New electroluminescent carbazole-containing conjugated polymer: Synthesis, photophysics, and electroluminescence

Polymer 55 (2014) 6220e6226

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New electroluminescent carbazole-containing conjugated polymer: Synthesis, photophysics, and electroluminescence
ra Cimrova  a, *, Christoph Ulbricht b, Vagif Dzhabarov a, Drahomír Výprachtický a, Ve Daniel Ayuk Mbi Egbe b a b

Institute of Macromolecular Chemistry, Academy of Sciences of the Czech Republic, Heyrovský Sq. 2, 162 06 Prague 6, Czech Republic Linz Institute for Organic Solar Cells, Physical Chemistry, Johannes Kepler University Linz, Altenbergerstr. 69, 4040 Linz, Austria

a r t i c l e i n f o

a b s t r a c t

Article history: Received 21 July 2014 Received in revised form 1 October 2014 Accepted 3 October 2014 Available online 13 October 2014

A new electroluminescent polymer, poly{9-(2-ethylhexyl)carbazole-2,7-diethynylene-alt-tris[2,5-bis(2ethylhexyloxy)-1,4-phenylenevinylene]} (PCzE-PPV), is synthesized, and its photophysical and electrochemical properties and electroluminescence (EL) are studied. In solution, an intense photoluminescence (PL) emission with a maximum at about 520 nm is observed. PL decay dynamics in solution are best described by a monoexponential function with a lifetime of 0.76 ns. Thin films exhibit an intense PL emission with a slightly red-shifted maximum at 532 nm compared to that in the solution spectra. The polymer oxidizes and reduces quasi-reversibly. The ionization potential (HOMO level) of 5.3 eV and the electron affinity (LUMO level) of 2.80 eV are evaluated from cyclovoltammetric measurements. The electrochemical bandgap value (2.45 eV) is in good agreement with the optical bandgap value. Using new polymer, light-emitting devices (LEDs) with a luminance higher than 3000 cd m
2 and low onset voltages at about 3 V are fabricated. The shape of EL spectra of the LEDs is similar to that of PL spectra of the thin films. © 2014 Elsevier Ltd. All rights reserved.

Keywords: Carbazole-containing conjugated polymer Synthesis Photophysics and electroluminescence

1. Introduction Carbazole-containing polymers and copolymers are of interest because of their interesting properties (hole-transporting, photoconductive, photoluminescent and electroluminescent) [1e3]. In particular, new conjugated copolymers with carbazole units incorporated in the polymer backbone are promising due to their potential for various applications in organic electronics, such as organic light-emitting devices (OLED), organic solar cells, organic field effect transistors (OFET), etc. [4e7] Recently, we also synthesized and studied novel luminescent fluorene-carbazole and low bandgap carbazole-thienothiadiazole copolymers [8e10], which were prepared by Suzuki coupling reaction of corresponding comonomers. In this paper, we report on the synthesis of a new carbazole-containing conjugated polymer in which the carbazole unit is incorporated into poly(arylene-ethynylene)-alt-poly(arylene-vinylene)'s (PAE-PAV's) structure. PAE-PAVs combine the very well established ethynylene- and vinylene-feature within a polymer. A broad collection of PAE-PAV polymers with various

* Corresponding author. Tel.: þ420 296809251. ). E-mail addresses: [email protected], [email protected] (V. Cimrova http://dx.doi.org/10.1016/j.polymer.2014.10.015 0032-3861/© 2014 Elsevier Ltd. All rights reserved.

arylene building blocks such as phenylenes, thiophenes or anthracene, as well as various side chain combinations, has been synthesized and investigated regarding their potential for OLED, organic photovoltaic (OPV), and OFET applications [11e14]. Here, we used 9-(2-ethylhexyl)-2,7-diiodocarbazole, whose synthesis we recently improved [15e17], and succeeded in preparing a new electroluminescent carbazole-containing polymer: poly{9-(2ethylhexyl)carbazole-2,7-diethynylene-alt-tris[2,5-bis(2ethylhexyloxy)-1,4-phenylenevinylene]} (PCzE-PPV). Its photophysical, electrochemical and electroluminescent properties were studied and are reported in this paper. 2. Experimental 2.1. Materials and methods All chemicals were obtained from SigmaeAldrich Co. and used as purchased. Solvents were dried with Na/LiAlH4 (tetrahydrofuran e THF), KOH (pyridine, diisopropylamine) or molecular sieve 4A (dimethylformamide e DMF) and distilled. 1 H NMR and 13C NMR spectra were measured in CDCl3 with an upgraded Bruker Avance DPX-300 spectrometer at 300.13 MHz and 75.45 MHz, respectively, using hexamethyldisiloxane as an internal

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standard. FT IR spectra were measured on a PerkineElmer Paragon 1000 PC Fourier transform infrared spectrometer by means of diamond attenuated total reflection (ATR). Size exclusion chromatography (SEC) measurements were performed using a Pump Deltachrom (Watrex Comp.) with a Midas autosampler and two columns of MIXED-B LS PL gel, particle size 10 mm. An evaporative light scattering detector (PL-ELS-1000 from Polymer Laboratories) was used; THF was the mobile phase. Polystyrene standards were used for calibration. Thermogravimetric analysis (TGA) was performed in nitrogen flow (50 ml min
1) at a heating rate of 10 K min
1 using a PerkineElmer TGA 7 Thermogravimetric Analyzer operated with Pyris 1 software. Thermal measurement was performed using a PerkineElmer DSC 8500 in closed aluminium pans with sample mass of 2.3 mg under nitrogen flow of 20 ml min
1. The calorimeter was calibrated with indium standard. 2.1.1. Sample preparation Thin films were prepared by spin coating from toluene solutions. Thin films were spin-coated onto fused silica substrates for optical studies or coated on a Pt wire electrode by dipping for electrochemical measurements. For polymer light-emitting devices (LED), polymer layers were prepared on indiumetin oxide (ITO) substrates covered with a thin layer of poly[3,4-(ethylenedioxy) thiophene]/poly(styrenesulfonate) (PEDOT:PSS). All polymer films were dried in a vacuum (10
3 Pa) at 353 K for 2 h. The ITO glass substrates were purchased from Merck (Germany) and PEDOT:PSS (CLEVIOS™ P VP AI 4083) from Heraeus Clevios GmbH (Germany). The 50 nm thick PEDOT:PSS layers were prepared by spin coating and dried in air at 396 K for 15 min. The calcium (20 nm) and, subsequently, 60e80 nm thick aluminium electrodes were vacuum-evaporated on top of the polymer films to form LEDs. Typical active areas of the LEDs were 4e8 mm2, precise values used for EL efficiency evaluation were determined by optical microscopy. Layer thicknesses were measured using a KLA-Tencor P-10 profilometer. 2.1.2. Cyclovoltammetric measurements Cyclic voltammetry (CV) was performed with a PA4 polarographic analyzer (Laboratory Instruments, Prague, CZ) with a threeelectrode cell. Platinum (Pt) wire electrodes were used as both working and counter electrodes. A non-aqueous Ag/Agþ electrode (Ag in 0.1 M AgNO3 solution) was used as the reference electrode. CV measurements were made in an electrolyte solution of 0.1 M tetrabutylammonium hexafluorophosphate (TBAPF6) in anhydrous acetonitrile under nitrogen atmosphere. Typical scan rates were 20, 50 and 100 mV s
1. 2.1.3. Photophysical measurements UVevis spectra were measured on a PerkineElmer Lambda 35 UV/VIS spectrometer. Solvents of spectroscopic grade were used. The absorption spectra of thin films were also measured in the glove box using fibre optics connected to the spectrophotometer. Steady-state PL spectra were recorded using a PerkineElmer LS55 Fluorescence spectrophotometer. The PL quantum yield of the polymer in solution was calculated relative to the fluorescein in 0.1 M NaOH, which was used as a standard (PL quantum yield 0.91) [18]. The time-resolved fluorescence was measured with a timeresolved fluorometer FL 900 CDT (Edinburgh Analytical Instruments, UK) using the time-correlated single-photon counting (TCSPC) method. Excitation of the samples was carried out with a nF 900 thyratron-controlled pulse lamp with a repetition frequency rate of 40 kHz. The lamp was filled with hydrogen (>99.995%) at 150 kPa. The intensity of the emission beam was adjusted so that 800 fluorescence photons (or less) per second (ca. 2% of repetition frequency) were observed. The time-resolved and lamp profile

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measurements were performed in L-format. The lamp pulse (profile) was measured using a scattering solution of Ludox (DuPont) closely spaced in time to a particular fluorescence decay. The theoretical fluorescence decay was analysed by deconvolution of the lamp pulse with the impulse response of the sample as described in our previous paper [19]. 2.1.4. Electroluminescence measurements EL spectra were recorded using an Acton Research Spectrograph with single photon-counting detection (SPEX, RCA C31034 photomultiplier). LEDs were supplied from a Keithley 237 source measure unit, which served to simultaneously record the current flowing through the sample. Currentevoltage and luminanceevoltage characteristics were recorded simultaneously using the Keithley 237 source measure unit and a Minolta LS110 Luminance Meter or a silicon photodiode with integrated amplifier (EG&G HUV-4000B) for the detection of total light output. A voltage signal from the photodiode was recorded with a Hewlett Packard 34401A multimeter. All thin film preparations and the device fabrication were done in a glove box under a nitrogen atmosphere. 2.2. Synthesis 2.2.1. Monomer synthesis 4-Ethynyl-2,5-bis(2-ethylhexyloxy)benzaldehyde (2) was synthesized from 2,5-bis(2-ethylhexyloxy)-4-(trimethylsilylethynyl) benzaldehyde (1) according to the procedure described in the literature [12]. 9-(2-Ethylhexyl)-2,7-diiodocarbazole (3) and 2,5-bis(2ethylhexyloxy)-1,4-xylylene-bis(diethylphosphonate) (5) were synthesized as described in our previous papers [17,12,20] respectively. 2,7-Bis[4-formyl-2,5-bis(2-ethylhexyloxy)phenylethynyl]-9-(2ethylhexyl)carbazole (4). A mixture of tetrahydrofuran (60 mL) and diisopropylamine (25 mL) was degassed with argon for 40 min. 4Ethynyl-2,5-bis(2-ethylhexyloxy)benzaldehyde (2) (1.28 g, 3.31 mmol), 9-(2-ethylhexyl)-2,7-diiodocarbazole (3) (840 mg, 1.581 mmol), tetrakis(triphenylphospine) palladium(0) (81 mg, 0.070 mmol), and copper(I) iodide (11 mg, 0.058 mmol) were added gradually. The reaction mixture was stirred at room temperature under argon for 42 h. The precipitated diisopropyl ammonium iodide was filtered off and the solvents were removed with a rotary evaporator. After column chromatography on silica gel (toluene/ heptane, 3:1 v.) and recrystallization (methanol/ethyl acetate), product 4 was obtained as a pale yellow powder (yield: 1.16 g, 73%). Anal. Calcd for C70H97O6N (1048.56): C 80.18, H 9.32, N 1.34; found: C 80.01, H 9.15, N 1.32. 1 H NMR (300.13 MHz, CDCl3, d): 10.46 (s, 2H, CHO), 8.04 (d, 2H, J ¼ 8.1 Hz, carbazole), 7.57 (s, 2H, carbazole), 7.42 (d, 2H, J ¼ 8.1 Hz, carbazole), 7.34 (s, 2H, aryl), 7.17 (s, 2H, aryl), 4.17 (d, 2H, J ¼ 6.9 Hz, NeCH2), 3.96 (t, 8H, J ¼ 5.1 Hz, 4  OeCH2), 2.11 (br, 1H, NeCeCH), 1.84e1.77 (m, 4H, 4  OeCeCH), 1.66e1.23 (m, 40H, 20  CH2), 1.01e0.85 (m, 30H, 10  CH3). 13 C NMR (75.45 MHz, CDCl3, d): 11.0 (1C), 11.4 (4C), 14.1 (5C), 23.1 (5C), 24.1 (4C), 24.4 (1C), 28.7 (1C), 29.2 (4C), 30.7 (4C), 30.9 (1C), 39.2 (1C), 39.6 (4C), 71.5 (1C, NeC), 71.7 (4C, OeC), all aliphatic, 85.7 (2C, C^C), 98.8 (2C, C^C), 109.6, 112.5, 117.1, 120.4, 120.6, 120.8, 122.9, 123.2, 124.8, 141.3, 154.0, 155.8, all 2C, aromatic, 189.2 (2C, CHO). 2.2.2. Polymer synthesis Poly{9-(2-ethylhexyl)carbazole-2,7-diethynylene-alt-tris[2,5bis(2-ethylhexyloxy)-1,4-phenylenevinylene]}, (PCzE-PPV). A solution of 2,7-bis[4-formyl-2,5-bis(2-ethylhexyloxy)phenylethynyl]9-(2-ethylhexyl)carbazole (4) (250 mg, 0.238 mmol) and 2,5-bis(2ethylhexyloxy)-1,4-xylylene-bis(diethylphosphonate) (5) (151 mg,

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Scheme 1. Synthesis of the carbazole building block 2,7-bis[4-formyl-2,5-bis(2-ethylhexyloxy)-phenylethynyl]-9-(2-ethylhexyl)carbazole (4).

0.238 mmol) in toluene (20 ml) was degassed by purging with argon. The reaction mixture was heated to 130  C and potassium tert-butylate (t-BuOK, 180 mg, 1.604 mmol) was added. Another portion of base (t-BuOK, 90 mg, 0.802 mmol) was added after 30 min. After 60 min, the reaction was quenched with benzyldiethylphosphonate (180 mg, 0.789 mmol) and the heating was stopped 10 min later. Toluene (50 mL), water (5 mL) and hydrochloric acid (10%, 20 mL) were then added. The organic layer was washed with brine (2  50 mL) and deionized water (4  50 mL). Residual water was removed by means of a DeaneStark apparatus. The hot, clear, bright yellow toluene solution was filtered and the toluene was evaporated. The material was dissolved in tetrahydrofuran and precipitated into ice-cooled methanol/water (3:1 v., 400 mL). After filtration the material was extracted (1 h) with methanol/diethyl ether (1:1 v., 150 mL) to remove low molecular weight fractions. The residue was dissolved in hot tetrahydrofuran and precipitated in cold methanol/water (3:1 v., 400 mL). Filtration (glass frit) and drying (oil pump) gave a bright yellow polymer (yield: 0.222 g, 68%). SEC (THF): Mn ¼ 37 700, Mw ¼ 61 000, PDI ¼ 1.62. Anal. Calcd for [C94H135O6N]n (1375.13)n: C 82.11, H 9.90, N 1.02; found: C 82.21, H 9.95, N 1.07. 1 H NMR (300.13 MHz, CDCl3, d): 8.01 (br, 2H, arom.), 7.55 (br, 4H, arom.), 7.40 (br, 2H, arom.), 7.20 (m, 2H, arom.), 7.08 (s, 4H, arom.),

6.83e6.73 (m, 2H, arom.), 3.95 (br, 12H, 6  OeCH2), 3.52 (br, 2H, NeCH2), 2.12 (br, 1H, NeCeCH), 1.83 (br, 6H, 6  OeCeCH), 1.50e1.35 (m, 56H, 28  CH2), 1.02 (t, 15H, J ¼ 7.2 Hz, 5  CH3), 0.90 (t, 27H, J ¼ 5.7 Hz, 9  CH3). 13 C NMR (75.45 MHz, CDCl3, d): 11.4, 14.2, 23.2, 24.2, 24.4, 29.2, 31.0, 39.8, all 7C aliphatic, 71.7 (4C, 2  C^C), 109.3 (3C), 112.7 (3C), 116.8 (3C), 120.3 (2C), 121.0 (3C), 122.5 (2C, C]C), 123.0 (2C, C]C), 127.4 (2C), 128.6 (2C), 141.3 (3C), 150.5 (3C), 151.2 (3C), 154.5 (3C), all backbone carbons. FT IR (ATR): n ¼ 2956, 2926, 2870, 2858, 1598, 1502, 1458, 1414, 1378, 1328, 1248, 1196, 1034, 968, 852, 804, 772, 726, 630, cm
1. FT IR, 1H and 13C NMR spectra of the synthesized 4 and PCzEPPV are available online in Supplementary Data. 3. Results and discussion 3.1. Synthesis The synthesis of 2,7-bis[4-formyl-2,5-bis(2-ethylhexyloxy)phenylethynyl]-9-(2-ethylhexyl)-carbazole (4) was accomplished by Sonogashira coupling reaction of 4-ethynyl-2,5-bis(2-ethylhexyloxy) benzaldehyde (2) and N-(2-ethylhexyl)-2,7-diiodocarbazole (3) with a palladium catalyst, a copper(I) cocatalyst, and an amine base according to Scheme 1.

Scheme 2. Synthesis of poly{9-(2-ethylhexyl)carbazole-2,7-diethynylene-alt-tris[2,5-bis(2-ethylhexyloxy)-1,4-phenylenevinylene]} (PCzE-PPV) by Horner-Wadsworth-Emmons reaction.

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1.4 1.2 1.0

dW/dlog(M)

Compound 2 was synthesized from hydroquinone as described in the literature [12]. N-(2-Ethylhexyl)-2,7-diiodocarbazole was synthesized by a two-step synthesis. In the first step, the 4,40 diiodobiphenyl was nitrated to a mixture of 4,40 -diiodo-2nitrobiphenyl and 4-iodo-40 -nitrobiphenyl. In the second step, the mixture of these compounds was converted by simultaneous carbazole ring closure and N-alkylation to the N-(2-ethylhexyl)-2,7diiodocarbazole by means of tris(2-ethylhexyl) phosphite. The details are described in our previous paper [17]. Synthesis of compound 4 proceeded smoothly if we used the diiodocarbazole 3 in contrast to using the dibromocarbazole derivative, for which the synthesis of compound 4 was not successful. This indicates that the use of the more reactive diiodocarbazole 3 is necessary for this kind of reaction. The alternating copolymer PCzE-PPV was synthesized by Horner-Wadsworth-Emmons (HWE) polycondensation of 4 and 2,5-bis(2-ethylhexyloxy)-1,4-xylylene-bis(diethylphosphonate) (5) according to the reaction shown in Scheme 2. The synthesis of the second comonomer 5 was performed according to standard procedure, starting from hydroquinone and including a sequence of etherification, bromomethylation and Arbuzov reaction, as described in our previous papers [12,20]. From a synthetic viewpoint, of course, the comonomers 4 and 5 alternate to a poly{2,7-bis[20 ,50 -bis(2-ethylhexyloxy)phenylethynyl]-9-(2ethylhexyl)carbazole-40 ,40 -diyl-alt-2,5-bis(2-ethylhexyloxy)-1,4phenylenevinylene} (Scheme 2, upper structure), which can be renamed using polymer nomenclature as poly{9-(2-ethylhexyl) carbazole-2,7-diethynylene-alt-tris[2,5-bis(2-ethylhexyloxy)-1,4phenylenevinylene]}, (Scheme 2, lower structure). This is possible because of the presence of the same 2,5-bis(2-ethylhexyloxy)-1,4phenylene structural units in both comonomers 4 and 5. The HWE reaction dialdehydes and bisphosphonates lead to the formation of double bounds with a high (E)-conformation ratio [21]. The advantage of the HWE synthetic route is that no metal complex mediator or catalyst is needed, in contrast to other synthetic routes for the preparation of conjugated polymers such as Yamamoto, Sonogashira, Suzuki, Heck or Stille coupling. The HWE reaction proceeded fast and PCzE-PPV polymer soluble in common solvents such as THF, toluene, chloroform or chlorobenzene with a rather high average molecular weight, Mn ¼ 37,700, Mw ¼ 61,000, and low polydispersity index PDI ¼ 1.62 was obtained. Its SEC curve is shown in Fig. 1. NMR and IR spectra proved the structures. The new PCzE-PPV polymer possesses a higher molecular weight and lower polydispersity when compared with analogous polymer, poly{anthracene-9,10-diethynylene-alt-tris[2,5-bis(2-ethylhexyloxy)-1,4phenylenevinylene]} (PAnE-PPV) with Mw ¼ 47,200 and PDI ¼ 2.98 [12].

0.8 0.6 0.4 0.2 0.0 3.0

3.5

4.0

4.5

5.0

5.5

6.0

Log(M) Fig. 1. SEC curve of PCzE-PPV. THF was the mobile phase. Polystyrene standards were used for calibration.

copolymers with various combinations of side chains including solely branched or solely linear ones. PAnE-PPV with solely 2ethylhexyl side chains exhibited a less tendency of aggregation due to bulky branched 2-ethylhexyls hindering intermolecular pep stacking [12,14]. 3.3. Photophysical properties The photophysical properties of the PCzE-PPV polymer were studied in solutions and thin films. Absorption spectra were measured in diluted toluene and chlorobenzene solutions and are displayed in Fig. 3. Absorption spectra exhibited a broad absorption band with maxima at 464 and 467 nm for toluene and chlorobenzene solutions, respectively, which corresponds to the absorption of the pep* transition of the conjugated backbone. The absorption spectra of the thin films were similar to the spectra measured in solutions. An example of the absorption spectrum of a 110-nm thick film is also shown in Fig. 3 together with the solution

100

3.2. Thermal properties

90 80

Weight (%)

The copolymer PCzE-PPV exhibited good thermal stability as proven by thermogravimetric analysis (TGA). A TGA curve measured in nitrogen at a heating rate of 10 K min
1 is shown in Fig. 2. A weight loss of about 1% was observed at high temperature at 331  C, 3% weight loss at 362  C, and 10% weight loss was detected at 394  C, indicating very good thermal stability of the new polymer in nitrogen. The decrease is a result of the decomposition of alkyl chains on benzene rings and on the carbazole units. The residual weight percentage in nitrogen at 800  C was 32%. This high value corresponds to the high aromatic content. The differential scanning calorimetry (DSC) measurement reveals a very small amount of crystalline or organized phase (<0.1%) melting at about 185  C. This finding is in agreement with the results obtained on the anthracene-containing poly(p-phenylene-ethynylene)-alt-poly(p-phenylene-vinylene)s (PPE-PPV)

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70 60 50 40 30

0

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O

Temperature ( C) Fig. 2. TGA curve of PCzE-PPV measured at a scan rate of 10 K min
1 in nitrogen.

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Normalized Absorbance

1.0

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Wavelength λ(nm) Fig. 3. Normalized UVevis absorption spectra of PCzE-PPV measured in diluted toluene (dashed) and chlorobenzene (dotted) solutions, and in a 110-nm thick film (solid).

absorption spectra. The coincidence of the normalized absorption and the fluorescence excitation spectra indicated that there were no detectable impurities absorbing or emitting in the used spectral region. A time resolved PL study was performed in PCzE-PPV chlorobenzene solution. The fluorescence decay was monoexponential. The fluorescence lifetime, t ¼ 0.76 ns, and the fluorescence quantum yield, Ff ¼ 0.84, were determined. The radiative deactivation constant kf ¼ Ff/t ¼ 1.1 ns
1 and the nonradiative deactivation constant krn ¼ (1 e Ff)/t ¼ 0.21 ns
1 were obtained. These results reveal that there were no significant impurities present, which is in accord with the results of the steady state PL study. The PCzE-PPV absorption (labsmax) and PL emission (lPLmax) maxima are distinctly blue-shifted compared with those of analogous anthracene-containing PAnE-PPV (labsmax ¼ 521 nm, lPLmax ¼ 580 nm in chloroform solution and labsmax ¼ 521e527 nm, lPLmax ¼ 598e600 nm in thin film) and also anthracene-containing PPE-PPV polymers with various side chains from whose PAnE-PPV with solely bulky branched 2-ethylhexyl exhibited the highest PL efficiency value [12e14]. The optical bandgap value of 2.39 eV determined from PCzE-PPV thin film spectrum is higher than the bandgap value 2 eV of the analogous PAnE-PPV. The new PCzEPPV exhibits the highest PL efficiency when compared with various PAE-PAVs including anthracene-containing PPE-PPVs [11e14]. 3.4. Electrochemical properties

spectra. Absorption maxima were located at 211, 256 and 466 nm. The absorption peaks in the short wavelength region (<400 nm) correspond to the carbazole unit absorption. PCzE-PPV exhibited intense photoluminescence in solution as well as in thin films. Fig. 4 shows PL emission spectra measured in toluene and chlorobenzene solutions and in a thin film. The emission spectra exhibited maxima at 516 nm and 520 nm for toluene and chlorobenzene, respectively. In the thin film spectrum the PL emission maximum located at 532 nm is slightly red-shifted as compared with the maximum in the PL spectra measured in solution. Both PL spectra, in thin film and in solution, exhibited vibration structure. The excitation spectra corresponded well to the

0.8 0.3

0.6 0.2

Current (mA)

Normalized PL Intensity

1.0

Cyclic voltammetry (CV) measurements were performed to obtain information on the electronic structure of the copolymer such as the highest occupied molecular orbitals (HOMO) and the lowest unoccupied molecular orbital (LUMO) energy levels and bandgap. A representative CV curve of a thin film coated on a Pt wire is displayed in Fig. 5. Quasi-reversibility in both oxidation and reduction were observed. The ionization potential (HOMO level), EIP, and electron affinity (LUMO level), EA, were estimated from onset potentials, Eonset, of the oxidation and reduction peaks on the basis of the reference energy level of ferrocene (4.8 eV below the vacuum level) using the equation EIP (EA) ¼ j
(Eonset
Eferr)
4.8j eV, where Eferr is the value for ferrocene vs. the Ag/Agþ electrode which was estimated as 0.05 eV. The EIP and EA values were evaluated as averages from CV curves measured at a scan rate of 50 mV s
1. The ionization potential and electron affinity corresponding to the positions of HOMO and LUMO levels (below the vacuum level) were

0.4

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Wavelength λ(nm)

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E vs. Ag/Ag+ (V) Fig. 4. Normalized photoluminescence emission spectra of PCzE-PPV measured in diluted toluene (dashed) and chlorobenzene (dotted) solutions, and in a 110-nm thick film (solid).

Fig. 5. Cyclic voltammogram of a PCzE-PPV thin film coated on a Pt wire recorded at a scan rate of 50 mV s
1.

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The new electroluminescent PCzE-PPV was used for an active layer in LEDs. LEDs with a hole-injecting electrode formed by ITO covered with a PEDOT:PSS layer and an electron-injecting electrode of calcium covered with aluminium (Ca/Al) were prepared and studied. The LEDs exhibited an intense EL emission with a maximum at 533 nm. The EL spectrum of the LED was similar to the thin film PL spectrum and is shown in Fig. 6. The LEDs exhibited high luminance values, over 3000 cd m
2. An example of the typical dependence of the current and the luminance on the applied voltage measured on the ITO/PEDOT:PSS/ PCzE-PPV/Ca/Al LED is shown in Fig. 7. EL emission started at 2 V and a steep increase in EL occurred at 3 V (i.e. at an electric field intensity <3  107 V m
1), which was associated with the steep current increase. The height of the injection barriers, the density of the trap sites, and the mobility of the charge carriers are important for the current behaviour. In the currentevoltage characteristics of the LEDs under study, two regimes can be distinguished. The dependence of the current on the applied voltage at lower voltages (electric fields) is sub-linear. At higher voltages, when EL emission starts the current increases steeply with increasing applied voltage V, j ~ Vm, with m about 11. The EL intensity, IEL, increased even more steeply with the applied voltage, IEL ~ Vn, with n about 15. The position of the PCzE-PPV transport levels was determined from cyclovoltammetric measurements. The IP value of 5.3 eV fit well with that of PEDOT:PSS. The ITO work function usually varies from 4.7 to 4.9 eV; therefore, the PEDOT:PSS layer is used to reduce the expected height of the injection barrier for holes. The injection barrier for electrons is also small because the PCzE-PPV electron

1.0

Normalized Intensity

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Luminance (cd m )

3.5. Electroluminescence

4

10

Current density (mA cm )

determined as 5.3 and 2.85 eV, respectively. The electrochemical bandgap value, Eelc g ¼ 2.45 eV, was evaluated. Its value is in good agreement with the optical bandgap value of 2.39 eV. The new PCzE-PPV possesses a slightly higher ionization potential and a lower electron affinity resulting in a higher bandgap value when compared with various PAE-PAVs exhibiting ionization potential values 5.1e5.2 eV, electron affinity values 3e3.4 eV and bandgap values 1.8e2.2 eV [12e14].

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Voltage U(V) Fig. 7. Dependence of current density (solid circles) and luminance (open circles) on the applied voltage measured on ITO/PEDOT:PSS/PCzE-PPV/Ca/Al LED.

affinity fit well to the calcium work function. The similarity of the EL and PL thin film spectra as well as their currentevoltage characteristics indicate that charge trapping does not play an important role in the EL process. The PCzE-PPV LED luminance efficiency of 2.6 cd A
1, the power efficiency of 1.35 lm W
1 and the external quantum efficiency estimation of about 0.5% are much better than those of anthracenecontaining PPE-PPV LEDs with red-shifted EL (maxima at 600e620 nm) and belong to the higher ones of LEDs made of analogous PAE-PAVs and of other relatively efficient polymer LEDs of similar simple device configuration [5,14,22]. The obtained EL results on the non-optimized simple LEDs are interesting and promising for further polymer or device optimization. An improvement of EL efficiency can be achieved in multilayer devices, by modification of injection electrodes or in polymer blend LEDs using various blends of EL polymer with appropriate electroluminescent or charge transporting polymers in order to make charge injection and transport more balanced [19,23e30]. EL enhancements up to two order magnitude were demonstrated [19,23e26]. Carbazole-containing polymers are reported as hosts in efficient electrophospohorescent LEDs [30]. Therefore possible use of new PCzE-PPV as a host in such LEDs is also interesting. In addition, polymer optimization can be done by replacing side chains (their nature, length and distribution) which influence the PL and EL efficiency and photovoltaic properties [14,22].

0.6 4. Conclusions

0.4

0.2

0.0 400

500

600

700

800

Wavelength λ(nm) Fig. 6. Normalized EL spectrum of ITO/PEDOT:PSS/PCzE-PPV/Ca/Al LED measured at 6 V.

A new electroluminescent carbazole-containing poly(aryleneethynylene)-alt-poly(arylene-vinylene) polymer was synthesized by Horner-Wadsworth-Emmons polycondensation reaction of carbazole-based dialdehyde, 2,7-bis[4-formyl-2,5-bis(2ethylhexyloxy)phenylethynyl]-9-(2-ethylhexyl)carbazole, and 2,5bis(2-ethylhexyloxy)-1,4-xylylene-bis-(diethylphosphonate). In the solution, an efficient PL emission with a maximum at 516e520 nm was observed. PL decay dynamics in solution were best described by a monoexponential function with a lifetime 0.76 ns. A 15-nm red-shift of the PL maximum in the thin film compared with that in solution was observed, indicating an aggregation in the solid state. It was found that the polymer oxidized and reduced quasi-reversibly. The ionization potential EIP ¼ 5.3 eV

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and electron affinity EA ¼ 2.80 eV were evaluated. Using these values an electrochemical bandgap of 2.45 eV was determined. Its value is in good agreement with the optical bandgap value (2.39 eV). Polymer LEDs with a hole-injecting indiumetin oxide covered with PEDOT:PSS and an electron-injecting calcium/ aluminium electrode were fabricated. The LEDs emitted light with a maximum at 533 nm. The polymer LEDs exhibited a luminance higher than 3000 cd m
2 at 6 V. Typical onset voltages were about 3 V. EL spectra were similar to the thin film PL spectrum. Acknowledgements We thank the Czech Science Foundation for supporting this work by grants 13-26542S and P106/12/0827. Daniel A. M. Egbe and Christoph Ulbricht also acknowledge the financial support of the Deutsche Forschungsgemeinschaft (DFG) in the framework of priority program SPP1355. Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.polymer.2014.10.015. References [1] Grazulevicius JV, Strohriegl P, Pielichowski J, Pielichowski K. Prog Polym Sci 2003;28:1297e353. http://dx.doi.org/10.1016/S0079-6700(03)00036-4. [2] Mullen K, Scherf U. Organic light emitting devices: synthesis, properties and applications. Wiley-VCH; 2006. http://dx.doi.org/10.1002/3527607986. [3] Li Z, Meng H. Organic light emitting materials and devices. CRC Taylor&Francis; 2007. [4] Lim E. Int J Photoenergy 2013. http://dx.doi.org/10.1155/2013/607826. Article ID 607826. [5] Guo X, Baumgarten M, Müllen K. Prog Polym Sci 2013;38:1832e908. http:// dx.doi.org/10.1016/j.progpolymsci.2013.09.005. [6] Park SH, Roy A, Beaupre S, Cho S, Coates N, Moon JS, et al. Nat Photon 2009;3: 297e302. http://dx.doi.org/10.1038/nphoton.2009.69. [7] Boudreault P-LT, Najari A, Leclerc M. Chem Mater 2011;23:456e69. http:// dx.doi.org/10.1021/cm1021855.  V, Kmínek I, Výprachtický D. Macromol Symp 2010;295:65e70. [8] Cimrova http://dx.doi.org/10.1002/masy.200900060.

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