This journal is c the Owner Societies 2011 Phys. Chem. Chem. Phys.

Cite this: DOI: 10.1039/c1cp20290a

Uncovering the role of the ZnS treatment in the performance of quantum

dot sensitized solar cellsw

Nestor Guijarro,*aJose M. Campina,

aQing Shen,

bcTaro Toyoda,

b

Teresa Lana-Villarrealaand Roberto Gomez*

a

Received 2nd February 2011, Accepted 9th May 2011

DOI: 10.1039/c1cp20290a

Among the third-generation photovoltaic devices, much attention is being paid to the so-called

Quantum Dot sensitized Solar Cells (QDSCs). The currently poor performance of QDSCs seems

to be efficiently patched by the ZnS treatment, increasing the output parameters of the devices,

albeit its function remains rather unclear. Here new insights into the role of the ZnS layer on

the QDSC performance are provided, revealing simultaneously the most active recombination

pathways. Optical and AFM characterization confirms that the ZnS deposit covers, at least

partially, both the TiO2 nanoparticles and the QDs (CdSe). Photoanodes submitted to the

ZnS treatment before and/or after the introduction of colloidal CdSe QDs were studied by

electrochemical impedance spectroscopy, cyclic voltammetry and photocurrent experiments. The

corresponding results prove that the passivation of the CdSe QDs rather than the blockage of

the TiO2 surface is the main factor leading to the efficiency improvement. In addition, a study of

the ultrafast carrier dynamics by means of the Lens-Free Heterodyne Detection Transient Grating

technique indicates that the ZnS shell also increases the rate of electron transfer. The dual role of

the ZnS layer should be kept in mind in the quest for new modifiers for enhancing the

performance of QDSCs.

Introduction

The need for renewable and low-cost energies has boosted an

impressive research in the field of photovoltaics, which

includes the so-called Quantum-Dot sensitized Solar Cells

(QDSCs).1 The potential of QDSCs lies in the unique

properties of quantum dots,2,3 namely, (1) high extinction

coefficient, (2) easily tunable band gap and (3) the possibility

of generating more than one e�–h+ pair per photon absorbed

(multiple exciton generation), which would lead to achieve

quantum yields over 100%, as recently reported by Sambur

et al.4 Unfortunately, the best energy conversion efficiencies

reported so far are still quite modest, revealing both the poor

understanding of the fundamental processes controlling the

efficiency and the remarkable difficulties found in the prepara-

tion of nanoscaled hybrid assemblies. Nevertheless, in the

past few years, an encouraging improvement in the overall

performance of liquid electrolyte QDSCs has been attained by

means of new routes of sensitization (e.g. different modes of

attachment,5 cosensitization6–8), optimized counter-electrodes

(nano-sulfide/carbon composites9), and also photoanode

post-treatments (such as ZnS7 or SiO210 treatments, dipole

adsorption,11 fluoride ion insertion,12 modification induced by

CdCl213 or annealing14).

One of the most widely extended post-treatments consists of

the deposition of ZnS over the sensitized electrode, taking

advantage of the straightforward preparation and the striking

enhancement of the efficiency achieved. This strategy was

pioneered by Yang et al. in 2002, proving that a ZnS layer

grown by SILAR (Successive Ionic Layer Adsorption and

Reaction) over a PbS/CdS sensitized TiO2 electrode not only

prevented chalcogenide photocorrosion, but also improved the

output parameters of the cell.7 However, owing to the poor

energy conversion reported, this procedure was unregarded for

several years, until some of us managed to almost double

the efficiencies of CdSe-based solar cells by means of this

method.12,15 Thereafter, ZnS deposition has arisen as a current

treatment in the preparation of QDSCs. In fact, it has been

successfully applied to TiO2 electrodes sensitized by CdSe

either grown by chemical bath deposition12 or adsorbed from

a Institut Universitari d’Electroquımica i Departament de QuımicaFısica, Universitat d’Alacant, Apartat 99, E-03080 Alacant, Spain.E-mail: nestor.guijarro@ua.es, roberto.gomez@ua.es;Fax: +34 965903537; Tel: +34 965903748

bDepartment of Engineering Science, Faculty of Informatics andEngineering, The University of Electro Communications,1-5-1 Chofugaoka, Chofu, Tokyo 182-8585, Japan

c PRESTO, Japan Science and Technology Agency (JST),4-1-8 Honcho Kawaguchi, Saitama 332-0012, Japanw Electronic supplementary information (ESI) available: Determina-tion of the potential to carry out the photocurrent transients. Electro-chemical impedance spectroscopy: equivalent circuit description andfittings to the experimental data. Effect of immersion in CH2Cl2 onTiO2/ZnS electrode voltammograms. See DOI: 10.1039/c1cp20290a

PCCP Dynamic Article Links

www.rsc.org/pccp PAPER

Dow

nloa

ded

by U

nive

rsid

ade

do P

orto

(U

P) o

n 02

Jun

e 20

11Pu

blis

hed

on 3

1 M

ay 2

011

on h

ttp://

pubs

.rsc

.org

| do

i:10.

1039

/C1C

P202

90A

View Online

Phys. Chem. Chem. Phys. This journal is c the Owner Societies 2011

colloidal dispersions.16 Moreover, until now, the best perfor-

mance in CdSe-sensitized solar cells (conversion efficiency

of 4.22%8) has been obtained by combining this treatment

with TiO2 cosensitization with layers of CdS and CdSe prepared

by chemical bath deposition (CBD).

An attempt to unravel the role of this coating can be found

in a previous paper from Toyoda’s group.15 Specifically, the

efficiency enhancement was ascribed to both the blockage of

the TiO2 surface (reducing the leakage of electrons injected

into the oxide toward the electrolyte) and the passivation of

the QD surface states (preventing electron trapping). Recent

articles dealing with QDSCs using ZnS coatings are in agree-

ment with these ideas.6,8 In other cases, the improvement in

the performance of the devices is fundamentally attributed to

either the passivation of QDs14 or the blockage of the TiO2

surface.17 This paper aims at contributing to the understanding

of the ZnS role.

Colloidal solutions of QDs overcoated with a capping layer

of another semiconductor (core–shell QDs) have been success-

fully synthesized. These onion-like QDs have been extensively

studied in photoluminescence (PL) experiments.18,19 In fact,

the reported increase in the PL quantum yield when ZnS is

capping CdSe QDs clearly indicates that the coating reduces

efficiently the density of surface states (passivation of QDs).20

However, very few reports focus on the sensitization of wide

band gap oxides with core–shell QDs. Several groups have

measured the electron injection rate constants from CdSe/ZnS

core–shell QDs to TiO2, revealing similar results to those

obtained when using plain CdSe QDs. A more systematic

study is needed, because the comparison among results

obtained with different techniques and QDs with disparate

capping ligands is not straightforward.21 In any case, in spite

of the large band gap of the ZnS outer shell, efficient electron

and hole injection are observed.

We present here a new approach to untangle the role of the

ZnS coating in the enhancement of the solar cell performance.

Different deposition sequences of the ZnS layer and the CdSe

QDs allow us to analyze separately the blockage of the

TiO2/electrolyte interface and the passivation of the CdSe

QD surface, and therefore, to assess the dominant mechanism

in the improvement of the solar cell characteristics. The

characterization of the modified electrodes is carried out by

means of cyclic voltammetry and Electrochemical Impedance

Spectroscopy (EIS) to follow the blockage of the TiO2 surface,

whereas the effect of passivating the CdSe QDs is clearly

revealed by photocurrent experiments. In addition, optical

characterization using emission and absorption spectra of

ZnS-modified electrodes is presented. Topographic investi-

gation of the ZnS layer is performed by means of atomic

force microscopy (AFM) measurements. The Lens-Free

Heterodyne Detection Transient Grating (LF-HD-TG)

technique (with resolution in the sub-picosecond range) is

employed to investigate the effect of the ZnS coating on the

electron injection rate.

Experimental

1. Synthesis of CdSe QDs. Colloidal dispersions of CdSe QDs

capped with trioctylphosphine were prepared by following the

solvothermal route proposed by Wang et al.,22 which allows us

to select the size of the QDs by controlling the reaction time.

In this study, the reaction time was fixed at 15 h.

2. Preparation of TiO2 electrodes. Nanoporous TiO2

electrodes were prepared by spreading (doctor blading)

7 mL per cm�2 of an aqueous slurry of Degussa P25 over the

substrates, and subsequently sintering the samples at

450 1C in an oven for 1 h. The slurry was prepared by grinding

a mixture of 1 g of TiO2 powder, 2.0 mL of H2O, 30 mL of

acetylacetone (99+%, Aldrich), and 20 mL of Triton X100

(Aldrich). The thickness of the film was measured to beB5 mmby means of SEM. As substrates, either F–SnO2 (FTO) coated

glass or thermally treated Ti foil, were employed. The latter

was obtained by heating Ti foil at 550 1C for 1 h in air, in order

to grow a nanocrystalline TiO2 compact layer on the surface.

The area of the electrodes was 3 cm2 in all cases.

3. Sensitization of electrodes and ZnS treatment. CdSe-

sensitized TiO2 samples were prepared by direct adsorption

of presynthesized QDs. The procedure relies on the immersion

of the electrode in a CH2Cl2 (99.6%, Sigma Aldrich) CdSe QD

dispersion as described elsewhere.23 In this case, the soaking

time was fixed at 2 h and 30 min in all cases. The ZnS coating

was applied by an optimized successive ionic layer adsorption-

reaction (SILAR) method (2 cycles), as proposed by Diguna

et al.,12 whereby the electrode is alternatively dipped in 0.5 M

Zn(CH3COO)2 and 0.5 M Na2S aqueous solutions for 1 min,

washing thoroughly with water prior to each immersion to

remove the excess of non-adsorbed/unreacted ions. Different

electrodes have been prepared by changing the deposition order

of the ZnS layer and the CdSe QDs. Accordingly, the electrodes

are termed as follows below: TiO2; TiO2/CdSe; TiO2/CdSe/ZnS;

TiO2/ZnS; TiO2/ZnS/CdSe; TiO2/ZnS/CdSe/ZnS.

4. Optical characterization. Absorption and emission spectra

were obtained for both colloidal dispersions and modified

electrodes. UV-Vis absorption spectra were recorded with a

Shimadzu UV-2401PC spectrophotometer. Concretely, the

diffuse reflectance spectra of modified TiO2 electrodes were

measured by means of an integrating sphere using BaSO4

(Wako) as background. A Kubelka–Munk transformation

was undertaken to facilitate the analysis of the reflectance

data. Emission spectra were measured by means of a

FluoroMax-A spectrofluorometer equipment. The excitation

wavelength was fixed at 450 nm in all the cases.

5. Electrochemical measurements. Electrochemical measure-

ments were performed at room temperature in a three-

electrode cell equipped with a fused silica window and using

a computer-controlled Autolab PGSTAT30 potentiostat.

All potentials were measured against and referred to a

Ag/AgCl/KCl (sat) reference electrode, whereas a Pt wire

was used as the counter electrode. A N2-purged 1 M Na2S +

0.1 M S + 1 M NaOH aqueous (ultrapure water) solution

(polysulfide) was used as the working electrolyte. The full

output of a 150 W Xe arc lamp (Osram) equipped with a

UV-filter (cut-off l o 380 nm) was employed for electrode

illumination, yielding an irradiance of 73.0 mW cm�2, recorded

with an optical power meter (Oriel model 70310) equipped

with a photodetector (thermo Oriel 71608). A slightly lower

irradiance is expected on the electrode surface, due to light

absorption in the electrolyte. Cyclic voltammetry was carried

Dow

nloa

ded

by U

nive

rsid

ade

do P

orto

(U

P) o

n 02

Jun

e 20

11Pu

blis

hed

on 3

1 M

ay 2

011

on h

ttp://

pubs

.rsc

.org

| do

i:10.

1039

/C1C

P202

90A

View Online

This journal is c the Owner Societies 2011 Phys. Chem. Chem. Phys.

out in the dark at a sweep rate of 20 mV s�1. Photocurrent

experiments were performed by applying a constant potential

of �0.6 V, in which a negligible dark current was observed

(see ESIw for details). EIS measurements were undertaken in

the dark, using a frequency range from 10 mHz to 10000 Hz, a

potential of �1.0 V and a perturbation of 10 mV. EIS data

were fitted following the model proposed by Sutter et al.24 to

obtain the charge transfer resistance (RCT) at the electrode/

electrolyte interface (see ESIw for details).

6. Morphological characterization by AFM. AFM measure-

ments were carried out by means of a Nanoscope III (Digital

Instruments) operated at room temperature in air. Images

were obtained in the tapping mode using silicon tips at a

driving frequency of B270 kHz. A rutile TiO2 (110) single

crystal was purchased from Commercial Crystal Laboratories,

Inc. In order to define an atomically smooth surface, the single

crystal was treated as described previously.23 The ZnS treat-

ment was performed following the same experimental proce-

dure as for the nanoporous TiO2 electrodes.

7. Sub-pico-second time-resolved carrier dynamics. The

principle and setup of the LF-HD-TG technique have been

described in depth elsewhere.25,26 In this experiment, the

laser source was a titanium/sapphire laser (CPA-2010,

Clark-MXR Inc.) with a wavelength of 775 nm, a repetition

rate of 1 kHz, and a pulse width of 150 fs. The light was

splitted into two parts. One of them was used as a probe pulse.

The other was used to pump an optical parametric amplifier

(OPA) (a TOAPS from Quantronix) to generate light pulses

with a wavelength tunable from 290 nm to 3 mm used as pump

light in the LF-HD-TG measurement. In this study, the pump

pulse wavelength was 470 nm and the probe pulse wavelength

was 775 nm.

Results and discussion

1. Optical characterization

The absorption and emission spectra of the colloidal dispersion

of CdSe QDs used in this work are depicted in Fig. 1. The well

defined band (1st excitonic peak) observed in the absorption

spectrum evidences a narrow size distribution centered at

ca. 3.5 nm.27 The photoluminescence spectrum shows a sharp

peak centered at around 580 nm and a broad, low intensity

band at higher wavelengths. The differences between both

spectra are not unexpected, taking into account that each

one involves different mechanisms. As it is widely known,

albeit not well-understood, the main PL band is clearly shifted

toward lower energies with respect to the excitonic peak in the

absorption spectra. These results suggest that prior to electron–

hole radiative recombination, either an energy relaxation via

surface states or the splitting of the HOMO occurs.28 In any

case, the intense PL band could be associated to radiative

recombination between electrons and holes located around the

LUMO and HOMO energetic levels, respectively. The broad

emission band located at higher wavelengths (650–800 nm) has

been widely described in the literature and attributed to

recombination from trap states without further characterization.18

Recently, a collection of exhaustive photoluminescence studies

based on a controlled modification of the QD surface with

amines and thiols supports such a notion.29,30

Fig. 2A shows the Kubelka–Munk transformation of the

diffuse reflectance spectra obtained for the TiO2 electrode

before and after sensitization with CdSe QDs and once the

ZnS treatment was applied. The strong signal recorded at

wavelengths lower than 400 nm is given by TiO2 absorption.

Upon adsorption of QDs, the electrode response is extended

toward higher wavelengths, matching quite well that recorded

Fig. 1 Absorption and emission spectra of the colloidal dispersion of

CdSe QDs in CH2Cl2 employed in this work. PL excitation wave-

length: 450 nm.

Fig. 2 Kubelka–Munk transformation of the diffuse reflectance

spectra for TiO2 electrodes before and after modification with CdSe

and ZnS (A). Comparison among emission spectra of the colloidal

solution and the TiO2, TiO2/CdSe and TiO2/CdSe/ZnS thin films (B).

PL excitation wavelength: 450 nm.

Dow

nloa

ded

by U

nive

rsid

ade

do P

orto

(U

P) o

n 02

Jun

e 20

11Pu

blis

hed

on 3

1 M

ay 2

011

on h

ttp://

pubs

.rsc

.org

| do

i:10.

1039

/C1C

P202

90A

View Online

Phys. Chem. Chem. Phys. This journal is c the Owner Societies 2011

for the colloidal dispersion. Eventually, the ZnS treatment

induces both a slight red shift (B10 nm) in the spectrum and a

loss of definition of the 1st excitonic peak. These findings can

be ascribed to a lower degree of quantum confinement, which

is in agreement with the existence of a ZnS shell deposited on

the CdSe QDs, as suggested previously.17 The fact that the

QDs are capped with TOP molecules does not preclude the

ZnS deposition.

The emission spectrum of the colloidal dispersion of QDs is

compared with those obtained for TiO2 electrodes before and

after modification with QDs (Fig. 2B). The large signal

recorded at wavelengths below 500 nm is due to excitation

light reflected in the sample. Upon QD adsorption on TiO2,

their PL undergoes a sharp drop, because of the appearance of

a new competitive pathway for electron leakage from the QDs,

which hinders the radiative recombination observed in the

colloidal medium. Once a QD is attached to the TiO2, the

band alignment favors electron transfer from the excited QDs

to the oxide, leading to an effective charge separation that

quenches the QD intrinsic radiative recombination. It should

be stressed that, probably, ZnS only coats the outer surface of

the QDs (that exposed to the electrolyte), in contrast to the

situation in presynthesized CdSe/ZnS core–shell QDs.

On the other hand, the growth of a ZnS shell onto the CdSe

QDs apparently increases their photoluminescence. Similar

results have been extensively discussed in the research with

colloidal dispersions and were ascribed to the reduction of the

trap state density in the presence of a ZnS shell.18 In our case,

as illustrated in Chart 1, non-radiative recombination present

in both colloidal (A) and TiO2-attached plain QDs (B) would

be suppressed partly by the ZnS treatment (C), which may

favor competitive processes such as radiative recombination

and electron injection. As shown, the main emission bands for

CdSe-sensitized TiO2 electrodes before and after the ZnS

treatment are centered at around 600 nm, that is, they are

shifted B20 nm from that of the colloidal dispersion. This is

not unexpected taking into account the change of the medium

surrounding the QDs, and the loss of quantum confinement

associated to the deposition of a ZnS shell. As mentioned

above, a similar red shift is also discerned in the absorption

spectra after the application of the ZnS coating.

2. Morphological characterization

To study the morphology of the ZnS layer deposited on TiO2,

a single crystal was employed as a substrate instead of the

typical nanoporous substrate. Difficulties in imaging the

modification of a nanoporous TiO2 surface derived from its

high roughness can be easily solved by exploiting the

characteristic flat surface of a single crystal.23 AFM images

of a rutile (110) single crystal before and after the ZnS

treatment are given in Fig. 3. Prior to the ZnS coating, the

surface morphology of the bare single crystal is characterized

by terraces and monoatomic steps. Conversely, after coating

with ZnS, the surface becomes heterogeneously covered by

rather flat nanoparticles around 10–15 nm in diameter (Fig. 3B

and D). At this point, it should be noted that no Raman signal

was detected for ZnS deposited on either FTO or TiO2,

pointing to the amorphous nature of these nanoparticles. It

is noteworthy that after only two deposition cycles the amount

of ZnS is quite high, leading to a significant coverage of the

TiO2 surface.

3. Electrochemical studies

As mentioned before, there is still no consensus on the role of

the ZnS treatment in the enhancement of the cell efficiency.

Both the blockage of the TiO2 surface and the passivation of

the QDs seem to be implied. Here, we propose a new approach

aimed to separately quantify the effect of both mechanisms,

based on the sequential modification of TiO2 electrodes with

CdSe QDs followed by ZnS or vice versa, and their in-depth

characterization by means of cyclic voltammetry, electro-

chemical impedance spectroscopy and photocurrent experiments.

All the experiments were carried out in a three-electrode

configuration, which permits us to investigate the behavior

of the photoanode without the interference of the counter

electrode, as it happens in a two electrode configuration. These

experiments were performed for TiO2 electrodes modified in

different ways, in an aqueous electrolyte containing the redox

couple S2�/S (polysulfide)31 (Fig. 4 and 5).

Fig. 4A shows the cyclic voltammograms obtained for the

bare TiO2 electrodes and for CdSe QD-sensitized electrodes

before and after being submitted to the ZnS treatment. At

potentials more negative than �0.9 V, faradaic currents

associated to the reduction of polysulfide by TiO2 electrons

develop in agreement with I–V curves of QDSCs presented

elsewhere.16 Therefore, it is not unreasonable to connect the

magnitude of this faradaic current to the fraction of bare TiO2

surface directly exposed to the electrolyte. As observed, the

deposition of the CdSe QDs triggers a drop in the current,

which is further diminished by the application of the ZnS layer.

Fig. 4B illustrates the Nyquist plot of the electrode sequentially

modified following the classical procedure. Impedance spectra

are characterized by a single semi-arc. The Bode phase

plots (see Fig. S2, ESIw) obtained in the accumulation region

(�1.0 V) for the nanoparticulate TiO2 semiconductor electrodes

prepared in this work displayed two peaks, even when they

Chart 1 Routes of radiative recombination and trapping in surface/interfacial states (SS), for carriers photogenerated in CdSe QDs: in colloidaldispersion (A), after attachment to TiO2 (B) followed by ZnS treatment (C). The arrow width qualitatively indicates the number of carriersinvolved in the corresponding pathway.

Dow

nloa

ded

by U

nive

rsid

ade

do P

orto

(U

P) o

n 02

Jun

e 20

11Pu

blis

hed

on 3

1 M

ay 2

011

on h

ttp://

pubs

.rsc

.org

| do

i:10.

1039

/C1C

P202

90A

View Online

This journal is c the Owner Societies 2011 Phys. Chem. Chem. Phys.

were modified with ZnS and/or CdSe (and independently of

the order of deposition). This evidence clearly indicates the

need of a two-time-constant equivalent circuit for the successful

fitting of the experimental data, such as the equivalent circuit

proposed by Sutter et al.24 employed herein. The calculated

charge transfer resistance (RCT) is displayed in Fig. 4C. It

increases upon the adsorption of the QDs and, additionally,

after the ZnS treatment. CdSe QDs can physically block part

of the oxide surface preventing its contact with the electrolyte.

On the other hand, the hydrophobic capping molecules that

surround the QDs can efficiently preclude the approach of

hydrophilic solution species. Mora-Sero et al. reported an

important blockage of the TiO2 surface when a layer of CdSe

was deposited by CBD.17 In contrast to the results presented

here, poor blockage was obtained when using directly adsorbed

colloidal QDs, likely due to the very low QD coverage attained

in comparison to that obtained in CBD-sensitized specimens.

On the other hand, recombination of TiO2 electrons with the

electrolyte could also occur via CdSe QDs (see below). Such a

recombination, together with that occurring directly through

the oxide/solution interface, would be efficiently impeded by

the ZnS coating because it acts not only as a physical barrier

(as the QDs), but also as a potential barrier owing to the

large band gap of ZnS, giving the so-called ‘‘type I’’ band

alignment.20

In a separate series of experiments, a TiO2 electrode was

modified sequentially by ZnS, CdSe and, again, by ZnS. The

voltammograms in polysulfide solution in the different stages

are depicted in Fig. 5A. After coating with ZnS, the cathodic

current falls dramatically. There is, however, a slight increase

Fig. 3 AFM images of a rutile (110) single crystal before (A and C) and after the ZnS treatment (B and D).

Dow

nloa

ded

by U

nive

rsid

ade

do P

orto

(U

P) o

n 02

Jun

e 20

11Pu

blis

hed

on 3

1 M

ay 2

011

on h

ttp://

pubs

.rsc

.org

| do

i:10.

1039

/C1C

P202

90A

View Online

Phys. Chem. Chem. Phys. This journal is c the Owner Societies 2011

of the cathodic current after the subsequent QD adsorption.

Finally, it sharply decreases upon the application of a second

ZnS layer. The impedance spectra for these electrodes

are given in Fig. 5B. The charge transfer resistance values

obtained from appropriate fittings24 are given in Fig. 5C.

It is worth noting the apparent ‘‘de-blockage’’ observed

both in voltammograms andRCT values after the QD attachment.

The QDs may behave as leakage centers since the energetic

position of the CdSe conduction band (CB) edge, besides the

probable existence of QD surface states energetically located

close to the TiO2 conduction band edge and/or surface states,

would favor recombination with the electrolyte,32 especially if

the ZnS layer is partially removed where the QD adsorption is

taking place (see Chart 2). Moreover, a minor contribution

from a partial detachment of the ZnS layer during long-term

immersion of the electrode in CH2Cl2 QD colloidal dispersions

cannot be discarded (see Fig. S5, ESIw).The drop in the cathodic current as well as the increase in

RCT after a second ZnS treatment is not unexpected. In fact, as

observed in the voltammograms, the previous treatments with

Fig. 4 Cyclic voltammetry (A), electrochemical impedance spectra

(B) and charge transfer resistance (RCT) (C) for TiO2 electrodes

sequentially modified with CdSe QDs and ZnS. All measurements

were performed in 1 M Na2S + 0.1 M S + 1 M NaOH aqueous

electrolyte, using thermally treated titanium foil as a substrate for the

electrode. Voltammetry was recorded at 20 mV s�1. EIS data were

obtained in the range from 10 mHz to 10 000 Hz, at an applied

potential of �1.0 V and the oscillation amplitude was 10 mV. RCT

values were calculated by fitting the EIS data.

Fig. 5 Cyclic voltammetry (A), electrochemical impedance spectra

(B) and charge transfer resistance (RCT) (C) for TiO2 electrodes

sequentially modified with ZnS, CdSe QDs and a second treatment

of ZnS. Experiments were carried out as described in Fig. 4.

Dow

nloa

ded

by U

nive

rsid

ade

do P

orto

(U

P) o

n 02

Jun

e 20

11Pu

blis

hed

on 3

1 M

ay 2

011

on h

ttp://

pubs

.rsc

.org

| do

i:10.

1039

/C1C

P202

90A

View Online

This journal is c the Owner Societies 2011 Phys. Chem. Chem. Phys.

ZnS and CdSe QDs yield an almost completely blocked oxide

surface. The subsequent ZnS treatment mainly increases the

thickness of the ZnS layer and blocks the QD surface. Both

facts prevent recombination with the electrolyte and lead to a

rise in RCT. At this point it is interesting to compare the results

of RCT obtained for the different series of electrodes discussed

so far. The charge transfer resistance for the TiO2/CdSe

electrode has a value of 2.47 kO, similar to that found for

the TiO2/ZnS/CdSe electrode (2.80 kO), but significantly

larger than that of the bare TiO2 electrode (0.7 kO). This

suggests that, irrespective of the presence of the ZnS layer, the

QDs could operate as electron leakage centers. More impor-

tantly, the final coating with ZnS leads to a drastic increase in

RCT in both cases, mainly due to the passivation of the QD

surface. However, the values of RCT are 5.81 and 7.47 kO for

the TiO2/CdSe/ZnS and TiO2/ZnS/CdSe/ZnS electrodes,

respectively. This indicates that the second ZnS layer leads

to a better blockage of the TiO2 surface.

Photocurrent experiments carried out with the electrodes

previously characterized by cyclic voltammetry are presented

in Fig. 6. As shown in Fig. 6A, upon the ZnS treatment, there

is a dramatic increase in the photocurrent. When a first ZnS

layer is deposited prior to sensitization of the electrode with

CdSe (TiO2/ZnS/CdSe in Fig. 6B), the resulting photocurrent

is slightly higher than that obtained for the TiO2/CdSe

electrode (Fig. 6A). As the QD coverage degree is similar in

both cases, the photocurrent increase is likely due to a better

blockage of the oxide surface in the presence of the ZnS layer,

as discussed previously. A significant increase in the photo-

current is recorded after covering the electrode with a second

layer of ZnS (TiO2/ZnS/CdSe/ZnS in Fig. 6B). Previously

deposited ZnS and CdSe QDs should have efficiently blocked

the oxide surface. Therefore, the effect observed for the

additional layer of ZnS should be linked to the passivation

of the QD surface. These results clearly indicate that both the

blockage of the TiO2 surface and the passivation of the QDs

are involved in the enhancement of the photoanode

performance, albeit the passivation of the QDs seems to be

more important. In this context, it is interesting to remark the

existence of a direct correlation between jph and RCT (Fig. 6C),

revealing that, as the resistance for recombination at the

electrode–electrolyte interface increases, more electrons can

be collected. Therefore, under the experimental conditions

employed in this study, the behavior of the photoanodes is

mainly determined by recombination with the electrolyte.

From a more practical viewpoint, the introduction of an

additional ZnS layer between the oxide and the QDs improves

the performance of the photoanode and, possibly, that of the

complete solar cell.

4. Ultrafast carrier dynamics

Once evidenced the effect of the ZnS layer on electron

recombination, a sub-pico-second time resolved technique

(LF-HD-TG) has been employed to uncover an eventual

effect of the ZnS coating on the rate of electron injection.

Chart 2 Energy diagram for TiO2/ZnS/CdSe assembly in polysulfideat an applied potential of �1.0 V vs. Ag/AgCl. The arrows representthe pathway for the electron recombination with polysulfide via QDsurface states or conduction band.

Fig. 6 Photocurrent experiments for TiO2/CdSe and TiO2/CdSe/ZnS

electrodes (A) or TiO2/ZnS/CdSe and TiO2/ZnS/CdSe/ZnS electrodes

(B). Stationary photocurrent vs. RCT (Fig. 4 and 5) (C). Electrolyte:

1 M Na2S + 0.1 M S + 1 M NaOH. Measurements of photocurrent

were performed at a constant potential of �0.6 V. The electrodes were

illuminated with white light using a UV-filter (cut-off l o 380 nm),

irradiance = 73.0 mW cm�2.

Dow

nloa

ded

by U

nive

rsid

ade

do P

orto

(U

P) o

n 02

Jun

e 20

11Pu

blis

hed

on 3

1 M

ay 2

011

on h

ttp://

pubs

.rsc

.org

| do

i:10.

1039

/C1C

P202

90A

View Online

Phys. Chem. Chem. Phys. This journal is c the Owner Societies 2011

As discussed in detail in recent reports,33 the TG signal is

proportional to the change of the refractive index (Dn(t)) of thesample upon an excitation pulse (pump beam). The excitation

photogenerates free electrons and holes inside the QDs, which

according to Drude’s model cause the refractive index to

change. The value of Dn(t) is proportional to the density of

carriers via their effective masses in the material. The TG decay

can be associated to the depopulation (trapping, transfer and

recombination) of the free carriers, but, in our case, due to the

low pump intensities (2 mW), only one body processes are

monitored, i.e. hole trapping and electron transfer and trapping.

Experiments are done in air and thus photogenerated holes

cannot be transferred.

Fig. 7 shows the TG signal for a CdSe-sensitized TiO2

electrode before and after the application of ZnS. After

covering the sample with ZnS, the TG signal, which is

proportional to the density of free carriers, drops faster than

in the absence of the ZnS coating. Therefore, the photo-

generated free carrier lifetime is substantially diminished in

the presence of the ZnS layer. According to recent studies, it

seems appropriate to fit the relaxation of the TG signal (y) to a

double exponential decay (eqn (1)). Fitting parameters are

summarized in Table 1

y = A1e�t/t1 + A2e

�t/t2 (1)

As treated in-depth by us recently, the fast signal decay

(A1, t1) can be ascribed to electron injection from QDs in

direct contact with the oxide surface (and hole trapping),

whereas the slow decay (A2, t2) is associated to electron

injection from QDs located in outer layers (as a result of

QD aggregation) and electron trapping in surface states.33 As

shown in Table 1, both t1 and t2 decrease upon the application

of the ZnS layer, pointing to an enhancement in electron

injection. Taking into account that hole trapping is unaffected

by surface modification as proved by Klimov et al.,34 the

decrease in t1 should be ascribed to a more efficient electron

injection after covering with ZnS. Such an enhancement in

electron injection would also extend to the slow component as

deduced from the decrease observed in the value of t2.Admittedly, the slow TG decay also contains the contribution

of electron trapping, which is not favored by the ZnS treat-

ment according to the PL results (see above). This should lead

to an increase in t2, which is not observed because the

dominant effect is the enhancement of the slow component

of electron injection. The change of the ratio A1/A2 is also

remarkable. Prior to the ZnS treatment its value is 1.02,

whereas afterward it rises to 1.58, indicating that fast electron

transfer becomes more important after the ZnS treatment.

The enhancement observed in electron transfer may be due

to a shift in the QD energy bands. It also suggests that the ZnS

layer may be able to passivate the electron traps generated at

QD/QD interfaces, which may play a role in the slow compo-

nent of the electron transfer. To our knowledge, this is the first

time that the influence of the ZnS shell on the electron

injection rate has been unveiled. Sambur and Parkinson21

have recently emphasized the importance of core–shell QDs

to improve the stability of the corresponding solar cells. It may

happen that the deposited ZnS layer yields better perfor-

mances than using pre-synthesized core–shell QDs, since the

route of electron injection is not hampered by an intermediate

barrier layer of ZnS.

Conclusion

Several techniques have been applied to QD-sensitized photo-

anodes to clarify the role of the ZnS treatment. Optical

characterization demonstrates that the ZnS layer covers the

CdSe QDs and passivates their surface states, bringing out

presumably a rough core–shell structure, i.e. ZnS probably

coats the QD surface exposed to the electrolyte, leaving

unaltered the TiO2/CdSe interface. On the other hand, AFM

images obtained with a rutile single crystal demonstrate that

the ZnS coating efficiently covers the TiO2 surface. A new

approach alternating the order of the different coatings

(QDs and ZnS) has been used to decipher the role of the

ZnS layer. Our findings evidence that both the blockage of

the TiO2 surface and the passivation of the QDs are involved,

the latter being more important. Concretely, electrochemical

measurements demonstrate that the QDs are also effective in

blocking the TiO2 surface. Although electron leakage to the

electrolyte would also occur via the CdSe QDs. Such a leakage

would be mostly suppressed by the ZnS coating. This is

evidenced by an increase in both the charge transfer resistance

(associated to recombination) and the photocurrent, in

contact with a polysulfide solution, once the ZnS is applied

on the TiO2/CdSe electrode, regardless of the previous deposi-

tion of another ZnS layer. Complementarily, the ZnS layer

would also avoid direct recombination of TiO2 electrons with

the electrolyte. Finally, the ultrafast carrier dynamics study

reveals that upon coating the CdSe QD with the ZnS shell,

electron injection is also favored, revealing for the first time

the double role of the ZnS layer in improving the performance

Fig. 7 TG signal for the CdSe-sensitized electrode before and after

the treatment with ZnS. The signals were normalized to their maximum

value.

Table 1 TG signal fitting parameters according to eqn (1)

Sample A1 t1/ps A2 t2/ps

TiO2/CdSe 0.48 � 0.01 4.3 � 0.2 0.47 � 0.01 209 � 11TiO2/CdSe/ZnS 0.55 � 0.02 3.9 � 0.4 0.35 � 0.01 148 � 16

Dow

nloa

ded

by U

nive

rsid

ade

do P

orto

(U

P) o

n 02

Jun

e 20

11Pu

blis

hed

on 3

1 M

ay 2

011

on h

ttp://

pubs

.rsc

.org

| do

i:10.

1039

/C1C

P202

90A

View Online

This journal is c the Owner Societies 2011 Phys. Chem. Chem. Phys.

of the photoanode. All these results provide a better insight

into the role of modifiers such as ZnS, which should open new

ways for the enhancement of the QDSC performance.

Acknowledgements

This work was supported by the Ministerio de Ciencia e

Innovacion of Spain, under the projects CONSOLIDER

HOPE CSD2007-00007 andMAT2009-14004 (Fondos FEDER).

Part of this research was supported by PRESTO program,

Japan Science and Technology Agency (JST), Grant in Aid for

Priority Area (470) (No.21020014), Scientific Research

(No.21310073) from the Ministry of Education, Sports,

Science and Technology (MEXT) of the Japanese Government.

N.G. thanks the Spanish Ministry of Education for the award

of an FPU grant.

Referencesand notes

1 S. Ruhle, M. Shalom and A. Zaban, ChemPhysChem, 2010, 11, 2290.2 A. J. Nozik, Physica E (Amsterdam), 2002, 14, 115.3 A. J. Nozik, Chem. Phys. Lett., 2008, 457, 3.4 J. B. Sambur, T. Novet and B. A. Parkinson, Science, 2010, 330, 63.5 N. Guijarro, T. Lana-Villarreal, Q. Shen, T. Toyoda andR. Gomez, J. Phys. Chem. C, 2010, 114, 21928.

6 Y.-L. Lee, B.-M. Huang and H.-T. Chien, Chem. Mater., 2008, 20,6903.

7 S.-M. Yang, C.-H. Huang, J. Zhai, Z.-S. Wang and L. Jiang,J. Mater. Chem., 2002, 12, 1459.

8 Y.-L. Lee and Y.-S. Lo, Adv. Funct. Mater., 2009, 19, 604.9 M. Deng, Q. Zhang, S. Huang, D. Li, Y. Luo, Q. Shen, T. Toyodaand Q. Meng, Nanoscale Res. Lett., 2010, 5, 986.

10 Z. Liu, M. Miyauchi, Y. Uemura, Y. Cui, K. Hara, Z. Zhao,K. Sunahara and A. Furube, Appl. Phys. Lett., 2010, 96, 233107.

11 E. M. Barea, M. Shalom, S. Gimenez, I. Hod, I. Mora-Sero,A. Zaban and J. Bisquert, J. Am. Chem. Soc., 2010, 132, 6834.

12 L. J. Diguna, Q. Shen, J. Kobayashi and T. Toyoda, Appl. Phys.Lett., 2007, 91, 023116.

13 I. Gur, N. A. Fromer, M. L. Geier and A. P. Alivisatos, Science,2005, 310, 462.

14 S.-Q. Fan, D. Kim, J.-J. Kim, D. W. Jung, S. O. Kang and J. Ko,Electrochem. Commun., 2009, 11, 1337.

15 Q. Shen, J. Kobayashi, L. J. Diguna and T. Toyoda, J. Appl. Phys.,2008, 103, 084304.

16 S. Gimenez, I. Mora-Sero, l. Macor, N. Guijarro, T. Lana-Villarreal, R. Gomez, L. J. Diguna, Q. Shen, T. Toyoda andJ. Bisquert, Nanotechnology, 2009, 20, 295204.

17 I. Mora-Sero, S. Gimenez, F. Fabregat-Santiago, R. Gomez,Q. Shen, T. Toyoda and J. Bisquert, Acc. Chem. Res., 2009, 42,1848.

18 M. A. Hines and P. Guyot-Sionnest, J. Phys. Chem., 1996, 100,486.

19 D. Chen, F. Zhao, H. Qi, M. Rutherford and X. Peng, Chem.Mater., 2010, 22, 1437.

20 P. Reiss, M. Potiere and L. Li, Small, 2009, 5, 154.21 J. B. Sambur and B. A. Parkinson, J. Am. Chem. Soc., 2010, 132,

2130.22 Q. Wang, D. Pan, S. Jiang, X. Ji, L. An and B. Jiang, J. Cryst.

Growth, 2006, 286, 83.23 N. Guijarro, T. Lana-Villarreal, I. Mora-Sero, J. Bisquert and

R. Gomez, J. Phys. Chem. C, 2009, 113, 4208.24 P. Pu, H. Cachet and E. M. M. Sutter, Electrochim. Acta, 2010, 55,

5938.25 L. J. Diguna, Q. Shen, A. Sato, A. Katayama, T. Sawada and

T. Toyoda, Mater. Sci. Eng., C, 2007, 27, 1514.26 Q. Shen, M. Yanai, K. Katayama, T. Sawada and T. Toyoda,

Chem. Phys. Lett., 2007, 442, 89.27 W. W. Yu, L. Qu, W. Z. Guo and X. G. Peng, Chem. Mater., 2003,

15, 2854.28 V. I. Klimov, A. A. Mikhailovsky, S. Xu, A. Malko, J. A.

Hollingswoth, C. A. Leatherdale, H. J. Eisler and M. G. Bawendi,Science, 2000, 290, 314.

29 C. F. Landes, M. Braun and M. A. El-Sayed, J. Phys. Chem. B,2001, 105, 10554.

30 D. R. Baker and P. V. Kamat, Langmuir, 2010, 26, 11272.31 The mixture of Na2S and S in aqueous electrolyte gives rise to a

complex family of molecular chains described as Sx2� with wide

distribution of lengths, the so-called polysulfide.32 G. Hodes, J. Phys. Chem. C, 2008, 112, 17778.33 N. Guijarro, Q. Shen, S. Gimenez, I. Mora-Sero, J. Bisquert,

T. Lana-Villarreal, T. Toyoda and R. Gomez, J. Phys. Chem. C,2010, 114, 22352.

34 V. I. Klimov, Ch. J. Schwarz, D. W. McBranch, C. A. Leatherdaleand M. G. Bawendi, Phys. Rev. B: Condens. Matter, 1999, 60,R2177.

Dow

nloa

ded

by U

nive

rsid

ade

do P

orto

(U

P) o

n 02

Jun

e 20

11Pu

blis

hed

on 3

1 M

ay 2

011

on h

ttp://

pubs

.rsc

.org

| do

i:10.

1039

/C1C

P202

90A

View Online