High Mobility Thiazole–Diketopyrrolopyrrole Copolymer

Nov 5, 2012 - New donor–acceptor copolymers incorporating both a strong electron-accepting diketopyrrolopyrrole unit and a weak electron-deficient ...
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High Mobility Thiazole−Diketopyrrolopyrrole Copolymer Semiconductors for High Performance Field-Effect Transistors and Photovoltaic Devices Selvam Subramaniyan, Felix Sunjoo Kim, Guoqiang Ren, Haiyan Li, and Samson A. Jenekhe* Department of Chemical Engineering and Department of Chemistry, University of Washington, Seattle, Washington 98195-1750, United States S Supporting Information *

ABSTRACT: New donor−acceptor copolymers incorporating both a strong electron-accepting diketopyrrolopyrrole unit and a weak electron-deficient thiazolothiazole or benzobisthiazole moiety were synthesized, characterized, and found to exhibit very high charge carrier mobility. Stille coupling copolymerization gave copolymers having moderate number-average molecular weights of 17.0−18.5 kDa with polydispersities of 3.3−4.0 and optical band gaps of 1.22−1.38 eV. High performance p-channel field-effect transistors were obtained using the thiazolothiazole-linked copolymers, PDPTT and PDPTTOx, giving hole mobilities of 0.5 and 1.2 cm2/(V s), respectively, with on/off current ratios of 105 to 106. In contrast, the benzobisthiazole-linked copolymer PDPBT had a substantially lower field-effect mobility of holes (0.005 cm2/(V s)) due to its amorphous solid state morphology. Bulk heterojunction solar cells fabricated by using one of the thiazolothiazole-linked copolymer, PDPTT, as electron donor and PC71BM acceptor show a power conversion efficiency of 3.4% under 100 mW/cm2 AM1.5 irradiation in air.

1. INTRODUCTION Solution-processable conjugated polymers are of growing interest for the development of large area and printable fieldeffect transistors1−13 and low-cost solar cells.14−22 Although remarkable progress has been made in the design and synthesis of p-type polymer semiconductors, their performance in terms of hole mobilities and photovoltaic properties remains to be fully optimized and maximized. An important approach in the development of p-type polymer semiconductors is the donor− acceptor (D−A) copolymer architecture, which by virtue of its modular nature can facilitate the tuning of absorption bands, charge carrier mobilities, and HOMO/LUMO energy levels, depending on the choice of the electron-donating (D) and electron-accepting segments (A).23 Among recent examples of D−A copolymers found to be good p-type semiconductors for developing high performance field-effect transistors include those containing donor moieties such as thienothiophene10a,11b and cyclopentadithiophene3a and acceptors such as diketopyrrolopyrrole (DPP),8−11 thiazolothiazole (TT),12 and benzothiadiazole (BTz).3a Further fine-tuning of the electronic structure and charge transport properties can be facilitated by the D−A copolymer architecture by the utilization of two or more different acceptor or donor units in the backbone: (D− A 1 −D−A 2 −D) or (A−D1 −A−D 2 −A). This strategy is exemplified by copolymers incorporating both DPP and benzothiadiazole as the electron-deficient acceptor units, resulting in high mobility ambipolar charge transport in the copolymer semiconductors.7c,e D−A copolymers incorporating DPP and oligothiophene segments have been extensively studied and found to have © 2012 American Chemical Society

broad absorption bands in the UV−vis/near-IR region, low optical band gaps, enhanced π−π stacking/crystallinity, and high charge carrier mobilities.8 Furthermore, the molecular πstacking and the charge transport properties of DPP-based copolymers were found to be strongly dependent on incorporation of a fused aromatic comonomer in the repeat unit.6a,b,7d,10b DPP-based copolymers have also been explored as the electron donor component in bulk heterojunction solar cells showing promising power conversion efficiencies.20 On the other hand, thiazolothiazole (TT)12 and benzobisthiazole (BT)13 have also emerged as promising electron-accepting units in the development of p-type polymer semiconductors for high performance OFETs12,13 and solar cells showing promising power conversion efficiency.17,18 In this paper, we report the synthesis, charge transport, and photovoltaic properties of three new p-type polymer semiconductors, which have a D−A structure and contain both thiazolothiazole (or benzobisthiazole) and diketopyrrolopyrrole as the electron-accepting moieties. The molecular structures of the new conjugated copolymers are shown in Chart 1: poly[3,6dithiene-2-yl-2,5-di(2-hexyldecyl)pyrrolo[3,4-c]pyrrole-1,4dione-5′,5″-diyl-alt-(2,5-bis(3-ethylhexylthiophen-2-yl)thiazolo[5,4-d]thiazole] (PDPTT); poly[3,6-dithiene-2-yl-2,5-di(2hexyldecyl)pyrrolo[3,4-c]pyrrole-1,4-dione-5′,5″-diyl-alt-2,5bis(3-octyloxythiophen-2-yl)-thiazolo[5,4-d]thiazole] (PDPTTOx); and poly[3,6-dithiene-2-yl-2,5-di(2-hexyldecyl)Received: August 6, 2012 Revised: October 22, 2012 Published: November 5, 2012 9029

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dissolved in anhydrous chlorobenzene (38 mL), and the flask was heated at reflux for 3 days under an argon atmosphere. Then the heating was reduced to 50 °C; the reaction mixture was poured into 200 mL of methanol containing 6 mL of hydrochloric acid and stirred for 5 h. The black precipitate was collected via filtration and was further purified by Soxhlet extraction with methanol and hexane. Then the residue was extracted with chloroform, evaporated, and dried to yield a dark brown solid with a metallic appearance (400 mg, 71%). 1H NMR (CDCl3): 8.71−8.91 (m, 4H), 6.85−7.20 (m, 2H), 3.82−4.30 (m, 4H), 2.71−3.02 (m, 4H), 0.87−2.10 (m, 92H). GPC: Mn = 18.0 kDa, PDI = 3.3. Synthesis of Poly[3,6-dithiene-2-yl-2,5-di(2-hexyldecyl)pyrrolo[3,4-c]pyrrole-1,4-dione-5′,5″-diyl-alt-2,5-bis(3-octyloxythiophen-2yl)thiazolo[5,4-d]thiazole] (PDPTTOx). A mixture of dibromo compound 1 (337 mg, 0.37 mmol) and distannyl compound 3 (330 mg, 0.37 mmol) and catalyst tris(dibenzylideneacetone) dipalladium(0) (7 mg, 0.007 mmol) and tri-o-tolylphosphine (9 mg, 0.03 mmol) in anhydrous chlorobenzene (10 mL) was heated at 120 °C for 72 h under an argon atmosphere. The temperature was then slowly reduced to 55 °C. Then the reaction mixture was poured into 200 mL of methanol containing 5 mL of hydrochloric acid and stirred for 6 h. The brown precipitate was collected via filtration and was further purified by Soxhlet extraction with methanol, hexane, and dichloromethane. The remaining solid obtained was dried under vacuum to afford PDPTTOx in 65% yield. 1H NMR (CDCl3, 300 MHz, ppm): δ 8.30−9.11 (m, 4H), 6.04−6.93 (m, 2H), 3.50−4.50 (m, 8H), 0.81− 2.10 (m, 92H). GPC: Mn = 17.0 kDa, PDI = 4.0. Synthesis of Poly[3,6-dithiene-2-yl-2,5-di(2-hexyldecyl)pyrrolo[3,4-c]pyrrole-1,4-dione-5′,5″-diyl-alt-2,5-bis(3-dodecylthiophen-2yl)benzo[1,2-d;4,5-d′]bisthiazole] (PDPBT). In a 50 mL round-bottom flask, a mixture of dibromo compound 1 (380 mg, 0.42 mmol) and ditin compound 4 (428 mg, 0.42 mmol) and catalyst tris(dibenzylideneacetone)dipalladium(0) (8 mg, 0.008 mmol) and trio-tolylphosphine (10 mg, 0.017 mmol) in anhydrous chlorobenzene (20 mL) was heated at reflux for 30 h. Then the heating was reduced to 50 °C; the reaction mixture was poured into 200 mL of methanol containing 5 mL of hydrochloric acid and stirred for 5 h. The black precipitate was collected via filtration and was further purified by Soxhlet extraction with methanol and hexane. Then the residue was further extracted with chloroform, evaporated, and dried to yield a dark brown solid (520 mg, 86%). GPC: Mn = 18.5 kg/mol, PDI = 3.6. 1 H NMR (CDCl3, δ ppm): 9.11−8.72 (m, 2H), 7.61−8.51 (m, 2H), 6.90−7.50 (m, 4H), 3.90−4.10 (m, 4H), 2.60−3.10 (m, 4H), 0.82− 1.91 (m, 108H). GPC: Mn = 18.5 kDa, PDI = 3.6. 2.2. Characterization. 1H NMR spectra were obtained from Bruker AV300 at 300 MHz using CDCl3 as the solvent. Mass spectra were recorded on Bruker Esquire LC/ion trap mass spectrometer. Gel permeation chromatography (GPC) analysis was performed using Polymer Lab Model 120 gel permeation chromatograph (DRI/high sensitivity refractive index detector and PL-BV400HT viscometer) against polystyrene standards in chlorobenzene at 60 °C. Differential scanning calorimetry (DSC) analysis was measured on a TA Instruments Q100 under N2 at a heating rate of 10 °C/min. TGA thermograms were recorded on a TA Instruments Q50 TGA at a heating rate of 20 °C/min. UV−vis absorption spectra were recorded on a PerkinElmer model Lambda900 UV/vis/near-IR spectrophotometer. Solution UV−vis absorption spectra were taken in 10−6 M concentration in chloroform. Cyclic voltammetry was done on an EG&G Princeton Applied Research potentiostat/galvanostat (Model 273A). Data were analyzed by using a Model 270 Electrochemical Analysis System Software on a PC. A three-electrode cell was used, using platinum wire electrodes as both counter and working electrodes. Silver/silver ion (Ag in 0.1 M AgNO3 solution, Bioanalytical System, Inc.) was used as a reference electrode. Ferrocene/ferrocenium (Fc/Fc+) was used as an internal standard. The potential values obtained in reference to Ag/Ag+ were converted to the saturated calomel electrode (SCE) scale. A thin film of each polymer was coated onto a platinum electrode from a concentrated solution in chloroform (10 mg/mL) and dried in vacuum for 2 h. All solutions were purged with N2 for 20 min before each experiment. X-

Chart 1. Molecular Structures of New Thiazole− Diketopyrrolopyrrole Copolymers

pyrrolo[3,4-c]pyrrole-1,4-dione-5′,5″-diyl-alt-2,5-bis(3-dodecylthiophen-2-yl)benzo[1,2-d;4,5-d′]bisthiazole] (PDPBT). The main purposes for incorporating thiazolothiazole or benzobisthiazole ring along with (DPP) into the same D−A copolymers are (i) to achieve optimum HOMO/LUMO energy levels to facilitate good hole injection but not electron injection since either BT or TT ring is a weaker electronwithdrawing group compared to DPP ring; (2) to achieve an extended heteroarene structure in the backbone to facilitate enhanced intermolecular interactions and thus enhanced charge carrier mobility; (3) to enhance oxidative and thermal stability,7b,11b and mechanical properties given the superior properties of known BT and TT polymers;24 and (4) to attain optimum band gaps and optical absorption for good light harvesting. The new copolymers were found to combine small optical band gaps (1.2−1.4 eV), broad absorption bands, high charge carrier mobility, and good oxidative and thermal stability with excellent film forming properties. As p-channel semiconductors in field-effect transistors, the thiazolothiazole-linked copolymers (PDPTT and PDPTTOx) exhibit a hole mobility as high as 1.2 cm2/(V s) with excellent on/off current ratios (>105). Bulk heterojunction (BHJ) solar cells incorporating the new copolymers as the donor component with [6,6]-phenyl C71-butyric acid methyl ester (PC71BM) as the acceptor component have a power conversion efficiency of 2.2−3.4% under 100 mW/cm2 AM1.5G irradiation in air.

2. EXPERIMENTAL SECTION 2.1. Materials and Synthetic Procedures. Materials. All commercially obtained reagents were used without further purification. Synthetic Procedures. 3,6-Bis(5-bromothiophen-2-yl)-2,5-bis(2hexyldecyl)-2,5-dihydropyrrolo[3,4-c]pyrrole-1,4-dione (1)5c was purchased from Solarmer Inc. and was used without further purification. Compound 2,5-bis[3-(2-ethylhexyl)-5-trimethylstannanylthiophen-2yl]thiazolo[5,4-d]thiazole (2),17a 2,5-bis(3-octyloxy-5-trimethylstannanylthiophen-2-yl)thiazolo[5,4-d]thiazole (3), and 2,6-bis(5-trimethyltin-3-dodecylthiophen-2-yl)benzo[1,2-d;4,5-d′]bisthiazole (4)18a were synthesized according to previously reported methods. Syntheisis of Poly[3,6-dithiene-2-yl-2,5-di(2-hexyldecyl)pyrrolo[3,4-c]pyrrole-1,4-dione-5′,5″-diyl-alt-2,5-bis(3-ethylhexylthiophen2-yl)thiazolo[5,4-d]thiazole] (PDPTT). In a 100 mL round-bottom flask, the starting materials 1 (398 mg, 0.4 mmol) and 2 (376 mg, 0.4 mmol) and catalyst tris(dibenzylideneacetone)dipalladium (0) (8 mg, 0.009 mmol) and tri-o-tolylphosphine (11 mg, 0.04 mmol) were 9030

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Scheme 1. Synthetic Route to PDPTT, PDPTTOx, and PDPBT

ray Diffraction (XRD) data were collected from a Bruker-AXS D8 Focus diffractometer with Cu Kα beam (40 kV, 40 mA) in θ−2θ scans (0.02 Å step size, 1 s/step). Polymer thin films for XRD were prepared by drop-casting from a concentrated solution (6−10 mg/mL) in 1,2dichlorobenzene onto a glass plate followed by annealing for 10 min at 180 °C and drying in a vacuum oven (60 °C) for overnight. 2.3. Fabrication and Characterization of Field-Effect Transistors. Field-effect transistors were fabricated on a heavily n-doped silicon substrate with a 200 nm thick silicon dioxide layer (capacitance density, Ci = 17 nF/cm2) that acted as a gate electrode with a gate insulator, by following our previous report.5b A monolayer of octyltrichlorosilane (OTS8) was formed by spin-coating from a chloroform solution (4 mM) at 3 krpm for 10 s, followed by thermal annealing at 100 °C in air. The polymer semiconductor was deposited onto the substrate by spin-coating at 2−3 krpm for 60 s from a solution (10 mg/mL) in 1,2-dichlorobenzene (PDPTT and PDPTTOx) and chloroform (PDPBT). The thin films were annealed at 200 °C for 10 min on a hot plate under an argon environment. Gold source-drain electrodes were deposited onto the polymer films by vacuum thermal evaporation through a shadow mask. The transistors have a top-contact and bottom-gate geometry with a channel width (W) of 1 mm and a length (L) of 100 μm (W/L = 10). Current− voltage (I−V) characteristics of the transistors were measured under a nitrogen atmosphere by using a probe station and an HP4145B semiconductor parameter analyzer. Field-effect mobility (μ) and threshold voltage (Vt) were calculated from five devices by using plots of |Ids|1/2 vs Vgs and a saturation equation, Ids = (μWCi)(Vgs − Vt)2/ (2L). 2.4. Fabrication and Characterization of Solar Cells. 10 mg/ mL polymer solutions and 60 mg/mL PC71BM (NanoC) solutions were prepared in 1,2-dichlorobenzene (ODCB), respectively, and passed through a 0.45 μm filter before use. Blends of polymer:PC71BM were prepared by mixing respective solutions at weight ratios of 1:2. A 2.5 vol % processing additive, 1,8-diiodooctane (DIO), was added before spin-coating. Solar cells used for photovoltaic and external quantum efficiency (EQE) measurements have a basic device structure of ITO/PEDOT:PSS/active layer/LiF/Al, where the active layer is a blend film spin-coated from polymer:PC71BM (1:2 w:w) solution. The ITO-coated glass (10 Ω/square, Shanghai B. Tree Tech, China) substrates were cleaned sequentially with acetone, deionized water, and isopropyl alcohol in an ultrasonic bath. The 40 nm PEDOT:PSS (Clevios P VP AI 4083) layer was spin-coated on top of ITO and dried at 150 °C for 10 min under vacuum. Each blend was spin-coated on top of the PEDOT:PSS layer for 18 s in a glovebox to make the active polymer blend layer of ∼100 nm. The devices were then dried in a vacuum chamber for 30 min. The devices were loaded in a thermal evaporator (BOC Edwards, 306), where a cathode consisting of 1.0 nm LiF and 80 nm Al was deposited through a shadow mask under high vacuum (8 × 10−7 Torr) to produce four solar cells with an active area of 9 mm2 each, per substrate. The current density−voltage (J−V) curves of solar cells were measured using an HP4155A semiconductor

parameter analyzer under laboratory ambient air condition. An AM1.5 illumination at 100 mW/cm2 was provided by a filtered Xe lamp and calibrated by using an NREL-calibrated Si diode.19c The external quantum efficiency (EQE) was measured using a QEX10 solar cell quantum efficiency measurement system (PV Measurements, Inc.) and was calibrated with a NREL-certified Si diode before measurement.

3. RESULTS AND DISCUSSION 3.1. Synthesis and Characterization. The thiazole− diketopyrrolopyrrole copolymers PDPTT, PDPTTOx, and PDPBT were designed to have a common thiophene− thiazolothiazole (or benzobisthiazole)−bithiophene−DPP repeating unit in the backbone. The DPP moiety has hexyldecyl side chains at the nitrogens whereas the thiophene rings adjacent thiazolothiazole have ethylhexyl (PDPTT) and octyloxy (PDPTTOx) side chains and the thiophene rings adjacent benzobisthiazole (PDPBT) have linear dodecyl side chains (Chart 1). The new D−A conjugated polymers were synthesized by Stille coupling polymerization of distannane comonomer 2 or 317a or 418a and 3,6-bis(5-bromothiophen-2yl)-2,5-bis(2-hexyldecyl)-2,5-dihydropyrrolo[3,4-c]pyrrole-1,4dione (1) in the presence of Pd2dba3 and P(tol)3 as catalyst and ligand, respectively, in anhydrous chlorobenzene (Scheme 1). Copolymers PDPTT and PDPTTOx were highly soluble (10− 15 mg/mL) in common organic solvents such as chloroform, tetrahydrofuran, chlorobenzene, and 1,2-dichlorobenzene at room temperature. However, PDPBT which contains benzobisthiazole rings was soluble (8−10 mg/mL) in chloroform at room temperature but only soluble in chlorobenzene and 1,2dichlorobenzene at high temperatures (60−100 °C). The molecular weight (Mn and Mw) and polydispersity index (PDI = Mw/Mn) of the copolymers were obtained by performing gel permeation chromatography (GPC) relative to polystyrene standards in chlorobenzene at 60 °C. The copolymers had moderate number-average molecular weight (Mn) that varied from 17.0 kDa in PDPTTOx and 18.0 kDa in PDPTT to 18.5 kDa in PDPBT. The weight-average molecular weight was also comparable for the three copolymers (Mw = 59.4−66.6 kDa) as was the PDI of 3.3−4.0. The molecular weight of these copolymers could probably be increased further by optimization of the polymerization conditions such as the monomer concentration in solution, catalyst/ligand ratio, and temperature. The thermal behavior of the thiazole−diketopyrrolopyrrole copolymers was investigated by differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). The copolymers showed onset thermal decomposition temperatures (Td) in the range of 378−396 °C under N2, 9031

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Figure 1. Absorption spectra of PDPBT, PDPTT, and PDPTTOx: (a) in dilute (10−6 M) chloroform solutions and (b) as thin films.

Table 1. Molecular Weight, Thermal, Optical, and Redox Properties of Copolymers

a

polymer

Mn (kDa)

PDI

Td (°C)

λmaxa (nm)

Egopt (eV)

Eoxonset (V)

Eredonset (V)

HOMO (eV)

LUMO (eV)

Egel (eV)

PDPBT PDPTT PDPTTOx

18.5 18.0 17.0

3.6 3.3 4.0

383 396 378

743, 673 749, 688 818, 769

1.33 1.38 1.22

0.91 0.81 0.74

−0.92 −0.80 −0.90

5.31 5.21 5.14

3.48 3.60 3.50

1.83 1.61 1.64

In thin film.

Figure 2. Cyclic voltammograms of PDPBT, PDPTT, and PDPTTOx thin films in 0.1 M Bu4NPF6 solution in acetonitrile at a scan rate of 40 mV/s: (a) oxidation waves and (b) reduction waves.

shifts to 818 nm in thin film and results in an optical band gap of 1.22 eV. This large red-shift in the absorption maximum (∼69 nm) of PDPTTOx compared to PDPTT films arises from the strong electron-donating nature of its octyloxy side chains. The absorption spectra of the benzobisthiazole-linked copolymer, PDPBT, in solution and as a thin film were nearly identical to those of PDPTT, showing an optical band gap (Egopt) of 1.33 eV (Table 1). The similarity of the optical absorption spectra of PDPTT and PDPBT suggest that the intramolecular charge transfer interactions in PDPTT are not significantly altered when the secondary electron-accepting moiety is changed from thiazolothiazole to benzobisthiazole. 3.3. Redox Properties and HOMO/LUMO Energy Levels. The electronic (HOMO/LUMO) energy levels of the D−A copolymers were estimated from cyclic voltammetry.25 Oxidation and reduction cyclic voltammograms (CVs) of the copolymers are shown in Figure 2. The onset oxidation (Eoxonset) and reduction (Eredonset) potentials are summarized in Table 1. All the copolymers showed irreversible oxidation waves as shown in Figure 2a. The onset oxidation potentials

indicating that they have good thermal stability (Figure S1, Supporting Information). DSC scans of the samples up to 300 °C showed that there is no glass or melt transitions in this temperature range (Figure S2). 3.2. Optical Properties. The normalized optical absorption spectra of copolymers PDPBT, PDPTT, and PDPTTOx in dilute chloroform solutions (∼10−6 M) and as thin films are shown in Figure 1. In both solutions and thin films, all the three copolymers are characterized by a broad absorption band that extends from ∼500 to 900−1000 nm. This absorption band appears to originate from the strong intramolecular charge transfer (ICT) interactions between the acceptor and donor moieties.23 In dilute solution, the thiazolothiazole-linked copolymer, PDPTT, showed an absorption maximum (λmax) at 736 nm and vibronic shoulder at 688 nm (Figure 1a). The thin film absorption spectrum has a similar line shape except that the absorption maximum is at 749 nm with a vibronic shoulder at 688 nm, resulting in an absorption edge optical band gap (Egopt) of 1.38 eV. In solution, PDPTTOx has a broad absorption with a maximum centered at 794 nm, which red9032

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not show diffraction peaks, indicating that PDPBT film is far more disordered in the solid state. 3.5. Organic Field-Effect Transistors (OFETs). The charge transport properties of the new copolymer semiconductors were investigated by fabricating and characterizing organic field-effect transistors (OFETs) with bottom-gate and top-contact (BGTC) geometry and gold source/drain electrodes. Uniform films of PDPTT and PDPTTOx were easily deposited onto the OTS8-treated, hydrophobic, SiO2 dielectric substrate by spin-coating from a solution in a high-boiling-point solvent, 1,2-dichlorobenzene (bp = 181 °C). However, because of the poor wetting of PDPBT solution in 1,2-dichlorobenzene on the hydrophobic OTS8/SiO2 substrate, we instead used chloroform (bp = 61 °C) solution to fabricate PDPBT OFETs. The thickness of the polymer semiconductor layer in the OFETs was 80−100 nm. Figure 4 shows the output curves and transfer characteristics of the OFETs based on PDPTTOx, PDPTT, and PDPBT. Each of the three copolymer semiconductors exhibited unipolar p-channel field-effect transistor characteristics. High current modulation was observed with on/off current ratio (Ion/Ioff) of 105−106 for the thiazolothiazole-linked copolymers, PDPTTOx and PDPTT. In the case of the benzobisthiazole-linked copolymer, PDPBT, the on/off current ratio was slightly lower at 104−105. n-Channel operation was not observed for any of the new copolymers, and thus there is no evidence of electron transport characteristics from the given device architecture. The unipolar p-type characteristics of the presented copolymers is due to the suitable HOMO energy levels (−5.1 to −5.3 eV) for hole injection and transport and rather high-lying LUMO energy levels (−3.5 to −3.6 eV), which presents a significant barrier for electron injection and transport. It is to be noted that some DPP-based D−A copolymer semiconductors exhibit ambipolar field-effect charge transport5c,6b,7a,e while others exhibit unipolar p-channel fieldeffect charge transport.8a,10a Hole mobility (μh) and threshold voltage (Vt) were calculated from the plots of square root of the drain current (|Ids|1/2) with respect to the gate voltage (Vgs) in the saturation region, as shown in Figure 4b,d,f. The thiazolothiazole-linked polymers, PDPTTOx and PDPTT, have a high hole mobility of 1.2 and 0.5 cm2/(V s), respectively. This carrier mobility is comparable to that of recently reported high mobility unipolar p-type polymer semiconductors.8a PDPBT, the benzobisthiazole-linked copolymer, on the other hand, has a rather low hole mobility of 0.005 cm2/(V s). The origin of the orders of magnitude (100−300×) smaller carrier mobility in PDPBT thin films, compared to the thiazolothiazole-linked polymers PDPTT and PDPTTOx, is not obvious given the similarity in molecular weight (Table 1), molecular weight polydispersity, and HOMO−LUMO energy levels. A possible explanation of the large difference in carrier mobility between the thiazolothiazole (TT)-linked copolymers and the benzobisthiazole (BT)-linked one is the large mismatch in the size between BT and diketopyrrolopyrrole (DPP) and thus poor solid state molecular packing and crystallinity in the case of PDPBT. The electrical parameters, including the average hole mobility (μh), the threshold voltage (Vt), and the on/off current ratio, of the OFETs are summarized in Table 2. One of the interesting features of PDPTTOx and PDPTT OFETs is the gate voltage (Vgs) dependence of the hole mobility. PDPTTOx OFETs showed higher field-effect mobility of >1 cm2/(V s) when the gate voltage is in the

(Eoxonset) of copolymers PDPBT, PDPTT, and PDPTTOx are 0.91, 0.81, and 0.74 V, respectively, from which the HOMO levels are estimated (EHOMO = eEoxonset + 4.4) to be 5.31, 5.21, and 5.14 eV, respectively (Table 1). The 0.1−0.2 eV lower lying HOMO energy level of PDPBT compared with the other copolymers can be understood to come from the weaker electron-accepting ability of benzobisthiazole unit.12b,13 The observed HOMO energy levels of 5.14−5.31 eV suggest that hole injection into these copolymers could be optimum from high work function electrodes such as gold (φ = 5.1 eV).26 The reduction waves seen in the CVs of the D−A copolymers were also irreversible as shown in Figure 2b. The onset reduction potential (Eredonset) of the copolymers was quite similar in the range of −0.8 to −0.9 V, irrespective of whether there is thiazolothiazole or benzobisthiazole linkage in the backbone. From this, the LUMO energy level was calculated (ELUMO = eEredonset + 4.4) to be 3.50−3.60 eV (Table 1). This value of the LUMO energy level suggests that electron injection into the copolymers may not be feasible from gold electrodes. Electron5b or ambipolar2b charge transport could thus be ruled out in these copolymers. The electrochemical band gap (Egel = Eoxonset −Eredonset) was found to be 1.61−1.83 eV, which is much larger than the observed 1.22−1.38 eV optical band gap. The discrepancy of 0.2−0.5 eV in the measured electrochemical and optical band gaps could be due to the exciton binding energy27 of the polymer thin films. 3.4. Morphology of Copolymer Films. To provide further insight into the morphology of the copolymers, X-ray diffraction measurement was performed on drop-cast films (from 10 mg/mL solutions in 1,2-dichlorobenzene) deposited onto a glass substrate, annealed at 180 °C for 10 min. Figure 3

Figure 3. X-ray diffraction patterns of PDPBT, PDPTT, and PDPTTOx films on glass substrates.

shows the XRD patterns of PDPBT, PDPTT, and PDPTTOx films. Weak diffraction peaks corresponding to a d-spacing of 2.1 and 2.5 nm due to the (100) lamellar reflections were observed for the thiazolothiazole-linked copolymers PDPTT and PDPTTOx, respectively, indicating relatively weak crystallinity in the thin films. These results reveal that there is no significant effect on morphology of the thiazolothiazolelinked copolymers, when branched ethylhexyl side chains on the thiophene rings of PDPTT were replaced by linear octyloxy side chains of PDPTTOx. On the other hand, the XRD pattern of benzobisthiazole-linked PDPBT, which has the large benzobisthiazole linkage and linear dodecyl side chains, did 9033

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Figure 4. Output curves (a, c, e) and transfer characteristics (b, d, f) of OFETs measured in a nitrogen-filled drybox: PDPTTOx (a, b), PDPTT (c, d), and PDPBT (e, f).

Such a large Vgs dependence of field-effect mobility in polymer OFETs has also been reported and attributed to contact effects.28 PDPBT OFETs did not show such a gate-voltage dependence. 3.6. Photovoltaic Properties. We investigated the photovoltaic properties of the thiazole−DPP copolymer:PC71BM blends by fabricating and characterizing bulk heterojunction (BHJ) solar cells that have the basic device structure of ITO/ PEDOT:PSS/active layer/LiF/Al. The active layer is a spincoated film composed of thiazole−DPP copolymer:PC71BM blend with an optimal weight ratio of 1:2 and processed with 2.5 vol % 1,8-diiodooctane (DIO) and 97.5 vol % 1,2-

Table 2. Electrical Parameters of Field-Effect Transistors polymer

μh (cm2/(V s))

Vt (V)

Ion/Ioff

PDPTTOx PDPTT PDPBT

1.23 0.45 0.0052

−23.8 −29.3 −14.3

105−106 105−106 104−105

range of −35 to −45 V. As |Vgs| further increases, the mobility decreases to 0.2−0.5 cm2/(V s). PDPTT OFETs also showed a similar behavior. Higher mobility (>0.4 cm2/(V s)) was observed when Vgs is in the range of −50 to −60 V, whereas the mobility decreases to ∼0.1 cm2/(V s) with higher |Vgs|.

Figure 5. (a) Current density−voltage characteristics of PDPTT:PC71BM (1:2), PDPTTOx:PC71BM (1:2), and PDPBT:PC71BM (1:2) solar cells with a structure of ITO/PEDOT/polymer:PC71BM/LiF/Al under 100 mW/cm2 1.5AM illumination in air. (b) EQE spectra of the solar cells, measured on devices with an active area of 9 mm2. 9034

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mA/cm 2 for PDPTT:PC 71 BM, and 7.68 mA/cm 2 for PDPTTOx:PC71BM. These Jsc values calculated from the EQE spectra are 5−8% lower than the Jsc values obtained in direct J−V measurement due to spectral mismatch between the simulated light source and the AM1.5 solar spectrum and possibly also due to degradation of solar cells during measurement in air. We used bright-field transmission electron microscope (BFTEM) imaging to investigate the nanomorphology of the thiazole−DPP copolymer:PC71BM blend thin films peeled off directly from the solar cells whose photovoltaic properties are discussed above. The images shown in Figure 6 were acquired at a slightly defocused condition to enhance the phase contrast between the polymer and fullerene, and under this focusing condition, fullerene domains appear darker than polymer domains due to the higher density of the fullerene. The copolymer:PC71BM blend films all showed a bicontinuous nanomorphology, with well-woven copolymer nanowire networks embedded in the fullerene matrix. For example, TEM images of PDPTT:PC71BM blend films showed copolymer nanowires with width of 16−18 nm and length of several hundred nanometers19b and interconnected fullerene domains with size of ∼100−200 nm. Selected area electron diffraction (SAED) patterns shown as inset of Figure 6 indicated that the copolymers and PC71BM formed semicrystalline domains, as shown by the Debye−Scherrer diffraction rings.19c

dichlorobenzene. The solar cells were fabricated in a glovebox and tested under AM1.5 solar illumination at 1 sun (100 mW/ cm2) in ambient air. The current density (J)−voltage (V) curves for the devices are shown in Figure 5a. The photovoltaic parameters of the devices, including the open circuit voltage (Voc), the short-circuit current density (Jsc), and fill factor (FF), are collected in Table 3. A significant decrease in Voc of the BHJ Table 3. Photovoltaic Parameters of Polymer:PC71BM BHJ Solar Cells blend (1:2 w:w) PDPBT:PC71BM PDPTT:PC71BM PDPTTOx: PC71BM

Voc (V)

Jsc (mA/cm2)

FF

ηmax (%)

ηave (%)

0.71 0.70 0.52

6.79 8.03 8.31

0.56 0.60 0.51

2.72 3.38 2.16

2.65 ± 0.07 3.27 ± 0.15 2.08 ± 0.08

solar cells is seen as the donor is changed from PDPBT (0.71 V) to PDPTT (0.70 V) and to PDPTTOx (0.52 V). The decrease in Voc can be partially understood by considering the decreasing energy offset between the donor HOMO and acceptor (LUMO)29 as one goes from PDPBT to PDPTTOx. In addition, the nanoscale phase-separated morphology of the donor/acceptor blend, which influences the charge photogeneration and charge carrier recombination, may also affect the observed Voc in the solar cells.30 On the other hand, Jsc shows an increasing trend, changing from 6.79 mA/cm2 in PDPBT:PC71BM to 8.31 mA/cm2 in PDPTTOx:PC71BM blends. This variation in Jsc can in part be explained on the basis of the relative difference in the charge carrier mobilities of PDPTT, PDPTTOx, and PDPBT. Solar cells made from PDPTT:PC71BM combine a high fill factor of 0.60 with a high Voc (0.7) and moderate short-circuit current density (8.03 mA/ cm2), resulting in a photovoltaic efficiency of 3.4% PCE. The photovoltaic efficiency of the other copolymers (PDPBT and PDPTTOx) was lower at 2.2−2.7% PCE. The external quantum efficiency (EQE) spectrum or action spectrum of each thiazole−DPP copolymer:PC71BM blend solar cell system is shown in Figure 5b. The photoresponse of the photodiodes turned on at near-IR wavelengths (900−1000 nm), demonstrating light harvesting of the near-IR photons due to the small band gaps of the thiazole−DPP copolymers. The photoresponse of PDPTTOx:PC71BM blends in particular begins at ∼1000 nm, showing a higher EQE in the wavelength range of 800−1000 nm compared to the other copolymers in the series. This may also partly explain the observed enhanced Jsc in PDPTTOx:PC71BM solar cells. The Jsc calculated from the EQE spectra was 6.43 mA/cm2 for PDPBT:PC71BM, 7.62

4. CONCLUSIONS New donor−acceptor copolymers containing both a strong electron-accepting diketopyrrolopyrrole (DPP) moiety and a weak electron-accepting thiazolothiazole or benzobisthiazole moiety have been synthesized with moderate number-average molecular weights (17.0−18.5 kDa) with polydispersity indexes of 3.3−4.0. The new thiazolothiazole−diketopyrrolopyrrole copolymers, PDPTT and PDPTTOx, were found to be unipolar p-type semiconductors, exhibiting a high carrier mobility of 0.5 and 1.2 cm2/(V s), respectively, and excellent on/off current ratio (>105) in field-effect transistors. Bulk heterojunction solar cells incorporating blends of the copolymers with PC71BM had power conversion efficiencies of 2.2−3.4%. TEM imaging of the copolymer:PC71BM blend thin films showed that the copolymers self-assembled into nanowires of 16−18 nm width and length of several hundred nanometers. These results demonstrate that the thiazole− diketopyrrolopyrrole copolymers are promising high mobility p-type semiconductors for developing high-performance OFETs and polymer solar cells.

Figure 6. TEM images of copolymer:PC71BM thin films peeled from solar cells and their SAED patterns (1:2) are shown as insets. 9035

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ASSOCIATED CONTENT

S Supporting Information *

TGA thermograms and DSC scans of the new copolymers and absorption spectra of copolymer:PC71BM blends. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The NSF (DMR-0805259) and Solvay S. A. supported the synthesis and study of the field-effect charge transport properties of the donor−acceptor copolymers. Study of the bulk heterojunction solar cells was supported by the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Material Sciences, under Award DE-FG0207ER46467.

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