Synthesis of metal-free organic dyes containing
tris(dodecyloxy)phenyl and dithienothiophenyl units and a
study of their mesomorphic and photovoltaic properties
Muthaiah Shellaiah
a, Hsiao-Ping Fang
a, Yu-Ling Lin
a, Ying-Chan Hsu
b,
Jiann-T
’suen Lin
b, Hong-Cheu Lin
a,*aDepartment of Materials Science and Engineering, National Chiao Tung University, Hsinchu 30049, Taiwan, ROC bInstitute of Chemistry, Academia Sinica, Taipei, Taiwan, ROC
a r t i c l e i n f o
Article history:
Received 26 October 2012
Received in revised form 2 January 2013 Accepted 4 January 2013
Available online 11 January 2013 Keywords:
3,4,5-Tris(dodecyloxy)phenyl Dye-sensitized solar cell Dithienothiophene Metal-free organic dye
a b s t r a c t
In this study we synthesized three metal-free organic dyes (Cpd11, Cpd16, and Cpd22) featuring 3,4,5-tris(dodecyloxy)phenyl and cyanoacrylic acid moieties as electron-donor and electron-acceptor/ anchoring units, respectively, linked through various dithienothiophenyl conjugated spacers. Cpd16 exhibits mesomorphic properties, confirmed through polarizing optical microscopy, differential scanning calorimetry, and X-ray diffraction (XRD), due to the appropriate ratio of the lengths of itsflexible chain to its rigid core. Molecular modeling of Cpd16, and its d-spacing determined from XRD data, verified the existence of a tilt angle in the SmC phase. Among these metal-free organic dyes, a dye-sensitized solar cell incorporating Cpd16 exhibited the best performance, presumably because of its better packing and its mesomorphic properties; the power conversion efficiency was 3.72% (Voc¼0.58 V; Jsc¼9.98 mA cm2; FF¼0.65) under simulated AM 1.5 irradiation (100 mW cm2).
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1. Introduction
The development of novel materials for use in organic opto-electronic devices, such as dye-sensitized solar cells (DSSCs),1has become a popular research topic in the quest for low-cost, green materials for sustainable use and a decrease in demand for fossil fuels and nuclear power. DSSCs based on Ru-photosensitizers,2,3 such as cis-bis(isothiocyanato)bis(2,20-bipyridyl-4,40 -dicarbox-ylato)-ruthenium(II) (N3)4and related derivatives, have been ap-plied very successfully with high power conversion efficiencies (PCEs) of 9e12%.4e9Recently, it has been demonstrated that DSSCs can also be constructed from metal-free organic dyes.10Because of the high cost of rare Ru metal and the relatively low molar extinction coefficients and tedious purification of Ru-photosensitizes,5
metal-free organic sensitizers have become increasingly attractive and widely developed.11,12Nevertheless, the ability to reach higher ef-ficiencies when using metal-free organic dyes remains a challenge, although great progress has been made in thisfield.13e16The key
characteristics for a dye to be used in a DSSC are high absorption over a wide range of the solar spectrum with high molar extinction co-efficients, efficient charge separation, redox stability, and suitable
functional groups to interact with the electron sink (TiO2). Metal-free organic dyes featuring a donor/acceptor structural design were synthesized have particularly wide absorption ranges for DSSC applications.11e19 Liquid-crystalline (or mesomorphic) properties can be introduced to organic dyes when 3,4,5-tris(dodecyloxy) phenyl segments are incorporated, resulting in potential applica-tions in flexible electronic materials.20e25 Kato et al.21 reported
conjugated oligothiophene-based polycatenar liquid crystal mate-rials exhibiting electrochromism upon applying an oxidative po-tential, with layered smectic and columnar structures capable of enhancing hole mobilities up to 0.01 cm2V1s1. Park et al.22also reported that the mesomorphic organization of a
dicyanodistyr-ylbenzene-based molecule could improve its
fluorescence-emitting and semiconducting properties. Therefore, great efforts have been taken toward the preparation and characterization of photo- and electro-active structures based on mesogenic units. The use of various aromatic segments (e.g.,fluorene,26,27thiophene28,29)
as spacers in organic dyes can improve the photocurrent generation and intramolecular charge separation. Dithieno[3,2-b:20,30 -d]thio-phene (DTT)-conjugated dyes have also exhibited excellent DSSC efficiencies.30
In addition, the DTT unitda sulfur-rich (three S atoms) and electron-rich building blockdincreases the planarity of the dye, resulting in longer
p
-conjugation.31,32Because of restricted intra-molecular rotation in fused-ring structures, such as DTT,p
-orbital overlap in such conjugated molecules could be maximized to* Corresponding author. Tel.: þ886 3 5712121x55305; fax: þ886 3 5724727; e-mail address:[email protected](H.-C. Lin).
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enhance intermolecular charge transport.33,34 In this study, we prepared dyes featuring a 3,4,5-tris(dodecyloxy)phenyl unit21and a cyanoacrylic acid as electron donor and acceptor units, re-spectively, with various spacers inserted as the conjugated bridge (Fig.1). For example, we introduced a fused-DTT unit as a conjugated spacer through which electrons could be transferred efficiently from the donor to the acceptor. In addition to the DTT unit (as a main structure of the conjugated spacer), we also employed bithiophene and bithiazole units to extend the conjugated lengths and, thereby, affect the electron mobilities and absorption spectra. Furthermore, bithiophene unit enhanced the liquid crystallinity of the dye. To increase solubility, we inserted alkyl chains onto bithiazole hetero-cyclic rings. We were aware, however, that the presence of thio-phene rings in the electron-rich segment might not result in sufficient separation between the highest occupied molecular or-bital (HOMO) and lowest unoccupied molecular oror-bital (LUMO); in addition, too many alkyl chains would affect the packing between the layers the molecule, induce molecular aggregation, increase steric hindrance, and decrease charge transfer. If the structure had too many rigid rings, however, its solubility would be impacted, making it harder to dissolve in common solvents with low boiling points and, thereby, complicating device fabrication. Therefore, we also compared the effects of bithiophene35e37and bithiazole38units. The design of the conjugated system can not only affect the ab-sorption range but also further influence the electron transfer from the excited state to the TiO2.39In this study, we suspected that better molecular arrangements and stacking would result if the donor/ acceptor molecules exhibited enhanced liquid crystallinity There-fore, we prepared series of metal-free organic dyes containing DTT37 units with good coplanarity and investigated their mesomorphic and photovoltaic properties.
2. Results and discussion 2.1. Optical properties
Fig 2 displays the UVevis absorption and normalized photo-luminescence (PL) spectra of Cpd11, Cpd16, and Cpd22 as solutions in THF (105M); Table 1lists their corresponding data. The ab-sorption spectra reveal that the signals for Cpd16 and Cpd22 were red-shifted relative to those of Cpd11 after the insertion of the bithiophene and bithiazole units, respectively, to lengthen the conjugated linking structures. The maximum absorption peaks for Cpd11, Cpd16, and Cpd22 at 443, 476, and 473 nm, respectively, resulted from intramolecular charge transfer (ICT); that is, for the transition from the 3,4,5-tris(dodecyloxy)benzene donor to the cyanoacrylic acid acceptor. The spectra of the dyes Cpd16 and Cpd22 both featured weak
p
ep
* transition bands, at 386 and 374 nm, respectively, whereas that of Cpd11 featured only a single intense band at 443 nm (seeTable 1). As expected, the elongatedp
-conjugations in Cpd16 and Cpd22 resulted in narrowerp
ep
* en-ergy gaps and spectral red shifts for thep
ep
* transitions. Because the inserted bithiophene and bithiazole units extended theconjugation lengths in Cpd16 and Cpd22, both dyes exhibited red-shifted and broader absorptions than those of Cpd11.40Since Cpd11 had the narrowest absorption wavelength (Fig. 2a), it also displayed the worst photovoltaic performance among the tested
dyes. The molar extinction coefficients of Cpd16 (476 nm;
ε¼7.50104M1cm1) and Cpd22 (473 nm;ε¼7.70104M1cm1) at their maximum absorptions are lower than that of Cpd11 (443 nm;ε¼9.90104M1cm1), because the inserted bithiophene and bithiazole units decreased the coplanarity of the acceptor and
Fig. 1. Chemical structures of the dyes Cpd11, Cpd16, and Cpd22.
Fig. 2. (a) UVevis absorption spectra and (b) normalized photoluminescence (PL) spectra of metal-free organic dyes in THF solutions (105M).
Table 1
Absorption, emission, and electrochemical properties of the tested dyes Dye labs(nm)a [ε (M1cm1] lPL a (nm) Stokes Shiftb (nm) Egc (eV) Eoxd (V) HOMOe (eV) LUMOf (eV) Cpd11 443 (99,000) 557 114 2.45 1.15 5.70 3.25 Cpd16 386 (35,700) 624 149 2.19 0.71 5.26 3.07 476 (75,000) Cpd22 374 (58,000) 610 135 2.15 0.77 5.32 3.17 473 (77,000)
aAbsorption and PL emission wavelengths recorded in dilute THF solution
(105M) at room temperature.
bStokes shift calculated from the difference betweenl absandlPL. cOptical band gap obtained from the equation E
g opt¼1240/l
onset. dE
oxis the oxidation potential. eE
HOMO¼[(Eox0.25)4.8] eV, where 0.25 V is the value for ferrocene versus Ag/
Agþ and 4.8 eV is the energy level of ferrocene below the vacuum.
fE
donor moieties and, therefore, decreased the degree of charge transfer.41,42Furthermore, in comparison with conventional ruthe-nium complexes (e.g., N3;ε¼1.52104M1cm1),4the molar
ex-tinction coefficients of the dyes are relatively large, indicating that they have good light harvesting ability. The bathochromic shifts upon proceeding from Cpd11 to Cpd16 (33 nm) and from Cpd11 to Cpd22 (30 nm) presumably resulted from the extended
p
-conju-gations.Fig. 2b reveals that when THF solutions of Cpd11, Cpd16, and Cpd22 were excited at 443, 475, and 475 nm, respectively, the resulting PL spectra featured weak emissions with Stokes shifts in the range 114e148 nm, with the PL emissions of dyes following similar trends to those in their absorption spectra.2.2. Electrochemical properties
The electrochemical properties of dyes can be obtained using cyclic voltammetry (CV);Table 1andFig. 3present the relevant CV data and representative cyclic voltammograms, respectively, for Cpd11, Cpd16, and Cpd22. We determined the HOMO energy levels of these dyes from their corresponding irreversible oxidation peaks. The cyclic voltammograms of the dyes Cpd11, Cpd16, and Cpd22 featured irreversible oxidation waves with oxidation potentials of 1.15, 0.71, and 0.77 V, respectively (Fig. 3). The HOMO energy level of a dye must be more positive (>0.3 eV) than the electrolyte iodine redox potential if it is to accept electrons effectively.43e46 The HOMO energy levels of our dyes were in the range from5.70 to 5.26 eV with respect to the I/I3redox couple (4.60 eV vs vac-uum). Because of the absence of reduction peaks, we could not determine the LUMO energy levels of these dyes from the CV traces, but we could elucidate them by subtracting the optical band gaps from the HOMO energy levels. The resulting LUMO energy levels, in the range from3.07 to 3.25 eV, are higher than the conduction band edge (4.0 eV vs vacuum); therefore, the electron injection process is energetically favorable. Relative to Cpd11 (containing only a simple fused dithienothiophene spacer), the dyes Cpd16 (with one more bithiophene unit) and Cpd22 (with one more bithiazole unit) both had smaller oxidation potentials (Eox). Therefore, the shortest spacer in Cpd11 (possessing the shortest conjugation length) resulted in it having the highest oxidation potential and the largest optical band gap (Eg) among our tested dyes. The optical band gaps of Cpd11, Cpd16, and Cpd22 were 2.45, 2.19, and 2.15 eV, respectively, which suggested that Cpd22 possibly might have a higher PCE value. However, in contrast to Cpd11 (without a donor bithiophene linkage) and Cpd22 (with an ac-ceptor bithiazole linkage), the improved electron injection of Cpd16
might be arisen from the donor bithiophene linkage and hence to achieve a higher PCE value. On the other hand, since the HOMO and LUMO levels of Cpd16 obtained from CV measurements were found to be higher than those of Cpd11 and Cpd22 inTable 3, and hence a greater electron injection process was favorable to obtain a higher PCE efficiency in later photovoltaic measurements.
2.3. Mesomorphic properties
Table 2lists the phase transition temperatures and enthalpies of the dyes Cpd11, Cpd16, and Cpd22, as characterized using differ-ential scanning calorimetry (DSC). The polarizing optical micros-copy (POM) image inFig. 4reveals that Cpd16 possessed a tilted
Fig. 3. Cyclic voltammograms of the dyes Cpd11, Cpd16, and Cpd22 (in THF), recorded at a scan rate of 100 mV s1.
Table 2
Phase transition temperatures and enthalpies of the dyes Cpd11, Cpd16, and Cpd22 Phase transition (C, [DH (J g1)])a,b Cpd11 Heating Cr1105.83[3.67] Cr2155.7[6.01] Cr3165.65[8.3]Iso Cooling Iso 146.21[7.88] Cr3130.46[1.9] Cr287.83[3.17] Cr1 Cpd16 Heating Cr1182.99[4.9] Cr2218.12[19.34] SmC 242.52[2.01]Iso Cooling Isoc235.2 SmC 175.10[2.04] Cr 2142.78[1.54] Cr1 Cpd22 Heating Cr 202.91[4.7]Iso Cooling Iso 194.97[3.3] Cr
aData determined through DSC from second heating/first cooling run at a
scan-ning rate of 5C min1. b Abbreviations: Cr
13, different crystalline modifications; SmC, tilted smectic
phase; Iso, isotropic liquid state.
c Isotropic temperature determined through POM.
Fig. 4. (a) Optical texture of the nematic phase in the dye Cpd16 at 225C (cooling), observed through POM, and (b) XRD intensity plotted with respect to angle for the dye Cpd16 at 225C.
smectic (SmC) phase, with a broken focal conic fan texture at 225 C. The mesomorphic properties of Cpd16 were confirmed using powder X-ray diffraction (XRD); after thermal annealing at 225C for 10 min, Cpd16 exhibited a primary diffraction feature in the low angle region ofFig. 4, with a sharp peak at a value of 2
q
of 1.3 (corresponding to a d-spacing of 50.6 A). Fig. 5 presents a possible packing motif (side-view) for Cpd16; this model suggests that the dye molecules stacked with bilayer packing and may have trivial interdigitated arrangements as a result of hydrogen bonding interactions between terminal carboxyl (COOH) units. Using Chemdraw software for simulation, we calculated the theoretical molecular length of Cpd16 to be 38.44 A; its hydrogen-bonded dimer would, therefore, have a length ofw75 A. The d-spacing of 50.6 A for Cpd16, determined using XRD, suggested a tilted smectic molecular arrangement (e.g., SmC phase) inFig. 5. The broad peak in the wide angle region at a value of 2q
of 15inFig. 4corresponds to a d-spacing of 4.6 A, which we assign to the lateral distance between the conjugated backbones, as has been reported for other similarp
-conjugated polymers presenting long pendants,47e49 al-though this broad peak might also have contained some contri-butions from the lateralp
ep
stacking of the dye planes.50The broad XRD halos inFig. 4suggest, however, thatp
ep
stacking in Cpd16 occurred only in very small areas; that is, it mainly possessed an amorphous structure.512.4. Photovoltaic properties of DSSCs
Fig. 6displays incident photon-to-current conversion efficiency (IPCE) and photocurrentevoltage (IeV) curves of DSSCs based on the dyes Cpd11, Cpd16, Cpd22, and N719. FromFig. 6b, we char-acterized the photovoltaic parameters of the DSSCs (Table 3), namely their open-circuit photovoltages (VOC), short-circuit pho-tocurrent densities (JSC), fill factors (FFs), and solar-to-electrical energy conversion efficiencies (
h
). The power conversion ef ficien-cies (PCEs, i.e.,h
) followed the trend Cpd16 (h
¼3.72)>Cpd22 (h
¼2.82)>Cpd11 (h
¼2.69). The highest PCE was that of the DSSC incorporating Cpd16, mainly because it had the highest short cur-rent density (Jsc¼9.98 mA cm2), which reveals more electrons were transferred from the excited state of the dye and injected into the conduction band of TiO2; the DSSCs incorporating the three dyes each had similar values of Vocand FF. The IPCE spectrum of Cpd16 (Fig. 6a) featured the broadest response in the range 300e750 nm with a maximum IPCE value of 64%; this behavior is consistent with its DSSC having the highest PCE (h
¼3.72; with VOC¼0.58 V; JSC¼9.98 mA cm2; and FF¼0.65). Thus, the highest PCE (h
¼3.72) for the device incorporating Cpd16 resulted from itshigh short current intensity (Jsc¼9.98 mA cm2) and broadest and most-intense IPCE spectrum (toward the longer wavelength re-gion), both of which presumably resulted from the longer conju-gated structure induced by this dye’s additional bithiophene linker. The different PCE values of Cpd11, Cpd16, and Cpd22 may be at-tributed to the following reasons; (i) Cpd16 and Cpd22 were bridged through a donor bithiophene linkage and an acceptor bithiazole linkage, respectively, in contrast to Cpd11; (ii) the steric effect induced by the lateral alkyl chains of the bithiazole (a) unit might affect the conjugation of Cpd22; (iii) as noticed inFig. 4, the greater packing nature of Cpd16 enhanced the electron injection to TiO2and also enhanced JSCto obtain a higher PCE value.
3. Conclusion
We have synthesized three new metal-free organic dyes (Cpd11, Cpd16, and Cpd22), each featuring a tris(dodecyloxy)phenyl moiety (a common unit in liquid crystalline structures) as an electron
Fig. 5. Molecular model of the dye Cpd16.
Fig. 6. (a) IPCE plots and (b) IeV curves of DSSCs fabricated using the dyes Cpd11, Cpd16, Cpd22, and N719.
Table 3
Cell performance of Cpd11, Cpd16, Cpd22, and N719-sensitized solar cells DSSC dyea V oc(V) Jsc(mA cm2) FF (%) h(%) Cpd11 0.57 6.85 0.70 2.69 Cpd16 0.58 9.98 0.65 3.72 Cpd22 0.60 6.77 0.70 2.82 N719 0.70 15.41 0.65 7.04
donor, a cyanoacrylic acid moiety as an electron acceptor/anchoring group, and a DTT-based spacer to bridge the donor and acceptor moieties. To extend the length of conjugation, we appended a bithiophene or bithiazole moiety to the DTT unit to enhance the capacity for charge transfer and increase the range of absorption. The dye Cpd16 exhibited mesomorphic properties, resulting from the appropriate ratio of the lengths of itsflexible chain to its rigid core; molecular modeling of Cpd16, and its d-spacing value de-termined using XRD, verified the existence of a tilt angle in the SmC phase. In addition, among the tested dyes, the DSSC exhibiting the best performance was that incorporating Cpd16, presumably be-cause of its superior packing as a result of its mesomorphic prop-erties. This DSSC exhibited a maximum PCE of 3.72% (Voc¼0.58 V; Jsc¼9.98 mA cm2; FF¼0.65) under simulated AM 1.5 irradiation (100 mW cm2).
4. Experimental section 4.1. General information
4.1.1. Materials. Chemicals and solvents were of reagent grade and purchased from Aldrich, ACROS, TCI, or Lancaster Chemical. Tol-uene, tetrahydrofuran (THF), dimethylformamide (DMF), and dichloromethane (DCM) were dried and distilled prior to use. N-Bromosuccinimide (NBS) was recrystallized from distilled water and dried under vacuum. All other chemicals were used without further purification. The synthetic routes for all dyes are presented inScheme 1. 1-Bromo-1,2,3-tris-n-dodecyloxybenzene (1),15 tri-n-butyl(dithieno[3,2-b:20,30-d]thiophen-2-yl)stannane (2),32
5-(5-bromothiophen-2-yl)thiophene-2-carbaldehyde (6),11,27 and
2-(5-bromo-4-hexylthiazol-2-yl)-4-hexylthiazole-5-carbaldehyde (7)26,43were synthesized according to literature procedures. 4.1.2. Synthesis
4.1.2.1. 2-(3,4,5-Tris-n-dodecyloxyphenyl)dithieno[3,2-b:20,30-d] thiophene (3). A solution of 1 (2.34 g, 1.00 mmol) and 2 (1.60 g, 3.29 mmol) in dry toluene (50 mL) in a 100-mL, three-neck, round-bottom flask was deoxygenated with N2 for 30 min. Pd(PPh3)4 (110 mg, 0.02 mmol) was added and then the mixture was heated at 110C for 2 days. The organic layer was extracted with DCM; the extracts were dried over anhydrous MgSO4. Column chromatog-raphy (SiO2; hexanes/CH2Cl2, 3:1) provided the title compound (55.6%).
4.1.2.2. 6-[3,4,5-Tris(dodecyloxy)phenyl]dithieno[3,2-b:20,30-a] thiophene-2-carbaldehyde (4). A 250-mL three-neck flask contain-ing anhydrous DMF (0.1 mL, 1 mmol) was cooled in an ice bath and then POCl3(0.1 mL, 5 mmol) was added dropwise over 30 min. A solution of 3 (0.82 g, 0.1 mmol) in 1,2-dichloroethane (30 mL) was added to the solution and then the mixture was heated at 90C for 24 h. This solution was cooled to room temperature, poured into ice water, and neutralized to pH 6e7 through dropwise addition of saturated aqueous NaOH. The mixture was partitioned between CH2Cl2and water. The organic layer was dried (MgSO4) and con-centrated under reduced pressure. The crude product was purified through column chromatography SiO2; (CH2Cl2/hexane, 1:1) to give a yellow solid (0.7 g, 82%).
4.1.2.3. 6-[3,4,5-Tris(dodecyloxy)phenyl]-2-tributylstannyl dithieno [3,2-b:20,30-a]thiophene (5). n-BuLi (2.5 M in hexane, 0.59 mL, 1.5 mmol) was added over 1 h to a stirred solution of 3 (0.83 g, 0.1 mmol) in dry THF (20 mL) in a 250-mLflask at 78C. The mixture was then warmed slowly (1 h) to room temperature under an ambient environment with stirring. After the mixture was had been re-cooled to78C, Bu3SnCl (0.53 mL, 1.2 mmol) was added slowly. The mixture was then stirred at ambient temperature for 18 h, followed by the addition of water (100 mL). The aqueous phase was extracted with CH2Cl2(200 mL); the combined organic phases were dried (MgSO4) and concentrated under a reduced pressure. The crude product was purified through column chromatography (SiO2; CH2Cl2/hexane, 2:5) to give a pale-yellow oil.
4.1.2.4. 5-(5-6-[3,4,5-Tris(dodecyloxy)phenyl]dithieno[3,2-b:20,30 -a]thiophylthiophen-2-yl)thiophene-2-carbaldehyde (8). A 250-mL two-neck flask containing 5 (1.1 mmol, 0.85 g), 6 (0.77 mmol, 0.21 g), and Pd(PPh3)4(0.03 mmol, 0.026 g), in toluene (15 mL) was heated at 90C for 24 h. The mixture was the partitioned between CH2Cl2and water. The organic phase was dried (MgSO4) and con-centrated under reduced pressure. The residue was purified through column chromatography (SiO2; CH2Cl2/hexane, 1:1) to give a red-orange solid (0.5 g, 64.5%).
4.1.2.5. 5-(5-6-[3,4,5-Tris(dodecyloxy)phenyl]dithieno[3,2-b:20,30 a ] t h i o p h e n yl 2 ( 4 h e x yl t h i a z o l 2 yl ) 4 h e x yl t h i a z o l e 2 -carbaldehyde) (9). A 250-mL, two-neck flask containing 5 (0.85 g, 1.1 mmol), 7 (0.21 g, 0.77 mmol), and Pd(PPh3)4 (0.026 g, 0.03 mmol), in toluene (15 mL) was heated at 90C for 24 h. The mixture was partitioned between CH2Cl2and water and then the organic phase was dried (MgSO4) and concentrated under reduced pressure. The residue was purified through column chromatogra-phy (SiO2; EtOAc/hexane, 1:10) to give a red-orange solid (0.5 g, 64.5%).
4.1.2.6. 3-{6-[3,4,5-Tris(dodecyloxy)phenyl]dithieno[3,2-b:20,30-a] thiophene-2-yl}-2-cyanoacrylic acid (Cpd11). A mixture of 4 (0.70 g, C12H25O C12H25O C12H25O Br 1 S S S SnBu3 C12H25O C12H25O C12H25O S S S 3 C12H25O C12H25O C12H25O S S S 4 CHO C12H25O C12H25O C12H25O S S S S S CHO S S CHO Br C12H25O C12H25O C12H25O S S S SnBu3 N S N S C6H13 C6H13 CHO Br C12H25O C12H25O C12H25O S S S N S N S C6H13 C6H13 CHO 9 + 2 i ii iii 6 5 7 iv v 8 4, 8, 9 vi Cpd11, Cpd16, Cpd22
Scheme 1. Synthetic routes of dyes. (i, iv, v) Pd(PPh3)4, toluene, reflux. (ii) POCl3, DMF,
0.08 mmol), cyanoacetic acid (0.12 g, 1.41 mmol), ammonium ace-tate (0.063 g, 0.08 mmol), and glacial AcOH (60 mL) was heated overnight at 110C with efficient stirring. The red solution was cooled to induce a precipitate, which was filtered off and thor-oughly washed with water and MeOH, to give a red solid (79.8%).
4.1.2.7. 3-{5-(5-6-[3,4,5-Tris(dodecyloxy)phenyl]dithieno[3,2-b:20,30 -a]thiophylthiophen-2-yl)thiophenyl}-2-cyanoacrylic acid (Cpd16). Pre-pared, using the same procedure as that described for Cpd11, as a dark-brown solid (73.2%).
4.1.2.8. 3-{5-(5-6-[3,4,5-Tris(dodecyloxy)phenyl]dithieno[3,2-b:20,30 -a]thiophyl}-2-(4-hexylthiazol-2-yl)-4-hexylthiazole-2-cyanoacrylic acid) (Cpd22). Prepared, using the same procedure as that described for Cpd11, as a dark-brown solid (73.2%).
4.1.3. Measurement and characterizations. 1H NMR spectra were recorded using a Varian unity 300 MHz spectrometer, with DMSO-d6and CHCl3as solvents. Elemental analyses were performed using a HERAEUS CHNeOS RAPID elemental analyzer. UVevis absorption spectra of dilute THF solutions (105M) were recorded using an HP G1103A spectrophotometer; photoluminescence (PL) spectra were recorded using a Hitachi F-4500 spectrophotometer. Cyclic vol-tammetry (CV) was performed at room temperature using a BAS 100 electrochemical analyzer, a standard three-electrode
electro-chemical cell, and a 0.1 M tetrabutylammonium
hexa-fluorophosphate (TBAPF6) solution (in THF), with a scanning rate of 100 mV s1. During CV measurements, the solutions were purged with N2for 30 s. In each case, a carbon coating rod was the working electrode, a platinum wire was the counter electrode, and a silver wire was the quasi-reference electrode; a Ag/AgCl (3 M KCl) elec-trode served as the reference elecelec-trode for all potentials quoted herein. The redox couple of ferrocene/ferrocenium ion (Fc/Fcþ) was used as an external standard. The corresponding HOMO and LUMO energy levels were calculated from the onset oxidation potential (Eox/onset) and UVevis absorption edge (Egopt), respectively. Meso-phasic textures were characterized through POM using a Leica DMLP equipped with a hot stage.
4.1.3.1. XRD characterization. Synchrotron powder XRD was performed at beamline BL17A of the National Synchrotron Radia-tion Research Center (NSRRC), Taiwan; the wavelength of the X-rays was 1.33366 A. The powder samples were packed into a capillary tube and heated with a heat gun, the temperature controller of which was programmed by a PC with a PID feedback system. The scattering angle,
q
, was calibrated using a mixture of silver behenate and silicon.4.1.3.2. TiO2 paste preparation. The TiO2 precursor and the electrode were fabricated using previously reported procedures34 with an autoclave temperature of 240C. The precursor solution was prepared according to the following procedure: 0.1 M HNO3 (430 mL) under vigorous stirring was slowly combined with Ti(C3H7O)4 (72 mL). After hydrolysis, the mixture was heated at 85 C in a water bath and stirred vigorously for 8 h to achieve peptization. The mixture was cooled to room temperature and the resultant colloid wasfiltered; the filtrate was then heated in an autoclave at 240 C for 12 h to grow the TiO2 particles. The colloid was cooled to room temperature and vibrated ultrasonically for 10 min. The TiO2 colloid was concentrated to 13 wt %, followed by the addition of 30 wt % (with respect to TiO2weight) of poly(ethylene glycol) (PEG; MW¼20,000 g mol1) to prevent the film from cracking while drying.
4.1.3.3. Device fabrication. The TiO2 paste was deposited on a FTO glass substrate (dimensions: 0.50.5 cm2) using the glass rod
method. Polyester tape (3 M) was used as an adhesive on two edges of the FTO glass. The tape was removed after the TiO2paste had been spread on the FTO using a glass rod and then the TiO2paste was dried in air at room temperature for 1 h. The TiO2-coated FTO was heated to 500 C at a heating rate of 10C min1and then maintained at that temperature for 30 min before cooling to room temperature. After repeating the procedure above to control the thickness of the TiO2film, the final coating was performed using TiO2 pastes containing different sizes (300 and 20 nm; 30 and 70 wt %, respectively) of light-scattering TiO2particles; the samples were then heated at 500C. The thicknesses of the TiO2films were
measured using a profilometer (Dektak3, Veeco/Sloan
In-struments). The density of each adsorbed dye was calculated from the concentration difference of each solution before and after TiO2 film immersion. The TiO2 electrode with a geometric area of 0.25 cm2was immersed overnight in a MeCN/tert-butanol (1:1, v/v) solution of 3104 M cis-di(thiocyanato)bis(2,20-bipyridyl-4,40 -dicarboxylato)-ruthenium(II) bis(tetrabutylammonium) (N719, Solaronix SA) or in a THF solution containing 3104M organic sensitizers. Thermally platinized FTO was used as the counter electrode; its active area was controlled to 0.36 cm2using adhered polyester tape having a thickness of 60
m
m. After rinsing with MeCN or THF, the photoanode was placed on top of the counter electrode and tightly clipped together to form a cell. Electrolyte was injected into the space and then the cell was sealed with Torr Seal cement (Varian). The electrolyte comprised 0.5 M LiI, 0.05 M I2, and 0.5 M 4-tert-butylpyridine (TBP) in MeCN. The photovoltage transients of the assembled devices were recorded using a digital oscilloscope (LeCroy, WaveSurfer 24Xs). Pulsed laser excitation was applied using a Q-switched Nd:YAG laser (Continuum, model Minilite II) operated at 532 nm, with a 1 Hz repetition rate and a 5-ns pulse width at half-height. The beam size was slightly larger than 0.50.5 cm2to cover the area of the device. The photovoltage of each device was adjusted by incident pulse energy to be 40 mV.4.1.3.4. Device measurements. A 0.60.6 cm2 cardboard mask was clipped onto the device to constrain the illumination area. Photoelectrochemical characterization of the DSSCs was performed using an Oriel Class A solar simulator (Oriel 91195A, Newport). The photocurrentevoltage characteristics of the DSSCs were recorded using a potentiostat/galvanostat (CHI650B, CH Instruments) at a light intensity of 1.0 sun, calibrated using an Oriel reference solar cell (Oriel 91150, Newport). The monochromatic quantum ef fi-ciency was recorded through a monochromator (Oriel 74100, Newport) under short-circuit conditions. The intensity of each wavelength was in the range from 1 to 3 mW cm2.
Acknowledgements
The powder XRD measurements are supported by beamline BL17A1 (charged by Dr. Jey-Jau Lee) of the National Synchrotron Radiation Research Center (NSRRC) in Taiwan. Thefinancial sup-ports of this project provided by the National Science Council of Taiwan (ROC) through NSC 2113-M-009-006-MY2, NSC 99-2221-E-009-008-MY2, and National Chiao Tung University through 97W807 are acknowledged.
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