Fine-Tuning of Molecular Energy Level of Alternating Copolymers Onthe basis of [1,2,5]Thiadiazolo[3,4-g]quinoxaline Derivatives forPolymer PhotovoltaicsYoonkyoo Lee and Won Ho Jo*

WCU Hybrid Materials Program and Department of Materials Science and Engineering, Seoul National University, Seoul 151-742,Korea

*S Supporting Information

ABSTRACT: A series of low-bandgap alternating copolymersconsisting of electron-accepting [1,2,5]thiadiazolo[3,4-g]-quinoxaline (TQ) derivatives and electron-donating fluoreneor carbazole were synthesized via the Suzuki coupling reaction.For the purpose to fine-tune the molecular energy level ofalternating copolymers and thus to improve charge transferbetween polymers and PCBM, two different TQ derivativessubstituted with strongly electron-donating butoxy group orweakly electron-donating thienyl group were synthesized andused as a building block of alternating copolymers.Copolymers with butoxy-substituted TQ have proper lowest unoccupied molecular orbital (LUMO) energy levels for effectivecharge dissociation between polymer and PCBM, whereas the LUMO levels of copolymers with thienyl-substituted TQ are tooclose to that of PCBM to be effective for charge dissociation. The power conversion efficiency was achieved up to 2.17%, which isthe highest value among the TQ-based polymer solar cells, when the blend of copolymer with butoxy-substituted TQ and [6,6]-phenyl-C71-butyric acid methyl ester was used as an active layer material in bulk heterojunction solar cells.

■ INTRODUCTIONIn recent years, low-bandgap alternating conjugated copolymersbased on the internal donor−acceptor (D−A) interaction haveattracted great interest because their electronic properties caneasily be tuned by a proper combination of D and A units, andtheir absorption ranges can be extended to longer wave-lengths.1−7 Among a wide variety of acceptor units, [1,2,5]-thiadiazolo[3,4-g]quinoxaline (TQ) has been a promisingbuilding block for synthesis of low-bandgap conjugatedpolymers because of the strong electron-withdrawing propertyof four imine nitrogens in the TQ unit. Using TQ derivatives,several research groups have synthesized D−A type low-bandgap copolymers and reported the photovoltaic propertiesof those copolymers.8−10 Among TQ derivatives, 6,7-diphenyl-[1,2,5]thiadiazolo[3,4-g]quinoxaline has been widely used as abuilding block for alternating conjugated copolymers. However,most of the alternating copolymers composed of TQ derivativeand fluorene or carbazole have shown poor photovoltaicperformance with a power conversion efficiency (PCE) lowerthan 1%.11−13 This low efficiency can be attributed to the low-lying LUMO level of the copolymers (−3.9 to −4.0 eV) whichis very close to that of PCBM (−4.0 eV). Since the LUMO−LUMO offset between the donor polymer and PCBM shouldbe at least higher than 0.3 eV for effective charge dissociation,the small LUMO−LUMO offset between TQ-based polymersand PCBM may prevent effective charge separation, whichcauses a poor photovoltaic performance. In this sense, it isrequired to raise the LUMO level of TQ-based polymers in

order to promote the charge transfer between donor polymerand PCBM and thus to achieve better photovoltaic perform-ance.One feasible approach to control the LUMO level of the

donor−acceptor type alternating copolymer is to tune theenergy level of acceptor unit in the copolymer. In general, thehybridized LUMO level of D−A type alternating copolymers islocated nearly at the LUMO level of acceptor moiety, while thehybridized HOMO level is governed by the HOMO level ofdonor moiety.14,15 In the present work, we have synthesizednew TQ derivatives substituted with electron-donating thienylor butoxy group to diminish the electron deficiency of TQ unit,thereby weakening the electron affinity of TQ unit. Since theelectron-donating ability of butoxy group is stronger thanthienyl group, it is expected that TQ with butoxy group(BOTQ) exhibits higher LUMO level than TQ with thienylgroup (THTQ). When a fluorene or carbazole derivative wascoupled to each of the two TQs to synthesize four alternatingcopolymers, it was clearly observed that the LUMO levels ofthe copolymers were fine-tuned by the LUMO levels of TQunits. One of BOTQ-based copolymers, poly(N-9′-heptade-canyl-2,7-carbazole-alt-6,7-dibutoxy-4,9-bis(4-hexylthien-2-yl)-[1,2,5]thiadiazolo[3,4-g]quinoxaline) (PCBOTQ), showed thebest photovoltaic performance with a PCE of 2.17%, which is

Received: December 23, 2011Revised: March 19, 2012Published: March 27, 2012

Article

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the highest value among the TQ-based polymer solar cells,when blended with [6,6]-phenyl-C71-butyric acid methyl ester(PC71BM).

■ EXPERIMENTAL SECTIONMaterials. All reagents were obtained from Aldrich unless

otherwise specified and used as received. Tetrahydrofuran(THF) was dried over sodium/benzophenone under nitrogenand freshly distilled prior to use. 5,6-Diamino-4,7-bis(4-hexylthien-2-yl)-2,1,3-benzothiadiazole16 (1) and 2,7-bis-(4′,4′,5′,5′-tetramethyl-1′,3′,2′-dioxaborolan-2′-yl)-N-9″-heptade-canylcarbazole17 (5) were synthesized by following themethods reported in the literature. PC71BM was obtainedfrom Nano-C. Poly(3,4-ethylenedioxythiophene):poly-(styrenesulfonate) (PEDOT:PSS) (Clevios P VP AI 4083)was purchased from H.C. Stark and passed through a 0.45 μmPVDF syringe filter before spin-coating.Synthesis of 4,9-Bis(4-hexylthien-2-yl)-6,7-di(thien-2-

yl)-[1,2,5]thiadiazolo[3,4-g]quinoxaline (2a). To a solutionof the compound 1 (0.5 g, 1.0 mmol) in 30 mL of ethanol/acetic acid (9:1, v/v), 2,2′-thenil (0.244 g, 1.1 mmol) was addedin one portion. After stirring the mixture at room temperaturefor 4 h, the solvent was removed under reduced pressure. Theresidue was purified by column chromatography on silica gel(1:1 chloroform/hexane as eluent) to afford the product as adark green solid. Yield: 0.55 g (80%). 1H NMR (300 MHz,CDCl3): δ (ppm) 8.73 (s, 2H), 7.61 (d, 2H), 7.55 (d, 2H), 7.30(s, 2H), 7.06 (t, 2H), 2.78 (t, 4H), 1.80 (m, 4H), 1.44−1.27(m, 12H), 0.91 (t, 6H). 13C NMR (125 MHz, CDCl3): δ(ppm) 152.00, 145.79, 143.10, 141.92, 135.63, 134.95, 134.30,131.52, 130.74, 127.61, 126.65, 120.71, 32.02, 30.86, 30.81,29.40, 22.91, 14.36. Elemental Anal. Calcd for C36H36N4S5 (%):C, 63.13; H, 5.26; N, 8.18; S, 23.43. Found (%): C, 63.25; H,5.22; N, 8.13; S, 23.40.Synthesis of 6,7-Dibutoxy-4,9-bis(4-hexylthien-2-yl)-

[1,2,5]thiadiazolo[3,4-g]quinoxaline (2b). To the com-pound 1 (1.11 g, 2.22 mmol) in 100 mL of acetic acid, dibutyloxalate (0.92 mL, 4.44 mmol) was added. The mixture wasallowed to react at 110 °C for 48 h. The solvent was removedunder reduced pressure, and a small amount of THF wasadded. Precipitation from n-hexane yielded the red product.Yield: 0.72 g (48%). 1H NMR (300 MHz, CDCl3): δ (ppm)8.71 (s, 2H), 7.83 (s, 2H), 3.98 (t, 4H), 2.74 (t, 4H), 1.73 (m,8H), 1.43−1.25 (m, 16H), 0.93−0.86 (m, 12H). 13C NMR(125 MHz, CDCl3): δ (ppm) 161.50, 159.85, 157.56, 150.39,144.21, 136.04, 131.24, 122.14, 70.34, 31.96, 30.88, 30.73,29.92, 29.39, 29.33, 22.88, 16.32, 14.33. Elemental Anal. Calcdfor C36H48N4O2S3 (%): C, 65.04; H, 7.23; N, 8.43; S, 14.48.Found (%): C, 65.11; H, 7.22; N, 8.37; S, 14.58.Synthesis of 4,9-Bis(5-bromo-4-hexylthien-2-yl)-6,7-

di(thien-2-yl)-[1,2,5]thiadiazolo[3,4-g]quinoxaline (3a).The compound 2a (0.31 g, 0.44 mmol) was dissolved in 20mL of THF, to which N-bromosuccinimide (NBS) (0.156 g,0.88 mmol) was added at 0 °C in the dark. The mixture wasstirred for 1 h at 0 °C, and the temperature was slowly raised toroom temperature. After stirring the mixture for an additional 1h, the solvent was evaporated under reduced pressure. Theresidue was purified by column chromatography on silica gel(1:1 chloroform/hexane as eluent) to afford the product as adark green solid. Yield: 0.36 g (95%). 1H NMR (300 MHz,CDCl3): δ (ppm) 8.67 (s, 2H), 7.64 (d, 2H), 7.47 (d, 2H), 7.07(t, 2H), 2.68 (t, 4H), 1.73 (m, 4H), 1.42−1.25 (m, 12H), 0.92(t, 6H). 13C NMR (125 MHz, CDCl3): δ (ppm) 151.30,

146.03, 141.87, 141.34, 135.50, 134.36, 133.74, 131.98, 131.09,127.66, 119.49, 118.01, 31.98, 30.09, 29.92, 29.39, 22.95, 14.39.Elemental Anal. Calcd for C36H34N4S5Br2 (%): C, 51.30; H,4.04; N, 6.65; S, 19.04. Found (%): C, 51.41; H, 4.14; N, 6.67;S, 19.12.

Synthesis of 6,7-Dibutoxy-4,9-bis(5-bromo-4-hexylth-ien-2-yl)-[1,2,5]thiadiazolo[3,4-g]quinoxaline (3b). Thecompound 2b (0.81 g, 1.2 mmol) was dissolved in 30 mL ofTHF, to which NBS (0.427 g, 2.4 mmol) was added in the dark.After stirring the mixture at room temperature for 3 h, thesolvent was removed under reduced pressure. The residue waspurified by column chromatography on silica gel (2:1chloroform/hexane as eluent) to afford the product as a redcompound. Yield: 0.88 g (89%). 1H NMR (300 MHz, CDCl3):δ (ppm) 8.62 (s, 2H), 3.92 (t, 4H), 2.71 (t, 4H), 1.69 (m, 8H),1.44−1.25 (m, 16H), 0.92−0.87 (m, 12H). 13C NMR (125MHz, CDCl3): δ (ppm) 157.83, 151.63, 149.39, 142.56, 141.75,136.05, 131.05, 111.58, 70.24, 31.91, 31.75, 30.65, 29.94, 29.45,29.24, 22.85, 16.35, 14.32. Elemental Anal. Calcd forC36H48N4O2S3Br2 (%): C, 52.55; H, 5.60; N, 6.81; S, 11.70.Found (%): C, 52.66; H, 5.56; N, 6.75; S, 11.72.

Synthesis of Poly(9,9-dioctylfluorene-alt-4,9-bis(4-hexylthien-2-yl)-6,7-di(thien-2-yl)-[1,2,5]thiadiazolo[3,4-g]quinoxaline) (PFTHTQ). 9,9-Dioctylfluorene-2,7-diboronicacid bis(1,3-propanediol) ester (4) (77.6 mg, 0.139 mmol),compound 3a (120.0 mg, 0.139 mmol), and 3 drops of Aliquat336 were dissolved in a mixture of toluene (12 mL) andaqueous K2CO3 solution (2 M, 3 mL). The solution wasflushed with N2 for 20 min, and then 8.0 mg of Pd(PPh3)4 wasadded. The reaction mixture was stirred at 100 °C for 48 h, andthen 2-thienylboronic acid (4.9 mg) and 2-bromothiophene(3.8 mg) were added in order to end-cap the polymer chain.After being cooled to room temperature, the resulting mixturewas poured into methanol. The crude product was filteredthrough a Soxhlet thimble and then subjected to Soxhletextraction with methanol, hexane, acetone, and chloroform.The polymer was recovered from the chloroform fraction andprecipitated into methanol to afford the product as a dark greensolid. Yield: 121.1 mg (78%). 1H NMR (300 MHz, CDCl3): δ(ppm) 8.90 (s, 2H), 7.88 (d, 2H), 7.70−7.59 (m, 8H), 7.09 (s,2H), 2.94 (s, 4H), 2.16 (s, 4H), 1.86 (s, 4H), 1.47−1.37 (m,12H), 1.16 (br, 24H), 0.90 (t, 6H), 0.77 (t, 6H). ElementalAnal. Calcd for C65H74N4S5 (%): C, 72.87; H, 6.91; N, 5.23; S,14.98. Found (%): C, 72.54; H, 6.94; N, 5.17; S, 14.89.

Synthesis of Poly(9,9-dioctylfluorene-alt-6,7-dibu-toxy-4,9-bis(4-hexylthien-2-yl)-[1,2,5]thiadiazolo[3,4-g]-quinoxaline) (PFBOTQ). The compounds 4 (81.2 mg, 0.146mmol) and 3b (120.0 mg, 0.146 mmol) were dissolved in amixture of THF (12 mL) and aqueous K2CO3 solution (2 M, 3mL). The solution was flushed with N2 for 20 min, and then 8.4mg of Pd(PPh3)4 was added. After the reaction mixture wasstirred at 70 °C for 48 h, 2-thienylboronic acid (4.9 mg) and 2-bromothiophene (3.8 mg) were added in order to end-cap thepolymer chain. After being cooled to room temperature, theresulting mixture was poured into methanol. The crude productwas filtered through a Soxhlet thimble and then subjected toSoxhlet extraction with methanol, hexane, acetone, andchloroform. The polymer was recovered from the chloroformfraction and precipitated into methanol to afford the product asa dark red solid. Yield: 104.6 mg (65%). 1H NMR (300 MHz,CDCl3): δ (ppm) 8.82 (s, 2H), 8.01−7.58 (m, 6H), 3.88 (s,4H), 2.80 (s, 4H), 2.17−1.85 (m, 12H), 1.33−1.14 (m, 40H),0.90−0.77 (m, 18H). Elemental Anal. Calcd for C65H86N4O2S3

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(%): C, 74.27; H, 8.19; N, 5.33; S, 9.16. Found (%): C, 74.40;H, 8.25; N, 5.25; S, 9.05.Synthesis of Poly(N-9′-heptadecanyl-2,7-carbazole-

alt-4,9-bis(4-hexylthien-2-yl)-6,7-di(thien-2-yl)-[1,2,5]-thiadiazolo[3,4-g]quinoxaline) (PCTHTQ). PCTHTQ wassynthesized by following the same procedure as the synthesis ofPFTHTQ. 2,7-Bis(4′,4′,5′,5′-tetramethyl-1′,3′,2′-dioxaborolan-2′-yl)-N-9″-heptadecanylcarbazole (5) (159.2 mg, 0.232 mmol)and compound 3a (200 mg, 0.232 mmol) were used asmonomers. Yield: 212.3 mg (72%). 1H NMR (300 MHz,CDCl3): δ (ppm) 8.92 (s, 2H), 8.24 (d, 2H), 7.88 (s, 2H),7.83−7.76 (m, 6H), 7.08 (s, 2H), 4.71 (s, 1H), 2.99 (s, 4H),2.16 (s, 4H), 2.05−1.89 (m, 4H), 1.48 (s, 4H), 1.48−1.36 (m,8H), 1.14 (br, 24H), 0.90 (t, 6H), 0.79 (t, 6H). Elemental Anal.Calcd for C65H75N5S5 (%): C, 71.87; H, 6.91; N, 6.45; S, 14.77.Found (%): C, 71.92; H, 6.99; N, 6.39; S, 14.67.Synthesis of Poly(N-9′-heptadecanyl-2,7-carbazole-

alt-6,7-dibutoxy-4,9-bis(4-hexylthien-2-yl)-[1,2,5]-thiadiazolo[3,4-g]quinoxaline) (PCBOTQ). PCBOTQ wassynthesized by following the same procedure as the synthesis ofPFBOTQ. Compounds 5 (100.1 mg, 0.146 mmol) and 3b(120.0 mg, 0.146 mmol) were used as monomers. Yield: 107.8

mg (60%). 1H NMR (300 MHz, CDCl3): δ (ppm) 8.86 (s,2H), 8.18−7.53 (m, 6H), 4.65 (s, 1H), 3.88 (s, 4H), 2.81 (s,4H), 2.16 (s, 4H), 2.01−1.58 (m, 8H), 1.43−1.16 (m, 40H),0.90−0.77 (m, 18H). Elemental Anal. Calcd for C65H87N5O2S3(%): C, 73.22; H, 8.17; N, 6.57; S, 9.03. Found (%): C, 73.40;H, 8.10; N, 6.45; S, 9.05.

Characterizations. The chemical structures of materialsused in this study were identified by 1H NMR (Avance DPX-300) and 13C NMR (Avance DPX-500). Elemental analysis wasperformed on EA1110 (CE Instrument) elemental analyzer.Molecular weight and its distribution were measured by gelpermeation chromatography (Waters) equipped with arefractive index detector using THF as an eluent, where thecolumns were calibrated against standard polystyrene samples.The optical absorption spectra were obtained by a UV−visspectrophotometer (Lambda 25, Perkin-Elmer). Cyclic voltam-metry was carried out on potentiostat/galvanostat (VMP 3,Biologic) in an electrolyte solution of 0.1 M tetrabutylammo-nium hexafluorophosphate (Bu4NPF6) in dichloromethane at ascan rate of 50 mV/s. The onset potential of each cyclicvoltammogram was determined by the intersection point of twotangential lines.18,19 Three-electrode cell was used for all

Figure 1. Synthetic route of alternating copolymers.

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experiments. Platinum wires (Bioanalytical System Inc.) wereused as both counter and working electrodes, and silver/silverion (Ag in 0.1 M AgNO3 solution, Bioanalytical System Inc.)was used as a reference electrode. TEM images were obtainedfrom a JEOL JEM1010 at an accelerating voltage of 80 kV.Fabrication and Characterization of Photovoltaic

Devices. Polymer solar cells were fabricated on ITO glasscleaned by stepwise sonication in acetone and IPA, followed byO2 plasma treatment for 10 min. PEDOT:PSS was spin-coatedon the ITO glass at 4000 rpm for 1 min and annealed at 120 °Cfor 30 min to yield 40 nm thick film. A mixture of polymer andPC71BM were dissolved in anhydrous chlorobenzene or o-dichlorobenzene (30 mg/mL) and spin-coated on the top ofthe ITO/PEDOT:PSS film at 700−800 rpm for 60 s. Thetypical thickness of the active layer was 80−90 nm. Lithiumfluoride (0.5 nm) and aluminum (100 nm) were evaporatedunder vacuum lower than 10−6 Torr on the top of active layerthrough a shadow mask. The effective area of cell was ca. 4mm2. The current−voltage (J−V) curves of the device wereobtained on a computer-controlled Keithley 4200 sourcemeasurement unit under AM 1.5G (100 mW/cm2) simulatedby an Oriel solar simulator (Oriel 91160A). The light intensitywas calibrated using a NREL-certified photodiode prior to eachmeasurement. The external quantum efficiency (EQE) wasmeasured using Polaronix K3100 IPCE measurement system(McScience). The light intensity at each wavelength wascalibrated with a standard single-crystal Si cell.

■ RESULTS AND DISCUSSION

Synthesis and Characterization. The general syntheticroute for monomers and polymers are outlined in Figure 1.Considering that the substitution of the electron-donatinggroup on 6- and 7-positions of TQ unit may reduce theelectron deficiency of TQ, two different TQ derivatives (2a and2b in Figure 1) were synthesized by condensation reaction ofthe diamino compound (1) with 2,2′-thenil or dibutyl oxalate.Bromination of monomers 2a and 2b with NBS afforded twofinal monomers, THTQ (3a) and BOTQ (3b), respectively. Afluorene or carbazole derivative was coupled to each of two TQderivatives via the palladium-catalyzed Suzuki coupling reactionto synthesize four alternating copolymers. The number-averagemolecular weight (Mn) and polydispersity index (PDI) of thepolymers are listed in Table 1. All the polymers synthesized inthis study are highly soluble in common organic solvents suchas THF, chloroform, chlorobenzene, and dichlorobenzene atroom temperature. When thermal transition behavior of thepolymers was examined by differential scanning calorimetry(DSC), all the polymers did not exhibit distinct glass transitionsand melting points up to 350 °C, probably due to the molecularrigidity of polymer chain and amorphous nature of thepolymers.

Optical Properties. The absorption spectra of thepolymers in chloroform solution and thin film are shown inFigures 2a and 2b, respectively. All of the polymers showed two

absorption peaks, a common feature of donor−acceptor typealternating copolymers,20−22 where the absorption peak atshorter wavelength is attributed to π−π* transition of theconjugated backbone while the absorption peak at longerwavelength corresponds to the intramolecular charge transferfrom the donor to the acceptor unit in the alternatingcopolymer. When the maximum absorption wavelengths ofthe polymers in solution are compared with those in solid state(Figure 2a vs Figure 2b), it reveals that all the polymerssynthesized in this study show pronounced peak broadeningand red shift of absorption edges in solid state, indicating thatthe polymer chains are highly aggregated in solid state. Anotherfeature to be noted is that the absorption spectra of BOTQ-based copolymers are blue-shifted compared to those ofTHTQ-based copolymers. The optical bandgaps (Eg(opt)) ofBOTQ-based copolymers as determined from the onset of theirabsorption spectra are ∼1.5 eV, while those of THTQ-based

Table 1. Characteristics of Polymers

absorption

polymer Mn (kg/mol) PDI λmax(CHCl3) (nm) λmax(film) (nm) Eg(opt)a (eV) HOMO (eV) LUMO (eV) Eg(ec)

b (eV)

PFTHTQ 26.5 1.63 393, 761 405, 813 1.19 −5.11 −3.82 1.29PFBOTQ 21.6 1.51 376, 540 383, 564 1.56 −5.14 −3.56 1.58PCTHTQ 22.2 1.57 389, 761 413, 841 1.15 −5.14 −3.83 1.31PCBOTQ 25.4 1.41 369, 532 375, 564 1.52 −5.16 −3.57 1.59

aDetermined from the onset of UV−vis absorption spectra. bDetermined from the cyclic voltammetry.

Figure 2. UV−vis absorption spectra of the copolymers in chloroformsolution (a) and solid state (b).

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copolymers are much lower (∼1.2 eV). Since the bandgap ofD−A type alternating copolymers are generally dependentupon the electronic push−pull interaction of the donor andacceptor units, it is obvious that the different electrondeficiency of TQ unit, which is controlled by the electron-donating power of substituent, results in different bandgap ofthe resulting alternating copolymers. Because the butoxy grouphas strong electron-donating nature, the introduction of abutoxy group to TQ makes TQ unit less electron deficient, andas a result it weakens the electron-withdrawing property of TQ,leading to weak intramolecular charge transfer, while theintroduction of weakly electron-donating thienyl group to theTQ unit does not significantly weaken the electron-withdrawingproperty of TQ, and therefore the TQ unit is still stronglyelectron deficient, resulting in lower bandgap of THTQ-basedcopolymers than BOTQ-based copolymers.Electrochemical Properties. The electrochemical proper-

ties of the monomers and polymers are determined from theoxidation and reduction cyclic voltammograms, as shown inFigure 3, and are summarized in Table 1. The HOMO energy

levels of compounds were calculated using the equationHOMO (eV) = −[Eox − E1/2(ferrocene) + 4.8], where Eox isthe onset oxidation potential of the compound andE1/2(ferrocene) is the onset oxidation potential of ferroceneversus Ag/Ag+. The LUMO energy levels were also estimatedby using the equation LUMO (eV) = −[Ered − E1/2(ferrocene)

+ 4.8], where Ered is the onset reduction potential ofcompound. When the energy levels of the synthesized TQsare compared, it is realized that the LUMO level of BOTQ issignificantly higher than that of THTQ, while the HOMOlevels of two TQs are almost same. The strongly electron-donating butoxy group makes TQ unit less electron deficientand hence lowers the electron affinity, resulting in the up-shifted LUMO level of TQ. As the electron-donating power ofthienyl group is weaker than that of butoxy group, the LUMOlevel of THTQ is lower than that of BOTQ. When the energylevels of the synthesized alternating copolymers are examined,all the polymers have similar HOMO energy levels while theLUMO energy levels of BOTQ-based copolymers are about 0.2eV higher than those of THTQ-based copolymers, which isvery consistent with the difference in LUMO levels betweenTHTQ and BOTQ. The LUMO energy levels of THTQ-basedcopolymers are around −3.8 eV, which is slightly higher thanthe LUMO energy level of diphenyl-substituted TQ-basedcopolymers (−3.9 to −4.0 eV).8,12 This is because the thienylgroup is electron-donating while the phenyl group has anelectron-withdrawing nature. However, the LUMO levels of theTHTQ-based copolymers are still close to that of PCBM (−4.0eV), which might prevent effective charge transfer from thedonor polymer to PCBM. On the other hand, excitons areexpected to be easily dissociated at the interface betweenBOTQ-based copolymers and PCBM because the LUMOlevels of BOTQ-based polymers (−3.6 eV) are sufficientlyhigher than that of PCBM. Hence, it is concluded that BOTQ-based copolymers are a better candidate for the donor polymerin the active layer of photovoltaic device.

Photovoltaic Properties. The polymer solar cells werefabricated with a layered configuration of glass/ITO/PEDOT:PSS/polymer:PC71BM/LiF/Al. The J−V character-istics of photovoltaic devices prepared from polymer:PC71BM(1:2 w/w) using chlorobenzene or o-dichlorobenzene as aprocessing solvent are represented in Figure 4, and their

photovoltaic parameters are summarized in Table 2. Whenphotovoltaic properties of BOTQ- and THTQ-based copoly-mers are compared, BOTQ-based devices exhibit superiorphotovoltaic performance than THTQ-based devices: Thedevices of PFBOTQ and PCBOTQ exhibit PCEs of 1.22% and2.17%, respectively, while their THTQ-based counterparts,PFTHTQ and PCTHTQ, show PCEs of 0.44% and 0.34%,respectively. The difference in PCE between the two types ofpolymers is attributed largely to the difference in the short-

Figure 3. Cyclic voltammograms of two TQ derivatives (a) and thecorresponding copolymers (b).

Figure 4. Current−voltage (J−V) characteristics of polymer:PC71BM(1:2 w/w) BHJ solar cells under AM 1.5G condition.

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circuit current density (Jsc): The Jscs of BOTQ-based devicesare much higher than those of THTQ-based devices. The Jscdifference may arise from two sources: First, BOTQ-basedcopolymers show strong absorption around 600 nm wavelengthregion where the maximum photon flux from the sun is found,while THTQ-based copolymers shows very weak absorption atthat region, as shown in Figure 2. Second, the small LUMO−LUMO offset between THTQ-based copolymers and PCBMmay not provide sufficient driving force for charge separation atthe interface, while BOTQ-based copolymers have relativelyhigh-lying LUMO level which is beneficial for effective chargetransfer from the polymer to PCBM.To better understand the effect of energy level of donor

polymers on the generation of photocurrent, the externalquantum efficiency (EQE) of the devices under monochro-matic light is measured (Figure 5). As compared with the

THTQ-based devices, the BOTQ-based devices show higherEQE values and broad spectral response up to 780 nm.Particularly, the device from PCBOTQ:PC71BM shows arelatively a high EQE of 52% at 520 nm, whereas theTHTQ-based devices exhibit low EQE below 20%. It is worthnoting that the EQE values of THTQ-based devices are below5% at 700−900 nm wavelength region where the polymersshow the maximum absorption. This significant lack ofphotoresponse in the long-wavelength region where theTHTQ-based copolymers show the maximum absorptionmay be attributed to the low LUMO level of the THTQ-based copolymers. Since a LUMO−LUMO offset of 0.3−0.4eV is necessary for efficient electron transfer from the donorpolymer to PCBM,23 the small offset between the LUMO ofthe copolymer and the LUMO of PCBM prevents efficientelectron transfer; thereby, only a small fraction of absorbedphotons are dissociated into charge carriers.Another important feature to be noted is that the fill factor

(FF) of THTQ-based devices is much lower than that ofBOTQ-based devices. It is previously reported that the

nanoscale phase-separated morphology of the active layer andthe balanced charge mobility between hole and electron iscrucial for reducing the recombination rate and hence achievinghigh FF.24−27 When the morphology of polymer:PC71BMblend films is characterized by TEM, all the films show thenanoscale phase separation, which can provide the pathway forcharge carriers to pass through (Figure 6). Since the

morphological features of the blends are almost identical, itcan be concluded that the difference in FF between THTQ-and BOTQ-based devices does not arise from the morpho-logical effect. When hole and electron mobilities were measuredfrom the space charge limited current (SCLC) J−V curves asobtained in the dark using hole-only devices (ITO/PEDOT:PSS/polymer:PC71BM/Au) and electron-only devices(Al/polymer:PC71BM/Al), respectively, as shown in Figure 7,hole mobilities of the BOTQ-based devices are 3−4-fold higherthan those of the THTQ-based devices (Table 3), while theelectron mobilities (μe) of four devices were almost same. Sincethe BOTQ-based devices show better balance between holeand electron mobility (μh/μh = 1.8−2.1) while THTQ-baseddevices show a large discrepancy between hole and electronmobility (μh/μh = 6.1−8.0), as listed in Table 3, BOTQ-baseddevices may have less recombination loss in the device thanTHTQ-based devices, leading to higher fill factor.

■ CONCLUSIONSWe have synthesized a series of D−A low-bandgap alternatingcopolymers composed of electron-donating carbazole orfluorene and electron-accepting [1,2,5]thiadiazolo[3,4-g]-quinoxaline derivatives by the palladium-catalyzed Suzukicoupling reaction. It has been found that the optical,electrochemical, and photovoltaic properties of the copolymersare strongly dependent upon the electron-withdrawing propertyof TQ which is an electron-accepting unit in alternatingcopolymers. The copolymers with butoxy-substituted TQ(BOTQ) has sufficiently high-lying LUMO energy levelcompared to that of PCBM, which facilitates charge

Table 2. Photovoltaic Properties of Devices Tested underStandard AM 1.5G Condition

polymer:PC71BM(1:2 w/w) solvent

Voc(V)

Jsc(mA/cm2) FF

PCE(%)

PFTHTQ DCB 0.58 2.30 0.33 0.44PFBOTQ CB 0.64 4.76 0.40 1.22PCTHTQ DCB 0.56 1.91 0.32 0.34PCBOTQ CB 0.65 7.43 0.45 2.17

Figure 5. External quantum efficiency spectra of polymer:PC71BMsolar cells.

Figure 6. TEM images of polymer:PC71BM blend films.

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dissociation and thus leads to high photocurrent, whereas thecopolymers with thienyl-substituted TQ (THTQ) have low-lying LUMO energy level which is close to that of PCBM,resulting in lower photocurrent. The device fabricated from ablend of PCBOTQ and PC71BM exhibits a PCE of 2.17% withan open-circuit voltage of 0.65 V, a short-circuit current densityof 7.43 mA/cm2, and a fill factor of 45% under AM 1.5condition (100 mW/cm2), which is the highest value amongthe TQ-based polymer solar cells. Our approach for fine-tuningof molecular energy levels of conjugated polymers can be usefulfor rational design of conjugated polymers for various electronicdevices.

■ ASSOCIATED CONTENT*S Supporting Information1H NMR spectra of THTQ, BOTQ, and polymers. Thismaterial is available free of charge via the Internet at http://pubs.acs.org.

■ AUTHOR INFORMATIONCorresponding Author*Ph +82-2-880-7192; Fax +82-2-876-6086; e-mail whjpoly@snu.ac.kr.NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSThe authors thank the Ministry of Education, Science andTechnology (MEST), Korea, for financial support through theGlobal Research Laboratory (GRL) and the World ClassUniversity (WCU) programs.

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Figure 7. Dark J−V characteristics for hole-only (a) and electron-only(b) devices of polymer:PC71BM blends. The solid lines are the bestlinear fit of the data points.

Table 3. Charge Carrier Mobilities of Polymer:PC71BMBlends

polymer:PC71BM(1:2 w/w)

hole mobility, μh(cm2/(V s))

electron mobility, μe(cm2/(V s)) μe/μh

PFTHTQ 8.05 × 10−6 4.91 × 10−5 6.1PFBOTQ 2.31 × 10−5 4.76 × 10−5 2.1PCTHTQ 7.33 × 10−6 5.88 × 10−5 8.0PCBOTQ 2.98 × 10−5 5.50 × 10−5 1.8

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