a conjugated pyridyl H-Acceptor side-chain polymer and various carbazole-based
H-Donor dyes bearing symmetrical cyanoacrylic acids for organic solar cells
Duryodhan Sahu
a, Harihara Padhy
a, Dhananjaya Patra
a, Dhananjay Kekuda
b, Chih-Wei Chu
b,c,
I-Hung Chiang
a, Hong-Cheu Lin
a,*aDepartment of Materials Science and Engineering, National Chiao Tung University, Hsinchu, Taiwan, ROC bResearch Center for Applied Sciences, Academia Sinica, Taipei, Taiwan, ROC
cDepartment of Photonics, National Chiao Tung University, Hsinchu, Taiwan, ROC
a r t i c l e i n f o
Article history: Received 12 August 2010 Received in revised form 4 October 2010 Accepted 10 October 2010 Available online 26 October 2010 Keywords:
H-bonded polymer network Solar cell
Self-assembly
a b s t r a c t
A series of novel hydrogen-bonded (H-bonded) cross-linking polymers were generated by complexing various proton-donor (H-donor) solar cell dyes containing 3,6- and 2,7-functionalized electron-donating carbazole cores bearing symmetrical thiophene linkers and cyanoacrylic acid termini with a proton-acceptor (H-proton-acceptor) side-chain homopolymer carrying pyridyl pendants (with 1/2 M ratio of H-donor/ H-acceptor). The supramolecular H-bonded structures between H-donor dyes and the H-acceptor side-chain polymer were confirmed by FTIR measurements. The effects of the supramolecular architecture on optical, electrochemical, and organic photovoltaic (OPV) properties were investigated. From DFT (density functional theory) calculations, the optimized geometries of organic dyes reflected that the carbazole cores of H-donor dyes were coplanar with the conjugated thiophenes and cyanoacrylic acids, which is essential for strong conjugations across the donor-acceptor units in D1eD4 dyes. Under 100 mW/cm2of
AM 1.5 white-light illumination, bulk heterojunction (BHJ) OPV cell devices containing an active layer of H-bonded polymers (PDFTP/D1eD4) as an electron donor blended with [6,6]-phenyl C61-butyric acid
methyl ester (PCBM) as an electron acceptor in a weight ratio of 1:1 were explored. From the preliminary investigations, the OPV device containing 1:1 weight ratio of H-bonded polymer PDFTP/D2 and PCBM showed the best power conversion efficiency (PCE) value of 0.31% with a short-circuit current (Jsc) of
1.9 mA/cm2, an open-circuit voltage (V
oc) of 0.55 V, and afill factor (FF) of 29%, which has a higher PCE
value than the corresponding H-donor D2 dye (PCE ¼ 0.15%) or H-acceptor PDFTP homopolymer (PCE¼ 0.02%) blended with PCBM in 1:1 weight ratio.
Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction
In order to overcome the growing global energy needs, extensive researches on environment friendly and renewable sources of ener-gies have been made during the past decade[1,2]. As an ecologically sustainable and renewable source of energies, solar energy led to a greater attention for the scientists to develop solar energy conver-sion devices[3,4]. Since then,
p
-conjugated oligomers and polymers were applied as advanced materials for the development of organic photovoltaic (OPV) devices for future applications[5,6]. After the breakthrough work of Heeger and coworkers in 1995[7], bulk het-erojunction (BHJ) solar cells became a low-cost,flexible, andlarge-area processible[8,9]alternatives to silicon-based solar cells, though their efficiencies are not comparable to those of silicon-based tech-nologies. In fact, different solar cell architectures have been devel-oped, including dye sensitized solar cells[10e12], donor-acceptor BHJs of polymer blends[13e19]and block copolymers[20], supra-molecular ensemble of small molecule/small molecule [21] etc. However, the interests on oligomers with easy purification processes and lack of problems in molecular weight distributions etc. are generally auxiliary to their polymer analogues due to the better solvent processabilities andfilm morphologies in photovoltaic cells [22]. Therefore, in order to get the advantages of both oligomeric and polymeric properties, an attractive approach would be the well-defined supramolecular architectures of
p
-conjugated oligomers with the processabilities of polymers[23].Since conjugated polymers with electron donor-acceptor (D-A) architectures had proved as the highly efficient solar cell polymers * Corresponding author. Tel.: þ8863 5712121x55305; fax: þ8863 5724727.
E-mail address:[email protected](H.-C. Lin).
0032-3861/$e see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymer.2010.10.018
that built intramolecular charge transfer (ICT) transitions from the electron donor to acceptor and thus to lower the band gaps of solar cell polymers [24]. Carbazole-based oligomers and polymers as semiconducting materials have been attracted attentions for its good chemical stability, easy functionalization with a large variety of substituents to modulate the carbazole properties without increasing the steric hindrance, and cheap availabilities of starting materials towards organic materials applications of organic light-emitting diodes (OLEDs) [25], organic field effect transisters (OFETs)[26]and organic photovoltaics (OPVs)[27]. The electron rich nitrogen and sulphur atoms in carbazole and thiophene units respectively enhance their potentials as efficient p-type trans-porting moieties in those applications. After the successful synthesis of N-alkyl-2,7-diiodocarbazole by Leclerc at.el[28], 2,7-functionalized carbazole became the promising candidate with more planar and well delocalized
p
-electron structure for the applications of organic photovoltaics[29]. There are many reports of carbazole-based polymers or oligomers in thefield of bulk het-erojunction (BHJ) organic photovoltaics (OPVs)[30]. However, in order to get high power conversion efficiency (PCE) values, self-assembles ofp
-conjugated oligomers with processable polymers into well-defined and organized D-A supramolecular structures remain a challenge [23]. Previously, Meijer and coworkers employed a supramolecular H-bonded polymer containing oligo(p-phenylene vinyene) and ureido-pyrimidinone units (as H-bonded groups) for the applications in OPV devices[22a,31]. Furthermore, we reported on supramolecular assemblies of H-bonded side-chain polymers which were prepared by complexation of light-emitting dendrimers or solar cell dyes bearing carboxylic acids as proton-donors (H-proton-donors) with side-chain polymers bearing pyridyl pendants as proton-acceptors (H-acceptors) for the applications in OLEDs and OPVs [32]. In general, different attempts of utilizing conjugated oligomers/polymers or both have been made to design electron donor-acceptor architectures to enhance theintramolecular charge transfer interactions from the electron donors to the electron acceptors and thus to increase the PCE values.
As shown in Fig. 1, a series of symmetrical carbazole-based conjugated dyes (D1eD4) containing carbazole cores functional-ized at two different (i.e., 3,6- and 2,7-substituted) positions linked to cyanoacrylic acid termini (as double H-donors) via
p
-conjugated oligo-thiophenes were synthesized, which were complexed with a side-chain homopolymer (PDFTP) bearing pyridyl pendants (as H-acceptors). Then, double H-donors of D1eD4 dyes and H-acceptors of side-chain homopolymer PDFTP can be self-assembled into H-bonded cross-linking polymers, and the schematic illustra-tion is demonstrated in Fig. 2. The supramolecular (H-bonded) polymer networks (PDFTP/D1eD4) of carbazole-based H-donor dyes (D1eD4) complexed with side-chain H-acceptor homopol-ymer (PDFTP) were confirmed by FTIR spectroscope. The active layer of blended polymers of H-bonded polymer networks PDFTP/ D1eD4 as electron donors and (6,6)-phenyl C61-butyric acid methylester (PCBM) as an electron acceptor (in a weight ratio of 1:1) were subjected to BHJ photovoltaic investigations (under AM 1.5 irradi-ation, 100 mW/cm2). From the preliminary results, the solar cell device containing H-bonded polymer network PDFTP/D2 produced the best PCE value of 0.31%, which was obviously doubled the PCE value (0.15%) of the active layer of D2 dye alone blended with PCBM, fabricated and measured under similar conditions.
2. Experimental 2.1. Materials
Chemicals and solvents were reagent grades and purchased from Aldrich, ACROS, TCI, Strem, Fluka, and Lancaster Chemical Co. THF and dichloromethane were distilled over sodium/benzophe-none and calcium hydride respectively and freshly distilled before
use. Tetra-n-butyl ammonium hexafluorophosphate (TBAPF6) was
recrystallized twice from absolute ethanol and further dried for two days under vacuum. N-bromosuccinimide was recrystallized from distilled water and dried under vacuum. The other chemicals were used without further purification. Chromatography was performed with Merck silica gel (mesh 70e230) and basic aluminum oxide, deactivated with water. The chemical structures for all products were confirmed by1H NMR spectroscopy, mass spectra (FAB) and
elemental analyses. 2.2. Measurements
1H NMR spectra were recorded on a Varian unity 300 MHz
spectrometer using d-DMSO as solvents. Elemental analyses were performed on a HERAEUS CHN-OS RAPID elemental analyzer. Fourier transform infrared (FT-IR) spectra were performed a Nicolet 360 FT-IR spectrometer. Thermo gravimetric analyses (TGA) were conducted on a Du Pont Thermal Analyst 2100 system with a TGA 2950 thermo gravimetric analyzer at a heat-ing rate of 20C/min under nitrogen. Gel permeation chroma-tography (GPC) analyses were conducted with a Water 1515 separations module using polystyrene as a standard and THF as an eluent. UVevisible absorption spectra were recorded in dilute THF solutions (105 M) on an HP G1103A spectrophotometer. Fourier transform infrared (FTIR) spectra of samples dispersed in KBr disks were recorded on a PerkineElmer Spectrum 100 Series[32a].
Thin films of UVevis were spin-coated on quartz substrates from THF solutions with a concentration of 1 wt%. Cyclic voltam-metry (CV) measurements were performed using a BAS 100 electrochemical analyzer with a standard three-electrode electro-chemical cell in a 0.1 M tetrabutylammonium hexafluorophosphate (TBAPF6) solution at room temperature with a scanning rate of
100 mV/s. During the CV measurements, the solutions were purged with nitrogen for 30 s. In H-bonded cross-linking polymers, a carbon working electrode coated with a thin layer of H-bonded polymer, and for dyes in solution (THF) a platinum wire as the counter electrode, and a silver wire as the quasi-reference electrode were used, and Ag/AgCl (3 M KCl) electrode was served as a refer-ence electrode for all potentials quoted herein. The redox couple of ferrocene/ferrocenium ion (Fc/Fcþ) was used as an external stan-dard. The corresponding highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) levels were calculated using Eox/onsetand Ered/onset. The onset potentials
were determined from the intersections of two tangents drawn at the rising currents and background currents of the cyclic voltam-metry (CV) measurements.
2.3. Quantum chemistry computation
The predicted structures of the molecules were optimized by using B3LYP hybrid functional[33]and 6-31G* basis sets[34]. For each of the molecules, a number of conformational isomers were examined and the one with the lowest energy was used. All of the analyses were performed under Gaussian 03 (G03) (revision E.01) program package[35]by using density functional theory (DFT). 2.4. Fabrication and characterization of OPV devices
Solar cells were fabricated on indium tin oxide (ITO)-coated glass substrates through a following procedure: The ITO coated glass substrate wasfirst cleaned with a detergent and sonicated in an ultrasonic bath, and they were dried overnight in an oven. After this cleaning procedure, the substrates were subjected to UV ozone cleaning for 15 min. PEDOT: PSS (Baytron PH) was spin coated onto the substrates at 4000 rpm. Thefilms were dried on a hotplate at 120 C for 30 min. The samples were then transferred to the nitrogenfilled glove box for the active layer deposition. A solution containing hydrogen-bonded polymer networks (PDFTP/Dx, where x vary from 1 to 4) and PCBM were prepared (2 wt%, 1:1) and were kept overnight digitally controlled hotplate at 60C for the uniform mixing. The solutions were used for the active layer deposition and were spin coated on the substrates at 1500 rpm for 60 s. Thefilms were allowed to dry in a covered Petri dish. No thermal annealing treatment was given to the active layers prior to the cathode deposition. Finally, a 20 nm Ca and 50 nm Al cathodes were deposited using a thermal evaporator at a base pressure of 1 106
Torr. The device area was 0.1 cm2. The devices were then trans-ferred to the nitrogenfilled glove box. The current-voltage char-acteristics of the devices were measured using HP 4156 semiconductor parameter analyzer. Air mass 1.5 Global (AM 1.5 G) solar simulator was used for the photo illumination of the devices. 2.5. Synthesis of monomer and dyes
Carbazole-based donor dyes D1eD4 (as shown inFig. 1) were synthesized from 9-hexyl-3,6-bis(4,4,5,5-tetramethyl-1,3,2-dioxa-borolan-2-yl)-9H-carbazole(D1eD2)[36]and 9-(heptadecan-9-yl)-2,7-bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-9H-carbazole (D3eD4)[37]. Intermediates 7e10 were synthesized by the similar Suzuki coupling conditions, and one of the synthetic procedures (for intermediate 7) has been described. The monomers D1eD4 were acquired from the previous intermediates via Knoevenagel condensation reaction. Their synthetic routes are shown inScheme S1and detailed procedures for intermediates are described in the Fig. 2. Schematic illustration of H-bonded polymer networks (PDFTP/D1eD4) containing H-donor dyes (D1eD4) and H-acceptor side-chain polymer (PDFTP).
supporting information. Monomer DFTP was synthesized from 2,7, dibromofluorene as a starting material and polymerized with free radical polymerization using AIBN as an initiator are shown in Scheme 1. The1H NMRs of the monomer DFTP and side-chain homopolymer (PDFTP) are shown inFig. 3and the detailed proce-dures for monomer DFTP and homopolymer PDFTP are described as follows:
2.5.1. 2,7-Dibromo-9,9-dihexyl-9H-fluorene (1)
A mixture of 2,7-dibromofluorene (10 g, 30.86 mmol) and potassium-tert-butoxide (10.37 g, 92.58 mmol) were stirred in 120 ml of dry THF under nitrogen for 1 h, then 1-bromohexane (11 ml, 78.00 mmol) was added drop wise and refluxed for overnight. Excess of THF was removed by rotary evaporator. Next, the compound was extracted with dichloromethane and washed
with water followed by a little brine. Then, the organic layer was dried over anhydrous MgSO4 (worked up). The solvent was
removed by rotary evaporator, and the crude product was purified by column chromatography (silica) using hexane as an eluent to yield a white solid (13.40 g, 88%).1H NMR (300 MHz, CDCl3),
d
(ppm): 7.50 (s, 2H), 7.46 (d, J¼ 1.5 Hz, 2H), 7.44 (d, J ¼ 1.8 Hz, 2H), 1.93e1.88 (m, 4H), 1.17 (m, 4H), 1.13e1.03 (m, 12H), 0.78 (t, J¼ 6.9 Hz, 6H).
2.5.2. 2,2’-(9,9-Dihexyl-9H-fluorene-2,7-diyl)dithiophene (2) Compound 1 (5 g, 10.15 mmol), thiophen-2-ylboronic acid (3.3 g, 25.78 mmol), K2CO3(4.2 g, 30.40 mmol) were reacted in 300 ml of
toluene and ethanol (3:1) and degassed for 10 min then Pd(PPh3)4
(293 mg, 0.253 mmol) was added and then the resulting mixture was stirred under reflux for 4 h. The reaction mixture was worked
up, then the excess solvent was concentrated under vacuum. Subsequently, the compound was purified by column chromatog-raphy (silica) using hexane as an eluent to yield a white solid (4.8 g, 96%).1H NMR (300 MHz, CDCl3),
d
(ppm): 7.69 (d, J¼ 7.8 Hz, 2H),7.63e7.57 (m, 4H) 7.40 (d, J ¼ 3 Hz, 2H), 7.30 (d, J ¼ 5.2 Hz, 2H), 7.13e7.10 (m, 2H), 2.05e2.00 (m, 4H), 1.14e1.06 (m, 12H), 0.78e0.69 (m, 10H).
2.5.3. 5,5 ’-(9,9-Dihexyl-9H-fluorene-2,7-diyl)bis(2-bromothiophene) (3)
Compound 2 (5 g, 10 mmol) was dissolved in 120 ml of DMF, to that N-bromosuccinimide (4.1 g, 23.05 mmol, freshly purified by recrystallization from water) was added portion wise and stirred for overnight at room temperature, sequentially. The reaction mixture was worked up and the solvent was removed under vacuum. Then, the compound was purified by column chroma-tography (silica) using hexane as an eluent to yield a white solid (6.1 g, 93%).1H NMR (300 MHz, CDCl3),
d
(ppm): 7.66 (d, J¼ 7.8 Hz,2H), 7.50 (d, J¼ 1.5 Hz, 1H) 7.48 (d, J ¼ 1.5 Hz, 1H), 7.45 (d, J ¼ 1.5 Hz, 2H), 7.11 (d, J¼ 3.9 Hz, 2H), 7.05 (d, J ¼ 3.9 Hz, 2H), 2.01e1.96 (m, 4H), 1.14e1.04 (m, 12H), 0.77e0.63(m, 10H).
2.5.4. 10-(4-Bromo-phenoxy)-decanol (4)
4-Bromophenol (10 g, 57.8 mmol), 10-bromo decanol (16.45 g, 69.36 mmol), and K2CO3(23.96 g, 173.33 mmol) was dissolved in
150 ml of dry acetone and stirred under reflux for 24 h. After cooling to room temperature, the potassium salt wasfiltered off and the solvent was removed by rotary evaporator followed by similar work up procedure, then the excess solvent was removed using rotary evaporator. The crude product was purified by column chromatography (silica) using mixture of hexane and ethyl acetate (4:1) as an eluent, to get low melting semi solid (15.2 g, 80%).1H
NMR (300 MHz, CDCl3),
d
(ppm): 7.33 (d, J¼ 9.0 Hz, 2H), 6.75 (d,J¼ 9.0 Hz, 2H), 3.89 (t, J ¼ 6.3 Hz, 2H), 3.62 (t, J ¼ 6.6 Hz, 2H), 1.78e1.69 (m, 2H), 1.59e1.50 (m, 2H), 1.41e1.29 (m, 12H).
2.5.5. [10-(4-Bromo-phenyl)-decyloxy]-tert-butyl-dimethyl-silane (5) Mixture of compound 4 (5 g, 15.18 mmol) and imidazole (2.00 g, 30.30 mmol) were dissolved in 100 ml of dry dichloromethane and degassed for 10 min. Tert-butyl-chloro-dimethyl-silane (4.57 g, 30.30 mmol) was added at 0C under nitrogen atmosphere and stirred further to react for 4 h at room temperature, then the reacting mixture was worked up and the excess solvent was removed using rotary evaporator. Subsequently, the crude product was purified by column chromatography (silica) using mixture of hexane and dichloromethane (4:1) as eluent, to get a colorless liquid (5.5 g, 82%).1H NMR (300 MHz, CDCl3),
d
(ppm): 7.34 (d, J¼ 12 Hz, 2H), 6.76(d, J¼ 8.1 Hz, 2H), 3.90 (t, J ¼ 15 Hz, 2H) 3.60 (t, J ¼ 15 Hz, 2H),1.73(m, 2H), 1.30e1.51 (m, 14H), 0.89 (s, 9H), 0.05 (s, 6H).
2.5.6. 2-{4-[10-(Tert-butyl-dimethyl-silanyloxy)-decyl]-phenyl}-4,4,5,5-tetramethyl-[1,3,2] dioxaborolane (6)
5 g of compound 5 (11.27 mmol) was dissolved in 100 ml of dry THF and the solution was cooled to 78C under nitrogen
atmo-sphere then 9 ml of n-BuLi (2.5 M in hexane, 22.54 mmol) was added. The solution was warmed up to room temperature and stirred for 2 h and cooled again to 78 C, then 6.13 ml
(22.54 mmol) of 2-Isopropoxy-4,4,5,5-tetramethyl- [1,3,2] dioxa-borolane was injected promptly in to theflask and the resulting mixture was stirred overnight at room temperature. The reaction was quenched by adding 50 ml of water then extracted with dichloromethane, and a little of brine. Thereafter the organic solvent was dried over anhydrous MgSO4 and the solvent was
removed by rotary evaporator. The crude product was purified by Fig. 3.1H NMR spectra of monomer DFTP and H-acceptor polymer PDFTP in CDCl
column chromatography (silica) using mixture of hexane and dichloromethane (4:1) as eluent to yield a low melting white solid (3.5 g, 63%).1H NMR (300 MHz, CDCl3),
d
(ppm): 7.72 (d, J¼ 8.4 Hz, 2H), 6.87(d, J ¼ 8.7 Hz, 2H), 3.95(t, J ¼ 12.9 Hz, 2H), 3.57(t, J¼ 13.5 Hz, 2H), 1.73(qn, J ¼ 6.9 Hz, 2H), 1.54e1.22 (m, 26H), 0.86 (s, 9H), 0.02 (s, 6H). 2.5.7. (10-(4-(5-(7-(5-Bromothiophen-2-yl)-9,9-dihexyl-9H-fluoren-2-yl)thiophen-2-yl)phenoxy)decyloxy) (tert-butyl) dimethylsilane (7)Mixture of compound 3 (3 g, 4.56 mmol), 6 (1.9 g, 3.9 mmol), and K2CO3(0.7 g, 5.03 mmol) were dissolved in 80 ml of toluene
and ethanol (3:1) and degassed for 10 min. Then Pd(PPh3)4(115 mg,
0.10 mmol) was added and the resulting mixture was stirred under reflux for overnight. The solvent was removed under vacuum, and after being worked up the solvent was concentrated by rotary evaporator. Then, the compound was purified by column chroma-tography (silica) using a mixture of hexane and dichloromethane (9:1) as an eluent, to get a yellow solid (2.00 g, 55%) 1H NMR (300 MHz, CDCl3),
d
(ppm): 7.66 (d, J¼ 7.8 Hz, 2H), 7.59e7.53(m, 5H)0.7.36 (dd, J¼ 3.6 Hz, J ¼ 1.2 Hz, 1H), 7.31 (d, J ¼ 3.9 Hz, 1H), 7.27 (dd, J¼ 5.1 Hz, J ¼ 1.2 Hz, 1H), 7.18 (d, J ¼ 3.9 Hz, 1H), 7.10e7.08 (m, 1H), 6.90 (d, J¼ 9 Hz, 2H), 3.97 (t, J ¼ 6.6 Hz, 2H), 3.58(t, J ¼ 13.5 Hz, 2H), 2.03e1.97 (m, 4H), 1.83e1.76 (m, 2H), 1.51e1.25 (m, 16H), 1.13e1.05 (m, 10H), 0.88 (s, 9H), 0.75e0.67 (m, 10H), 0.035 (s, 6H).
2.5.8. 4-(Thiophen-2-yl) pyridine (8)
Mixture of compound 4-iodopyridine (2 g, 9.75 mmol), thio-phen-2-ylboronic acid (1.5 g, 11.7 mmol), K2CO3(1.8 g, 13 mmol)
were dissolved in 120 ml of toluene and ethanol (3:1) and degassed for 10 min then Pd(PPh3)4(225 mg, 0.195 mmol) was added and
then the resulting mixture was stirred under reflux for overnight. The solvent was removed under vacuum followed by work up procedure, then the solvent was concentrated under vacuum. Thereafter, the compound was purified by column chromatography (Al2O3) using mixture of hexane and dichloromethane (9:1) as
eluent, to get a white solid (1.4 g, 86%)1H NMR (300 MHz, CDCl3),
d
(ppm): 8.57 (d, J¼ 6 Hz, 2H), 7.50e7.46 (m, 3H), 7.40 (d, J ¼ 5.1 Hz, 1H), 7.12 (t, J¼ 4.2 Hz, 1H).
2.5.9. 4-(5-Bromothiophen-2-yl) pyridine (9)
4-(thiophen-2-yl)pyridine (8) (1 g, 6.2 mmol) was dissolved in 50 ml of dichloromethane, to that, bromine (0.63 ml,12.4 mmol) with 10 ml of dichloromethane was added slowly for 10 min and stirred for additional 15 min at room temperature. The reaction was quenched with 50 ml of 10% K2CO3 followed by extraction from
dichloro-methane and dried over anhydrous MgSO4. Subsequently, the solvent
was concentrated by rotary evaporator. After that, the compound was purified by column chromatography on Al2O3 using mixture of
hexane and dichloromethane (10:1) as eluent, to get white solid (1.2 g, 81%)1H NMR (300 MHz, CDCl3),
d
(ppm): 8.60 (d, J¼ 6 Hz, 2H),7.45 (d, J¼ 6 Hz, 2H), 7.30 (d, J ¼ 3 Hz, 1H), 7.11 (d, J ¼ 3 Hz, 1H). 2.5.10. 2-Tri-n-butylstannyl-5-(4-pyridyl) thiophene (10)
4-(5-bromothiophen-2-yl) pyridine (9) (0.3 g, 1.24 mmol) was dissolved in 80 ml of THF and the solution was cooled to - 78C. 0.75 ml (2.5 M in hexane, 1.87 mmol) of n-BuLi was added over a period of 3 min under nitrogen atmosphere. After stirring the solution for 30 min at - 78C, Bu3SnCl (0.5 ml, 1.87 mmol) was
added and the reaction mixture was stirred for further 1 h. It was then allowed to warm up to room temperature and stirred for additional 3 h then the reaction was quenched with water (30 ml). THF was removed in rotary evaporator and worked up, then the solvent was removed in rotary evaporator. Afterward, the compound was purified by column chromatography(Al2O3) using
a mixture of hexane and ethyl acetate (5:1) as eluent, to get a yellow oil (0.45 g, 80%) 1H NMR (300 MHz, CDCl3),
d
(ppm): 8.56 (d,J¼ 4.8 Hz, 2H), 7.60 (d, J ¼ 3.3 Hz, 1H), 7.48 (d, J ¼ 6.3 Hz, 2H), 7.19 (d, J¼ 3.3 Hz, 1H), 1.64e1.53 (m, 6H), 1.39e1.26 (m, 6H), 1.17e1.11 (m, 6H), 0.90 (t, J¼ 7.5 Hz, 9H).
2.5.11. 4-(50-(7-(5-(4-(10-(tert-Butyldimethylsilyloxy)decyloxy) phenyl)thiophen-2-yl)-9,9-dihexyl-9H-fluoren-2-yl)-2,20
-bithiophen-5-yl)pyridine (11)
Mixture of compound 7 (2 g, 2.12 mmol), compound 10 (1 g, 2.22 mmol) and Pd(PPh3)4 (0.3 g, 0.26 mmol) were added to
toluene (200 ml) and degassed for 10 min. Then the mixture was heated to reflux for 24 h under nitrogen atmosphere. The solvent was removed under vacuum. After the similar work up procedure, the solvent was concentrated by rotary evaporator, and then the compound was purified by column chromatography (Al2O3) using
dichloromethane as eluent, to get a yellow solid (1.3 g, 60%)1H NMR (300 MHz, CDCl3),
d
(ppm): 8.57 (d, J¼ 6.3 Hz, 2H), 7.70 (d, J ¼ 3 Hz,1H), 7.67 (d, J¼ 3 Hz, 1H), 7.62e7.50(m, 10H), 7.32 (d, J ¼ 3.6 Hz, 2H), 7.25 (d, J¼ 5.7 Hz, 1H), 7.18 (d, J ¼ 3.9 Hz, 1H), 6.90 (d, J ¼ 9 Hz, 2H), 3.96 (t, J¼ 6.9 Hz, 2H), 3.58 (t, J ¼ 6.6 Hz, 2H), 2.13e2.05 (m, 4H), 1.80e1.76 (m, 2H), 1.49e1.28 (m, 16H), 1.11e1.05 (m, 10H), 0.88 (s, 9H), 0.75e0.67 (m, 10H), 0.042 (s, 6H).
2.5.12. 10-(4-(5-(9,9-Dihexyl-7-(50-(pyridin-4-yl)-2,20 -bithiophen-5-yl)-9H-fluoren-2-yl)thiophen-2-yl)phenoxy)decan-1-ol (12)
Compound 11 (0.9 g, 0.88 mmol) was dissolved in 30 ml of dry THF, to this 1.75 ml (1.76 mmol) of 1M TBAF was added under an ice-cold water bath. The reaction mixture was stirred for 10 min then further 12 h at room temperature. THF was removed by rotary evaporator, the product was extracted with excess of dichloro-methane followed by brine wash .The organic solvent was dried over anhydrous magnesium sulphate then the solvent was removed using rotary evaporator. The crude product was purified by column chromatography on Al2O3 using mixture of
dichlor-omethane:THF (20:1) as an eluent to yield a yellow solid (0.65 g, 81%)1H NMR (300 MHz, CDCl3),
d
(ppm): 8.70 (d, J¼ 5.4 Hz, 2H),8.05 (d, J¼ 3.9 Hz, 2H), 7.96 (d, J ¼ 6.3 Hz, 2H), 7.82e7.73 (m, 4H), 7.66e7.54 (m, 7H), 7.40 (d, J ¼ 3.6 Hz, 1H), 6.95 (d, J ¼ 8.7 Hz, 2H), 3.99 (t, J¼ 6.6 Hz, 2H), 3.64 (t, J ¼ 6.6 Hz, 2H), 2.06e2.01 (m, 4H), 1.82e1.77 (m, 2H), 1.59e1.53 (m, 2H), 1.47e1.43 (m, 14H), 1.15e1.07 (m, 10H), 0.77e0.72 (m, 10H).
2.5.13. 10-(4-(5-(9,9-Dihexyl-7-(50-(pyridin-4-yl)-2,20 -bithiophen-5-yl)-9H-fluoren-2-yl)thiophen-2-yl)phenoxy)decyl methacrylate (DFTP)
To a schlenk tube, compound 12 (1.0 g, 1.10 mmol), vinyl methacrylate (0.37 g, 3.3 mmol), 1,3-dichloro-1,1,3,3-tetrabu-tyldistannoxane (0.30 mg, 0.05 mmol), and 2,6-di-tert-butyl-4-methylphenol (24 mg, 0.110 mmol) in dry THF (2 mL) were purged with nitrogen for 15 min at room temperature. The tube was sealed and stirred at 50C for 2 days. After cooling to room temperature, the reaction mixture was extracted using dichloromethane, and the extract was washed with water, dried over Mg2SO4, and then
evaporated. The crude product was purified by column chroma-tography using dichloromethane as eluent and then washed with hexane. to yield yellow solid (0.84 g, 81%) 1H NMR (300 MHz, CDCl3),
d
(ppm): 8.60 (d, J¼ 5.7 Hz, 2H), 7.68 (d, J ¼ 7.8 Hz, 2H),7.62e7.56 (m, 8H) 7.47e7.44 (m, 2H), 7.34e7.32 (m, 2H), 7.25 (d, J¼ 2.7 Hz,1H), 7.21 (d, J ¼ 2.7 Hz, 1H), 6.95 (d, J ¼ 8.7 Hz, 2H), 3.99 (t, J¼ 6.6 Hz, 2H), 3.64 (t, J ¼ 6.6 Hz, 2H), 2.06e2.01 (m, 4H), 1.95 (s, 3H) 1.82e1.77 (m, 2H), 1.59e1.53 (m, 2H), 1.47e1.43 (m, 14H), 1.15e1.07 (m, 10H), 0.77e0.72 (m, 10H). MS (FAB): m/z [Mþ] 975;
calcd m/z [Mþ] 974.46. Anal. Calcd for C62H71NO3S3: C, 76.42; H,
collected, washed with diethyl ether, and dried under high vacuum. Yield: 1.2 g (80%). Mn¼ 25128 and PDI (Mw/Mn)¼ 1.72 (by GPC).1H
NMR (300 MHz, CDCl3),
d
(ppm): 8.51 (br, s, 2H), 7.55e7.46 (br, m,4H), 7.32e7.11 (br, m, 10H), 6.95e6.91 (br, m, 2H), 6.81 (br, s, 2H), 3.99 (br, m, 2H), 3.83 (br, m, 2H), 1.95 (br, m, 4H), 1.72 (br, m, 4H), 1.33 (br, m, 12H), 0.98 (br, m, 12H), 0.64 (br, m, 10H). Anal. Calcd for C62H71NO3S3: C, 76.42; H, 7.34; N, 1.44; Found: C, 75.89; H, 7.11; N,
2.05.
2.6.2. Preparation of H-Bonded polymer networks (PDFTP/D1eD4) Proton-acceptor polymer PDFTP and proton-donor dye (D1eD4) 2:1 M ratio were dissolved in minimum amount of THF to make a clear solution. Then the solvent was evaporated under ambient temperature, and was followed by drying in a vacuum oven at 60C for several hours. The complexation of donor and acceptor through hydrogen bonding occurred during the solvent evaporation. Similar procedure was followed for all the H-bonded polymers.
3. Results and discussion 3.1. Structural characterization
All H-donor dyes D1eD4, monomer DFTP, and H-acceptor side-chain homopolymer PDFTP were satisfactorily characterized by1H NMR, FAB, and elemental analyses. The weight average molecular weight (Mw) of H-acceptor polymer PDFTP was determined by gel permeation chromatography (GPC) with THF as the eluting solvent and polystyrene as a standard. Fig. 3shows1H NMR spectra of monomer DFTP and polymer PDFTP. The number average molec-ular weight (Mn) and polydispersity index (PDI) of polymer PDFTP were 25128 g/mol and 1.72, respectively. The decomposition temperatures (Td) of H-bonded polymer networks PDFTP/D1-D4
and H-acceptor polymer PDFTP were measured by TGA and summarized inTable 1. H-bonded cross-linking polymers showed good thermal stabilities with decomposition temperatures in the
shows the IR spectra of H-donor dye D1, H-acceptor homopol-ymer PDFTP, and H-bonded polhomopol-ymer network PDFTP/D1. In contrast to the characteristic OeH stretching vibrations at 2490 and 2650 cm1of pure D1 dye, the weaker OeH bands observed at 2500 and 1900 cm1are indicative of strong hydrogen bonding between the pyridyl groups of H-acceptor polymer PDFTP and carboxylic acids of H-donor D1 dye[32,38]. In addition, the C]O stretching vibration at 1723 cm1of H-bonded polymer network PDFTP/D1 showed that the carbonyl group was in a less associated state than that in pure D1 which appeared at 1678 cm1 [32b]. The above results implied that the hydrogen bonds were formed between H-acceptor polymer PDFTP and H-donor D1 dye in the solid state of H-bonded polymer network PDFTP/D1. The other H-bonded cross-linking polymers also have the similar consequences as the H-bonded structure of H-H-bonded polymer network PDFTP/D1 demonstrated here.
3.3. Optical properties
The normalized UVevis absorption spectra of H-acceptor poly-mer PDFTP and H-donor dyes D1eD4 in THF solutions (105M) are shown inFig. 5and their data are listed inTable 1. H-donor dyes D1eD4 showed the maximum absorption wavelengths in the range from 468 to 480 nm and were attributed to the intramolecular charge transfer (ICT) from the donor carbazole core to the cya-noacrylic acid termini. Regardless of the increased conjugation lengths (with one more symmetrical thiophene unit) from D1 to D2
Table 1
Photophysical Properties and Optical Band gaps of Donor Dyes (D1eD4), H-Acceptor Polymer (PDFTP), and H-Bonded Polymer Networks (PDFTP/D1eD4).
Compound labs,sol(nm)a labs,film(nm)a lonset,abs(nm)b Eg,opt(eV)b Td(C)
D1 470 513 646 1.91 e D2 468 513 663 1.87 e D3 480 530 639 1.94 e D4 470 526 655 1.89 e PDFTP 414 416 480 2.57 e PDFTP/D1 e 426 586 2.11 381 PDFTP/D2 e 463 648 1.91 384 PDFTP/D3 e 445 604 2.05 386 PDFTP/D4 e 431 598 2.07 386
aAbsorption spectra were recorded in THF solutions.
bEstimated from the onset wavelengths of absorption spectra in solidfilms.
Fig. 4. FT-IR spectra of H-acceptor polymer PDFTP, H-donor dye D1, and H-bonded polymer network PDFTP/D1 at room temperature.
and from D3 to D4, the presence of lateral alkyl chains in the thiophene units resulted in 2 and 10 nm blue shifts, respectively, which was presumably increased the disorder in the conjugated system of the dyes due to the steric hindrance imparted by those alkyl side chains. However, the onset absorption wavelengths were increased by the increased conjugation effect[39]. Relative to the solution state inFig. 5, the solidfilm absorption spectra of dyes (D1eD5) inFig. 6a were red-shifted by a range of 43e56 nm (see Table 1), thus the red-shifts of the absorption peaks suggest the enhanced
p
-p
stacking in terms of inter-chain characteristics[40] in solid state. As anticipated, the red-shifts from D1 to D3 (10 nm in solutions and 17 nm in solid films) and D2 to D4 (2 nm in solutions and 13 nm in solid films) were possible due to the enhancedp
-p
stacking assisted by the coplanar 2,7-carbazole cores of D3 and D4 over non-planar 3,6-carbazole cores of D1 and D2 [29e]. Evidently, the poorp
-p
stacking interactions in H-acceptor polymer PDFTP, due to presence of longer flexible alkyl chain resulted in negligible (2 nm) red shift in the absorption spectrum of solidfilm compared with that in solution. As shown inFig. 6b, the maximum absorption peak of H-bonded polymer networks PDFTP/ D1eD4 on solid quartz films were blue shifted in contrast to those of H-donor dyes (D1eD4) and the blue shifts (blue shifted to a range of 50e87 nm) were ascribed to the dilution effect of acceptor polymer as solid solvent for dyes as solutes in the H-bonded cross-linking polymers. However, the broader absorption spectra of H-bonded cross-linking polymers (compared with PDFTP) extending to longer wavelength regions (Fig. 6b) favored better light harvesting and increased the photocurrent response region. In addition, the broadening of absorption spectra in the H-bonded cross-linking polymers increased theirl
onset, thus thedecrease of optical band gaps in solidfilms was observed. Among all H-bonded polymer networks, PDFTP/D2 has the longest broadening of the absorption spectrum. Thus, the lowest band gap (1.91 eV) of H-bonded polymer network PDFTP/D2 was observed, which further reflected as the most superior performance among all photovoltaic devices. Furthermore, it is supported by the fact that there were significant PL quenching observed in the H-bonded polymer networks (seeFig. S1of the supporting information) and the degrees of quenching were consistent with their PCE values (PDFTP/D2> PDFTP/D1 > PDFTP/D3 > PDFTP/D4). The largest PL quenching of PDFTP/D2 was a symptom of better photo-induced charge transfer in H-bonded polymer network PDFTP/D2, which will reflect later to show the best PCE value.
3.4. Electrochemical properties
Cyclic voltammetric (CV) method was used to investigate the electrochemical characteristics of H-donor dyes (D1eD4), H-acceptor polymer PDFTP, and the H-bonded polymer networks (PDFTP/D1eD4). The relevant CV data of dyes are presented in Fig. 5. Normalized UVevisible absorption spectra of donor dyes D1eD4 and
H-acceptor polymer PDFTP in THF solutions.
Fig. 6. Normalized UVevisible absorption spectra of (a) donor dyes (D1eD4) (b) H-acceptor polymer (PDFTP) and H-bonded polymer networks (PDFTP/D1eD4) in solid films coated onto fused quartz plates.
Table 2
Electrochemical Properties of H-Acceptor Polymer (PDFTP) and H-Bonded Polymer Networks (PDFTP/D1eD4).
Polymer Eox/onset(V)a Ered/onset(V)a HOMO (eV)b LUMO (eV)b Eg,cv(eV)c
PDFTP 1.28 0.90 5.63 3.45 2.21
PDFTP/D1 1.16 0.94 5.51 3.41 2.10
PDFTP/D2 1.02 0.83 5.37 3.52 1.85
PDFTP/D3 1.06 0.96 5.41 3.39 2.02
PDFTP/D4 1.14 0.83 5.49 3.52 1.97
aOxidation and reduction potentials were measured by cyclic voltammetry in
solidfilms.
bEstimated from the onset potentials using empirical equations: HOMO/
LUMO¼ [(Eox/red onset E1/2 (ferrocene)þ 4.8)] eV, where 4.8 eV is the energy level of
ferrocene below the vacuum level.
Table S1andFig. S2(see the supporting information), and those of polymers are also illustrated inTable 2andFig. 7. Cyclic voltam-mograms of the dyes are measured in deoxygenated THF solutions containing 0.1 M TBAPF6at 25C in volts versus Ag/AgNO3(0.01 M
in MeCN; the scan rate is 100 mV s1). All dyes (D1eD4) exhibited quasi-reversible oxidation and reduction potential (seeFig. S2of the supporting information). The calculated HOMO and LUMO levels of those H-donor dyes D1eD4 were found in the range of 5.44 to 5.56 eV and 3.31 to 3.34 eV, respectively, with electrochemical band gap of 2.11e2.22 eV. Cyclic voltammograms of H-acceptor polymer PDFTP and H-bonded polymer networks PDFTP/D1eD4 were measured in solid films with Ag/AgCl as a reference electrode, which were calibrated by ferrocene (E1/2
(ferrocene)¼ 0.45 mV vs Ag/AgCl). The HOMO and LUMO levels of all
polymers were estimated by the oxidation and reduction potentials from the reference energy level of ferrocene (4.8 eV below the vacuum level) according to the following equation [41]: EHOMO/ LUMO ¼ [-(Eonset - 0.45) - 4.8] eV. The HOMO/LUMO levels of
H-bonded polymer networks PDFTP/D1eD4 were estimated to be
2.02, and 1.97 eV, correspondingly. The attained LUMO and HOMO levels in these H-bonded polymer networks with good air stabili-ties are suitable as a donor for electron injection and transporting to PCBM acceptor[42]. Moreover, electrochemical band gaps (Eg,cv)
were very close to those obtained from the absorption spectra of solid polymerfilms (Eg,opt). To gain insight into the geometries of
H-donor dyes D1eD4, we performed DFT (density functional theory) calculations using Gaussian 03 program package. The optimized geometries in ground state and the dihedral angles formed between the carbazole-thiophene and thiophene-cyanoacrylate units are shown inFig. S3and Table S2of the supporting infor-mation. From the calculated dihedral angles, it is noticeable that the thiophene units are found to be coplanar with the cyanoacrylic acid groups, which are important characteristics in the conceptions of materials in photovoltaic applications.
3.5. Photovoltaic properties
To demonstrate the potential application of the H-bonded polymer networks (PDFTP/D1eD4) as electron donors in photo-voltaic devices, we fabricated devices by spin-coating from 2 wt% N-methylpyrrolidone (NMP) solutions of polymer blends contain-ing H-bonded polymer networks PDFTP/D1eD4 (as electron donors) and PCBM (as an electron acceptor) in 1:1 weight ratio with a configuration of ITO/PEDOT:PSS/H-bonded polymer net-works:PCBM (1:1 w/w)/Ca/Al (under AM 1.5 irradiation, 100 mW/ cm2).Fig. 8shows the current density versus voltage (JeV) curves and the results are illustrated inTable 3. As shown inTable 3, the short-circuit current density (Jsc), open-circuit voltage (Voc), fill
factor (FF), and PCE values of the OPV devices containing H-bonded Fig. 7. Cyclic voltammograms of H-bonded polymer networks PDFTP/D1eD4 in
acetonitrile at a scan rate of 100 mV/s.
Fig. 8. Current density-voltage curves of illuminated solar cells incorporating H-bonded polymer networks PDFTP/D1eD4 with PCBM in 1:1 (w/w) ratio under AM 1.5G, 100 mW/cm2.
Fig. 9. Current density-voltage curves of illuminated solar cells incorporating D2, D3, or PDFTP with PCBM in 1:1 (w/w) ratio under AM 1.5G, 100 mW/cm2.
polymer networks PDFTP/D1eD4 were in the range of 1.13e1.9 mA/cm2, 0.55e0.59 V, 24e30%, 0.16e0.31%, respectively.
Due to the negligible difference in their HOMO values, all the devices containing H-bonded polymer networks (PDFTP/D1eD4) showed similar Vocvalues. As described before, due to the longest
broadening of absorption spectra in PDFTP/D2, it had the best light harvesting and the highest photocurrent response and hence to appeal a highest Jscvalue (1.9 mA/cm2) among all H-bonded
poly-mer networks (PDFTP/D1eD4). As a contributing parameter for the net PCE value, thefill factors of the OPV devices were low, which could be associated with the large series resistance of the devices. In addition, the behavior of the solar cell properties can be well analyzed from the surface topography of the active layer. On the whole, the OPV device containing blended H-bonded polymer network PDFTP/D2:PCBM resulted the best overall PCE value of 0.31% with Jsc¼ 1.9 mA/cm2, Voc¼ 0.55 V, and FF ¼ 29%, which was
mainly because of its broader absorption and better utilization of the solar spectrum, and also supported by the largest quenching of PL spectra (see Fig. S1 of the supporting information) which increased the photo-induced charge transfer in H-bonded polymer network PDFTP/D2[14b,19c]. In general, the poor solubilities of H-bonded polymer networks might be one of the reasons for the lower PCE values. In order to demonstrate the contribution of supramolecular structures in H-bonded cross-linking polymers, we fabricated OPV devices with only H-donor D2eD3 dyes or H-acceptor polymer PDFTP blended with PCBM in 1:1 weight ratio measured under similar conditions as those of the blended H-bonded cross-linking polymers and the J-V curves are shown in Fig. 9and the data are listed inTable 4. Interestingly, it is noticeable that the PCE values of D2 (0.15%), D3 (0.11%), and PDFTP (0.02%) were much lower than those of H-bonded polymer networks PDFTP/D2 (0.31%) and PDFTP/D3 (0.24%). In general, the approach in self-assembly of H-donor dyes with H-acceptor polymers via hydrogen bonding was proven to enhance OPV properties by the unique non-covalent-bonded practical applications.
4. Conclusions
In order to get the advantage from physical properties of both small molecules and polymers in OPV devices, the concept of supramolecular architectures by complexation of H-donor dyes (D1eD4) with a side-chain H-acceptor homopolymer (PDFTP) was applied to produce H-bonded cross-linking polymers. Photo-physical properties revealed that the absorption spectra of H-bonded polymer networks showed broader wavelength ranges and higher
l
onsetvalues to appeal better light harvesting, though theyshowed blue shifts in their respective absorption maxima (
l
max).The H-bonded polymer networks (PDFTP/D1eD4) had much higher PCE values than the individual H-donor dyes (D1eD4) or H-acceptor polymer (PDFTP). The H-bonded polymer network PDFTP/D2 containing D2 dye showed the best photovoltaic performance with a PCE value of 0.31%, which is double of the PCE value (0.15%) in the corresponding D2 dye, fabricated and measured under similar conditions. Thus, the results by complexation of small
molecules and polymers via H-bonding are very encouraging for the future research on donor-acceptor supramolecular architec-tures of oligomeric dyes H-bonded with processable
p
-conjugated polymers in organic solar cell applications.Appendix. Supplementary material
Supplementary data related to this article can be found online at doi:10.1016/j.polymer.2010.10.018.
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