Journal of Industrial and Engineering Chemistry 20 (2014) 474–479

Fabrication of thin film nanocrystalline TiO2 solar cells usingruthenium complexes with carboxyl and sulfonyl groups

Sule Erten-Ela a,*, Kasim Ocakoglu b

a Solar Energy Institute Ege University, Bornova, 35100 Izmir, Turkeyb Advanced Technology Research & Application Center, Mersin University, Ciftlikkoy Campus, TR-33343 Yenisehir, Mersin, Turkey

A R T I C L E I N F O

Article history:

Received 23 November 2012

Accepted 7 May 2013

Available online 15 May 2013

Keywords:

Dye sensitized solar cell

Nanocrystalline

Ruthenium complex

Renewable energy

A B S T R A C T

Two ruthenium complexes with carboxyl and sulfonyl groups have been synthesized, [RuII(L1)2(NCS)2]

RuIIbis(4,7-diphenyl-1,10-phenanthroline-disulfonic acid disodium salt)-di(thiocyanate) [K313],

[RuII(L1)2(dcbpy)] RuII bis(4,7-diphenyl-1,10-phenanthroline-disulfonic acid disodium salt)(4,40-

dicarboxy-2,20-bipyridyl) [K314] as photosensitizers. UV–vis, fluorescence emission, AFM and CV

measurements are also supplied for ruthenium complexes. Photovoltaic properties of dye sensitized

nanocrystalline semiconductor solar cells based on Ruthenium complexes which bear carboxyl and

sunfonyl groups have been tested under standard AM 1.5 sunlight. Under the standard global AM 1.5

solar conditions, K314 and K313-sensitized solar cells demonstrate short circuit photocurrent

densities of 14.92 mA/cm2 and 11.23 mA/cm2 and overall conversion efficiencies of 5.09% and 4.02%,

respectively.

� 2013 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights

reserved.

Contents lists available at SciVerse ScienceDirect

Journal of Industrial and Engineering Chemistry

jou r n al h o mep ag e: w ww .e lsev ier . co m / loc ate / j iec

1. Introduction

The sun is our primary source to use of the photovoltaic effect insolar cells in the search for sustainable renewable energy sources.New generation solar cells employing thin film technology arepromising technology [1–3]. Solar cells based on nanocrystallinetitanium dioxide sensitized with an organic or metalorganic dyeshave attracted a great deal of interest since a 1991 researchbreakthrough. This type of organic solar cells are notable forachieving solar power-to-electricity conversion efficiencies ex-ceeding 12% in the laboratory that implies low manufacturing costs[4]. The TiO2 film is generally deposited over a glass substrate witha conductive layer such as fluorine-doped tin oxide (FTO) by screenprinting or doctor blading technique. Dye sensitized solar cellsbased on nanocrystalline TiO2 electrodes have attracted intensiveinterest for scientific and industrial applications due to their highphoto to electricity conversion efficiency and low production cost[5,6]. Also, metaloxide nano-particle layers are of extensive usein the organic photovoltaics (OPV), as a hole blocking layer ininverted geometry bulk-heterojunction (BHJ) solar cells [7,8]. Oneof the essential strategies for improving the performance of solarcells is provided by modification of the organic or metalorganic

* Corresponding author. Tel.: +90 232 3111231; fax: +90 232 3886027.

E-mail addresses: suleerten@yahoo.com, suleerten@ege.edu.tr (S. Erten-Ela).

1226-086X/$ – see front matter � 2013 The Korean Society of Industrial and Engineer

http://dx.doi.org/10.1016/j.jiec.2013.05.004

dyes. Numerous sensitizers such as metal-free organic dyes [9–12],nonruthenium metal dyes [13–16], and ruthenium dyes, [17–24]have been employed to improve energy conversion efficiency.Recently, the best energy conversion efficiency of over 12% (13.1%at 0.5 Sun) was achieved by Yella et al. using a cobalt(II/III)-basedtris(bipyridyl)tetracyanoborate complex as the redox mediator incombination with a custom synthesized donor–p-bridge–acceptorzinc porphyrin dye and another organic cosensitizer [4].

To further increase the efficiency of these cells, much effort hasbeen directed toward the development of highly efficient solarcells based on different dyes [25]. There are several essentialdesign requirements for an efficient sensitizer. The LUMO of thedye must be sufficiently high in energy to promote efficient chargeinjection into the TiO2 film and the HOMO should be sufficientlylow in energy for efficient regeneration of the oxidized dye by thehole-transport material (HTM) [26,27].

In this paper, functionalized ruthenium complexes which havecarboxyl and sulfonyl groups are reported. Their photovoltaicproperties have been tested in dye sensitized solar cells tounderstand the relationship between the molecular structures andefficiencies. Standard global AM 1.5 solar conditions, it is foundthat K314-sensitized solar cell shows higher efficiency and mostlypromising. K314 based dye sensitized solar cell exhibits shortcircuit photocurrent density of 14.92 mA cm�2, open circuitvoltage of 570 mV, filling factor of 0.62 and overall conversionefficiencies of 5.09%. Schematic drawing of DSSC is shown in Fig. 1.

ing Chemistry. Published by Elsevier B.V. All rights reserved.

Fig. 1. Schematic drawing of DSSCs.

S. Erten-Ela, K. Ocakoglu / Journal of Industrial and Engineering Chemistry 20 (2014) 474–479 475

2. Experimental

2.1. Materials

All organic solvents are supplied from Merck and Fluka, andused as received. [RuCl2(p-cymene)]2, 4,7-diphenyl-1,10-phenan-throline-disulfonic acid disodium salt (L1), 1,10-phenanthroline-5,6-dione (L2), 4,40-dicarboxy-2,20-bipyridyl (dcbpy), SephadexLH-20 are purchased from Aldrich.

2.2. Materials characterization

The UV–vis absorption spectra of ruthenium complexes arerecorded in a 1 cm path length quartz cell by using Analytic JENA S600. The infrared (IR) spectra are obtained by using PerkinElmer,FT-IR/MIR-FIR (ATR) spectrophotometer. 1H NMR spectra aremeasured on a Bruker 400 MHz spectrometer. Cyclic voltammetrymeasurements of the complex are taken by using CH-Instrument660 B Model Potentiostat equipment. Atomic force microscopy(AFM) studies are carried out in non-contact mode using a ParkSystem XE-100 SPM instrument. Dye sensitized solar cells arecharacterized by current–voltage (J–V) measurements. All cur-rent–voltage (J–V) are done under 100 mW/cm2 light intensity andAM 1.5 conditions. 450 W Xenon light source (Oriel) is used to givean irradiance of various intensities. J–V data collection is made byusing Keithley 2400 Source-Meter and LabView data acquisitionsoftware.

2.3. Synthesis of ruthenium complexes

All commercially available chemicals and solvents are usedwithout further purification. Synthetic details and characterizationof ruthenium complexes are described below. Molecular structuresof ruthenium complexes are shown in Fig. 2. And ground stateoptimization of complexes are supplied in Fig. 3.

2.3.1. Synthesis of RuII bis(4,7-diphenyl-1,10-phenanthroline-

disulfonic acid disodium salt)-di(thiocyanate), [RuII(L1)2(NCS)2],

[K313]

[RuCl2(p-cymene)]2 (0.1 g, 0.163 mmol) is dissolved in DMF(50 mL) under inert atmosphere, and 4,7-diphenyl-1,10-phenan-throline-disulfonic acid disodium salt (L1) (0.351 g, 0.654 mmol)then added. The reaction mixture is heated to 80 8C for 12 h withconstant stirring. To this reaction flask excess of NH4SCN (0.988 g,13 mmol) is added to the reaction mixture and the refluxcontinued for another 4 h. Finally, the reaction mixture is cooledto room temperature and the solvent is removed by using a rotaryevaporator under vacuum. The solid is extracted with methanol,filtered and dried under vacuum. On a Sephadex LH-20 column thecrude complex is purified with methanol as an eluent. The crudecomplex is re-crystallized from MeOH/Ethyl acetate. 216 mg of theproduct with 93% yield is obtained. Anal. Calc. For RuC50H28N6-

Na4O12S6: C, 46.55; H, 2.19; N, 6.51. Found: C, 46.48; H, 2.18; N,6.48%. FT-IR (ATR, cm�1): 2100 (n-NCS), 2046 (n-NCS), 1377 (n-S55O), 1033 (n-S–O), 786 (n-NCS). 1H NMR (400 MHz, D2O) d ppm:

8.71 (s, 1H, NCHCH), 8.61 (s, 1H, NCHCH), 7.58 (m, J = 1.4 Hz, 4H,CCHCHC), 7.45 (q, J = 1.1 Hz, 2H, NCHCHC), 7.38 (q, J = 1.2 Hz, 2H,NCHCHC), 7.22 (m, J = 2.4 Hz, 4H, CCHCHCSO3Na), 7.17 (m,J = 2.6 Hz, 4H, CCHCHCSO3Na), 6.91 (m, J = 2.4 Hz, 4H,CCHCHCSO3Na), 6.84 (m, J = 2.4 Hz, 4H, CCHCHCSO3Na).

2.3.2. Synthesis of RuIIbis(4,7-diphenyl-1,10-phenanthroline-

disulfonic acid disodium salt)(4,40-dicarboxy-2,20-bipyridyl),

[RuII(L1)2(dcbpy)], [K314]

K314 is synthesized according to the method given in theliterature [28]. 181 mg, 78% yield. Anal. Calc. For RuC60H36N6Na4O16S4: C, 50.81; H, 2.56; N, 5.93. Found: C, 50.79;H, 2.39; N, 5.88%. MS (MALDI): m/z = 1419.1 [M+H]+. FT-IR (ATR,cm�1): 3048 (n-C–H), 2098 (n-NCS), 1719 (n-C55O), 1211 (n-S55O),(n-C–O), 1006 (n-S–O), 837 (n-NSC). 1H NMR (400 MHz, D2O) dppm: 9.56 (t, J = 1.7 Hz, 1H, NCHCHC, dcbpy), 9.43 (t, J = 1.3 Hz, 1H,NCHCHC, dcbpy), 8.73 (s, 1H, NCHCH, L1), 8.62 (s, 1H, NCHCH, L1),8.10 (t, J = 1.1 Hz, 1H, CCHC, dcbpy), 8.02 (t, J = 1.3 Hz, 1H, CCHC,dcbpy), 7.78 (d, J = 3.6 Hz, 1H, NCHCHC, dcbpy), 7.73 (d, J = 3.8 Hz,1H, NCHCHC, dcbpy), 7.62 (m, J = 1.4 Hz, 4H, CCHCHC, L1), 7.44 (q,J = 1.1 Hz, 2H, NCHCHC, L1), 7.40 (q, J = 1.2 Hz, 2H, NCHCHC, L1),7.24 (m, J = 2.4 Hz, 4H, CCHCHCSO3Na, L1), 7.18 (m, J = 2.6 Hz, 4H,CCHCHCSO3Na, L1), 6.95 (m, J = 2.4 Hz, 4H, CCHCHCSO3Na, L1),6.89 (m, J = 2.4 Hz, 4H, CCHCHCSO3Na, L1). 1H NMR spectra ofruthenium complexes are shown in Fig. 4.

2.4. Electrochemical, absorption and emission studies

The cyclic voltammograms of ruthenium complexes are takenusing a CH-Instrument 660 B Model electrochemical analyzer. Theredox potentials of the complexes are measured using a three-electrode cell comprising a platinum wire counter electrode,working electrode, and an Ag/AgCl reference electrode. DMF isused as a solvent and the supporting electrolyte is tetrabutyla-monium hexafluorophosphate (TBAPF6), 0.1 M. Ferrocene is addedto each sample solution at the end of the experiments andferrocenium/ferrocene redox couple is used as an internalpotential reference.

2.5. Photovoltaic characterization

2.5.1. Solar cell fabrication and characterization

A nanocrystalline TiO2 photoelectrode is prepared by screenprinting on conductive glass. Preparation and characterization ofdouble-layer TiO2 electrode are previously described by Wang andco-workers [29]. FTO (SnO2:F, Pilkington TEC-15; Rsheet:15O/&),electrically conductive oxide-coated glasses are used as transpar-ent electrodes. The construction of the dye sensitized solar celldevice requires first cleaning of the fluorine doped tin oxide (FTO)coated glass substrates in a detergent solution using an ultrasonicbath for 15 min, rinsed with water and ethanol. Both sensitizershave been used to manufacture solar cell devices to explorecurrent–voltage characteristics using 7(transparent) + 5(scatter-ing) mm TiO2 transparent layers. TiO2-coated electrodes, aftersintering at 450 8C for 30 min and cooling to 100 8C, and then TiO2

N

N

N

N

COOH

COOH

Ru

N

NaO3S

N

NaO3S

SO3Na

SO3Na

N

N

N

N

Ru

N

NaO3S

N

NaO3S

SO3Na

SO3Na

C S

CS

K314

K313

Fig. 2. Chemical structures of the ruthenium complexes.

Table 1UV–vis apsorption and emission properties of ruthenium complexes measured in

DMF.

lmax, (nm) (e/104 M�1 cm�1) Emission lmax, (nm)

L (p–p*) 4d–p*

K313 281 (8.29) 436 (1.22), 463 (1.31) 625

K314 278 (9.30) 481 (1.33) 635

S. Erten-Ela, K. Ocakoglu / Journal of Industrial and Engineering Chemistry 20 (2014) 474–479476

electrodes are immersed into the solution of 0.5 mM rutheniumcomplexes in a mixture of DMF: acetonitrile: tert-butanol (volumeratio: 3:1:1) containing and kept at room temperature overnight.Ruthenium complexes adsorbed TiO2-coated glasses are washedwith pure chlorobenzene. Pt-counter electrode is deposited on the

FTO glass by coating with a drop of H2PtCl6 solution with the heattreatment at 395 8C for 15 min to give photoanode. The dye-covered TiO2 electrode and Pt-counter electrode are assembledinto a sandwich type cell. The DSSCs have an active area of0.16 cm2 and electrolyte composed of 0.6 M N-methyl-N-butyl-imidazolium iodide (BMII) + 0.1 M LiI + 0.05 M I2 + 0.5 M 4-tert-butylpyridine (TBP) in acetonitrile. Photovoltaic experiments areperformed under AM 1.5 irradiation (100 mW cm�2). J–V datacollection is made by using Keithley 2400 Source-Meter andLabView data acquisition software.

3. Results and discussion

3.1. Absorption and emission properties

The absorption and emission spectra of the complexesmeasured in DMF are shown in Fig. 5, and the energy maximaand absorption coefficients are summarized in Table 1. Thespectrum of complexes K313 and K314 in DMF shows two metal-to-ligand charge transfer (MLCT) bands which are well-resolved[18,19] at 436 and 463 nm for K313 and 481 nm for K314. Theobtained results are very close to the reported MLCT absorptivitiesof other similar complexes [25]. The bands at 281 nm for K313 and278 nm for K314 are assigned to the intraligand p–p* transition ofdcbpy, L1 and L2 ligands. The emission spectra of the complexesare recorded in DMF at room temperature. Emission of rutheniumpolypyridine complexes usually occurs from the lowest-lying3MLCT excited state [26–29]. K313 and K314 in DMF solution at298 K exhibit a luminescence consisting of a single band with amaximum at 625 nm (lex: 470 nm) and 635 nm (lex: 470 nm),respectively.

3.2. Electrochemical properties

The electrochemical behaviors of the complexes have beenstudied in DMF by cyclic voltammetry (Fig. 6). Results are shown inTable 2. All redox potentials are calibrated vs. SCE. The cyclicvoltammograms of the complexes K313 and K314 show quasi-reversible couples at 1.50 V and 1.10 V, respectively, which can bereadily assigned to the Ru(II/III) couple. The difference in theoxidation potentials of the complexes from each other and ananalogous 2,20-bipyridine ruthenium complex (reported 0.55 V vs.SCE) is due to the p-accepting nature of dcbpy ligand. Thereduction peaks of the complexes are observed – 1.54 V and 1.19 Vfor K313 and K314, respectively. The difference between thereductions related to the number of carbonyl and sulfonyl groupsand different electron accepting behavior of the ligands [18]. It canbe seen from the calculated HOMO–LUMO energy levels of thecomplexes from the electrochemical measurements in Table 2. Theelectrochemical measurements of these complexes show thatthe LUMO levels are suitable for an electron injection into theconduction band of the titanium oxide. To gain insight into theelectronic properties of these dyes, electron density maps of theirfrontier orbitals (HOMO and LUMO) were calculated using densityfunctional theory (DFT) [30–32]. Theoretical EHOMO and ELUMO

values of complexes are in good agreement with the experimentaldata (Fig. 7).

Fig. 3. Ground state optimization at B3LYP/LANL2DZ.

Fig. 4. 1H NMR spectra of ruthenium complexes, K313 (a), K314 (b).

S. Erten-Ela, K. Ocakoglu / Journal of Industrial and Engineering Chemistry 20 (2014) 474–479 477

Fig. 6. Cyclic voltammogram of ruthenium complexes.

0,0 0,2 0,4 0,6 0,8 1,0

5

0

-5

-10

-15 K313 under illumination K314 under illumination K314 in dark K313 in dark Z907 in dark Z907 under illumination

J (m

A/c

m2 )

Voltage (V)

η= 5.09 % Isc = 14 .92 mA/cm2 Voc = 570 mV η= 4.02 % Isc = 11 .23 mA/cm2 Voc = 570 mVη= 5.72 % Isc = 11 .54 mA/cm2 Voc = 772 mV

Fig. 8. J–V curve of TiO2 based dye sensitized solar cells.

300 40 0 50 0 60 0 70 0 80 0 90 0

0

1000

2000

3000

0

1000

2000

3000

Nor

mal

ized

Em

issi

on D

ensi

ty (a

.u.)

Waveleng th (nm)

Nor

mal

ized

Opt

ical

Den

sity

(a.u

.) K3 13 K3 14

Fig. 5. UV–vis Absorption (solid lines) and Steady-State Fluorescence Spectra

(dashed lines). Of K313 and K314 Measured in DMF, lex: 470 nm for K313, lex:

470 nm for K314.

Fig. 7. Schematic representation of frontier orbitals of K314 and K313, ALONG.

WITH ISODENSITY PLOts of HOMO and LUMO orbitals. The experimental and

calculated energy levels are displayed in solid and dashed line, respectively.

S. Erten-Ela, K. Ocakoglu / Journal of Industrial and Engineering Chemistry 20 (2014) 474–479478

3.3. Photovoltaic performance

J–V characteristics of the ruthenium complexes K313 and K314-based dye sensitized solar cell in dark and under illumination areshown in Fig. 8 and results are summarized in Table 3. Understandard global AM 1.5 solar irradiation, DSSC based on K313 andK314 exhibited a Jsc: 11.23 mA cm�2, Voc: 570 mV and a FF: 0.65with an overall conversion efficiency of h: 4.02% and Jsc:14.92 mA cm�2, Voc: 570 mV and a FF: 0.62 with an overallconversion efficiency of h: 5.09% respectively. Also Z907 is testedin the same condition with the yield of 5.72%. The higher efficiencyof the K314 sensitized cell compared to the K313 can be attributed

Table 2Redox potentials and EHOMO and ELUMO levels of ruthenium complexes.

Eoxidationa (Volt) Ereduction

b (Volt) Eferrocene

K313 1.50 �1.54 0.47

K314 1.10 �1.19 0.45

a First oxidation potentials of Ru complexes.b Reduction potentials of Ru complexes.c Potentials of ferrocene, internal reference electrode.d HOMO energy level of Ru complexes.e LUMO energy level of Ru complexes.f Energy Band Gap of Ru complexes.

to higher electron injection yield and slower charge recombinationrate due to bulky sulfonyl groups avoiding the aggregation on TiO2

surface. The aggregation on the TiO2 surface increases theintermolecular charge transfer between dye molecules anddecreases the charge transfer from dye molecules to TiO2

electrode. Furthermore, the structure of K314 is similar to thatof the well-known dye Z907, except that the hydrophobic alkylchains of Z907 are replaced by bulky sulfonyl groups whichsuppress charge recombination. Fig. 9 shows the IPCE spectra ofruthenium complexes. All of the dyes scan efficiently convertvisible light to photocurrent in the region from 300 nm to 850 nm.

c (Volt) EHOMOd (eV) ELUMO

e (eV) EBand gapf

5.83 2.79 3.04

5.45 3.16 2.29

Table 3Photovoltaic performances of the compounds under 100 mW/cm2 light intensity

and AM1.5 global radiation.

Sample h (%) Voc (mV) Isc (mA/cm2) FF

K314 5.09 570 14.92 0.62

K313 4.02 570 11.23 0.65

Z907 5.72 772 11.54 0.66

300 40 0 50 0 60 0 70 0 80 0 90 00

10

20

30

40

K313 Z907 K314

IPC

E (%

)

Waveleng th (nm )

Fig. 9. IPCE spectra of ruthenium complexes.

S. Erten-Ela, K. Ocakoglu / Journal of Industrial and Engineering Chemistry 20 (2014) 474–479 479

The external IPCE performance of the DSSCs with Z907 is higherthan the K313 and K314. The external IPCE of K314 reaches amaximum 19% at 500 nm. The external IPCE of K313 is lower thanK314 at 500 nm.

4. Conclusion

Two ruthenium complexes are synthesized for dye sensitizedsolar cells applications. The products are characterized by 1H NMR,UV–vis, FTIR, CV and AFM to determine the photophysical andelectrochemical properties. A solar-to-electric conversion efficien-cies of 5.09% and 4.02% for K314 and K313 are achieved with theseruthenium dyes under the standard AM 1.5 conditions. K314complex comprising carboxy and sulphoxy groups reported in thispaper can show similar behaviors like Z907 reduces back electrontransfer due to the bulky sulphoxy groups.

Acknowledgements

We acknowledge financial support from Scientific and Techno-logical Research Council of Turkey (TUBITAK, Grants: 110M803and 112M809) and to Alexander von Humboldt Foundation (AvH).I thank Mechanical Engineer MSc. Cagatay Ela for proofreading andhis fruitful advices.

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