue, green, and red light, respectively. A double-layer device with the configuration of ITO/PEDOT/polymer/Ca/Al using PFB as the active layer showed a threshold voltage of 5 V, a maximum brightness of 2030 cd/m2, and a maximum current efficiency of 0.35 cd/A. Using PFG as the active layer, the device exhibited a threshold voltage of 6 V, a maximum brightness of 6447 cd/m2, and a maximum current efficiency of 1.27 cd/A. Using PFR as the active layer, the device showed a threshold voltage of 4 V, a maximum brightness of 2135 cd/m2, and a maximum current
efficiency of 0.16 cd/A. Better electroluminescent performance was also found based on different design of device structures. Due to photo-crosslinking property of oxetane groups, the UV-exposed thin films are insoluble in common organic solvents. A device comprised of blue, green, and red-emissive pixels was successfully fabricated by spin-coating and photo-lithographic processes. In addition, a white light-emitting device with CIE coordinate of (0.34, 0.33) was achieved by blending PFR into a host material PFB as the active layer.VC 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 516–524, 2010
KEYWORDS:conjugated polymers; fluorescence; photopolymeri-zation; PLED
INTRODUCTION Conjugated polymers have received broad interest recently in academic as well as in industrial research communities.1These polymer materials represent promising technology for large, flexible, and lightweight device applica-tions, such as polymer light-emitting diode (PLED),2 organic thin-film transistor (OTFT),3 and photovoltaic cell.4 The PLED device was first demonstrated by the Cambridge group in 1990 by using poly(p-phenylenevinylene) (PPV) as emit-ting layer.5Thep–p* transitions of conjugated polymers with aromatic and/or heterocyclic units generally lead to the opti-cal band gap with an absorption peak of 300–600 nm.6The color purity, quantum efficiency (QE) of light emission, turn-on voltage, and stability of the devices must be optimized for PLEDs to be applicable to commercial light-emitting devices. A large number of electroactive- and photoactive-conjugated polymers have been introduced during the past few years,7 such as PPV,8 poly(p-phenylene) (PPP),9 polythiophene (PT),10and polyfluorene (PF).11
PFs, with their high photoluminescence (PL) and electrolumi-nescence (EL) efficiencies are among the most promising candidates as blue light-emitting polymers.12,13 PFs can also be used as host material for internal color conver-sion14,15and polarized light emission16,17 for exploiting the
liquid crystalline nature of most of the PFs. Although PFs serve as excellent materials for light-emitting application,
the so-called keto effect usually occurs with alkyl chains at the C-9 position,18,19 which brings some drawbacks. The keto-defect form at higher temperatures or under long-term bias operation. The formation of fluorenone produces an additional emission band at longer wavelengths around 540 nm and changes the emissive color of the original PF. Besides, the aggregate/excimer formation also reduces de-vice performance. Several modifications were proposed to reduce the aggregation and keto effect. The introduction of a spiro structure at the C-9 position is an effective design to suppress keto defect.
The fabrication of PLEDs by processing the active materials from solution promises to be less expensive than that of OLEDs, where deposition of the active layers requires the use of vacuum technology. The solution process used in PLEDs has three approaches. The first approach is spin coat-ing which can easily produce large size devices with low cost. However, pixilated matrix displays are difficult to fabri-cate because of solution mixing problem. The second approach is ink-jet printing which is promising manufacture multicolor display.20 In the other way, the wettability of the substrate has to be preadjusted (patterned by photoresist technique) to place the drops at the precise location (wetting of pixel, but not of bank). Often inhomogeneous films are obtained after drying. The third approach is
photo-Correspondence to: C.-S. Hsu (E-mail: [email protected])
lithography. By choosing suitable photo-crosslinking groups into polymer chain and mask designed, the light-emitting polymer can be crosslinked photochemically to produce in-soluble polymer networks in desired areas.21
In this article, we demonstrate the synthesis and characteri-zation of three fluorene-based copolymers containing pend-ent oxetane groups. The thermal, electrochemical, optical, and other relevant physical properties of the synthesized polymers were systematically studied. Double-layer PLED devices with the configuration of ITO/PEDOT/polymer/Ca/Al were also fabricated to study their device performance. Because of the photoresist-type properties, a PLED prototype with three emission colors on one substrate was successfully fabricated.
EXPERIMENTAL
Characterization Methods
FTIR spectra were recorded on a Perkin-Elmer Spectrum One FTIR spectrometer with KBr pellets. 1H NMR spectra were measured with a Varian 300 MHz spectrometer. Gel permeation chromatography (GPC) data assembled from a Viscotek T50A Differential Viscometer and a LR125 Laser Refractometer and three columns in series were used to measure the molecular weights of polymers relative to poly-styrene standards at 35C. Differential scanning calorimetry (DSC) was performed on a Perkin-Elmer Pyris Diamond DSC instrument at a scan rate of 10C min1. Thermal gravimet-ric analysis (TGA) was undertaken on a Perkin-Elmer Pyris 1 TGA instrument with a heating rate of 10C min1. UV–Vis absorption spectra were obtained with an HP 8453 diode array spectrophotometer. PL emission spectra were obtained using ARC SpectraPro-150 luminescence spectrometer. A degassed toluene solution was used to determine the PL quantum yields (uf). The concentration was adjusted to
reduce the absorbance of the solution below 0.1, and the ex-citation was performed at the corresponding kex,max. A
solu-tion of quinine sulfate in 1 N H2SO4, which has aufof 0.546
(kabs¼ 365 nm), was used as a standard. Cyclic
voltammet-ric measurements of the material were made in acetonitrile with 0.1 M tetrabutylammonium hexafluorophosphate (TBAPF6) as the supporting electrolyte at a scan rate of 80
mV s1. Platinum wires were used as both the counter and working electrodes, and silver/silver ions (Ag in 0.1 M AgNO3 solution, from Bioanalytical Systems, Inc.) was used
as the reference electrode. Ferrocene was used as an internal standard, and the potential values were obtained and con-verted to vs SCE (saturated calomel electrode).
Materials
Starting materials such as 2,7-dibromofluorene, n-butyl lithium, and 2-isopropoxy-4,4,5,5-tetramethyl-1,3,2-dioxabor-olane were purchased from Aldrich. All other chemicals, reagents, and solvents were used as received from commer-cial sources without further purification, except tetrahy-drofuran (THF) and toluene which were distilled over sodium-benzophenone and calcium hydride, respectively. The Suzuki-coupling catalyst reagent
tetrakis(triphenylphosphi-ne)palladium(0) was purchased from TCI and handled under inert argon atmosphere. The syntheses of monomers M1 and M2 are shown in Scheme 1. Monomers M3–M5 were synthe-sized according to the previous literatures.22–24
Synthesis of 2,7-Dibromo-9-fluorenone (1)
To a solution of fluorenone (50 g, 0.28 mole) and FeCl3
(2.28 g, 14 mmole) in CHCl3(300 mL), bromine (35 L, 0.69
mole) in CHCl3 (100 mL) were slowly added. The reaction
mixture was stirred at 25C for 48 h, and then poured into sodium thiosulfate solution (300 mL) until the red color of bromine disappeared. The organic layer was dried over MgSO4, and the solvent was removed in vacuo. The residue
was purified by column chromatography (SiO2, hexane) to
yield 15.71 g (81%) yellow solid, mp: 204C.1H NMR (300 MHz, DMSO, d, ppm): 7.81 (m, 4H, aromatic protons), 7.73 (s, 2H, aromatic protons).
Synthesis of 2,7-Dibromo-9,90-bis(4-hydroxyphenyl)] Fluorine (2)
To a solution of 2,7-dibromofluorenone (5 g, 14.8 mmole) and phenol (10 g, 106.3 mmole) under nitrogen atmosphere, Eaton’s reagent (70 mL) was slowly added. The reaction mix-ture was stirred at 100 C for 1 h and then poured into water. The organic layer was dried over MgSO4and
concen-trated in vacuo. The residue was purified by column chroma-tography (silica gel, EA:hexane ¼ 1:3 as eluent) to yield 6.1 g (81%) white solid, mp: 59 C. 1H NMR (300 MHz, DMSO,
d, ppm): 9.41 (s, 2H, aromatic protons), 7.89 (d, 2H, aromatic protons), 7.57 (dd, 2H, aromatic protons), 7.47 (d, 2H, aro-matic protons), 6.87 (d, 4H, aroaro-matic protons), 6.65 (d, 4H, aromatic protons).
Synthesis of 2,7-Dibromo-9,9-di(4-hexylphenyl)-9 H-fluorene (3)
To a solution of (2) (5 g, 9.84 mmole), potassium hydride (1.66 g, 29.52 mmole) and potassium iodide (0.33 g, 1.97 mmole) in acetonitrile (40 mL) was added bromohexane (4.05 g, 24.6 mmole) at 80 C. The reaction mixture was refluxed for 24 h, and then poured into water. The organic layer was dried over MgSO4, and the solvent was removed in
vacuo. The residue was purified by column chromatography (silica gel, hexane as eluent) to yield 5.3 g (89%) white solid, mp: 41C.1H NMR (300 MHz, CDCl3,d, ppm): 7.56–6.73 (m,
14H, aromatic protons), 3.91–3.86 (t, 4H, OACH2
A(CH2)4ACH3), 2.30–1.20 (m, 16H, OACH2 A(CH2)4ACH3),
0.94–0.82 (t, 6H,A(CH2)4ACH3).
Synthesis of 2-[9,9-Di[4-(hexyloxy)phenyl]-7-(4,4,5,5-tetramethyl-1,3,2- dioxaborolan-2-yl)-9H-2-fluorenyl]-4,4,5,5-tetramethyl-1,3,2-dioxaborolane (M1)
To a cooled solution (78C) of (3) (5 g, 7.39 mmole) in 20 mL dry THF was added 1.6 M n-butyllithium/hexane (11.55 mL, 18.48 mmole) dropwise. The mixture was kept at 78
C for and stirred 1 h. A solution of
2-isopropoxy-4,4,5,5-tet-ramethyl-1,3,2-dioxaborolane (4.52 mL, 22.17 mmol) was then slowly added in 10 min and the reaction mixture was stirred overnight. After extracting with ether and water, the collected organic phase was dried over MgSO4 and
from hexane to yield 3.5 g (61%) white crystals, mp: 245C.
1
H NMR (300 MHz, CDCl3,d, ppm): 7.77–6.69 (14H, aromatic
ring), 3.88–3.84 (t, 4H, OACH2A(CH2)4ACH3), 1.71–1.68 (m,
16H, OACH2A(CH2)4ACH3), 1.29 (s, 24H, CH3), 0.86–0.84 (t,
6H,A(CH2)4ACH3).
Synthesis of 3-(((6-Bromohexyl)oxy)methyl)-3-methyloxetane (4)
A mixture of tetrabutylammonium chloride (TBACl) (0.5 g), hexane (120 mL), 50 wt % NaOH water solution (120 mL), dibromohexane (60.25 mL, 391.6 mmole), and 3-methyl-3-oxetanemethanol (10 g, 97.91 mmole) was stirred at room temperature for 24 h, followed by refluxing at 75 C for additional 4 h. After cooling the reaction mixture to room temperature, the mixture was extracted with hexane and water. The organic phase was washed with saturated NaCl(aq), dried over MgSO4, and concentrated to give the crude product. Further purification was carried out by distil-lation in vacuum (2 mmHg, 90 C) to give 18.2 g (70.32 %) yellow oil. 1H NMR (300 MHz, CDCl3, d, ppm): 0.97 (s,
3H,ACH3), 1.26–1.76 (m, 6H, CH2A(CH2)3ACH2), 3.26–3.36
(m, 6H, ACH2 AOACH2A(CH2)4ACH2ABr), 4.19–4.38 (m,
4H, ring protons).
Synthesis of 3-({[6-(4-{3,6-Dibromo-9-[4-({6-[(3-methyl- 3-oxetanyl)methoxy]hexyl}oxy)phenyl]-9H-9-fluorenyl}-phenoxy)hexyl]methyl}-3-methyloxetane (M2)
To a solution of M1 (6.9 g, 13.58 mmole), potassium hydride (2.29 g, 40.74 mmole) and potassium iodide (0.46 g, 2.72 mmole) in acetonitrile (50 mL) was added (4) (9 g, 2.72 mmole) slowly. The reaction mixture was then refluxed for
24 h. After cooling to room temperature, the mixture was poured into water. The organic layer was dried over MgSO4
and concentrated in vacuo. The crude product was purified by column chromatography (silica gel, EA:hexane ¼ 1:5 as eluent) to yield 9.1 g (76.4%) colorless oil. 1H NMR (300 MHz, CDCl3,d, ppm): 7.93–6.80 (m, 14H, aromatic protons),
4.33–4.14 (s, 8H, ring protons), 3.91–3.87 (t, 4H), 3.42–3.27 (m, 8H, ACH2AOACH2), 1.98–1.14 (m, 16H, CH2A(CH2)3
ACH2), 0.84–0.82 (s, 6H,ACH3).
General Synthetic Procedure for Polymers PFB, PFG, and PFR
Three polymers PFB, PFG, and PFR which emit blue, green, and red light, respectively, were prepared via palladium-cata-lyzed Suzuki-coupling reaction.25Scheme 2 outlines the syn-theses of the three synthesized polymers. A typical synthetic procedure for PFB is given as follows. To a 50 mL two-necked round flask was added M2 (1 g, 1.07 mmole), M1 (0.59 g, 0.85 mmole), M3 (0.37 g, 0.84 mmol), and 15 mL of toluene. Pd(PPh3)4, K2CO3(aq) (2.0 M), and aliquat 336 were
then added under an argon atmosphere. The mixture was refluxed with vigorous stirring at 85 C for 4 days. N,N-Bis(4-methylphenyl)-N-(4-bromophenyl)amine and N,N-bis(4-methylphenyl)- N-[4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)phenyl] amine were sequentially added into the mixture and stirred for additional 6 h. After cooling to room tempera-ture, the reaction mixture was poured into methanol to pre-cipitate polymer which was purified by dissolving in THF and reprecipitated from methanol several times. After drying under vacuum for 24 h, the polymer was obtained as a light-green solid with yield of 62–90%.
Polymer PFB
1H NMR (300 MHz, CDCl
3,d, ppm): 0.83–2.15 (m, alkyl
pro-tons), 3.40–3.88 (m, ACH2AOACH2A), 4.30–4.47 (m, ring
protons), 6.71–7.76 (m, aromatic protons). Elemental Anal. Calcd. for C143H165O10N1: C, 83.87; H, 7.63; N, 0.68. Found:
C, 81.11; H, 7.56; N, 0.96. Polymer PFG
1
H NMR (300 MHz, CDCl3,d, ppm): 0.83–1.70 (m, alkyl
pro-tons), 3.40–3.86 (m, ACH2AOACH2A), 4.30–4.47 (m, ring
protons), 6.71–7.73 (m, aromatic protons). Elemental Anal. Calcd. for C4179.5H4973O397N1S0.5: C, 81.76; H, 7.84; N, 0.02.
Found: C, 78.94; H, 7.57; N, 0.66. Polymer PFR
1
H NMR (300 MHz, CDCl3,d, ppm): 0.71–1.70 (m, alkyl
pro-tons), 3.40–3.86 (m, ACH2AOACH2A), 4.30–4.47 (m, ring
protons), 6.71–7.71 (m, aromatic protons). Elemental Anal. Calcd. for C338.5H390O25N4S2.5: C, 81.80; H, 7.41; N, 1.13.
Found: C, 79.77; H, 7.33; N, 1.45.
Device Fabrication
Double-layer devices were fabricated as sandwich structures between calcium (Ca) cathodes and indium tin oxide (ITO) anodes. ITO-coated glass substrates were cleaned sequen-tially in ultrasonic baths of detergent, 2-propanol/deionized water (1:1 volume) mixture, toluene, deionized water, and acetone. A 50 nm-thick hole injection layer of poly(ethylene-dioxythiophene) (PEDOT) doped with poly (styrenesulfonate) (PSS) was spin-coated on top of ITO from a 0.7 wt % disper-sion in water and dried at 150C for 1 h in a vacuum. Thin films of synthesized polymers were spin-coated from toluene solutions onto the PEDOT layer and dried at 100 C for 1 h in a vacuum. The polymer solution contains diphenyiodo-nium hexafluoroarsenate (6 wt %) as photo initiator. The thickness of the active layer was 50 nm. The active layer was irradiated (standard handheld UV lamp, k ¼ 302 nm) for about 20 s in inert gas atmosphere at room temperature. Afterward, to further advance the crosslinking process in the growing network, the active layer was annealed at 120 C for 30 min. The active layer was rinsed with THF and finally heated to 120 C for 15 min to remove radical cations SCHEME 2Synthesis of polymers PFB, PFG, and PFR.
formed upon polymerization. The resulting polymer net-works were found to be insoluble in any common solvent. Finally, 35 nm Ca and 100 nm Al electrodes were made through a shadow mask onto the polymer films by thermal evaporation using an AUTO 306 vacuum coater (BOC Edwards, Wilmington, MA). Evaporations were carried out typically at base pressures lower than 8 107 torr. The active area of each EL device was 4 mm2.
Synthesis and Characterization
The monomers M1 and M2 containing two phenyl groups at C-9 position instead of alkyl chain were designed to suppress the keto effect. Triphenylamine(TPA) compounds are well-known hole-transporting materials. TPA-containing monomer M3 was introduced to fine-tune the EL emission wavelength of polyfluorenes in the range 450–470 nm and substantially improve the EL performance. Monomer M4 containng benzo-thiodiazole (BTDZ) structure was incorporated and yellow-green light was emitted via energy transfer from fluorene to BTDZ moiety. Monomer M5 containing BTDZ and two addi-tional thiophene rings to elongate the conjugated length and thus the emitting wavelength can be further shifted to red light region. The chemical structures of the resulting poly-mers were confirmed by 1H NMR and elemental analyses. The actual ratio calculated from elemental analyses of C, H, and N was in good agreement with the feeding ratios of the monomers. Slight deviations might be caused by two end-capping reagents (N,N-bis(4-methylphenyl)-N-(4-bromo-phenyl) amine and N,N-bis(4-methylphenyl)-N-[4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-phenyl] amine. The ob-tained polymers are completely soluble in common organic solvents, such as toluene, THF, and chloroform. The aver-age molecular weights are summarized in Table 1. It can be seen that Mn and Mw values of these polymers are in
the range from 1.0 104to 2.2 104and from 1.2 104 to 3.42 104, respectively, with polydispersity index less
than 2.
Thermal Properties
Thermal stability of conjugated polymers is an important fac-tor for PLED application, which is generally evaluated by DSC and TGA techniques. The thermal decomposition tem-peratures (Td, 5% weight loss) and the glass transition
tem-peratures (Tg) of three polymers are also listed in Table 1.
All three polymers show a Tg value above 80 C, which is
higher than that of poly(9,9-dioctylfluorene) (Tg ¼ 51 C).26
It is well-known that high Tgvalue is essential for polymers
to be used as emissive materials. In addition, these polymers also exhibit high Td value over 380 C, indicating good
potential use in light-emitting devices. Optical Properties
The fluorescent behavior of a polymer fundamentally depends on the band gap between the HOMO and LUMO, which depends on delocalization of p-electrons along the polymer backbone. The characteristic peak in the emission spectrum arises as an excited electron relaxes into vibronic energy level of the HOMO. The optical spectra of conjugated polymers are usually broad, which is due to the coupling of several different vibronic modes. Figure 1 shows the normal-ized UV–Vis absorption and PL spectra of three polymers in thin-film state. The wavelengths of UV–Vis absorption and PL emission maxima of these polymers in dilute toluene solu-tion (105 M) and in film state are summarized in Table 2. The absorptions located in the range from 381 to 387 nm were due to p–p* transition of conjugated backbones. For polymers PFB and PFG, no shoulder peak in UV–Vis absorp-tion region was observed. For polymer PFR, two slight shoulders (460 and 525 nm) were found, resulting from the BTDZ and thiophene-BTDZ-thiophene moieties. The blue emission of polyfluorene around 440 nm could be effectively quenched by introducing BTDZ group into the polymer main chain, tuning emission to green light region around 517 nm. It also reveals the effective energy transfer from photo-TABLE 1Average Molecular Weights and Thermal Properties
of Polymers PFB, PFG, and PFR Polymer Mn(104)a Mw(104) PDI Td(C)b Tg(C)c PFB 1.0 1.2 1.15 371 90 PFG 1.85 3.42 1.8 385 102 PFR 2.2 2.44 1.11 399 83 a
Determined by GPC by eluting with THF, by comparison with polysty-rene standards.
b
Temperature at which a 5% weight loss occurred was determined at a heating rate of 10C/min under a nitrogen atmosphere.
c
The value ofTgwas determined at a heating rate of 20C/min under a nitrogen atmosphere.
FIGURE 1UV–Vis absorption and PL spectra of polymers PFB, PFG, and PFR in thin-film state.
TABLE 2 Optical Properties of Polymers PFB, PFG, and PFR
Polymer
UV(kmax(nm)) PL(kmax(nm))
PLeff(%) Toluene Toluene Film Toluene Film
PFB 380 383 442 448 88
PFG 387 395 419 (440) 517 87
generated excitons on the polyfluorene segment to BTDZ groups. By incorporating thiophene-BTDZ-thiophene moieties into polymer main chains, PL emission was further red-shifted to 640 nm in thin-film state. Table 2 also outlines the PL quantum efficiencies of three polymers as measured by an integrating sphere. These polymers exhibit moderate PL QE values from 23 to 87% in toluene solution.
Annealing Experiment
According to the previous literature,27when the temperature inside PLED devices exceeds 86 C under a high
forward-bias condition, the optical properties, especially lumines-cence efficiency and color stability of light-emitting material are strongly affected. PLED devices using polyfluorene as light-emitting layer usually show a green emission under a high forward-bias condition. The origin of the green emission in polyfluorene-based conjugated polymers has been mostly attributed to aggregation and/or excimer formation or keto defects caused from the formation of fluorenone units in the PF backbone.28–31The utility of polyfluorenes is thus limited because of the tendency to undergo interchain aggregation. Furthermore, when the solid films of the polymers are main-tained under the higher temperature above Tg, the
keto-defect would appear and emit green light around 550 nm. This is because the fluoreone group exhibits lower energy band gap and the energy transfer from fluorenyl ring to flu-oreone moiety is very efficient. To investigate the thermal stability and the keto effect of the prepared polymers, thin films were annealed at 250 C in air for 3 h. For compari-son, a known polyfluorene derivative PFC6 without phenyl ring at C-9 position of fluorene ring was also prepared and tested. The chemical structure of PFC6 is shown in Chart 1.
FIGURE 2PL spectra of polymers (A) PFC6 and (B) PFB in thin-film state before and after annealing at 250C for 3 h in air.
CHART 1Chemical structure of PFC6.
FIGURE 3 FTIR spectra of polymers PFC6 and PFB (A) at 25C (B) after annealing at 250C for 3 h.
Figure 2(A,B) shows the normalized photoluminescence (PL) spectra of polymers PF1 and PFB films at room temperature and thermally annealed at 250C for 3 h. In Figure 2(A), the PFC6 film annealed at 250C for 3 h showed a strong green emission at a longer wavelength of about 559 nm, which is attributed to the fluoreone emission when compared with the fresh films with max emission kmaxband located at 448 nm.
Turning to polymer PFB, the PL spectrum of PFB showed a shoulder around 500 nm under similar heating conditions. However, the max emission kmax remained at 448 nm,
sug-gesting better thermal stability against annealing process. It is generally thought that defect phenomena are generally appeared for those alkyl-substituted polyfluorene, such as PFC6. This unchanged PL curve of polymer PFB strongly hints that polyfluorene with phenyl group attached at C-9 position may inhibit the aggregation of polymer chains. Figure 3 shows the FTIR spectra of PF1 and PFB films after annealed at 250
C for 3 h. No obvious fluorenone (C¼¼O) characteristic IR
peak at around 1720 cm1 peak was found for PFB. The above observations suggest that incorporation of phenyl group at C-9 position could effectively suppress the keto formation and aggregation of polymer chains.
Electrochemical Properties
The electrochemical behaviors of these polymers were inves-tigated by cyclic voltammetry (CV). The corresponding high-est-occupied molecular orbital (HOMO) and lowest-unoccu-pied molecular orbital (LUMO) energy levels were estimated from the onset of the redox potentials. HOMO and LUMO energy levels of the chromophores were calculated according to empirical formulaeEHOMO¼ (Eoxþ 4.4) (eV) and ELUMO
¼ (Ered þ 4.4) (eV).32 From the difference of the onset
potentials and the optical band gaps (Eg) calculated from the onsets of UV absorption spectra, the energy levels, the oxidation, and reduction potentials were determined and are summarized in Table 3. Polyfluorenes usually show an onset in oxidation curve around 1.4 V, which can be assigned to the p-doping potential of polyfluorene main chains.
Mean-while, the electrochemical oxidation of polymers exhibit peak at 1.8 V versus Ag/Agþ, and the characteristic peak is origi-nated from phenyl-fluorene units. However, the BTDZ moiety in polymers PFG and PFR is more electron-deficient and eas-ier to be reduced when compared with PFB. Since BTDZ has a smaller gap than polyfluorene, the excitation energy on the fluorene segments can be transferred to BTDZ unit effi-ciently. For PFR, the HOMO and LUMO energy levels were shifted lower because of adding two thiophene groups on both sides of BTDZ moiety. The incorporation of thiophene-BTDZ-thiophene moiety onto polymer chains increases the conjugated length and reduces the optical band gap as well. Electroluminescent Properties
The EL properties of the copolymers were investigated using a double-layer device with the configuration of ITO/PEDOT/ polymer/Ca/Al. The maximum EL emission bands and CIE’1931 coordinates of the three devices were recorded in Table 4. It is clear that PFB, PFG, and PFR devices emitted blue, green, and red light as expected. The turn-on voltage is measured to be 4–5 V. The PFB device showed a maximum brightness of 2030 cd/m2(at 12 V) and a maximum current efficiency of 0.35 cd/A (303 mA/cm2). The PFG device revealed a maximum brightness of 6447 cd/m2 (at 11 V) and a maximum current efficiency of 1.27 cd/A (507 mA/ cm2). The PFR device showed a maximum brightness of 2135 cd/m2 (at 13 V) and a maximum yield of 0.16 cd/A (1334 mA/cm2). It is worthy to note that PFB and PFR
devi-ces emitted deep blue and red light, from CIE’1931 coordi-nates of (0.15, 0.14) and (0.68, 0.32), respectively. Since these light-emitting polymers contain photo-crosslinable oxe-tane groups, photo-lithography process was substantially investigated to obtain blue, green, and red pixelated devices. For this purpose, organic layers were irradiated by UV light through a shadow mask. After curing, the films were devel-oped by immersing them into THF to dissolve nonirradiated part. By sequentially developing green, blue, and red part through photo-lithographic process, three different light TABLE 3Electrochemical Properties of PFB, PFG, and PFR
Sample Optical Band Gapa(eV) Eox.onsetb(eV) Ered.onsetb(eV) HOMOc(eV) LUMOc(eV)
PFB 2.85 (435) 1.35 1.5 5.75 2.9
PFG 2.85 (435) 1.37 1.48 5.77 2.92
PFR 1.87 (663) 1.39 0.48 5.79 3.92
a
The optical band gap estimated from the onset wavelength (value in parentheses) of UV–Vis spectra of the polymer film.
b
TheEoxand theEredare the onset potentials of oxidation and reduc-tion, respectively.
c
Calculated from the empirical formula,E(HOMO)¼ (Eoxþ 4.40) (eV), E(LUMO)¼ (Eredþ 4.40) (eV).
TABLE 4Device Performance of Polymers PFB, PFG, and PFR
Polymer Turn-On Voltage (V) Max Brightness, cd/m2@ V Yieldmax, cd/A EL Spectrum, kmax(nm) CIE 1931 (x, y) PFB 5 2,030 0.35 450 (0.15, 0.14) PFG 6 6,447 1.27 520 (0.28, 0.49) PFR 4 2,135 0.16 656 (0.68, 0.32)
emitted on the same substrate was successfully achieved, as shown in Figure 4.
White Polymer Light-Emitting Diode
White light organic light-emitting devices (WOLEDs) have attracted a lot of interest because of their potential applica-tions in solid-state-lighting, maskless full color OLED fabrica-tion with color filter, as well as backlights for liquid crystal displays. Here, we present an easy and effective way to pro-duce white light PLED devices by blending method. Blue light-emitting PFB was used as host matrix, while 2% PFR was selected as guest material. A similar double-layer device with the configuration of ITO/PEDOT/98% PFB þ 2% PFR/ Ca/Al was fabricated. Pure white light with CIE’1931 coordi-nate (0.34, 0.33) was revealed under bias operation of 11 V. Figure 5 shows the EL emission spectra of the device under different voltage. It should be noted that the device emitted dominate red light at lower voltage of 7 V and blue light at
higher voltage over 13 V. Figure 6 depicts luminance-volt-age-current efficiency characteristics of the device, showing a maximum brightness of 1500 cd/m2and a maximum current efficiency of 0.46 cd/A.
CONCLUSIONS
In conclusion, three oxetane-containing polyfluorene deriva-tives which emit blue, green, and red light were synthesized and characterized. FTIR and PL spectral experiments demon-strate that the incorporation of the phenyl group at C-9 posi-tion of fluorene moiety could effectively suppress the keto-defect. Because of the photo-crosslinking property of oxetane groups, the UV-exposed thin films are not soluble in common organic solvents. A device containing blue, green, and red-emissive pixels was fabricated by spin-coating and photo-li-thography processes. In addition, a pure white light-emitting device was demonstrated by blending 2% PFR into host materials PFB as the active layer, with CIE coordinate of (0.34, 0.33).
REFERENCES AND NOTES
1 Heeger, A. J. Solid State Commun 1998, 107, 673–679. 2 Ziemelis, K. Nature 1999, 399, 408–411.
3 Dang, T. T. M.; Park, S. J.; Park, S. J.; Park, J. W.; Chung, D. S.; Park, C. E.; Kim, Y. H.; Kwon, S. K. J Polym Sci Part A: Polym Chem 2007, 45, 5277–5284.
4 Bundgaard, E.; Krebs, F. C. Sol Energy Mater Sol Cells 2007, 91, 954–985.
5 Burroughes, J. H.; Bradley, D. D. C.; Brown, A. R.; Marks, R. N.; Mackay, K.; Friend, R. H.; Burns, P. L.; Holmes, A. B. Nature 1990, 347, 539–541.
6 Kim, D. Y.; Cho, H. N.; Kim, C. Y. Prog Polym Sci 2000, 25, 1089–1139.
7 Kraft, A.; Grimsdale, A. C.; Holmes, A. B. Angew Chem Int Ed 1998, 37, 402–428.
FIGURE 4Photograph of an RGB (red, green, blue) device. Dimensions of the pixels are 20 20 (mm mm).
FIGURE 5EL spectra of the device: ITO/PEDOT/blend of PFB, PFG, and PFR/Ca/Al at the different voltages.
FIGURE 6I-V-L curves of the device with ITO/PEDOT/blend of PFB, PFG, and PFR/Ca/Al configuration.
8 Chen, K. B.; Chen, M. H.; Yang, S. H.; Hsieh, C. H.; Hsu, C. S.; Chen, C. C.; Chieh, H. J. J Polym Sci Part A: Polym Chem 2006, 44, 5378–5390.
9 Kranzelbinder, G.; Leising, G. Rep Prog Phys 2000, 63, 729–762.
10 Pei, J.; Yu, W. L.; Ni, J.; Lai, Y. H.; Huang, W.; Heeger, A. J. Macromolecules 2001, 34, 7241–7248.
11 Kameshima, H.; Nemoto, N.; Endo, T. J Polym Sci Part A: Polym Chem 2001, 39, 3143–3150.
12 Pei, Q.; Yang, Y. J Am Chem Soc 1996, 118, 7416–7417. 13 Leclerc, M. J Polym Sci Part A: Polym Chem 2001, 39, 2867–2873.
14 Virgili, T.; Lidzey, D. G.; Bradley, D. D. C. Adv Mater 2000, 12, 58–62.
15 Lane, P. A.; Palilis, L. C.; O’Brien, D. F.; Giebeler, C.; Cadby, A. J.; Lidzey, D. G.; Campbell, A. L.; Blau, W.; Bradley, D. D. C. Phys Rev B 2001, 63, 235206-1–8.
16 Oda, M.; Nothofer, H.-G.; Lieser, G.; Scherf, U.; Meskers, S. C. J.; Neher, D. Adv Mater 2000, 12, 362–365.
17 Contoret, A. E. A.; Farrar, S. R.; Jackson, P. O.; Khan, S. M.; May, L.; O’Neill, M.; Nicholls, J. E.; Kelly, S. M.; Richards, G. J. Adv Mater 2000, 12, 971–974.
18 Scherf, U.; List, E. J. W. Adv Mater 2002, 14, 477–487. 19 Wu, F. I.; Reddy, D. S.; Shu, C. F.; Liu, M. S.; Jen, A. K.-Y. Chem Mater 2003, 15, 269–274.
20 Gans, B.-J. G.; Duineveld, P. C.; Schubert, U. S. Adv Mater 2004, 16, 203–213.
21 Mu¨ller, C. D.; Falcou, A.; Reckefuss, N.; Rojahn, M.; Wieder-hirn, V.; Rudati, P.; Frohne, H.; Nuyken, O.; Becker, H.; Meerholz, K. Nature 2003, 421, 829–833.
22 Ranger, M.; Rondeau, D.; Leclerc, M. Macromolecules 1997, 30, 7686–7691.
23 Millard, I. S. Synth Met 2000, 111, 119–123.
24 Pilgram, K.; Zupan, M.; Sliles, R. J Heterocycl Chem 1970, 7, 629–633.
25 Miyaura, N.; Suzuki, A. Chem Rev 1995, 95, 2457–2483. 26 Ding, J.; Day, M.; Robertson, G.; Roovers, J. Macromole-cules 2002, 35, 3474–3483.
27 Zhou, X.; He, J.; Liao, L. S.; Lu, M.; Ding, X. M.; Hou, X. Y.; Zhang, X. M.; He, X. Q.; Lee, S. T. Adv Mater 2000, 12, 265–269. 28 Gaal, M.; List, E. J. W.; Scherf, U. Macromolecules 2003, 36, 4236–4237.
29 Bliznyuk, V. N.; Carter, S. A.; Scott, J. C.; Kla¨rner, G.; Miller, R. D.; Miller, D. C. Macromolecules 1999, 32, 361–369.
30 Pei, J.; Yu, W. L.; Huang, W.; Heeger, A. J. Chem Commun 2000, 1631–1632.
31 Yu, W. L.; Pei, J.; Cao, Y.; Huang, W.; Heeger, A. J. Chem Commun 1999, 1837–1838.
32 Leeuw, D. M.; Simenon, M. M. J.; Brown, A. R.; Einer-hand, R. E. F. Synth Met 1997, 87, 53–59.
