Dyes: Synthesis, Characterization, and Photovoltaic Applications
SO-LIN HSU, CHIA-MIN CHEN, KUNG-HWA WEI
Department of Materials Science and Engineering, National Chiao Tung University, Hsinchu 30010, Taiwan
Received 7 August 2010; accepted 12 August 2010 DOI: 10.1002/pola.24311
Published online 5 October 2010 in Wiley Online Library (wileyonlinelibrary.com).
ABSTRACT:We have synthesized and characterized two new carbazole-based conjugated polymers, PCDCN and PCDTA, incorporating two strong light-absorbing organic dyes. These polymers exhibit relatively low band gaps (1.5 eV) and broad absorption ranges (from 300 to 700 nm). We fabri-cated polymer solar cells incorporating these polymers as donors and [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) as the acceptor. At a blending ratio of 1:4, we
obtained power conversion efficiencies, under simulated AM 1.5 (100 mW/cm2) conditions, of 2.31% and 2.47% for the PCDCN- and PCDTA-based devices, respectively. VC 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 5126–5134, 2010
KEYWORDS:blends; conjugated polymers; donor/acceptor; pho-tovoltaic cells; polycarbazole; polycondensation
INTRODUCTION Harvesting energy directly from sunlight using photovoltaic cells is potentially one of the most effi-cient ways of solving future global energy crises and envi-ronmental pollution problems. Polymeric solar cells (PSCs) can be used to produce lightweight, large-area, flexible devi-ces at low cost because they can be fabricated using solution coating or roll-to-roll processing.1–3 In recent years, many conjugated polymers have been developed and blended with [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM) to form
the bulk heterojunction (BHJ)-type solar cells.3Among these materials, poly(3-hexylthiophene) (P3HT) is the state-of-the-art p-type material, providing power conversion efficiencies (PCEs) of up to 5%.3–9Unfortunately, the optical band gap of
P3HT (2 eV) is too large to absorb solar energy in the near-IR region. Furthermore, the energy offset between the highest occupied molecular orbital (HOMO) of P3HT and the lowest unoccupied molecular orbital (LUMO) of PC61BM
is relatively small; therefore, the open circuit voltage (Voc) of
the PSCs is limited.10 A conjugated polymer possessing a narrow optical band gap can be obtained by either rising its HOMO energy level or lowering its LUMO energy level. The latter approach will, however, inevitably decrease the LUMO energy difference between the donor and acceptor materials, thereby weakening the driving force of exciton dissocia-tion.11–13 Moreover, a high-lying HOMO energy level might
also decrease the magnitude ofVocof a BHJ device, according
to the semiempirical estimation equation proposed by Scharber et al.10Consequently, molecular engineering of new conjugated polymers should be performed not only to reduce the band gap of the polymers but also to modulate their HOMO and LUMO energy levels to optimal values. One of the
most effective strategies toward this goal is to directly alter-nate a conjugated electron-rich donor (D) unit with a conju-gated electron-deficient acceptor (A) unit in a polymer back-bone.14–16 Following this approach, various donor–acceptor (D–A) conjugated polymers have exhibited promising per-formance, with PCEs as high as 4–6%.17–23
Push/pull D-p-A organic compounds, comprising an electron-releasing donor unit, ap-conjugated bridge, and an electron-withdrawing acceptor unit, have been used extensively as active materials in the field of nonlinear optics.24–28Because efficient intramolecular charge transfer (ICT) occurs from the donor unit to the acceptor unit on excitation, such chromo-phores are strong light-absorbing dyes possessing a broad absorption window extending to the near-IR region. More importantly, the band gap of D-p-A chromophores can be con-trolled simply by adjusting the electron-donating strength of the donor and the electron-withdrawing strength of the acceptor units in the dye. As a result, D-p-A chromophores have been used as effective photoactive materials in dye-sen-sitized solar cells.29–34It is envisaged that incorporation of D-p-A organic dyes into p-type conjugated polymers might be a useful strategy for greatly enhancing the light-harvesting abil-ity of solar cells. Recently, several new conjugated polymers with pendent D-p-A side chains have been synthesized for use in high-performance PSCs.35,36By copolymerization with fluorene or silafluorene segments in an alternating manner, the diphenylamino groups in D-p-A organic dyes are embed-ded into the main chains of the polymers. We envisioned that utilization of other electron-rich conjugated building blocks in the main chain would allow further optimization of the intrin-sic properties of this class ofp-type conjugated polymer. Correspondence to: K.-H. Wei (E-mail: [email protected])
Because of its excellent thermal and photochemical stability, relatively high hole mobility, and good solubility in common organic solvents, the 2,7-carbazole unit has emerged as a promising electron-donating moiety for the construction of D-A polymers.37–41 For example, devices based on poly(2,7-carbazole)-alt-dithienylbenzothiadiazole, synthesized by Leclerc and coworkers,19,22,42,43 have exhibited values of V
oc
as high as 0.89 V and PCEs of up to 6%.43,44
In this article, we report the synthesis and characterization of two-dimensional conjugated polymers: PCDCN and PCDTA, denoted from 2,7-carbazole-based polymers using dicyanovinyl group (DCN) and diethyl thiobarbituric acid (DTA) as acceptors, respectively. Both alternating copolymers comprise 2,7-carba-zole units and pendent D-p-A organic dye units. The D-p-A or-ganic dye inPCDCN features a diphenylamino group as the do-nor unit, a styrylthiophene unit as the p-bridge, and a dicyanovinyl group as the acceptor unit; in contrast, the dye in PCDTA features a thiobarbital group as the acceptor unit. We fabricated BHJ PSC devices using the polymers as the electron donor and PC71BM as the acceptor and investigated their
pho-tovoltaic properties. EXPERIMENTAL Materials
Malononitrile, 1,3-diethyl-2-thiobarbituric acid, phosphorus oxychloride, triphenylamine, N-bromosuccinimide (NBS), and tetrakis(triphenylphosphine)palladium [Pd(PPh3)4] were
pur-chased in reagent grade from Aldrich, Acros, TCI, or Lancas-ter Chemical and used as received. [6,6]-Phenyl-C71-butyric
acid methyl ester (PC71BM) was purchased from Nano-C.
Tet-rahydrofuran (THF), toluene, and N,N-dimethylformamide (DMF) were purified prior to use. All other solvents, includ-ing 1,2-dichlorobenzene and chloroform, were purchased from Aldrich, J. T. Baker, or Tedia and used as received. Characterization Techniques
1
H and13C NMR spectra were recorded using a Varian Unity-300 spectrometer. The molecular weights of the polymers were measured through gel permeation chromatography, using a Waters 2414 differential refractometer and a Waters Styragel column (polystyrene standards, THF as the eluent). UV–vis absorption spectra were recorded using an HP 8453 spectro-photometer. Photoluminescence spectra were recorded using a Hitachi F-4500 luminescence spectrometer. Thermogravimetric analysis (TGA) was performed under a N2atmosphere using a
Du Pont TGA 2950 instrument (heating rate: 10C/min). Differ-ential scanning calorimetry (DSC) was performed under a N2
atmosphere using a Perkin–Elmer Pyris DSC1 instrument (heat-ing rate: 10C/min). Cyclic voltammetry (CV) was performed using a BAS 100 electrochemical analyzer and a conventional three-electrode cell: a carbon glass electrode coated with the polymer thin film functioned as the working electrode, a Pt wire as the counter electrode, and Ag/Agþ(0.01 M AgNO3) as
the reference electrode. Tetrabutylammonium hexafluorophos-phate (0.1 M) in acetonitrile was the electrolyte for the CV measurements; the curves were calibrated using ferrocene as the standard (HOMO energy level of4.8 eV with respect to the zero vacuum level). The topographies of the polymer/PC71BM
films were measured through atomic force microscopy (AFM) in tapping mode, using a Digital Instruments Nanoscope IIIa apparatus under ambient conditions.
Synthetic Procedures
2-(2-{4-[N,N-Di(4-bromophenyl)amino]phenyl}ethenyl)thien-5-al (M1)35,36 and 2,7-bis(4,4,5,5-tetramethyl-1,3,2-dioxa-borolane-2-yl)-N-90-heptadecanylcarbazole (M2)45 were
pre-pared using reported procedures. The synthetic procedures for the preparation of the monomerM1 and the copolymers PCDCN and PCDTA are presented in Scheme 1.
4-(Diphenylamino)benzaldehyde (1)46,47
Phosphorus oxychloride (13.8 g, 89.7 mmol) was added drop-wise to DMF (6.65 g, 89.7 mmol) cooled at 0C in a three-neck 100-mL round-bottom flask and then the mixture was stirred at 0C for 1 h. A solution of triphenylamine (20.0 g, 81.5 mmol) in 1,2-dichloroethane was added via syringe and then the mix-ture was heated at 90C for 2 h. After cooling to room tempera-ture, the reaction mixture was washed sequentially with water (2 200 mL), saturated NaHCO3(1 200 mL), and water
again (1 200 mL). Following extraction with CH2Cl2(2 200
mL), the organic layer was dried (MgSO4) and concentrated.
The crude product was recrystallized (EtOH) to yield1 (12.8 g, 57%). 1 H NMR (300 MHz, CDCl3, ppm):d 9.81 (s, 1 H), 7.68 (d, J ¼ 8.4 Hz, 2 H), 7.34 (t, J ¼ 7.5 Hz, 4 H), 7.19–7.17 (m, 6 H), 7.02 (d,J ¼ 8.4 Hz, 2 H). 4-[N,N-Di(4-bromophenyl)amino]benzaldehyde (2)
NBS (17.2 g, 96.6 mmol) was added portionwise to a solution of 1 (12.0 g, 43.9 mmol) in chloroform (110 mL) in an ice-water cooling bath (0C) and then the mixture was stirred for 1 h. After removing the ice bath, the mixture was heated under reflux for 16 h. The resulting mixture was extracted with CH2Cl2 and then the combined extracts were washed with
water and brine. The organic phases were collected, dried (MgSO4), and concentrated under reduced pressure. The crude
product was recrystallized (MeOH) to yield2 (15.6 g, 82%).
1 H NMR (300 MHz, CDCl3, ppm):d 9.84 (s, 1 H), 7.71 (d, J ¼ 8.8 Hz, 2 H), 7.46–7.43 (m, 4 H), 7.06–7.00 (m, 6 H). 2-(2-{4-[N,N-Di(4-bromophenyl)amino]phenyl}ethenyl) thiophene (3) Diethyl (2-methylthiophene)phosphonate (1.41 g, 6.03 mmol) was added to a solution of potassium tert-butoxide (0.680 g, 6.03 mmol) in dry THF (15 mL) at 0 C and then the mixture was stirred for 1 h. A solution of2 (2.00 g, 4.64 mmol) in dry THF was added via syringe and the resulting mixture was then stirred at room temperature for 12 h. The reaction mixture was poured into water and extracted with CH2Cl2. The organic layer was washed with water, dried
(MgSO4), and concentrated under reduced pressure; the
crude product was purified chromatographically (SiO2;
EtOAc/hexane, 1:10). Recrystallization (CH2Cl2/hexane)
afforded3 as a yellowish solid (0.96 g, 41%).
1
H NMR (300 MHz, CDCl3, ppm):d 7.37–7.34 (m, 6 H), 7.19–
7.11 (m, 2 H), 7.05–6.94 (m, 8 H), 6.89–6.84 (m, 1 H). Anal.
Calcd: C, 56.38; H, 3.35; N, 2.74. Found: C, 56.24; H, 3.72; N, 2.84.
2-(2-{4-[N,N-Di(4-bromophenyl)amino]phenyl}ethenyl) thien-5-al (M1)
Phosphorus oxychloride (2.25 g, 14.7 mmol) was added drop-wise to DMF (1.07 g, 14.7 mmol) in a two-neck 100-mL round-bottom flask at 0C and then the mixture was stirred for 1 h. A solution of 3 (3.00 g, 5.87 mmol) in 1,2-dichloroethane was added via syringe and then the mixture was heated at 90C for 12 h. After cooling to room temperature, the reaction was quenched with saturated aqueous NaHCO3and extracted with
CH2Cl2. The combined organic phases were washed with water,
dried (MgSO4), and concentrated under reduced pressure. The
crude product was purified chromatographically (SiO2; EtOAc/
hexane, 1:10) to yieldM1 as a brown solid (3.01 g, 95%).
1
H NMR (300 MHz, CDCl3, ppm):d 9.85 (s, 1 H), 7.66 (d, J ¼
3.9 Hz, 1 H), 7.39 (d, J ¼ 8.7 Hz, 6 H), 7.13–6.96 (m, 9 H). Anal. Calcd: C, 55.68; H, 3.18; N, 2.60. Found: C, 55.43; H, 3.49; N, 2.56.
PCCHO
A mixture of M1 (164 mg, 0.30 mmol), M2 (200 mg, 0.30 mmol), K2CO3 (221 mg, 1.6 mmol), H2O (1 mL), and Aliquat
336 (36 mg) in toluene (5 mL) was degassed and then Pd(PPh3)4(7 mg, 0.006 mmol) was added at 60C. The mixture
was stirred and heated under reflux for 24 h under a N2
atmos-phere. Phenylboronic acid (37 mg, 0.03 mmol) was then added and the mixture was heated under reflux for a further 3 h; bro-mobenzene (0.03 mL, 0.03 mmol) was then added and the ture was heated under reflux for another 3 h. The reaction mix-ture was poured into MeOH (100 mL) and the resulting orange precipitate was collected through filtration and then purified in a Soxhlet apparatus with MeOH, acetone, and hexane for 24 h to remove any residual oligomers and catalyst residues, to pro-vide the purified polymer (182 mg, 76%).
1H NMR (300 MHz, CDCl
3, ppm):d 9.83 (s, 1 H), 8.18–8.10
(m, 2 H), 7.75–7.12 (m, 20 H), 4.67 (s, 1 H), 2.38 (s, 2 H), 1.98 (s, 2 H), 1.23–1.12 (m, 24 H), 0.77(t, J ¼ 6 Hz, 6 H). Anal. Calcd: C, 82.82; H, 7.47; N, 3.58. Found: C, 81.64; H, 7.56; N, 3.81.
PCDCN
Pyridine (0.085 mL) was added to a solution ofPCCHO (85 mg, 0.10 mmol) and malononitrile (250 mg, 3.78 mmol) in chloroform (8.5 mL) with a two-neck 25-mL round-bottom flask. The mixture was stirred at room temperature for 24 h and then poured into MeOH (100 mL). The black precipitate was filtered off and purified through repeated precipitation (twice) from THF solution into MeOH (71 mg, 79%).
1 H NMR (300 MHz, CDCl3, ppm):d 8.18–8.10 (m, 2 H), 7.72– 7.12 (m, 21 H), 4.67 (s, 1 H), 2.38 (s, 2 H), 1.98 (s, 2 H), 1.23–1.12 (m, 24 H), 0.77 (t,J ¼ 6 Hz, 6 H). Anal. Calcd: C, 82.36; H, 7.03; N, 6.74. Found: C, 80.71; H, 7.70; N, 6.40. PCDTA
Pyridine (0.085 mL) was added to a solution ofPCCHO (85 mg, 0.10 mmol) and 1,3-diethyl-2-thiobarbituric acid (758
mg, 3.78 mmol) in chloroform (8.5 mL) in a two-neck 25-mL round-bottom flask. The mixture was stirred at room tem-perature for 24 h and then poured into MeOH (100 mL). The black precipitate was filtered off and purified through repeated (twice) precipitation from THF solution into MeOH (90 mg, 86%).
1
H NMR (300 MHz, CDCl3, ppm):d 8.61 (s, 1 H), 8.18–8.10
(m, 2 H), 7.78–7.14 (m, 20 H), 4.62–4.56 (m, 5 H), 2.38 (s, 2 H), 1.98 (s, 2 H), 1.48–1.12 (m, 30 H), 0.77 (t, J ¼ 6 Hz, 6 H). Anal. Calcd: C, 77.13; H, 7.10; N, 5.81. Found: C, 75.74; H, 7.68; N, 5.86.
Photovoltaic Device Fabrication
The current density–voltage (J–V) properties were deter-mined for devices possessing the sandwich structure: indium tin oxide (ITO)/poly(3,4-ethylenedioxythiophene) (PEDOT)/ polymer:PC71BM/Ca/Al. First, an ITO-coated glass substrate
was etched with acid and cleaned using detergent, deionized water, acetone, and isopropyl alcohol, respectively. The cleaned substrate was treated with ozone plasma for 5 min and then a 30-nm-thick layer of PEDOT (Batron P AL 4083, HC Stark) was deposited through spin coating. The PEDOT-coated substrate was heated at 150C for 30 min to ensure that all of the solvents had been removed. The active layers of the devices were spin coated (1500 rpm) from 1,2-dichlorobenzene/chloroform (1:1, v/v) solutions containing a polymer and PC71BM at mixed ratios ranging from 1:1 to 1:4
(w/w). The thickness of each polymer/PC71BM layer was
100 nm. After thermal annealing of the active layers at 120
C for 10 min, a layer of Ca (35 nm) was vapor deposited as
the cathode and then a layer of Al (100 nm) was deposited as a protecting layer under a base pressure of less than 1 106 torr. The effective area of each resulting device was 0.04 cm2. Testing of the sample devices was performed under simulated AM 1.5 G illumination (100 mW/cm2) using
a Xe lamp-based Newport 66902 150 W solar simulator equipped with an AM 1.5 filter as the white light source. An OPHIR thermopile 71964 instrument was used to determine that the optical power at the sample was 100 mW/cm2. The
J–V characteristics of all of the samples were measured under a N2 atmosphere using a Keithley 236 source
mea-surement meter. The external quantum efficiencies (EQEs) were measured using a Keithley 236 source measure unit coupled with an Oriel Cornerstone 130 monochromator; the light intensity at each wavelength was calibrated using an OPHIR 71580 diode.
TABLE 1 Molecular Weights and Thermal Properties of the Polymers Polymer Mn (g/mol) Mw (g/mol) PDI Td(C) Tg(C) PCCHO 11,017 50,675 4.59 398 145 PCDCN 11,049 50,957 4.61 402 157 PCDTA 6,466 22,284 3.45 312 166 ARTICLE
RESULTS AND DISCUSSION Synthesis and Characterization
Scheme 1 presents the chemical structures and synthetic routes toward the copolymersPCDCN and PCDTA. We copo-lymerized the monomersM1 and M2 through Suzuki cross-coupling polycondensation to afford the precursor polymer PCCHO in reasonable yield of 76%. After polymerization for 24 h, end-capping reactions were performed using bromo-benzene and phenylboronic acid to increase the stability of the polymer. The final polymers, PCDCN and PCDTA, were obtained through Knoevenagel condensations by treat-ing the aldehyde-functionalized precursor polymer with
malononitrile and diethylthiobarbituric acid, respectively. The reason to complete the targeted polymers via postfunc-tionalization of the precursor polymer is that the final D-p-A organic dyes are subject to decomposition in the presence of base used in Suzuki cross-coupling condition. According to size exclusion chromatography experiments (monodisperse polystyrene standards, THF as the solvent), our purified PCCHO had a number-average molecular weight (Mn) of
11,017 g/mol and a polydispersity index (PDI) of 4.59. Table 1 summarizes the physical properties of our three polymers. The values of Mn of PCDCN and PCDTA are 11,049 g/mol
and 6466 g/mol, respectively, with corresponding PDIs of FIGURE 11H NMR spectra of the polymers: (a) PCCHO, (b) PCDCN, and (c) PCDTA.
FIGURE 2UV–vis absorption spectra of PCDCN and PCDTA. [Color figure can be viewed in the online issue, which is avail-able at wileyonlinelibrary.com.]
FIGURE 3Cyclic voltammograms of the polymer films, meas-ured from acetonitrile solutions containing 0.1 M Bu4NPF6at a scan rate of 50 mV/s. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]
4.61 and 3.45, respectively. Notably, PCDTA possessed a lower molecular weight than its precursor polymer,PCCHO, presumably because strong dipole–dipole interactions between the organic dye units led to packing of the side chains and, thereby, disturbed the GPC results. Hence, the molecular weight based on PCCHO (Mn¼ 11,017 g/mol) is
more reliable for estimating the actual molecular weight of PCDTA. The chemical structures of both polymers were veri-fied through 1H NMR spectroscopic and elemental analyses. Figure 1 presents the1H NMR spectra of our three polymers. The signal of the aldehydic proton ofPCCHO at 9.83 ppm is absent in the spectra of both PCDCN and PCDTA; mean-while, signals of olefinic proton appear at 7.72 ppm for PCDCN and 8.61 ppm for PCDTA, confirming the transforma-tion of the aldehyde units to the targeted acceptor groups. The solubilities of PCDCN and PCDTA are good in 1,2-dichlorobenzene, chloroform, and THF; therefore, we readily obtained uniform thin films of the polymers through spin coating for further study of PSC devices.
Thermal Properties
We performed DSC and TGA under a N2 atmosphere to
determine the thermal properties ofPCDCN and PCDTA (Ta-ble 1). TGA indicated that both polymers exhibit good ther-mal stability, with 5%-weight-loss temperatures (Td) of 402 C for PCDCN and 312 C for PCDTA. The glass transition
temperatures (Tg) ofPCDCN and PCDTA are 157C and 166
C, respectively. Good thermal stability and high values ofT g
are important parameters for polymers incorporated in PSC devices, because they provide resistance against the defor-mation or degradation of the active layers.
Optical Properties
The absorption spectra of all studied polymers were meas-ured in both dilute THF and in the thin films (Fig. 2), and the correlated optical parameters were summarized in Ta-ble 2. Both polymers exhibit two characteristic bands in the absorption spectra; the first absorption peak at 385 nm represents the p–p* transition of their conjugated main chains, while the second absorption peaks at longer wave-lengths are attributed to the ICT from the diphenyl amino groups in the conjugated main chains to the acceptor groups in the pendent side chains. The ICT absorption peak of PCDTA (at 567 nm) is much red-shifted compared with that of PCDCN (at 519 nm), indicating that the accepting strength of DTA unit is stronger than that of DCN unit. It is also noteworthy that the ICT bands of absorption spectra shift toward longer wavelengths from the solution to the solid states, whereas the p–p* transition bands are essen-tially unchanged. This result suggests that strong packing between the D-p-A conjugated side chains due to electro-static interactions occurs in the solid state, whereas the intermolecular interactions between the main chains of poly-mers are relatively weak. The optical band gaps (Eopt
g ),
calcu-lated from the absorption onsets in their films, are 1.51 eV forPCDCN and 1.47 eV for PCDTA.
Electrochemical Properties
CV was used to estimate the HOMO and LUMO energy levels of our conjugated polymers (Figure 3). We calculated the HOMO energy levels ofPCDCN and PCDTA to be 5.22 and 5.24 eV when using ferrocene (4.8 eV below the vacuum) TABLE 2Optical and Electrochemical Properties of the Polymers
Polymer kmax(nm) Solution kmax(nm) Film Eopt g (eV) a EEC
g (eV) EHOMO(eV) ELUMO(eV)
PCDCN 383, 519 385, 550 1.51 1.55 5.22 3.66
PCDTA 384, 567 388, 592 1.47 1.56 5.24 3.68
a
The value ofEopt
g was calculated from the edge of the absorption spectrum of the film.
FIGURE 4J–V characteristics of devices having the configura-tion ITO/PDOT:PSS/polymer:PC71BM/Ca/Al. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]
TABLE 3 Photovoltaic Properties of Polymer Solar Cells Polymer:PC71BM (Weight Ratio) Voc(V) Jsc (mA/cm2) FF PCE (%) PCDCN:PC71BM (1:1) 0.60 6.95 0.36 1.52 PCDCN:PC71BM (1:2) 0.66 7.32 0.37 1.81 PCDCN:PC71BM (1:3) 0.68 7.55 0.38 1.95 PCDCN:PC71BM (1:4) 0.70 8.22 0.40 2.31 PCDTA:PC71BM (1:1) 0.37 5.62 0.34 0.74 PCDTA:PC71BM (1:2) 0.58 8.13 0.34 1.61 PCDTA:PC71BM (1:3) 0.66 8.62 0.37 2.12 PCDTA:PC71BM (1:4) 0.68 8.80 0.41 2.47 ARTICLE
as the internal standard. The similar HOMO energy levels can be ascribed to the identical main chain structure of the polymers. The LUMO energy levels are approximately located at3.66 eV for PCDCN and 3.68 eV for PCDTA, which are positioned 0.2–0.3 eV above the LUMO level of the PC71BM
acceptor (3.8 eV) to ensure energetically favorable electron transfer.
Photovoltaic Properties
To investigate the photovoltaic properties of PCDCN and PCDTA, we fabricated BHJ PSC devices having the structure of ITO/PEDOT:PSS/polymer:PC71BM/Ca/Al. The copolymer:
PC71BM blend active layers, in which the copolymer was used
as the donor and PC71BM as the acceptor, were spin coated
from 1,2-dichlorobenzene/chloroform (1:1, v/v) solutions at a concentration of 7.5 mg/mL. The PC71BM was chosen as the
acceptor because of its high absorption coefficient in the region from 440 to 530 nm, which could complement the absorption valley of the polymers.48Because the weight ratio between the
copolymer and PC71BM in the active layer affects the device
performance, we studied PSC devices incorporating blending ratios ranging from 1:1 to 1:4 (polymer:PC71BM). The optimal
ratio was 1:4; Figure 4 presents theJ–V characteristics of the resulting device measured under simulated AM 1.5 G (100 mW/cm2) illumination conditions. A PCE of up to 2.47% with a value ofVocof 0.68 V, a value ofJscof 8.80 mA/cm
2
, and a fill factor (FF) of 0.41 was obtained for the device based on PCDTA:PC71BM (1:4). In thePCDCN system, a best
perform-ance—a value ofVocof 0.70 V, a value ofJscof 8.22 mA/cm2, a
FF of 0.40, and a PCE of 2.31%—was also obtained at a poly-mer:PC71BM ratio of 1:4. Table 3 summarizes the performances
of devices prepared from the two polymers at different blend-ing ratios. The values ofVocwere quite similar for the solar cells
incorporating PCDCN and PCDTA because of the relatively FIGURE 5EQE spectra of PCDCN- and PCDTA-based solar cells
illuminated under monochromatic light.
FIGURE 6Topographic AFM images (scale: 2 2 lm2) of devices incorporating PCDCN/PC71BM blends at weight ratios of (a) 1:1, (b) 1:2, (c) 1:3, and (d) 1:4. Their corresponding roughness are determined to be 1.92, 1.45, 1.25, and 1.07 nm, respectively.
small difference in the HOMO energy levels of these two poly-mers; however, the higher value ofJscfor the device
incorporat-ingPCDCN might be ascribed to its broader absorption in the range 600–700 nm, as supported by the spectra of EQE in Fig-ure 5. On increasing the amount of PC71BM, the photocurrents
(Jsc) of the devices incorporating either PCDCN or PCDTA
increased. We attribute this phenomenon to the sufficient num-ber of electron and hole percolation pathways that were formed to allow more efficient charge separation.22,49Figure 5 presents the EQE spectra of the best two devices; the curves reveal great coherence with the absorptions of the active layers. Although relative to the PCDCN blended layer, the PCDTA blended layer exhibited lower absorption near 500 nm, it main-tained slightly higher intensities in the range 620–720 nm so that the device based onPCDTA provided a higher photocurrent (8.80 mA/cm2) than that based onPCDCN (8.22 mA/cm2). The morphology of the active layer in the device plays an im-portant role in determining the device performance and was thus investigated using tapping-mode AFM. The topography images ofPCDCN:PC71BM blends are shown in the Figure 6. It
is interesting to observe that as the blending ratio of PC71BM
increases from 1:1 to 1:4, the root-mean-square roughness decreases from 1.92 to 1.07 nm. Increasing the content of PC71BM in the blend might prevent the polymer from severe
aggregation by diluting the electrostatic interactions between the organic dyes, thereby decreasing the surface roughness. As a result, the improved photovoltaic performance of the PSC devices may be correlated to the decreased intermolecular packing between the organic dyes in the polymers.
CONCLUSIONS
We have used Suzuki coupling to synthesize two new carba-zole-based conjugated polymers, PCDCN and PCDTA, incor-porating strong acceptor groups linked through a triphenyl-amine backbone, for use in PSCs. These two polymers exhibit high solubility, adequate thermal stability, and the broadband absorptions from 300 to 700 nm. According to the CV measurements, PCDCN and PCDTA possessed very similar band gap energies (1.55 eV and 1.56 eV, respec-tively). The best performing devices incorporating PCDCN/ PC71BM and PCDTA/PC71BM at the same blending ratio
(1:4), exhibited promising PCEs of 2.31% and 2.47%, respec-tively, accompanied by values ofJscof 8.22 mA/cm2and 8.80
mA/cm2, respectively. To further optimize the optical and electrical properties, future molecular engineering of D-p-A dye-based polymers will be focused on selecting appropriate acceptor units for positioning at the ends of the side chains. The authors thank the National Science Council for financial support.
REFERENCES AND NOTES
1 Brabec, C. J.; Sariciftci, N. S.; Hummelen, J. C. Adv Funct Mater 2001, 11, 15–26.
2 Krebs, F. C.; Gevorgyan, S. A.; Alstrup, J. J Mater Chem 2009, 19, 5442–5451.
3 Gu¨nes, S.; Neugebauer, H.; Sariciftci, N. S. Chem Rev 2007, 107, 1324–1338.
4 Hoppe, H.; Sariciftci, N. S. J Mater Chem 2006, 16, 45–61. 5 Thompson, B. C.; Fre´chet, J. M. J. Angew Chem Int Ed Engl 2008, 47, 58–77.
6 Li, G.; Shrotriya, V.; Huang, J. S.; Yao, Y.; Moriarty, T.; Em-ery, K.; Yang, Y. Nat Mater 2005, 4, 864–868.
7 Ma, W. L.; Yang, C. Y.; Gong, X.; Lee, K. H.; Heeger, A. J. Adv Funct Mater 2005, 15, 1617–1622.
8 Yip, H.-L.; Hau, S. K.; Baek, N. S.; Ma, H.; Jen, A. K.-Y. Adv Mater 2008, 20, 2376–2382.
9 Hau, S. K.; Yip, H.-L.; Ma, H.; Jen, A. K.-Y. Appl Phys Lett 2008, 93, 233304-1–233304-3.
10 Scharber, M. C.; Mu¨hlbacher, D.; Koppe, M.; Denk, P.; Wal-dauf, C.; Heeger, A. J.; Brabec, C. J. Adv Mater 2006, 18, 789–794.
11 Koster, L. J. A.; Mihailetchi, V. D.; Blom, P. W. M. Appl Phys Lett 2006, 88, 093511-1–093511-3.
12 Brabec, C. J.; Winder, C.; Sariciftci, N. S.; Hummelen, J. C.; Dhanabalan, A.; van Hal, P. A.; Janssen, R. A. J. Adv Funct Mater 2002, 12, 709–712.
13 Halls, J. J. M.; Cornil, J.; dos Santos, D. A.; Silbey, R.; Hwang, D.-H.; Holmes, A. B.; Bre´das, J. L.; Friend, R. H. Phys Rev B 1999, 60, 5721–5727.
14 Kroon, R.; Lenes, M.; Hummelen, J. C.; Blom, P. W. M.; de Boer, B. Polym Rev 2008, 48, 531–582.
15 Cheng, Y. J.; Yang, S. H.; Hsu, C. S. Chem Rev 2009, 109, 5868–5923.
16 Li, Y. F.; Zou, Y. P. Adv Mater 2008, 20, 2952–2958.
17 Mu¨hlbacher, D.; Scharber, M.; Morana, M.; Zhu, Z.; Waller, D.; Gaudiana, R.; Brabec, C. Adv Mater 2006, 18, 2884–2889. 18 Wang, E.; Wang, L.; Lan, L.; Luo, C.; Zhuang, W.; Peng, J.; Cao, Y. Appl Phys Lett 2008, 92, 033307-1–033307-3.
19 Blouin, N.; Michaud, A.; Gendron, D.; Wakim, S.; Blair, E.; Neagu-Plesu, R.; Belleteˆte, M.; Durocher, G.; Tao, Y.; Leclerc, M. J Am Chem Soc 2008, 130, 732–742.
20 Baek, N. S.; Hau, S. K.; Yip, H.-L.; Acton, O.; Chen, K.-S.; Jen, A. K.-Y. Chem Mater 2008, 20, 5734–5736.
21 Hou, J. H.; Chen, H.-Y.; Zhang, S. Q.; Li, G.; Yang, Y. J Am Chem Soc 2008, 130, 16144–16145.
22 Park, S. H.; Roy, A.; Beaupre´, S.; Cho, S.; Coates, N.; Moon, J. S.; Moses, D.; Leclerc, M.; Lee, K.; Heeger, A. J. Nat Pho-tonics 2009, 3, 297–302.
23 Liang, Y. Y.; Feng, D. Q.; Wu, Y.; Tsai, S. T.; Li, G.; Ray, C.; Yu, L. P. J Am Chem Soc 2009, 131, 7792–7799.
24 Liu, S.; Haller, M. A.; Ma, H.; Dalton, L. R.; Jang, S.-H.; Jen, A. K.-Y. Adv Mater 2003, 15, 603–607.
25 Ma, H.; Liu, S.; Luo, J.; Suresh, S.; Liu, L.; Kang, S. H.; Hal-ler, M.; Sassa, T.; Dalton, L. R.; Jen, A. K.-Y. Adv Funct Mater 2002, 12, 565–574.
26 Burland, D. M.; Miller, R. D.; Walsh, C. A. Chem Rev 1994, 94, 31–75.
27 He, M.; Leslie, T. M.; Sinicropi, J. A. Chem Mater 2002, 14, 4662–4668.
28 He, M.; Leslie, T. M.; Sinicropi, J. A.; Garner, S. M.; Reed, L. D. Chem Mater 2002, 14, 4669–4675.
29 Chen, K.-F.; Hsu, Y.-C.; Wu, Q.; Yeh, M.-C. P.; Sun, S.-S. Org Lett 2009, 11, 377–380.
30 Teng, C.; Yang, X.; Yang, C.; Tian, H.; Li, S.; Wang, X.; Hag-feldt, A.; Sun, L. J Phys Chem C 2010, 114, 11305–11313. 31 Im, H.; Kim, S.; Park, C.; Jang, S.-H.; Kim, C.-J.; Kim, K.; Park, N.-G.; Kim, C. Chem Commun 2010, 46, 1335–1337. 32 Tian, H.; Yang, X.; Pan, J.; Chen, R.; Liu, M.; Zhang, Q.; Hag-feldt, A.; Sun, L. Adv Funct Mater 2008, 18, 3461–3468.
33 Erten-Ela, S.; Yilmaz, M. D.; Icli, B.; Dede, Y.; Icli, S.; Akkaya, E. U. Org Lett 2008, 10, 3299–3302.
34 Qin, P.; Zhu, H.; Edvinsson, T.; Boschloo, G.; Hagfeldt, A.; Sun, L. J Am Chem Soc 2008, 130, 8570–8571.
35 Huang, F.; Chen, K. S.; Yip, H. L.; Hau, S. K.; Acton, O.; Zhang, Y.; Luo, J.; Jen, A. K.-Y. J Am Soc Chem 2009, 131, 13886–13887.
36 Duan, C.; Cai, W.; Huang, F.; Zhang, J.; Wang, M.; Yang, T.; Zhong, C.; Gong, X.; Cao, Y. Macromolecules 2010, 43, 5262–5268.
37 Wakim, S.; Aich, B.-R.; Tao, Y.; Leclerc, M. Polym Rev 2008, 48, 432–462.
38 Morin, J.-F.; Drolet, N.; Tao, Y.; Leclerc, M. Chem Mater 2004, 16, 4619–4626.
39 Kobayashi, N.; Koguchi, R.; Kijima, M. Macromolecules 2006, 39, 9102–9111.
40 Grigalevicius, S.; Ma, L.; Xie, Z.-Y.; Scherf, U. J. Polym Sci Part A: Polym Chem 2006, 44, 5987–5994.
41 Xie, L.-H.; Deng, X.-Y.; Chen, L.; Chen, S.-F.; Liu, R.-R.; Hou, X.-Y.; Wong, K.-Y.; Ling, Q.-D.; Huang, W. J. Polym Sci Part A: Polym Chem 2009, 47, 5221–5229.
42 Blouin, N.; Michaud, A.; Leclerc, M. Adv Mater 2007, 19, 2295–2300.
43 Wakim, S.; Beaupre´, S.; Blouin, N.; Aich, B.-R.; Rodman, S.; Gaudiana, R.; Tao, Y.; Leclerc, M. J Mater Chem 2009, 19, 5351–5358.
44 Chu, T. Y.; Alem, S.; Verly, P. G.; Wakim, S.; Lu, J.; Tao, Y.; Beaupre´, S.; Leclerc, M.; Be´langer, F.; De´silets, D.; Rod-man, S.; Waller, D.; Gaudiana, R. Appl Phys Lett 2009, 95, 063304-1–063304-3.
45 Blouin, N.; Leclerc, M. Acc Chem Res 2008, 41, 1110–1119. 46 Lee, T. H.; Tong, K. L.; So, S. K.; Leung, L. M. Synth Met 2005, 155, 116–124.
47 Lee, S. K.; Hwang, D.-H.; Jung, B.-J.; Cho, N. S.; Lee, J.; Lee, J.-D.; Shim, H.-K. Adv Funct Mater 2005, 15, 1647–1655. 48 Yao, Y.; Shi, C. J.; Li, G.; Shrotriya, V.; Pei, Q. B.; Yang, Y. Appl Phys Lett 2006, 89, 153507-1–153507-3.
49 Gadisa, A.; Mammo, W.; Andersson, L. M.; Admassie, S.; Zhang, F.; Andersson, M. R.; Ingana¨s, O. Adv Funct Mater 2007, 17, 3836–3842.
