Effects of Co-adsorbate and Additive on the Performance of Dye-Sensitized Solar Cells: A Photophysical Study

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Solar Energy Materials & Solar Cells 91 (2007) 1426–1431

Effects of co-adsorbate and additive on the performance

of dye-sensitized solar cells: A photophysical study

Kun-Mu Lee

a

, Vembu Suryanarayanan

b

, Kuo-Chuan Ho

a,b,

,

K.R. Justin Thomas

c

, Jiann T. Lin

c

a_{Institute of Polymer Science and Engineering, National Taiwan University, Taipei 10617, Taiwan} b_{Department of Chemical Engineering, National Taiwan University, Taipei 10617, Taiwan}

c_{Institute of Chemistry, Academia Sinica, 115 Nankang, Taipei, Taiwan}

Available online 9 May 2007

Abstract

The effects of deoxycholic acid (DCA) in a dye solution as a co-adsorbate and guanidinium thiocyanate (GuSCN) in an electrolyte as an additive, on the photovoltaic performance of dye-sensitized solar cells (DSSCs) based on organic dye containing thiophene and ﬂuorine segments (FL dye1) and black dye (BD) were investigated. The presence of DCA, up to 2 mM, increases both the photovoltage and photocurrent of the DSSC incorporating BD and further addition leads to decrease in the photocurrent. On the other hand, in the case of the DSSC containing FL dye1, the photocurrent decrease and photovoltage increase with the increase of DCA concentration. The pulsed laser measurement showed that electron lifetime (te) of BD is higher than that of FL dye1 on a TiO2electrode and the electron

diffusion coefﬁcient (De) of the latter is higher than that of the former where there is little deviation for both teand Dewith the addition

of the DCA. The addition of GuSCN into electrolytes does enhance the photovoltage for DSSCs containing either FL dye1 or BD. This can be explained by the further adsorption of guanidinium cations following the dye adsorption that facilitates the self-assembly of dye molecules so as to either reduce the dark current or positively shift the conduction band edge of the TiO2. The value of teincreases and

the value of Dedecreases as a result of GuSCN addition.

Keywords: Dye-sensitized solar cells; Additive; Co-adsorption; Electron transfer

1. Introduction

Dye-sensitized solar cells (DSSCs), composed of a dyed oxide semiconductor, redox electrolyte and counter elec-trode, have proved themselves as the most promising low-cost alternative for the effective conversion of light energy into electrical energy [1]. In these devices, conversion efﬁciency is achieved by ultra-fast injection of an electron from a photoexcited dye into the conduction band of TiO2

and subsequently dye regeneration and holes transporta-tion to the counter electrode.

To improve the performance of the DSSC, several kinds of additives have been added into dye solutions. For example, Kay and Gra¨tzel [2]found that the addition of cholic acid (CA) with porphyrin derivatives improved both the photocurrent and photovoltage of the solar cell. Further, other co-adsorbates such as hexadecylmalonic acid (HDMA)[3], 1-decyl phosphonic acid (DPA)[4]and 3-phenylpropionic acid (PPA)[5]were used with sensitizer onto nanocrystalline TiO2 ﬁlm. Co-grafting of the two

amphiphiles will result in the formation of mixed mono-layer which should be more tightly packed than when the sensitizer is adsorbed alone, providing a more effective insulated barrier for the back electron transfers from TiO2

conduction band to triiodide electrolyte [3–6]. Such unwanted redox process is retarded by the hydrophobic spacer and as a result, the dark current is reduced producing high VOC. Moreover, guanidinium thiocyanate www.elsevier.com/locate/solmat

Corresponding author. Department of Chemical Engineering, Na-tional Taiwan University, Taipei 10617, Taiwan. Tel.: +886 2 2366 0739; fax: +886 2 2362 3040.

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(GuSCN) has been also used as additive in organic electrolytes [7,8], as it remarkably improves the photo-voltage of solar cell due to reduction in the dark current, and increases the conversion efﬁciency of about 11%[7].

Detailed studies on the solar performance, spectroscopic measurements and electron transfer kinetics of DSSCs based on black dye (BD) (Scheme 1a) have been reported

[9]. Recently, we constructed highly efficient DSSCs based on FL dye1 containing thiophene and fluorine segments (Scheme 1b), and good solar energy-to-electricity conver-sion efficiency was achieved (5.23%) under simulated AM 1.5 (100 mW/cm2) light energy source[10]. Based on these reports, the influences of deoxycholic acid (DCA) as a co-adsorbate in the dye solution and GuSCN as an additive in the electrolyte were investigated on the solar cell perfor-mance along with FL dye1 and BD.

2. Experimental

Anhydrous LiI, I2, poly(ethylene glycol) (PEG), and

4-tertiary butyl pyridine (TBP) were obtained from Merck. GuSCN, DCA and titanium (IV) isopropoxide (+98%) were purchased from Acros and used as such. CH3CN and

tertiary butanol were purchased from Merck and water molecules were removed by putting molecular sieves (4 A˚) into the solvent. FL dye1 was synthesized in the laboratory based on our previous report [10] and BD was the commercial product obtained from Solaronix S.A., Au-bonne, Switzerland. 1-Methyl-3-propylimidazolium iodide (MPII) was synthesized and its purity was checked by1H NMR.

The preparation of TiO2 precursor and the electrode

fabrication were carried out according to the procedures mentioned in literatures [8,9] except after autoclave treatment, where the solution was concentrated to

13 wt% and PEG (M.W. 20,000) was added to the TiO2

paste to prevent the ﬁlm from cracking during drying. The TiO2paste was coated on a ﬂuorine-doped tin oxide (FTO)

glass plate (Rsh¼25 O/square, Sinonar Corp., Hsinchu,

Taiwan) using glass rod.

An active area of 0.25 cm2 was selected from sintered electrode and the electrodes were immersed in 3 104M solution of either BD or FL dye1 containing different concentrations of the DCA in acetonitrile and tertiary butanol (in the volume ratio of 1:1) for 24 h. Pt (100 nm thick) sputtered on FTO glass was used as the counter electrode. The electrolytes was composed of 0.8 M 1-methyl-3-propylimidazolium iodide (MPII)/0.1 M LiI/ 0.05 M I2/0.5 M TBP in CH3CN. GuSCN was added to

the electrolyte for few studies.

The photoelectrochemical characterizations of the DSSCs were carried out by using an AM 1.5 simulated light radiation. The light source was a 450 W Xe lamp (Oriel, no. 6266) equipped with a water-based IR filter and AM 1.5 filter (Oriel, no. 81075). The photovoltage transients of assembled devices were recorded with a digital oscilloscope (LeCroy, model LT322). Pulsed laser excitation was applied by a frequency-doubled Q-switched Nd:YAG laser (Spectra-Physics laser, model Quanta-Ray GCR-3-10) with 2 Hz repetition rate at 355 and 532 nm, and 7 ns pulse width at half height. The beam size was slightly larger than 0.25 cm2to cover the area of the device with an incident energy of 1 mJ/cm2. The average electron lifetime can be estimated approximately by fitting a decay of the open-circuit voltage transient with exp (t/te), where

t is the time and te is an average time constant before

recombination. Diffusion coefﬁcient of electron was estimated by ﬁtting a decay of the current transient with exp (t/tc) which was derived from the equation of

continuity for electrons in the conduction band[11], where t and tc are the time and average time constant,

respectively. Then, the apparent diffusion coefﬁcient of electron can be estimated using the equation

De¼

w2

2:35tc

, (1)

where w is the ﬁlm thickness and the factor 2.35 arises from the geometry of the diffusion problem.

UV–vis absorption data was measured by UV–vis spectra (Jasco, V-570). Photoelectrochemical characteris-tics and the AC-impedance measurements of the DSSCs were recorded with a potentiostat/galvanostat (PGSTAT 30, Autolab, Eco-Chemie, The Netherlands) under con-stant light illumination of 100 mW/cm2. The applied bias voltage and AC amplitude were set at open-circuit voltage of the DSSCs at 10 mV between the FTO–Pt counter electrode and the FTO–TiO2-dye working electrode,

respectively, starting from the short-circuit condition and the frequency range explored was 10 mHz–65 kHz. The impedance spectra were analyzed by an equivalent circuit model interpreting the characteristics of the DSSCs[12].

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3. Results and discussion

UV–vis spectra of (a) BD and (b) FL dye1 adsorbed on TiO2 photoanode at various concentrations of DCA are

shown in Fig. 1. For BD without DCA, the absorption peaks at 330–340 nm were assigned to the p–p* absorption band. In Fig. 1a, two metal-to-ligand charge transfer (MLCT) transition bands were observed at 417 and 617 nm, as reported in previous literature [9]. The single absorption peak observed for FL dye1 in Fig. 1bmay be associated with p–p* charge transfer transition [10]. With the additions of DCA on both dyes adsorbed on the TiO2

electrode, no shift in absorption peak was observed; however the absorption maxima on TiO2surface decreased

with increasing concentration of the DCA. A decrease of about 60% and 30% of absorption was observed for the increase in the DCA concentrations of 20 and 30 mM,

respectively, for both BD and FL dye1 (Fig. 1a and b). The co-adsorbate DCA, like the dye, is able to adsorb on the TiO2surface through the carboxyl group. The

co-adsorp-tion of the DCA would decrease the amount of adsorbed dye on the TiO2 surface. Strong interaction between the

adsorbed dye molecules and the oxide molecules in the TiO2 surface leads to aggregate formation [13] and

consequently, broadening of the absorption spectrum was observed during the dye absorption. This might be correlated with dye–TiO2 interactions, dye–dye

interac-tions, or both, and co-adsorption of DCA diminished these interactions on the TiO2surface[14,15].

The photovoltaic performances of DSSCs based on BD and FL dye1 under AM 1.5 irradiation (100 mW/cm2) at various DCA concentrations were summarized inTable 1. For DSSCs made with BD and DCA co-absorbate, with increase in the DCA concentration, open-circuit voltage

400 500 600 700 0.0 0.4 0.8 1.2 20 mM 0 mM Abs (a.u.) BD BD+ 2 mM DCA BD+ 5 mM DCA BD+20 mM DCA 400 500 600 700 0.0 0.4 0.8 1.2 0 mM 30 mM Abs (a.u.) FL dye1 FL dye1+5 mM DCA FL dye1+10 mM DCA FL dye1+20 mM DCA FL dye1+30 mM DCA Wavelength (nm) Wavelength (nm) BD+10 mM DCA

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Table 1

The photovoltaic performance of the DSSCs based on BD and FL dye1 containing different concentrations of DCA Sample no. Dye Concentration of DCA in dye

solution (mM) JSC(mA/cm 2 ) VOC(V) Z (%) FF 1 FL dye1 0 16.96 0.689 7.18 0.615 2 5 14.92 0.731 7.14 0.655 3 10 14.24 0.723 6.80 0.660 4 20 13.52 0.723 6.38 0.650 5 30 12.16 0.747 6.17 0.679 6 BD 0 10.76 0.628 4.33 0.641 7 2 11.24 0.653 4.69 0.640 8 5 9.28 0.671 4.09 0.657 9 10 9.68 0.677 4.48 0.684 10 20 8.00 0.689 3.87 0.702 11 w/o dye 20 0.34 0.585 0.12 0.587 12 bare TiO2 — 0.31 0.463 0.07 0.483 0 5 10 15 20 25 1 2 3 4 Electron lifetime (ms) BD FLdye1 0 5 10 15 20 25 4 8 12 16 20 24

Electron diffusion coefficient (10

-5 cm 2/s) BD FL dye1 Concentration of DCA (mM) Concentration of DCA (mM)

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(VOC) improved very well (for the addition from 0 to

20 mM of DCA, the VOC increased from 0.63 to 0.69 V).

Short-circuit current density (JSC) increased slightly with

2 mM of DCA and further increase in DCA concentration led to decrease in the JSC value. The improvement in the

photovoltage has to do with the suppression of the electron recombination between the injected electrons and I3

ions on the TiO2surfaces; while the photocurrent increases as

the co-adsorbate reduces the dye aggregation either by suppressing the quenching processes of the energy transfer or shifting the conduction band edge of the TiO2 ﬁlms

positively. A different trend in the photovoltaic properties had been observed for the DSSCs based on FL dye1 and DCA co-absorbate, where the JSCvalues decreased and the

VOC improved with the increase in the concentration of

DCA (Table 1).

We had also compared the cell performance of the DSSCs based on bare TiO2 (sample no. 12) and DCA

anchored TiO2 without incorporating dye molecules

(sample no. 11) and found that the JSC, VOCand ﬁll factor

(FF) were higher for the latter (0.34 mA/cm2, 0.585 V, and 0.587) when compared to the former (0.31 mA/cm2, 0.463 V, and 0.483) (Table 1). This indicates that adsorp-tion of the DCA obviously increases the value of both the VOCand FF in the DSSC.

Laser pulse-induced photovoltage transients were re-corded for different concentrations of the DCA (co-adsorbed) on the TiO2electrode and the resultant electron

lifetime (te) and electron diffusion coefﬁcient (De) for BD

and FL dye1 are shown inFig. 2a and b, respectively. The te of BD was higher than that of FL dye1 in all

concentrations of the DCA and it increased slightly with increase in the concentration of the DCA for both the dyes (Fig. 2a). On the other hand, the electron diffusion coefﬁcient (De) in the TiO2 electrode is higher for FL

dye1 than that of BD (Fig. 2b).

The effect of GuSCN as additive on the photovoltaic performance of the DSSC was ﬁrst reported by Gra¨tzel

[7,16]. It was concluded that co-adsorption of GuSCN with dyes such as N 719 and N3 improved the VOCof the DSSC

obviously due to the reduction of dark current[7]. In this study, we have also investigated the effect of addition of 0.2 M of GuSCN in the same electrolyte and the results are shown in Table 2. The optimum concentrations of the DCA taken for this study with the BD and FL dye1 were 2 and 5 mM, respectively, based on previous results (see

Table 1). With the addition of 0.2 mM of GuSCN, the VOC

showed a remarkable increase for the cell containing both the BD and FL dye1 systems though the cell did not contain any DCA (compare sample nos. 1 and 6 inTable 1

with sample nos. 10 _{and 3}0 _in _{Table 2}_{). In the presence}

of DCA, the VOC of the solar cell containing both the

dyes was found to be very high (compare sample nos. 2 and 7 in Table 1with sample nos. 20 _{and 4}0 _in _{Table 2}_).

This effect could be further conﬁrmed from I vs. V curve obtained from the solar cell containing GuSCN without light (dark current) (Fig. 3). In this ﬁgure, the onset potential for the reduction of triiodide shows negative shift which indicates that the addition of GuSCN reduces the electron recombination between the TiO2

surface and I3

[16]. Consequently, the enhancement of photovoltage by adding GuSCN as additive that not only

Table 2

The effect of addition of GuSCN in the electrolyte on the photovoltaic performance of the DSSC (GuSCN ¼ 0.2 M)

Sample no. Dye JSC(mA/cm2) VOC(V) Z (%) FF

10 _{FL dye1 only} _9.52 _0.755 _4.69 _0.650 20 _{FL dye1+5 mM DCA 9.40} _0.791 _5.32 _0.715 30 _{BD only} _8.00 _0.725 _4.12 _0.710 40 _{BD+2 mM DCA} _8.20 _0.755 _4.41 _0.712 -0.6 -8 -4 0

Current density (mA/cm

2) FL dye1 FL dye1/ DCA FL dye1/ DCA-GuSCN Potential (V) -0.4 -0.8 -1.0 -0.2 0.0

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shield the surface against recombination but also shift the band edge to negative potential.

On the other hand, the JSCas well as the cell efﬁciency of

the DSSC was lower in the presence of the DCA and/or GuSCN than their absence (compare Tables 1 and 2). From Table 2, it is understandable that the addition of GuSCN in the electrolyte somehow decreases the efﬁciency of the solar cell slightly. Moreover, the efﬁciency of the solar cell with FL dye1 was always higher than the BD irrespective of different additives. Hence, for further studies the effects of DCA and GuSCN on electron transport mechanism by laser pulse-induced transient photovoltage, FL dye1 was alone taken and the results are shown in Table 3. The addition of GuSCN in the absence of the DCA increased the Deand there was only

little decrease in the electron lifetime (te). However, in the

DCA co-adsorption system, although the DCA can increase the electron lifetime (te), the addition of the

GuSCN in electrolyte decreases the electron lifetime (te)

and increases the Degently.

Consequently, it is noted that the addition of the DCA in dye solution and GuSCN in electrolyte increase the VOC

and lower the JSC of the solar cell, whereas both behave

differently in their electron transport on the TiO2 (DCA

increases the electron lifetimes and decrease the Deand the

GuSCN functions in vice versa). 4. Conclusions

The effect of addition of the DCA and GuSCN on the photovoltaic performance of the DSSCs based on the FL dye1 and BD was studied. Small increase in the VOCand

JSC of the DSSC based on the BD was noted with the

addition of 2 mM of the DCA and further addition increased the VOC only. However, a decrease in the JSC

and increase in the VOC of the DSSC was observed

incorporating FL dye1 for the addition of the DCA. The enhancement in the photovoltage may be correlated with the suppression of the electron recombination between the

injected electrons and I3 ions on the TiO2 surfaces. The

pulsed laser measurement showed that te of the BD is

higher than that of the FL dye1 on the TiO2electrode and

the Deof the later is higher than that of the former where

there is little deviation for both teand Dewith the addition

of the DCA. The addition of GuSCN into electrolytes enhances the photovoltage for the DSSCs containing either FL dye1 or BD in which it increases Deand decreases te.

Acknowledgments

This work was ﬁnancially supported by the Academia Sinica, Taipei, Taiwan, the Republic of China, under Grant AS-94-TP-A02. Helpful discussions with Professor Tien-Yau Luh, of Department of Chemistry, National Taiwan University, are appreciated. We also want to thank Professor King-Chuen Lin and his research group members of Department of Chemistry, National Taiwan University, for the help in making the pulsed laser apparatus available to us.

References

[1] B. O’Reagen, M. Gra¨tzel, Nature 353 (1991) 373. [2] A. Kay, M. Gra¨tzel, J. Phys. Chem. 97 (1993) 6272.

[3] P. Wang, S.M. Zakeeruddin, P. Comte, R. Charvet, R. Humphry-Baker, M. Gra¨tzel, J. Phys. Chem. B 107 (2003) 14336.

[4] P. Wang, S.M. Zakeeruddin, R. Humphry-Baker, J.E. Moser, M. Gra¨tzel, Adv. Mater. 15 (2003) 2101.

[5] P. Wang, S.M. Zakeeruddin, R. Humphry-Baker, M. Gra¨tzel, Chem. Mater. 16 (2004) 2694.

[6] J. He, G. Benko¨, F. Korodı´, T. Polivka, R. Lomoth, B. A˚kermark, L. Sun, A. Hagfeldt, V. Sundstro¨m, J. Am. Chem. Soc. 124 (2002) 4922. [7] M. Gra¨tzel, J. Photochem. Photobiol. A: Chem. 164 (2004) 3. [8] Z.-S. Wang, K. Hara, Y. Dan-oh, C. Kasada, A. Shinpo, S. Suga, H.

Arakawa, H. Sugihara, J. Phys. Chem. B 109 (2005) 3907. [9] K. Hara, T. Nishikawa, M. Kurashige, H. Kawauchi, T. Kashima, K.

Sayama, K. Aika, H. Arakawa, Sol. Energy. Mater. Sol. Cells 85 (2005) 21.

[10] K.R. Justin Thomas, J.T. Lin, Y.-C. Hsu, K.-C. Ho, Chem. Commun. (2005) 4098.

[11] J. van de Lagemaat, A.J. Frank, J. Phys. Chem. B 105 (2001) 11194. [12] C. Longo, J. Freitas, M.A. De Paoli, J. Photochem. Photobiol. A:

Chem. 159 (2003) 33.

[13] M.C. Bernard, H. Cachet, P. Falaras, A. Hugot-Le Goff, M. Kalbac, I. Lukes, N.T. Oanh, T. Stergiopoulos, I. Arabatzis, J. Electrochem. Soc. 150 (2003) E155.

[14] P.V. Kamat, Chem. Rev. 93 (1993) 267.

[15] K. Hara, Y. Dan-oh, C. Kasada, Y. Ohga, A. Shinpo, S. Suga, K. Sayama, H. Arakawa, Langmuir 20 (2004) 4205.

[16] M. Gra¨tzel, J. Photochem. Photobiol. C: Photochem. Rev. 4 (2003) 145.

Table 3

Comparison of electron lifetime and electron diffusion coefﬁcient on the TiO2electrode containing DCA in the absence and presence of GuSCN

FL-a FL-b FL-c FL-d

te(ms) 0.92 0.52 1.35 0.70

De(cm2/s) 2.96 105 5.32 105 2.05 105 3.13 105

FL-a: FL dye1 only; FL-b: FL dye1+0.2 M GuSCN; FL-c: FL dye1+0.5 M DCA; FL-d: FL dye1+0.2 M GuSCN+0.5 M DCA.