Incorporating Perylene Moiety into Poly(phenothiazine-co-bithiophene

Mar 5, 2008 - In the 1H NMR spectrum of P0, there is a triplet peak at δ 3.81 ppm (peak 1), which is attributed to the N-10 α-methylene protons of t...
2 downloads 0 Views 248KB Size
Download PDF
3590

J. Phys. Chem. B 2008, 112, 3590-3596

ARTICLES Incorporating Perylene Moiety into Poly(phenothiazine-co-bithiophene) Backbone for Higher Charge Transport Weihua Tang, Lin Ke, and Zhi-Kuan Chen* Institute of Materials Research and Engineering, 3 Research Link, Singapore 117602, Singapore ReceiVed: October 8, 2007; In Final Form: January 8, 2008

Low band gap π-conjugated polymers composed of phenothiazine, bithiophene, and perylene moieties were prepared in high yields by using a palladium-catalyzed Suzuki coupling reaction. The polymers were characterized by NMR, gel permeation chromatography, and elemental analysis. The characterizations revealed that high-molecular weight (weight-average molecular weight up to 42 400 g/mol) polymers were thermally stable with a decomposition temperature in the region of 338-354 °C and their glass transition temperatures (Tg) ranging from 124 to 136 °C. All polymers demonstrated broad optical absorption in the region of 300550 nm with efficient blue-green light emission. The absorption was broadened further (for ca. 50 nm) when the perylene moiety was incorporated. Cyclic voltammograms displayed that the p- and n-doping processes of all the polymers were partially reversible and that electrochemical band gaps were as low as -2.30 eV with the incorporation of a perylene moiety. The hole mobility of polymers was evaluated by using the spacecharge-limited current model with a device structure of ITO/PEDOT:PSS/polymer/Ca. The results show that the incorporation of perylene is beneficial for improving the hole mobility of the conjugated polymers.

Introduction Because of their extended π-electron systems, conjugated polymers have received continuing interest as a result of their potential applications such as polymer light-emitting diodes (PLEDs),1-6 field-effect transistors,7-10 photovoltaic solar cells,11-15 and electrochromic devices.16-19 The broad research efforts have focused on the structure manipulation of conjugated polymers to tailor their electronic and material properties for proposed applications. The key in the advancement of the organic electronic field is the development of methodologies to attain precise control over the polymer electronic structure to achieve a desired band gap and suitable HOMO (highest occupied molecular orbital) and LUMO (lowest unoccupied molecular orbital) energy levels.20 Equally important is the control of a material’s physical properties to obtain novel solution processable polymers by using industrially relevant techniques. The general approaches to realize such objectives are either functionalization of conjugated moieties16,21-25 or incorporation of other conjugated building blocks into the polymer chain26-28 or even the development of a network structure of cross-linked conjugated polymers.29-31 Among the conjugated polymers, phenothiazine-based polymers have been widely investigated for their applications in various electronic and electrochemical devices19,32-37 since they are good electron-donor and hole transport materials due to the presence of electron-rich sulfur and nitrogen heteroatoms. However, only a few papers have reported the photovoltaic performance of phenothiazine-based polymers.28,32 In our early * Corresponding author. E-mail: [email protected].

work, we synthesized conjugated polymers by incorporating a nonplanar phenothiazine ring with narrow energy band gap building blocks such as bithiophene and thieno[3,2-b]thiophene for photovoltaic solar cells.37 However, the absorption of the polymers only covers the range of 300 to ∼450 nm in the UV and visible sunlight spectrum. To broaden the absorption spectra of the polymers, in this paper, we synthesized a new series of phenothiazine-based copolymers by the direct incorporation of low band gap perylene.38 We investigated the effect of the content of perylene on the optical, electrochemical, and hole mobility in detail. These perylene-incorporated copolymers achieved high hole transport mobilities and broader absorption spectra. These results will be helpful to us in developing promising new photovoltaic materials. Experimental Procedures Measurements. 1H NMR and 13C NMR spectra were recorded on a Bruker 400 MHz DPX FT-NMR spectrometer. Elemental analyses were performed with a PerkinElmer 2400 CHNS elemental analyzer. The number- and weight-average molecular weights of the polymers were determined by gel permeation chromatography (GPC) on a Waters 2696 instrument with THF as the eluent and monodisperse polystyrene as the standard. Thermal gravimetric analyses (TGA) were conducted under a nitrogen atmosphere at a heating rate of 20 °C/min on a DuPont 2050 analyzer. Differential scanning calorimetry (DSC) was performed on a TA 2920 modulated DSC instrument at a heating rate of 20 °C/min and a cooling rate of 10 °C/min. UV-vis absorption spectra were recorded on a Shimadzu UV 3101 spectrophotometer and photoluminescence (PL) spectra

10.1021/jp7098152 CCC: $40.75 © 2008 American Chemical Society Published on Web 03/05/2008

Poly(phenothiazine-co-bithiophene) Backbone on a PerkinElmer LS-50B luminescence spectrometer. Cyclic voltammetry (CV) measurements were conducted on an Autolab/PGSTAT30, with a three-electrode cell in a solution of Bu4NBF4 (0.10 M) in dichloromethane at a scan rate of 100 mV/s at room temperature under the protection of argon. Device Fabrication and Characterization. Single-layer PLED devices ITO/PEDOT:PSS/polymer/Ca were fabricated to investigate their charge transport and emission properties. The device preparation and characterization procedure can be found in a previous publication.39 The current-voltage and luminancevoltage curves were measured with a dynamic multimeter (Keithley DMM 2001) and a source meter (Keithley 3A 2420) under the control of LabView software. Materials. Phenothiazine, perylene, tetrakis(triphenylphosphine)palladium, bromine, 1-bromohexane, and toluene (99.8%, anhydrous) were purchased from Aldrich. All chemicals were used without further purification. 3,6-Dibromo-10-hexylphenothiazine and 3,9(10)-dibromoperylene were prepared following a reported procedure.33,38 Synthesis of 3,7-dibromo-10-hexylphenothiazine.33 Yield: 56%. 1H NMR (400 MHz, CDCl3): δ(ppm) 7.22-7.25 (m, 4H), 6.68 (d, J ) 8.4 Hz, 2H), 3.75 (t, J ) 7.2 Hz, 2H), 1.74 (tt, J ) 7.2 Hz, J ) 6.8 Hz, 2H), 1.28-1.43 (m, 6H), 0.87 (t, J ) 6.8 Hz, 3H). 13C NMR (100 MHz, CDCl3): δ(ppm) 144.59, 130.50, 130.12, 126.97, 117.04, 115.17, 48.09, 31.74, 27.09, 26.87, 22.89, 14.25. MS (MALDI): m/z ) 441.1 (calcd for C18H19Br2NS: 440.9). Synthesis of 3,9(10)-dibromoperylene.38 Yield: 58%. 1H NMR (400 MHz, CDCl3): δ(ppm) 7.46-7.62 (m, 2H), 7.707.79 (m, 2H), 7.97-8.03 (m, 2H), 8.11-8.16 (d, J ) 8.4 Hz, 2H), 8.18-8.27 (m, 2H). MS (MALDI): m/z ) 410.1 (calcd for C20H10Br2: 410.1). General Procedure of Polymerization via Suzuki Coupling Reaction.40 To a 1:1 mixture of diborolanyl bithiophene, dibromo phenothiazine or dibromo perylene (or both according to feeding ratio), and tetrakis(triphenylphosphine)palladium [Pd(PPh3)4] (2 mol %) was added a degassed mixture of toluene and 2 M potassium carbonate aqueous solution (3:2 in volume). The reaction mixture was vigorously stirred at 85-90 °C for 72 h under the protection of nitrogen. After the mixture was cooled to room temperature, it was poured into a vigorously stirred mixture of methanol and deionized water (10:1 in volume). The powder solid was collected by filtration and washed sequentially with methanol, water, and methanol. The polymers were further purified by Soxhlet extraction in acetone for 2 days to remove oligomers and catalyst residues. The polymers were obtained after drying in vacuo at 50 °C. Yields: ∼65-78%. Poly[3,7-(10-hexylphenothiazine)-co-bithiophene] (P0). 5,5′Bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-2,2′bithiophene (0.50 equiv) and 3,7-dibromo-10-hexylphenothiazine (0.50 equiv) were used in the polymerization. Brownish yellow solid (yield: 65%). 1H NMR (400 MHz, CDCl3): δ(ppm) aromatic; 7.36-7.42 (m, 2H), 7.02-7.25 (m, 4H), 6.84 (m, 2H), 6.69 (m, 2H), aliphatic; 3.81 (t, N-CH2, J )7.2 Hz, 2H), 1.86-1.79 (m, 2H), 1.52-1.23 (m, 6H), 0.89 (t, J ) 6.8 Hz, 3H). 13C NMR (100 MHz, CDCl3): δ(ppm) 144.58, 130.37, 130.08, 125.21, 124.92, 124.75, 123.90, 123.42, 116.92, 116.02, 48.09, 31.94, 27.09, 26.87, 23.02, 14.41. Anal. calcd for (C26H23S3N)n: C, 70.07; H, 5.20; S, 21.59; N, 3.14. Found: C, 69.96; H, 5.18; S, 21.45; N, 3.22. P1. Red solid (67%). 5,5′-Bis(4,4,5,5-tetramethyl-1,3,2dioxaborolan-2-yl)-2,2′-bithiophene (0.50 equiv), 3,7-dibromo10-hexylphenothiazine (0.40 equiv), and 3,9(10)-dibromo-

J. Phys. Chem. B, Vol. 112, No. 12, 2008 3591 perylene (0.10 equiv) were used in the polymerization. 1H NMR (400 MHz, CDCl3): δ(ppm) aromatic; 8.29-8.22 (m, 1.0H), 7.65-7.53 (m, 1.3H), 7.38-7.34 (m, 2.5H), 7.22-7.02 (m, 7.2H), 6.86-6.73 (m, 2.8H), aliphatic; 3.86 (t, N-CH2, J ) 7.2 Hz, 2H), 1.87-1.79 (m, 2H), 1.46-1.25 (m, 6H), 0.87 (t, J ) 6.8 Hz, 3H). Anal. calcd for [(C26H23S3N)0.8[(C28H14S2)0.2]n: C, 72.16; H, 4.86; S, 20.43; N, 2.55. Found: C, 72.14; H, 4.85; S, 20.39; N, 2.62. P2. Red solid (62%). 5,5′-Bis(4,4,5,5-tetramethyl-1,3,2dioxaborolan-2-yl)-2,2′-bithiophene (0.50 equiv), 3,7-dibromo10-hexylphenothiazine (0.25 equiv), and 3,9(10)-dibromoperylene (0.25 equiv) were used in the polymerization. 1H NMR (400 MHz, CDCl3): δ(ppm) aromatic; 8.28-8.20 (m, 3.8H), 7.66-7.52 (m, 3.4H), 7.38-7.31(m, 3.7H), 7.22-7.02 (m, 7.8H), 6.85-6.72 (m, 2.2H), aliphatic; 3.86 (t, N-CH2, J ) 7.2 Hz, 2H), 1.88-1.79 (m, 2H), 1.26-1.49 (br, 6H), 0.87 (t, J ) 6.8 Hz, 3H). 13C NMR (100 MHz, CDCl3): δ(ppm) 144.57, 142.62, 141.31, 136.48, 133.36, 129.21, 128.59, 128.26, 127.50, 125.13, 124.74, 124.20, 123.37, 121.25, 120.41, 115.87, 48.12, 32.25, 29.63, 27.26, 23.02, 14.42. Anal. calcd for [(C26H23S3N)0.5[(C28H14S2)0.5]n: C, 75.40; H, 4.33; S, 18.64; N, 1.63. Found: C, 75.26; H, 4.36; S, 18.71; N, 1.67. P3. Red solid (yield: 70%). 5,5′-Bis(4,4,5,5-tetramethyl1,3,2-dioxaborolan-2-yl)-2,2′-bithiophene (0.50 equiv), 3,7dibromo-10-hexylphenothiazine (0.10 equiv), and 3,9(10)dibromoperylene (0.40 equiv) were used in the polymerization. 1H NMR (400 MHz, CDCl ): δ(ppm) aromatic; 8.28-8.21 (m, 3 15.4H), 7.66-7.52 (m, 9.0H), 7.38-7.31 (m, 2.5H), 7.22-7.02 (m, 11.4H), 6.85-6.72 (m, 2.3H), aliphatic; 3.86 (t, N-CH2, J ) 7.2 Hz, 0.4H), 1.88-1.79 (m, 0.4H), 1.49-1.26 (br, 1.2H), 0.87 (t, J ) 6.8 Hz, 0.6H). Anal. calcd for [(C26H23S3N)0.2[(C28H14S2)0.8]n: C, 78.78; H, 3.78; S, 16.77; N, 0.67. Found: C, 78.71; H, 3.80; S, 16.80; N, 0.69. P4. Red solid (yield: 78%). 5,5′-Bis(4,4,5,5-tetramethyl1,3,2-dioxaborolan-2-yl)-2,2′-bithiophene (0.50 equiv) and 3,9(10)-dibromoperylene (0.50 equiv) were used in the polymerization. 1H NMR (400 MHz, CDCl3): δ(ppm) aromatic; 8.278.18 (m, 4H), 8.14-7.70 (m, 2H), 7.64-7.52 (m, 4H), 7.257.18 (m, 2H), 7.10-6.99 (m, 2H). 13C NMR (100 MHz, CDCl3): δ(ppm) 131.21, 129.24, 128.59, 128.26, 127.89, 124.89, 124.48, 124.19, 121.35, 120.51. Anal. calcd for (C28H14S2)n: C, 81.13; H, 3.40; S, 15.47. Found: C, 81.08; H, 3.36; S, 15.28. Results and Discussion Synthesis and Characterization of Polymers. The syntheses of the polymers were carried out using the palladium-catalyzed Suzuki coupling reactions between dibromoaryl and diborolanylaryl reagents (Scheme 1).40 The reference polymer P0 is an alternate copolymer of phenothiazine (PT) and bithiophene (BT). Another reference polymer P4 is an alternate copolymer of perylene (PER) and bithiophene. The polymers P1-P3 are random copolymers of PT, BT, and PER, as shown in Scheme 1. The ratio of x/y was modulated by controlling the monomer feeding ratio of 3,6-dibromo-10-hexylphenothiazine and 3,9(10)-dibromoperylene. The actual values of x/y were determined on the basis of elemental analyses and 1H NMR spectra of polymers. In the 1H NMR spectrum of P0, there is a triplet peak at δ 3.81 ppm (peak 1), which is attributed to the N-10 R-methylene protons of the PT ring, while this triplet peak appears at δ 3.86 ppm in the 1H NMR spectra of P1-P3. In the 1H NMR spectrum of P4, there is a multiple peak at δ 8.278.18 ppm (peak 2), which is attributed to four aromatic protons of the PER ring, while this multiple peak appears at δ 8.28-

3592 J. Phys. Chem. B, Vol. 112, No. 12, 2008

Tang et al.

SCHEME 1: Synthesis and Molecular Structures of Copolymers, Where Bithiophene Units Couple Perylene at Both 3,9- and 3,10 Positions

8.21 ppm in 1H NMR spectra of P1-P3. Therefore, the ratio of x/y in P1-P3 could be determined by the ratio of the integral areas of peak 1 to peak 2, as shown in Figure 1. The values of x/y determined in this way were 3.90 for P1, 1.05 for P2, and 0.26 for P3, which are in good agreement with the values determined by the nitrogen content of the elemental analysis. The weight-average molecular weights (Mw) of the synthesized polymers were determined by GPC using polystyrene as the standard and were found to range from 35 000 to 42 400 with polydispersity indexes (PDI ) Mw/Mn) ranging from 1.7 to 2.7 (Table 1). Interestingly, perylene-containing polymers (P1-P4) possess lower Mw and larger PDI values than P0, probably due to the steric hindrance of perylene and the copolymerization competition between two dibromo aryl compounds when coupling with diborolanyl bithiophene. The polymers are all readily dissolved in chloroform, THF, and toluene.

Thermal Properties. Figure 2 shows the TGA plots of polymers P0-P4. The onset decomposition temperatures of these polymers are around 290 °C in nitrogen. Thermal induced phase transition behavior of the polymers was investigated with DSC under nitrogen atmosphere. As listed in Table 1, the glass transition temperature (Tg) of the polymers fell in the range of 124-140 °C. Interestingly, among all synthesized five polymers, only P3 demonstrates semicrystallization behavior. The crystallization behavior was investigated by applying a four-cycle heating and cooling process, with the second-cycle DSC trace shown in Figure 3. Obviously, P3 displays Tg at 123.8 °C and two melting temperatures at 192.7 and 218.6 °C. One single unsymmetrical crystallization peak at 158.6 °C was observed tailing toward the lower temperature side. The crystallization temperatures (Tcr) at four sequential heating and cooling cycles are illustrated in the inset of Figure 3. It is evident that the crystallization peak shifted toward the lower temperature side

Poly(phenothiazine-co-bithiophene) Backbone

J. Phys. Chem. B, Vol. 112, No. 12, 2008 3593

Figure 1. 1H NMR spectra of P0-P2 and P4 for the determination of composition.

TABLE 1: Molecular Weights and Thermal Properties of Polymers ratio (BT/PT/PER) polymers

feed

producta

Mw b

P0 P1 P2 P3 P4

50:50:0 50:40:10 50:25:25 50:10:40 50:0:50

50:50:0 50:39.8:10.2 50:25.6:24.4 50:10.3:39.7 50:0:50

42 400 39 100 37 100 35 000 37 300

Mw/Mn DPc Td d (°C) Tg e (°C) 1.7 2.7 2.4 2.6 2.1

56 33 36 32 43

353 341 354 340 338

140 132 128 124 136

a Calculated from results of elemental analysis and 1H NMR. Molecular weights determined by GPC using polystyrene standards in THF solutions. c Degree of polymerization, reference to Mn. d Decomposition temperature, determined by TGA in nitrogen, based on 5% weight loss. e Glass transition temperature, determined by DSC in nitrogen at 20 °C/min heat rate and 10 °C cooling rate.

b

Figure 3. DSC trace for P3. Inset shows the crystalline temperatures (Tcr) at four heat/cooling cycles, with Tcr decreasing with increasing heating/cooling cycles.

Figure 2. Thermograms of P0-P4 measured under a nitrogen atmosphere.

with a longer heating and cooling history, mainly due to more ordered crystalline rearrangement upon heating. The relatively high decomposition temperature and glass transition temperature of the polymers are adequate for their applications in optoelectronic devices such as PLED and photovoltaic solar cells (PSCs).

Figure 4. UV-vis absorption spectra of P0-P4 in chloroform solutions (∼1 × 10-5 M).

Optical Properties. Figure 4 shows the absorption spectra of the polymers in chloroform solutions. Interestingly, perylene-

3594 J. Phys. Chem. B, Vol. 112, No. 12, 2008

Figure 5. Photoluminescence spectra of P0-P4 solutions in chloroform (∼1 × 10-5 M) at room temperature.

Figure 6. Cyclic voltammograms of P3 and P4 in dichloromethane solution of 0.1 M Bu4NBF4 with a scan rate of 100 mV/s.

containing polymers (i.e., P1-P4) were observed to cover a broad absorption wavelength ranging from 250 to 550 nm. Especially for P2, its absorption spectrum shows a broad plateau in the region of 300-450 nm. As compared to the absorption spectra of P0 and perylene,37,38 obviously, the absorption of P1P4 in the visible region (400-460 nm) results from the incorporation of perylene into the polymer main chain. When the content of perylene in the polymers from P1 to P4 was increased, the absorbance at 400-460 nm increased and that at 300-400 decreased. In dilute CHCl3 solutions, P0-P2 show two absorption maxima in the UV and visible region (i.e., 359 and 418 nm for P0, 343 and 428 nm for P1, and 342 and 456 nm for P2). By contrast, polymers P3 and P4 display single strong absorption maxima at 470 and 455 nm, respectively. PL spectra of polymers in CHCl3 solutions are depicted in Figure 5. The fluorescence spectra of each polymer were identical when the polymer (P0-P2) was excited at their maximum absorption wavelengths. Exception for P0, polymers P1-P4 displayed broad emission spectra in the region of 400750 nm. The emission maximum of P0 was observed to be 535 nm, whereas random copolymers P1-P3 displayed two maxima in the emission spectra (i.e., 491 and 626 nm for P1, 529 and 624 nm for P2, and 520 and 615 nm for P3). Alternative copolymer P4 displays three emission maxima at 446, 472, and 603 nm, respectively. The multiple peaks in the emission spectrum arise because an excited electron can relax into one of several vibronic energy levels of the HOMO. Because of the coupling to vibronic modes and the variations in polymer chain configuration, the emission spectra of conjugated polymers of P1-P4 are broad.

Tang et al.

Figure 7. Current-voltage data from single-layer devices with P0P3.

Electrochemical Properties. The electrochemical behavior of the polymers was investigated by CV. CV was performed in a solution of Bu4NBF4 (0.10 M) in dichloromethane at a scan rate of 100 mV/s at room temperature under the protection of argon. A platinum plate was used as a working electrode. A platinum wire was used as the counter electrode, and a Ag/ AgNO3 electrode was used as the reference electrode. The results of the electrochemical measurements are summarized in Table 2. All cyclic voltammograms of P0-P4 exhibited partial reversibility in both n-doping and p-doping processes. The onset potentials for p-doping (oxidation) are observed to fall in the region of 0.66-0.70 V. On the other hand, the onset potentials for n-doping (reduction) range from -1.58 to -1.82 V. From the onset potentials of the oxidation and reduction processes, the band gaps of P0-P4 are estimated to be 2.52, 2.31, 2.25, 2.27, and 2.36 eV, respectively. It is evident that the incorporation of perylene to the polymer backbone results in a narrow band gap when the band gaps of P1-P3 are compared to that of P0. These band gaps are in good agreement with those measured from the onset wavelength of the UV spectra (Table 2). The HOMO and LUMO energy levels were also calculated according to the reported equations, IP ) -([Eonset]ox + 4.4) eV and EA ) -([Eonset]red + 4.4) eV, where [Eonset]ox and [Eonset]red are the onset potentials for the oxidation and reduction processes of polymers.41 The HOMO energy levels of P0-P4 were thus calculated to be -5.10, -5.09, -5.07, -5.06, and -5.08 eV, respectively, whereas the LUMO energy levels of P0-P4 were calculated to be -2.58, -2.78, -2.82, -2.79, and -2.72 eV, respectively. As compared to P0, P1-P3 exhibited lower oxidation potentials (by 0.01-0.04 V) and higher reduction potentials (by 0.14-0.24 V), resulting in lower band gaps (by 0.16-0.25 eV). This is evidence due to the increase in the effective conjugation length along the polymer main chain caused by the incorporation of perylene. Hole Mobility Investigation. It is generally known that a higher charge mobility and better absorption in the visible sun spectrum are critical for photovoltaic application of conjugated polymers.42-44 For the hole transporting materials, the hole mobility is very important for the photovoltaic application of the conjugated polymers. The main purpose of incorporating perylene into the polymer backbone of P0 is for broadening the absorption spectra (Figure 4) and improving the hole mobility of the polymers. Here, the hole mobility of the perylene-containing polymers (P1-P3) was evaluated from the

Poly(phenothiazine-co-bithiophene) Backbone

J. Phys. Chem. B, Vol. 112, No. 12, 2008 3595

TABLE 2: Optical and Electrochemical Properties of Polymers in Chloroform Solutions polymers

λmaxabs (nm)a

λedge (nm)b

λmaxem (nm)

P0 P1 P2 P3 P4

418 (359) 428 (343) 456 (342) 470 455

502 540 548 542 531

535 491, 626 529, 614 520, 615 446, 472, 603

Eoxonset (V)/HOMO (eV)

Eredonset (V)/LUMO (eV)

0.70/-5.10 0.69/-5.09 0.67/-5.07 0.66/-5.06 0.68/-5.08

-1.82/-2.58 -1.62/-2.78 -1.58/-2.82 -1.61/-2.79 -1.68/-2.72

optical Egopt (eV)c

electrochemical Egec (eV)

2.47 2.30 2.26 2.29 2.33

2.52 2.31 2.25 2.27 2.36

a 1 × 10-5 M in anhydrous chloroform, wavelength of maximum absorbance. The data in parentheses are the wavelength of shoulder peaks. λedge is the onset value of the absorption spectrum in the long wavelength range. c Optical band gap (Egopt) was obtained from the empirical formula Eg ) 1240/λedge (eV).

b

(VT ) ∼2.5 V). The maximum brightness of P0-P3 was very low, and the highest brightness (∼200 cd/m2) was observed for P2 and no more than 10 cd/m2 brightness was achieved for P0, P1, and P3. Conclusion

Figure 8. Luminance-voltage characteristics of devices from P0P3.

space-charge-limited current (SCLC) model45 with a single-layer PLED device structure of ITO/PEDOT:PSS/polymer/Ca. The film thickness of P1-P3 was 90 nm. The current densityvoltage data are shown in Figure 7. In the case of charge mobility, the current-voltage relation can be approximated by the following equation:45

J = (9/8)0µ0V2 exp(0.89xV/E0L)/L3 where J is the current density, µ0 is the zero-field mobility, E0 is the characteristic field,  is the dielectric constant of the polymer, o is the permittivity of free space, L is the thickness of the polymer film, V ) Vappl - Vbi, Vappl is the applied voltage, and Vbi is the built-in potential (Vbi ) 1.9 V for our devices with Ca as the cathode). Polymers with a higher current density in single-layer PLED possess a higher hole mobility according to the equation.29 As shown in Figure 7, the current density of the polymers increased quickly with the applied voltage. For example, at 10 V applied voltage, the current density was as high as 390, 554, 563, and 741 mA/cm2, respectively, for P0P3. Obviously, perylene-containing random copolymers (P1P3) possess a higher current density than the alternating copolymer (P0) composed of phenothiazine and bithiophene moieties. Among them, P3 possessed the highest current density, which can be attributed to its partial crystallization (Figure 3). Better structural ordering and alignment of polymer chains obtained in P3 facilitate more efficient interchain and intrachain charge transport, as compared to amorphous P0-P2. The higher current density was largely attributed to hole transport along the polymer backbone since only weak emission from the sample was detected (Figure 8). The weak emission from the polymer with the Ca cathode indicates some electron injection but with electrons carrying less than 1% of the device current.45 The devices P0-P3 showed very low turn-on voltages

We synthesized a series of new low band gap conjugated polymers with a heterocyclic aryl structure using phenothiazine, bithiophene, and perylene as building blocks. The polymers were characterized by NMR, GPC, and elemental analysis. Optical and electrochemical characterizations revealed that all these polymers displayed broad absorption spectra (ca. 300-550 nm) and had low band gaps with the incorporation of the perylene unit. Single-layer PLED devices with the configuration ITO/ PEDOT:PSS/polymer/Ca were fabricated for polymers P0-P3. On the basis of the SCLC model, the insertion of the perylene moiety was beneficial to improve the hole mobility of the conjugated polymers. Only weak emission was observed for the devices, making these polymers a promising candidate for photovoltaic applications. References and Notes (1) Friend, R. H.; Gymer, R. W.; Holmes, A. B.; Burroughes, J. H.; Marks, R. N.; Taliani, C.; Bradley, D. D. C.; Dos Santos, D. A.; Bre`das, J. L.; Lo¨gdlund, M.; Salaneck, W. R. Nature (London, U.K.) 1999, 397, 121. (2) Kraft, A.; Grimsdale, A. C.; Holmes, A. B. Angew. Chem., Int. Ed. 1998, 37, 403. (3) Son, S.; Dodabalapur, A.; Lovinger, A. J.; Galvin, M. E. Science (Washington, DC, U.S.) 1995, 269, 376. (4) Berggren, M.; Ingana¨s, O.; Gustafsson, G.; Rasmusson, J.; Andersson, M. R.; Hjerberg, T.; Wennerstrom, O. Nature (London, U.K.) 1994, 372, 444. (5) Pei, Q.; Yang, Y. J. Am. Chem. Soc. 1996, 68, 147. (6) Hancock, J. M.; Gifford, A. P.; Zhu, Y.; Lou, Y.; Jenekhe, S. A. Chem. Mater. 2006, 18, 4924. (7) Sirringhaus, H.; Tessler, N.; Friend, R. H. Science (Washington, DC, U.S.) 1998, 280, 1741. (8) Ong, B. S.; Wu, Y. L.; Liu, P.; Gardner, S. AdV. Mater. 2005, 17, 1141. (9) Li, Y.; Wu, Y.; Liu, P.; Birau, M.; Pan, H.; Ong, B. S. AdV. Mater. 2006, 18, 3029. (10) McCulloch, I.; Heeney, M.; Bailey, C.; Genevicius, K.; MacDonald, I.; Shkunov, M.; Sparrowe, D.; Tierney, S.; Wagner, R.; Zhang, W.; Chabinyc, M. L.; Kline, R. J.; McGehee, M. D.; Toney, M. F. Nat. Mater. 2006, 5, 328. (11) Hou, J.; Tan, Z.; Yan, Y.; He, Y.; Yang, C.; Li, Y. J. Am. Chem. Soc. 2006, 128, 4911. (12) Scharber, M. C.; Muhlbacher, D.; Koppe, M.; Waldauf, C.; Heeger, A. J.; Barbec, C. J. AdV. Mater. 2006, 18, 789. (13) Kim, Y.; Cook, S.; Tuladhar, S. M.; Choulis, S. A.; Nelson, J.; Durrant, J. R.; Bradley, D. D. C.; Giles, M.; McCulloch, I.; Ha, C.-S.; Ree, M. Nat. Mater. 2006, 5, 197. (14) Beek, W. J. E.; Sloof, L. H.; Wienk, M. M.; Kroon, J. M.; Jassen, R. A. AdV. Funct. Mater. 2005, 15, 1703. (15) Koetse, M. M.; Sweelssen, J.; Hoekerd, K. T.; Schoo, H. F. M.; Veenstra, S. C.; Kroon, J. M.; Yang, X.; Loos, J. Appl. Phys. Lett. 2006, 88, 83504. (16) Thompson, B. C.; Kim, Y.-G.; McCarley, T. D.; Reynolds, J. R. J. Am. Chem. Soc. 2006, 128, 12714. (17) Ko, H. C.; Kim, S.; Lee, H.; Moon, B. AdV. Funct. Mater. 2005, 15, 905.

3596 J. Phys. Chem. B, Vol. 112, No. 12, 2008 (18) Sonmez, G.; Meng, H.; Wudl, F. Chem. Mater. 2004, 16, 574. (19) Fungo, F.; Jenekhe, S. A.; Bard, A. J. Chem. Mater. 2003, 15, 1264. (20) Roncali, J. Chem. ReV. 1997, 97, 173. (21) McCullough, R. D.; Ewbank, P. E.; Loewe, R. S. J. Am. Chem. Soc. 1997, 119, 633. (22) Bernier, S.; Garreau, S.; Bera-Aberem, M.; Gravel, C.; Leclerc, M. J. Am. Chem. Soc. 2002, 124, 12463. (23) Zhou, E.; Chen, H.; Tan, Z.; Yang, C.; Li, Y. J. Polym. Sci., Part A: Polym. Chem. 2006, 44, 4916. (24) Rhomas, S.; Zhang, C.; Sun, S.-S. J. Polym. Sci., Part A: Polym. Chem. 2005, 43, 4280. (25) Ashraf, R. S.; Klemm, E. J. Polym. Sci., Part A: Polym. Chem. 2005, 43, 6445. (26) Zhu, Y.; Champion, R. D.; Jenekhe, S. A. Macromolecules 2006, 39, 8712. (27) Shi, C.; Yao, Y.; Yang, Y.; Pei, Q. J. Am. Chem. Soc. 1996, 118, 8980. (28) Jenekhe, S. A.; Lu, L.; Alam, M. M. Macromolecules 2001, 34, 7315. (29) Zhou, E.; Tan, Z.; Yang, Y.; Huo, L.; Zhou, Y.; Yang, C. Y. Macromolecules 2007, 40, 1831. (30) Hittinger, E.; Kokil, A.; Weder, C. Angew. Chem., Int. Ed. 2004, 43, 1808. (31) Weder, C. Chem. Commun. (Cambridge, U.K.) 2005, 5378.

Tang et al. (32) Cho, N. S.; Park, J. H.; Lee, S. K.; Lee, J.-H.; Shim, H.-K.; Park, M.-J.; Hwang, D.-H.; Jung, B.-J. Macromolecules 2006, 39, 177. (33) Alam, M. M.; Jenekhe, S. A. Chem. Mater. 2002, 14, 4775. (34) Lim, E.; Jung, B.-J.; Shim, H.-K. J. Polym. Sci., Part A: Polym. Chem. 2006, 44, 243. (35) Jung, Y. K.; Lee, J.; Lee, S. K.; Cho, H.-J.; Shim, H.-K. J. Polym. Sci., Part A: Polym. Chem. 2006, 44, 4611. (36) Alam, M. M.; Jenekhe, S. A. Chem. Mater. 2002, 14, 4775. (37) Tang, W.; Kietzeke, T.; Vemulamada, P.; Chen, Z.-K. J. Polym. Sci., Part A: Polym. Chem. 2007, 45, 5266. (38) Klarner, G.; Lee, J.-I.; Davey, M. H.; Miller, R. D. AdV. Mater. 1999, 11, 115. (39) Tang, W.; Ke, L.; Tan, L.; Kietzke, T.; Chen, Z.-K. Macromolecules 2007, 40, 6164. (40) Miyaura, N.; Suzuki, A. Chem. ReV. 1995, 95, 2457. (41) de Leeuw, D. M.; Simenon, M. M. J.; Brown, A. R.; Einerhand, R. E. F. Synth. Met. 1997, 87, 53. (42) Huang, J. S.; Li, G.; Yang, Y. Appl. Phys. Lett. 2005, 87, 112105. (43) Savenije, T. J.; Kroeze, J. E.; Yang, X. N.; Loos, J. AdV. Funct. Mater. 2005, 15, 1260. (44) Verilhac, J. M.; LeBlevennec, G.; Djurado, D.; Rieutord, F.; Chouiki, M.; Travers, J. P.; Pron, A. Synth. Met. 2006, 156, 815. (45) Malliaras, G. G.; Salem, J. R.; Brock, P. J.; Scott, C. Phys. ReV. B: Condens. Matter Mater. Phys. 1998, 58, 13411.