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Performance improvement of dye-sensitizing solar cell by semi-rigid triarylamine-based donors

Chengyou Wang,Jing Li,Shengyun Cai,Zhijun Ning,Dongmei Zhao,Qiong Zhang,Jian-Hua Su *

Key Laboratory for Advanced Materials and Institute of Fine Chemicals,East China University of Science and Technology,Shanghai 200237,PR China

a r t i c l e i n f o

Article history:

Received 28September 2011Received in revised form 5November 2011

Accepted 7November 2011

Available online 17November 2011Keywords:

Dye-sensitized solar cell Semi-rigid triarylamine Synthesis

Characterization

Nanocrystalline TiO 2High ef ?ciency

a b s t r a c t

Novel organic dyes (IDB and ISB dyes),which contain 5-phenyl-iminodibenzyl (IDB )and 5-phenyl-iminostilbene (ISB )as electron donors and a cyanoacrylic acid moiety as an electron acceptor and an anchoring group,connected with a thiophene as a p -conjugated system,have been synthesized and used as the sensitizers for dye-sensitized solar cells (DSSCs).The photophysical and electrochemical properties of the dyes were investigated by absorption spectrometry,cyclic voltammetry and density functional theory calculations.As demonstrated,the IDB and ISB unit exhibited stronger electron-donating ability and broader absorption spectra when coated onto TiO 2.The DSSC based on ISB-2consisting of ISB unit produced 5.83%of h (J sc ?13.14mA cm à2,V oc ?0.64V,and ff ?0.68)under 100mW cm à2simulated AM 1.5G solar irradiation.

1.Introduction

As a potential alternative to conventional inorganic photovoltaic devices,dye-sensitized solar cells (DSSCs),the so-called Gr?tzel cells [1],have attracted ever-increasing attention in the past decades.Several ruthenium-based sensitizers,such as N3,N719and black dye [2e 5],have achieved remarkable power conversion ef ?ciency of 10e 11%under AM 1.5G irradiation.Recent attentions have focused on metal-free organic dyes as DSSC sensitizers because of their strong molar absorption coef ?cient,ease of structure modi ?cation,and low material cost [6].Most metal-free dyes used for highly ef ?cient DSSCs follow a donor e (p -spacer)e acceptor (D e p e A)architecture [7].Generally,coumarin [8e 10],indoline [11e 14],tri-phenylamine [15e 20],tetrahydroquinoline [21,22],and carbazole [23e 27]have been widely employed as the electron donor unit with good performance.Carboxylic acid,cyanoacrylic acid [28e 32],and rhodamine [33e 35]moieties are often introduced into the (D e p e A)system as electron acceptors to ful ?ll these requirements described above.The donors and the acceptors are linked by various p -conjugated spacers such as polyene,oligophenylenevinylene,oligothiophene and their derivatives.A series of indoline dyes have shown an impressive high ef ?ciency of 9%for DSSCs [32,36].

Among various electron donors,triarylamine moieties have been investigated widely due to their prominent electron-donating ability,hole-transport properties,and prevention of direct charge recombination between TiO 2and I 3àby the bulky aryl group covering TiO 2surfaces [37].Much work has been done to optimize the structure of triarylamine to improve the performance of the cells in our group [16,17,23,38].However,one problem of triphe-nylamine that need to be addressed is the rotation of the phenyl rings,which causes serious energy loss.In our previous research,it was observed that as the increase of the triphenylamine units,the ef ?ciency of the dyes decreases seriously.Hence,it can be specu-lated that the ef ?ciency of the dye can be effectively increased if the phenyl rings were locked to prevent the rotation.Iminodibenzyl and iminostilbene with two phenyl rings connected by alkyl and ethylene chains have been widely used in materials of organic light-emitting diodes (OLEDs),which contribute to the achieve-ment of high luminescence quantum yield [39,40],however,there are no report of utilizing iminodibenzyl and iminostilbene moiety as electron donor for the organic sensitizers of DSSCs.Here,we report the synthesis and characterization of four new organic dyes that contain 5-phenyl-iminodibenzyl (IDB )and 5-phenyl-iminos-tilbene (ISB )as the electron donors,thiophene as p -conjugation linkage,and cyanoacrylic acid as the electron acceptor and anchoring group,shown in Fig.1.Thiophene and alkene as the p -conjugated systems between the donor and the acceptor were

*Corresponding author.Tel./fax:t862164252288.E-mail address:bbsjh@https://www.sodocs.net/doc/0e1855531.html, (J.-H.

Su).

Contents lists available at SciVerse ScienceDirect

Dyes and Pigments

journal h omepage:www.elsevier.co m/lo

cate/dyepig

Dyes and Pigments 94(2012)40e 48

extended to adjust the molecular HOMO and the LUMO energy levels of the dyes,hence red-shifting and broadening the absorp-tion spectra with the number of p -conjugations introduced [41].On the other hand,it was found that the bridge between the two rings can affect the performance of the sensitizer signi ?cantly.To understand the effect of the con ?guration to the performance of the sensitizer,TDDFT calculation was performed.2.Experimental

2.1.General synthetic procedure and spectroscopic measurements All chemicals were used as received from commercial sources without puri ?cation.Solvents for chemical synthesis such as dichloromethane (CH 2Cl 2),dimethylformamide (DMF)and tetra-hydrofuran (THF)were puri ?ed by distillation.All chemical reac-tions were carried out under nitrogen atmosphere.1H NMR and 13C NMR spectra were recorded on Brucker AM-400MHz instruments with tetramethylsilane as internal standard.HRMS were performed using a Waters LCT Premier XE spectrometer.The absorption spectra of the dyes in solution and adsorbed on TiO 2?lms were measured with a Varian Cary 500spectrophotometer.2.2.Synthesis

2.2.1.5-Phenyl-10,11-dihydro-5H-dibenzo[b,f]azepine (3)

A mixture of iminodibenzyl (1,4g,20.5mmol),iodobenzene (4.89g,23.9mmol),copper powder (0.30g,4.6mmol),potassium carbonate (6.93g,50.2mmol),and [18]crown-6(0.33g,1.25mmol)was heated in 1,2-dichlorobenzene (100ml)at 180 C for 48h under an atmosphere of argon.The inorganic components were removed by ?ltration after cooling.Then the solvent was distilled under reduced pressure,and the crude product was puri ?ed by column chromatography on silica (DCM/PE ?1/3,v/v)to give a white solid,3(3.57g,13.2mmol,64.4%).1H NMR (400MHz,CDCl 3,d ):7.45e 7.38(m,2H),7.27e 7.18(m,6H),7.09(dd,J ?8.8,7.3Hz,2H),6.70(t,J ?7.3Hz,1H),6.58(dd,J ?8.8,0.9Hz,2H),2.98(s,4H).

2.2.2.5-Phenyl-5H-dibenzo[b,f]azepine (4)

The synthesis method resembled that of compound 3and the crude compound was puri ?ed by column chromatography on silica (DCM/PE ?1/3,v/v)to yield a white solid,4(2.98g,11.1mmol,60.8%).1H NMR (400MHz,CDCl 3,d ):7.54e 7.46(m,4H),7.44(d,J ?7.4Hz,2H),7.39e 7.29(m,2H),7.04e 6.95(m,2H),6.82(s,2H),6.68(t,J ?7.3Hz,1H),6.26(dd,J ?8.8,0.9Hz,2H).

2.2.

3.5-(4-Bromophenyl)-10,11-dihydro-5H-dibenzo[b,f]azepine (5)

Compound 3(1.94g,7.16mmol)was dissolved in DMF (50mL).While the solution was being stirred at 0 C,N -bromosuccinimide (1.89g,10.74mmol)was dissolved in DMF (50mL)and added dropwise to it.And the reaction mixture was then warmed to room temperature.After being stirred for 12h,the reaction mixture was poured into water and extracted with diethyl ether,and the combined extracts were washed with brine,dried over anhydrous magnesium sulfate,and ?ltered.The solvent was removed by rotary evaporation.The crude product was dissolved in hexane,and the undissolved solid was ?ltered.The ?ltrate was evaporated to dryness,and the resulting solid was washed with ethanol to yield a white solid,5(1.56g, 4.47mmol,62.4%).1H NMR (400MHz,DMSO,d ):7.39e 7.27(m,8H),7.24(d,J ?8.9,2H),6.37(d,J ?8.9,2H),2.91(s,4H).

2.2.4.5-(4-Bromophenyl)-5H-dibenzo[b,f]azepine (6)

The synthesis method resembled that of compound 5and the resulting solid was washed with ethanol to yield a white solid,6(1.25g,3.58mmol,68.5%).1H NMR (400MHz,CDCl 3,d ):7.56e 7.41(m,6H),7.40e 7.32(m,2H),7.12e 7.00(m,2H),6.82(s,2H),6.12(d,J ?9.1Hz,2H).

2.2.5.5-(4-(10,11-Dihydro-5H-dibenzo[b,f]azepin-5-yl)phenyl)thiophene-2-carbaldehyde (7)

A mixture of compound 3(850mg,2.44mmol),Pd(PPh 3)4(10mg,0.01mmol),K 2CO 3(270mg,1.96mmol),THF (10mL)and H 2O (5mL)was heated to 45 C under nitrogen atmosphere for 30min.A stirred solution of 5-formyl-2-thiophene-boronic acid (756mg,4.90mmol)in THF (5mL)was added slowly,and the mixture was re ?uxed for further 12h.After cooling to room temperature,the mixture was extracted with DCM.The organic portion was combined and removed by rotary evaporation.The residue was puri ?ed by column chroma-tography on silica (DCM/PE ?1/2,v/v)to give a yellow solid,7(434mg,1.14mmol,46.8%).1H NMR (400MHz,DMSO,d ):9.75(s,1H),7.62(d,J ?3.8Hz,1H),7.48(d,J ?8.9Hz,2H),7.44e 7.38(m,2H),7.37e 7.26(m,7H),6.47(d,J ?8.9Hz,2H),2.94(s,4H).

2.2.6.5-(4-(5H-Dibenzo[b,f]azepin-5-yl)phenyl)thiophene-2-carbaldehyde (8)

The synthesis method resembled that of compound 7and the crude compound was puri ?ed by column chromatography on silica (DCM/PE ?1/2,v/v)to yield a yellow solid,8(352mg,0.93mmol,49.2%).1H NMR (400MHz,DMSO,d ):9.80(s,1H),7.92(d,J ?4.0Hz,1H),7.64e 7.54(m,6H),7.51e 7.43(m,4H),7.42(d,J ?4.0Hz,1H),6.95(s,2H),6.18(d,J ?8.9Hz,

2H).

Fig.1.Molecular structures of IDB and ISB dyes.

C.Wang et al./Dyes and Pigments 94(2012)40e 4841

2.2.7.4-(10,11-Dihydro-5H-dibenzo[b,f]azepin-5-yl)benzaldehyde(9)

Phosphorus oxychloride(0.55mL, 5.89mmol)was added dropwise to DMF(10mL)at0 C,and the mixture was stirred for 1h at this https://www.sodocs.net/doc/0e1855531.html,pound3(795mg,2.93mmol)was added and the reaction mixture was heated to80 C for8h.The mixture was subsequently cooled to room temperature,poured into ice water,carefully neutralized with NaOH and extracted with DCM.The combined organic extract was dried over anhydrous MgSO4and?ltered.Solvent was removed by rotary evaporation and the residue was puri?ed by column chromatography on silica (DCM/PE?1/1,v/v)to give a yellow solid,9(720mg,2.41mmol, 82.1%).1H NMR(400MHz,CDCl3,d):9.73(s,1H),7.61(d,J?9.0Hz, 2H),7.43e7.34(m,2H),7.32e7.21(m,6H),6.64(d,J?8.9Hz,2H), 3.00(s,4H).

2.2.8.4-(5H-Dibenzo[b,f]azepin-5-yl)benzaldehyde(10)

The synthesis method resembled that of compound9and the crude compound was puri?ed by column chromatography on silica (DCM/PE?1/1,v/v)to yield a yellow solid,10(985mg,3.32mmol, 78.4%).1H NMR(400MHz,CDCl3,d):9.69(s,1H),7.55e7.46(m,8H), 7.43e7.37(m,2H),6.86(s,2H),6.35(d,J?8.9Hz,2H).

2.2.9.5-(2-(4-(10,11-Dihydro-5H-dibenzo[b,f]azepin-5-yl)phenyl) vinyl)thiophen[41]

A mixture of compound9(650mg, 2.17mmol),t-BuOK (292mg,2.61mmol)and dry THF(20mL)was stirred at ambient temperature under nitrogen atmosphere for1h.2-Thienylmethyl triphenylphosphonium chloride(1290mg, 3.25mmol)was dis-solved in THF and added dropwise to the solution,and the reaction mixture was stirred for1h at ambient temperature,whereupon the mixture was heated to re?ux for24h.The reaction mixture was allowed to cool to ambient temperature and a molar excess of water was added.The mixture was concentrated by rotary evap-orator and the water phase was extracted with DCM.The organic phase was dried over MgSO4,?ltered through a plug of silica gel (DCM)and a crude intermediate was obtained(720mg, 1.90mmol).The crude product was used in the next step without further puri?cation.

2.2.10.5-(2-(4-(10,11-Dihydro-5H-dibenzo[b,f]azepin-5-yl)phenyl) vinyl)thiophen-2-carbaldehyde(11)[41]

5-(2-(4-(10,11-Dihydro-5H-dibenzo[b,f]azepin-5-yl)phenyl) vinyl)thiophen(720mg, 1.90mmol)was dissolved in dry THF (20mL)and was cooled toà78 C under nitrogen atmosphere.n-Butyl lithium(0.75mL,2.5M hexane solution)was added dropwise over10min and the mixture was stirred atà78 C for1h.The mixture was allowed to warm to0 C and stirred for30min.The mixture was once again cooled toà78 C and DMF(0.17mL, 2.09mmol)was added.The reaction mixture was allowed to warm to ambient temperature and stirred for2h.The reaction was quenched by the addition of aqueous HCl(10%,100mL)and extracted with DCM.The combined organic extract was dried over anhydrous MgSO4 and?ltered.Solvent was removed by rotary evaporation and the residue was puri?ed by column chromatography on silica(DCM/ PE?1/1,v/v)to give an orange solid,11(347mg,0.85mmol,45.0%). 1H NMR(400MHz,CDCl

3,d):9.77(s,1H),7.52(d,J?3.9Hz,1H),7.42 (d,J?7.6Hz,2H),7.29e7.18(m,6H),7.11(d,J?8.7Hz,2H),7.06(d, J?3.9Hz,1H),6.63(d,J?12.0Hz,1H),6.53(d,J?8.7Hz,2H),6.45(d, J?12.0Hz,1H),3.00(s,4H).

2.2.11.5-(2-(4-(5H-Dibenzo[b,f]azepin-5-yl)phenyl)vinyl)thiophen-2-carbaldehyde(12)

The synthesis method resembled that of compound11and the crude compound was puri?ed by column chromatography on silica (DCM/PE?1/1,v/v)to yield a yellow solid,12(298mg,0.79mmol,47.8%).1H NMR(400MHz,DMSO,d):9.82(s,1H),7.89(d,J?3.0Hz, 1H),7.56(dd,J?15.9,7.4Hz,6H),7.46(t,J?6.9Hz,2H),7.29(d, J?8.0Hz,3H),7.16(d,J?16.1Hz,1H),7.09(d,J?16.1Hz,1H),6.94 (s,2H),6.14(d,J?8.2Hz,2H).

2.2.12.3-(5-(4-(10,11-Dihydro-5H-dibenzo[b,f]azepin-5-yl)phenyl) thiophen-2-yl)-2-cyanoacrylic acid(IDB-1)

A mixture of compound7(120mg,0.31mmol),2-cyanoacetic acid(29mg,0.34mmol),piperidine(0.5mL)and THF(15mL) was heated to re?ux under nitrogen atmosphere for6h.Solvent was removed by rotary evaporation and the residue was puri?ed by column chromatography on silica(DCM/ethanol?20/1,v/v)to give a red solid,IDB-1(99mg,0.22mmol,71.3%).1H NMR(400MHz, DMSO,d):7.96(s,1H),7.58(d,J?3.8Hz,1H),7.47(d,J?8.9Hz,2H), 7.44e7.40(m,2H),7.39e7.27(m,7H),6.50(d,J?8.9Hz,2H),2.95 (s,4H).13C NMR(100MHz,DMSO,d):149.40,148.99,142.46,137.66, 136.39,134.31,131.13,129.33,127.60,127.32,126.96,122.35,122.10, 118.49,112.43,29.93.HRMS(ESI,m/z):[MtH]tcalcd for C28H21N2O2S,449.1324;found,449.1321.

2.2.1

3.3-(5-(4-(5H-Dibenzo[b,f]azepin-5-yl)phenyl)thiophen-2-

yl)-2-cyanoacrylic acid(ISB-1)

The synthesis method resembled that of compound IDB-1and the crude compound was puri?ed by column chromatography on silica(DCM/ethanol?20/1,v/v)to yield a red solid,ISB-1(102mg, 0.23mmol,72.3%).1H NMR(400MHz,DMSO,d):8.35(s,1H),7.88 (d,J?4.1Hz,1H),7.68e7.53(m,6H),7.52e7.39(m,5H),6.96(s, 2H),6.21(d,J?8.9Hz,2H).13C NMR(100MHz,DMSO,d):163.95, 153.64,149.61,145.76,141.55,141.14,135.55,132.80,130.51,130.30, 130.16,129.67,127.69,127.06,122.74,122.23,117.06,111.82.HRMS (ESI,m/z):[MàH]àcalcd for C28H17N2O2S,445.1011;found, 445.1006.

2.2.14.3-(5-(4-(10,11-Dihydro-5H-dibenzo[b,f]azepin-5-yl)styryl) thiophene-2-yl)-2-cyanoacrylic acid(IDB-2)

The synthesis method resembled that of compound IDB-1and the crude compound was puri?ed by column chromatography on silica(DCM/ethanol?10/1,v/v)to yield a red solid,IDB-2(90mg, 0.19mmol,68.7%).1H NMR(400MHz,DMSO,d):8.14(s,1H),7.65 (d,J?3.9Hz,1H),7.44e7.33(m,6H),7.30(ddd,J?13.9,7.0,1.8Hz, 4H),7.24e7.16(m,2H),7.00(d,J?16.1Hz,1H),6.44(d,J?8.9Hz, 2H),2.94(s,4H).13C NMR(100MHz,DMSO,d):164.11,149.70, 148.87,142.55,142.05,137.68,137.42,134.10,131.32,131.12,129.39, 128.15,127.54,127.29,125.83,125.61,118.73,117.17,112.21,29.94. HRMS(ESI,m/z):[MtH]tcalcd for C30H23N2O2S,475.1480;found, 475.1485.

2.2.15.3-(5-(4-(5H-dibenzo[b,f]azepin-5-yl)styryl)thiophene-2-yl)-2-cyanoacrylic acid(ISB-2)

The synthesis method resembled that of compound IDB-1and the crude compound was puri?ed by column chromatography on silica(DCM/ethanol?10/1,v/v)to yield a purple solid,ISB-2 (103mg,0.22mmol,70.5%).1H NMR(400MHz,DMSO,d):13.59(s, 1H),8.40(s,1H),7.88(d,J?4.1Hz,1H),7.58(tt,J?7.8,4.1Hz,6H), 7.50e7.41(m,2H),7.31(dd,J?6.4,5.0Hz,3H),7.21(d,J?16.1Hz, 1H),7.08(d,J?16.1Hz,1H),6.95(s,2H),6.14(d,J?8.9Hz,2H).13C NMR(100MHz,DMSO,d):163.86,153.32,149.14,146.30,141.76, 141.52,135.63,133.06,132.91,130.50,130.31,130.11,129.78,128.10, 127.59,126.05,125.82,116.80,116.73,111.55.HRMS(ESI,m/z): [MàH]àcalcd for C30H19N2O2S,471.1167;found,471.1156.

2.3.Theoretical calculation

Density functional theory(DFT)calculations were conducted by using the B3LYP hybrid functional for the geometry optimizations.

C.Wang et al./Dyes and Pigments94(2012)40e48 42

The molecular orbital levels of HOMO and LUMO were achieved with the6-31G(d)basis set implemented in the Gaussian03 package.

2.4.Electrochemical measurements

The cyclic voltammograms were determined with a Versastat II electrochemical workstation(Princeton Applied Research)using a three-electrode cell with a Pt working electrode,a Pt wire auxiliary electrode,and a saturated calomel reference electrode in saturated KCl solution.The supporting electrolyte was0.1M TBAPF6 (tetra-n-butylammonium hexa?uorophosphate)in acetonitrile as the solvent.

2.5.General procedure for preparation

The dye-sensitized TiO2electrode was prepared by following the procedure reported in the literature[3].TiO2colloidal dispersion was made employing commercial TiO2(P25,Degussa AG,Germany) as material.Films of nanocrystalline TiO2colloidal on FTO were prepared by sliding a glass rod over the conductive side of the FTO, followed by calcinations at450 C for30min.Before immersion in the dye solution,these?lms were immersed into a40mM aqueous TiCl4solution at70 C for30min and washed with water and ethanol.Then the?lms were heated again at450 C followed by cooling to80 C and soaked in dye solutions(0.3mM in chloroform) for at least12h.The adsorbed TiO2electrode and Pt counter elec-trode were assembled into a sealed sandwich-type cell by heating with a hot-melt ionomer?lm(Surlyn1702,DuPont).The redox electrolyte was placed in a drilled hole in the counter electrode by capillary force,and was driven into the cell by means of vacuum back?lling.Finally,the hole was sealed using a UV-melt gum and a cover glass.Two electrolytes were used for device evaluation, in which one was composed of0.1M lithium iodide,0.6M 1,2-dimethyl-3-n-propylimidazolium iodide(DMPImI),0.1M I2,and 0.5M4-tert-butylpyridine(4-TBP)in acetonitrile,the other was composed of0.1M lithium iodide,0.6M methyl-propylimidazolum iodide(MPII)and0.05M I2in the mixed solvent of acetonitrile and 3-methoxypropionitrile(7:3,v/v).

2.6.Photoelectrochemical measurements

Photovoltaic measurements employed an AM1.5solar simulator equipped with a300W xenon lamp(Model No.91160,Oriel).The power of the simulated light was calibrated to100MW/cm2using a Newport Oriel PV reference cell system(Model91150V).I e V curves were obtained by applying an external bias to the cell and measuring the generated photocurrent with a Keithley model2400 digital source meter.The voltage step and delay time of photocur-rent were10mV and40ms,respectively.Cell active area tested with a mask of0.158cm2.The photocurrent action spectra were measured with an IPCE test system consists of a Model SR830DSP Lock-In Ampli?er and Model SR540Optical Chopper(Stanford Research Corporation,USA),a7IL/PX150xenon lamp and power supply,and a7ISW301Spectrometer.

2.7.Electrical impedance measurements

Electrical impedance experiments were performed in the dark with CHI660C electrochemical work station,with a frequency range from0.01Hz to100kHz and a potential modulation of10mV.The applied bias potential is held atà0.60V.

3.Results and discussion

3.1.Synthesis

The synthetic strategy,illustrated in Scheme1,involves the Ullmann coupling reaction,the Suzuki coupling reaction,the Knoevenagel reaction,and the Wittig reaction to extend

the

Scheme1.Synthesis of the sensitizers(IDB-1,ISB-1,IDB-2,and ISB-2).

Table1

Experimental data for spectral and electrochemical properties of the dyes.

Dyes l max(nm)a3(Mà1cmà1)a l max on TiO2(nm)b HOMO(V)vs.NHE c E0e0(eV)d LUMO(V)vs.NHE e IBD-142227,000420 1.12 2.08à0.96

ISB-147039,000418 1.19 2.05à0.86

IDB-246734,000466 1.05 2.03à0.98

ISB-249842,0004390.96 2.01à1.05

a Absorption maximum in CH

2Cl2solution(3?10à5M).

b Absorption maximum on TiO

2?lm.

c HOMO were measure

d in acetonitril

e with0.1M tetrabutylammonium hexa?uorophosphate(TBAPF

)as electrolyte(working electrode:FTO/TiO2/dye;reference elec-trode:SCE;calibrated with ferrocene/ferrocenium(Fc/Fct)as an external reference.Counter electrode:Pt).

d E

0e0was estimated from the absorption thresholds from absorption spectra of dyes adsorbed on the TiO2?lm.

e LUMO is estimated by subtracting E

0e0from the HOMO.

C.Wang et al./Dyes and Pigments94(2012)40e4843

thiophene linker.The IDB and ISB as electron donors were synthesized via the Ullmann coupling reaction of iminodibenzyl and iminostilbene with iodobenzene in the presence of copper powder,potassium carbonate and [18]https://www.sodocs.net/doc/0e1855531.html,pounds 5and

6were obtained by bromination reaction of IDB and ISB using N -bromosuccinimide (NBS)followed by the Michaelis e Arbuzor reaction [42,43].The Suzuki coupling reaction was made with the respective bromides and 5-formyl-2-thiophene-boronic acid to yield the aldehydes 7and 8.Aldehydes 9and 10were prepared from 3and 4by means of the Vilsmeier e Haack reaction [44,45].In the cases of linker extension by the Wittig reaction,formylation of the thiophene with n -BuLi and DMF was applied [41].Finally,the target products (IDB-1,ISB-1,IDB-2,and ISB-2)were synthesized via the Knoevenagel condensation reaction of the respective alde-hydes with cyanoacetic acid in the presence of piperidine,and their chemical structures were fully characterized with 1H NMR,13C NMR and HRMS.

3.2.Absorption properties in solutions and on TiO 2?lm

Table 1presents the experimental spectral and electrochemical properties of the IDB-1,ISB-1,IDB-2,and ISB-2dyes.Normalized adsorption spectra of the dyes in dichloromethane diluted solution and on TiO 2?lms were shown in Fig.2.In dichloromethane solu-tion,all these dyes exhibit a relatively broad and strong absorption band in the visible region corresponding to the intramolecular charge transfer (ICT)band (Fig.2a).The absorption peaks (l max )for IDB-1,ISB-1,IDB-2,and ISB-2located at 422,470,467and 498nm in diluted solution,respectively.Judging from that of IDB-1and ISB-1,the absorption band of IDB-2and ISB-2showed a large red-shift upon increased linker conjugation.Obviously,in the series of sensitizers containing the same cyanoacetic acid unit and electron transport channels,the ICT band is dependent upon the donor units.As listed in Table 1,the red-shift by 48or 31nm was observed when replacing IDB unit (IDB-1and IDB-2)with ISB unit (ISB-1and ISB-2)in the donor moiety,indicative of the more powerful electron-donating capability of the ISB unit than that of the IDB unit.

Generally,when sensitizers are anchored onto nanocrystalline TiO 2surface,the deprotonation and aggregation of the dye mole-cules affect UV-vis absorption pro ?les.Deprotonation and H-aggregates always result in blue-shift of absorption peak,while J-aggregates mainly lead to the red-shift of absorption peak.Like most D-p -A conjugated organic sensitizers,the ICT absorption peak of ISB dyes (ISB-1and ISB-2)on 6.5m m TiO 2?lms exhibit a large blue-shift (52and 59nm,respectively)with respect

Fig.2.Normalized absorption spectra of the IDB-1,ISB-1,IDB-2,and ISB-2dyes in CH 2Cl 2(a)and anchored on 6.5m m TiO 2?lms

(b).

Fig.3.The optimized geometries of the IDB-1,IDB-2,ISB-1,and ISB-2calculated with DFT on the B3LYP/6-31G(d)level.

C.Wang et al./Dyes and Pigments 94(2012)40e 48

measurements taken in dichloromethane solution(Table1).Since the aggregation tendency is not obvious within such thin?lms of TiO2,here the shift in absorption peak might be predominated by deprotonation effect.Unexpectedly,the ICT absorption peaks of IDB dyes(IDB-1and IDB-2)are slightly blue-shifted by only2nm from 422nm in solution to420nm on TiO2,and1nm from467nm in solution to466nm on TiO2,respectively.Thus iminodibenzyl unit in IDB dyes countervails the deprotonation effect.It is noteworthy that the absorption spectrum of the dyes adsorbed onto6.5m m TiO2?lms shows a markedly broad pro?le,which is bene?cial to light-harvesting(Fig.2b).

3.3.Theoretical calculation

To gain insight into the geometrical properties and scrutinize the charge transfer on excitation,the ground-state geometries of the protonated dyes have been optimized in the gas phase by DFT with the Gaussian09package[46],using the hybrid B3LYP[47] functional and the standard6-31G(d)basis set.The TDDFT calcu-lations were performed with MPW1K[48]functional and6-31G(d) basis set on the B3LYP optimized ground-state geometries.Solva-

tion effect was included in the TDDFT calculations in CH2Cl2with the nonequilibrium version of the C-PCM model[49].As illustrated in Fig.3,the dihedral angles between the phenyl B1planes and the phenyl B2planes of the ISB dyes are higher than those of the IDB dyes.The phenomenon is due to the rigid iminostilbene substituent which would increase the steric strain and cause the iminostilbene moiety to twist slightly out of the plane de?ned by phenyl B1.The angle formed between the phenyl B1plane and the phenyl B2plane of the ISB-2is as large as82 ,which can help to inhibit the close p e p aggregation.As illustrated in Fig.4,the HOMO orbital of the dyes is primarily located at the p-framework of the donor part and thiophene group,while the electron density of the LUMO is delo-calized over the thiophene group and anchoring group.Examina-tion of the frontier orbitals of all sensitizers suggests that the HOMO-LUMO excitation would shift the electron density distri-bution from the donor unit to the anchoring moiety,facilitating ef?cient photoinduced/interfacial electron injection from excited dyes to the TiO2electrode.

3.4.Electrochemical properties

Fig.5depicts the cyclic voltammogram(CV)of IDB-1,IDB-2,ISB-1,and ISB-2,measured with four sensitizers attached to6.5m m nanocrystalline TiO2?lms deposited on conducting FTO glass in MeCN containing0.1M tetra-n-butylammonium hexa-?uorophosphate(TBAPF6)with a0.05V sà1scan rate.CV was employed to evaluate the redox stability and oxidation potential of the dyes.As estimated from the absorption threshold of these dyes onto TiO2?lms,the excitation transition energy(E0e0)of IDB-1, IDB-2,ISB-1,and ISB-2is2.08,2.03,2.05and2.01eV,respectively (Table1).The HOMO values of IDB-1,IDB-2,ISB-1,and ISB-2cor-responding to their?rst redox potential were1.12,1.05,1.19and 0.96V vs.NHE,respectively.Observed from these electrochemical data of IDB-1and ISB-1,or IDB-2and ISB-2,it seems that the HOMO energy level is exclusively determined by donors.The esti-mated excited-state potential corresponding to the LUMO levels of IDB-1,IDB-2,ISB-1,and ISB-2,calculated from E HOMOàE0e0, areà0.96,à0.98,à0.86andà1.05V vs.NHE,respectively.Judging from the LUMO value,the four dyes are more negative than the bottom of the conduction band of TiO2(à0.5V),indicating that the electron injection process from the excited dye molecule to TiO2 conduction band is energetically permitted[50e52].

3.5.Performances of dye-sensitized solar cells

Action spectrum,incident photon-to-electron conversion ef?-ciency(IPCE)as a function of wavelength,was measured to eval-uate the photoresponse of photoelectrode in the whole

spectral Fig.4.Frontier molecular orbitals(HOMO and LUMO)of the IDB-1,ISB-1,IDB-2,and ISB-2calculated with DFT on the B3LYP/6-31G(d)

level.

Fig.5.Oxidative cyclic voltammetry plots of IDB-1,ISB-1,IDB-2,and ISB-2attached to

a6.5m m nanocrystalline TiO2?lm deposited on conducting FTO glass.

C.Wang et al./Dyes and Pigments94(2012)40e4845

region.The IDB-1,IDB-2,ISB-1,and ISB-2sensitizers have been used to manufacture solar cell devices by using8(transparent)t4 (scattering)m m TiO2layers.Fig.6shows the IPCE obtained with a sandwich cell by using0.1M lithium iodide,0.6M methyl-propylimidazolum iodide(MPII)and0.05M I2in the mixed solvent of acetonitrile and3-methoxypropionitrile(7:3,v/v)as redox electrolyte.The photocurrent action spectra of all sensitizers exhibit very high plateaus where IPCE values reach85%.Consid-ering the light absorption and scattering loss by the conducting glass,the maximum ef?ciency for absorbed photon-to-collected electron conversion ef?ciency is almost unity over a broad spec-tral range,suggesting a very high electron injection ef?ciency of these dyes.Solar cells based on IDB-1and ISB-1showed high IPCE (85%)but not broad spectra,resulting in lower overall ef?ciencies due to spectral limitations.Solar cells based on IDB-2and ISB-2 show broad IPCEs in accordance to the broad absorption spectrum achieved by increased linker conjugation.

Fig.7shows the current e voltage(J e V)curves of the DSSCs under standard global AM1.5G solar irradiation.The short-circuit photocurrent density(J sc),open-circuit voltage(V oc),and?ll factor(ff)of the solar cell based on ISB-2are13.14mA cmà2,649mV,and0.68,respectively,yielding an overall conversion ef?ciency(h)of5.83%.Under the same conditions,the device based on IDB-2shows lower J sc,leading to an inferior h value of5.21% (Table2).For a fair comparison,the N719-sensitized DSSC was also fabricated under the same conditions and yielded h value of7.47%. The higher J sc values of the IDB-2and ISB-2-based cells than those of the IDB-1and ISB-1-based cells are consistent with the values for the IPCE spectra due to the increased linker conjugation.In comparison with the IDB-1and IDB-2-based cells,the ISB-1and ISB-2-based cells show higher J sc,respectively,re?ecting the better sunlight-harvesting ability of the ISB

dyes.

Fig.7.Photocurrent e voltage curves of dyes IDB-1,IDB-2,ISB-1,and ISB-2sensitized

TiO2electrodes under irradiation of AM1.5G simulated solar light(100mW cmà2)

with liquid electrolyte.

Table2

DSSC performance data of the dyes.a

Dyes J sc/mA cmà2V oc/mV ff h(%)

IDB-110.676570.68 4.80

ISB-111.046650.72 5.27

IDB-211.306650.69 5.21

ISB-213.146490.68 5.83

N71916.106890.677.47

a Illumination:100mW cmà2simulated AM1.5G solar light.Electrolyte con-

tained0.1M LiI,0.6M MPII,0.1M I2,1.0M4-TBP in the mixed solvent of acetonitrile

and3-methoxypropionitrile(7:3,

v/v).

Fig.8.Impedance spectra of DSSCs based on IDB-1,ISB-1,IDB-2,and ISB-2dyes

measured atà0.60V bias in the dark.(a)Nyquist plots;and(b)Bode phase

plots. Fig.6.Photocurrent action spectra of the TiO2electrodes sensitized by IDB-1,ISB-1,

IDB-2,and ISB-2.

C.Wang et al./Dyes and Pigments94(2012)40e48

Electrochemical impedance spectroscopy(EIS)analysis was performed to elucidate the photovoltaic?ndings further.Fig.8 compares the impedance spectra for IDB-1,ISB-1,IDB-2,and ISB-2-sensitized cells measured in the dark under a forward bias ofà0.60V with a frequency range of0.1Hz to100kHz.The important differences for these organic dye-sensitized solar cells are their conductivity.The Nyquist plots(Fig.8a)show the radius of the middle semicircle to increase in the order ISB-2

4.Conclusion

In summary,four new organic sensitizers(IDB-1,ISB-1,IDB-2, and ISB-2)containing5-phenyl-iminodibenzyl(IDB)and5-phenyl-iminostilbene(ISB)as electron donors had been designed and synthesized for dye-sensitized solar cells.These dyes were successfully adsorbed on nanocrystalline anatase TiO2particles, and subsequently ef?cient dye-sensitized solar cells had been fabricated.It was found that the dyes containing5-phenyl-imi-nostilbene(ISB)as electron donors exhibited improved photovol-taic performance compared with the ones containing5-phenyl-iminodibenzyl(IDB)as electron donors.A solar cell device based on the sensitizer ISB-2yielded a higher overall conversion ef?ciency up to5.83%(J sc?13.14mA cmà2,V oc?0.64V,and ff?0.68)under 100mW cmà2simulated AM1.5G solar irradiation.The introduc-tion of increased linker conjugation appears to contribute to higher J sc and IPCE in DSSCs.Our?ndings demonstrate that the IDB and ISB organic sensitizers are promising for further improvement of the conversion ef?ciency of DSSCs owing to the original and versatile molecular design.

Acknowledgments

This work is supported by NSFC/China,National Basic Research 973Program(2006CB806200)and Scienti?c Committee of Shanghai.Wang CY thanks Prof.Hua JL,Dr.Wu.WJ,Zhang ZY, Huang XH,Zou Q,and Zhang HY in our laboratory for their assis-tance in the experiments.

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