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Solar Energy Materials & Solar Cells 94 (2010) 655–661

Contents lists available at ScienceDirect

Solar Energy Materials & Solar Cells

0927-02

doi:10.1

n Corr

E-m

journal homepage: www.elsevier.com/locate/solmat

Carbazole-based organic dye sensitizers for efficient molecular photovoltaics

Ceylan Zafer a,n, Burak Gultekin a, Cihan Ozsoy a, Cem Tozlu a,b, Banu Aydin a, Siddik Icli a

a Solar Energy Institute, Ege University, TR-35100 Izmir, Turkeyb Department of Physics, Art and Science Faculty, Mugla University, 48000-Mugla, Turkey

a r t i c l e i n f o

Article history:

Received 15 June 2009

Received in revised form

13 November 2009

Accepted 16 November 2009Available online 9 December 2009

Keywords:

Carbazolocarbazole

Sensitizer

Dye-sensitized solar cell

Nanostructured electrode

48/$ - see front matter & 2009 Elsevier B.V. A

016/j.solmat.2009.11.014

esponding author. Tel.: +90 232 3886023; fa

ail address: ceylan.zafer@ege.edu.tr (C. Zafer)

a b s t r a c t

Efficient dye sensitizers N,N0-dialkylcarbazolocarbazole derivatives BG-501 and BG-502 were

synthesized and characterized. UV–vis, 1H NMR and CV were used for the structural characterization.

The aim of the synthesis of these molecules is to improve some optical and electronic properties such as

molar absorption coefficient, spectral coverage and electron injection properties of the sensitizers.

Conjugated organic dyes exhibit higher molar extinction coefficients which allow harvesting more light

in thinner semiconductor layer. Carbazole derivative dyes are well known in the literature with their

thermal and photochemical stabilities and improved electron donor properties.

We have achieved promising photovoltaic conversion efficiencies with new dyes BG-501 and

BG-502 under standard conditions (AM1.5G, 100 mW. cm�2 light intensity). The conversion efficiencies

of solar cells are Z:3.18% and Z:2.49% with ionic liquid-based electrolyte for BG-501 and BG-502,

respectively.

& 2009 Elsevier B.V. All rights reserved.

1. Introduction

Since the report of O’Regan and Graetzel, on high powerconversion efficiency [1] dye-sensitized solar cells (DSSCs) basedon nanocrystalline TiO2 have attracted great attention in scientificresearch due to the potential advantages of low cost production,flexibility and transparency with comparison of silicon solar cell[3]. Over the past 15 years, remarkable progress has beenachieved in terms of performance and stability of solar cell [5].The dye sensitizer is a crucial part for the power conversionefficiency in the devices. Although there is no dye sensitizerreported that has a higher efficiency in comparison withruthenium complexes, in spite organic dyes have been attractingvery intensive research works, owing to ease on synthesis, highmolar absorption coefficient, adjustable absorption spectra, andinexpensive production techniques [2,3]. Recently, several groupshave developed metal-free organic sensitizers to overcome theprohibitive cost of ruthenium metal complexes, and the impress-ive photovoltaic performance has been obtained with someorganic indoline, coumarin, oligoene, hemicyanine and merocya-nine dyes reaching to efficiencies in the range of 5–9% [6–10]. Forexample DSSC with indoline dye exhibited 9% efficiency whichcan be attributed to the high molar extinction coefficient of thesensitizer, 68700 M�1 cm�1 in contrast to ruthenium bipyridylcomplexes generally have molar extinction coefficient lower than20000 M�1 cm�1. Organic compounds easily can be modified to

ll rights reserved.

x: +90 232 3886027.

.

increase molar extinction coefficients and spectral coverage. Thisallows light harvesting to be accomplished with thinner TiO2

films [4].Here we report the synthesis, optical, electrochemical and

photovoltaic characterization of two new N-alkyl substitutedcarbazolocarbazole derivative dyes with 3.2% conversion effi-ciency.

2. Experimental section

2.1. Synthesis and Spectroscopy:

All reactions were carried out under argon atmosphere. Allreagents were purchased from Sigma-Aldrich and used withoutfurther purification. 5,10-dihydrocarbazolo[3,4-c]carbazole wassynthesized using the procedure of previous reference [11]. 1Hand 13C NMR spectra were recorded on a Brunker 400 MHzspectrometer. The absorption and the fluorescence spectra wererecorded on an Analytic Jena S 600 UV spectrophotometer andPTIQM1 fluorescence spectrophotometer, respectively.

2.2. Electrochemistry:

Electrochemical properties of BG-501 and BG-502 werestudied by cyclic voltammetry (CV). CV measurements wererecorded by CH 660B model potentiostat from CH Instruments inthree electrode cell consist of platinum wire counter electrode(CE), glassy carbon electrode used as a working electrode (WE)

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C. Zafer et al. / Solar Energy Materials & Solar Cells 94 (2010) 655–661656

and Ag/AgCl electrode in 3 M KCl (aq) used as a referenceelectrode (RE). Measurements were carried out in 0.1 M TBAPF6

acetonitrile solution as supporting electrolyte. Sweep rate keptconstant at 0.1 V/s. The oxidation potential of Ferrocen/Ferroce-nium couple was used as an internal reference was exhibited atabout +0.41 V.

Electrochemical impedance spectra (EIS) of DSSCs weremeasured in the dark at �0.66 V forward bias to determinecharge transfer resistances (RCT) by using a Zahner IM6 Impe-dance Analyzer.

2.3. Fabrication of Solar Cells

Electrically conductive oxide-coated glasses were used astransparent electrodes. Dye-sensitized mesoporous TiO2-coatedelectrode was used as working electrode and platinized electrodewas used as counter electrode.

Anatase nanocrystalline TiO2 colloids were obtained from ahydrothermal sol–gel method as described in the previous report[12]. The TiO2 paste was coated on FTO-covered glasses by screenprinting technique and sintered at 450 1C for 30 min. The filmthicknesses were measured by Ambios XP Stylus Profiler andfound about 6 mm. The electrodes were dipped into dye solutionsof 0.5 mM BG-501 and BG-502 in chloroform and Z907 inacetonitrile: tert-butanol (1:1) mixture for overnight. Then theelectrodes were washed with solvent of the dye solutions.

The counter electrodes were platinized by thermal reductionof hexachloroplatinic acid with the concentration of 1% in2-propanol and annealed at 400 1C for 20 min. DSSCs wasconstructed with sandwich geometry. A thermoplastic polymerSurlyn-1702 which had a thickness of 50 mm was used as sealingmaterial. Then the space between two electrodes was filled withelectrolyte under vacuum using a pre-drilled small hole oncounter electrode. Then the hole was sealed by using piece ofSurlyn and cover glass. Electrolyte composition was 0.6 MN-methyl-N-propyl imidasolium iodide (PMII)+0.1 M LiI+0.05 MI2+0.5 M 4-tert-butyl pyridine (TBP) in 3-Methoxypropionitrile.Active areas of the solar cells were measured as 1 cm2.

2.4. Characterization of DSSC:

The cell performances were measured by using Keithley 2400Source-Meter Unit and Labview data acquisition software. Weemployed a 100 mW/cm2 AM 1.5 light intensity by a solarsimulator (KHS equipped with 750 W Xe lamp). The incident lightintensity was calibrated with a reference solar cell calibrated byFraunhofer ISE. The measurements were made at the same daywith the cell preparation and the averages of three consecutivemeasurements were taken. No further long-term stability test wasdone. The incident photon-to-current conversion efficiency (IPCE)spectra for the cells were measured on an IPCE measuring setup.

2.5. Synthesis

2.5.1. Synthesis of 5,10-dihydrocarbazolo[3,4-c]carbazole

5,10-dihydrocarbazolo[3,4-c]carbazole was synthesized accord-ing to procedure described by Zander et al.(11). 2, 7-dihydroxynaphthalene (2.5 g, 1. 56 mmol) and phenylhydrazin (3.5 g,4.17 mmol) were added into a round bottom flask and stirredvigorously at room temperature. After 30 min, 20 mL of K2CO3

solution (%36 percent) was added. The final mixture was refluxedfor 50 h. The crude product was extracted with 30 mL ofethylacetate for three times at the end of the reaction. Organicphase was evaporated and the pure product was obtained bycolumn chromatography using silica (ethlyacetate: hexane-2:3) in

a yield 25%. 1H NMR (d) in DMSO- d6: 7.08–7.10 Ar-H (dd),7.36–7.40 Ar-H (dd), 7.64–7.66 Ar-H(m), 7.64–7.66 Ar-H(m),7.97–8.0 Ar-H(d), 11.826N-H (s)

2.5.2. Synthesis procedure of N, N-dimethylcarbazolocarbazole

Bromomethyl (2 eq.) was added dropwise into a mixture ofcarbazolo carbazole (1 eq.)and NaH (2.5 eq.) in DMF, followed byrefluxing for 2 h. After the reaction, the mixture was poured intowater and extracted with n-hexane for three times. Organicsolvent was evaporated and the residue was purified by silica-gelcolumn chromatography using ethylacetate: hexane (1:4) aseluent to afford dimethylcarbazolocarbazole in a yield of 90%.1H NMR (d) in DMSO- d6: 4.48–4.53 Ar-C-H (m), 7.20–7.23 Ar-H(dd), 7.57–7.64 Ar-H (m), 7.57–7.64 Ar-H(m), 7.93–7.97 Ar-H(dd),7.98–8.02 Ar-H(dd),

2.5.3. Synthesis procedure of N, N-dioctylcarbazolocarbazole

The product was synthesized according to the procedure asdescribed above for the synthesis of N, N-Dimethylcarbazolocar-bazole(85%). 1H NMR (d) in DMSO- d6: 1.21- 1.50 C-H (m),4.45–4.51 Ar-C-H (m), 7.15–7.21 Ar-H (dd), 7.54–7.62 Ar-H (dd),7.54–7.62 Ar-H(dd), 7.91–7.96 Ar-H(dd), 7.97–8.0 Ar-H(dd)

2.5.4. Synthesis Procedure of N, N-dialkylcarbazolocarbazole-

2-carbaldehyde

Compounds were prepared by Vilsmeier formylations. Phosphorylchloride (5 eq.) was added dropwise to the DMF (20 eq.) cooled0 oC and stirred for 1 h. Then a solution of alkyl carbazolocarbazole (1 eq.) in DMF (15 mL) was added into the mixture.After that the final mixture was heated to 90 oC and stirred for 2 h.After the reaction, the mixture was poured into the iced water andextracted with chloroform for three times. The solvent wasremoved under reduced pressure and the residue was purified bysilica-gel column chromatography using chloroform as eluent togive the brown product.

2.5.5. Synthesis procedure of (N, N-dimethylcarbazolocarbazole-

2-yl)-isocyanoacrylic acid

A mixture of N, N-dimethylcarbazolocarbazole-2-carbalde-hyde (1 eq.) and cyanoacrylic acid (2 eq.) was vacuum-driedand added acetonitrile and piperidine(0.01 eq.). The solution wasrefluxed for 6 h after the reaction, the solvent was removed underreduced pressure and the residue was purified by silica-gelcolumn chromatography using chloroform as eluent to give aorange-solid in a yield of 75%. 1H NMR (d) in DMSO-d6: 4.40–4.50Ar-C-H (m), 7.16–7.20 Ar-H (dd), 7.52–7.62 Ar-H (m), 7.54–7.62Ar-H(m), 7.90–7.94 Ar-H(dd), 8.30–8.34 Ar-H(dd), 8.36 =C-H (s),10. 35 O-H (s)

2.5.6. Synthesis procedure of (N, N-dioctylcarbazolocarbazole-2-yl)-

isocyanoacrylic Acid

The product was synthesized according to the procedure asdescribed above for the synthesis of (N, N-dimethylcarbazolocar-bazole-2-yl)-isocyanoacrylic Acid (70%). 1H NMR (d) in DMSO-d6:1.21- 1.50 C-H (m), 4.45–4.51 Ar-C-H (m), 7.15–7.21 Ar-H (dd),7.54–7.62 Ar-H (dd), 7.54–7.62 Ar-H(dd), 7.91–7.96 Ar-H(dd),8.32–8.35 Ar-H(dd), 8.37 =C-H (s), 10. 33 O-H (s)

3. Results and discussion

Novel organic dyes, BG-501 and BG-502 were synthesized bythe stepwise synthetic procedures showed in Scheme 1.

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HOOH

NHNH2

HNNH

K2CO3, reflux,50 h

1-bromooctane, NaH

DMF, reflux, 8 h NN

bromomethyl, NaHDMF, reflux, 8 h

NN

NN

O

POCl3DMFreflux, 2h

NN

OPOCl3DMFreflux, 2h

NC COOH

PiperidineAcetonitrile, 6 h

NN

NCCOOH

NN

HOOCCN

NC COOH

Piperidine

Acetonitrile, 6 h

1

2

3

4

5

6

7

BG501

BG502

Scheme 1. Synthetic route of BG-501 and BG-502.

C. Zafer et al. / Solar Energy Materials & Solar Cells 94 (2010) 655–661 657

3.1. Spectroscopic Investigation

UV–vis and fluorescence spectra of dyes in chloroform andadsorbed on TiO2 film were shown in Fig. 1 and spectroscopicproperties were listed in Table 1. The absorption maxima of bothBG-501 and BG-502 were at 435 nm, attribute to the n�pn

transition of the molecule while the absorption coefficients (e)were calculated as 66 900 M�1 cm�1 and 68 300 M�1 cm�1,respectively. The fluorescence maxima of BG-501 and BG-502were at 561 nm and 556 nm, respectively. It showed that thelength of the alkyl chain did not affect the spectral propertiessignificantly as predicted. On the contrary, as seen from the Fig. 2,the absorption spectra of BG-501 and BG-502 on TiO2 film exhibitblue-shifted bands due to the interaction of the anchoring groupswith the titanium dioxide surface. Beside this, the absorption

band of BG-502 on TiO2 film is sharper than the absorption bandof BG-501 on TiO2 film. It can be attributed to the aggregation ofBG-501 on TiO2 surface and longer alkyl chains decrease theaggregation of BG-502.

3.2. Electrochemical Properties

Cylic voltammetry measurements were carried out in acetoni-trile solution phase for BG-501 and BG-502 dyes. The reductionand oxidation peaks were observed both BG-501 and BG-502 atthe same potential cycle. While the reduction peaks for BG-501and BG-502 observed at �1.32 V and �1.45 V, respectively,which attributed to cyano carboxylic acid group. The reductiononset of BG-501 shifted toward positive voltage with respect to

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3600.025

0.030

0.035

0.040

0.045

0.050

0.055

0.060

0.065

0.070

Nor

mal

ized

Flu

orec

senc

e (a

.u)

Opt

ical

Den

sity

(A)

Wavelength (nm)

Absorbtion of BG501 Fluorescence of BG501

0.00

0.02

0.04

0.06

0.08

0.10

Opt

ical

Den

sity

(A)

Wavelength (nm)

Absorbtion of BG502

Nor

mal

ized

Flu

ores

cenc

e (a

.u.) Fluorescence of BG502

2800.00.10.20.30.40.50.60.70.80.91.01.11.21.31.41.51.6

Opt

ical

Den

sity

(A)

Wavelength (nm)

Absorption of BG501 on TiO2

Absorption of BG 502 on TiO2

400 440 480 520 560 600 640 680 360 400 440 480 520 560 600 640 680

320 360 400 440 480 520 560 600 640 680

Fig. 1. Absorbtion and fluorescence spectra of, (a) BG-501, (b) BG-502 in chloroform and (c) absorbtion spectra of BG-501 and BG-502 adsorbed on TiO2.

Table 1Optical and electrochemical properties of molecules.

Dye kabsa/nm e/M�1 cm�1 kem

b/nm E0-0c/(eV) Eox

d/V Ered/V ELUMOe/eV EHOMO

f /eV

BG-501 435 66900 561 2.52 +0.95 �1.32 2,82 5,34

+1.26

BG-502 435 68300 556 2.52 +0.92 �1.45 2,79 5,31

+1.26

a Absorption spectra were measured in chloroform.b Emission spectra were measured in chloroform.c E0-0 was determined from intersection of absorptional and emission spectra in chloroform.d Redox potential of dyes was measured in CH3CN with 0.1 M TBAPF6 with the scan rated 100 mV s�1.e ELUMO was calculated by Eox�E0-0.

C. Zafer et al. / Solar Energy Materials & Solar Cells 94 (2010) 655–661658

BG-502. The reduction potential is increased by substitutingelectron accepting moiety into molecule structure. Due to thecarboxylic acid group has strong electron accepting property, thereduction of the molecule occurs on this group. The reason ofshifting reduction potential between BG-501 and BG-502 is due toelectron donating properties of alkyl chains attached to nitrogenatoms [13]. For this reason BG-502 is more difficult to reduce thanBG-501 because of alkyl chains. However the two oxidation peakswere observed for both of the BG-501 and BG-502. On thecontrary to the occurrence of reduction peaks, the oxidation peaksfor each dyes detected at almost the same potential. All redox

potentials were measured versus Ag/AgCl reference electrode andthe energy levels were calculated with respect to oxidationpotential of Fc/Fc+ reference at +0.41 V. The cyclic voltammo-grams of dyes shown in Fig. 2 and all electrochemical parametersof BG-501 and BG-501 are summarized in Table 1.

3.3. Photovoltaic Performance

Photovoltaic performances of the BG-501, BG-502 and Z907were shown in Fig. 3 and results were summarized in Table 2.

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Fig. 2. Cyclic Voltammograms of (a) BG-501 and (b)BG-502.

Fig. 3. I–V curves of FTO/TiO2/Dye/I�/I3�/Pt/FTO system, under the dark and

standard conditions (1,5AM, 100 mW/cm2).

Table 2DSSC performance parameters of dyesa.

Dye Jsc (mA/cm2) Voc (mV) FF M. Power (

BG-501 7.46 560 0.60 2.49

BG-502 8.40 660 0.57 3.18

Z907 15.29 600 0.46 4.20

a Performances of DSSCs were measured with 1 cm2 working area.

C. Zafer et al. / Solar Energy Materials & Solar Cells 94 (2010) 655–661 659

Under standard global AM 1.5 solar irradiation, the BG-501sensitized cell gave a short-circuit current density (Jsc) of 7.46 mAcm�2, open circuit voltage (Voc) of 0.56 V, and a fill factor of 0.60,corresponding to an overall conversion efficiency (Z) of 2.49%. Atsame conditions, DSSC based on BG-502 exhibited a Jsc: 8.40 mAcm�2, Voc: 0.66 V and a FF: 0.57 with an overall conversionefficiency of Z: 3.18%. Both of the dyes were compared withreference dye Z907 which yield an overall conversion efficiency of4.20%. The higher efficiency of the BG-502 sensitized cellcompared to the BG-501 can be attributed to higher electroninjection yield and slower charge recombination rate due tolonger alkyl chain avoiding the aggregation on TiO2 surface. Theaggregation on the TiO2 surface increases the intermolecularcharge transfer between dye molecules and decreases the chargetransfer from dye molecules to TiO2 electrode. These results in anincrease of charge transfer resistance of TiO2/dye/electrolyteinterface (Fig. 5). The octyl chain effected the Voc remarkably asreflected from the Voc enhancement of �100 mV from BG-501 toBG-502 (Table 2) which can be explained by (1) a shift of thepotential of TiO2 band and (2) electron lifetime (Fig. 6) [14,15].

Fig. 4, shows action spectra of monochromatic incidentphoton-to-current conversion efficiencies (IPCE) for DSSCs basedon BG-501 and BG-502. The IPCE spectra for dyes BG-501 andBG-502 show maxima at 440 and 430 nm, efficiencies about 27%and 38%, respectively. BG-502 exhibits large IPCE spectrum incomparison with BG-501 indicating better charge injectionproperties and less charge recombination with BG-502, whichcorrespond to photovoltaic data.

Electrical impedance spectroscopy (EIS) was performed toelucidate the photovoltaic results further and clarify the chargetransfer resistances (Rct) of TiO2/dye/electrolyte interface.

A typical EIS spectrum of DSSC exhibits three characteristicsemicircles in a Nyquist plot and three frequency peaks in a Bodeplot as shown in Figs. 5 and 6, respectively. The lower frequency

mW/cm2) Jmpp (mA/cm2) Vmpp (mV) Efficiency (%)

6.56 380 2.49

7.23 440 3.18

11.66 360 4.20

Fig. 4. Incident Photon-to-Current Conversion Efficiency (IPCE) spectra of BG-501

and BG-502.

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Fig. 5. Nyquist plots of BG-501 and BG-502 sensitized cells measured in the dark

under forward bias of �0.66 V.

Fig. 6. Bode Phase plots of BG-501 and BG-502 sensitized cells measured in the

dark under forward bias of �0.66 V.

C. Zafer et al. / Solar Energy Materials & Solar Cells 94 (2010) 655–661660

peak (right semicircle) corresponds to the Nernstian diffusionwithin the electrolyte; the middle-frequency peak (middlesemicircle) corresponds to the electron recombination at theTiO2/dye/ electrolyte interface and the high frequency peak (leftsemicircle) corresponds to the charge transfer at counterelectrode.

Fig. 5 compares the data for BG-501 and BG-502 sensitizedcells measured in the dark under forward bias of �0.66 V. As seenfrom the Nyquist plot in Fig. 5, charge transfer resistance (Rct) forBG-501 is higher than BG-502 indicating that electron recombi-nation is higher with BG-501. Higher electron injection yield andslower charge recombination rate in BG-502-based DSSC, yield inhigher short-circuit current (Jsc).

From the Bode Phase plot shown in Fig. 6, electron lifetimes forDSSCs based on BG-501 and BG-502 can be estimated [14]. Twopeaks located at middle and high frequency, correspond to largesemicircle and small semicircle in the Nyquist plot, respectively.The reciprocal of the peak frequency for the middle-frequencypeak is regarded as the electron lifetime since it represents thecharge transfer process at the TiO2/dye/electrolyte interface. It isclear that, the electron lifetime with dye BG-502 is much longer

than that obtained from BG-501. According to this result, it isevident that Voc increase from BG-501 to BG-502.

4. Conclusions

Two new carbazolocarbazole derivatives, BG-501 and BG-502with different alkyl moieties , were synthesized for DSSCapplications. The products were characterized by H1-NMR, C13-NMR, UV–vis, CV and EIS to determine the photophysical andelectrochemical properties. A solar-to-electric conversion effi-ciency of 3.18% and 2.49% are achieved with this novel dyes,compared to 4.2% for Z907 dye under the standard AM 1.5conditions. N-Alkyl substituents are significantly effective onphotovoltaic performance. Remarkable effect of octyl chain onVoc is interpreted by electron lifetime. EIS measurements depictsthat alkyl moieties on carbazole improves electron lifetime.

Further modifications on carbazolocarbazole core (attachingdonor moieties like thiophene or extending the p-conjugationsystem etc.) which may improve the solar cell performance are inprogress and will be published soon.

Acknowledgements

We acknowledge partial funding by the European Commission(FP6 MOLYCELL STREP project-SES-CT-2003-502783 and CAProject OrgaPVNET), Scientific and Technical Research Council ofTurkey (TUBITAK). We appreciate the project support funds of theState Planning Organization of Turkey (DPT).

Appendix. Supplementary material

Supplementary data associated with this article can be foundin the online version at doi:10.1016/j.solmat.2009.11.014.

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