Effect of Fluorine Substitution on Photovoltaic Properties of

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Effect of Fluorine Substitution on Photovoltaic Properties of Benzothiadiazole−Carbazole Alternating Copolymers Tomokazu Umeyama,†,‡ Yusuke Watanabe,† Evgenia Douvogianni,† and Hiroshi Imahori*,†,§ †

Department of Molecular Engineering, Graduate School of Engineering, and §Institute for Integrated Cell-Material Sciences (WPI-iCeMS), Kyoto University, Nishikyo-ku, Kyoto 615-8510, Japan ‡ PRESTO, Japan Science and Technology Agency, 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan S Supporting Information *

ABSTRACT: To investigate the effect of fluorine substitution on molecular and film structures, optical, electrochemical, and photovoltaic properties of a moderate bandgap polymer, poly(2,7-carbazole-alt-dithienylbenzothiadiazole) (PCDTBT) with deep HOMO energy level, a fluorinated analogue of PCDTBT (i.e., PCDTBT-F) has been developed for the first time by replacing two hydrogen atoms on benzothiadiazole (BT) units with two fluorine atoms. An analogous polymer, PCBBBT-F with additional hexylthiophenes between the thiophene and carbazole of PCDTBT-F, has also been prepared to overcome the poor solubility of PCDTBT-F. The PCBBBT-F film showed wide absorption bands in UV and visible regions with an optical bandgap of 1.82 eV that is smaller than that of PCDTBT (1.89 eV), whereas the film of PCDTBT-F exhibited blue-shifted absorption with a bandgap of 1.96 eV due to the low molecular weight arising from the deficient solubility. The HOMO energy level of PCDTBT-F is lower than that of PCDTBT, owing to the electron-withdrawing fluorination of the BT unit, whereas PCBBBT-F exhibited a higher HOMO level than PCDTBT, implying that the additional incorporation of electron-donating hexylthiophenes negated the fluorination effect. A bulk heterojunction (BHJ) polymer solar cell (PSC) that employed PCDTBT-F or PCBBBT-F as an electron donor and a fullerene derivative [70]PCBM as an electron acceptor yielded lower power conversion efficiencies of 1.29 and 1.98%, respectively, than that of PCDTBT (6.16%) due to the unfavorable film structures of PCDTBT-F:[70]PCBM resulting from the poor solubility and low molecular weight, as well as low crystallinities and limited exciton lifetimes, of the fluorinated polymers. These results provide valuable information on the elaborate design of PCDTBT-based polymers for the PSC applications.

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INTRODUCTION Polymer solar cells (PSCs) with bulk heterojunction (BHJ) structures have stimulated broad interest due to their potential advantages such as facile fabrication, low cost, lightweight, and flexibility.1,2 Considerable efforts have been made in recent years to pursue higher power conversion efficiencies (PCEs) through the design of new electron donors (i.e., π-conjugated polymers)3−5 and electron acceptors (i.e., fullerene derivatives),6 as well as device optimization.7−9 Donor material development continues to dominate research, although efforts to fullerene alternatives are notable.10,11 By taking into consideration that low bandgap polymers can harvest more solar photons, many chemists have recently devoted much effort to reducing the bandgap of conjugated polymers.3−5 This © 2013 American Chemical Society

approach has improved remarkably short-circuit current density (JSC) values up to >18 mA cm−2,12−14 whereas it generally counts against open-circuit voltage (VOC) values. A higher HOMO energy level to pursue a lower bandgap diminishes the difference between LUMO of an electron acceptor and HOMO of a donor, which has been considered as an important factor for determining the VOC values.15,16 A facile method to control HOMO−LUMO levels of polymers by combining electron-rich (donor) and electrondeficient (acceptor) moieties in their repeating units, forming Received: August 1, 2013 Revised: September 18, 2013 Published: September 19, 2013 21148

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PCE values of single-junction PSCs.31,32 On the other hand, Zhou et al. first reported the synthesis of a conjugated polymer with an incorporated fluorinated benzothiadiazole (BT) unit, and demonstrated its improved performance relative to the unsubstituted BT moiety in a PSC.33 The increase in device performance arose predominately from a deeper HOMO level which increased the open circuit voltage of the PSC. Other groups have also subsequently reported the use of the fluorinated BT unit which in general resulted in an improved photovoltaic performance.13,34−38 In this report, to investigate the effect of fluorine substitution on molecular and film structures, optical, electrochemical, and photovoltaic properties of PCDTBT, we combine 4,7-di(thien2-yl)-5,6-difluoro-2,1,3-benzothiadiazole (DTBT-F) with the carbazole unit for the first time to obtain a fluorinated analogue of PCDTBT, that is, PCDTBT-F, where two hydrogen atoms on the BT units are replaced by two fluorine atoms (Figure 1). In addition, PCBBBT-F with additional hexylthiophenes between the thiophene and carbazole of PCDTBT-F was also prepared to overcome the poor solubility of PCDTBT-F (Figure 1). We expected that the electron density on the benzene ring of BT would be decreased owing to the electronwithdrawing fluorine atoms, thereby lowering the HOMO and LUMO energy levels of PCDTBT-F and PCBBBT-F. Their optical, electrochemical, and photovoltaic properties were characterized in detail and compared with those of PCDTBT.

internal donor−acceptor (D−A) structures, has been intensively explored.3−5 Among the numerous building blocks, carbazole is one of the most prevalent electron-rich units owing to its fully aromatic fused ring with sufficient chemical and environmental stability.17 Various electron-deficient fusedheteroarenes were systematically copolymerized with 2,7carbazole toward rational design of photovoltaic conjugated polymers by Leclerc et al. in 2008.18 They found that the HOMO and LUMO energy levels are mainly correlated to carbazole and acceptor units, respectively, and symmetric acceptor units allow for a higher degree of crystalline order. In their study, poly[N-9′-heptadecanyl-2,7-carbazole-alt-5,5-(4′,7′di-2-thienyl-2′,1′,3′-benzothiadiazole)] (PCDTBT, Figure 1)

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EXPERIMENTAL SECTION Instruments. The microwave-assisted procedures were carried out with a CEM Discover BenchMate microwave. 1H NMR spectra were measured with a JEOL JNM-EX400 NMR spectrometer. Attenuated total reflectance (ATR) FT-IR spectra were recorded on a ThermoFisher Scientific Nicolet 6700 FT-IR. High-resolution mass spectra (HRMS) were obtained with a Thermo Fisher Scientific EXACTIVE. Elemental analyses with the analytical error of ±0.3% were conducted at Organic Elemental Analysis Research Center, Kyoto University. Gel permeation chromatography (GPC) measurements were carried out on a SHIMADZU Prominence equipped with JAIGEL-3HAF (Japan Analytical Industry) column using chloroform as an eluent calibrated with polystyrene standards. UV−vis−near-infrared (NIR) absorption spectra of solutions and films were measured with a Perkin-Elmer Lambda 900 UV/vis/NIR spectrometer. Steadystate fluorescence spectra were recorded on a HORIBA SPEX Fluoromax-3 spectrofluorometer. Atomic force microscopy (AFM) analyses were carried out with an Asylum Technology MFP-3D-SA in the AC mode. Optical micrographs were recorded using KH-7700 (Hirox). Thermogravimetric analysis (TGA) measurements were conducted with a SHIMADZU TGA-50 under flowing nitrogen at a scan rate of 2 °C min−1. Differential scanning calorimetry (DSC) analysis was made on a SHIMADZU DSC-60 under flowing nitrogen at a scan rate of 10 °C min−1. Photoemission yield spectroscopy in air (PYSA) measurements were conducted using an AC-3 (Riken Keiki). Xray diffraction (XRD) analyses were conducted by a Rigaku SmartLab 9 kW using Cu Kα radiation. The samples were coated on glass substrates. Photocurrent−voltage characteristics were measured by 2400 SourceMeter (Keithley) under an argon atmosphere and simulated solar light (100 mW cm−2, AM1.5) with OTENTO-SUN III solar simulator (Bunkoukeiki). Photocurrent action spectra were recorded with CEP2000RR (Bunkoukeiki).

Figure 1. Structures of PCDTBT, PCDTBT-F, and PCBBBT-F.

with a relatively low HOMO level (−5.5 eV) had superior potential for applications in PSCs. The initial PCE reached 3.6%17,18 in a typical BHJ device and has been subsequently improved, exceeding 6% by thickness optimization and nanomorphology control of the BHJ layer.19−21 However, the bandgap of PCDTBT is still larger than the value (1.5−1.7 eV) of the ideal polymers for PCE values exceeding 10%.15 Recently, to lower the bandgap of PCDTBT without sacrificing the low HOMO level, further different electron-accepting monomers, including phenylbenzotriazole,22 naphthothiadiazole,23 dialkoxylated benzothiadiazole,24 and quinoxaline-based units,25,26 have been incorporated into 2,7-carbazole based copolymers. In addition, ladder-typed analogues of carbazole with forced planarity were copolymerized with benzothiadiazole unit.27,28 However, the PCE values of the PSCs with these polymers still remain at most comparable to PCDTBT-based devices. Substitution by electron-withdrawing groups is an efficient approach to broaden the absorption and downward-shift of the HOMO−LUMO energy levels of the D−A conjugated polymers.29−40 In the electron-withdrawing substituents, fluorine atom substitution, as the smallest electron-withdrawing group in size, is ideal in downward-tuning the HOMO−LUMO energy levels without disturbing the planar molecular structure of the conjugated polymers.13,14,31−41 In particular, the D−A copolymers consisting of electron-donating benzodithiophene (BDT) and electron-accepting fluorine-substituted thieno[3,4b]thiophene with a carbonyl group showed one of the highest 21149

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Scheme 1. Synthesis of PCDTBT-F and PCBBBT-F

DFT Calculations. Geometry optimization and electronic structure calculations for the unit structures of the polymers were performed by using the B3LYP functional and 6-31G(d) basis set implemented in the Gaussian 03 program package.42 Molecular orbitals were visualized by Molstudio 3.0 software. Materials. 2,7-Bis(4′,4′,5′,5′-tetramethyl-1′,3′,2′-dioxaborolan-2′-yl)-N-9″-heptadecanylcarbazole (1), 5,6-difluoro-4,7-bis(5-bromo-2-thienyl)-2,1,3-benzothiadiazole (2), and poly[N-9′heptadecanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′benzothiadiazole)] (PCDTBT) were prepared according to the reported procedures.17,43 All other solvents and chemicals were of reagent grade quality, purchased commercially and used without further purification unless otherwise noted. Monomer Synthesis. 5,6-Difluoro-4,7-bis(3′-hexyl-[2,2′bithiophen]-5-yl)-2,1,3-benzothiadiazole. In a 50 mL dried 2neck round-bottom flask with a condenser, 2 (40 mg, 0.080 mmol), excess of 3-hexyl-2-trimethylstannylthiophene (81 mg, 0.24 mmol) and dry toluene (1 mL) were added. The mixture was then purged with argon for 15 min. Then, Pd(PPh3)4 (4.0 mg, 0.0035 mmol) was added, and the reaction mixture was heated to reflux for 48 h. The reaction mixture was then cooled to room temperature, and the solvent was evaporated. The crude orange product was purified by column chromatography with dichloromethane/hexane = 1/6 as eluent and recrystallized from isopropanol as purple solid. Yield: 40%. 1H NMR (400 MHz, CDCl3): δ 8.30 (d, 2H, thiophene), 7.29 (d, 2H, thiophene), 7.27 (d, 2H, α-H of hexylthiophene), 7.02 (d, 2H, β-H of hexylthiophene), 2.87 (t, 4H, Ar-CH2), 1.71 (m, 4H, ArCH2H2), 1.50−1.26 (m, 12H, -CH2-), 0.89 (t, 6H, -CH3). IR (ATR): 2921, 2852, 1586, 1478, 1440, 1419, 1346, 1295, 1229, 1177, 1051, 1000, 893, 824, 791 cm−1. HRMS (m/z): calcd for C34H34F2N2S5, 669.1366; found, 669.1367. Anal. Calcd. for C34H34F2N2S5: C, 61.05; H, 5.12; N, 4.19; F, 5.68; S, 23.96. Found: C, 61.17; H, 5.08; N, 4.17; F, 5.69; S, 23.73. 5,6-Difluoro-4,7-bis(5′-bromo-3′-hexyl-[2,2′-bithiophen]5-yl)-2,1,3-benzothiadiazole (3). The above compound (30 mg, 0.045 mmol) and N-bromosuccinimide (NBS; 17 mg, 0.098 mmol) were added into THF under stirring. The reaction mixture was stirred at a room temperature for 8 h, then washed with brine and dried over anhydrous sodium sulfate. The solvent was removed under a reduced pressure and the target compound was obtained by recrystallization from isopropanol as purple solid. Yield: 90%. 1H NMR (400 MHz, CDCl3): δ 8.28 (d, 2H, thiophene), 7.25 (d, 2H, thiophene), 6.99 (s, 2H, β-H of hexylthiophene), 2.81 (t, 4H, Ar-CH2), 1.67 (m, 4H, ArCH2H2), 1.50−1.26 (m, 12H, -CH2-), 0.89 (t, 6H, -CH3). IR (ATR): 2922, 2853, 1586, 1478, 1440, 1419, 1346, 1295, 1229, 1177, 1051, 1000, 893, 824, 791 cm−1. HRMS (m/z): calcd for C34H32Br2F2N2S5, 826.9556; found, 826.9538. Anal. Calcd. for C34H32Br2F2N2S5: C, 49.39; H, 3.90; N, 3.39; F, 4.60; S, 19.39;

Br, 19.33. Found: C, 49.20; H, 3.76; N, 3.60; F, 4.41; S, 19.38; Br, 19.48. Polymer Synthesis. PCDTBT-F. A test tube was charged with 1 (72.3 mg, 0.11 mmol), 2 (54.5 mg, 0.11 mmol), Pd2dba3 (dba: dibenzylideneacetone, 2.51 mg, 2.5% mmol), tri(otolyl)phosphine (6.69 mg, 20% mmol), and anhydrous oxylene (1 mL), and bubbled with argon for 10 min. The reaction mixture was stirred at 180 °C for 18 h under microwave irradiation. The cooled mixture was subsequently poured into methanol and the dark purple precipitate was collected on a membrane filter. The product was washed with methanol and hexane. Then the obtained solid was extracted with chloroform and reprecipitated with chloroform/methanol. Yield: 44%. 1H NMR (400 MHz, CDCl3): δ 8.37 (2H, thiophene), 8.11 (2H, carbazole), 7.91 (2H, carbazole), 7.75 (2H, carbazole), 7.58 (2H, thiophene), 4.68 (1H, NCH), 2.39 (4H, NCHCH2), 1.97 (4H, NCHCH2CH2), 1.35−1.03 (20H, -CH2-), 0.81 (6H, -CH3). IR (ATR): 3732, 2922, 2851, 2360, 2340, 1740, 1621, 1595, 1562, 1482, 1428, 1362, 1333, 1266, 1218, 1142, 1077, 998, 966, 893, 854, 796, 687 cm−1. PCBBBT-F. PCBBBT-F was prepared following the same procedure as described above using monomer 3 instead of 2. Yield: 68%. 1H NMR (400 MHz, CDCl3): δ 8.32 (2H, thiophene), 8.09 (2H, carbazole), 7.77 (2H, carbazole), 7.56 (2H, carbazole), 7.33 (2H, thiophene), 7.00 (2H, β-H of hexylthiophene), 4.63 (1H, NCH), 2.92−0.81 (60H, -CH2- and -CH3). IR (ATR): 2920, 2851, 2362, 1596, 1433, 1332, 1240, 1175, 1049, 998, 891, 832, 795 cm−1. Fabrication of Polymer Solar Cells. The PSC devices were fabricated as follows.44−47 Indium tin oxide (ITO) on a glass substrate with a sheet resistance of 5 Ω/sq (Geomatec) was used. The substrates were sonicated consecutively with acetone and ethanol for 15 min. After blow-drying and UVozone treatment, the substrates were spin-coated at 1000 rpm with poly(ethylene dioxythiophene) doped with polystyrene sulfonic acid (PEDOT:PSS, Clevios P VP AI 4083) and dried with a hot plate at 200 °C for 10 min. Under a nitrogen atmosphere, an active layer of polymer/[6,6]-phenyl-C71butyric acid methyl ester ([70]PCBM; American Dye Source, Inc.) film was formed by spin coating a o-dichlorobenzene solution of polymer/[70]PCBM (1:4 (w/w), [polymer] = 7.5 mg mL−1, [[70]PCBM] = 30 mg mL−1) at 1000 rpm for 1 min onto the ITO/PEDOT:PSS. Then, a TiOx layer was prepared by spin coating an ethanol solution of titanium(IV) isopropoxide (Wako Pure Chemical Industries, Ltd.) at 4000 rpm and then leaving for 30 min under ambient atmosphere. The titanium(IV) isopropoxide was hydrolyzed to yield the TiOx layer. Finally, an Al (The Nilaco Corporation) layer was deposited by thermal evaporation under vacuum (5 × 10−5 Pa) to yield the layered device structure (denoted as ITO/ 21150

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such as glass transition or melting up to 300 °C, implying that these polymers are thermally robust. Optical Properties, Energy Levels, and Film Structures. The optical properties of PCDTBT-F and PCBBBT-F were investigated by UV−vis absorption spectra in diluted chloroform solutions and film states spin-coated on glass substrates (Figure 2). Characteristics of the polymer

PEDOT:PSS/polymer:[70]PCBM/TiOx/Al). Schematic illustrations of the top and side views of the device are depicted in Figure S1 (see Supporting Information). The size of each device was 0.06 cm2 (0.2 × 0.3 cm2). During the photovoltaic measurements, a black mask was attached on the front of the ITO electrode to avoid the scattering effect of incident light.

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RESULTS AND DISCUSSION

Monomers and Polymers Syntheses and Thermal Properties. Monomers 1 and 2 were synthesized following the reported methods.17,43 PCDTBT-F was prepared via Suzuki coupling polymerization reaction of 1 and 2 using Pd2dba2 (dba: dibenzylideneacetone) as catalyst in toluene (Scheme 1) with microwave-assisted heating. During the polymerization, dark-red solid precipitated out and the fine particles could not be dissolved in any common organic solvents. The numberaverage molecular weight (Mn) and polydispersity index (PDI) of the soluble part of PCDTBT-F were estimated to be 4000 and 1.5, respectively, by gel permeation chromatography (GPC) measurement with polystyrene standards in chloroform. The limitation of the molecular weight is apparently caused by the poor solubility of PCDTBT-F irrespective of the branched alkyl chains on the carbazole unit. This is in stark contrast to PCDTBT with Mn of 90000 prepared by the same procedure.48 It is well-known that achieving high molecular weight is critical for excellent photovoltaic performance, because it facilitates the bicontinuous phase formation with fullerene derivatives and efficient charge transport.49 The introduction of fluorine atoms may significantly enhance the inter- and intrachain interactions of the polymer during the polymerization, limiting the elongation of the polymer chain. To improve the solubility of the resulting polymer, we further designed a fluorinated BT-based monomer with additional 3-hexylthiophenes (3 in Scheme 1). Monomer 3 was prepared from 2 by the conventional two-step method: (1) cross-coupling with 3-hexyl-2-trimethylstannylthiophene, and (2) bromination at α-positions of terminal thiophenes using NBS. The monomers 1 and 3 were copolymerized by the same procedure as above to yield PCBBBT-F (Scheme 1) as a darkred solid. In contrast with PCDTBT-F, no precipitation appeared during the polymerization. The obtained PCBBBTF showed excellent solubility in common organic solvents such as toluene, chloroform, chlorobenzene, o-xylene, and odichlorobenzene, which permits PCBBBT-F to be readily processed by solution technique for the film formation. Mn and PDI values of PCBBBT-F estimated by GPC were 13000 and 2.0, respectively, demonstrating the significant improvement in the molecular weight compared to PCDTBT-F. However, the Mn value of PCBBBT-F is still lower than that of PCDTBT although the solubility limitation was overcome. The reason is currently unclear, but the purity of monomer 3 may be responsible for the relatively low molecular weight of PCBBBTF. The thermal stabilities of PCDTBT-F and PCBBBT-F were investigated with thermogravimetric analysis (TGA). As in the case of PCDTBT,17,18 both polymers were thermally stable with decomposition temperatures (5% weight loss) more than 400 °C in nitrogen atmosphere (409 °C for PCDTBT-F and 407 °C for PCBBBT-F). The high-thermal stabilities of the obtained polymers are beneficial for the application to the active layers in PSCs. On the other hand, differential scanning calorimetry (DSC) did not display any transition temperatures

Figure 2. UV−vis absorption spectra of (a) chloroform solutions of PCDTBT-F (red line), PCBBBT-F (blue line, 12.5 μg mL−1), PCDTBT (black line), and (b) films of PCDTBT-F (red line), PCBBBT-F (blue line), and PCDTBT (black line). The spectra of PCDTBT-F and PCDTBT were normalized to those of PCBBBT-F for comparison.

Table 1. Optical and Electrochemical Data of Polymers abs. λmax (nm)

abs. λedge (nm)

polymer

solution

film

film

Egopta (eV)

HOMOb (eV)

LUMOc (eV)

PCDTBT-F PCBBBT-F PCDTBT

500 526 546

519 570 578

633 683 657

1.96 1.82 1.89

−5.54 −5.44 −5.48

−3.58 −3.62 −3.59

a

Determined from the onset wavelength in the absorption spectra of film samples. bDetermined by PYSA. cDetermined from HOMO levels and optical bandgaps.

absorptions are summarized in Table 1. The corresponding spectra of PCDTBT were also measured and in close agreement with the reported data.17 The dilute solutions of PCDTBT-F and PCBBBT-F revealed absorption bands in visible region with absorption peaks at 500 and 526 nm, respectively, arising from the π−π* transitions. These values are blue-shifted compared to that of the PCDTBT solution (546 nm). The relatively small molecular weights of PCDTBT-F and PCBBBT-F may limit the effective conjugation length of the polymer backbones, resulting in the blue-shift in solutions. In 21151

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donating hexylthiophenes is larger than that of the fluorination. The LUMO energy levels were determined to be −3.58 eV for PCDTBT-F and −3.62 eV for PCBBBT-F by adding the thinfilm optical band gap to the HOMO level. The energy levels of PCDTBT-F, PCBBBT-F, PCDTBT, [70]PCBM, buffer layers, and electrodes used in the PSC devices (vide infra) are illustrated in Figure 4.44−47 As it has been well recognized that

the film states, the absorption spectra of PCDTBT-F and PCBBBT-F displayed red-shifted maxima at 519 and 570 nm, respectively, relative to those in the solutions, as in the case of PCDTBT (Figure 2). These bathochromic shifts of the peaks are ascribed to the interchain associations and π−π stackings in the solid state. Note here that the red-shift of the PCDTBT-F film compared to the solution (0.09 eV) is less significant than that of the PCBBBT-F (0.18 eV) film. From the poor solubility of PCDTBT-F a certain degree of the interchain interactions is anticipated even in the diluted solution. The bathochromic shift of PCBBBT-F is even larger than that of PCDTBT (0.13 eV) and the peak positions of the PCBBBT-F and PCDTBT films become comparable (570 nm for PCBBBT-F and 578 nm for PCDTBT), suggesting more efficient interchain interactions in the film state of PCBBBT-F than PCDTBT. More importantly, the absorption band of the PCBBBT-F film is broad and the absorption edge (683 nm) is meaningfully red-shifted compared with those of the PCDTBT film exhibiting an onset at 657 nm. The replacement of two protons of BT units with electron-withdrawing fluorine atoms and incorporation of additional hexylthiophenes afford the more extended conjugation length and narrower optical bandgap in films states. On the other hand, the photoluminescence spectra of PCDTBT-F and PCBBBT-F in chloroform revealed emission in the visible region with peaks at 618 and 660 nm by the excitations at the absorption maxima (Figure S2, see Supporting Information). The emission intensity of the PCDTBT-F solution is larger than that of PCDTBT, whereas those of PCBBBT-F and PCDTBT are rather comparable. The films of PCDTBT-F and PCBBBT-F, however, exhibited emissions with much lower intensities than that of PCDTBT (Figure S3, see Supporting Information), suggesting the significant quenching of the polymer excited states by the interchain interaction. The reduced lifetime of the excited states, that is, excitons, in the polymer films could serve as a drawback for the use as donor materials in PSC devices, taking into account the excitons in the polymer domain of the BHJ polymer−fullerene composite films must migrate efficiently to polymer−fullerene interfaces for charge separation.50 The HOMO energy levels of the polymers were determined by using photoemission yield spectroscopy in air (PYSA) measurements (Figure 3). The HOMO energy level of

Figure 4. Energy level diagrams of PCDTBT-F, PCBBBT-T, PCDTBT, [70]PCBM, buffer layers, and electrodes used in PSC devices.

the binding energy of the excitons is about 0.3−0.5 eV, the LUMO energy level of the donor must be positioned above that of the acceptor (i.e., [70]PCBM) by 0.3−0.5 eV to ensure efficient photoinduced electron transfer (ET) from the donor excited state to the acceptor.1−5 Thus, the driving forces (−ΔG) for ET from the excited states of PCDTBT-F (−ΔG = 0.62 eV) and PCBBBT-F (−ΔG = 0.58 eV) to [70]PCBM are sufficient for efficient ET. The crystallinities and molecular organizations of the PCDTBT-F and PCBBBT-F films were investigated by X-ray diffraction (XRD) analyses of the spin-cast films onto glass substrates (Figure 5). The both films did not reveal any well-

Figure 5. In-plane XRD patterns of PCDTBT-F (red line), PCBBBTT (blue line), and PCDTBT (black line) films.

defined scattering patterns, indicating the macroscopically disordered, amorphous structure. In the case of PCDTBT, the spin-cast film showed a weak broad diffraction peak at 19.6°, corresponding to d-spacing of 0.45 nm. This peak reflects a π−π stacking between polymer chains.18 These observations suggest the inferior crystallinities of PCDTBT-F and PCBBBTF to that of PCDTBT. Theoretical Studies. To provide an in-depth insight into the energy levels, polymer structures and molecular orbital distributions of the fluorinated analogues of PCDTBT, density functional theory (DFT) calculations using B3LYP/6-31G*

Figure 3. Photoemission yield spectroscopy in air of PCDTBT-F (red circle), PCBBBT-F (blue square), and PCDTBT (black triangle).

PCDTBT-F is −5.54 eV, which is lower by 0.06 eV than that of PCDTBT (−5.48 eV).51 This result is consistent with the previous results that the fluorination of BT unit lowered the HOMO level.33−38 Contrary to our expectation, however, PCBBBT-F exhibited a higher HOMO level (−5.44 eV) by 0.04 eV than PCDTBT, implying that the effect of the electron21152

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Figure 6. Optimized geometry, HOMO/LUMO wave functions, and energy levels of the monomer units of PCDTBT-F, PCBBBT-T, and PCDTBT simulated by DFT calculations using B3LYP/6-31G* model. The dihedral angles between the neighboring blocks in the structure are inserted. The hexyl group was replaced with methyl group to shorten the calculation time.

the hole transport (PEDOT:PSS) and hole blocking (TiOx) layers (Figure S1). The detailed device-fabrication process is described in the Experimental Section. Optimal performance was obtained from 7.5 mg mL−1 polymer solutions in odichlorobenzene with a polymer:[70]PCBM ratio of 1:4 (w/ w). The thicknesses of the active layers were determined to be 80−90 nm. Figure 7 depicts the current−voltage characteristics

model were performed on the unit structure with isopropyl substituted carbazole for simplicity (Figure 6). The dihedral angle between the carbazole and adjacent thiophene in the monomer unit of PCDTBT-F (∼28°) is similar to that in PCDTBT (∼28°). The small dihedral angle is indicative of effective conjugation through π orbitals.52 The calculated HOMO energy values are −5.05 eV for the monomer unit of PCDTBT-F and −4.94 eV for that of PCDTBT. This is in agreement with the experimental result of deeping HOMO upon fluorination substitution, whereas the opposite trend in the calculated LUMO levels (−2.61 eV for PCDTBT-F and −2.50 eV for PCDTBT) can be rationalized by the difference in polymer chain lengths. On the other hand, by incorporating methylthiophene between the carbazole and thiophene, the dihedral angle slightly decreases (∼25°) in the monomer unit of PCBBBT-F (Figure 6). This higher planarity may cause the efficient extension of π-conjugation, thereby resulting in the lower HOMO−LUMO bandgap in the isolated state (2.12 eV for PCBBBT-F vs 2.44 eV for PCDTBT and PCDTBT-F). The experimental absorption measurement of PCBBBT-F in solution, however, showed a blue-shifted band compared to PCDTBT in solution (Figure 2a). This discrepancy can also be attributed to the difference in the polymer chain lengths as well as less electron delocalization of PCBBBT-F in comparison with PCDTBT in the HOMO level. It should be noted here that in the film states PCBBBT-F showed an absorption band with a comparable peak position and a red-shifted onset relative to PCDTBT (Figure 2b). The high planarity of PCBBBT-F may induce efficient interchain interaction, thereby resulting in the lower optical bandgap in the film state. On the other hand, the higher HOMO level of PCBBBT-F (−4.77 eV) relative to PCDTBT agrees with the experimental trend (Figure 4). In the monomer units of PCDTBT-F and PCDTBT, the HOMO wave function is delocalized along the whole conjugated backbones, and the density of states for LUMO is mainly localized on the electron-accepting BT units (Figure 6). This is in general agreement with typical D−A polymers with excellent photovoltaic properties in PSC devices. However, the HOMO wave function of PCBBBT-F is less distributed on the carbazole (C) unit than on the 5,6-difluoro-4,7-bis(3′-hexyl[2,2′-bithiophen]-5-yl)-2,1,3-benzothiadiazole (BBBT-F) unit in spite of the higher planarity. This localization could hamper the efficient hole-transport to the electrode after the charge separation. Photovoltaic Properties. BHJ-PSC devices were fabricated with the structure of ITO/PEDOT:PSS/polymer: [70]PCBM/TiOx/Al, where the active layer is placed between

Figure 7. Current−voltage characteristics of PSC devices based on PCDTBT-F:[70]PCBM (red line), PCBBBT-F:[70]PCBM (blue line), and PCDTBT:[70]PCBM (black line) films under illumination of AM 1.5, 100 mW cm−2 white light.

of the PSC devices under simulated AM 1.5G solar irradiation at 100 mW cm−2, and the photovoltaic parameters are listed in Table 2. The PSC device based on the PCBBBT-F:[70]PCBM Table 2. Photovoltaic Properties active layer

JSC (mA cm−2)

VOC (V)

FF

PCE (%)

PCDTBT-F:[70]PCBM PCBBBT-F:[70]PCBM PCDTBT:[70]PCBM

4.93 6.56 11.1

0.82 0.88 0.88

0.32 0.34 0.63

1.29 1.98 6.16

layer demonstrated a PCE value of 1.98% with an open circuit voltage (VOC) of 0.88 V, a short circuit current density (Jsc) of 6.56 mA cm−2, and a fill factor (FF) of 0.34, which are superior to those of the PCDTBT-F:[70]PCBM-based device (PCE = 1.29%, JSC = 4.93 mA cm−2, VOC = 0.82 V, FF = 0.32). Note that both photovoltaic performances are lower than that of the PCDTBT:[70]PCBM-based device (PCE = 6.16%, JSC = 11.1 mA cm−2, VOC = 0.88 V, FF = 0.63), which agrees well with the previous report using the same device configuration.19 Figure 8 illustrates the incident photon-to-current efficiency (IPCE) spectra of the PSC devices under white bias light illumination. The photocurrent action spectra of both PCDTBT-F: [70]PCBM and PCBBBT-F:[70]PCBM-based devices agree 21153

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microscopy (AFM) and digital microscopy measurements. In the AFM images, the PCDTBT-F:[70]PCBM film showed relatively smooth surface morphology with root-mean-square (rms) surface roughness of 0.53 nm compared with those of PCBBBT-F:[70]PCBM (rms = 0.65 nm) and PCDTBT: [70]PCBM (rms = 0.63 nm). All of them had nanoscale phase-separated structure as in the previous report.19 However, micrometer-sized phase separation was observed in the PCDTBT-F:[70]PCBM film by the digital microscopy. The micrometer-sized domains can be attributed to the aggregations of PCDTBT-F, where inefficient charge separation would occur, taking into account the limited diffusion length of exciton (∼10 nm).50 Furthermore, the irregular surface morphology of PCDTBT-F:[70]PCBM may induce the inhomogeous surface contact between the cathode (Al) and the active layer, thereby accelerating charge recombination at the interface and lowering the VOC value.16 The photovoltaic performance of the PCBBBT-F: [70]PCBM-based device as well as that with PCDTBT-F: [70]PCBM is inferior to that of the well-known PCDTBT: [70]PCBM-based device with the same configuration (Figure 7 and Table 2). Although the VOC values are comparable, JSC and FF of PCDTBT are higher than those of PCBBBT-F. Consistently, the IPCE values of PCDTBT exceed those of PCBBBT-F in the whole wavelength region of 400−750 nm (Figure 8). Note here that there is no apparent difference in the surface morphologies of PCBBBT-F:[70]PCBM and PCDTBT:[70]PCBM blended films on ITO/PEDOT:PSS, both of which exhibit smoothly distributed nanosized grains. One of the plausible reasons for the lower device performance of PCBBBT-F:[70]PCBM than PCDTBT:[70]PCBM is the less-organized structure of PCBBBT-F in the polymer domain (vide supra). The poor crystallinity of PCBBBT-F may lead to low charge-transport efficiencies and JSC values in comparison with PCDTBT. The ill-distributed electron density in the HOMO of PCBBBT-F (Figure 6) may also contribute to the poor charge mobility and the resultant low JSC value. In addition, faster nonradiative vibrational relaxation of the excited states of PCBBBT-F to the ground state, as seen in the film emission, would lead to the lower charge separation efficiency (vide supra). To take full advantage of the lower HOMO level of the fluorinated PCDTBT analogues for high-performance photovoltaic devices, it is important to optimize the molecular alignment in the films, which would be modulated by altering the side-chain structures and utilizations of additives.53

Figure 8. Photocurrent action spectra of PSC devices based on PCDTBT-F/:[70]PCBM (red line), PCBBBT-F:[70]PCBM (blue line), and PCDTBT:[70]PCBM (black line) films.

well with the absorption profiles, which coincides the sum of the absorptions of [70]PCBM and the polymer films (Figure S4, see Supporting Information). The absorbance of PCBBBTF:[70]PCBM film in the wavelength region of >550 nm is higher than that of PCDTBT-F:[70]PCBM owing to the smaller bandgap of PCBBBT-F, but the improvement is not prominent due to the considerable absorbance of [70]PCBM in this region. Although the film thickness of PCDTBT: [70]PCBM is comparable with those of other composite films, the absorbance is relatively high in the whole UV−vis region (Figure S4). This may result from the well-packed, more crystalline film structure of PCDTBT (vide supra). On the other hand, convolution of the spectral response with the photon flux of the AM 1.5G spectrum provided an estimate of the JSC value under irradiation. The calculated JSC values for the PCDTBT-F, PCBBBT-F and PCDTBT-based devices are 5.01, 7.04, and 11.6 mA cm−2, respectively. Because of the discrepancy between the IPCE results and the photon flux under AM 1.5 illumination, a mismatch less than 10% was present between the convolution and the solar simulator spectra. The VOC value is closely related to the energy difference between the HOMO of the donor and the LUMO of the acceptor.15 However, the VOC value of the PCDTBT-F-based device (0.82 V) is lower than that of the PCBBBT-F-based device (0.88 V), which do not fit the HOMO energy levels estimated by PYSA. This discrepancy may be explained in terms of the morphology of the blended film.16 Poor solubility and low molecular weight often cause a significant, undesirable impact on the interface resistance and the nano- to micrometer scale morphology, resulting in a low VOC, as well as JSC and FF. Figures 9 and 10 visualize the images of the blend films on ITO/PEDOT:PSS substrates obtained by atomic force

Figure 9. Tapping-mode atomic force micrographs of (a) PCDTBT-F:[70]PCBM, (b) PCBBBT-F:[70]PCBM, and (c) PCDTBT:[70]PCBM thin films spin-coated on ITO/PEDOT:PSS substrate. The color scale represents the height topography, with bright and dark representing the highest and lowest features, respectively. The rms surface roughnesses are (a) 0.53, (b) 0.65, and (c) 0.63 nm, respectively. 21154

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Figure 10. Optical microscope images of (a) PCDTBT-F:[70]PCBM, (b) PCBBBT-F:[70]PCBM, and (c) PCDTBT:[70]PCBM thin films spincoated on ITO/PEDOT:PSS substrate.

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CONCLUSION To investigate the effect of fluorine substitution on molecular and film structures, optical, electrochemical, and photovoltaic properties of the well-established conjugated polymer, PCDTBT, we have combined a fluorinated benzothiadiazole unit with carbazole in the polymer backbone for the first time to obtain fluorinated analogues of PCDTBT, that is, PCDTBTF and PCBBBT-F. The HOMO energy level of PCDTBT-F was lowered by the fluorination of benzothiadiazole unit, whereas the bandgap became large compared with that of PCDTBT due to the low molecular weight. PCBBBT-F exhibited narrower bandgap than PCDTBT, whereas the HOMO level was slightly higher than that of PCDTBT, implying that the additional incorporation of the electrondonating hexylthiophenes negates the fluorination effect. BHJPSC devices with PCDTBT-F:[70]PCBM and PCBBBT-F: [70]PCBM resulted in the lower PCE values (1.29 and 1.98%, respectively) than that of the PCDTBT-based device (6.16%). Unfavorable film structures, low crystallinities, and limited exciton lifetimes are responsible for the decrease in PCE. Considering the availabilities of fluorinated analogues of PCDTBT to lower the HOMO energy levels, there is still room to establish better efficiencies of BHJ-PSCs. The results obtained here provide valuable information on the elaborate design of PCDTBT-based polymers for the PSC applications.

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ASSOCIATED CONTENT

S Supporting Information *

Schematic illustrations for PSC devices, emission spectra of polymers, absorption spectra of polymer:[70]PCBM, and references with full list of authors. 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 This work is supported by New Energy and Industrial Technology Development Organization (NEDO). Authors thank Prof. Tetsuya Shishido (Tokyo Metropolitan University), Dr. Hisashi Yashiro and Mr. Shinichi Sato (Rigaku Corporation) for XRD measurements, and Prof. Takashi Sagawa (Kyoto University) for PYSA measurements.

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REFERENCES

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