Synthesis of Novel Dithienothiophene- and 2,7-Carbazole-Based
Conjugated Polymers and H-Bonded Effects on Electrochromic and
Photovoltaic Properties
Hsiao-Ping Fang,
1Jia-Wei Lin,
1I-Hung Chiang,
1Chih-Wei Chu,
2,3Kung-Hwa Wei,
1Hong-Cheu Lin
11Department of Materials Science and Engineering, National Chiao Tung University, Hsinchu, Taiwan, Republic of China 2
Department of Photonics, National Chiao Tung University, Hsinchu, Taiwan, Republic of China 3Research Center for Applied Sciences, Academia Sinica, Taipei, Taiwan, Republic of China
Correspondence to: C.-W. Chu (E-mail: [email protected]) or H.-C. Lin (E-mail: [email protected]) Received 14 June 2012; accepted 5 August 2012; published online 11 September 2012
DOI: 10.1002/pola.26336
ABSTRACT:Three kinds of dithienothiophene/carbazole-based conjugated polymers (P1–P3), which bear acid-protected and benzoic acid pendants in P2 and P3, respectively, were synthe-sized via Suzuki coupling reaction. Interestingly, P1–P3 exhib-ited reversible electrochromism during the oxidation processes of cyclic voltammogram studies, and P3 (with H-bonds) revealed the best electrochromic property with the most no-ticeable color change. According to powder X-ray diffraction (XRD) analysis, these polymers exhibited obvious diffraction features indicating bilayered packings between polymer back-bones and p-p stacking between layers in the solid state. Com-pared with the XRD data of P2 (without H-bands), H-bonds of P3 induced a higher crystallinity in the small-angle region (cor-responding to a higher ordered bilayered packings between polymer backbones), but with a similar crystallinity in the wide
angle region indicating a comparable p-p stacking distance between layers. Moreover, based on the preliminary photovol-taic properties of PSC devices (P1–P3 blended individually with PCBM acceptor in the weight ratio of 1:1), P3 (with H-bonds) possessed the highest power conversion efficiency of 0.61% (with Jsc¼ 2.26 mA/cm2, FF¼ 29.8%, and Voc¼ 0.9 V). In con-trast to P2 (without H-bands), the thermal stability, crystallinity, and electrochromic along with photovoltaic properties of P3 were generally enhanced due to its H-bonded effects.VC 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 50: 5011–5022, 2012
KEYWORDS:electrochromism; functionalization of polymers; heterocyclic conjugated polymer; solar cell; supramolecular structures; WAXS
INTRODUCTION Novel materials are developed for organic optoelectronic devices, such as polymeric solar cells (PSCs), which is a popular research topic in recent decades, because they are low cost and green materials for sustainable resour-ces to reduce consumptions of fossil energy and nuclear power.1In particularly, bulk heterojunction (BHJ) solar cells consisting of electron-donating conjugated polymers blended with electron-accepting fullerenes are fabricated in solid thin films.2 Up to now, regio-regular poly[2-methoxy-5-(3’,7’-dimethyloctyloxy)-p-phenylenevinylene] (MDMO-PPV)3 and poly(3-hexylthiophene) (P3HT)4as electron donors blended with [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) as an electron acceptor approached high power conversion effi-ciency (PCE) values of 5.0% in PSCs. More recently, the PCE values of BHJ solar cells using new low-band gap conjugated polymers have reached 6 to 8%.5,6The PCE values of BHJ so-lar cells were affected by, for example, the energy band gaps of polymers, which is related to the chemical structure of the conjugated polymers. In order to improve the thermal
stabilities of polymers by longer conjugation lengths and more rigid structures, novel heteroaromatic fused-ring deriv-atives, including fused dithienothiophene and carbazole units, are integrated into the polymer backbones. Carbazole unit, which is well known to be a good electron-donating moiety7 due to its fully aromatic structure, can improve chemical stability in contrast to fluorene unit, and thus poly(2,7-carbazole)s are attractive candidates for solar cells.8 New carbazole-based copolymers utilized in BHJ solar cells were also reported to achieve a PCE value of 4.3%.9 Accord-ing to all these results, the development of new polymers based on carbazole units should have interesting features for the photovoltaic applications. Dithieno[3,2-b:2’,3’-d]thiophene (DTT) unit is a sulfur-rich (three S atoms) and electron-rich building block to make the polymer backbones more rigid and coplanar, and thus to have longer p-conjugation and absorption lengths along with narrower band gaps.10,11(a) Due to the limiting intramolecular rotation in the fused-ring structures, such as DTT, the p-orbital overlaps in conjugated
molecules could be maximized to enhance intermolecular charge transports.11(b,c) In order to improve the solubility of poly(DTT), alkyl-substituted pendants were incorporated into the polymer backbones, which can be applied to or-ganic solar cells12-15and field effect transistors (FETs).15(b) Hydrogen bonds (H-bonds) are ideal noncovalent interac-tions to form self-assembled architectures due to their se-lectivity and directionality. A numerous advantages of H-bonded polymers, such as stronger light absorptions, lower HOMO levels, higher Voc values, higher hole mobilities, and higher crystallinities, were utilized for organic solar cells.16 Therefore, great efforts have been taken toward the prepa-ration and characterization of photo- and electroactive non-covalent assemblies based on hydrogen bonds (H-bonds). Wu¨rthner,16(a,b) El-ghayoury et al., and Jonkheijm et al.16(c,d)
reported H-bonded assemblies of perylene bisimide and melamine derivatives. In addition, El-ghayoury et al. reported a PCE value of 0.39% for PSCs by utilizing a H-bonded polymer containing oligo(p-phenylene vinyene) and ureido-pyrimidinone units.16(c)Because of several advantages in polymers, including low cost, easy processing, and tunable chemical properties, the conjugated polymers consisting of different heteroaromatic rings, such as thio-phene and carbazole, exhibit an electrochromic behavior as well as photovoltaic properties. Based on this concept, two different moieties, i.e., carbazole (M1 and M2) and fused dithienothiophene (M3), were applied into electron-donor monomers to synthesize fused-dithienothiophene-based polymers P1–P3, where P1 is a model polymer and the acid pendants of P3 are protected in P2 to compare the
H-bonded effects on their electrochromic behavior as well as photovoltaic properties.
EXPERIMENTAL
Materials
All chemicals and solvents were used as received; 2,7-dibromo-9-(heptadecan-9-yl)-9H-carbazole (1),7
2,7-dibromo-carbazole (4),7 and 3,5-didecanyldithieno[3,2-b:2’3’-d] thio-phene) (5)17 were synthesized according to the literature procedures. The synthetic routes of monomers 1–3 and poly-mers P1–P3 are shown in Schemes 1 and 2, and the syn-thetic procedures of their intermediates were described. Chemicals and solvents were reagent grades and purchased from Aldrich, ACROS, TCI, and Lancaster Chemical Co.
Toluene, tetrahydrofuran, and diethyl ether were distilled to keep anhydrous before use.
Measurements
1
H NMR spectra were recorded on a Varian Unity 300 MHz spectrometer using CDCl3 solvents. Elemental analyses were
performed on a HERAEUS CHN-OS RAPID elemental analyzer. Transition temperatures were determined by differential scanning calorimetry (DSC, Perkin-Elmer Pyris 7) with a heating and cooling rate of 10 C/min. Thermogravimetric
analyses (TGA) were conducted with a TA instrument Q500 at a heating rate of 10 C/min under nitrogen. Gel permea-tion chromatography (GPC) analyses were conducted on a Waters 1515 separation module using polystyrene as a standard and tetrahydrofuran (THF) as an eluent. UV-Vis absorption and photoluminescence (PL) spectra were recorded in dilute THF solutions (106M) on a HP G1103A and Hitachi F-4500 spectrophotometer, respectively. Solid films of UV-Vis and PL measurements were spin-coated on a quartz substrate from THF solutions with a concentration of 10 mg/mL. Cyclic voltammetry (CV) measurements were performed using a BAS 100 electrochemical analyzer with a standard three-electrode electrochemical cell in a 0.1 M tet-rabutylammonium hexafluorophosphate (TBAPF6) solution
(in dimethylformamide, DMF) at room temperature with a scanning rate of 100 mV/s. In each case, a carbon working electrode coated with a thin layer of these polymers, a plati-num wire as the counter electrode, and a silver wire as the quasi-reference electrode were used. Ag/AgCl (3 M KCl) elec-trode was served as a reference elecelec-trode for all potentials quoted herein. During the CV measurements, the solutions were purged with nitrogen for 30 s, and the redox couple ferrocene/ferrocenium ion (Fc/Fcþ) was used as an external standard. The corresponding HOMO levels in polymer solu-tions of P1–P3 could be calculated from Eox/onset values of
the electrochemical experiments (but no reduction curves, i.e., no Ered/onset values and LUMO levels, were obtained in
the CV measurements). Each onset potential in the CV meas-urements was defined by the intersection of two tangents drawn at the rising current and background current. The LUMO value of PCBM18was in accordance with the literature data. Film thickness and morphology were determined using a Veeco Nanoscope DI 3100 AFM microscope operating in tapping mode. Synchrotron powder X-ray diffraction (XRD) measurements were performed at beamline BL13A of the National Synchrotron Radiation Research Center (NSRRC), Taiwan, where the wavelength of X-ray was 1.026503 Å. The photovoltaic cell (PVC) device structure used in this study was a sandwich configuration of ITO/PEDOT:PSS/active layer/Ca/Al, where the active layer was made of electron donor polymers P1–P3 mixed with electron acceptor [6,6]-phenyl C61 butyric acid methyl ester (PCBM) in a weight
ra-tio1:1. The PVC devices were fabricated according to the pro-cedures similar to those of EL devices. An ITO-coated glass substrate was precleaned and treated with oxygen plasma before use. A thin layer (ca. 50 nm) of PEDOT:PSS was spin-coated on an ITO substrate and heated at 130 C for 1 h. Subsequently, the preliminary active layer (ca. 100–160 nm)
was prepared by spin coating from composite solutions of P1–P3: PCBM (w/w ¼ 1:1) in dichlorobenzene (10 mg/mL) by a spin rate of 500 to 800 rpm on the top of the PEDOT: PSS layer. The PVC devices were completed by deposition with a back electrode consisting of Ca (ca. 50 nm) and alu-minum (100 nm). The film thicknesses were measured by a profilometer (Dektak3, Veeco/Sloan Instruments Inc.). For PVC measurements, I–V curves were recorded under a solar simulator with AM 1.5 irradiation (at 100 mW/cm2). A 300 W xenon lamp (Oriel, #6258) with AM 1.5 filter (Oriel, #81080 kit) was used as the white light source, and the op-tical power at the sample was 100 mW/cm2 detected by Oriel thermopile 71964. The I–V characteristics were meas-ured using a CHI 650B potentiostat/galvanostat. The external quantum efficiency (EQE) was measured using a CHI 650B coupled with Oriel Cornerstone 260 monochromator. All PVC devices were prepared and measured under ambient conditions.
Synthesis and Characterization Materials
9-(Heptadecan-9-yl)-2,7-bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-9H-carbazole (M1). To a solution of compound 1 (2,7-dibromo-9-(heptadecan-9-yl)-9H-carbazole)7 (3.62 g, 6.42 mmol) in 150 mL of dry THF, n-butyllithium (2.5 M solution in hexane, 5.65 mL, 2.2 eq) was added dropwise, and then stirred to react at78C under nitrogen. After
reac-tion for 2 h at 78 C,
2-isopropoxy-4,4,5,5-tetramethyl-1,3,2-dioxaborolane (3.15 mL, 2.4 eq) was added carefully to the mixture solution at78 C and then the mixture was allowed
to warm up to react at room temperature overnight. The final solution was acidified with 100 mL of 10% HCl solution and stirred for 45 min at room temperature and the final solution was extracted with diethyl ether. The organic layer was dried over magnesium sulfate, and the solvent was evaporated. The solvent was removed under reduced pressure, and the residue was purified by recrystallization in methanol/acetone (ca. 10:1) to obtain the final product as a white crystal (2.69 g, yield: 64 %).1H NMR(CDCl3, 300 MHz): d 8.12 (d, J¼ 8.0 Hz,
2H), 8.02 (s, 1H), 7.89 (s, 1H), 7.66 (d, J ¼ 8.1 Hz, 2H), 4.73– 4.66 (m, 1H), 2.35–2.30 (m, 2H), 1.98–1.90 (m, 2H), 1.39–1.12 (m, 48H), 0.82 (t, J ¼ 7.0 Hz, 6H). MS (FAB): m/z [Mþ] 657; calcd. m/z [Mþ] 657. Anal. Calcd.: C, 74.89; H, 9.96; N, 2.13. Found: C, 74.42; H, 9.68; N, 2.26.
tert-Butyl hydroxybenzoate (2). Seven grams of 4-hydroxybenzoic acid (50.7 mmol) and 4-dimethylaminopyri-dine (0.62 g, 0.1 eq) were dissolved in 60 mL of dry THF and stirred in a two-necked flask. Then, 2-methylpropan-2-ol (60 mL, 2-methylpropan-2-ol) and N,N’-dicyclohexylcarbodii-mide (12.6 g, 1.2 eq) were added sequentially. The mixture was purged with nitrogen and vigorously stirred overnight at room temperature. Water was added after reaction, and the reaction mixture was extracted with dichloromethane. Consequently, the organic layer was separated and dried with magnesium sulfate. Solvent was removed under vac-uum, and the crude product was purified by chromatography using hexane: ethyl acetate (4:1) as eluent. Subsequently, the pure compound was obtained as a white powder. Yield: 3.95
g (40 %).1H NMR (CDCl3, 300 MHz): d 7.87 (d, J ¼ 8.7 Hz,
2H), 6.84 (d, J¼ 8.5 Hz, 2H), 6.44 (s, 1H), 1.59 (s, 9H). tert-Butyl 4-(6-bromohexyloxy) benzoate (3). tert-Butyl 4-hydroxybenzoate (3.95 g, 20.3 mmol), K2CO3 (8.43 g, 3
eq), and KI (0.2 g, 0.06 eq) were dissolved in acetone (150 mL) and stirred in a two-necked flask. The mixture was purged with nitrogen and refluxed overnight. Water was added after reaction, and the reaction mixture was extracted with dichloromethane. Consequently, the organic layer was separated and dried with magnesium sulfate. Solvent was removed under vacuum, and the crude product was purified by chromatography using hexane: ethyl acetate (30:1) as elu-ent. Subsequently, the pure compound was obtained as a col-orless oil. Yield: 6.48 g (89%). 1H NMR(CDCl3, 300 MHz): d
7.89 (d, J¼ 8.7 Hz, 2H), 6.86 (d, J ¼ 8.5 Hz, 2H), 3.98 (t, J ¼ 6.4 Hz, 2H), 3.40 (t, J¼ 6.8 Hz, 2H), 1.90–1.77 (m, 4H), 1.56 (s, 9H), 1.51–1.46 (m, 4H).
tert-Butyl 4-(6-(2,7-dibromo-9H-carbazol-9-yl)hexyloxy)-benzoate (M2). NaH (0.36 g, 1.8 eq) and 2,7-dibromo-car-bazole8(2.71 g, 8.34 mmol) were dissolved in THF (10 mL) and stirred in a two-necked flask and refluxed for 1 h. The solution of tert-butyl-4-(6-bromohexyloxy)benzoate (3.7 g, 1.3 eq) in THF (10 mL) was added dropwise and refluxed overnight. Water was added after reaction, and the reaction mixture was extracted with dichloromethane. Consequently, the organic layer was separated and dried with magnesium sulfate. Solvent was removed under vacuum, and the crude product was purified by chromatography using hexane: ethyl acetate (1:7) as eluent. Subsequently, the pure compound was obtained as a colorless oil. Yield: 2.90 g (60 %). 1H NMR(CDCl3, 300 MHz): d 7.90 (m, 4H), 7.53 (d, J ¼ 1.2 Hz,
2H), 7.34 (dd, J¼ 9.0, 1.6 Hz, 2H), 6.84 (d, J ¼ 8.9 Hz, 2H), 4.22 (t, J¼ 7.2 Hz, 2H), 3.96 (t, J ¼ 6.29 Hz, 2H), 1.91–1.75 (m, 4H), 1.59–1.44 (m, 13H). MS (EI): m/z [Mþ] 601; calcd m/z [Mþ] 601. Anal. Calcd.: C, 57.92; H, 5.28; N, 2.33. Found: C, 57.78; H, 5.04; N, 2.52.
2,6-Dibromo-3,5-dihexyldithieno[3,2-b:2’3’-d]thiophene (6). Compound 5 (3,5-dihexyldithieno[3,2-b:2’3’-d]thiophene)17 (1.96 g, 5.38 mmol) and NBS (2.39 g, 2.5 eq) were dissolved in 50 mL of DMF. The resulting solution was stirred to react overnight at room temperature under nitrogen. Water (50 mL) was then added, and the organic phase was extracted with ethyl acetate (100 mL) twice, washed with water, and dried with magnesium sulfate. After that, the solvent was removed under reduced pressure to obtain the product. The crude product was purified by column chromatography with hexane to obtain a pale yellow oil (2.51 g). Yield: 89 %. 1H
NMR(CDCl3, 300 MHz): d 2.72 (t, J¼ 7.5 Hz, 4H), 1.75–1.65
(m, 4H), 1.42–1.32 (m, 12H), 0.92–0.88 (m, 6H).
2,6-Dithienyl-3,5-dihexyldithieno[3,2-b:2’3’-d]thiophene (7). Compound 5, 3,5-dihexyldithieno[3,2-b:2’3’-d]thiophene (2.46 g, 4.71 mmol), Pd(PPh3)4(0.32 g, 0.06 eq), and
tribu-tyl(thiophen-2-yl)stannane (3.29 mL, 2.2 eq) were dissolved in toluene (25 mL) and stirred in a two-necked flask to reflux for 12 h. Solvent was removed under vacuum, and the crude product was purified by chromatography using hexane
as eluent. Subsequently, the pure compound was obtained as a yellow powder. Yield: 2.12 g (86 %). 1H NMR (CDCl3, 300
MHz): d 7.35 (dd, J ¼ 5.1, 1.0 Hz, 2H), 7.18 (t, J ¼ 1.0 Hz, 2H), 7.09 (dd, J ¼ 5.1, 3.6 Hz, 2H), 2.91 (t, J ¼ 8.1 Hz, 4H), 1.67–1.61 (m, 4H), 1.42–1.27 (m, 12H), 0.94–0.89 (m, 6H). 2,6-Bis(2’-bromothien-5’-yl) 3,5-dihexyldithieno[3,2-b:2’3’-d] thiophene (M3). About 2.12 g of 2,6-dithienyl-3,5-dihexyldi-thieno[3,2-b:2’3’-d]thiophene (4.01 mmol) and NBS (2.1 g, 2.1 eq) were dissolved in DMF (20 mL) and stirred in a flask. The mixture was vigorously stirred overnight at room temper-ature. Water was added after reaction, and the reaction mix-ture was extracted with ethyl acetate. Consequently, the or-ganic layer was separated and dried with magnesium sulfate. Solvent was removed under vacuum, and the crude product was purified by chromatography using hexane as eluent. Sub-sequently, the pure compound was obtained as a pale yellow powder. Yield: 2.0 g (73%). 1H NMR(CDCl3, 300 MHz):d 7.04
(d, J¼ 3.5 Hz, 2H), 6.9 (d, J ¼ 3.5 Hz, 2H), 2.85 (t, J ¼ 7.3 Hz, 4H), 1.75-1.73 (m, 4H), 1.41–1.26 (m, 12H), 0.92–0.89 (m, 6H). MS (EI): m/z [Mþ] 685; calcd. m/z [Mþ] 685. Anal. Calcd.: C, 48.98; H, 4.40. Found: C, 48.89; H, 4.63.
General Synthetic Procedures of Polymers P1–P319 The synthetic routes of polymers P1–P3 are shown in Scheme 2. All of the polymerization procedures were carried out through the palladium (0)-catalyzed Suzuki coupling reactions. In a 25 mL two-necked flask, 0.5 eq of M1 and M2 with a molar ratio of M2:M3 ¼ 0:0.5(P1) and 0.05:0.45 (P2, P3) were added into 5 mL of anhydrous toluene. The Pd (0) com-plex, tetrakis(triphenylphosphine)palladium (1 mol %), was transferred into the mixture under dry environment. Then, 2 M aqueous potassium carbonate and a phase transfer catalyst, that is, Aliquat 336 (several drops), were subsequently trans-ferred to the previous mixture via dropping funnel. The reac-tion mixture was stirred at 90 C for 2 days, and then both excess amounts of end-cappers (i.e., iodobenzene and phenyl-boronic acid) were correspondingly dissolved in 1 mL of anhy-drous toluene and reacted for 4 h. The reaction mixture was cooled to 40C and added slowly into a vigorously stirred mix-ture of methanol/water (10:1). The polymers were collected by filtration and reprecipitation from methanol. The crude polymers were further purified by washing with acetone for 3 days in a Soxhlet apparatus to remove oligomers and catalyst residues. The chloroform fractions (350–400 mL) were reduced to 40–50 mL under reduced pressure, and precipi-tated in acetone along with air-dried overnight finally.
P1
Following the general polymerization procedure, M1 (0.5 equiv) and M3 (0.5 equiv) were used in this polymerization to obtain a red powder. Yield: 67%. 1H NMR (ppm, CDCl
3): d
8.06–7.23 (br, 10H), 4.62 (br, 1H), 2.99 (br, 4H), 2.5-0.5 (br). Anal. Calcd.: C, 73.57; H, 7.69; N, 1.51. Found: C, 73.39; H, 7.64; N, 1.35.
P2
Following the general polymerization procedure, M1 (0.5 equiv), M2 (0.05 equiv), and M3 (0.45 equiv) were used in this polymerization to obtain a red powder. Yield: 67%. 1H
NMR (ppm, CDCl3): d 8.23-7.22 (br, 10H), 4.644 (br, 1H),
3.947 (br, 1H), 3.020 (br, 4H), 2.50–0.66 (br). Anal. Calcd.: C, 75.28; H, 7.49; N, 2.04. Found: C, 75.39; H, 7.44; N, 2.15. P3
P2 (200 mg) was dissolved in 20 mL toluene, and excess HCl solution was added slowly. The mixture was vigorously stirred for 2 days at 80C. The polymers were collected by following the general polymerization procedure to gain a red powder. Yield: 81%. 1H NMR (ppm, CDCl3): d 8.23–7.22 (br,
10H), 4.654 (br, 1H), 3.950 (br, 1H), 3.025 (br, 4H), 2.50– 0.66 (br). Anal. Calcd.: C, 74.84; H, 7.20; N, 2.13. Found: C, 74.79; H, 7.44; N, 2.15.
RESULTS AND DISCUSSION
Syntheses and Chemical Characterization
As outlined in Scheme 1, two monomers M1 and M2 based on carbazole moieties were prepared from 2,7-dibromo-9-(heptadecan-9-yl)-9H-carbazole (1) and 2,7-dibromo-carba-zole (4) moieties.7 In addition, M3 based on dithienothio-phene was prepared from 3,5-didecanyldithieno[3,2-b:2’3’-d]thiophene17(5) using a reduction procedure and followed by dibromination, which were described by Coppo et al.20 The electron-donating unit of compound 5 was prepared according to the literature procedures. Monomers M1–M3 were satisfactorily characterized by 1H NMR, 13C NMR, MS spectroscopies, and elemental analyses. Polymers P1–P3 were prepared successfully via Suzuki coupling, where P1 was produced by M1 and M3; P2 was synthesized by the copolymerization of monomer M1 with M2 and M3; and P3 was prepared by the deprotection of acid in P2. The syn-thetic procedures towards polymers P1–P3 are outlined in Scheme 2. Most polymers are partly soluble in organic sol-vents, such as chloroform, THF, and chlorobenzene at room temperature, and completely soluble in high boiling point solvents (e.g., chlorobenzene) at high temperatures. The yields and molecular weights of polymers P1–P3 determined by gel permeation chromatography (GPC) against polysty-rene standards in THF are summarized in Table 1. These results show that considerable molecular weights with high yields (50–81% after Soxhlet extractions) were obtained in these copolymers, where the weight-average molecular weights (Mw) of 19,700 to 53,100 with polydispersity indices
(PDI¼ Mw/Mn) of 1.2 to 2.7 were obtained.
The thermal stabilities and phase transition temperatures of polymers P1–P3 were characterized by thermogravimetric analyses (TGA) and differential scanning calorimetry (DSC) measurements under nitrogen atmosphere, and the thermal decomposition temperatures (Td) and glass transition
tem-peratures (Tg) are summarized in Table 1. It is apparent that
all polymers exhibited good thermal stabilities, which showed less than 5% weight loss upon heating to 380 to 428 C. Regarding DSC experiments, samples (weighted 1–5 mg) sealed in an aluminum pan were operated at 30 to 250
C under N
2 atmosphere with a scan rate of 10 C/min.
These polymers showed glass transition (Tg) temperatures at
130, 138, and 146 C for P1, P2, and P3, respectively. The Td and Tg values of P3 are higher than those of P2, which
were attributed to the rigid polymer networks of P3 formed by H-bonds (due to its carboxyl group).
Optical Properties
The optical absorption spectra of polymers P1–P3 in THF solutions (106 M) and solid films are shown in Figure 1, and their photophysical properties are demonstrated in Ta-ble 2. As can be seen, the absorption spectra of polymers
TABLE 1 Molecular Weights, Yields, and Thermal Data of Polymers P1–P3 Polymer Mn a Mw b PDIc Tgd (C) Tde (C) Yield (%) P1 16,600 19,700 1.2 130 421 71.4 P2 19,200 53,100 2.7 138 380 79.5 P3 18,200 50,100 2.7 146 428 70.5 a
Number average molecular weight.
b
Weight average molecular weight.
c
Polydispersity indices (PDI¼ Mw/Mn). d
Glass transition temperature.
e
Decomposition temperature at 5% weight loss.
FIGURE 1 Normalized optical absorption spectra of polymers P1–P3 in (a) solutions (THF) (106M) and (b) solid films (spin-coating from THF solutions).
P1–P3 covered broad wavelength ranges (300–600 nm) for both solutions and solid films. In addition, P1–P3 possessed similar maximum absorption wavelengths of 445 and 465 nm in THF solutions and solid films, respectively. Due to the p-p stacking of these polymer chains in solids, the maximum absorption wavelength (465 nm) of P1–P3 in solid films was red-shifted about 20 nm in contrast to that (445 nm) of their
solutions. As shown in Table 2, the optical band gaps (Eg,opt) of 2.24 to 2.25 eV in polymers P1–P3 could be determined by the cut-offs of the absorption spectra in solid films. The PL emission spectra of the polymers in solutions and solid films were illustrated in Figure 2. The photolumines-cence (PL) spectra of polymers P1–P3 in THF solutions and solid films were excited at incident wavelengths of 445 and 465 nm, respectively. Interestingly, in comparison with poly-mer P2 in Figure 2, the PL spectra of P3 containing carbox-ylic acid moieties were almost quenched in solution and com-pletely quenched in solid film [see Fig. 2(a,b), respectively]. The PL emission of P3 containing carboxylic acid moieties (COOH) is quenched because its dimeric H-bonded structure is formed to shorten the distance between chromophores and to result in the stacking phenomena to quench the PL emis-sion. In other words, the PL quenching phenomena of H-bonds in P3 induced by the carboxylic groups might stem from the intersystem crossing from the photo-excited singlet state to the triplet one, where both intramolecular (in solu-tions) and intermolecular (in solid films) energy transfers along the conjugated backbones might occur. The correspond-ing optical properties of these polymers in solid films, includ-ing the broad and strong optical absorptions, proposed their potential applications in photovoltaic cells described below. Electrochemical Properties
The electronic states, that is, highest occupied molecular or-bital (HOMO) and lowest unoccupied molecular oror-bital (LUMO) levels, of the polymers were investigated by cyclic voltammetry (CV) measurements in order to understand the charge injection processes in these polymers and their PSC devices. The oxidation cyclic voltammograms of P1–P3 in solid films and the corresponding electrochromic photos (with a distinct change from orange to black) of P3 are dis-played in Figure 3(a), where the electrochromic activities in P1 and P2 were not noticeable to be included due to the lack of H-bonds. The formal potentials and HOMO energy levels (estimated the average oxidation potentials from the electrochemical measurements) are summarized in Table 2. As shown in Figure 3(a), polymers P1–P3 showed one quasi-reversible oxidation process but no detectable reduc-tion behavior. Therefore, the HOMO energy levels of P1–P3 can be decided by their quasi-reversible oxidation peaks
TABLE 2 Photophysical Data in THF Solutions and Solid Films, Optical Band Gaps, Electrochemical Potentials, Energy Levels, and Band Gap Energies of Polymers P1–P3
Polymer kabs, Sola (nm) kabs, Filmb (nm) kPL,Filmb (nm) E1/2d (ox) EHOMO (eV)e ELUMO (eV)f Eg,opt (eV)g P1 445 465 546 0.89 5.58 3.34 2.24 P2 445 465 543 0.91 5.60 3.35 2.25 P3 445 465 –c 0.91 5.60 3.35 2.25 a
The absorption spectra were recorded in dilute THF solution at room temperature.
b
The absorption and PL films were spin-coated from 10 mg/1 mL THF solution.
c
PL peaks were not detectable due to the PL quenching behavior.
d
E1/2was the average value of oxidation.
e
EHOMO¼ [(E1/2 0.11) 4.8] eV where 0.11 V is the value for
ferro-cene versus Ag/Agþ and 4.8 eV is the energy level of ferroferro-cene below the vacuum.
f
LUMO¼ HOMO Eg,opt. g
Optical band gaps were estimated from the absorption spectra in solid films by using the equation of Eg¼ 1240/kedge
FIGURE 2 Normalized photoluminescence (PL) spectra of poly-mers P1-P3 in (a) solutions (THF) (106M), and (b) solid films (spin-coating from THF solutions).
correspondingly. The moderate onset oxidation potentials of P1–P3 occurred between 0.6 and 1.2 V from which the esti-mated HOMO levels of 5.58 to 5.60 eV were acquired
according to the following equation:21,22 EHOMO/LUMO ¼
[(Eonset (vs Ag/AgCl) Eonset (Fc/Fc þ vs Ag/AgCl) 4.8] eV,
where 4.8 eV is the energy level of ferrocene below the vac-uum level and Eonset (Fc/Fc þ vs Ag/AgCl) ¼ 0.11 eV. Due to
the lack of their reduction peaks, the LUMO energy levels of P1–P3 can not be determined, but the LUMO levels can be elucidated by subtracting the optical band gaps (Eg,opt) from the HOMO energy levels of P1–P3. The electrochemical reductions of polymers P1–P3 showed LUMO energy levels at about3.34 to 3.35 eV, which represent to possess high electron affinities and also make these polymers suitable donors for electron injection and transporting to PCBM acceptor (with 0.4 eV offsets in LUMO levels regarding PCBM with a LUMO level of3.75 eV,23
as shown in Fig. 4) for the bulk heterojunction polymer solar cell devices.24 As the potentials of P1–P3 were gradually increased to þ1.3 to 1.5 V in Figure 3(a), due to the higher stability and crystallinity induced by H-bonds, only the electrochromic color of P3 changed noticeably from orange (in the neutral state) to black (in the oxidation state) and the color change could be easily detected by naked eye. The absorption spectra (at var-ious applied potentials), and optical feature images along with CIE chromaticity diagram of P3 film under neutraliza-tion (0.0 V) and oxidaneutraliza-tion (1.3 V) states are demonstrated in Figure 3(b,c), where P3 illustrated a hypsochromic absorp-tion and an enhanced absorpabsorp-tion in the range of 500 to 800 nm during the oxidation process (from 0.0 to 1.3 V). There-fore, P3 is a good candidate for the electrochromic applica-tion due to its distinct color change with easy processibility in different solvents (in DCM, THF, and CHCl3).
X-Ray Diffraction (XRD) Analyses
As shown in Figure 5(a,b), powder X-ray diffraction (XRD) patterns of polymers P1–P3 were acquired to investigate the molecular organization and morphological change. The meas-urements were proceeded on drop-cast films prepared from 0.5 wt % solutions in THF after the thermal treatment of about 150 C for 10 min and were then cooled to room
FIGURE 3 (a) Cyclic voltammograms of polymers P1–P3 (in solid films) at a scan rate of 100 mV/s, (b) absorption spectra and optical images of P3 on ITO at various applied potentials (0 V–1.3 V), (c) CIE chromaticity diagram of P3 at ‘‘off’’ (0 V) and ‘‘on’’ (þ1.3 V) states.
FIGURE 4 Energy band diagrams with HOMO/LUMO levels of do-nor polymers P1–P3 in relation to the work functions of ITO and Al.
temperature. P1–P3 after thermal annealing exhibited sub-stantially a primary diffraction feature with a angle at 2h ¼2.9 (corresponding to a large d-spacing value of 20 Å for
P1) and 2h ¼2.8 (corresponding to a large d-spacing value
of 21 Å for P2–P3), which were assigned to a distance between the conjugated backbones separated by the long side chains as reported for other similar p-conjugated poly-mers with long pendants.25Moreover, a much stronger (100) XRD characteristic peak of P3 (with H-bonds) was observed in contrast to P2 (without H-bonds). The XRD data demon-strate that P3 possessed a larger crystallinity than P2 which was enhanced by the H-bonded interactions. In our previous studies, the crystallinities of the supramolecular polymers were much improved due to the presence of hydrogen bonds, which enhanced the self-assembled behavior and thus to induce higher PCE values of H-bonded polymers.26(a) Moreover, the higher liquid crystalline arrangements of smec-tic layers can be induced by the formation of supramolecular structures with highly ordered H-bonds.26(b,c) The possible packing motifs of polymers P1–P3 in the XRD measurements with three-dimensional layered and p-p stacked arrangements
are represented in Figure 5(c,d), which show a model that the alkyl side chains stack as bilayered packings within the same layer. On the other hand, the reflections in the wide angle region (corresponding to 3.9–4.5Å) are related to the p-p stacked distance between the polymer layers,27,28 which have the similar wide-angle d-spacing values and XRD intensities in all polymers P1–P3 to show their almost fixed p-p stacking distances (ca. 3.9–4.5Å) with comparable crystallinities. The diffraction features of polymers P1–P3 were often observed in the XRD patterns of the p-conjugated polymers.29Overall, the proposed model can explain the possible structural arrange-ments of the polymer chains in P1–P3, and the highest crystal-linity of P3 in the small and large angle regions of XRD pattern (corresponding to the bilayered packings between polymer backbones and the p-p stacked distance between the polymer layers, respectively) was induced by H-bonds.
Morphology
The AFM topographies of polymer blends (P1–P3: PCBM ¼ 1:1 w/w) were investigated by the casting films of dichloro-benzene solutions as shown in Figure 6, where the images
FIGURE 5 Powder X-ray diffraction (XRD) spectra of (a) P1 and (b) P2–P3; and schematic representations of proposed three-dimen-sional layered and p-p stacked arrangements of (c) P1 and (d) P2 in their XRD measurements (P3 is similar to P2).
were obtained by the tapping mode. Furthermore, the solid films of blended copolymers P1 and P3 showed a similar surface roughness with moderate root mean square (RMS) values about 18 nm. In comparison with blended polymers P1 and P3, blended polymer P2 revealed a rather uneven surface with a RMS roughness of 58.6 nm, which was attrib-uted to the aggregation of polymer chains due to their poor solubilities and lack of H-bonds, and thus to reduce the interface between donor (P2) and acceptor (PCBM) signifi-cantly. As shown in Table 3, owing to the unfavorable mor-phology for charge transport offered by poor solubility of P2, the PSC device based on P2 gave relatively low current densities (Isc) and thus the lowest PCE value. Therefore,
the PCE values of blended polymers (P1–P3: PCBM ¼ 1:1 w/w) are inversely proportional to their RMS roughnesses in AFM.
Photovoltaic Properties
The motivation for the design and syntheses of the conju-gated polymers is to look for new polymers for the applica-tion of PSCs. To investigate the potential use of polymers P1–P3 in PSCs, bulk heterojunction devices were fabricated from an active layer in which polymers P1–P3 were blended with PCBM. PSC devices with a configuration of ITO/PEDOT: PSS/P1–P3: PCBM (w/w¼ 1:1)/Ca/Al were fabricated. The PSC devices were measured under simulated AM 1.5 solar illumination for a calibrated solar simulator with an intensity of 100 mW/cm2. The preliminarily obtained properties are summarized in Table 3, and the typical I–V characteristics of all PSC devices are shown in Figure 7. Under monochromatic
illumination, the power conversion efficiency (PCE) values of 0.44 to 0.61% were obtained for the PSC devices composed of polymers P1–P3 with current density (Jsc), open circuit
voltage (Voc), and fill factor (FF) in the range of 2.02 to 2.27 FIGURE 6 AFM images obtained from solid films of P1–P3/PCBM (1:1 w/w).
TABLE 3 Photovoltaic Properties of PSC Devices Containing an Active Layer of Polymer: PCBM (1:1 w/w) with a Device Configuration of ITO/PEDOT: PSS/Polymer: PCBM/Ca/Ala Active Layer (Polymer:
PCBM¼ 1:1 w/w) Voc (V) Jsc (mA/cm2) FF (%) PCE (%) P1 0.73 2.27 0.33 0.54 P2 0.71 2.02 0.31 0.44 P3 0.90 2.26 0.31 0.61 a
Measured under AM 1.5 irradiation, 100 mW/cm2
.
FIGURE 7 I–V curves (under simulated AM 1.5 solar irradiation) of solar cells with an active layer of P1–P3: PCBM measured by (a) illuminated current and (b) dark current.
mA/cm2, 0.71 to 0.90 V, and 31 to 33%, respectively. The photovoltaic properties of the PSC devices containing fused dithienothiophene-based polymers P1–P3 were dependent on the solubility and film-forming quality of the polymers. As mentioned in our introductory content, some donor poly-mers containing dithienothiophene units (designed and pre-pared by Millefiorini et al.,12(b) Gong et al.,12(a) and Zhang et al.15(a)) illustrated lower open circuit voltages ( 0.8 V) and PCE values ( 0.4%) than those of P3, even certain do-nor-acceptor copolymers11(a),14 have worse photovoltaic properties than these developed polymers. Among these PSC devices containing P1–P3 in Figure 7, polymer P3 gave the best performance of PCE¼ 0.61% with Jsc ¼ 2.26 mA/cm2,
Voc ¼ 0.90 V, and FF ¼ 31%. The Voc values are normally
related to the HOMO energy levels of the polymers and the LUMO energy levels of the acceptors (e.g., PCBM).30 Though the HOMO energy levels of P1–P3 were similar, the Voc value of P3 was noticeably higher than those of P1–P2.
Therefore, the highest PCE value of P3 is associated with its highest Voc value and highest crystallinity induced by
H-bonds.
CONCLUSIONS
We have successfully synthesized three dithienothiophene/ carbazole-based conjugated polymers (P1–P3) by Suzuki coupling reaction. Interestingly, P1–P3 exhibited reversible electrochromism during the oxidation processes of cyclic vol-tammogram studies. Among P1–P3, polymer P3 (with H-bonds) revealed the best electrochromic property with the most noticeable color change. In powder X-ray diffraction (XRD) measurements, these polymers exhibited obvious dif-fraction features indicating distinct bilayered packings between polymer backbones and similar p-p stacking between layers in the solid state. Compared with the XRD data of P2 (without H-bands), H-bonds of P3 induced a higher crystallinity in the small angle region (corresponding to a higher ordered bilayered packings between polymer backbones), but with a similar crystallinity in the wide angle region indicating a comparable p-p stacking distance between layers. The potential applications of P1–P3 in bulk heterojunction photovoltaic solar cells (PSCs) were further investigated, where the PSC device containing P3 blended with PCBM (by a weight ratio of 1:1) had the optimum power conversion efficiency (PCE) up to 0.61% (with Jsc ¼
2.26 mA/cm2, FF¼ 29.8%, and Voc¼ 0.90 V). Due to the
H-bonded effects, polymer P3 possessed higher thermal decomposition temperature (Td), glass transition
tempera-ture (Tg), RMS smoothness, open circuit voltage (Voc), and
PCE value than P2. These polymers demonstrate a novel family of conjugated polymers along the path toward achiev-ing the electrochromic and PSC applications.
ACKNOWLEDGMENTS
The financial supports of this project provided by the National Science Council of Taiwan (ROC) through NSC 99-2113-M-009-006-MY2, NSC 99-2221-E-009-008-MY2, and National Chiao Tung University through 97W807 are acknowledged. The
pow-der XRD measurements are supported by beamline BL13A (charged by Ming-Tao Lee) of the National Synchrotron Radia-tion Research Center (NSRRC), in Taiwan.
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