Fluorene-Based Alternating Polymers ContainingElectron-Withdrawing Bithiazole Units: Preparation andDevice ApplicationsJAEMIN LEE,1,2 BYUNG-JUN JUNG,1,2* SANG KYU LEE,1,2 JEONG-IK LEE,3 HOON-JE CHO,1,2

HONG-KU SHIM1,2

1Department of Chemistry (BK21), Korea Advanced Institute of Science and Technology, 373-1 Guseong-dong,Yuseong-gu, Daejeon 305-701, Korea

2Center for Advanced Functional Polymers, Korea Advanced Institute of Science and Technology, 373-1 Guseong-dong,Yuseong-gu, Daejeon 305-701, Korea

3Basic Research Laboratory, Electronics and Telecommunications Research Institute, 161 Gajeong-dong,Yuseong-gu, Daejeon 305-350, Korea

Received 13 September 2004; accepted 17 November 2004DOI: 10.1002/pola.20659Published online in Wiley InterScience (www.interscience.wiley.com).

ABSTRACT: We report here the synthesis via Suzuki polymerization of two novel alter-nating polymers containing 9,9-dioctylfluorene and electron-withdrawing 4,4�-dihexyl-2,2�-bithiazole moieties, poly[(4,4�-dihexyl-2,2�-bithiazole-5,5�-diyl)-alt-(9,9-dioctylflu-orene-2,7-diyl)] (PHBTzF) and poly[(5,5�-bis(2�-thienyl)-4,4�-dihexyl-2,2�-bithiazole-5�,5�-diyl)-alt-(9,9-dioctylfluorene-2,7-diyl)] (PTHBTzTF), and their application toelectronic devices. The ultraviolet–visible absorption maxima of films of PHBTzF andPTHBTzTF were 413 and 471 nm, respectively, and the photoluminescence maximawere 513 and 590 nm, respectively. Cyclic voltammetry experiment showed an improve-ment in the n-doping stability of the polymers and a reduction of their lowest unoccu-pied molecular orbital energy levels as a result of bithiazole in the polymers’ mainchain. The highest occupied molecular orbital energy levels of the polymers were �5.85eV for PHBTzF and �5.53 eV for PTHBTzTF. Conventional polymeric light-emitting-diode devices were fabricated in the ITO/PEDOT:PSS/polymer/Ca/Al configuration[where ITO is indium tin oxide and PEDOT:PSS is poly(3,4-ethylenedioxythiophene)doped with poly(styrenesulfonic acid)] with the two polymers as emitting layers. ThePHBTzF device exhibited a maximum luminance of 210 cd/m2 and a turn-on voltage of9.4 V, whereas the PTHBTzTF device exhibited a maximum luminance of 1840 cd/m2

and a turn-on voltage of 5.4 V. In addition, a preliminary organic solar-cell device withthe ITO/PEDOT:PSS/(PTHBTzTF � C60)/Ca/Al configuration (where C60 is fullerene)was also fabricated. Under 100 mW/cm2 of air mass 1.5 white-light illumination, thedevice produced an open-circuit voltage of 0.76 V and a short-circuit current of 1.70mA/cm2. The fill factor of the device was 0.40, and the power conversion efficiency was0.52%. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 1845–1857, 2005Keywords: conducting polymers; conjugated polymers; light-emitting diodes (LED);photophysics

INTRODUCTION

The use of conjugated polymers in electronic de-vices as active components has produced a previ-ously unimaginable era in polymer electronics.

Organic and polymeric materials possess severaladvantages as active components over their sili-con counterparts, including their ease of prepara-tion, low processing temperature, and nearly un-limited variability. With the success of organiclight-emitting diodes and polymeric light-emit-ting diodes (PLEDs),1,2 polymer electronics is cur-rently expanding its applications to other devices,such as organic thin-film transistors,3 organic so-lar cells,4,5 and organic memory devices.6,7 Suchrapid progress is, of course, mainly based on the

*Present address: Corporate R&D Center, Samsung SDI,428-5 Gongsae-ri; Kiheung-eup, Yongin, Kyunggi-do 449-577,Korea

Correspondence to: H.-K. Shim (E-mail: hkshim@kaist.ac.kr)Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 43, 1845–1857 (2005)© 2005 Wiley Periodicals, Inc.

1845

development of new conjugated polymers and re-lated materials.

Among the various conjugated polymers devel-oped so far in this field, polyfluorene derivativesare currently attracting much attention.8,9 Thehigh photoluminescence (PL) and electrolumines-cence (EL) efficiencies of polyfluorene derivativesmake them promising candidates for use in PLEDapplications. Adding suitable comonomers to thepolyfluorene backbone is a facile method for ma-nipulating the electrical and optical properties ofthe resulting polymer. The possibility of polymerchain alignment with liquid crystallinity is also adistinctive feature of polyfluorenes.10,11

In the early stages of research into polyfluorenes,they were first developed as potential blue-light-emitting materials12–15 with a relatively large bandgap. The confinement of the conjugation length hasalso been known to be a good way of attaining blue-light emissions.16,17 More recently, the manipula-tion of the emission wavelength and band gap ofpolyfluorenes has been achieved through copoly-merization,18 and this makes polyfluorenes the onlyfamily of conjugated polymers that can emit thewhole range of visible light. In addition, electro-chemical redox stability is also worthy of consider-ation for device applications. Because the electricaland charge-transporting properties of polymers canbe controlled through copolymerization, many poly-fluorene copolymers containing either hole-trans-porting19,20 or electron-transporting,21–23 comono-mers have been synthesized and characterized. Inparticular, the addition of electron-withdrawingimine nitrogen to a conjugated polymer backbonegenerally enhances its electron-accepting propertiesand makes it susceptible to n-doping (reduction).24

Benzothiadiazole is, in that sense, a typical exampleof such imine nitrogen containing units. Polyflu-orene copolymers containing benzothiadiazole unitsshow excellent electron-transporting properties,25

and high-performance PLEDsmaking use of the co-polymers have also been reported.26,27 Because con-jugated polymers with stable n-doping propertiesare relatively uncommon, this imine nitrogen ap-proach provides a useful way of designing new con-jugated polymers.

Thiazole has an imine nitrogen in place of thecarbon atom at the 3-position of thiophene and,therefore, deserves attention because of its elec-tron-withdrawing properties. Polymers contain-ing the bithiazole moiety have been demonstratedto exhibit reversible n-doping properties.28,29 De-spite these unusual electrochemical properties,only a limited number of bithiazole-based poly-mers have been described, and results for their

applications in devices are rare.30,31 Metal compl-exation into the polymer main chain is an addi-tional advantage of these imine nitrogen contain-ing polymers.32,33

We report here the synthesis and properties ofnovel alternating conjugated polymers containing9,9-dioctylfluorene and electron-withdrawing 4,4�-dihexyl-2,2�-bithiazole (2) moieties, poly[(4,4�-dihexyl-2,2�-bithiazole-5,5�-diyl)-alt-(9,9-dioctylfluorene-2,7-diyl)] (PHBTzF) and poly[(5,5�-bis(2�-thienyl)-4,4�-dihexyl-2,2�-bithiazole-5�,5�-diyl)alt-(9,9-dioctylfluorene-2,7-diyl)] (PTHBTzTF). In addition tothe aforementioned advantages of polyfluorenes, theincorporation of bithiazole is expected to result inpolymers with reductive stability. Moreover, theemission color of these polymers can easily bemanipulated by the variation of the comonomer.In light-emitting-diode (LED) devices, PHBTzFand PTHBTzTF have been found to exhibit brightgreen and orange-red light, respectively. More-over, an experiment with PTHBTzTF as an elec-tron donor and fullerene (C60) as an electron ac-ceptor in an organic solar cell has shown thepotential of this new type of polymer for solar-cellapplications.

EXPERIMENTAL

Materials

2-Octanone, urea, dithiooxamide, N-bromosuccin-imide (NBS), magnesium, 2-bromothiophene, [1,1�-bis(diphenylphosphino)ferrocene]dichloropalla-dium(II) [Pd(dppf)Cl2], Aliquat 336, bromoben-zene, and phenylboronic acid were purchasedfrom Aldrich Co. Glacial acetic acid was pur-chased from Merck Co. Bromine, sodium carbon-ate, sodium chloride, and magnesium sulfatewere purchased from Junsei Co. Tetrakis(triph-enylphosphine)palladium(0) was purchased fromDNF Solution Co. 2,7-Bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-9,9�-dioctylfluorene (6)was synthesized according to a literature proce-dure.34 Analytical-grade solvents were usedthroughout the experiments, and all chemicalswere used without further purification.

Measurements

NMR spectra were recorded on a Bruker Avance400 spectrometer with tetramethylsilane as aninternal reference. Mass spectra were obtainedwith an Autospec Ultima spectrometer. Elemen-

1846 LEE ET AL.

tal analysis was performed with a Fisons EA-1110 elemental analyzer. The number-averagemolecular weight (Mn) and weight-average molec-ular weight (Mw) of the polymer were determinedby gel permeation chromatography (GPC) on aWaters GPC-150C instrument with tetrahydrofu-ran (THF) as the eluent and monodisperse poly-styrene as the standard. Thermogravimetricanalysis (TGA) and differential scanning calorim-etry of the polymer were performed under a ni-trogen atmosphere at a heating rate of 10 °C/minwith a Dupont 9900 analyzer. Ultraviolet–visible(UV–vis) spectra were measured with a JascoV-530 UV–vis spectrometer. PL spectra weremeasured with a Spex Fluorolog-3 spectroflu-orometer. Cyclic voltammetry (CV) measure-ments were performed on an Autolab/PGSTAT12at room temperature with a three-electrode cell ina solution of Bu4NBF4 (0.10 M) in acetonitrile ata scanning rate of 50 mV/s. Polymer films wereprepared via the dipping of platinum workingelectrodes into the polymer solutions and thenair-drying. A platinum wire was used as a counterelectrode, and an Ag/Ag� electrode was used as areference electrode. The film thickness was mea-sured with a Tencor Alpha-Step 500 surface pro-filer.

Device Fabrication and Characterization

A hole-injection layer of poly(3,4-ethylenedioxy-thiophene) doped with poly(styrenesulfonic acid)(PEDOT:PSS; Bayer Al 4083) was spin-coatedonto the prepatterned indium tin oxide (ITO) an-ode and dried. The polymer solution was thenspin-coated onto the PEDOT:PSS layer and dried.The Ca/Al (20 nm/100 nm) cathode was vacuum-deposited onto the polymer film through a shadowmask at a pressure below 1 � 10�6 Torr; thisyielded an active area of 0.04 cm2. For PLEDmeasurements, EL spectra were obtained with aMinolta CS-1000. The current–voltage–lumi-nance characteristics were measured with a cur-rent–voltage source (Keithley 238) and a lumines-cence detector (Minolta LS-100). Photo current–voltage measurements were performed under theAM 1.5, 100 mW/cm2 irradiation (1 sun) of aWXS-105H solar simulator (Wacom Co., Ltd.)with HP4140B and HP3456A digital voltmeters.The incident photon conversion efficiency (IPCE)spectra were measured via the recording of thespectrally resolved photocurrent via a lock-intechnique, normalized to the monochromaticpower flux density of the light source. A cali-

brated Si photodiode was used to determine thephotosensitivity.

Synthesis of the Monomers and Polymers

1-Bromo-2-octanone (1)

2-Octanone (40 mL, 0.251 mol), urea (25.0 g,0.417 mol), and glacial acetic acid (125 mL) wasplaced in a 250-mL, two-necked, round-bottomflask with an ice bath for cooling. A solution ofbromine (14.0 mL, 0.275 mmol) in glacial aceticacid (40 mL) was added dropwise to the flask, andthe solution was stirred overnight at room tem-perature. Water (250 mL) was then added to thesolution, and the solution was extracted with di-chloromethane. The combined extracts werewashed with 10% sodium carbonate and brineand then dried over anhydrous magnesium sul-fate. The pure product was obtained after vacuumdistillation (28.591 g, yield � 55%).

1H NMR (CDCl3, ppm, �): 3.88 (s, 2H), 2.62 (t,2H), 1.58 (m, 2H), 1.35–1.15 (m, 6H), 0.86 (t, 3H).13C NMR (CDCl3, ppm, �): 202.1, 39.7, 34.3, 31.4,28.6, 23.7, 22.4, 13.9. Electron-impact mass spec-trometry (EIMS) m/z: 206. Calcd.: 206.03.

4,4�-Dihexyl-2,2�-bithiazole (2)

Compound 1 (10.906 g, 52.66 mmol), dithiooxam-ide (3.165 g, 26.33 mmol), and absolute ethanol(140 mL) were placed in a 250-mL, two-necked,round-bottom flask equipped with a reflux con-denser. The solution was heated to reflux for 4 h,and after cooling, it was poured onto crushed ice.The mixture was extracted with dichloromethaneand then dried over anhydrous magnesium sul-fate. After the evaporation of the solvent, theproduct was obtained as a brown crystal (7.178 g,yield � 81%).

1H NMR (CDCl3, ppm, �): 6.92 (s, 2H), 2.76 (t,4H), 1.72 (m, 4H), 1.40–1.15 (m, 12H), 0.87 (t,6H). 13C NMR (CDCl3, ppm, �): 160.7, 159.1, 11.6,31.6, 31.5, 29.1, 28.9, 22.5, 14.0. EIMS m/z: 336.Calcd.: 336.17.

5,5�-Dibromo-4,4�-dihexyl-2,2�-bithiazole (3)

Compound 2 (1 g, 2.97 mmol) and NBS (1.335 g,7.43 mmol) were dissolved in a mixture of glacialacetic acid (10 mL) and N,N-dimethylformamide(10 mL). After 2 h of stirring in the dark, a graysolid precipitated in the reaction mixture. Theprecipitate was filtered, washed with methanol,and then dried to produce the dibromo product(1.292 g, yield � 88%).

FLUORENE-BASED ALTERNATING POLYMERS 1847

1H NMR (CDCl3, ppm, �): 2.72 (t, 4H), 1.68 (m,4H), 1.40–1.15 (m, 12H), 0.87 (t, 6H). 13C NMR(CDCl3, ppm, �): 159.9, 157.4, 106.8, 31.5, 29.5,28.8, 28.6, 22.6, 14.1. EIMS m/z: 492. Calcd.:491.99.

5,5�-Bis(2-thienyl)-4,4�-dihexyl-2,2�-bithiazole (4)

To a suspension of magnesium (0.793 g, 33 mmol)in anhydrous diethyl ether (5 mL), a solution of2-bromothiophene (4.89 g, 30 mmol) in anhydrousdiethyl ether (30 mL) was added dropwise. Thesolution of the Grignard reagent was added drop-wise to an ice-cooled suspension of compound 3(4.944 g, 10 mmol) and Pd(dppf)Cl2 (140 mg,0.2 mmol) in anhydrous diethyl ether (30 mL).The reaction mixture was stirred for 1 h at roomtemperature. It was then hydrolyzed with a sat-urated aqueous ammonium chloride solution, andthis was followed by the addition of diethyl ether.The organic layer was washed three times with200 mL of water and dried over sodium sulfate.After the removal of the solvent, reprecipitationfrom methanol gave the product (3.706 g, yield� 74%).

1H NMR (CDCl3, ppm, �): 7.37 (d, 2H), 7.17 (d,2H), 7.08 (t, 2H), 2.92 (t, 4H), 1.74 (m, 2H), 1,40–1.15 (m, 12H), 0.86 (t, 6H). 13C NMR (CDCl3,ppm, �): 157.8, 154.5, 133.0, 127.7, 127.5, 127.4,126.5, 31.6, 30.3, 29.4, 29.1, 22.6, 14.1. EIMS m/z:500. Calcd.: 500.14. ELEM. ANAL. Calcd. forC26H32N2S4: C, 62.35%; H, 6.44%; N, 5.59%; S,25.61%. Found: C, 60.49%; H, 6.58%; N, 5.40%; S,27.85%.

5,5�-Bis(5-bromo-2-thienyl)-4,4�-dihexyl-2,2�-bithiazole (5)

Compound 4 (0.5 g, 1 mmol) was dissolved in amixture of chloroform (4.9 mL) and glacial aceticacid (1.2 mL). NBS (0.365 g, 2.05 mmol) was thenadded to the solution and stirred for 1 h in thedark. The precipitate in the reaction mixture wasfiltered, washed with methanol, and then dried toproduce the dibromo product (0.468 g, yield� 71%).

1H NMR (CDCl3, ppm, �): 7.03 (d, 2H), 6.92 (d,2H), 2.86 (t, 4H), 1.72 (m, 4H), 1.40–1.15 (m,12H), 0.87 (t, 6H). 13C NMR (CDCl3, ppm): 158.0,155.1, 134.4, 130.6, 127.7, 126.7, 113.3, 31.6, 30.3,29.4, 29.1, 22.6, 14.1. EIMS m/z: 656. Calcd.:655.94. ELEM. ANAL. Calcd. for C26H30Br2N2S4: C,47.42%; H, 4.59%; N, 4.25%; S, 19.48%. Found: C,45.72%; H, 4.62%; N, 4.22%; S, 21.97%.

PHBTzF

Compound 3 (0.500 g, 1.01 mmol), compound 6(0.650 g, 1.01 mmol), tetrakis(triphenylphos-phine)palladium(0) (0.023 g, 0.0202 mmol), andphase-transfer catalyst Aliquat 336 (0.041 g,0.101 mmol) were dissolved in toluene (6 mL). A2 M aqueous solution of sodium carbonate(2.3 mL) was then added to the reaction mixture.The polymerization proceeded at 90 °C for 2 daysand was end-capped with bromobenzene and phe-nylboronic acid. The reaction mixture was cooledto room temperature and added to vigorouslystirred methanol (400 mL). The polymer fiberswere filtered and reprecipitated from methanolseveral times. The polymer was further purifiedby Soxhlet extraction with methanol to removeoligomers. The final polymer was obtained afterdrying at 50 °C in vacuo (0.506 g, yield � 69%).

1H NMR (CDCl3, ppm, �): 7.86–7.72 (bd, 2H),7.54–7.39 (bs, 4H), 3.10–2.64 (bs, 4H), 2.21–1.60(bd, 8H), 1.45–0.50 (m, 48H). FTIR (KBr, cm�1):2923 (CH stretching), 2852, 1466 (CH2 bending).ELEM. ANAL. Calcd. for C47H66N2S2: C, 78.06%; H,9.20%; N, 3.87%; S, 8.87%. Found: C, 77.92%; H,9.25%; N, 4.01%; S, 8.97%.

PTHBTzTF

The polymerization process was the same as thatfor PHBTzF, except that compound 5 was usedinstead of compound 3 (0.744 g, yield � 83%).

1H NMR (CDCl3, ppm, �): 7.80–7.66 (bd, 2H),7.66–7.48 (m, 4H), 7.42–7.30 (bd, 2H), 7.23–7.15(bd, 2H), 3.15–2.85 (b, 4H), 2,20–1.72 (bd, 8H),1.65–0.55 (m, 48H). FTIR (KBr, cm�1): 2923 (CHstretching), 2852, 1466 (CH2 bending). ELEM.ANAL. Calcd. for C55H70N2S4: C, 74.44%; H, 7.95%;N, 3.16%; S, 14.45%. Found: C, 75.35%; H, 7.82%;N, 3.31%; S, 14.64%.

RESULTS AND DISCUSSION

Synthesis and Characterization of the Polymers

The two alternating polymers were prepared withthe well-known Suzuki polymerization betweenthe diboronic ester of fluorene (6) and the bithia-zole-containing dibromide monomer (3 or 5). Forthe synthesis of 2, 2-octanone was brominated,and then the product, 1, was reacted with dithio-oxamide to produce 2.35 The treatment of com-pound 2 with NBS resulted in selective dibromi-nation at the � position of the bithiazole moiety,which yielded monomer 3. Another monomer with

1848 LEE ET AL.

a donor–acceptor–donor structure was also pre-pared for use in tuning the optical and electronicproperties of the polymers. The alternation ofelectron-donor and electron-acceptor groups in apolymer is known to reduce its band gap.36 Elec-tron-donating thiophene was therefore added toboth sides of each electron-withdrawing bithia-zole moiety through the Grignard reaction.2-Thienylmagnesium bromide was prepared from2-bromothiophene and magnesium, and then cou-pled with compound 3 in the presence of a cata-lyst, Pd(dppf)Cl2. The tetraheterocycle compound(4) was then brominated with NBS to producemonomer 5. The diboronic ester monomer (6) wasprepared according to the literature method, thatis, the alkylation of 2,7-dibromofluorene with bro-mooctane followed by boronation with 2-iso-propoxy-4,4,5,5-tetramethyl-1,3,2-dioxaborolaneafter lithiation. The synthetic routes and struc-tures of these monomers and polymers are shownin Schemes 1 and 2.

The chemical structures of the polymers wereconfirmed with NMR, FTIR, and elemental anal-ysis. The polymers were readily soluble in com-mon organic solvents, such as chloroform, chloro-benzene, THF, and toluene. Figure 1 shows the1H NMR spectra of the two monomers and the twopolymers. The triplet near 2.8 ppm in the mono-mer 1H NMR spectra arose from the benzylic pro-

tons of the bithiazole moiety, and the broad peaknear 2.0 ppm in the polymer 1H NMR spectraarose from the methylene protons adjacent to thefluorene 9-position. A broadening of the 1H NMRsignals in both the aromatic and aliphatic regionswas observed as a result of polymerization.

The molecular weights of the polymers were de-termined with GPC, with THF as an eluent andmonodisperse polystyrene as a standard. The Mnvalues of PHBTzF and PTHBTzTF were 18,400 and49,300, respectively, and their polydispersity indi-ces (PDIs) were 2.47 and 2.08, respectively. Thethermal stabilities of the polymers were evaluatedwith TGA under a nitrogen atmosphere. Both poly-mers exhibited good thermal stability up to 420 °Cwith less than 5% weight loss. The TGA thermo-grams of the polymers are shown in Figure 2. Thecharacterization results are summarized in Table 1.

Optical Properties of the Polymers

The optical properties of the two polymers in solu-tion (chloroform) and in thin films were investi-gated. In solution, PHBTzF exhibited maximum ab-sorption at 412 nm, whereas PTHBTzTF had alonger absorption maximum at 461 nm; the differ-ences between the PL emission spectra of the twopolymers were similar to those between the absorp-tion spectra. Under UV light, the PHBTzF solution

Scheme 1. Synthetic scheme of the monomers.

FLUORENE-BASED ALTERNATING POLYMERS 1849

emitted blue-greenish light with a PL maximum of489 nm, and the PTHBTzTF solution emitted yel-low-green light with a PL maximum of 541 nm. Thefluorescence quantum yield (often known as thesolution PL quantum efficiency) of each polymerwas also measured and calculated. Because theoverall external quantum efficiency of an LED de-vice is closely related to the innate fluorescencequantum yield of the emitting material, the fluores-cence quantum yield, which is the fraction of mole-cules that emit a photon after direct excitation, canbe used to estimate the efficacy of the emittingmaterial. The fluorescence quantum yields of thetwo polymers were measured in very dilute solu-tions (optical density � 0.04),37 for which interchaininteractions between the polymers could be ignored.The polymer solutions exhibited similar fluores-cence quantum yields: 0.40 for PHBTzF and 0.39 forPTHBTzTF. These fluorescence quantum yields arecomparable to those of other fluorene-based alter-nating polymers.38,39

The film-state optical properties were also in-vestigated. Uniform and transparent polymerfilms were prepared on quartz plates via the spincoating of the polymer solutions in chlorobenzene.Figure 3 shows the UV–vis absorption and PLemission spectra of the two polymer films. Theabsorption maxima of the PHBTzF and PTH-BTzTF films were 413 and 471 nm, respectively.

The onset wavelengths of the PHBTzF and PTH-BTzTF UV–vis absorption spectra were 476 and561 nm, respectively, and from these values, theoptical band gaps were calculated to be 2.60 eVfor PHBTzF and 2.21 eV for PTHBTzTF. Theemission maxima of the PL emission spectra werefound for the PHBTzF film at 513 nm and for thePTHBTzTF film at 590 nm. The thiophene ringextension in PTHBTzTF therefore resulted in anincrease in the effective conjugation length and ina decrease in the band gap.

Because there has been much research intofluorene-based alternating polymers, we cancompare our results with those of others anddetermine the effect of the addition of the bi-thiazole group. Among the various reports sofar, the results of Huang et al.40 are particu-larly noteworthy because they preparedpoly[2,7-(9,9-dihexylfluorene)-alt-5,5�(4,4�-didecyl-2,2�-bithiophene)], which has a struc-ture very similar to that of PHBTzF. This poly-mer, based on 4,4�-didecyl-2,2�-bithiophene, hasan absorption maximum at 403 nm and anemission maximum at 490 nm in the film state;the corresponding results for PHBTzF, which isbased on 2, are redshifted (413 and 513 nm,respectively) with respect to those of the bithio-phene-containing polymer. The absence of the �proton (3,3�-position) in bithiazole results in the

Scheme 2. Synthetic scheme of the polymers.

1850 LEE ET AL.

improved coplanarity of the backbone, whichthen increases the effective conjugation lengthof the polymer. This explains the differencesbetween the spectra of the bithiazole-containingpolymer and the bithiophene-containing poly-mer, which are also due to the electron-with-drawing properties of the imine nitrogen. Inaddition, the fluorescence quantum yield of PH-

BTzF is slightly higher. The optical propertiesof the polymers are summarized in Table 2.

Electrochemical Properties of the Polymers

The electrochemical properties of the polymerswere investigated with CV, which we used to de-termine the highest occupied molecular orbital(HOMO) and lowest unoccupied molecular orbital(LUMO) of each polymer. The HOMO and LUMOlevels of conjugated polymers are closely relatedto their charge-injection and transport properties,so it is important to determine the correct elec-trochemical properties of materials for use in elec-tronic device applications. Figure 4 shows the cy-clic voltammograms of the two polymers with re-

Figure 1. 1H NMR spectra of the monomers and polymers: (a) monomer 3, (b)PHBTzF, (c) monomer 5, and (d) PTHBTzTF. The solvent peaks are marked withasterisks.

Figure 2. TGA thermograms of the polymers.

Table 1. Physical Properties of the Polymers

Mn Mw PDI T5d (°C)a

PHBTzF 18,400 45,500 2.47 420PTHBTzTF 49,300 102,500 2.08 429

a Temperature at 5% weight loss.

FLUORENE-BASED ALTERNATING POLYMERS 1851

spect to the standard calomel electrode (SCE)potential. The cyclic voltammograms of the ho-mopolymer, poly(9,9-dioctylfluorene) (PFO), andof ferrocene are also shown in Figure 4 for com-parison. Note the perfect reversibility of the n-doping (reduction) processes of the two polymers.In contrast to the cyclic voltammograms of thefluorene homopolymer41 or the fluorene–bithio-phene copolymer,40 it is clear that the presence ofthe imine nitrogen enhances the reductive stabil-

ity of the polymers sufficiently that clear revers-ible n-doping (reduction) can be achieved. Fur-thermore, the polymers have lower reduction po-tentials than PFO, and this indicates the ease ofelectron injection for these polymers.

Another important feature is that the cyclicvoltammograms of the two polymers are almostidentical during their respective n-doping (reduc-tion) processes and are similar to that of the bi-thiazole homopolymer, poly(4,4�-dimethyl-2,2�-bi-thiazole-5,5�-diyl).28 On the other hand, there areclear differences between the p-doping (oxidation)processes of the two polymers. The p-doping (ox-idation) of PHBTzF was found to be irreversible,whereas PTHBTzTF was found to exhibit par-tially reversible oxidation. Because PHBTzF con-tains only fluorene and electron-withdrawing bi-thiazole units, it is closer to an n-type materialthan a p-type material. The oxidation potential ofPHBTzF was found to be slightly higher than thatof PFO. However, the presence of electron-donat-ing thiophenes on both sides of each bithiazole

Figure 3. UV–vis and PL spectra of the polymerfilms.

Table 2. Optical Properties of the Polymers

Solution Film

�max,abs

(nm)�max,em

(nm)�

(L/mol/cm)a �b�max,abs

(nm)�max,em

(nm)�onset,abs

(nm) Eg (eV)c

PHBTzF 412 489 2250 0.40 413 513 476 2.60PTHBTzTF 461 541 7170 0.39 471 590 561 2.21

a Measured at the maximum absorption wavelength.b Fluorescence quantum yield calculated with respect to the reference material, 9,10-diphenylanthracene (�std � 0.91 in

ethanol).37

c Band gap calculated from the film-state absorption onset wavelength.�max,abs � maximum absorption wavelength; �max,em � maximum emission wavelength; �onset,abs � the absorption onset

wavelength.

Figure 4. Cyclic voltammograms of the polymers.

1852 LEE ET AL.

moiety changes the electrical properties of PTH-BTzTF so that it is both p-type and n-type. More-over, the oxidation potential of PTHBTzTF is re-duced as a result of the thiophene ring extension,and the reduction potential is nearly preserved,with only a minute decrease. From these results,it is evident that the presence of bithiazole in thepolymer mainly affects its n-doping (reduction)process, and the thiophene ring extension mainlyaffects the oxidation potential of the polymer.

The HOMO levels were calculated to be�5.85 eV for PHBTzF and �5.53 eV for PTH-BTzTF. These values were calculated from theoxidation onset potentials of the polymers, theenergy level of ferrocene/ferrocenium beingtreated as �4.80 eV.42 In addition to this electro-chemical determination, we also measured theHOMO levels of the polymer films with a low-energy photoelectron spectroscope (Riken-KeikiAC-2).43 The measured values were �5.94 eV forPHBTzF and �5.48 eV for PTHBTzTF, and theseare in good agreement with the HOMO levels thatwe determined by electrochemical methods. Theband-gap energy values of the polymers deter-mined by electrochemical methods are, however,somewhat larger than those determined with op-tical methods, that is, from the onset wavelengthsof the UV–vis absorption spectra. This differencein the band gaps determined with optical andelectrochemical methods has previously been re-ported and is thought to be due to the presence ofan interface barrier between the polymer film andthe electrode surface used in the electrochemicaldetermination.41,44 However, it is apparent thatthe two polymers, PHBTzF and PTHBTzTF, havesimilar LUMO levels. PTHBTzTF is thus likely toexhibit better hole-injection and transport ability

than PHBTzF in electronic devices. The results ofthe electrochemical measurements are summa-rized in Table 3.

Characteristics of the PLED Devices

Conventional PLED devices with the ITO/PE-DOT:PSS (30 nm)/polymer (70 nm)/Ca (20 nm)/Al(100 nm) configuration were fabricated. Figure 5shows the EL spectra of the two polymers, whichare similar to the PL spectra of the correspondingpolymer films. PHBTzF emitted green light withan EL maximum at 515 nm and a shoulder at546 nm, and PTHBTzTF emitted orange-red lightwith an EL maximum at 591 nm and a shoulderat 630 nm. Figure 6 shows typical current–volt-age–luminance curves for the LED devices. ThePHBTzF device had a relatively high turn-on volt-age of 9.4 V and a maximum luminance of 210 cd/

Figure 5. EL spectra of the polymers.

Table 3. Electrochemical Properties of the Polymers

p-Doping (V) n-Doping (V)HOMO

(eV)LUMO

(eV)Epa Epc E1/2 Eonset Epc Epa E1/2 Eonset

PFO 1.35 1.24 1.30 1.28 �2.38 �2.26 �2.32 �2.28 �5.73a �2.15a

(�2.78)c

PHBTzF 1.52 1.41 �1.83 �1.78 �1.81 �1.77�5.85a �2.67a

(�5.94)b (�3.25)c

PTHBTzTF 1.19 1.13 1.16 1.09 �1.78 �1.63 �1.70 �1.67�5.53a �2.77a

(�5.48)b (�3.32)c

a Calculated from the onset potential with respect to the energy level of ferrocene/ferrocenium as �4.80 eV.b Measured from Riken-Keiki AC-2.c Estimated by subtraction of the optical band gap from the electrochemically determined HOMO level.Epa, Epc, E1/2, and Eonset stand for anodic peak potential, cathodic peak potential, the average of the anodic and cathodic peak

potentials, and onset potential, respectively.

FLUORENE-BASED ALTERNATING POLYMERS 1853

m2. In contrast, the PTHBTzTF device had aturn-on voltage of 5.4 V and a maximum lumi-nance of 1840 cd/m2. These differences in thedevice characteristics originated in the differ-ences between their electronic properties. PTH-BTzTF has a higher HOMO energy level (ca.�5.5 eV) and a lower hole-injection barrier, andthis resulted in its lower turn-on voltage.

In the current–voltage curve, it seems that thecurrent–voltage relationship obeys a power law, I� Vm�1 (where I is the current and V is the volt-age), fairly well.45 However, there is a shoulderaround 5 V for the PHBTzF device. Because thehole injection of the PHBTzF device was rela-

tively suppressed in a low-voltage regime, thisresulted in a shoulder in the initial stage of thecurrent–voltage curve. With an increase in theapplied voltage, hole injection became more flu-ent, and the current began to increase rapidly;this accompanied a rapid increase in luminance.The PTHBTzTF device, however, seemed to havea balanced injection of holes because the HOMOlevel was higher than that of PHBTzF.

Despite the different electronic properties ofthe polymers, the maximum external quantumefficiencies of the two devices were almost identi-cal, 0.16% for PHBTzF and 0.18% for PTHBTzTF.Because the two polymers have similar fluores-cence quantum yields, the differences betweenthe electric current that flowed through the de-vices was the main influence on the different lu-minance levels of the devices. The characteristicsof the PLED devices are summarized in Table 4.

Organic Solar-Cell Device Characteristics

The device configuration of a typical organic so-lar-cell device is very similar to that of a PLED,except that a blend of the conjugated polymerwith C60 is used as the active layer. The presenceof C60 results in charge separation and photoin-duced charge transfer from the electron-donatingconjugated polymer to the electron-accepting C60upon irradiation with light. It is generally knownthat low-band-gap polymers are more favorablefor use in organic solar-cell devices because of theresulting larger overlap of the polymer absorptionspectrum with the solar spectrum. However, onlya limited number of conjugated polymers, such asdialkoxy-poly(p-phenylene vinylene) (PPV) deriv-atives46 and some thiophene-containing poly-mers,47–49 have been used in organic solar-celldevices, and the development of new polymers forthis purpose is one of the most active areas in thisresearch field. Of the two polymers prepared inthis work, PTHBTzTF is particularly suited foruse in an organic solar-cell device because of its

Figure 6. Current–voltage–luminance curves of thePLED devices.

Table 4. PLED Device Characteristics of the Polymers

Von (V)a Lmax (cd/m2)b ELmax (nm)c CIE (x, y) EQEd (%)

PHBTzF 9.4 210 516 0.34,0.54 0.16 at 13.2 VPTHBTzTF 5.4 1840 591 0.56,0.44 0.18 at 10 V

a Turn-on voltage needed for 0.1 cd/m2.b Maximum luminance.c Maximum electroluminescence wavelength.d External quantum efficiency.

1854 LEE ET AL.

relatively low band gap and its electrochemicalstability with respect to both reduction and oxi-dation. To test whether PTHBTzTF could be usedas an electron donor, we prepared a compositethin film by spin-coating a 1:1 wt % solution ofPTHBTzTF and C60 in chlorobenzene onto aquartz substrate. Figure 7 shows the PL spectraof the polymer-only film and the polymer/C60 film.Complete PL quenching was observed as a resultof blending with C60, and this is generally attrib-uted to the different kinetics of charge transfer(�10�14 s)50 and recombination (�10�3 s).51 ThisPL quenching shows that PTHBTzTF can be usedas an electron donor in organic solar-cell devices.

Thus, we fabricated a preliminary organic solar-cell device with PTHBTzTF as an electron donorand C60 as an electron acceptor in the ITO/PEDOT:PSS (50 nm)/PTHBTzTF:C60 � 1:1 (50 nm)/Ca(20 nm)/Al (100 nm) configuration. The spectral re-sponse of the solar-cell device was obtained by themeasurement of the current–voltage curves uponmonochromatic illumination. Figure 8 shows theresults for the IPCE (the incident photon to col-lected electron efficiency, often called the externalquantum efficiency) of the solar-cell device, andthe UV–vis absorption spectrum of the compos-ite thin film (PTHBTzTF/C60) is also shown inFigure 8 for comparison. The IPCE spectrumand UV–vis absorption spectrum are wellmatched, and the IPCE reached up to about23% near the �–�* absorption maximum. Theabsorption around 340 nm originated from C60itself in the blend film. Figure 9 shows a linearplot of the current–voltage characteristics ofthe organic solar-cell device under white-lightillumination at 100 mW/cm2. A semilog plot ofthe current–voltage characteristics of the de-

vice in both a dark state and an illuminatedstate is also shown as an inset in Figure 9.Under white-light illumination, an open-circuitvoltage (VOC) of 0.76 V and a short-circuit cur-rent (ISC) of 1.70 mA/cm2 were obtained. The fillfactor (FF) of the device was 0.40, and the powerconversion efficiency (�PCE) was 0.52%. FF and�PCE were calculated as follows:

FF �Imax � Vmax

ISC � VOC(1)

�PCE(%) � �Pout

Pin� � 100

�FF � VOC � ISC

Pin� 100� (2)

where Pin is the incident light power, which inour experiment was 100 mW/cm2, Imax and Vmax

are the current and the voltage at the maximumpower point in the 4th quadrant of the current-voltage curve, and Pout is the electrical power ofthe device under illumination. These results arepreliminary and are meant only to examine thepotential of the newly synthesized polymer foruse in organic solar-cell devices. We believethat further efforts to optimize device perfor-mance, such as the control of the ratio of elec-tron donors to acceptors, the employment ofother electron acceptors, the investigation ofthe film morphology, the control of the layerthickness, and postproduction treatment, canimprove these solar-cell device characteristics.

Figure 7. PL spectra of a pure PTHBTzTF film and ablend film of PTHBTzTF and C60 (1:1 w/w).

Figure 8. IPCE spectrum of the solar cell and thecorresponding UV–vis absorption spectrum.

FLUORENE-BASED ALTERNATING POLYMERS 1855

CONCLUSIONS

Two novel alternating conjugated polymers basedon fluorene and bithiazole, PHBTzF and PTH-BTzTF, were designed and then synthesized withpalladium-catalyzed Suzuki polymerization. Bothpolymers were found to be thermally stable up to420 °C, and their good solubility in common or-ganic solvents makes them suitable for solutionprocessing, which is essential for a wide range ofdevice applications. The addition of the electron-withdrawing bithiazole units to the polyfluorenebackbone was found to result in electrochemicalstability during n-doping (reduction). Moreover,band-gap tuning of the polymers could beachieved through the alternation of thiopheneand bithiazole, and thus the emission colors ofthese polymers could also be controlled fromgreen to orange-red. PLED devices with thesepolymers exhibited excellent brightness, andtheir EL spectra were in good agreement withtheir film PL spectra. An organic solar-cell devicewas also fabricated with PTHBTzTF as an elec-tron donor and C60 as an electron acceptor. Thepreliminary results for this device show the po-tential of this kind of material for use in organicsolar-cell devices.

The authors appreciate Mr. Joonghwan Kwak and Prof.Koeng Su Lim (Department of Electrical Engineeringof the Korea Advanced Institute of Science and Tech-nology) for fruitful discussions and solar-cell experi-ments. This work was supported by the Korea Scienceand Engineering Foundation [R01-2003-000-10213-0(2003)], the Center for Advanced Functional Polymersof the Korea Science and Engineering Foundation, andthe BK21 of the Ministry of Education and HumanResources Development.

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