effect of a redox electrolyte in mixed solvents on the photovoltaic performance of a dye-sensitized...

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Solar Energy Materials & Solar Cells 90 (2006) 649–658

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Effect of a redox electrolyte in mixed solvents onthe photovoltaic performance of a dye-sensitized

solar cell

Atsushi Fukui, Ryoichi Komiya, Ryohsuke Yamanaka,Ashraful Islam, Liyuan Han�

Ecological Technology Development Center, Sharp Corporation, 282-1 Hajikami, Katsuragi,

Nara 639-2198, Japan

Received 19 October 2004; accepted 30 January 2005

Available online 31 August 2005

Abstract

The effect of the iodide/triiodide redox electrolyte in various organic solvents on the

photoelectrochemical properties of bis(tetrabutylammonium) cis-bis(thiocyanato)bis(4-car-

boxy-2,20-bipyridine-40-carboxylato)ruthenium(II)-sensitized nanocrystalline TiO2 solar cells

was studied. Solvents with large donor numbers dramatically enhanced the open-circuit

voltage (Voc), but usually reduced the short-circuit photocurrent density (Jsc). For a mixed

solvent of tetrahydrofuran (THF) and acetonitrile, Voc increased and the fill factor decreased

with increasing THF concentration, but Jsc remained relatively constant. As the partial charge

of the N or O atom of the solvent molecule increased, Voc increased, but Jsc was unchanged up

to a certain value of the partial charge (for THF, �0.46). For cells using 0.3M 4-tert-

butylpyridine and 20 vol% THF in the electrolyte, a short-circuit photocurrent density of

18.23mA cm�2, an open-circuit voltage of 0.73V, a fill factor of 0.73, and an overall

conversion efficiency of 9.74% were obtained.

r 2005 Elsevier B.V. All rights reserved.

Keywords: Dye-sensitized solar cell; Electrolyte; Organic solvent; Ionic conductivity; Dielectric constant

see front matter r 2005 Elsevier B.V. All rights reserved.

.solmat.2005.01.020

nding author. Tel.: +81745 63 3539; fax: +81 745 63 3306.

dress: [email protected] (L. Han).

www.elsevier.com/locate/solmat

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A. Fukui et al. / Solar Energy Materials & Solar Cells 90 (2006) 649–658650

1. Introduction

Gratzel et al. [1–4] have developed a highly efficient dye-sensitized solar cell(DSC) based on a nanocrystalline TiO2 thin-film electrode and an I�=I�3 redoxelectrolyte. In an attempt to improve the performance of the solar cell, someresearchers have studied the nanostructured semiconductor electrodes [3,5–7], themetal complex sensitizer [8–10], and the redox electrolyte [11–15]. But few haveinvestigated the influence of electrolytes on the photovoltaic performance ofthe DSC.

Kebede et al. [11] investigated the spectrophotometric properties of I�, I2, and theI�/I2 mixture in various organic solvents. They found that the spectral shift of the I2solutions is well correlated with the donor number (DN) of the solvent, andpostulated that the increase in the efficiency of the DSC by addition of pyridinederivatives to an I�=I�3 acetonitrile (AcN) solution was due to the formation of thedipyridine complex.

Hara et al. [12] studied the effect of the electrolyte, including the redox species,counter cation, and solvent composition, on the performance of mercurochrome-sensitized nanocrystalline TiO2 solar cells, and found that the short-circuitphotocurrent density (Jsc) increases with increasing Li+ molar conductivity andthat a large donor number leads to a large open-circuit voltage (Voc), while Jscdecreases sharply.

In this paper, we studied the effect of the I�=I�3 redox electrolyte in variousorganic solvents on the photoelectrochemical behavior of bis(tetrabutylammonium)cis-bis(thiocyanato)bis(4-carboxy-2,20-bipyridine-40-carboxylato)ruthenium(II)(N719)-sensitized nanocrystalline TiO2 solar cells, especially the effect of mixedsolvents in enhancing cell performance.

2. Experimental

TiO2 paste (Ti-nanoxide D/SP, Solaronix, Switzerland) was screen-printed on anF-doped SnO2 (FTO) substrate (about 12O/&, Nippon Sheet Glass) and thensintered at 500 1C for 1 h [16]. The film thickness is 30 mm and the amount ofadsorbed dye on TiO2 electrode is 1.66� 10�7mol/cm2.

The sintered electrodes were treated with 40mM TiCl4 aqueous solution at 70 1Cfor 20min to make a good mechanical contact between the TiO2 particles and theconducting glass matrix. After sintering at 500 1C and cooling to about 80 1C, theTiO2 electrodes were dye-coated by immersing them into a solution of N719 at roomtemperature overnight. The dye solution (0.4mM N719) was prepared in tert-butylalcohol/acetonitrile (1:1). A 3000 A-thick Pt film was deposited on the FTO substrateby the DC sputtering method, and the Pt-coated FTO substrate was used as thecounter electrode. The constituents of the electrolyte were 0.6M 2,3-dimethyl-1-propylimidazolium iodide (DMPII, Tomiyama Chemical), 0.1M LiI (Aldrich),0.05M I2 (Aldrich), and 0.3M 4-tert-butylpyridine (TBP, Aldrich, if necessary) invarious solvents.

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A. Fukui et al. / Solar Energy Materials & Solar Cells 90 (2006) 649–658 651

Commercially available stainless steel electrode cells were used for theconductivity measurements. The conductivity of the redox electrolyte prepared inthe cell (diameter, 8mm; thickness, 0.5mm) was determined at 298K by using afrequency-response analyzer (Solartron 1255B impedance analyzer) to measure thecomplex impedance in the frequency range 10Hz – 250 kHz (signal amplitude,5mV). The dielectric constant of the solvent was measured at 298K with thestainless steel electrode cells and the frequency-response analyzer described above.The photoelectrochemical properties of the two-electrode, sandwich-type closed cellwere measured with a solar simulator light source (AM 1.5, 100mWcm�2; WXS-155S-10, Wacom Denso Co., Japan) and a computer-controlled digital source meter(Keithley, Model 2400).

Acetonitrile (AcN, Aldrich), formamide (FA, Aldrich), N-methylpyrrolidone(NMP, Aldrich), N-methylformamide (NMF, Aldrich), dimethyl sulfoxide (DMSO,Aldrich), N,N-dimethylformamide (DMF, Aldrich), tetrahydrofuran (THF,Aldrich), and their mixtures were used as solvents for the redox electrolytes.

3. Results and discussion

Table 1 shows the effect of various organic solvents in the liquid electrolyte on thephotovoltaic performance of N719 dye-sensitized nanocrystalline TiO2 solar cells.The electrolyte composition was 0.6M DMPII, 0.1M LiI, 0.05M I2. Because of thevery low solubility of DMPII in THF, the cell performance could not be measured inthis solvent. Furthermore, with FA or NMF, the sensitizer partially desorbed fromthe TiO2 surface into the electrolyte after photovoltaic measurement. The largedielectric constants of FA and NMF led to desorption of the sensitizer, which maydecrease Jsc. Therefore, the optimum dielectric constant of the solvent was expectedto be 30–50.

Table 1

Effect of electrolyte in various solvents on the photovoltaic properties of dye-sensitized TiO2 solar cellsa

Solvent DN Dielectric

constant

Viscosity

(cP)

Jsc(mAcm�2)

Voc (V) FF Efficiency

(%)

Acetonitrile (AcN) 14.1 35.9 0.33d 18.34 0.603 0.678 7.50

Tetrahydrofuran (THF)b 20.0 7.6 0.47d — — — —

Formamide (FA)c 24.0 111.0 3.30e 7.68 0.697 0.660 3.53

N,N0-Dimethylformamide (DMF) 26.6 36.7 0.80e 11.72 0.744 0.627 5.47

N-Methylpyrrolidone (NMP) 27.3 32.2 1.65e 10.19 0.747 0.424 3.20

Dimethyl sulfoxide (DMSO) 29.8 46.5 2.00e 8.32 0.780 0.596 3.87

N-Methylformamide (NMF)c 49.0 182.4 1.73e 10.84 0.729 0.688 5.44

aElectrolyte composition: 0.6M DMPII, 0.1M LiI, 0.05M I2.bDMPII was not dissolved.cN719 dye partially desorbed from the TiO2 electrode.d303K.e298K.

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Table 1 also shows that solvents with large DNs enhanced Voc but usually reducedJsc, which agrees with the photovoltaic results of Hara et al. [6] for mercurochrome-sensitized nanocrystalline TiO2 solar cells. A large short-circuit photocurrent densityof 18.34mA cm�2 was obtained in AcN (DN 14.1), but the open-circuit voltage(0.60V) was smaller than that for other solvents. In DMSO (DN ¼ 29.8), the open-circuit voltage increased to 0.78V, but Jsc decreased to 8.32mA cm�2. The increasein open-circuit voltage with increasing DN may be due to the suppression of the darkcurrent at the semiconductor electrolyte junction, arising from the reduction oftriiodide by conduction band electrons. This behavior is explained by the diodeequation:

Voc ¼ ðnRT=F Þ lnððJsc=JoÞ � 1Þ,

where n is the ideality factor, whose value is between 1 and 2; Jsc and Jo are the short-circuit photocurrent and reverse saturation current, respectively; and R and F are theideal gas and Faraday constants, respectively. Owing to their relatively small size,solvent molecules can be adsorbed on the bare TiO2 surface of nanometer-sized, dye-impenetrable pores and block the active surface sites of the TiO2 electrode, which isresponsible for triiodide reduction. Adsorption of solvent molecules with large DNonto the free areas of the TiO2 surface leads to a negative shift in the conductionband level of the TiO2 electrode, in addition to the suppression of back electrontransfer from the TiO2 electrode to I�3 , and so leads to a large Voc.

The small photocurrents in solvents with large DNs are due to a decrease in thedriving force for the electron injection process from the LUMO of the N719 dye intothe conduction band of TiO2. Moreover, when the value of viscosity of solventsincreases (from 0.33 to 3.30 cP), Jsc decreases from 18.34 to 7.68mA cm�2. So, largeviscosity may decrease the ion mobility in electrolyte and have large influence on Jsc.

A high cell efficiency was observed in AcN due to its large Jsc value (Table 1). ButVoc in AcN was smaller than in other solvents because of efficient back electrontransfer from the TiO2 electrode to I�3 at the semiconductor electrolyte junction.Therefore, to improve cell performance, a solvent with a large DN (e.g., DMF, THF,NMP, or DMSO) was mixed with AcN to increase the Voc.

Fig. 1a and b shows the effect of the DNs of DMF/AcN (filled squares), THF/AcN (filled circle), NMP/AcN (open squares), and DMSO/AcN (open circles) mixedsolvents on Jsc and Voc in dye-sensitized TiO2 solar cells. The donor numbers of themixed solvents were estimated by the additivity rule, described by the followingequation:

Dmixed solvent ¼ w=100DAcN þ ð100� wÞ=100Dadded solvent.

Here, Dmixed solvent, DAcN, and Dadded solvent are the donor numbers of the mixedsolvent, AcN, and the added solvent, respectively, and w is the volume percent ofAcN in the mixed solvent. For DMF/AcN, NMP/AcN, and DMSO/AcN, Jscdecreased with increasing DN of the mixed solvent. The decrease of Jsc can beexplained by the upward shift of the conduction band level of the TiO2 electrodeowing to the adsorption of polar solvent molecules (DMF, DMSO, NMP) onthe TiO2 surface, and the decrease of the energy difference between the LUMO and

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2 3 4 5 6

7

9

10

11

12

13 1415

8

1

20

18

16

14

12

10

6

8

J sc (

mA

/ cm

2 )

10 20 30 40

DN(a)

2

3 413

14

15711

1210

9

5

8

1

6

(b)

10 20 30 40

DN

0.8

0.7

0.6

Voc

(V

)

0.5

Fig. 1. Dependence of (a) Jsc and (b) Voc of dye-sensitized TiO2 solar cells on the DN of various organic

solvents and their mixed solvents (1: AcN, 2: 10% THF/90% AcN, 3: 20% THF/80% AcN, 4: 30% THF/

70% AcN, 5: 50% THF/50% AcN, 6: 70% THF/30% AcN, 7: 10% DMSO/90% AcN, 8: 20% DMSO/

80% AcN, 9: 50% DMSO/50% AcN, 10: DMSO, 11: 50% DMF/50% AcN, 12: DMF, 13: 10% NMP/

90% AcN, 14: 20% NMP/80% AcN, 15: 30% NMP/70% AcN). Electrolyte composition: 0.6M DMPII,

0.1M LiI, 0.05M I2.


the conduction band, resulting in a decrease of the driving force for electroninjection, and/or the increase of viscosity, resulting in the decrease of the ionmobility in electrolyte.

In contrast, for THF/AcN mixed solvent, Jsc remained relatively constant withincreasing DN. Therefore, the DN-independent behavior of Jsc in THF/AcN impliesthat the conduction band level of the TiO2 electrode does not change markedly afteradsorption of THF on the TiO2 surface.

For DMF/AcN, DMSO/AcN, NMP/AcN, and THF/AcN, Voc increased withincreasing DN of the mixed solvent (Fig. 1b). In a mixed solvent containing 10 vol%of DMF, DMSO, NMP, or THF, Voc was markedly higher than that in 100 vol%AcN, and Voc gradually increased with increasing DN of the mixed solvent.However, in THF/AcN, Voc remained relatively constant for mixtures with420 vol% of THF in AcN. Upon addition of 20 vol% of THF (DN 20) to AcN(DN 14.1), Voc increased from 0.60 to 0.65V without a decrease in Jsc, whichindicates that THF molecules, when adsorbed on the bare TiO2 surface, suppressback electron transfer from the TiO2 electrode to I�3 without changing theconduction band level of the TiO2 electrode. However, for DMF/AcN, DMSO/AcN, and NMP/AcN, Voc increased and Jsc decreased with increasing DN. Theseresults indicate that DMSO (DN 29.8), DMF (DN 26.6), and NMP (DN 27.3)molecules adsorbed on the bare TiO2 surface, suppressed back electron transfer fromthe TiO2 electrode to I�3 , and negatively shifted the conduction band level of the TiO2

electrode, resulting in a marked increase of Voc.To estimate the electron-donating extent of the lone-pair electrons in the nitrogen

or oxygen atoms of the solvent molecules, we calculated their partial charges at the

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1

2

3

4

56

1 2 3

5

4

6

20

18

16

14

12

10

8

6

4

2

0

J sc

(mA

cm

-2)

-0.2 -0.4 -0.6 -0.8 -1 -1.2

Partial charge of the O/N atom

0.8

0.7

0.6

0.5

0.4

Voc

(V

)

Fig. 2. Dependence of Jsc and Voc on the calculated partial charge of the N or O atom of the solvent

molecule. The partial charge is calculated for solvent molecules in AcN because in pure solvents, the

deposition of dye from TiO2 surfaces and viscosity of solvents influence on Jsc and Voc, and the effects of

the partial charge of molecule solvents does not appear (1: AcN, 2: 0.3M TBP in AcN, 3: 20% THF/80%

AcN, 4: 20% NMP/80% AcN, 5: 20% DMF/80% AcN, 6: 20% DMSO/80% AcN).


PM5 level using the MOPAC system available in the CAChe package. Fig. 2illustrates the influence of the partial charge on the N or O atom of the solventmolecule on the Jsc and Voc of the cells. As the partial charge on the N or O atomincreased, Voc increased, but Jsc was unchanged up to a certain value of the partialcharge (for THF, �0.46) and then decreased with further increase in the partialcharge. The larger the partial charge on the N or O atom in the solvent molecule, theeasier and more often the solvent can be adsorbed onto the Lewis acid sites of theTiO2 surface, which would cause a higher Voc by suppressing back electron transferfrom the TiO2 electrode to I�3 . Solvent molecules with a high value of partial charge(�0.5) would also lead to a negative shift in the conduction band edge of TiO2,resulting in a decrease in the driving force for electron injection from the excited dyeand thus a decrease in Jsc. In the case of THF/AcN mixed solvent, interactionbetween TiO2 electrode and the THF molecules may be small because of their lowvalue of partial charges (Fig. 2). So, THF molecules, when adsorbed on the bareTiO2 surface, suppress back electron transfer from the TiO2 electrode to I�3 withoutmarked change of the conduction band level of the TiO2 electrode.

Fig. 3a shows the dependence of the cell properties on the DMSO:AcN mixingratio. Voc increased and Jsc decreased with increasing DMSO concentration in theDMSO/AcN mixed solvent. In contrast, the fill factor (FF) increased from 0.68 to0.75 when the DMSO concentration in the DMSO/AcN mixed solvent was0–10 vol%; but above 10 vol% DMSO, FF decreased from 0.75 to 0.58. Theviscosity of AcN is 0.33 cP at 303K and that of DMSO is 2.00 cP at 298K. Thus, thedecrease in FF above 10 vol% DMSO may be due to the increase in the viscosity ofthe liquid electrolyte, which may decrease in ionic conductivity. The increase in FF inthe range 0–10 vol% DMSO is due to the large decrease in Jsc after addition of

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Table 2

Effects of THF and TBP on the photovoltaic properties of dye-sensitized TiO2 solar cells

Solvent mixed

with ACN

Vol% of

solvent

Concentration

of TBP (M)

Jsc(mAcm�2)

Voc (V) FF Efficiency

(%)

— 0 0.0 18.34 0.603 0.678 7.50

DMSO 10 0.0 13.79 0.734 0.750 7.59

DMSO 20 0.0 13.02 0.740 0.700 6.81

THF 10 0.0 18.23 0.637 0.671 7.79

THF 20 0.0 18.45 0.652 0.665 8.00

THF 30 0.0 18.43 0.651 0.656 7.87

THF 0 0.3 18.14 0.716 0.696 9.03

THF 10 0.3 18.13 0.719 0.723 9.42

THF 20 0.3 18.23 0.730 0.732 9.74

THF 30 0.3 18.18 0.725 0.711 9.38

20

18

16

14

12

10

8

6

18

16

14

12

10

8

6

4

2

J sc

(mA

/ cm

-2),

Eff

i. (%

)

J sc

(mA

/ cm

-2),

Eff

i. (%

)

0

Volume of DMSO in DMSO :AcN=x:100-x (vol%) Volume of THF in THF :AcN=x:100-x (vol%)

0.8

0.7

0.6

0.5

Voc

(V),

FF

0.8

0.7

0.6

0.5

Voc

(V

), F

F

20 40 60 80 0 20 40 60 80100

(a) (b)

Fig. 3. Dependence of the photocurrent–photovoltage characteristics of dye-sensitized TiO2 solar cells on

the mixing ratio of (a) DMSO/AcN and (b) THF/AcN (filled circles: Jsc, filled squares: Voc, open circles:

eff., open squares: FF). Electrolyte composition: 0.6M DMPII, 0.1M LiI, 0.05M I2.


DMSO. Cell efficiency was not improved with increasing DMSO concentration,owing to the marked decrease of Jsc upon addition of DMSO (Table 2). Similarresults were obtained for the DMF/AcN and NMP/AcN mixed solvents.

Fig. 3b shows the dependence of the cell properties on the THF:AcN mixing ratio.Voc increased and FF decreased with increasing THF concentration in the THF/AcNmixed solvent, but Jsc remained relatively unchanged with increasing THFconcentration (Table 2). Because there is little difference between the viscosities ofTHF (0.55 cP at 293K) and AcN (0.33 cP at 303K), the decrease in FF is probablynot due to an increase in solution viscosity. The ionic conductivity at 298Kdecreased with increasing volume percent of THF in the THF/AcN mixed solvent(Fig. 4). This decrease was not due to an increase in the viscosity of the mixed solventbut rather to a decrease in the dielectric constant, resulting in a decrease in thedissociation of LiI and DMPII, and thus decreases in the concentrations of I� andI�3 . The small conductivity in the THF/AcN mixed solvent would lower FF. The

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40

35

30

25

20

15

10

5

0

Die

lect

ric

cons

tant

0 10 20 30 40 50 60

Volume of THF in THF :AcN =x:100-x(vol%)

25

20

15

10

5

0

Ioni

c co

nduc

tivity

(m

S/ c

m2 )

Fig. 4. Dependence of the dielectric constant and the ionic conductivity on the mixing ratio of THF/AcN

(filled circles: dielectric constant; open circles: ion conductivity).


decrease in the dielectric constant with increasing THF concentration in the THF/AcN mixed solvent is also shown in Fig. 4. With increasing THF concentration inthe THF/AcN mixed solvent, the cell efficiency gradually increased, attained a peakwith a maximum at 20 vol% THF, and then decreased again. A short-circuitphotocurrent density of 18.45mA cm�2, an open-circuit voltage of 0.65V, a fillfactor of 0.67, and an overall conversion efficiency of 8.0% were obtained using anelectrolyte composition of 0.6M DMPII, 0.1M LiI, 0.05M I2 in a 20 vol% THF/80 vol% AcN mixed solvent (Table 2).

Gratzel and co-workers reported that adding TBP to the I�=I�3 redox electrolyte inAcN increased the Voc [2]. The enhanced solar cell performance was due to theadsorption of TBP at the bare TiO2 surface and the suppression of back electrontransfer from the TiO2 electrode to I�3 . Adsorption of TBP at the TiO2 surface iscaused by interaction between the Ti(IV) ion, which has Lewis acidity, and the loneelectron pair of TBP. We also constructed a cell using an electrolyte composition of0.6M DMPII, 0.1M LiI, 0.05M I2, and 0.3M TBP in AcN. Fig. 5a shows thephotocurrent–photovoltage characteristics obtained with a sandwich-type closed cellunder simulated AM 1.5 solar light illumination. After TBP was added to theelectrolyte, Voc increased to 0.716V, FF to 0.696, and efficiency to 9.0% (Fig. 5a).

The dependence of the cell properties on the THF:AcN mixing ratio for the sameelectrolyte composition (0.6M DMPII, 0.1M LiI, 0.05M I2, 0.3M TBP) was studied(Table 2). The presence of THF in the electrolyte solution further improved both Voc

and FF, and thus cell efficiency. A short-circuit photocurrent density of18.23mA cm�2, an open-circuit voltage of 0.73V, a fill factor of 0.73, and anoverall conversion efficiency of 9.74% were obtained with a cell using 0.3M TBPand 20 vol% of THF in the electrolyte (Fig. 5a). The incident photon-to-currentconversion efficiency (IPCE) of such a cell is plotted as a function of wavelength inFig. 5b. The IPCE value in the plateau region is about 85%. Considering light lossesin the conducting glass, the efficiency of electric current generation is practically100% in this wavelength region (around 550 nm). Further studies are in progress to

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Cur

rent

Den

sity

(m

A/ c

m2 )

20

18

16

14

12

10

8

6

4

2

0

90

80

70

60

50

40

30

20

10

00 0.2 0.4 0.6 0.8

Voltage (V)

IPC

E (

%)

Wavelenght (nm)

300 400 500 600 700 800 900

(a) (b)

Fig. 5. (a) Photocurrent–photovoltage characteristics of dye-sensitized TiO2 solar cells using AcN (broken

line) and 20% THF/80% AcN mixed solvent (solid line). Electrolyte composition: 0.6M DMPII, 0.1M

LiI, 0.05M I2, 0.3M TBP. Performance parameters are listed in Table 2. (b) Photocurrent action spectrum

of dye-sensitized TiO2 solar cells using 20% THF/80% AcN mixed solvent. The incident photon-to-

current conversion efficiency is plotted as a function of wavelength. A sandwich-type cell configuration

was used to measure this spectrum.


understand the detailed mechanism of the enhancement of the photovoltaicperformance due to the presence of THF in the electrolyte.

4. Conclusion

The effect of the iodide/triiodide redox electrolyte in various organic solvents onthe photovoltaic performance of N719 dye-sensitized nanocrystalline TiO2 solar cellswas investigated. Solvents with large donor numbers (DNs) enhanced the open-circuit voltage (Voc), but usually reduced the short-circuit photocurrent density (Jsc).The decrease of Jsc may be due to the upward shift of the conduction band level ofthe TiO2 electrode due to the adsorption of the high-DN (426) DMSO, DMF, orNMP molecules on the TiO2 surface, thus decreasing the driving force for electroninjection. For cells using a mixed solvent of THF (DN 20) and AcN (DN 14.1), Voc

increased, but Jsc remained nearly constant, as the concentration of THF increased,and thus the overall conversion efficiency increased. The combined effects of THFand the presence of TBP in the electrolyte led to a maximum energy conversionefficiency of 9.74% under AM 1.5 irradiation.

Acknowledgments

This work was supported by New Energy and Industrial Technology DevelopmentOrganization (NEDO) under Ministry of Economy Trade Industry. We are grateful

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to Professor Hironori Arakawa of Tokyo university of Science for helpfulsuggestions.

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