Inorganica Chimica Acta 405 (2013) 252–257

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Inorganica Chimica Acta

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The first application of water-soluble ruthenium phenanthrolinecomplex for dye sensitized solar cells from aqueous solution usingPEDOT:PSS counter electrode versus platinum counter electrode

0020-1693/$ - see front matter � 2013 Elsevier B.V. All rights reserved.http://dx.doi.org/10.1016/j.ica.2013.06.010

⇑ Corresponding author. Tel.: +90 232 3111231; fax: +90 232 3886027.E-mail addresses: suleerten@yahoo.com, sule.erten@ege.edu.tr (S. Erten-Ela),

kasimocakoglu@gmail.com, kasim.ocakoglu@mersin.edu.tr (K. Ocakoglu).

Sule Erten-Ela a,⇑, S. Gokhan Colak b, 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 a b s t r a c t

Article history:Received 18 January 2013Received in revised form 4 June 2013Accepted 5 June 2013Available online 13 June 2013

Keywords:Ruthenium complexNanostructured TiO2

Dye sensitized solar cell

Photovoltaic properties of RuII(4,7-diphenyl-1,10-phenanthroline-disulfonic acid disodium salt)-bis(2,20-bipyridine), [Ru(L1)(bpy)2](PF6)2, [K403] related to its adsorption behaviour onto the nanoporous tita-nium oxide are investigated in association with its water-soluble structure thanks to disulfonic acid diso-dium salt groups. Dye sensitized solar cell is fabricated from aqueous solution of water-solubleruthenium complex. Devices are fabricated in a sandwich geometry using PEDOT:PSS counter electrodeversus platinum counter electrode. Poly(3,4-ethylendioxythiophene)-poly(styrene sulfonate) (PED-OT:PSS) is coated on FTO conducting glass substrate by spin coating process and used as dye sensitizedsolar cell’s counter electrode. Our device performance results show that PEDOT:PSS counter electrode inDSSC has reasonably good catalytic activity and can be used instead of platinum counter electrode. Theenergy conversion efficiency is significantly favourable for DSSC using PEDOT:PSS counter electrode. Theoptical and electrochemical behaviours of K403 are studied. Also cyclic voltammograms and estimated bydensity functional theory studies (DFT) are studied for calculating EHomo and ELumo energy levels of ruthe-nium complex.

� 2013 Elsevier B.V. All rights reserved.

1. Introduction

One of the present technological main challenges is to addressthe problem of the increasing global energy demand using renew-able energy. Dye sensitized solar cells (DSSCs) are attracting wide-spread interest for their low cost and high efficiency. In dyesensitized solar cells, the sensitizer is an important component,harvesting solar energy and converting it to electric current. Ruthe-nium(II) complexes as sensitizers have attracted considerableattention in scientific research and for practical applications dueto their high overall conversion efficiencies, ease of fabricationcompared to silicon based photovoltaic solar cells [1–6]. The broadmetal to ligand charge transfer (MLCT) absorption, strong drivingforce of the charge injection and regeneration, slow recombinationof photoinjected electrons and appropriate localization of the fron-tier orbitals of ruthenium(II) sensitizers are important determi-nants of DSSC performance. Various synthetic strategies havebeen employed in the molecular engineering of sensitizers, suchas extending the conjugation in the ligand. Platinum (Pt) film is

commonly used as the counter electrode because of Pt has a goodcatalytic activity with I3

�which reducing it to iodide. Platinum (Pt)is an expensive material, cheaper alternative instead of platinumhave been intensively investigated. These materials include carbonblack, carbon nanotubes or conductive polymers [7–11]. Poly(3,4-ethylendioxythiophene)–poly(styrene sulfonate) (PEDOT:PSS) isone of the conductive polymers which is considered for use as aDSSC counter electrode. This is because of its unique properties:good catalytic activity, good conductivity and comparatively lowerprice than platinum. Many scientists observe that the pure PED-OT:PSS counter electrode generate lower cell efficiency than plati-num based DSSC [12–16].

In this work, new Ruthenium(II) complex comprising 1,10-phe-nanthroline-disulfonic acid disodium salt, [K403] has been synthe-sized. Water soluble property of ruthenium complex gives anadvantage to fabricate dye sensitized solar cell from aqueous solu-tion. This is the first time solar cell efficiency is reported for oper-ating solar cell from aqueous solution using water-solubleruthenium complex. Furthermore, dye sensitized solar cells arefabricated using platinum counter electrode and PEDOT:PSS coun-ter electrode because of the good catalytic activity and lower priceof PEDOT:PSS counter electrode. Our device performance resultsshow that PEDOT:PSS counter electrode in DSSCs has reasonablygood catalytic activity and can be used instead of platinum counter

O2+

PF6

S. Erten-Ela et al. / Inorganica Chimica Acta 405 (2013) 252–257 253

electrode. Electrochemistry, optical properties and DFT calcula-tions of ruthenium complex are also carried out in this work.

N

N

Ru

N

N

N

N

S

O

O

O Na

S

O

O Na

Fig. 1. Structure of ruthenium(II) complex.

2. Experimental

2.1. Materials

All materials and solvents are reagent grade and are used as re-ceived. All organic solvents are purchased from Merck and Fluka.Dichloro(p-cymene)ruthenium(II)dimer, 2,20-bipyridine (bpy),4,7-diphenyl-1,10-phenanthroline-disulfonic acid disodium salt(L1), NH4PF6 and Sephadex LH-20 are purchased from Aldrich.

2.2. Materials characterization

The UV–Vis absorption spectra of synthesized dyes are recordedin a 1 cm path length quartz cell by using Analytic JENA S 600. Theinfrared (IR) spectra are obtained by using Perkin-Elmer, FT-IR/MIR-FIR (ATR) spectrophotometer. Luminescence spectra of thesamples are recorded at room temperature using a Varian CaryEclipse spectrophotometer. 1H NMR spectra are measured on aBruker 400 MHz spectrometer. Cyclic voltammetry measurementsof synthesized ruthenium complex are taken by using CH-Instru-ment 660 B Model Potentiostat equipment. Dye sensitized solarcells are characterized by current–voltage (J–V) and IPCE measure-ments. All current–voltage (J–V) are done under 100 mW/cm2 lightintensity and AM 1.5 conditions. 450 W Xenon light source (Oriel)is used to give an irradiance of various intensities. J–V data collec-tion is made by using Keithley 2400 Source-Meter and LABVIEW dataacquisition software.

2.3. Synthesis of ruthenium complex

2.3.1. Synthesis of RuII(4,7-diphenyl-1,10-phenanthroline-disulfonicacid disodium salt)-bis(2,20-bipyridine), [Ru(L1)(bpy)2](PF6)2, [K403]

Dichloro(p-cymene)ruthenium(II) dimer (65 mg, 0.106 mol)and 2,20-bipyridine (bpy) (66 mg, 0.425 mol) are dissolved in dryDMF (30 mL) under inert atmosphere, and heated to 80 �C for 4 hwith constant stirring. Then, 4,7-diphenyl-1,10-phenanthroline-disulfonic acid disodium salt (L1) (125 mg, 0.212 mol) is added tothe reaction mixture and heated to 150 �C for 4 h. The reactionmixture is cooled to room temperature and the solvent is removedby using a rotary evaporator under vacuum. The solid is dissolvedin EtOH, and saturated solution of NH4PF6 in EtOH is added to thereaction mixture. The precipitate is collected by filtration. On aSephadex LH-20 column the crude complex is purified with meth-anol as an eluent. 205 mg, 78% yield. Anal. Calc. for RuC44H30N6Na2-

O6S2: C, 55.63; H, 3.18; N, 8.85. Found: C, 55.61; H, 3.15; N, 8.82%.FT-IR (ATR, cm�1): 3628, 3322, 3248, 3066, 2625, 2854, 1739, 1651,1634, 1623, 1607, 1588, 1535, 1424, 1216, 1178, 1127, 1036, 823,765. 1H NMR (CD3OD) d ppm: 1H NMR (CD3OD) d ppm: 8.91–8.66(m, 4H, NCHCH), 8.33 (br s, 2H, NCHCH), 7.99 (br s, 2H, NCHCHC),7.89 (br s, 4H, NCCH, bpy), 7.70 (br s, 2H, CCHCHC), 7.44–7.37 (m,8H, NCHCHCHCHC), 7.16–7.03 (m, 8H, CCHCHCSO3Na). Molecularstructure of complex is shown in Fig. 1.

2.4. Electrochemical studies

The cyclic voltammogram is collected using a CH-Instrument660 B Model electrochemical analyzer. The redox potential of thecomplex is measured using a three-electrode apparatus comprisedof a platinum wire counter electrode, glassy carbon electrode asworking electrode, and an Ag/AgCl reference electrode. The sup-porting electrolyte is tetrabutylamonium hexafluorophosphate(TBAPF6), 0.1 M. Ferrocene is added to each sample solution at

the end of the experiments and ferrocenium/ferrocene redox cou-ple is used as an internal potential reference. EHOMO and ELUMO en-ergy levels of complex are calculated according to previousliterature [7].

2.5. Solar cell fabrication and photovoltaic characterization

Water-soluble RuII(4,7-diphenyl-1,10-phenanthroline-disul-fonic acid disodium salt)-bis(2,20-bipyridine), [Ru(L1)(bpy)2](PF6)2

has been used as photosensitizer in liquid DSSC. TiO2 coated FTOsubstrates are immersed in a 0.5 mM aqueous solution of theruthenium dye complex in water at room temperature overnight,and dried under a flow of nitrogen. The active solar cell area is0.16 cm2. The counter electrodes are prepared on FTO glasses(TEC 8; Hartford Glass) catalyzed with platinum (Platisol, Solaro-nix) and PEDOT:PSS, separately. The cell is sealed using a Surlyn(60 lm, Solaronix) and the 0.6 M N-methyl-N-butyl-imidazoliumiodide (BMII) + 0.1 M LiI + 0.05 M I2 + 0.5 M 4-tert-butylpyridine(TBP) in acetonitrile as redox electrolyte solution is introducedthrough pre-drilled holes in the counter electrode. The filling holesare sealed using Surlyn and a microscope cover glass. Current–volt-age (J–V) curves are obtained using Keithley measurement unit andthe light source consisted of an Oriel Xe-lamp. Schematic drawingof TiO2 based dye sensitized solar cell is shown in Fig. 2.

3. Results and discussion

3.1. Absorption and emission properties

The normalized absorption and emission spectra of the complexmeasured in water are shown in Fig. 3 and the energy maxima andabsorption coefficients are summarized in Table 1. The complexshows a broad absorption band in the 350–550 nm region. Twohigh-energy bands at 280 and 313 nm are assigned to the intrali-gand p–p⁄ transition of bpy and L1 ligands. The lower energy partof the absorption spectrum of the complex is dominated by MLCTtransitions with maxima at 450 nm. The emission spectra of thecomplex K403 in water exhibits a luminescence consisting of a sin-gle band with a maximum at 620 nm (kex: 400 nm).

3.2. Electrochemical properties of ruthenium complex

To investigate the electron transfer from the excited dye mole-cule to the conductive band (Ecb) of TiO2, cyclic voltammetry isperformed for the dye on glassy carbon electrode measured in0.1 M tetrabutylammonium hexafluorophosphate solution inwater/acetonitrile at a scan rate of 50 mV s�1. The EHOMO and ELUMO

O 2

FTO

Gla

ss N

N

Ru

N

N

N

N

S

O

O

ONa

S

O

ONa

2+

PF6

Pedot, J:1.30 mA/cm2

Pt, J:3.41 mA/cm

H2O

TiO2

Fig. 2. Schematic drawing of TiO2 based dye sensitized solar cell.

300 400 500 600 700 8000.00.0

0.4

0.80.8

1.2

0.00.0

0.4

0.80.8

1.2

K403 in water

Wavelength (nm)

Nor

mal

ized

Em

issi

on D

ensi

ty (a

.u.)

Nor

mal

ized

Opt

ical

Den

sity

(a.u

.)

Fig. 3. UV–Vis absorption (solid line) and steady-state luminescence spectra(dashed line) of K403 measured in water, kex: 400 nm.

Table 1UV–Vis absorption and emission properties of ruthenium complex measured inwater.

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

L(p–p⁄) 4d–p⁄

K403 280(5.22), 313(1.32) 450(0.71) 620

Fig. 4. Cyclic voltammogram of ruthenium complex.

Table 2Redox potentials and EHOMO and ELUMO levels of Ru complex.

Eoxidationa

(V)Ereduction

b

(V)Eferrocene

c

(V)EHOMO

d

(eV)ELUMO

e

(eV)Eband

gapf

K403 1.21 �0.92 0.21 5.8 3.67 2.13

a Oxidation potential of Ru complex.b Reduction potential of Ru complex.c Potential of ferrocene, internal reference electrode.d HOMO energy level of Ru complex.e LUMO energy level of Ru complex.f Energy band gap of Ru complex.

254 S. Erten-Ela et al. / Inorganica Chimica Acta 405 (2013) 252–257

energy levels of the dye are measured from the onset oxidation andreduction potentials. The electrochemical behaviors of the complexare shown in Fig. 4. All redox potentials are calibrated versus SCE.The cyclic voltammogram of the complex K403 shows quasi-reversible couple at 1.21 V, which can be readily assigned to theRu(II/III) couple. The reduction peak of the complex is observed0.92 V. It can be seen from the calculated EHOMO and ELUMO energylevels of the complex from the electrochemical measurements inTable 2. The electrochemical measurements of the complex showthat the LUMO level is suitable for an electron injection into theconduction band of the TiO2.

3.3. Photovoltaic performance of DSSCs

Fig. 5 represents the J–V characteristics of the DSSCs and the de-tailed photovoltaic performance parameters are listed in Table 3.The photovoltaic performances of dye sensitized solar cells aremeasured at 100 mW/cm2 under simulated AM 1.5 G solar lightconditions. Under standard global AM 1.5 solar irradiation, the

0.15.00.06

4

2

0

-2

-4

-6

-8

-10

-12

K403_Pt in dark K403_Pt under illumination K403_PEDOT under illumination K403_PEDOT_dark Z907_under illumination Z907 in dark

J (m

A/cm

2 )

Voltage (V)

Fig. 5. J–V curves of TiO2 based dye sensitized solar cells.

Table 3Photovoltaic performance of TiO2 based dye sensitized solar cells.

Jsc (mA cm�2) Voc (mV) FF g (%)

K403_Pt 3.41 327 0.46 0.55K403_PEDOT 1.30 364 0.64 0.29Z907 11.54 772 0.66 5.72

300 400 500 600 700 800 900

0

10

20

30

40

50 K403_Pt_EQE K403_Pt_IQE K403_Pedot_EQE K403_Pedot_IQE Z907_EQE

IPC

E (%

)

Wavelength (nm)

Fig. 6. Incident photon to charge carrier efficiency of dye sensitized solar cells.

Fig. 7. Ground state optimization of K403 at B3LYP/LANL2DZ.

S. Erten-Ela et al. / Inorganica Chimica Acta 405 (2013) 252–257 255

K403 sensitized solar cell with Pt counter electrode gives a shortcircuit photocurrent density (Jsc) 3.41 mA/cm2, an open-circuitvoltage (Voc) of 327 mV, and a fill factor (FF) of 0.46, correspondingto an overall conversion efficiency of 0.55%. Under the same condi-tions, K403 sensitized solar cell with PEDOT:PSS counter electrodegives a Jsc value of 1.30 mA/cm2, a Voc of 364 mV, and an FF of 0.64,corresponding to an overall conversion efficiency of 0.29%.

K403 sensitized solar cell with Pt counter electrode shows en-hanced photocurrent and higher energy conversion efficiencyaccording to K403 sensitized solar cell with PEDOT:PSS counterelectrode. Photovoltaic performance of PEDOT:PSS based DSSCshows that PEDOT:PSS counter electrode has good catalytic activityto reduce I3

� to iodine. PEDOT:PSS counter electrode which hasgood catalytic activity can be used intead of expensive platinumcounter electrode. Fig. 6 shows the action spectra of internal andexternal incident photon-to-current conversion efficiencies (IPCE)for platinum based DSSCs and PEDOT:PSS based DSSC using K403ruthenium complex. All of the dye scan efficiently convert visiblelight to photocurrent in the region from 300 to 700 nm. The Exter-nal IPCE performance of the DSSCs with K403_Pt is higher than theK403_PEDOT:PSS due to best catalytic activity. Its external IPCEreaches a maximum 6% at 450 nm, although the external IPCE ofK403_Pt is lower than K403_PEDOT:PSS in the wavelength regionof 300–700 nm.

3.4. Density functional theory (DFT) calculation

All ground-state geometry optimization calculations are per-formed with the Gaussian 03 program package [17] employingthe DFT method with Becke’s three parameter hybrid functional[18] and Lee–Yang–Parr’s gradient corrected correlation functional(B3LYP) [19]. The LanL2DZ basis set [20] and effective core poten-tial are used for the Ru atom and basis set is applied for all otheratoms. In addition, in order to obtain the vertical excitation ener-gies of the complex, single point time dependent density functionaltheory (TD-DFT) using B3LYP is utilized with LANL2DZ basis set byusing its ground state DFT geometries. Fig. 7 represents the ground

state optimization of ruthenium complex. The molecular orbital(MO) plots of the [K403] molecule involving in the lowest eighthenergy CT transitions (S0?S1, S0?S2, S0?S3, S0?S5, S0?S6,S0?S7, S0?S8 and S0?S9) and two energy LE transitions(S0?S4 and S0?S10) are displayed in Fig. 8. As clearly seenfrom Fig. 8, HOMO is completely localized on the donor [Ru(bpy)(phen)] groups and LUMOs are localized on the acceptor[Ru(bpy)(phen)(phenyl)NaSO3], [Ru(bpy)(phen) (phenyl)], [Ru(b-py)(phen)], [NaSO3] and [(phenyl)NaSO3] groups and it is clear thatthe HOMO–LUMO transitions of [K403] will have p?p⁄ character.As a result, the computational studies revealed that the [K403]complex have thermodynamic stabilities at the ground state inthe gas phase. Density functional theory (DFT) calculations of thecompound’s ground state indicate that the highest occupied MOslocalized on [Ru(bpy)(phen)] group energetically match the semi-conductor valence band; the lowest unoccupied MOs lie abovethe conduction band edge and are [Ru(bpy)(phen)(phenyl)NaSO3],[Ru(bpy)(phen) (phenyl)], [Ru(bpy)(phen)], [NaSO3] and [(phenyl)NaSO3] groups p⁄ in character. Various excited states of thesecomplexes are identified using time-dependent density functionaltheory (TD-DFT). The TD-DFT calculations reveal that the S0?S1 tran-sitions for all complexes are pure charge transfer transitions fromdonor to acceptor molecules between HOMO–LUMO of the complex.

4. Conclusion

Water-soluble ruthenium phenanthroline complex is synthe-sized and used for the fabrication of DSSCs. Dye sensitized solar

(a)

HOMO HOMO-1 HOMO-2 HOMO-3

HOMO-4 HOMO-5

(b)

LUMO LUMO+1 LUMO+2 LUMO+3

LUMO+4 LUMO+5 LUMO+6 LUMO+7

LUMO+8 LUMO+9 LUMO+10

Fig. 8. Schematic representation of the molecular orbitals of K403 obtained from DFT (B3LYP/LANL2DZ) method.

256 S. Erten-Ela et al. / Inorganica Chimica Acta 405 (2013) 252–257

cells based platinum and PEDOT:PSS counter electrodes are fabri-cated from the aqueous solution of ruthenium complex. PED-OT:PSS and platinum based DSSCs exhibit overall conversionefficiencies of 0.29% and 0.55%, respectively. Results show thatPEDOT:PSS counter electrode is promising for dye sensitized solarcell application.

Acknowledgements

We acknowledge financial support from The Scientific andTechnological Research Council of Turkey (TUBITAK) (Grants:110M803 and 112M809) in the framework of European Science

Foundation (ESF-EUROCORES-EuroSolarFuels-10-FP-006), andAlexander von Humboldt Foundation (AvH). I thank to MSc. Caga-tay ELA for his unending support and valuable advices.

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