Copolymers Presenting 1,3,4-Oxadiazole Units and Their Application to
Photovoltaic Cells
HSU-SHEN WANG, MING-SHIN SU, KUNG-HWA WEI
Department of Materials Science and Engineering, National Chiao Tung University, 300 Hsinchu, Taiwan, Republic of China
Received 8 March 2010; accepted 4 May 2010 DOI: 10.1002/pola.24117
Published online in Wiley InterScience (www.interscience.wiley.com).
ABSTRACT:We have used Grignard metathesis polymerization to prepare poly(3-hexylthiophene)-based copolymers containing electron-withdrawing 4-tert-butylphenyl-1,3,4-oxadiazole-phenyl moieties as side chains. We characterized these copolymers using 1H and 13C nuclear magnetic resonance spectroscopy, thermogravimetric analysis, and gel permeation chromatogra-phy. The band gap energy of copolymer was determined from the onset of the optical absorption. The quenching effects were observed in the photoluminescence spectra of the copolymers
incorporating pendant electron-deficient 1,3,4-oxadiazole moi-eties on the side chains. The photocurrents of devices were enhanced in the presence of an optimal amount of the 1,3,4-oxa-diazole moieties, thereby leading to improved power conversion efficiencies.VC 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 3331–3339, 2010
KEYWORDS:conjugated polymers; copolymerization; hetero-atom-containing polymers; oxadiazole; photovoltaic cell
INTRODUCTION Conjugated polymers possessing extended delocalized p electrons are being investigated intensively for their potential uses in such organic optoelectronic devices as light-emitting diodes (LEDs), thin film transistors, and photo-voltaic cells incorporating bulk heterojunctions.1–9 Bulk
het-erojunctions based on blends of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester
(PCBM) have recently reached power conversion efficiencies of 4–5% under standard solar illumination (AM 1.5G, 100 mW cm2).10–14 Research into conjugated polymers contain-ing electron donor–acceptor pairs in the polymeric main chain and/or side chains has become quite attractive recently15–22because such system exhibits tunable electronic
properties and enlarged spectral absorption ranges. Alterna-tively, the introduction of a side-chain electron-acceptor unit—usually a conjugated species that absorbs a wavelength different from that of the conjugated polymer—can increase the breadth of the wavelengths of light absorbed. Further-more, the photogenerated excitons can be dissociated into electrons and holes more efficiently in these types of conju-gated polymer because of the internal field produced by the inherent dipole moment resulting from the donor–acceptor molecular structure, with subsequent charge transfer to nearby n-type nanoparticles (e.g., PCBM). In previous studies, we developed a new class of conjugated polymers containing side chain–tethered conjugated acceptor moieties that not only absorb light more effectively but also exhibit enhanced
charge transfer ability—two desirable properties for photo-voltaics applications.23–25
Most polythiophene derivatives are good hole-transport materials but exhibit low electron mobility. Oxadiazole-con-taining polymers have been widely applied as electron-trans-porting and hole-blocking materials in organic electronic devices26–29 (e.g., organic LEDs) because of the strong elec-tron-withdrawing ability of the heterocycle, their good ther-mal and chemical properties,30–32 and the tenability of the charge carrier mobility of attached materials.33,34Oxadiazole units have previously been incorporated as side chains on poly(phenylenevinylene) (PPV) and polyfluorene (PF) deriva-tives that have then been incorporated into photovoltaic cells without PCBM.35 The performance of these devices was improved as a result of increases in the electron mobility and rate of exciton dissociation in the photoactive layer. Fur-thermore, PPV derivatives featuring branched oxadiazole units within the PPV main chain have been applied as a PCBM blend to provide a photovoltaic cell exhibiting a PCE of 1.6%.36 In contrast, the photovoltaic behavior of oxadia-zole-attached polythiophene copolymers has not been reported previously.
In this study, we synthesized a donor–acceptor thiophene-type copolymer presenting electron-withdrawing 1,3,4-oxa-diazole moieties as side chains for application in photovoltaic cells. Schemes 1 and 2 display our synthetic approach
Additional Supporting Information may be found in the online version of this article. Correspondence to: K.-H. Wei (E-mail: [email protected]) Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 48, 3331–3339 (2010)VC2010 Wiley Periodicals, Inc.
toward a thiophene monomer modified with an electron-withdrawing moiety and its subsequent polymerization. EXPERIMENTAL
Materials
2,5-Dibromo-3-hexylthiophene (M1) and 4,4,5,5-tetramethyl-2-(thiophen-3-yl)-1,3,2-dioxaborolane were synthesized according to literature procedures.37–41 The synthetic routes toward monomer 2 (M2) and polymers P05–P20 are pre-sented in Schemes 1 and 2; procedures for the syntheses of their intermediates are described below. The 3 M ether solu-tion of methylmagnesium bromide was purchased from TCI. PCBM was purchased from Nano-C. Regioregular P3HT was purchased from Rieke Metals (4002-E). Poly(ethylenedioxy-thiophene)/poly(styrenesulfonate) (PEDOT/PSS) was pur-chased from Baytron (P VP A1 4083). All other chemicals and solvents were purchased in reagent grade from Aldrich, Acros, TCI, or Lancaster Chemical and used as received. Measurement and Characterization
1
H and13C nuclear magnetic resonance (NMR) spectra were recorded using a Varian Unity-300 NMR spectrometer. Ther-mogravimetric analysis (TGA) of the polythiophene deriva-tives was performed using a TA Instruments Q500 apparatus operated at a heating rate of 20C min1under a N2 flow.
Differential scanning calorimetry (DSC) was performed using a Perkin–Elmer Pyris 7 instrument operated at a heating rate of 20 C min1 under N2 flow; samples were heated
from 30 to 200 C, cooled to 20 C, and then heated again from 30 to 200C. Elemental analysis (EA) of the polymers was performed using a Heraeus CHN-OS Rapid instrument. UV–vis spectra were measured using an HP 8453 diode array spectrophotometer. PL spectra were recorded using a Hitachi F-4500 luminescence spectrometer. We used a Waters chro-matography unit interfaced to a Waters 2414 differential re-fractometer. Three 5-lm Waters styragel columns were con-nected in series in decreasing order of pore size (104, 103, and 102 Å); THF was the eluent and standard polystyrene
samples were used for calibration. Cyclic voltammetry (CV) was performed using a BAS 100 electrochemical analyzer, operated at a potential scan rate of 50 mV s1; the redox behavior of each polymer was investigated using a solution of 0.1 M tetrabutylammonium hexafluorophosphate in anhy-drous acetonitrile as the electrolyte; the potentials were measured against a Ag/Agþ(0.01 M AgNO3) reference
elec-trode; ferrocene/ferrocenium ion (Fc/Fcþ) was used as the internal standard. The onset potentials were determined from the intersection of two tangents drawn at the rising and background currents of the cyclic voltammogram. The topographies of the polymer/PCBM films were measured through tapping-mode atomic force microscopy (AFM) using a Digital Instruments Nanoscope IIIa apparatus under ambi-ent conditions; AFM samples were prepared by spin-coating solutions of polymer/PCBM blends in chlorobenzene onto in-dium tin oxide (ITO)/PEDOT substrates, followed by anneal-ing at 150C for 10 min.
Device Fabrication and Characterization of Polymer Solar Cell
Current density–voltage (J–V) measurements were performed using devices having a sandwich structure [ITO/PEDOT:PSS/ polymer:PCBM (1:1, w/w)/Ca/Al]. The ITO-coated glass sub-strate was precleaned and treated with oxygen plasma before use. The polymer/PCBM layers were spin-coated from their corresponding dichlorobenzene solutions (20 mg mL1) at 1500 rpm. The thickness of the polymer/PCBM layers was 100 nm. The active layers of the devices were thermally annealed at 150 C for 10 min before electrode deposition. Using a base pressure of <1 106Torr, layers of Ca (20 nm) and Al (100 nm) were vacuum-deposited to form the anode. The effective area of one cell was 0.04 cm2. The devices were tested under simulated AM 1.5G irradia-tion (100 mW cm2) using a Xe lamp-based Newport 66902 150-W solar simulator equipped with an AM1.5 filter as the white light source; the optical power at the sample was 100 mW cm2, detected using an OPHIR thermopile 71964. The J–V characteristics were measured using a Keithley 236 source-measure unit. 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.
SCHEME 1Synthetic routes toward M1 and M2.
5-(4-Bromophenyl)-2H-tetrazole (1)
A mixture of 4-bromobenzonitrile (5.00 g, 27.6 mmol), so-dium azide (5.35 g, 82.3 mmol), NH4Cl (4.40 g, 82.2 mmol),
and DMF (15 mL) was heated for 4 h under N2 in an oil
bath maintained at a bath temperature of 150C. After cool-ing to room temperature, the solution was poured into 1 N HCl (150 mL) and stirred for 30 min. After filtering, the solid product was washed with water (3 300 mL) and dried to yield1 (5.30 g, 85%).
1
H NMR (300 MHz, DMSO-d6, ppm): 7.20 (d, J ¼ 8.7 Hz, 2H),
7.04 (d, J ¼ 8.7 Hz, 2H).13C NMR (75 MHz, DMSO-d6, ppm):
155.9, 133.2, 129.6, 125.6, 123.9. HRMS-EI (m/z): [Mþ] Calcd. for C7H5BrN4, 223.9698; Found, 223.9692.
2-(4-Bromophenyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (2)
4-tert-Butylbenzoyl chloride (3.25 g, 16.5 mmol) was added dropwise to a solution of 1 (2.50 g, 11.1 mmol) in pyridine (30 mL) and then the mixture was heated for 6 h under N2
in an oil bath maintained at 130 C. After cooling to room temperature, the solution was poured into water (300 mL). The solid product was filtered off, washed with water (5 300 mL), and dried to yield2 (3.25 g, 82%).
1 H NMR (300 MHz, CDCl3, ppm): 8.04 (d, J ¼ 8.7 Hz, 2H), 7.99 (d, J ¼ 8.4 Hz, 2H), 7.66 (d, J ¼ 8.7 Hz, 2H), 7.54 (d, J ¼ 8.4, 2H), 1.34 (s, 9H). 13C NMR (75 MHz, CDCl 3, ppm): 164.2, 163.2, 155.1, 132.5, 128.6, 126.6, 126.3, 125.6, 122.6, 120.5, 34.9, 30.8. HRMS-EI (m/z): [Mþ] Calcd. for C18H17BrN2O, 356.0524; Found, 356.0527.
2-(4-tert-Butylphenyl)-5-[4-(thiophen-3-yl)phenyl]-1,3,4-oxadiazole (3)
A total of 2 M aqueous potassium carbonate (11 mL) was added via syringe to a solution of 2 (3.00 g, 8.40 mmol), 4,4,5,5-tetramethyl-2-(thiophen-3-yl)-1,3,2-dioxaborolane (1.80 g, 8.53 mmol), and tetrakis(triphenylphosphine)palladium (2 mol %) in toluene (22 mL) in a 50-mL two-neck flask. The mixture was then stirred overnight at 90 C under N2. After
cooling to room temperature, the solution was washed with water and back-extracted with EtOAc. The organic layer was dried (MgSO4) and concentrated to yield3 (2.70 g, 90%). 1 H NMR (300 MHz, CDCl3, ppm): 8.17 (d, J ¼ 8.1 Hz, 2H), 8.08 (d, J ¼ 8.1 Hz, 2H), 7.77 (d, J ¼ 8.1 Hz, 2H), 7.61–7.59 (m, 1H), 7.56 (d, J ¼ 8.4 Hz, 2H), 7.48–7.43 (m, 2H), 1.38 (s, 9H). 13C NMR (75 MHz, CDCl3, ppm): 164.6, 164.2, 155.3, 141.0, 138.8, 127.4, 126.8, 126.7, 126.2, 126.0, 124.3, 122.5, 121.7, 121.1, 35.1, 31.1. HRMS-EI (m/z): [Mþ] Calcd. for C22H20N2OS, 360.1296; Found, 360.1287.
2-(4-tert-Butylphenyl)-5-[4-(2,5-dibromothiophen-3-yl)phenyl]-1,3,4-oxadiazole (M2)
N-Bromosuccinimide (NBS, 2.00 g, 11.2 mmol) was added portionwise to a solution of 3 (2.00 g, 5.55 mmol) in THF (20 mL) and acetic acid (20 mL) and then the mixture was stirred and heated at 80 C for 6 h. After cooling to room temperature, the solution was washed sequentially with water (2 200 mL), saturated NaHCO3(1 200 mL), and
then water again (1 200 mL). After extraction with EtOAc, the organic layer was dried (MgSO4) and concentrated to
yield the monomerM2 (2.64 g, 92%).
1 H NMR (300 MHz, CDCl3, ppm): 8.18 (d, J ¼ 8.7 Hz, 2H), 8.06 (d, J ¼ 8.7 Hz, 2H), 7.67 (d, J ¼ 8.4 Hz, 2H), 7.55 (d, J ¼ 8.4 Hz, 2H), 7.07 (s, 1H), 1.37 (s, 9H).13 C NMR (75 MHz, CDCl3, ppm): 164.8, 163.9, 155.4, 140.8, 137.1, 131.3, 129.1, 127.0, 126.8, 126.0, 123.5, 121.0, 111.8, 108.7, 35.1, 31.1. HRMS-EI (m/z): [Mþ] Calcd. for C22H18Br2N2OS, 517.9507;
Found, 517.9457.
Preparation of Polythiophene Derivatives
All polymers were synthesized through Grignard metathesis polymerization in THF according to procedures similar to those described in the literature.37–39The Grignard metathe-sis polymerizations ofM1 and M2 are presented in Scheme 2. Poly(3-hexylthiophene)
CH3MgBr (1.50 mL, 4.50 mmol) was added via syringe to a
stirred solution of 2,5-dibromo-3-hexylthiophene (1.60 g, 4.50 mmol) in freshly distilled THF (80 mL) in a three-neck 100-mL round-bottom flask. The solution was heated under reflux for 2 h and then Ni(dppp)Cl2(12 mg, 0.02 mmol) was
added. The mixture was stirred at 95C for 2.5 h before the reaction was quenched through the addition of MeOH. The solid polymer was washed with MeOH and hexane within a Soxhlet extractor; it was then dissolved through Soxhlet extraction with CHCl3, the solvent was evaporated, and the
residue dried under vacuum to yield P3HT (0.51 g, 61%). The weight-average molecular weights (Mw) and
polydisper-sity index (PDI) are 15.9 kg mol1and 1.26, respectively.
1
H NMR (300 MHz, CDCl3, ppm): 6.96 (s, 1H), 2.79 (t, 2H);
0.89–0.81, 1.43–1.15, and 1.72–1.45 (m, 11H). 13C NMR (75
MHz, CDCl3, ppm): 139.8, 133.7, 130.4, 128.5, 31.9, 30.6,
29.6, 29.4, 22.7, 14.2. Anal. Calcd: C, 72.23; H, 8.49. Found: C, 72.10; H, 8.30.
P05
CH3MgBr (0.820 mL, 2.46 mmol) was added via syringe to a
stirred solution of 2,5-dibromo-3-hexylthiophene (760 mg, 2.33 mmol), M2 (63.5 mg, 0.122 mmol), and freshly distilled THF (40 mL) in a three-neck 100-mL round-bottom flask. The solution was heated under reflux for 2 h and then Ni(dppp)Cl2(6 mg, 0.01 mmol) was added. The mixture was
stirred at 95 C for 2.5 h and then the reaction was quenched through the addition of MeOH. The solid polymer was washed with MeOH and hexane within a Soxhlet extrac-tor; it was then dissolved through Soxhlet extraction with CHCl3, the solvent was evaporated, and the residue dried
under vacuum to yieldP05 (202 mg, 46%). The Mwand PDI
are 25.7 kg mol1and 1.36, respectively.
1 H NMR (300 MHz, CDCl3, ppm): 8.3–7.9 (br, 0.25H), 7.7–7.4 (m, 0.25H), 6.96 (s, 1.09H), 2.78 (br, 2.12H), 1.66–1.36 (m, 7.31H), 0.88 (s, 2.98H). 13C NMR (75 MHz, CDCl 3, ppm): 140.1, 133.9, 130.7, 129.3, 128.8, 128.5, 127.3, 127.0, 126.3, 31.9, 31.4, 30.7, 29.7, 29.5, 22.9, 14.4. Anal. Calcd: C, 72.30; H, 8.32; N, 0.39. Found: C, 70.15; H, 7.83; N, 0.39.
P15
A mixture of CH3MgBr (0.820 mL, 2.46 mmol),M1 (680 mg,
2.08 mmol), and M2 (191 mg, 0.368 mmol) in THF (40 mL) was copolymerized using the method described for the prep-aration ofP05 to give P15 (215 mg, 44%). The Mwand PDI
are 19.6 kg mol1and 1.36, respectively.
1 H NMR (300 MHz, CDCl3, ppm): 8.3–7.9 (br, 0.64H), 7.7–7.4 (m, 0.65H), 6.96 (s, 1.08H), 2.78 (t, 1.75H), 1.66–1.36 (m, 7.38H), 0.89 (s, 3.1H). 13C NMR (75 MHz, CDCl3, ppm): 140.1, 133.9, 130.7, 129.3, 128.8, 128.5, 127.0, 126.3, 125.5, 31.9, 31.4, 30.7, 29.7, 29.5, 22.9, 14.4. Anal. Calcd: C, 72.45; H, 7.98; N, 1.17. Found: C, 69.91; H, 7.72; N, 1.24. P20 A mixture of CH3MgBr (0.820 mL, 2.46 mmol),M1 (640 mg,
1.96 mmol), and M2 (254 mg, 0.488 mmol) in THF (40 mL) was copolymerized using the method described for the prep-aration ofP05 to give P15 (218 mg, 43%). The Mwand PDI
are 13.8 kg mol1and 1.24, respectively.
1 H NMR (300 MHz, CDCl3, ppm): 8.3–7.9 (br, 0.66H), 7.7–7.4 (m, 0.72H), 6.96 (s, 0.75H), 2.78 (br, 1.33H), 1.66–1.36 (m, 9.7H), 0.89 (s, 1.83H). 13C NMR (75 MHz, CDCl3, ppm): 140.1, 133.9, 130.7, 129.3, 128.8, 128.5, 127.0, 126.3, 125.5, 31.9, 31.4, 30.7, 29.7, 29.5, 22.9, 14.4. Anal. Calcd: C, 72.53; H, 7.80; N, 1.56. Found: C, 70.33; H, 7.70; N, 2.03.
RESULTS AND DISCUSSION
Schemes 1 and 2 illustrate the synthetic routes that we fol-lowed for the preparation of the monomers and copolymers. Starting from 4-bromobenzonitrile, Compounds 1 and 2 were prepared via tetrazole routes with relative high yields and simple workup procedures.42 The Suzuki coupling of Compound 2 with 4,4,5,5-tetramethyl-2-(thiophen-3-yl)-1,3,2-dioxaborolane furnished the desired Compound 3, which we then brominated with NBS to generate the mono-mer M2. As indicated in Scheme 2, the copolymers were obtained through Grignard metathesis polymerization using various monomer M1/M2 mixtures. The resulting copoly-mers P05–P20 are soluble in common organic solvents, including toluene, THF, CHCl3, and chlorobenzene. We
char-acterized the synthesized monomers and copolymers using
1
H and13C NMR spectroscopy and mass spectrometry. In the1H NMR spectrum of
P15, the peak at 6.96 ppm (CH proton of the thiophene ring) is absent, whereas broad peaks at 7.9–8.3, 7.4–7.6 (CH protons on the phenyl group), and 0.8–3.0 (hexyl chain and tert-butyl group protons) ppm
con-firm that the copolymer ofM1 and M2 had formed. We con-firmed that the copolymers and the self-made P3HT pos-sessed head-to-tail (regioregular) configurations because the signal at 6.96 ppm was present in the spectra without any other peaks nearby.33,34 Table 1 displays the actual ratio of polymers, molecular weights, degradation temperatures, and glass transition temperatures of all of our synthesized copolymers. The actual content of oxadiazole pendant side chain groups for P05, P15, and P20 as estimated from EA is 5, 16, and 26 mol %, respectively. The number molecular weights (Mn) of our polymers ranged from 11.2 to 18.9 kg
mol1, with PDIs ranging from 1.24 to 1.36. Each copolymer exhibited outstanding thermal stability, with 5% weight losses temperatures (Td) greater than 350 C under N2
atmosphere. The glass transition temperatures (Tg) increased
from 55.8 C for the self-made P3HT to 92.1 C for P20, because of the stronger interchain interactions among the copolymers.
Optical Properties
Figure 1(a) displays UV–vis spectra of the polymers in THF solution (9.6 105 M). The small peak at 304 nm was caused by the presence of conjugated 1,3,4-oxadiazole moi-eties that were not fully coplanar with the polythiophene chain, owing to steric hindrance. The dihedral angle between the plane of oxadiazole moiety and that of thiophene is 32.11 as determined by molecular modeling (ChemBio3D, see Supporting Information Fig. S2.). The absorption maxi-mum wavelength of comonomer (Compound 3) is 314 nm, which is closed to the absorption of 1,3,4-oxadiazole moi-eties in copolymer. The p–p* transitions were responsible for the maximum absorptions (kmax) occurring at445 nm for
P3HT and at 446 nm for P15. Figure 1(b) displays UV–vis spectra of the polymers in the solid state with film thickness 90 nm on quartz. For P3HT, the peak of the p–p* transition had red-shifted from 445 nm in solution to 521 nm in the solid state; for P15, the red-shift was from 446 to 518 nm. These data indicate that efficient p stacking and intermolecu-lar interactions occurred in the films. The vibronic absorp-tion shoulders in the P3HT andP05–P20 film are a manifes-tation of high degree of p–p stacking of thiophene planes. Introducing the 1,3,4-oxadiazole moieties to the polymer side chain, however, weakens this p–p stacking and therefore the vibronic shoulders, causing P15 to have less red-shifted shoulders than that of P3HT. The optical band gaps (Eopt
g ) of
P3HT and P05–P20, estimated from the onsets of the absorptions in their solid films, are quite close to one another, in the range 1.89–1.91 eV as shown in the Table 2. TABLE 1Polymerization Data and Thermal Properties of the Copolymers
Polymer Feed Ratiom:n Actual Ratioa Yield (%) Mn(103) Mw(103) PDI DSC (Tg,C) TGA (Td,C)
P3HT 0:n 0:n 68 12.6 15.9 1.26 55.8 381.2
P05 0.5:9.5 0.5:9.5 65 18.9 25.7 1.36 86.9 364.7
P15 1.5:8.5 1.6:8.4 56 14.5 19.6 1.36 91.8 360.4
P20 2:8 2.6:7.4 62 11.2 13.8 1.24 92.1 355.9
a
Figure 2 displays photoluminescence (PL) spectra of polymer films for self-made P3HT and copolymer P05–P20. The thickness of film is 90 nm on ITO substrate. The PL spectra are recorded at an excitation wavelength of 450 nm and quenched relative to that of pure P3HT, with the degree of quenching increasing upon increasing the content of 1,3,4-oxadiazole units in the copolymer.
Electrochemical Properties
We used CV to investigate the redox properties of the copolymers and, thereby, estimate the energy levels of their highest occupied molecular orbitals (HOMOs) and lowest unoccupied molecular orbitals (LUMOs). Figure 3 displays the electrochemical behavior of the copolymers in solid films; Table 2 summarizes the relevant data. The 1,3,4-oxa-diazole-containing copolymers P05–P20 exhibited reversible reductions; their onset potentials (1.74–1.76 V) were slightly lower than that of the self-made P3HT (1.80 V). The CV traces of P05–P20 feature reversible oxidations; we assign the onset potentials at 0.34–0.38 V (i.e., very close to 0.37 V) to oxidation of the self-made P3HT. From the onset potentials of the copolymers, we estimated HOMO energy levels of the self-made P3HT andP05–P20 to be 5.17, 5.18, 5.17, and 5.14 eV, respectively, with LUMO energy levels of 3.00, 3.05, 3.04, and 3.06 eV, respectively, according to the energy level of the ferrocene reference (4.8 eV below vac-uum level)43; the electrochemical band gaps of the self-made
P3HT and P05–P20 were 2.17, 2.13, 2.13, and 2.08 eV, respectively. For the copolymers P05–P20, the introduction of electron-withdrawing groups on the side chains resulted in slightly lower LUMO energy levels and lower electrochem-ical band gaps relative to those of the self-made P3HT. Photovoltaic Properties
Figure 4 displays the photocurrents of diodes having the structure ITO/PEDOT:PSS/polymer:PCBM (1:1, w/w)/Ca/Al that we illuminated under AM 1.5 G conditions (100 mW cm2), as well as the dark currents measured for self-made P3HT/PCBM and P15/PCBM blends. The short-circuit cur-rent density (Jsc) increased upon increasing the content of
1,3,4-oxadiazole moieties, presumably because of enhanced light absorption at lower wavelengths. Table 3 lists the short-circuit current densities, open-circuit voltages, and PCEs of the heterojunction polymer photovoltaic cells. The devices based on the self-made P3HT/PCBM and P05-P15/PCBM exhibited open-circuit voltages (Voc) of 0.59–0.65
V. Although these values are related to the difference between the HOMO energy level of the copolymers and the LUMO energy level of PCBM,44 they are also influenced by
many other factors, including solvent effects and the misci-bility of copolymer and PCBM.
The short-circuit current density (Jsc) of the device
contain-ing the copolymer featurcontain-ing 15 mol % 1,3,4-oxadiazole units (P15) and PCBM was 8.80 mA cm2, an improvement of FIGURE 1Normalized optical absorption spectra of the
self-made P3HT, the copolymers P05–P20, and comonomer (Com-pound 3) in (a) THF solution (9.6 105M) and (b) self-made P3HT and the copolymers P05–P20 as thin films (the thickness of film is 90 nm on quartz). [Color figure can be viewed in the online issue, which is available at www.interscience.wiley.com.]
TABLE 2Optical and Redox Properties of the Copolymers
Polymer kmax(nm), Solution kmax(nm), Film Eoptg (eV) a
Eox(V) Ered(V) HOMO (eV) b LUMO (eV)b Self-made P3HT 445 521 (553,605) 1.91 0.37 1.80 5.17 3.00 P05 446 521 (546,601) 1.91 0.38 1.75 5.18 3.05 P15 446 518 (545,600) 1.90 0.37 1.76 5.17 3.04 P20 446 517 (545,600) 1.89 0.34 1.74 5.14 3.06 a
Estimated from the onset wavelength absorptions of the solid films. b
36% from the value of 6.45 mA cm2measured for the de-vice containing self-made P3HT. The PCE of the dede-vice pre-pared from the self-made P3HT and PCBM was 2.05%, in agreement with the values reported in the literature.45 Although the PCE of a device that was prepared using com-mercially available high-molecular-weight P3HT (Mn ¼
33,000, 198 repeating units) and PCBM was much higher, that is, 3.40% under the same processing condition as shown in Table 3, we are comparing the effect of different molecular structures on their photovoltaic devices perform-ance at the same effective polymer chain length—the average number of repeating units in the polythiophene-oxadiazole FIGURE 2PL spectra of films of the self-made P3HT and the copolymers P05, P15, and P20, recorded at an excitation wave-length of 450 nm. The thickness of film is 90 nm on ITO sub-strate. [Color figure can be viewed in the online issue, which is available at www.interscience.wiley.com.]
FIGURE 3Cyclic voltammograms of films of the self-made P3HT and the copolymers P05, P15, and P20, recorded at a scan rate of 50 mV s1. [Color figure can be viewed in the online issue, which is available at www.interscience.wiley.com.]
FIGURE 4Current density–voltage characteristics of illuminated (AM 1.5G, 100 mW cm2) polymer photovoltaic cells incorpo-rating PCBM blends of the self-made P3HT and the copolymers P05, P15, and P20. [Color figure can be viewed in the online issue, which is available at www.interscience.wiley.com.]
FIGURE 5(a) EQEs of solar cells incorporating copolymer P15/ PCBM and the self-made P3HT/PCBM blends. (b) Absorption spectra of the copolymer P15/PCBM and the self-made P3HT/ PCBM at blend ratios of 1:1. [Color figure can be viewed in the online issue, which is available at www.interscience.wiley.com.]
copolymerP15 (Mn¼ 14,500) and the self-made P3HT (Mn
¼ 12,600) were 75 and 76, respectively. Nevertheless, the PCE of the P15/PCBM device was 2.50%, 22% higher than the PCE of 2.05% of the device featuring P3HT/PCBM as the active layer. The shape of the EQE curve of the P15/PCBM device in Figure 5(a) resembles the shape of the absorption spectrum of the active layer in Figure 5(b), suggesting that the absorbed photons contributed to the induced photocur-rent. The EQEs ofP15/PCBM were at least 10% higher than those of the self-made P3HT/PCBM at wavelengths in the range 320–460 nm. Although the oxadiazole side chains pro-vide the absorption at short wavelength region (around 304 nm), they also weaken the degree of p–p stacking of thio-phene planes, resulting in weaker vibronic absorption shoulders. Therefore, the content of oxadiazole side chains
should be optimized to provide an additional absorption region and to possess suitable degree of polymer chain ordering for better device performance. The copolymer P05 only has a small absorption at 304 nm but with decreased chain ordering, see Figure S4 in the Supporting Information. The copolymerP20 provides much larger absorption at 304 nm, but the excess oxadiazole moieties decreased chain ordering more intensively. Therefore, the device by copoly-mer P15 shows the optimal performance. Figure S4 shows the synchrotron grazing-incidence X-ray diffraction patterns of self-made P3HT andP05–P15.
Figure 6 displays the surface morphologies determined from AFM measurements. Samples of the self-made P3HT and the copolymer P05–P20/PCBM (1:1 w/w) blended films were spin-coated from their corresponding chlorobenzene solu-tions and then annealed at 150 C for 10 min, that is, condi-tions identical to the procedure used to fabricate the active layers of the devices. The root-mean-square (RMS) rough-ness of the self-made P3HT and P05–P20 blends was 1.33, 1.531, 0.88, and 1.44 nm, respectively. The larger RMS value for theP20/PCBM film suggests its significantly large phase separation. This suitable phase separation and surface rough-ness facilitated the improved charge transport and carrier collection efficiency, resulting in reduced charge recombina-tion and an increased short-circuit current density. From the AFM images, we assume that the homogeneous morphology of P15 may have had a significant influence on the device performance.
CONCLUSIONS
We have used Grignard metathesis polymerization to prepare a series of thiophene-based copolymers through conjugation with electron-withdrawing 1,3,4-oxadiazole moieties in the
FIGURE 6Topographic AFM images of films of PCBM blends (1:1, w/w) with (a) the self-made P3HT and (b–d) the copoly-mers (b) P05, (c) P15, and (d) P20. Image size: 2.5lm 2.5 lm.
TABLE 3Photovoltaic Properties of Polymer Photovoltaic Cells Incorporating Blends (1:1, w/w) of P3HT/PCBM and P05–P20/ PCBM Polymer Jsca (mA cm2) Vocb(V) FFc(%) gd(%) Self-made P3HT (Mn¼ 12,600) 6.45 0.63 50.0 2.05 P05 7.40 0.59 40.7 1.79 P15 8.80 0.65 43.8 2.50 P20 3.39 0.60 40.8 0.83 Commercial P3HT (Mn¼ 33,000) 9.79 0.59 59.2 3.42 a
Short-circuit current density. b
Open-circuit voltage. c
Fill factor. d
polymer side chains. We observed PL quenching for the copolymers incorporating pendant electron-deficient 1,3,4-oxadiazole moieties on their side chains. The photocurrents of devices were enhanced in the presence of an optimal amount of the 1,3,4-oxadiazole moieties, thereby leading to improved power conversion efficiencies. The photovoltaic de-vice based on the copolymerP15 and PCBM exhibited a PCE of 2.50% under AM 1.5 illumination (100 mW cm2). The EQE of the device incorporating this polythiophene present-ing side chain–tethered 1,3,4-oxadiazole units was greater than that of the device incorporating low-molecular-weight P3HT; as a result, its short-circuit current density was also much higher.
The authors thank the National Science Council for the financial support (project NSC 97-2120M-009-006) and Yao-Te Chang and So-Lin Hsu for assistance with the synthesis of the polymers.
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