Department of Chemistry, Pusan National University, Busan 609-735, South KoreaBusan Center, Korea Basic Science Institute, Busan 609-735, South Korea
r t i c l e i n f o
rticle history:eceived 26 December 2011eceived in revised form 8 February 2012ccepted 10 February 2012vailable online 19 February 2012
eywords:lectrochemistry
a b s t r a c t
A novel conjugated polymer bridged a benzene ring between polymer backbone and carboxylic acid,poly(2,2′:5′,2′′-terthiophene-3′-p-benzoic acid) (pTTBA) has been synthesized and applied to an elec-trochromic device and a polymer-sensitized solar cell. The pTTBA is characterized as a photo sensitizer ona TiO2 layer, and the cell performance is compared with that of poly(5,2′:5′,2′′-3′-carboxylic acid) (pTTCA),which does not contain the benzene ring. The conductivity of pTTBA is determined to be 0.24 S cm−1 at1.4 V. The spectroelectrochemical study shows that pTTBA switches to an opaque dark-blue in its oxidizedstate from transmissive yellow in its reduced state with a contrast of 53%. The polymer film exhibits a good
optical switching time within 0.82 s. The pTTBA film has a band gap energy of 1.98 eV. The impedanceresults show that the pTTBA is the enhanced electron transfer process in the TiO2/polymer/electrolyteinterface in comparison with that of pTTCA. A photovoltaic cell is assembled with pTTBA coated on theTiO2 electrode as an anode and Pt-counter electrode as a cathode. The electrolyte is used a propionitrilesolution containing I−/I3
− as a redox couple. The higher energy conversion efficiency was achieved withthe pTTBA based dye solar cell (3.97%) (active layer = 0.24 cm2).
. Introduction
Of the various �-conjugated polymers, polythiophene (PT) isne of the most important and widely studied polymers becausef its high stability in air and potential applications [1,2]. PTs haveeen used in a variety of applications such as organic field-effectransistors (OFETs) [2], polymer light-emitting diodes (PLEDs) [3],olar cells [4], and chemical sensors [5], because of their ability toe processed, environmental stability, thermal stability, and goodlectronic properties. However, the high oxidation potential of theonomers and the �–� coupling between the thiophene rings dur-
ng polymerization cause these polymers to have poor chemicalnd physical properties. To overcome this limitation, 3-substitutednd 3,4-disubstituted PTs have been synthesized using differentunctional groups [6,7]. However, the resultant structures mustontain large numbers of thiophene ring that are twisted far outf conjugation as a result of steric hindrance between 3- or/and,4-substituents (functional groups). To avoid such problems, we
ried to synthesize terthiophenes containing a functional groups a unit of polymer backbone instead of thiophene. These com-ounds can be easily applied for use in the electrochromic devices,
solar cell electrode materials, and other various applications. Wepreviously studied the applications of 3′-substituted terthiophenederivatives [8–11]. These polymers show chemical stability in theiroxidized states, high optical contrast values, and fast switchingtimes between redox states. The electrochemical and spectroscopicproperties of these polymers also vary significantly according to thedegree of extended conjugation between the consecutive repeatedunits. Controlling the band gap of conjugated polymers, one impor-tant property, is an essential application for the electrochromismdevices and solar cell systems. The band gap is controlled by fivecontributions via bond-length alternation, aromaticity, conjugationlength, substituent effects, and intermolecular interactions relatedto the conjugated polymer backbone [12]. Since the emitted light(color) depends on the band gap of the �–�* transition; there-fore, the modification of the structure will affect the electronicproperties, which will consequently change the emitted color andintensity.
It has been previously demonstrated that PTs can be applied inelectrochromic devices (ECDs) on the basis of their well-definedelectrochromic behaviors [13]. Most ECDs materials can be appliedin a variety of photovoltaic devices. Thus, it is interesting to applythese polymers for use in the electrochromic devices and dye sen-
sitized solar cells (DSSCs) [10,14,15]. Of the available solar cells,DSSCs based on nanocrystalline TiO2 is an essential type of pho-tovoltaic cell, since the properties of photosensitizers, such as theabsorptivity of photons in the visible region (absorption color) and
he electron-transfer rate are important factors in determining thehotovoltaic cell performance. Recently, the interest in polymer-ensitizers, such as polythiophenes, is increasing because of theirarious advantages, including their diverse of molecular structure,nd high molar extinction coefficient. To date, the highest energyonversion efficiency of these polymer dye cells is approximately.32% using pTTCA [10]. Although the energy conversion efficiencyf these cells is lower than that of ruthenium complex dyes [16], itas potential applicability to the solar cell because of the increas-
ng efficiency through the change of structure of these polymers10,14,17]. Thus, we tried to improve the low energy conversionfficiency of the cells using a polyterthiophene backbone bearing aarboxylic acid bridged with a conjugated benzene ring. The role ofhe carboxylic acid groups was very strongly absorbed onto the TiO2ayer through the covalent C O Ti bond formation [18]. The TTBAonsists of the terthiophene backbone moiety acting as an electrononor and carboxylic acid moiety acting as an acceptor, and thewo functional groups are connected by a �-conjugated benzeneing. A conjugated benzyl moiety can help enhance electron trans-er from terthiophene backbone moiety to carboxylic acid moiety.urthermore, this configuration is expected to be more reactiveith resonance effect, significantly reduces steric hindrance with
ulky group of terthiophene moiety, and avoids the aggregation ofonjugate polymer dye on the TiO2 surface, which contributes thenhancement of the electron transfer property.
In the present study, we synthesized and characterized a 3′-ubstituted polyterthiophene derivative (pTTBA) containing anlectro-withdrawing p-benzoic acid group as a potential materialor solar cells. The structure of the monomer was analyzed by 1HMR, 13C NMR, FT-IR and mass spectroscopy. The resulting polymerlm was electrochemically and spectroelectrochemically charac-erized by cyclic voltammetry (CV), conductivity, in situ UV–vispectroscopy, and electrochromic behavior studies. The solar cellerformance of pTTBA was compared with pTTCA, which does notontain the benzyl moiety in the polymer backbone structure. Elec-rochemical impedance spectroscopy (EIS) was also employed tonvestigate the charge-transfer processes of polymer dye solar cells.
e have studied its performance both as a polymer photovoltaicell and as an electrochromic display function.
. Experimental
.1. Chemicals
All chemical regents were purchased from Sigma–Aldrich Co.USA), and were used as received. All aqueous solutions wererepared in double distilled water, which was obtained from ailli-Q water-purifying system (18 M� cm). 3′-Bromo-2,2′:5′,2′′-
erthiophene (BTT) was synthesized by the method in the literature13]. All reactions and manipulations were carried out under N2ith the use of standard inert atmosphere and Schlenk techniques.hromatography for product separation and purification was car-ied out using silica gel for flash columns, 70–230 mesh. Theuorine-doped SnO2 (FTO, 2.2 mm 15 � sq−1), TiO2 (Ti-NanoxideT), the electrolyte (Iodolyte PN-50), the Pt paste (platisol), and
he hot-melt film (SX 1170-60) were purchased from SolaronixSwitzerland).
.2. Measurements
The 1H and 13C NMR spectra were obtained in deuterated chlo-
oform (CDCl3) using a Bruker Advance 300. Chemical shifts areiven in parts per million (ppm) with tetramethylsilane (TMS)s an internal standard. Mass spectra were recorded on a JMS-00 double focusing mass spectrometer (JEOL, Japan) The FT-IR
Acta 67 (2012) 201– 207
spectrum was recorded using a JASCO FT-IR spectrometer. Theabsorption spectrum was recorded in a dilute dichloromethanesolution with a Shimadzu UVPC-3101 spectrometer. The emissionspectrum was measured in a dilute dichloromethane solution witha PerkinElmer LS50B Fluorescence spectrophotometer. The atomicforce microscopy (AFM) images were obtained in ambient condi-tions using a Multimode AFM device (Veeco Metrology) equippedwith a Nanoscope IV controller (Veeco). The cyclic voltammetry(CV) was performed using a potentiostat/galvanostat, Kosentech,Model PT-2 (South Korea). A quartz crystal microbalance (QCM)experiment was conducted using a SEIKO EG&G model QCA 917and a PAR model 263A potentiostat/galvanostat. All measure-ments were carried out at room temperature with a conventionalthree-electrode configuration consisting of a platinum (Pt) workingelectrode, a platinum wire counter electrode, and Ag/AgCl referenceelectrode. The solvent in all experiments was dichloromethane, andthe supporting electrolyte was 0.1 M tetrabutylammonium per-chlorate (TBAP) dried at a vacuum oven (1.33 × 10−3 Pa). In situUV–vis spectroscopic spectra were obtained from a UV–vis spec-trometer assembled to a CCD detector, a xenon flash lamp, and abifurcated optical fiber from Ocean Optics Co. An electrochemicalcell with a quartz window was used for the in situ experiment. Themethod for obtaining the absorption spectra was the same as theone previously report in the literature [19]. The impedance spectrawere measured with the EG&G Princeton Applied Research PAR-STAT 2263 at an open circuit voltage from 100 kHz to 100 mHz at asampling rate of five points per decade (AC amplitude: 10 mV). Thephotovoltaic measurement was performed using an air mass (AM)1.5 solar simulators that were equipped with a 150 W xenon lamp(Model 92251A, NewPort). The power of the simulated light wascalibrated to 100 mW cm−2 using a reference Si photodiode, whichwas measured at the solar-energy institute (NREL, USA). The short-circuit photocurrent of the reference Si solar cell was calibrated tothe average data from NREL under the one-sun condition. CurvesI–V were obtained by measuring the generated photocurrent usinga Keithley Model 2400 digital source meter.
2.3. Synthesis of monomer
3′-Bromo-2,2′:5′,2′′-terthiophene (BTT) was prepared accordingto a previously reported method [13]. A degassed diethyl ethersolution (15 mL) of BTT (3.27 g, 10.0 mmol) was slowly reactedwith tetramethylethylenediamine (TMEDA) (1.8 mL, 12.0 mmol)and 1.6 M n-BuLi (7.5 mL, 12.0 mmol) under N2 atmosphere at−83 ◦C. The reaction mixture was stirred for 1 h at −83 ◦C, andtrimethyl borate (2.2 mL, 20.0 mmol) diluted in diether ether(30 mL) was added. The mixture was allowed to warm to room tem-perature and was stirred for 4 h after 2 M HCl (20 mL) was added.After 1.5 h of additional stirring, crude 2,2′:5′,2′′-terthiophene-3′-boronic acid (TTB) was precipitated out in white solids, washedwith water and dried under reduced pressure. The crude TTB, 4-bromobenzonitrile (1.7 g, excess), Pd(PPh3)4 (0.116 g, 1.0 mol%),toluene (20 mL) and a nitrogen degassed aqueous solution of 2 MK2CO3 (8 mL, 16.0 mmol) were added and stirred for 10 min underN2. The mixture was heated at 80 ◦C for 24 h and monitored via thinlayer chromatography (TLC) for reaction completion. Toluene wasthen evaporated using a rotary evaporator and the product wasextracted with dichloromethane, washed with water, and driedover MgSO4. The removal of the solvent afforded the crude prod-uct as brown oil, which was then precipitated out in methanol. Theresidue was purified by chromatography on silica gel eluted with
ydroxide (1.0 g, 17.8 mmol) in ethoxyethanol:water (5:1) wasefluxed for 24 h and acidified with excess hydrochloric acid (12 M).he mixture was allowed to cool, forming yellow precipitates. Therecipitate was filtered off washed with water, and dried in aacuum. Recrystallization from ethanol provided 2.93 g (80%) ofellow needles. Mp: 127–128.3 ◦C. IR (KBr) 250–3400 cm−1 (br,H), 1660 cm−1 (C O), 1510 cm−1, 1417 cm−1 (C S in thiophene);H NMR (CDCl3) ı: 6.22 (br, s, 1H), 6.91–7.08 (m, 2H), 7.17–7.28m, 3H), 7.50 (d, 1H), 7.83 (d, 1H); 13C NMR ı: 124.7, 124.7, 126.1,26.8, 127.2, 127.4, 127.7, 128.5, 128.8, 129.8, 133.4, 134.1, 135.2,35.4, 137.9, 138.6, 167.4; MS (EI) mz−1 = 368 (M+).
.4. Photovoltaic cell fabrication
A transparent nanocrystalline layer was formed on the FTO glasslate using the doctor blade-printing TiO2 paste and dried for 1 ht 25 ◦C. Then, the layer was heated under airflow for 30 min at50 ◦C. Afterward, the TiO2 electrodes were immersed in a 50 mMiCl4 aqueous solution, dried at 70 ◦C for 30 min, and sintered at50 ◦C for 30 min. The thickness of the TiO2 film coated on the FTOlass was 3.5 �m. The TiO2/FTO electrode was used as the work-ng electrode, Ag/AgCl (in saturated KCl) was used as the referencelectrode, and a platinum plate was used as the counter electrode.he electropolymerization of TTBA on the TiO2 electrode was sep-rately carried out in a 0.1 M TBAP/CH2Cl2 solution containing theonomer from a voltage of 0.0 to 1.4 V using the potential cyclingethod. The area of the active polymer layer was 0.24 cm2. The
t-counter electrode was deposited on the FTO glass by coatinghe surface with a drop of Pt solution. The surface was treated at50 ◦C for 15 min. The polymer-covered TiO2 and Pt-count elec-rodes were assembled into a sealed sandwich-type cell througheating at 100 ◦C using a hot-melt film with a thickness of 60 �m
n the space between the electrodes. A drop of the redox elec-rolyte was placed in the hole on the back of the counter electrode.he electrolyte was composed of an imidazolium iodide derivative,ithium iodide, and pyridine derivative in a propionitrile solution.
c route to pTTBA.
3. Results and discussion
3.1. Synthesis and spectroscopic properties
The starting material, BTT was synthesized from 2,3,5-tribromothiophene in a one-step procedure, as is illustrated inScheme 1. Then, BTT is treated with n-BuLi and trimethyl boratein Et2O under a N2 atmosphere at −83 ◦C to give TTB. The Suzukicoupling of TTB with 4-bromobenzonitrile in a refluxing two-phasesolution of toluene in the presence of K2CO3 and Pd(PPh3)4 afforded3′-(4-benzonitrile)-2,2′:5′,2′′-terthiophene (BNTT) in overall 48%yield. Then, BNTT with KOH in a refluxing mixture solution ofethoxyethanol/water (5:1) afforded TTBA in an overall 80% yield.The product is characterized by spectroscopic methods (FT-IR,NMR, and mass spectrophotometry), which corroborated the iden-tity of the proposed structure. In the FT-IR spectrum, BNTT showsa medium-intensity nitriles band and a C S stretching absorp-tion of thiophene at 2215 cm−1 and 1495 cm−1, respectively. TheTTBA exhibits a carbonyl stretching band at 1644 cm−1 and a broadhydroxy stretching band at 2500–3400 cm−1. In the 1H NMR spectraof TTBA, the proton of benzoic acid resonates at 6.22 ppm (car-boxylic acid proton) and 7.50–7.83 ppm (benzene ring protons). TheMS data of TTBA coincides with the expected formula.
The UV–vis absorption and PL emission spectra are recordedfor the TTBA monomer in a dichloromethane solution (Fig. 1). Theabsorption maximum is 344 nm for TTBA with an additional peakat the low-wavelength of 288 nm having a small side band, whichare attributed to the �–�* transition of the �-conjugated segmentsand benzoic acid units. The TTBA emits photons in the blue region(�max = 484 nm) with a large stoke shift of about 140 nm. Theseresults suggested that the intramolecular energy was transferredfrom thiophene-based terthiophene to the benzoic acid group.
3.2. Electropolymerization and characterization of pTTBA andpTTCA
The electrochemical behavior of TTBA was investigated usingcyclic voltammetry (CV). The electropolymerizations of the TTBAand TTCA monomers were carried out in a 0.1 M TBAP/CH2Cl2
204 D.-M. Kim et al. / Electrochimica
Fd
s1iaai+i
Fap
ig. 1. UV–vis absorption and photoluminescence spectra of TTBA inichloromethane solution.
olution containing 1.0 mM monomers from 0.0 to 1.4 V at00 mV s−1. The anodic electropolymerization of TTBA monomers
n CH2Cl2 is shown in Fig. 2(A). The CV exhibits an oxidation peakt about +1.3 V due to the monomer oxidation to form the polymer,
nd the polymer reduction peak immediately appears at +0.9 V dur-ng the cathodic scan. As shown in Fig. 2(A), the peak currents at0.9 V and +1.3 V increase with the potential cycle number, whichndicates the formation and growth of the polymer film. A blue
ig. 2. (A) CV for the electropolymerization of TTBA in a 0.1 M TBAP dichloromethane solut the redox peak current vs. scan rates. Inset: The scan rate increases from 10, 20, 30, 40,TTBA film; images size is 1.0 �m × 1.0 �m.
Acta 67 (2012) 201– 207
color film of an electrochemically active polymer is formed on thePt electrode surface, over the potential of +1.1 V in the anodic direc-tion during repetitive potential cycling in the potential region from0.0 to +1.4 V. Fig. 2(B) shows cyclic voltammograms recorded atdifferent scan rates for the polymer film in a 0.1 M TBAP blanksolution. The peak current is directly proportional to the scan rate,which indicates the involvement of the surface-confined species.This suggests that the thickness of the film is smaller than the dif-fusion layer thickness of counter anions on the cyclic voltammetrictime scale. The oxidation peaks shift to the more positive poten-tial at the scan rates higher than 10 mV s−1 because of the quasireversibility of the redox process. Similar results were observed forthe pTTCA polymer film in the previous report [13].
The morphology of the polymer film after electropolymerizationwas studied by atomic force microscopy (AFM) using the tappingmode. It shows a homogeneous composition of small particles ofthe polymer film. The particle size of pTTBA film is determined to be73.2 ± 22.3 nm. The difference in the root mean square (RMS) sur-face roughness between the pTTBA films was small, as is depictedin Fig. 2(C), which was 1.75 nm.
3.3. In situ conductivity
The in situ conductivity measurement was performed in adichloromethane solution at the various applied potentials. First,the resistance was measured for the polymer bridged across a∼20 �m gap between the split gold electrodes in a solution con-taining 0.1 M TBAP, while the potential was varied from 0.0 to
+1.4 V. The resistances and conductivity were plotted as a func-tion of the applied potential as shown in Fig. 3. In this case,when the potential increases from +0.5 to +1.0 V, the conductiv-ity is clearly increased; however, as the potential increases from
tion. (B) CVs recorded for the pTTBA film in a 0.1 M TBAP dichloromethane solution 50, 100, 150, 200, 250, 300 to 350 mV s−1 in the ascending order. (C) AFM image of
D.-M. Kim et al. / Electrochimica
Fp
+ptbcaa
of the polymer in a polaron state. In addition, an absorption band at
Fr
ig. 3. Resistance and conductivity of pTTBA measured as a function of appliedotential in dichloromethane containing 0.1 M TBAP.
1.0 to +1.4 V, the conductivity remains steady. In the polythio-hene system, the conductivity of the polymer film increases untilhe polaron/bipolaron population reaches a maximum and thenecomes constant when the quinoid form is generated [20]. The
onductivity of the pTTBA film is determined to be 0.24 S cm−1
t +1.4 V, which is 1.4 times higher than that of pTTCA without benzene ring in the structure.
ig. 4. In situ UV–vis absorption spectra of the pTTBA as a function of applied potential inate was 5 mV s−1. (C) Switching study for pTTBA film monitored at 798 nm, when double
Acta 67 (2012) 201– 207 205
3.4. In situ spectroelectrochemical behavior
Fig. 4(A) shows a series of in situ UV–vis absorption spectrarecorded for the polymer film in a 0.1 M TBAP solution withouta monomer, which is grown on the platinum electrode by cyclingthe potential from 0.0 to 1.4 V five times. The spectra are recordedwhile the potential is scanned from 0.0 to 1.4 V at a scan rate of5 mV s−1. As shown in Fig. 4(A), upon starting the oxidation, thepTTBA film exhibits a color change from yellow (at 0.0 V) to darkblue (at 1.4 V). A very strong absorption band appears at 472 nm,which corresponds to the �–�* transition of the photons for pTTBAgrown on the electrode, as shown in Fig. 4(B). The intensity of thisband gradually decreases as the applied potential becomes morepositive (from 0.0 V to +1.4 V), which means that the absorptionband at 472 nm is related to the electronic transition of the polymerfilm in the neutral state. When the potential reaches to 0.8 V, a newband corresponding to the polarons appears at about 847 nm. Themaximum absorption of this band shifts to a shorter wavelength of798 nm as the potential goes from 0.8 to 1.0 V. The shift of the max-imum absorption band may have occurred because of the stacking
798 nm decreases greatly and new bands appear at about 993 nmas the potential goes from 1.0 to 1.4 V, which is mainly related tothe formation of the dication (bipolaron). In the case of pTTCA,
0.1 M TBAP/CH2Cl2 solution where (A) 3D and (B) 2D spectrum. The potential scan step pulse was applied between 0.0 and +1.4 V.
206 D.-M. Kim et al. / Electrochimica Acta 67 (2012) 201– 207
FT
t�ttfpN
3
ardfitstTw
3
tu3oHbtr
Fig. 6. (A) Nyquist plots for pTTCA and pTTBA dye solar cell at open-circuit voltage(−0.65 V). Inset: Equivalent circuit for polymer dye solar cells. (B) Comparison of the
TT
ig. 5. Frontier molecular orbital of the HOMO and LUMO levels of (A) TTBA and (B)TCA.
he absorption bands at 430, 795, and 900 nm correspond to the–�* transition band, polaron, and bipolaron formations, respec-
ively. The red shift of absorption band of pTTBA compared withhat of pTTCA indicates that more conjugated polymer of pTTBAormed comprehensively. Generally, the bipolaron state formed onoly(terthiophene) film is known to be observed in the visible andIR region (over 1000 nm) of absorption spectra [6].
.5. Switching time
Switching time of the color change is one of the important char-cteristics of electrochromic devices, which is defined as the timeequired for switching between its neutral state (at 0.0 V) and oxi-ized state (at +1.4 V). While the applying potential to the polymerlm is switched, the percentage transmittance at 798 nm is moni-ored as a function of time, as shown in Fig. 4(C). When the doubletep pulse potential is applied to the polymer film from 0.0 to +1.4 V,he switching time is 0.82 s in both forward and reverse directions.he polymer film shows a faster switching time than the pTTCA,hich indicates a fast electronic state transition.
.6. Molecular orbital calculations
To get an insight into the molecular structure and electron dis-ribution of TTBA and TTCA, their geometries have been optimizedsing density functional theory (DFT) calculation at DFT/B3LYP/6-1G* level of theory using gauss view (5.0.8) program. The frontierrbital of the HOMO and LUMO of the TTBA is shown in Fig. 5. The
OMO orbital is delocalized over the � system at the terthiopheneackbone. In the case of LUMO orbital, electrons move the towardshe acceptor moiety from therthiophene moiety through a benzeneing to the compared with the TTCA, which is located at junction
able 1he fitting values of the equivalent circuit element solar cell sensitized by polymer dye (p
Rs , � Rp1, � Q1 Rp2, �
Y0, mhg n1
pTTCA 10.6 4.3 1.5 × 10−3 1.1 15.5
pTTBA 10.9 6.0 2.1 × 10−3 0.8 3.4
J–V characteristics of TTCA, and pTTBA as a sensitizer on a nanocrystalline TiO2 solarcell. The illumination intensity of 100 mW cm−2 with AM 1.5 and the active area of0.24 cm2 were applied.
between terthiophene and acceptor moiety. The LUMO orbital ofthe TTBA with a benzene ring shows a higher electron density dis-tribution than the TTCA without a benzene ring. Hence, the TTBAunits can more effective electron injection from the LUMO level tothe TiO2 conduction band in comparison to the TTCA.
3.7. Application to a polymer-sensitized solar cell
Electrochemical impedance spectroscopy (EIS) was employedto investigate the charge-transfer processes in the polymer dyesolar cells at the open-circuit voltage (−0.65 V) in the dark con-dition. Fig. 6(A) shows the Nyquist plots obtained from pTTCA-and pTTBA-dye solar cells. The first semicircles in the spec-tra correspond to the charge-transfer processes occurring at the
Pt/electrolyte (Rp1). The second semicircles are composed of twocomponents, which correspond to the charge-transfer process atthe TiO2/polymers/electrolyte interface (Rp2) and in the elec-trolyte (Rp3). From the fitted values in Table 1, the pTTCA dye
olar cell exhibits impedance values (Rp2) of 15.5 �, while theTTBA shows the smaller impedance value of 3.43 �. In the casef pTTBA, we observed the enhanced electron transfer processn the TiO2/polymer/electrolyte interface through the conjugatedromatic ring and well-oriented terthiophenes backbone in thetructure of benzoic carboxylated polyterthiophene. This result isxpected to improve the low energy conversion efficiency of theolymer dye solar cells in comparison with pTTCA.
From the in situ UV–vis spectroscopy and cyclic voltammetrytudies of pTTBA, we determined the HOMO, LUMO, and band gapnergy levels of pTTBA to be −5.53, −3.55, and 1.98 eV, respectively.he effects of electropolymerization parameters (monomer con-entration, scan rate, and number of cycles) for the polymer layersn the TiO2 were investigated according to preliminary optimiza-ion experiments [10]: TTBA monomer concentration at 4 mM, scanate at 100 mV s−1, and number of cycle at 6 cycles. To determine themount of dye onto the TiO2 layer, the quartz crystal microbalanceQCM) study with a TiO2 coated Au electrode was carried out underhe optimized condition. The amount of pTTBA and pTTCA wasetermined to be 3.15 × 10−7 mol cm−2, and 1.25 × 10−7 mol cm−2,espectively. The photocurrent density–voltage (J–V) curves of theTTBA-sensitized solar cells are shown in Fig. 6(B). To evaluate ouresults, a standard solar call fabricated by using a well known ruthe-ium complex (N3). The energy conversion efficiency of the solarell using N3 from our system was 8.59%, while those of Grätzeleported in 1993 was 10% [21]. The pTTBA-sensitized solar cellhows a short-circuit current of 10.47 mA cm−2, an open-circuitoltage of 0.59 V, and a fill factor of 64.23%. The energy conver-ion efficiency of 3.97% is obtained with a cell area of 0.24 cm2
nder an AM 1.5 solar simulated light irradiation of 100 mW cm−2.he stability of pTTBA sensitized solar cell showed almost noecrease in energy conversion efficiency after 2000 h at 40 ◦C.therwise, pTTCA without benzyl moiety exhibited much smallernergy conversion efficiency (2.32%) with short-circuit current of.78 mA cm−2 [10]. In the case of pTTBA, the carboxylate groupf the polymer film can effectively anchor on the surface of theanocrystalline TiO2. Moreover, a conjugated benzyl moiety canelp to enhance the intermolecular charge transfer between theiO2 surface and the polymer backbone.
. Conclusion
Poly(2,2′:5′,2′′-terthiophene-3′-p-benzoic acid) (pTTBA) is a
romising candidate for electrochromic device and polymer-ensitized solar cell applications because of its synthetic versatility,ast switching time, good stability, and high coloration efficiency.he cyclic voltammetry, in situ UV–vis, and in situ conductivity
[[
[
Acta 67 (2012) 201– 207 207
measurements of the pTTBA film show the unique properties of aconducting polymer. The spectroelectrochemical experiments forthe pTTBA distinguished the neutral state (�–�* transition) of theabsorption band at 472 nm and the oxidized states (the polaronand bipolaron states) of the absorption bands at 847 and 993 nm,respectively. The pTTBA film has the band gap energy of 1.98 eV,which can be used for an effective electron transporting mate-rial. The pTTBA exhibited higher energy conversion efficiency thanthat of pTTCA. In the case of pTTBA, a conjugated benzyl moi-ety can help enhance intermolecular charge transfer between TiO2surface and polymer backbone. The maximum energy conversionefficiency of the pTTBA solar cell is 3.97%. We believe that the mod-ified monomer structure would lead to more effective the use ofconducting polymers in photovoltaic cells.
Acknowledgement
This work was supported by the Researcher Program throughNRF grant funded by the MEST (2010-0029128).
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