ISSN 1463-9076

Physical Chemistry Chemical Physics

www.rsc.org/pccp Volume 14 | Number 47 | 21 December 2012 | Pages 16165–16488

1463-9076(2012)14:47;1-J

COMMUNICATIONGrätzel, Mhaisalkar et al.A selective co-sensitization approach to increase photon conversion effi ciency and electron lifetime in dye-sensitized solar cells

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16182 Phys. Chem. Chem. Phys., 2012, 14, 16182–16186 This journal is c the Owner Societies 2012

Cite this: Phys. Chem. Chem. Phys., 2012, 14, 16182–16186

A selective co-sensitization approach to increase photon conversion

efficiency and electron lifetime in dye-sensitized solar cellsw

Loc H. Nguyen,zab Hemant K. Mulmudi,zac Dharani Sabba,ac

Sneha A. Kulkarni,a

Sudip K. Batabyal,aKazuteru Nonomura,

abMichael Gratzel*

bdand

Subodh G. Mhaisalkar*abc

Received 23rd August 2012, Accepted 4th October 2012

DOI: 10.1039/c2cp42959d

Ruthenium-based C106 and organic D131 sensitizers have been

judicially chosen for co-sensitization due to their complementary

absorption properties and different molecular sizes. Co-sensitization

yields a higher light-harvesting efficiency as well as better dye

coverage to passivate the surface of TiO2. The co-sensitized devices

C106 + D131 showed significant enhancement in the performance

(g = 11.1%), which is a marked improvement over baseline devices

sensitized with either D131 (g = 5.6%) or C106 (g = 9.5%). The

improved performance of the co-sensitized cell is attributed to the

combined enhancement in the short circuit current, open circuit

voltage, and the fill-factor of the solar cells. Jsc is improved because

of the complementary absorption spectra and favorable energy level

alignments of both dyes; whereas, Voc is improved because of the

better surface coverage helping to reduce the recombination and

increase the electron life time. The origins of these enhancements

have been systematically studied through dye desorption, absorption

spectroscopy, and intensity modulated photovoltage spectroscopy

investigations.

Dye sensitized solar cells (DSSC) have gained much attention

from both industry and academic research during the previous

decades due to their cost effective manufacturing capability

and high photon-to-electron conversion efficiencies.1–7 The

key tunable factors to obtain a high efficiency DSSC are to

maximize the light-harvesting efficiency as well as to reduce

charge recombination.5,6 Enhancement of light-harvesting

efficiency may be achieved through incorporation of dyes that

have broad absorption bands in the near-IR of the solar

spectrum.8–17 However, a disadvantage of using a single

sensitizer with wide absorption spectra is its difficulty to

transfer photo-excited electrons from the sensitizer to the

TiO2 photoanode when the conduction band of TiO2 approaches

the LUMO of the dye.9–16 Consequently, another approach is to

co-sensitize the metal oxide photoanode with multiple sensitizers

having both complementary absorption spectra and matched

energy levels simultaneously to obtain panchromatic absorp-

tion.18,19 This approach was demonstrated in tandem device

structures.20,21 However, the method resulted in limited

improvement in the photocurrent because of the extensive loss

of incident light due to the light diffusion by the front

Pt-loaded counter-electrode. Therefore, co-sensitization strategies

were investigated to load the different dyes on a single TiO2

electrode.22–24 For example, Park et al. have reported repeated

adsorption–desorption processes mimicking the concept of the

mobile phase and the stationary phase in a column chromato-

graph, to selectively position different dyes on a single TiO2

film.24 However, the low efficiency (4.8%) obtained and the

complexity of this method limited their practical application.

Apart from these approaches, a stepwise selective dye adsorption

is a muchmore inexpensive and simpler way for co-sensitization of

multiple sensitizers on a single TiO2 film.25–27 Here, we report a

promising DSSC system using a ruthenium-based dye as a

primary sensitizer for co-sensitization via a stepwise adsorption

approach having the potential to further enhance the device

performance significantly.

The ruthenium-based C106 sensitizer, which is one of the

best dyes for DSSC in terms of device performance,28 was

chosen as the primary sensitizer due to its wide light absorp-

tion over the visible regime of the solar spectrum, high

absorption coefficient (18 500 M�1 cm�1 at ca. 550 nm) and

long hydrophobic chains with a strong steric effect reducing dye

aggregation.28–30 The organic dye D131 with its P-conjugated

indoline ring was chosen as the second sensitizer to enhance

the device performance on the C106 platform due to its small

and relatively coplanar structure, and a very high extinction

coefficient in the 400–500 nm spectral range (56 200 M�1 cm�1

at 460 nm)31 compared to C106 as well as other organic

dyes.31–34 Due to these complementary properties of the dyes,

the ruthenium-based C106 dye and organic D131 dye were used

a Energy Research Institute@NTU (ERI@N), NanyangTechnological University, Research Techno Plaza, Singapore 637553.E-mail: subodh@ntu.edu.sg; Fax: +65-6790-9081;Tel: +65-6790-4626

bCenter for Nanostructured Photosystems (CNPS), NanyangTechnological University, Research Techno Plaza, Singapore 637553

c School of Materials Science & Engineering, Nanyang TechnologicalUniversity, Singapore 639798

d Ecole Polytechnique Federale de Lausanne, Laboratory of Photonicsand Interfaces, CH-1015 Lausanne, Switzerland.E-mail: michael.graetzel@epfl.ch; Tel: +41 21 69 33112,+41 21 69 33115

w Electronic supplementary information (ESI) available: Detaileddevice fabrication, photovoltaic measurements and dispersion in thevalues of J–V characteristics for individual as well as co-sensitizeddevices. See DOI: 10.1039/c2cp42959dz These authors have equally contributed.

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This journal is c the Owner Societies 2012 Phys. Chem. Chem. Phys., 2012, 14, 16182–16186 16183

as co-sensitizers in order to enhance light-harvesting efficiency

as well as dye coverage on the surface of TiO2. These two

factors could possibly lead to improvements not only in terms

of increased photo-excited electron injection, but also may

lower charge recombination, thus improving conversion

efficiency.

In general, there are two approaches for co-sensitization: the

cocktail approach35,36 uses a mixture of dye solutions with

certain molar ratios of the two dyes, whereas the stepwise

approach25–27,37 accomplishes sequential adsorptions of the

two sensitizers. Since the chosen primary sensitizer C106 yields

some of the highest reported device efficiencies,28–30 the objective

of co-sensitization is to further enhance device performance by

co-adsorbing the D131 sensitizer on the platform of saturated

C106 sensitized TiO2. One possible limitation of the cocktail

approach is the competition between the small, higher mobility

D13131–34 with the C106 dye, which is likely to result in

diminished adsorption of the primary sensitizer. Therefore, to

maximize the adsorption of the C106 dye, the TiO2 photoanodes

were first dipped into C106 dye solution, with the gaps in the

C106 coverage being subsequently filled with the D131 dye. The

co-sensitization route employed a stepwise fabrication approach,

which accomplishes adsorption of two dyes on the TiO2

photoanodes in a consecutive manner (details of device

fabrication are provided in ESIw). Fig. 1 shows the J–V

characteristics of the representative solar cells fabricated in

this study. Devices sensitized with only C106 yielded a Jsc of

19.6 mA cm�2, a Voc of 740 mV, a FF of 65% and a resultant Zof 9.5 � 0.4%. Devices sensitized with the C106 dye alone

have been previously reported to yield photocurrents, open

circuit voltage, fill factors and efficiencies of 19.78 mA cm�2,

758 mV, 77.9% and 11.7% respectively.29 It is evident from

the results of our study that although we could not match the

fill factor and efficiency values, photocurrent obtained in our

work is of the same order as previously reported.29 The D131

devices yielded a Jsc of 11.9 mA cm�2, a Voc of 733 mV, a FF

of 65% and a Z of 5.6 � 0.1%. Surprisingly, the co-sensitized

(C106 + D131) devices yielded significant enhancement in the

device performance (Jsc = 20.6 mA cm�2; Voc = 766 mV;

FF = 70%; Z = 11.1 � 0.2%) over the individually sensitized

C106 and D131 devices with improvements in both photo-

current and open circuit voltages. The focus of our work is on

the use the C106 and D131 sensitized device data as a baseline

for comparison and understand the origins of the relative

increments in Jsc and in particular Voc. The data in the inset

table of Fig. 1 represent an average of six devices from different

batches fabricated under identical conditions and the detailed

dispersion in the values is reported in ESIw (S1–S3).

To understand the origin of Jsc enhancement, UV-vis

spectroscopy has been carried out to study the combined

light-harvesting effect of the two sensitizers. Fig. 2a and b

indicates the absorption spectra of D131 and C106 in DMF

solution (a), and D131, C106 and C106 +D131 on 2 mm thick

D18-NRT TiO2 nanoparticle films (b). Both the spectra clearly

reveal that D131 possesses higher absorption intensity in the

450–500 nm region, whereas the absorption of C106 is rela-

tively lower in this region. Therefore, upon co-sensitizing C106

with D131 the absorption spectrum of the co-sensitized film

demonstrates a panchromatic feature to provide an increased

light-harvesting effect. Fig. 2c shows the energy diagram and

operating schematic of a co-sensitized DSSC.28,30,32 Since the

LUMO levels of both C106 (�3.38 eV) and D131 (�2.16 eV)

lie above the conduction band edge of TiO2 (�4.0 eV) the

electron injection from the photo-excited dye states to TiO2

is thermodynamically favorable. Therefore, co-sensitization

enhances photon absorption and carrier generation across

the whole visible portion of the solar spectrum thereby making

the dye combination more effective than its individual con-

stituent. The enhanced light-harvesting efficiency accounts for

the increase in IPCE up to 94% in the 400–500 nm spectral

region (Fig. 2d), corresponding to an internal quantum

efficiency of nearly unity. As a result, the obtained Jsc for

the C106 + D131 co-sensitized device is 20.6 mA cm�2, which

is almost 5% higher than that of C106 and 73% higher than

that of D131.

To quantify the dye loading on the TiO2 surface, the

amount of dye adsorbed on the 15 mm-thick sensitized TiO2

photoanode was carefully measured. Upon dipping the

co-sensitized photoanodes for 2 min into a 0.02 M NaOH

solution, desorption of D131 occurs predominantly compared

to that of C106. The minor amounts of C106 desorbed were

calculated by measuring the absorption spectra of the NaOH

solution and monitoring the peak at 550 nm. The residual

C106 dye on the photoelectrode was subsequently desorbed in

a 0.1 M tetramethylammonium hydroxide (TMAH) acetonitrile–

water solution (volume ratio 1 : 1). The calculated dye adsorp-

tion of individual dyes on the TiO2 film for various dipping

times is provided in Fig. 3a. As the figure indicates, both the

dyes reach an equilibrium loading after about 4 h for C106 and

at around 2 h for D131. The saturation loading was B1.2 �10�7 mol cm�2 for C106 and B1.9 � 10�7 mol cm�2 for D131

respectively. The amount of C106 adsorbed on the TiO2

surfaces was relatively constant even for longer dipping times,

indicating that the bulky C106’s adsorption was saturated

after 4 h of dye-soaking as well as the dye aggregation between

Fig. 1 Current density–voltage characteristics (J–V) of devices sensi-

tized with D131, C106, and C106 + D131 under one-sun AM-1.5G

irradiation; TiO2 photoanodes thickness 15 mm, active area 0.16 cm2.

The inset shows the detailed photovoltaic parameters of the devices.

Note that the data in the inset table represent an average of six devices

from different batches fabricated under identical conditions and the

detailed dispersion in the values is reported in ESIw (S1–S3).

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16184 Phys. Chem. Chem. Phys., 2012, 14, 16182–16186 This journal is c the Owner Societies 2012

C106’s molecules during a long dye soaking process was very

unlikely, justifying the choice of 12 h dipping time in our

experiments. Fig. 3b shows the relative and total amount of dyes

loaded onto the TiO2 photoelectrode during the co-sensitization

approach. Every sample represented in Fig. 3b has been

dipped for 12 h in C106 solution before being loaded with

D131 for different dipping times. The concentration of D131

loaded onto the C106 sensitized photoanode saturates after 4 h

Fig. 2 (a) and (b) UV-Vis absorption spectra of: D131 and C106 in DMF solution (a), and D131, C106 and C106 + D131 on transparent TiO2

films (b). (c) Operating schematic of co-sensitized DSSC.28,30,32 (d) Incident-photon-to-electron conversion efficiency (IPCE) action spectra of

devices sensitized with D131, C106, and C106 + D131 under identical fabrication conditions.

Fig. 3 (a) Concentration of dye loaded onto the TiO2 photoelectrode for various dipping times of C106 and D131. The plotted values are the

average values of samples from 3 different batches. (b) Concentration of dye loaded onto co-sensitized TiO2 photoelectrodes monitored for

different D131 dipping times. All the photoanodes were prepared by dipping in a C106 solution for 12 h before varying the dipping time for the

co-sensitization of D131. (c) and (d) Schematic illustrating molecular structure and adsorption sites of C106 and D131 dyes on TiO2.

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This journal is c the Owner Societies 2012 Phys. Chem. Chem. Phys., 2012, 14, 16182–16186 16185

of dipping (in contrast to 2 h for individual sensitization).

Notably, the concentration of D131 on the co-sensitized

photoanode was only 0.9 � 10�7 mol cm�2 (vs. 1.9 �10�7 mol cm�2 for individual sensitization) indicating that

there is a lower number of binding sites left on the TiO2

surface after C106 adsorption. The total number of C106 dye

molecules on the co-sensitized samples reduces slightly (from

1.4 � 10�7 mol cm�2 to 1.2 � 10�7 mol cm�2) with increasing

dipping time due to their solubility in the D131 sensitizing

solvent (ACN, TBA). In spite of this, the total amount of dye

molecules (both C106 and D131) was significantly increased to

2.1 � 10�7 mol cm�2. The results thus reveal that the total dye

coverage has been improved by inserting small-sized D131 dye

molecules into the gaps within the C106 saturated TiO2 films,

as visualized in Fig. 3c and d. C106 is adsorbed on the TiO2

surface via its carboxylic group just as a conventional dye,28–30

whereas D131 is adsorbed via not only its carboxylic group

but also via the nitrogen in the cyano group as discussed

elsewhere.33 Consequently, the anchoring sites on the TiO2

nanocrystals are different for both dyes, thus enhancing their

adsorption–dispersion on the TiO2 surface.33 On the other

hand, D131 is expected to link perpendicularly to the TiO2

surface since the molecular structure is practically coplanar

(y anchor E0) thus giving rise to a denser packing and

coverage in tandem with C106.34

To understand the enhancement in Voc, Intensity Modu-

lated Photovoltage Spectroscopy (IMVS) measurements were

carried out. The improvement in the TiO2 coverage could have

consequences on the recombination of the electrons in TiO2

with the electrolyte. The dependence of electron recombina-

tion lifetime (t) under open circuit conditions on the light

intensity for individual- and co-sensitized devices is shown in

Fig. 4. Electrochemical impedance spectroscopy (EIS) has

been carried out for those samples confirming the same trend

as the data shown in Fig. 4. Nyquist plots for all the three

samples are provided in ESIw (Fig. S4). The co-sensitized

device has a longer electron life-time compared to that of each

individual dye sensitized devices, indicating that the electron

recombination with the oxidized species of the electrolyte has

been reduced in the case of co-sensitized devices. This effect

explains the enhancement in Voc. The uphill energy offsets by

the HOMOs of both C106 (�5.05 eV) and D131 (�5.16 eV)

relative to that of iodide (�4.65 eV) supply a thermodynamic

driving force for dye regeneration.28,30,32 Owing to very

fast regeneration of dyes by iodide, the back reaction of the

electrons in the TiO2 film with I3� is the predominant recom-

bination channel compared to the recapture of injected electrons

by the dye molecules attached on the TiO2 surface.29,38,39 Since

the exposed TiO2 is a well-recognized electron recombination

center,40 the better dye coverage in our co-sensitized system

helps to passivate the TiO2 surface, and thus reduces the

recombination due to electron back-transfer between TiO2 and

I3�, ultimately increasing open-circuit voltage (Voc) compared to

those of individual sensitized devices.

Combined enhancement in both Jsc and Voc yields 17%

higher energy conversion efficiency upon co-sensitization (Z =

11.1 � 0.2%) than that for C106 (Z = 9.5 � 0.4%) and 98%

higher than that for D131 (Z = 5.6 � 0.1%) baseline devices.

Surface engineering as well as collection of photons in the

visible region are key parameters to realize high efficiency

dye-sensitized solar cells. Solar cell efficiency of over 11.1%

represents one of the best results for the co-sensitized DSSC

with further enhancements possible with approaches that

could combine dye design and alternative redox couples.

We have demonstrated a methodology for high perfor-

mance DSSC by a facile sequential co-sensitization method.

The co-sensitizers were selectively distributed on the TiO2

electrode due to their different molecular sizes and anchoring

groups. Jsc is improved because of the complementary absorp-

tion spectra and favorable energy level alignments of both

dyes; whereas, Voc is improved because of the better surface

coverage helping to reduce the recombination due to electron

back-transfer between the injected electron in TiO2 and I3�

ions. The enhancements in both Jsc and Voc lead to the

improvement in overall conversion efficiency of the device.

The fabrication of this type of co-sensitized DSSC requires one

additional dye dipping process, but broadens the possibilities

to enhance the performance of the existing visible sensitizers.

The simplicity in device fabrication and improved efficiencies

suggest promising prospects for the co-sensitization approach to

be widely adopted for high performance, large-scale production

of DSSC devices.

Acknowledgements

Funding from National Research Foundation (NRF), Singapore,

is acknowledged (CRP Award No.: NRF-CRP4-2008-03).

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