Efficient dye-sensitized solar cells based on cosensitized metal free organic dyes withcomplementary absorption spectraG. D. Sharma, Surya Prakash Singh, P. Nagarjuna, J. A. Mikroyannidis, R. J. Ball, and Rajnish Kurchania Citation: Journal of Renewable and Sustainable Energy 5, 043107 (2013); doi: 10.1063/1.4811797 View online: http://dx.doi.org/10.1063/1.4811797 View Table of Contents: http://scitation.aip.org/content/aip/journal/jrse/5/4?ver=pdfcov Published by the AIP Publishing

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Efficient dye-sensitized solar cells based on cosensitizedmetal free organic dyes with complementary absorptionspectra

G. D. Sharma,1,a) Surya Prakash Singh,2,a) P. Nagarjuna,2

J. A. Mikroyannidis,3 R. J. Ball,4 and Rajnish Kurchania5

1R & D Center for Engineering and Science, JEC Group of colleges JEC Campus,Kukas, Jaipur 303101, Rajasthan, India2Inorganic and Physical Chemistry Division, CSIR-Indian Institute of Chemical Technology,Uppal Road, Tarnaka, Hyderabad 500607, India3Chemical Technology Laboratory, Department of Chemistry, University of Patras,GR-26500 Patras, Greece4Department of Architecture and Civil Engineering, University of Bath, Bath BA2 7AY,United Kingdom5Department of Physics, Maulana Azad National Institute of Technology (MANIT),Bhopal 462051, Madhya Pradesh, India

(Received 23 January 2013; accepted 7 June 2013; published online 8 July 2013)

Dye sensitized solar cells (DSSCs) were fabricated using two metal-free organic

dyes, TA-St-CA and D, with complementary absorption bands in the visible and

near infrared regions. The power conversion efficiency (PCE) of the cosensitized

DSSC (6.26%) was improved when compared to DSSCs based on individual dyes

TA-St-CA (4.98%) or D (4.22%). The PCE was further enhanced up to 7.19%

when deoxycholic acid (DCA) was added to the mixed dye solution as a

coadsorbant. The enhancement of PCE observed in the cosensitized DSSC by

addition of DCA was attributed to both suppression of dye aggregation and

prevention of the backward electron transfer from the conduction band of TiO2.VC 2013 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4811797]

INTRODUCTION

Low cost dye-sensitized solar cells (DSSCs) have been of great interest as an alternative to

conventional solar cells based on silicon.1 A DSSC is a photoelectrochemical device which con-

tains a dye sensitized nanocrystalline photoanode, iodine/iodide redox coupled electrolyte, and

counter electrode. Among these three components, the sensitizer plays an important role on the

power conversion efficiency (PCE) as well as device stability. Power conversion efficiencies

greater than 11% have been reported for DSSCs sensitized with polypyridyl ruthenium com-

plexes2 and 12.3% for DSSCs sensitized with push-pull porphyrin dyes with cobalt (II/III)

based redox electrolyte.3 However, Ru based sensitizers have the following disadvantages: (a)

high cost and negative environmental impact, (b) the metal to ligand charge transfer (MLCT)

band of these dyes normally have high molar extinction coefficients, and therefore, a thicker

film of nanocrystalline TiO2 is needed for the fabrication of DSSCs.

Metal-free organic dyes have attracted much attention for application as alternatives to

metal complex sensitizers. Their unique advantages include low cost, high molar extinction

coefficient, tunable absorption, and electrochemical properties via suitable molecular design. In

recent years, the performance of DSSCs based on metal-free dyes has been improved and

showed PCEs in the range of 8%–10%.4 These values of PCE are lower than that for the

DSSCs based on metal based sensitizers. Because of a lower photocurrent, determined by the

a)Authors to whom correspondence should be addressed. Electronic addresses: gdsharma273@gmail.com and

spsingh@iict.res.in

1941-7012/2013/5(4)/043107/7/$30.00 VC 2013 AIP Publishing LLC5, 043107-1

JOURNAL OF RENEWABLE AND SUSTAINABLE ENERGY 5, 043107 (2013)

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light harvesting efficiency of the dye molecules, metal free dyes do not have a broad band

absorption covering most of the visible to near infrared (NIR) region of the solar spectrum. To

improve the light harvesting efficiency of the DSSCs, the organic sensitizer should posses a

broadly extended absorption spectrum ranging from 350 nm to the near infrared region, in order

to attain overlap with the solar spectrum. It is difficult to design a single metal free dye which

posses this property; therefore, the panchromatic approach of cosensitization is an effective

method of enhancing the performance of DSSCs. A combination of two or more dyes with

complementary light absorption covers the whole solar spectrum and, therefore, increases the

light harvesting efficiency. Several research groups have investigated the DSSCs based on

cosensitization of two or more dyes and found that the PCE of these devices had been

improved significantly with respect to DSSCs based on individual sensitizers.5 Han et al. has

reported a very high PCE (11.4%) for DSSCs based on a cosensitized system of metal-free dye

and black dye.5(g) A record high PCE of 12.3% was reported with two metal containing porphy-

rin dyes as co-sensitizers with the use of a Co (II/III) based redox electrolyte.3 There are many

reports for the use of co-sensitized systems consisting of solely metal-free dyes. Nazeeruddin

et al. and Torres et al. have used NIR dyes (SQ or TT1) together with JK2 for the cosensitized

DSSC and achieved PCEs of over 7%.6 A PCE of 8.65% was reported by Choi et al. for cosen-

sitized DSSCs using JK2 and SQ1 dyes with a Al2O3 coated TiO2 photoanode.7

In this work, two metal free dyes having complementary absorption spectra were studied

and are denoted TA-St-CA and D. The PCE has been significantly improved up to 6.26% using

the cosensitization of these two dyes. This is attributed to the suppression of dye aggregation

upon cosensitization and increased light harvesting efficiency. PCE has been further improved

up to 7.19% by addition of deoxycholic acid (DCA) as a coadsorbant in the DSSC, attributed

to increases in both Jsc and Voc.

EXPERIMENTAL DETAILS

A TiO2 photoanode of thickness 12 lm was prepared using the doctor blade technique as

described in our earlier communication.9 The dye solutions of concentration 5 �10�4 M were

prepared from the dyes TA-St-CA in acetonitrile/tert-butyl alcohol (1:1, v/v) and D in dimethyl

sulfoxide (DMSO). Subsequently, the dyes were mixed together in different volume ratios,

therefore, producing cosensitized dye solutions and deposited onto the TiO2 films by immersion

overnight at room temperature. DCA (20 mM) was added to the dye solution to act as a coad-

sorbent thus preventing the aggregation of dye molecules. After dyeing, a 25 lm Surlyn spacer

was placed around the photoelectrode and Pt coated fluorine doped tin oxide (FTO) counter

electrode to facilitate heat sealing of the two electrodes together. One drop of electrolyte solu-

tion (LiI (0.05 M), I2 (0.5 M), dimethylpropyl-benzimidiazole iodide (DPMII) (0.6 M), 4-tert-

butylpyridine (TBP) (0.5 M) in acetonitrile solution) was inserted from the hole in between the

photoanode and counter electrode. The current-voltage characteristics of the devices under illu-

mination were measured using a Keithley source meter. A xenon light source coupled with an

optical filter was used to give an irradiance of 100 mW/cm2 at the surface of the device. The

incident photon to current efficiency (IPCE) spectra of the DSSCs was measured using a mono-

chromator, and the resultant photocurrent was measured with a Keithley electrometer which

was interfaced with a computer using LABVIEW software. Electrochemical impedance spectra

(EIS) measurements were carried out by applying bias equivalent to the Voc of the DSSCs and

recorded over a frequency of 1 mHz to 105 Hz with an AC amplitude of 10 mV, in darkness.

The above measurements were carried out using a potentiostat (PGSTAT 30 Autolab Eco-

Chemie), equipped with a frequency response analyzer (FRA). DSSCs based on both individual

dyes were also manufactured for comparison.

RESULTS AND DISCUSSION

The dyes TA-St-CA8 and D

9 were synthesized using previously reported methods, and

their chemical structures are shown in Figure 1. The dye TA-St-CA contains triphenylamine as

donor and cyanoacrylic acid as acceptor moiety and a p-conjugated oligophenylene unit. The

043107-2 Sharma et al. J. Renewable Sustainable Energy 5, 043107 (2013)

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other metal free dye D contains one dithienylthienothiadiazole central unit and two cyanoacrylic

acid anchoring side groups. The optical absorption spectra of the individual and mixed dyes are

shown in Figure 2. The dye TA-St-CA showed an absorption maxima around 412 nm with

molar extinction coefficient 2.8 �104 M�1 cm�1. The other dye D showed an absorption maxi-

mum around 612 nm with molar extinction coefficient 1.28� 104 M�1 cm�1 and a second

weaker absorption peak around 410 nm with molar extinction coefficient of 0.6� 104

M�1 cm�1. The mixed dye shows a broad absorption spectrum covering a wide range in the

visible region extending up to 780 nm with superior light harvesting efficiency compared to the

individual dyes.

The dyes TA-St-CA and D were identified for cosensitization, since they have complemen-

tary absorption spectral bands. Dye D compensates for the deficit in the NIR region, and dye

TA-St-CA compensates for the absorption of D in the visible region below 450 nm. DSSCs

formed from TA-St-CA and D solutions of the same concentration in different volume ratios

were fabricated. Performance was found to improve with decrease in the concentration of D,

i.e., when the volume concentration of TA-St-CA:D was 4:1. The current-voltage characteris-

tics for the DSSCs based on TA-St-CA, D and TA-St-CA:D (4:1) dye systems, under illumina-

tion, are shown in Figure 3 with the performance parameters in Table I. The highest PCE of

6.26% was achieved with lowest amount of D. This can be attributed to (i) the high molar

extinction coefficient of D leading to absorption saturation at relatively low concentrations and

(ii) a low concentration of D may avoiding dye aggregation.10 The enhancement in the PCE is

mainly due to the increase of Jsc. Values of 9.85 mA/cm2 and 10.18 mA/cm2 were recorded for

FIG. 1. Chemical structure of dyes TA-St-CA, D and coadsorbant DCA.

FIG. 2. Absorption spectra of D, TA-St-CA and TA-St-CA þD in solution.

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dyes D and TA-St-CA, respectively, and 12.40 mA/cm2 for TA-St-CA1D. The total amount of

dye adsorbed onto the TiO2 surface for all three DSSCs was estimated and found to be higher

for each cosensitized system compared to the individual dyes. The larger total amount of dye

adsorbed is believed to be one of the main reasons for the observed higher Jsc value of DSSCs

based on the cosensitized system. As shown in Figure 4, the IPCE spectra of the DSSCs based

on the TA-St-CA 1 D cover a fairly broad range from 350 to 700 nm, which is in accordance

with the film spectra of TA-St-CA 1 D. The improvement in the PCE of DSSCs based on

cosensitized dye systems is mainly attributed to the total amount of dye adsorption. In addition,

the suppression of TA-St-CA dye aggregation at the TiO2 surface also contributed to observed

improvements in the solar cell performance to some extent.

The energy level, i.e., highest occupied molecular orbital (HOMO) and lowest unoccupied

molecular orbital (LUMO) levels of both the dyes were estimated from cyclic voltammetry and

density functional theory (DFT) calculations as reported in our earlier communications.8,9 The

ground state oxidation potential, i.e., HOMO energy level (�5.76 eV) of both the dyes is suffi-

ciently low compared to the redox potential of I�3 =I� (�5.20 eV)11 for efficient regeneration of

oxidized dyes through reaction with iodide. The higher excited state oxidation potentials, i.e.,

LUMO energy levels of both sensitizers were �3.70 eV and �3.53 eV for D and TA-St-CA,

respectively. Compared to the conduction band edge (�4.2 eV)12 of nanocrystalline TiO2 elec-

trons from both the dyes will be injected into the nanocrystalline TiO2 film.

The IPCE spectra of the DSSCs sensitized with individual dyes and cosensitized dyes are

shown in Figure 4. The DSSC sensitized with D only showed a broad IPCE spectrum in the

longer wavelength region, i.e., from 510 nm to 750 nm, and displayed the highest IPCE value

(53%) at wavelength 610 nm. This suggests that after the absorption of light by sensitizer D in

the wavelength region above 510 nm, electrons are injected from the LUMO level of D into the

conduction band of TiO2 and transported to the external circuit leading to the generation of

FIG. 3. Current-voltage (J-V) characteristics of the DSSCs sensitized with D, TA-St-CA, TA-St-CA þ D, and TA-St-CA

þ D with DCA.

TABLE I. Photovoltaic parameters of dye-sensitized solar cells.

Dye Jsc (mA/cm2) Voc (V) FF PCE (%)

D 9.85 0.63 0.68 4.22

TA-St-CA 10.18 0.68 0.72 4.98

TA-St-CA1D 12.40 0.66 0.76 6.26

TA-St-CA1D with DCA 12.96 0.73 0.76 7.19

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photocurrent. The absorption spectra of D, Figure 2, shows an absorption peak around 400 nm,

but the value of IPCE, Figure 4, is negligible. Since the I�3 has a similar peak around 400 nm

with a high molar extinction coefficient than D around this wavelength, we can assume that the

negligible value of IPCE in the shorter wavelength region is due to competitive light absorption

between I�3 and dye D. Most of the light is absorbed by the I�3 and does not contribute to the

photocurrent generation in the DSSC. On the other hand, the DSSC sensitized with TA-St-CA

showed a broad absorption spectrum in the shorter wavelength region, i.e., from 350 nm to

540 nm and displayed the maximum IPCE value (61%) in the wavelength range from 400 nm to

500 nm. The DSSC based on cosensitized dyes, i.e., TA-St-CA showed a broad IPCE spectrum

over a large spectral range 350 nm to 720 nm. This is consistent with the absorption spectra of

TA-St-CA 1 D cosensitized dyes, and both the dyes are contributing to the photocurrent gener-

ation. The loss of light absorption by I�3 has been suppressed by the use of both dyes. These

results indicated that it is possible to achieve a high IPCE in the visible and near infrared wave-

length region by developing the suitable combination of metal free dyes with strong light

absorption.

The energies for maximum light harvesting have also been calculated from the IPCE spectrum

of DSSCs based on cosensitized dyes. Values of 2.95 eV correspond to 420 nm for TA-St-CA and

1.98 eV corresponding 625 nm for D dye. The energy levels associated with maximum light harvest-

ing are also offset between TA-St-CA and D which may be beneficial for reducing the inter-dye

recombination13 and leading to an increase in photocurrent.

Electrochemical impedance spectroscopy was used to understand the charge transfer prop-

erty and estimate the electron lifetime for the DSSCs sensitized with TA-St-CA, D and the

cosensitized TA-St-CA 1 D system. The Bode plots of DSSCs sensitized with D, TA-St-CA,

and cosensitized TA-St-CA 1 D are shown in Figure 5. The middle frequency peak (in the fre-

quency region 1–100 Hz) has been used to estimate the electron lifetime in the TiO2, using the

expressions ¼ 1=2pfpeak.14 As shown in Figure 5, the frequency phase angle peak of the DSSC

sensitized with TA-St-CA1D is lower than that for the DSSCs sensitized with either of the

individual dyes. As a result, the lifetime of electrons in the conduction band was enhanced for

the cosensitized dye system compared to systems with individual dyes. The longer electron life

time leads to enhancement in the Jsc and thus an improvement in the overall PCE of the DSSC

sensitized with the mixed dyes.

In the DSSC based on the cosensitized system, the overall PCE has been improved but is

still low. This low value means that dye aggregation at the TiO2 surface was not fully sup-

pressed. Coadsorption of TA-St-CA and D was carried out using mixed dye solutions

FIG. 4. IPCE spectra of the DSSCs sensitized with D, TA-St-CA, TA-St-CA þ D, and TA-St-CA þ D with DCA.

043107-5 Sharma et al. J. Renewable Sustainable Energy 5, 043107 (2013)

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containing 20 mM of DCA. The J-V characteristics under illumination of this DSSC are pre-

sented in Figure 3, and photovoltaic parameters are complied in Table I. Upon addition of

DCA as a coadsorbant in the mixed dye solution, the overall PCE was significantly enhanced

from 6.26% to 7.19%. The Jsc value was largely increased from 12.40 to 12.96 mA/cm2, by the

addition of DCA, although the total amount of dye loading decreased from 1.86� 10�7 to

1.80� 10�7 mol/cm2. As shown in the IPCE spectrum (Figure 4), the IPCE values increased over

the whole visible range. These results indicate that addition of DCA further suppresses the dye

aggregation. Additionally, the value of Voc also increased from 0.66 to 0.73 V, by the addition of

DCA, suggesting that the backward electron transfer from the conduction band of TiO2 has been

also reduced. Figure 5 shows that as the electron lifetime in the TiO2 conduction band of DSSCs

with mixed dyes is significantly improved by the DCA. The peak frequency in the bode plot is

shifted further to the lower frequency side for DSSCs sensitized with mixed dyes. The enhance-

ment of the photovoltaic performance of the cosensitized DSSC by addition of DCA is due to both

suppression of dye aggregation effectively and prevention of the backward electron transfer from

the conduction band of TiO2.

CONCLUSION

Two metal free dyes TA-St-CA and D with complementary absorption spectra in the visi-

ble and near infrared regions of the solar spectrum have been used for the fabrication of cosen-

sitized DSSCs. The PCE of DSSCs based on cosensitization have been achieved up to 6.26%

which was higher than that of DSSCs sensitized with the individual dyes TA-St-CA (4.98%) or

D (4.22%). The increase in PCE has been attributed to the enhancement of Jsc associated with

the higher light harvesting efficiency for cosensitized dyes and also suppression of dye aggrega-

tion at the TiO2 surface. The PCE of the cosensitized DSSC was further improved by the addi-

tion of DCA, which disrupted the remaining dye aggregation and prevented the backward elec-

tron transfer from the conduction band of TiO2. This was evidenced by the increase in the

electron lifetime estimated from the Bode plot of electrochemical impedance spectra.

ACKNOWLEDGMENTS

The authors (G.D.S., R.K., and R.J.B.) wish to thank the UK India Education and Research

Initiative (UKIERI II) coordinated by the British Council, New Delhi, India, for financial support

through a Thematic Partnership. P.N. thanks CSIR-JRF for providing junior research fellowship.

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FIG. 5. Bode plots of electrochemical impedance spectra of DSSCs sensitized with D, TA-St-CA, TA-St-CA þ D, and TA-

St-CA þ D with DCA.

043107-6 Sharma et al. J. Renewable Sustainable Energy 5, 043107 (2013)

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