Donor–Acceptor Semiconducting Polymers Containing

Synthesis and photovoltaic properties of two donor–acceptor polymers containing benzodithiophene with 3,3′,5-trihexylbithienyl substituents are re...
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Donor−Acceptor Semiconducting Polymers Containing Benzodithiophene with Bithienyl Substituents Ruvini S. Kularatne, Prakash Sista, Hien Q. Nguyen, Mahesh P. Bhatt, Michael C. Biewer, and Mihaela C. Stefan* Department of Chemistry, University of Texas at Dallas, 800 West Campbell Road, Richardson, Texas 75080, United States S Supporting Information *

ABSTRACT: Synthesis and photovoltaic properties of two donor−acceptor polymers containing benzodithiophene with 3,3′,5-trihexylbithienyl substituents are reported. Benzo[c][1,2,5]thiadiazole and 5-hexylthieno[3,4-c]pyrrole-4,6-dione were used as acceptor building blocks for the synthesis of donor−acceptor polymers. The photovoltaic properties of the synthesized donor−acceptor polymers were investigated in bulk heterojunction solar cells with [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) acceptor.

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INTRODUCTION Benzodithiophene semiconducting polymers have garnered considerable interest in the past few years due to their good performances in both organic field-effect transistors and polymeric solar cells.1,2 In addition to having a planar backbone, the benzodithiophene core offers the flexibility of attaching different substituents on the central benzene core to fine-tune the energy levels of the polymer.2−4 Various research groups have exploited this property and attached different substituents such as alkyl, alkoxy, and thioalkyl to the benzene core of benzodithiophene.2,3,5−9 Our group reported the synthesis and electronic properties of benzodithiophene polymers with alkyl phenylethynyl substituents.10 More recently, the Yang group reported the synthesis of donor−acceptor polymers containing benzodithiophene with a conjugated thienyl substituent.11 Huo showed that replacing the alkoxy with alkylthienyl substituents on the BDT core increases the thermal stability, gives broader absorption spectra, lowers the HOMO and LUMO energy levels, and improves the photovoltaic properties of the polymers.12−14 Furthermore, the attachment of thienyl substituents to the BDT can result in increasing the solubility of the polymer as it provides the ability to attach a larger number of alkyl substituents on the thiophene ring.15 For example, Cao attached four alkyl chains on the BDT building block which improved the solubility of the resulting polymer.15 Recent reports have shown that benzodithiophene semiconducting polymers displayed power conversion efficiencies (PCE) in excess of 7% in bulk heterojunction (BHJ) solar cells.16,17 Donor−acceptor polymers with lower band gaps are highly desirable for organic solar cell applications. Additionally, the ability to improve the open-circuit voltage (Voc) has attracted much attention. Since the Voc of a BHJ solar cell is directly proportional to the difference between the HOMO of the donor and the LUMO of the acceptor, lowering the HOMO © XXXX American Chemical Society

level of the donor polymer will be a crucial factor in enhancing the Voc.18 Moreover, a donor−acceptor copolymer with a deeper HOMO level will enhance the oxidative stability of the polymer. Hou and co-workers synthesized a donor−acceptor polymer with 5-alkylthiophene-substituted BDT with a deeplying HOMO of −5.3 eV and obtained an air-stable donor polymer that gave a Voc of 0.88 V in bulk heterojunction solar cells.12 To generate highly soluble semiconducting polymers with higher molecular weight and broader absorption in the visible region, we targeted the synthesis of benzodithiophene with bithienyl substituents. The bithienyl substituent allows the attachment of up to six alkyl substituents per benzodithiophene repeating unit, which in turn should allow the synthesis of higher molecular weight polymers with good solubility in organic solvents. The conjugated bithienyl substituents perpendicular to the polymer backbone is also expected to provide a broader absorption in the visible region. Additionally, by incorporating this building block in donor−acceptor structures, a deeper HOMO energy level can be obtained, which is expected to generate higher Voc values in bulk heterojunction solar cells.

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EXPERIMENTAL SECTION

Materials. All commercial chemicals were purchased from Aldrich Chemical Co., Inc., and were used without further purification unless otherwise noted. All reactions were conducted under purified nitrogen. The polymerization glassware and syringes were dried at 120 °C for at least 24 h before use and cooled under a nitrogen atmosphere. THF was distilled from sodium/benzophenone ketyl. 2-Bromo-3-hexylthReceived: August 1, 2012 Revised: September 5, 2012

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nitrogen atmosphere at −78 °C, 0.639 mL of 2.5 M n-BuLi in hexane (1.47 × 10−4 mol) was added dropwise. Upon the addition the solution turned to a bright green color. The reaction was stirred at −78 °C for 2 h, at which 1.50 mL of 1 M trimethyltin chloride in hexane (1.50 × 10−4 mol) was added to the reaction mixture. The reaction was stirred at −78 °C for 15 min and was allowed to warm to room temperature. The solvent was evaporated, the obtained yellow oil was diluted with chloroform (200 mL), and the solution was washed with water (5 × 200 mL), dried with anhydrous magnesium sulfate, filtered, and concentrated to obtain the product (0.76 g, 5.63 × 10−4 mol, 82%). 1H NMR (CDCl3, 270 MHz) δ: 0.40 (s, 18H), 0.87 (m, 9H), 1.33 (m, 18H), 1.68 (m, 6H), 2.58 (m, 4H), 2.79 (t, 2H), 6.66 (s, 1H), 7.36 (s, 1H), 7.77 (s, 1H). 13C NMR (CDCl3, 270 MHz) δ: −8.25, 14.23, 22.70, 29.01, 29.28, 29.32, 30.37, 30.85, 31.54, 31.68, 31.74, 122.31, 125.71, 130.39, 137.27, 139.56, 142.07, 142.07, 142.25, 142.56, 143.26, 145.85. Synthesis of Polymer P1. Benzodithiophene monomer 3 (404 mg, 2.99 × 10−4 mol), 4,7-dibromobenzo[c][1,2,5]thiadiazole (88 mg, 2.99 × 10−4 mol), and tetrakis(triphenylphosphene)palladium(0) (9.9 mg, 9.86 × 10−6 mol) were added to a three neck round-bottomed flask under a nitrogen atmosphere. Toluene (20 mL) and DMF (0.5 mL) were added to dissolve the monomers and the catalyst. The reaction mixture was heated at reflux for 3 h, at which time another 20 mL of toluene was added to the reaction mixture. The polymerization was stopped after 30 h by precipitating the polymer in methanol. The polymer was filtered and was purified by Soxhlet extractions with methanol, ether, hexane, and dichloromethane with successive drying after each extraction. The polymer was obtained from the dichloromethane fraction as a dark blue solid upon the evaporation of the solvent (268 mg, 77% yield). 1H NMR (CDCl3, 270 MHz) δ: 0.72 (br, 6H), 0.82 (br, 12H), 1.28 (m, 24H), 1.36 (m, 12H), 1.67 (m, 12H), 2.63 (m, 8H), 2.75 (m, 4H), 6.65 (m, 2H) 7.44 (br, 2H), 7.50 (br, 1H), 7.90 (br, 1H), 9.00 (br, 2H). SEC: Mn = 40 800 g/mol; PDI = 4.4. Synthesis of Polymer P2. Benzodithiophene monomer 3 (252 mg, 1.867 × 10−4 mol), 1,3-dibromo-5-hexylthieno[3,4-c]pyrrole-4,6dione (74 mg, 1.867 × 10−4 mol), and tetrakis(triphenylphosphene)palladium(0) (6.29 mg, 6.2 × 10−6 mol) were added to a three neck round-bottom flask under a nitrogen atmosphere. Toluene (20 mL) and DMF (0.5 mL) were added to dissolve the monomers and the catalyst. The reaction mixture was heated at reflux for 3 h, at which time another 20 mL of toluene was added to the reaction mixture. The polymerization was stopped after 36 h by precipitating in methanol. The polymer was filtered and was purified by Soxhlet extractions with methanol, ether, hexane, and chloroform with successive drying after each extraction. A dark red solid polymer was obtained after evaporation of the chloroform extract (176 mg, 75% yield). 1H NMR (CDCl3, 270 MHz) δ: 0.70 (br, 3H), 0.80 (br, 18H), 1.25 (m, 42H), 1.54 (m, 16H), 2.59 (m, 4H), 2.65 (m, 4H), 2.74 (m, 4H) 3.61 (m, 2H), 6.63 (m, 2H), 7.47 (m, 2H), 8.85 (m, 2H). SEC: Mn = 29 000 g/mol; PDI = 2.5. Analysis. 1H and 13C NMR spectra were recorded at room temperature using either a 270 MHz JEOL or a 500 MHz Bruker spectrometer, as indicated, and were referenced to residual protio solvent (CHCl3: δ 7.27 ppm). The data are reported as follows: chemical shifts are reported in ppm on the δ scale, multiplicity (br = broad, s = singlet, d = doublet, t = triplet, q = quartet, m = multiplet). GC-MS was obtained on an Agilent 6890-5973 GC/MS workstation. The GC column was a Hewlett-Packard fused silica capillary column cross-linked with 5% phenylmethylsiloxane. Helium was the carrier gas (1 mL/min). The following conditions were used for all GC/MS analyses: injector and detector temperature, 250 °C; initial temperature, 70 °C; temperature ramp, 10 °C/min; final temperature, 280 °C. Analytical thin layer chromatography was performed on EM reagents 0.25 mm silica gel 60-F plates. Liquid chromatography was performed using flash chromatography of the indicated solvent system on Select silical gel (SiO2) 230−400 mesh. Molecular weights of the synthesized polymers were measured by size exclusion chromatography (SEC) analysis on a Viscotek VE 3580 system equipped with ViscoGEL columns (GMHHR-M), connected

iophene and 4,8-dihydrobenzo[1,2-b:4,5-b′]dithiophene-4,8-dione were prepared according to the literature.19,20 Synthesis of 3,3′-Dihexyl-2,2′-bithiophene. Magnesium turnings (0.218 g, 0.009 mol) were added to a three-neck round-bottomed flask and were kept under vacuum for 2 h. The vacuum was canceled with nitrogen followed by the addition of 2-bromo-3-hexylthiophene (2.0 g, 0.0080 mol) and anhydrous ether (60 mL). The reaction was initiated by heating the flask and by adding several drops of 1,2dibromoethane. After 30 min the reaction mixture turned to a gray color, indicating the formation of 2-magnesiobromo-3-hexylthiophene (flask A). In another round-bottomed flask [1,3-bis(diphenylphosphino)propane]dichloronickel(II) (0.1626 g, 0.0003 mol), 2-bromo-3-hexylthiophene (2.0 g, 0.0080 mol), and anhydrous diethyl ether (60 mL) were added under a nitrogen atmosphere (flask B). The 2-magnesiobromo-3-hexylthiophene from flask A was cannulated to flask B. The reaction mixture was stirred overnight at 40 °C. The reaction mixture was extracted into ethyl acetate, and the organic phase was washed with water, brine, dried over anhydrous magnesium sulfate, filtered, and concentrated to obtain a brown oil which was further purified by column chromatography (eluent hexane) to obtain a clear oil of 3,3′-dihexyl-2,2′-bithiophene (2.10 g, 0.0063 mol, 78%). 1H NMR (CDCl3, 270 MHz) δ: 0.78 (t, 6H), 1.16 (m, 12H), 1.47 (m, 4H), 2.42 (t, 4H), 6.89 (d, 2H), 7.21 (d, 2H). 13C NMR (CDCl3, 270 MHz) δ: 14.14, 22.65, 28.87, 29.18, 30.78, 31.71, 125.29, 128.60, 128.79, 142.42. GC-MS: m/z = 334.3. Synthesis of 3,3′,5-Trihexyl-2,2′-bithiophene (2). 3,3′-Dihexyl-2,2′-bithiophene (1.80 g, 0.005 38 mol) was diluted in 125 mL of THF followed by the dropwise addition of n-BuLi in hexane (2.5 M) (0.005 28 mol, 2.29 mL) under a nitrogen atmosphere at 0 °C. The reaction mixture was stirred at 0 °C for 30 min, followed by the addition of 1-bromohexane (1.132 mL, 0.008 07 mol), and was heated at reflux for 2 h. The reaction was stopped and was diluted with diethyl ether (200 mL). The organic phase was washed with water (4 × 200 mL) and brine (200 mL) and dried with anhydrous magnesium sulfate and concentrated to obtain a oil, which was further purified by column chromatography (eluent hexane) to obtain a yellow color oil (0.89 g, 0.002 mol, 40%). 1H NMR (CDCl3, 270 MHz) δ: 0.87 (m, 9H), 1.26 (m,12H), 1.33 (m, 6H), 1.52 (m,4H), 1.68 (m, 2H), 2.42 (t, 2H), 2.51 (t, 2H), 2.76 (t, 2H), 6.63 (s, 1H), 6.93 (s, 1H), 7.24 (d, 1H). 13 CNMR (CDCl3, 270 MHz) δ: 14.16, 22.67, 28.89, 28.97, 29.04, 29.20, 29.24, 30.32, 30.79, 31.54, 31.68, 31.74, 124.95, 125.84, 128.53, 129.5, 142.03, 142.05, 145.67. Synthesis of 4,8-Bis(3,3′,5′-trihexyl-[2,2′-bithiophen]-5-yl)benzo[1,2-b:4,5-b′]dithiophene (2′). 3,3′,5-Trihexyl-2,2′-bithiophene (0.80 g, 0.0019 mol) was dissolved in 60 mL of dry THF followed by the dropwise addition of n-BuLi in hexane (2.5 M) (0.002 20 mol, 0.95 mL) under a nitrogen atmosphere at 0 °C. The reaction mixture was stirred at 0 °C for 30 min, followed by the addition of 4,8dihydrobenzo[1,2-b:4,5-b′]dithiophen-4,8-dione (0.190 g, 0.000 864 mol). After the addition the reaction mixture was heated at reflux for 1 h. The reaction mixture was cooled to room temperature, and tin chloride dihydrate (0.985 g, 0.004 37 mol) dissolved in 20 mL of 20% HCl was added. The reaction mixture was heated at reflux for 2 h. The solvent was evaporated to obtain a yellow oil, which was dissolved in diethyl ether (200 mL), and the solution was washed with water (4 × 200 mL), dried with anhydrous magnesium sulfate, filtered, and concentrated to obtain a yellow oil, which was further purified by column chromatography (eluent hexane) to obtain 0.78 g of the product (7.6 × 10−4 mol, 80%). 1H NMR (CDCl3, 270 MHz) δ: 0.87 (m, 9H), 1.33 (m, 18H), 1.68 (m, 6H), 2.58 (m, 4H), 2.79 (t, 2H), 6.66 (s, 1H), 7.34 (s, 1H), 7.47 (d, 2H), 7.72 (d, 2H). 13CNMR (CDCl3, 270 MHz) δ: 14.19, 22.71, 22.68, 28.98, 29.09, 29.21, 29.25, 29.31, 30.36, 30.82, 30.91, 31.55, 31.69, 31.78, 123.97, 125.44, 130.73, 136.48, 138.85, 138.99, 142.23, 142.35, 145.99. Anal. Calcd for C62H86S6: C,72.80%; H, 8.41%; S, 18.79%. Found: C, 73.12%; H, 8.46%; S, 18.42%. Synthesis of 2,6-(Trimethyltin)-4,8-bis(3,3′,5′-trihexyl-[2,2′bithiophen]-5-yl)benzo[1,2-b:4,5-b′]dithiophene. 4,8-Bis(3,3′,5′-trihexyl-[2,2′-bithiophen]-5-yl)benzo[1,2-b:4,5-b′]dithiophene (0.7 g, 6.83 × 10−4 mol) was dissolved in 50 mL of dry THF. Under a B

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Scheme 1. Synthesis of Benzodithiophene with the 3,3′,5-Trihexylbithienyl Substituents

to a refractive index (RI) detector. A GPC solvent/sample module (GPCmax) was used with HPLC grade THF as the eluent, and calibration was based on polystyrene standards. Running conditions for SEC analysis were as follows: flow rate = 1.0 mL/min, injector volume = 100 μL, detector temperature = 30 °C, column temperature = 35 °C. All the polymer samples were dissolved in THF, and the solutions were filtered through PTFE filters (0.45 μm) prior to injection. The UV−vis spectra of polymer solutions in chloroform solvent were carried out in 1 cm cuvettes using an Agilent 8453 UV−vis spectrometer. Thin films of polymer were obtained by evaporation of chloroform from polymer solutions on glass microscope slides. The films for the determination of absorption coefficients were deposited by spin-casting solutions of 5 mg/mL of polymer in chloroform. CV was obtained with a BAS CV-50W voltammetric analyzer (Bioanalytical Systems, Inc.). Electrochemical grade tetrabutylammonium perchlorate (TBAP) was used without further purification. Acetonitrile was distilled over calcium hydride and collected over molecular sieves. The electrochemical cell was comprised of a platinum electrode, a platinum wire auxiliary electrode, and an Ag/AgCl reference electrode. Acetonitrile solutions containing 0.1 M TBAP were placed in a cell and purged with argon. A drop of the polymer solution was evaporated in ambient air. The film was immersed into the electrochemical cell containing the electrolyte, and the oxidation potential was observed and recorded. All electrochemical shifts were standardized to the ferrocene redox couple at 0.474 V. Preparation of Solar Cell Devices. Glass substrates coated with ITO were purchased from Luminescence Technology Corp. (Taiwan) and were patterned using standard photolithography. The substrates were cleaned by sonication for 20 min in acetone, methanol, toluene, and isopropyl alcohol. The substrates were subjected to UV/ozone treatment for 20 min prior to use. After the ozone treatment, poly(3,4ethylenedioxythiophene):poly(styrenesulfonate) PEDOT−PSS was spin-coated on the substrates (4000 rpm, 1740 rpm/s, 90 s). The substrates were annealed at 120 °C for 10 min under a nitrogen atmosphere. Polymer/PCBM blends were prepared in chloroform with a total blend concentration of 12 mg/mL. For the solar cells with 1,8-diiodooctane (DIO) additive the blends were prepared by adding DIO dissolved in chloroform to a polymer/PCBM blend such that the total concentration was 12 mg/mL. These blends were spin-coated (2000 rpm, 60 s) on the PEDOT−PSS treated substrate. Films of 10 nm Ca and 100 nm Al were thermally evaporated on the substrates at a rate of 2.5 Å/s through a shadow mask to obtain the solar cell devices. IV testing was carried out under a controlled nitrogen atmosphere using a Keithley 236, model 9160 interfaced with LabView software. The solar simulator used was a THERMOORIEL equipped with a 300 W xenon lamp; the intensity of the light was calibrated to 100 mW cm−2 with a NREL certified Hamamatsu silicon photodiode. The active area of the devices was 0.1 cm2. The active layer film thickness was measured using a Veeco Dektak VIII profilometer. Tapping Mode Atomic Force Microscopy (TMAFM). was carried out on the active area of the solar cell devices. AFM studies were carried out on a VEECO-dimension 5000 scanning probe microscope with a hybrid xyz head equipped with Nano-Scope

Software. AFM images were obtained using silicon cantilevers with nominal spring constant of 42 N/m and nominal resonance frequency of 300 kHz (OTESPa). Image analysis software Nanoscope 7.30 was used for surface imaging and image analysis. All the AFM measurements were taken of the active area of solar cell devices and were conducted under ambient conditions. All cantilever oscillation amplitude was equal to ca. 375 mV, and all images were acquired at 2 Hz scan frequency. Sample scan area was 5 μm. Field-Effect Mobility Measurements. of the synthesized polymers were performed on thin-film transistors with a common bottom-gate, bottom-contact configuration. Highly doped, n-type silicon wafers with a resistivity of 0.001−0.003 Ω cm were used as substrates. 200 nm of thermal oxide (SiO2) was grown at 1000 °C. Chromium metal (5 nm) followed by 100 nm of gold was deposited by E-beam evaporation as source-drain metals. The source-drain pads were formed by photolithographically patterning the metal layer. The SiO2 on the backside of the wafer was etched with buffered oxide etchant (7:1 BOE from J.T. Baker) to generate the common bottom gate. The resulting transistors had a channel width of 475 μm and channel length ranging from 2 to 80 μm. The measured capacitance density of the SiO2 dielectric was 17 nF/cm2. After the SiO2 on the backside was removed, the devices were cleaned under UV-ozone for 6 min using a Technics Series 85 RIE etcher and stored under vacuum. This process removes any residual organics on the substrate. Prior to the polymer deposition, the devices were treated with n-octyltrichlorosilane (OTS). For the surface treatment, the devices were rinsed sequentially with deionized water, acetone, hexanes, and chloroform and placed in a glass container in a solution of 8 × 10−3 M noctyltrichlorosilane silane in dried toluene. The sealed container was placed in a glovebox at ambient temperature for 48 h. After 48 h, the devices were taken out of the glovebox and rinsed with freshly distilled toluene before baking them at 80 °C for 30 min in a vacuum oven. The devices were allowed to cool under vacuum. The polymer films were deposited in air by drop-casting 4−5 drops of 1 mg/mL of polymer solution (in chloroform) filtered through a 0.2 μm PTFE filter using a 25 μL syringe. The devices were allowed to dry in a Petri dish saturated with chloroform. The devices were annealed under vacuum for 30 min at 120 °C prior to measurements. The devices were allowed to cool down to room temperature under vacuum after annealing. A Keithley 4200-SCS semiconductor characterization system was used to probe the devices. The probe station used for electrical characterization was a Cascade Microtech Model Summit Microchamber. When measuring current−voltage curves and transfer curves, VGS was scanned from +20 to −100 V. All the measurements were performed at room temperature in air.

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RESULTS AND DISCUSSION In this paper we describe the synthesis of a new benzodithiophene building block containing 3,3′,5-trihexylbithienyl substituents. This building block increases the conjugation along the side chain to ensure a broader absorption in the visible region and enhances the solubility of the synthesized polymers due to the attachment of six alkyl substituents per benzodithiophene repeating unit. We are also C

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Scheme 2. Synthesis of the Donor−Acceptor Polymers

Table 1. Molecular Weight as Well as Optical and Electronic Energy Levels of Polymers polymer

Mna (g/mol)

PDI

λmaxb (nm)

λmaxc (nm)

HOMOd (eV)

LUMOe (eV)

Egf (eV)

Egg (eV)

P1 P2

40 800 29 000

4.4 2.5

348, 589 366, 563

354, 593, 644 371, 561, 608

−5.53 −5.52

−3.57 −3.48

1.97 2.04

1.69 1.70

Determined by SEC (THF eluent). bAbsorption of chloroform solution. cAbsorption of films. dEstimated from the onset of oxidation wave of cyclic voltammogram. eEstimated from the onset of reduction wave of cyclic voltammograms. fElectrochemical band gap calculated from cyclic voltammograms. gOptical band gap calculated from the onset of the UV−vis spectra of the films. a

Figure 1. UV−vis absorption spectra of (a) polymer P1 and (b) polymer P2: red, film; blue, solution (chloroform solvent).

generated the benzodithiophene precursor which was converted to its distannylated derivative (3, Scheme 1). The donor−acceptor polymers were synthesized by the Stille coupling polymerization of distannylated monomer 3 with 4,7dibromobenzo[c][1,2,5]thiadiazole (4, Scheme 2) and 1,3dibromo-5-hexylthieno[3,4-c]pyrrole-4,6-dione (5, Scheme 2) to generate polymers P1 and P2, respectively (Scheme 2). Polymer P1 had a molecular weight of 40 800 g/mol, while polymer P2 had a molecular weight of 29 000 g/mol (Table 1). Both synthesized polymers had good solubility in common organic solvents such as chloroform, toluene, and THF. The UV−vis absorption spectra of polymers P1 and P2 in chloroform and film are shown in Figure 1. Both polymers display similar absorption spectra. The UV−vis spectra of both polymers P1 and P2 display two absorption maxima at ∼350 and ∼560 nm and a shoulder peak at ∼600 nm. The absorption maxima at ∼350 nm is due to the 3,3′,5-trihexyl-2,2′-bithienyl

reporting here the synthesis and photovoltaic properties of two donor−acceptor polymers containing benzodithiophene with 3,3′,5-trihexylbithienyl substituents donor. Benzo[c][1,2,5]thiadiazole and 5-hexylthieno[3,4-c]pyrrole-4,6-dione were used as acceptor building blocks for the synthesis of donor− acceptor polymers. These strong acceptor units with the conjugated bithienyl substituents on the BDT moiety were expected to generate a donor−acceptor polymer with low-lying HOMO energy level, which in turn should give relatively high values for the Voc in BHJ solar cell devices. 3,3′,5-Trihexyl-2,2′-bithiophene precursor (2, Scheme 1) was synthesized by Kumada cross-coupling of 2-bromo-3-hexylthiophene (1, Scheme 1) and 2-magnesiobromo-3-hexylthiophene followed by lithiation and reaction with 1-bromohexane.19,21 The reaction of 4,8-dihydrobenzo[1,2-b:4,5-b′]dithiophen-4,8-dione with lithiated 3,3′,5-trihexyl-2,2′bithiophene followed by the aromatization using tin chloride D

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Figure 2. I−V curve for the best bulk heterojunction solar cell device of polymer: (a) P1 (P1:PCBM = 1:3); (b) P2 (P2:PCBM = 1:4).

Table 2. Photovoltaic Properties of Polymers P1 and P2 polymer

[P]:PCBM weight ratio

Voc (V)

Isc (mA/cm2)

FF

PCEa (%)

P1

1:1 1:2 1:3 1:4 1:1 1:2 1:3 1:4

1.04 0.98 0.99 0.97 0.93 0.99 0.89 0.91

2.00 4.96 5.36 4.91 1.06 3.21 3.26 3.22

0.24 0.30 0.47 0.43 0.23 0.31 0.34 0.45

0.50 1.46 2.54 2.08 0.23 1.00 1.00 1.33

P2

(0.42) (1.24) (2.38) (1.89) (0.19) (0.88) (0.90) (1.18)

thicknessb (nm) 59.5 79.5 85.8 77.4 80.2 65.1 69.8 83.9

a

The data outside of the parentheses represent the maximum measured PCE values, and the data inside the parentheses represent the average measured PCE values. bThickness of the polymer/PCBM active layer blend.

decomposition temperature for both polymers is around 400 °C, which indicates a good thermal stability. The photovoltaic properties of polymers P1 and P2 were investigated in BHJ solar cells with [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) acceptor. The solar cells were fabricated using a conventional device structure ITO/ PEDOT:PSS/polymer:PCBM/Ca/Al and tested under AM1.5G illumination. The polymer/PCBM blends were prepared in chloroform maintaining a total blend concentration of 12 mg/mL. To determine the optimal blending ratio, the polymers were tested in BHJ solar cell at [P]:[PCBM] weight ratios ranging from 1:1 to 1:4 (Table 2). Upon the increase of polymer/PCBM weight ratio from 1:1 to 1:3 the PCE of the polymer P1 increased from 0.59% to 2.54%, and the shortcircuit current density (Jsc) increased from 2.00 to 5.36 mA/ cm2. A maximum PCE of 2.54% was measured for polymer P1 for a weight ratio [P1]:[PCBM] = 1:3. The measured Voc varied between 0.97 and 1.04 V, which represent a relatively good agreement with the theoretically predicted values calculated from the HOMO of the donor polymer and LUMO of PCBM acceptor.18 The polymer P1 gave exceptionally large Voc values which is an important factor in achieving large PCEs since the PCE is given by the product between Voc, Jsc, and fill factor (FF). A maximum PCE of 1.33% was measured for polymer P2 at a weight ratio of 1:4. Similar to polymer P1, the Voc values ranged from 0.89 to 0.99 V. While the Voc values obtained for polymer

substituents while the absorption band in the visible region is due to the conjugation along the polymer backbone. A significant feature of the UV/vis spectra of the films vs the chloroform solutions is the broadening of the absorption bands and the increase in the intensity of the shoulder peaks. A relatively low red-shift was observed for the polymer films as compared to the solution (Table 1), indicating that the polymers have a rigid-rod confirmation in both solution and solid state.22,23 The estimated absorption coefficients of polymers P1 and P2 are 3.84 × 104 and 3.80 × 104 cm−1, respectively (Supporting Information). The HOMO and LUMO energy levels of polymers P1 and P2 were estimated by cyclic voltammetry from the onset of the oxidation and the reduction curves, respectively. The HOMO and the LUMO energy levels of the polymer P1 were determined as −5.53 and −3.57 eV and that of the polymer P2 as −5.52 and −3.48 eV, respectively (Table 1). Both polymers P1 and P2 have a band gap of ∼2.0 eV. The HOMO energy levels of polymers P1 and P2 are lower than the air oxidation threshold (ca. −5.27 eV), indicating good air stability.24,25 Moreover, due to the lower HOMO energy level of these polymers, a higher Voc for BHJ could be potentially obtained.18 The thermal stability of the polymers P1 and P2 were investigated using thermogravimetric analysis (TGA) (thermograms shown in Supporting Information). The onset of E

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3).12,20,22 The decrease of PCE upon addition of DIO could be due to the preferential interaction of DIO with the insulating alkyl substituents of the benzodithiophene repeat unit which affects the morphology of the blends. Moreover, it has been shown that the effect of DIO on the solar cell performance is dependent on the solubility of the polymer and also on the chain stacking ability of the donor polymer.23 These factors may also contribute to a decrease of PCE. The surface morphology of the active layer of BHJ solar cells was investigated by tapping mode atomic force microscopy (TMAFM). TMAFM analysis of the devices with the highest PCEs was preformed. The 3D-TMAFM images of P1/PCBM and P2/PCBM blends are shown in Figure 3 and display a smoother surface morphology for P1/PCBM blend (rms = 0.368 nm) as compared to the P2/PCBM blend (rms = 0.855 nm). This result is consistent with our previous report in which we observed that smoother blend films displayed better performance in bulk heterojunction solar cells.11,21 TMAFM analysis was also performed for the solar cell devices of both P1 and P2 with 1% DIO additive (Supporting Information and Figure 3). A comparison of the TMAFM images of P1/PCBM blend with 0% and 1% DIO indicates that the film obtained from 1% DIO has a more rougher surface morphology (rms = 0.432 nm) as compared to those obtained without DIO (rms = 0.368 nm), thereby reducing the PCEs of the devices. The same trend was observed for P2/PCBM blends where the film obtained from 1% DIO had a rougher morphology (rms = 2.19 nm) as compared to blend without DIO (rms = 0.855 nm). The field-effect mobilities of the polymers P1 and P2 were measured in bottom-gate bottom-contact field-effect transistors (OFET).5,29 The IDS−VDS curve family was recorded at different gate voltages. The field-effect mobility (μ) was obtained by plotting IDS1/2 vs VGS in the saturation regime (Figure 4), using the equation

P2 are comparable to P1, the Jsc values are almost twice lower. The larger decrease of Jsc observed for polymer P2 is most likely due to the unfavorable morphology of the active layer blend. Moreover, the higher PCEs measured for polymer P1 are most probably due to its higher molecular weight as compared to polymer P2 (Table 1). To attempt further optimization of the photovoltaic performance of the polymers P1 and P2, 1,8-diiodooctane (DIO) was used as an additive as previously reported by other research groups.26−28 It has been shown that the high boiling point DIO does not evaporate completely during spin-coating, and it can interact with either the donor polymer or the PCBM acceptor phase in the blend.27 We used the DIO additive for the weight ratios that gave the best performing BHJ solar cells, i.e., [P1]:[PCBM] = 1:3 and [P2]:[PCBM] = 1:4 (Table 3). In Table 3. Photovoltaic Properties of Polymers P1 and P2 with 1,8-Diiodooctane Additive polymer P1a

P2b

DIOc (%)

Voc (V)

Isc (mA/cm2)

FF

PCEd (%)

0 1 3 5 0 1 3 5

1.00 0.98 0.78 0.79 0.91 0.85 0.92 0.55

5.36 5.32 4.88 4.78 3.22 2.85 2.48 3.90

0.47 0.41 0.31 0.34 0.45 0.35 0.40 0.27

2.54 2.17 1.21 1.28 1.33 0.86 0.91 0.58

(2.38) (1.65) (1.15) (1.03) (1.18) (0.77) (0.80) (0.43)

thicknesse (nm) 85.8 67.5 69.5 91.3 83.9 86.9 80.9 67.2

a

[P1]:[PCBM] = 1:3. b[P2]:[PCBM] = 1:4. cWeight % DIO. dThe data in the parentheses represent the average measured PCE values. e Thickness of the polymer/PCBM blend.

contrast to previous reports, the PCE of both polymers P1 and P2 decreased upon the addition of DIO additive (Table

Figure 3. 3D TMAFM phase images of solar cell devices: (a) polymer P1/PCBM (1:3 w/w) active layer without additives; (b) polymer P1/PCBM (1:3 w/w) active layer with 1,8-diiodooctane; (c) polymer P2/PCBM (1:4 w/w) active layer without additives; (d) polymer P2/PCBM (1:4 w/w) active layer with 1,8-diiodooctane; scan size 5 × 5 μm (spin-cast from chloroform). F

dx.doi.org/10.1021/ma301624t | Macromolecules XXXX, XXX, XXX−XXX

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Figure 4. Current−voltage characteristic of polymer P1: (a) output curves at different gate voltages; (b) transfer curves at VDS = −50 V (μ = 7.3 × 10−4 cm2/(V s), VT = 2.1 V, on/off ratio = 102, W = 475 μm, L = 20 μm). Current−voltage characteristic of polymer P2: (c) output curves at different gate voltages; (d) transfer curves at VDS = −50 V (μ = 9.5 × 10−5 cm2/(V s), VT = −6.5 V, on/off ratio = 10, W = 475 μm, L = 20 μm).

μ=

⎤ IDS 2L ⎡ ⎢ ⎥ WCi ⎣ (VGS − VT)2 ⎦

tuning to generate highly soluble semiconducting polymers for use in solar cells.

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where IDS is the source-drain current, W is the channel width, L is the channel length, Ci is the capacitance of the dielectric, VGS is the gate voltage, and VT is the threshold voltage. The polymer P1 had mobility of 7.3 × 10−4 cm2/(V s), and the polymer P2 had a mobility of 9.5 × 10−4 cm2/(V s).

ASSOCIATED CONTENT

S Supporting Information *

1 H and 13C NMR spectra of monomers and polymers, cyclic voltammograms of polymers, thermograms of polymers, TMAFM images of polymer blends. This material is available free of charge via the Internet at http://pubs.acs.org.

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CONCLUSION A novel benzodithiophene monomer with bithienyl substituents has been synthesized. Stille coupling of the benzodithiphene monomer with 4,7-dibromobenzo[c][1,2,5]thiadiazole and 1,3-dibromo-5-hexylthieno[3,4-c]pyrrole-4,6-dione generated two donor−acceptor polymers P1 and P2, respectively. Six alkyl substituents were attached per each benzodithiophene repeating unit generating relatively higher molecular weight polymers with good solubility in organic solvents. Both polymers showed relatively good photovoltaic properties: P1 with a PCE of 2.54% and P2 with a PCE of 1.33%. In addition, the lower HOMO energy levels of polymers P1 and P2 resulted in high Voc values up to 1.04 V. The surface morphology of the polymer/PCBM blends in solar cell devices suggested that smoother films gave a higher PCE. While the photovoltaic performance of the reported polymers is modest, the novel benzodithiophene with 3,3′,5-trihexylbithienyl substituents building block is highly versatile, and it allows further structural

AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support for this project from NSF (Career DMR0956116) and Welch Foundation (AT-1740) is gratefully acknowledged. We gratefully acknowledge the NSF-MRI grant (CHE-1126177) used to purchase the Bruker Advance III 500 NMR instrument.

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REFERENCES

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