High Efficiency CdS/CdSe Quantum Dot Sensitized Solar Cells with Two ZnSe Layers

Abstract

CdS/CdSe quantum dot sensitized solar cells (QDSCs) have been intensively investigated; however, most of the reported power conversion efficiency (PCE) is still lower than 7% due to serious charge recombination and a low loading amount of QDs. Therefore, suppressing charge recombination and enhancing light absorption are required to improve the performance of QDSCs. The present study demonstrated successful design and fabrication of QDSCs with a high efficiency of 7.24% based on CdS/CdSe QDs with two ZnSe layers inserted at the interfaces between QDs and TiO2 and electrolyte. The effects of two ZnSe layers on the performance of the QDSCs were systematically investigated. The results indicated that the inner ZnSe buffer layer located between QDs and TiO2 serve as a seed layer to enhance the subsequent deposition of CdS/CdSe QDs, leading to higher loading amount and covering ratio of QDs on the TiO2 photoanode. The outer ZnSe layer located between QDs and electrolyte behave as an effective passivation layer which not only reduces the surface charge recombination, but also enhances the light harvesting.

Full text

High Efficiency CdS/CdSe Quantum Dot Sensitized Solar Cells with
Two ZnSe Layers
Fei Huang,
†,‡
Lisha Zhang,
‡
Qifeng Zhang,
†
Juan Hou,
†
Hongen Wang,
†
Huanli Wang,
‡
Shanglong Peng,
†
Jianshe Liu,*
,‡
and Guozhong Cao*
,†
†
Department of Materials Science and Engineering, University of Washington, Seattle, Washington 98195-2120, United States
‡
College of Environmental Science and Engineering, Donghua University, Shanghai 201620, P.R. China
*
SSupporting Information
ABSTRACT: CdS/CdSe quantum dot sensitized solar cells
(QDSCs) have been intensively investigated; however, most of
the reported power conversion efficiency (PCE) is still lower
than 7% due to serious charge recombination and a low
loading amount of QDs. Therefore, suppressing charge
recombination and enhancing light absorption are required
to improve the performance of QDSCs. The present study
demonstrated successful design and fabrication of QDSCs with
a high efficiency of 7.24% based on CdS/CdSe QDs with two
ZnSe layers inserted at the interfaces between QDs and TiO2
and electrolyte. The effects of two ZnSe layers on the performance of the QDSCs were systematically investigated. The results
indicated that the inner ZnSe buffer layer located between QDs and TiO2serves as a seed layer to enhance the subsequent
deposition of CdS/CdSe QDs, which leads to higher loading amount and covering ratio of QDs on the TiO2photoanode. The
outer ZnSe layer located between QDs and electrolyte behaves as an effective passivation layer, which not only reduces the
surface charge recombination, but also enhances the light harvesting.
KEYWORDS: CdS/CdSe quantum dots, ZnSe buffer layer, ZnSe passivation layer, enhanced quantum dots loading,
reduced charge recombination, quantum dot-sensitized solar cell
1. INTRODUCTION
To solve the increasing serious energy crisis and environmental
pollution problems, the development of photovoltaic devices
for solar power conversion is of great significance. Dye
sensitized solar cells (DSCs) have attracted plenty of research
during the past 25 years because of their inexpensive, simple
fabrication process and high efficiency; however, it is not easy
to obtain a low cost commercial dye that can absorb the whole
light spectrum region.
1−6
Perovskite absorbers in the form of
thinner layers can enable complete light absorption.
7
In the past
few years, perovskite solar cells have been a hot spot in solar
cell research fields; however, they are sensitive to the
environment (oxygen, moisture, UV light, and temperature)
because of their low chemical stability.
8−10
Because of their
good stability, size-dependent tunable band gap, large
extinction coefficient, and multiple exciton generation (MEG)
with single-photon absorption, narrow band gap semiconductor
quantum dots have been widely explored as sensitizers for
quantum dots sensitized solar cells (QDSCs).
11−14
In view of
the MEG effect, the theoretical photovoltaic conversion
efficiency of QDSCs can reach up to 44%.
15
However, the
highest power conversion efficiency of QDSCs was still lower
than 12%.
16−19
Considerable efforts have been put forward to improve the
performance of QDSCs to meet the demand of effective solar
power conversion devices, mainly including (1) introducing
passivation layer (i.e., ZnS, or ZnSe) on the QDs’surfaces to
suppress recombination processes in the photoelectrode/QDs/
electrolyte interfaces
20,21
or introducing wide band gap
semiconductors (i.e., SiO2, TiO2) on the surface of photo-
electrode to reduce electron losses,
22,23
(2) adopting ex-situ
synthesized high quality QDs with reduced surface trap
states,
24,25
(3) choosing narrow band gap semiconductors to
extend the absorption range to longer wavelength or near-
infrared region,
26,27
(4) enhancing the deposition amount of
homogeneous distributed QDs,
28
and (5) introducing tran-
sition metal ion dopants such as Mn2+ to modify the
optoelectronic properties of QD sensitizers.
29−31
Significant
improvement in the performance of QDSCs has been
demonstrated with these methods.
Taking the advantages of a wide light absorption range,
superior electron transfer properties, good stability, easy
synthesis, CdS/CdSe cosensitized QDSCs have attracted a lot
of attention in the research community.
32−36
However, CdS
and CdSe QDs were always synthesized via a low temperature
synthesis method such as chemical bath deposition (CBD) and
Received: October 9, 2016
Accepted: November 29, 2016
Published: November 29, 2016
Research Article
www.acsami.org
© XXXX American Chemical Society ADOI: 10.1021/acsami.6b12842
ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

successive ionic layer adsorption and reaction (SILAR) or
electrodeposition method.
6,34,37,38
The QDs obtained by the
low-temperature chemical processes contain a large number of
surface trap states, which can act as charge recombination
centers. It is very difficult to get a sufficient and evenly
deposition of QDs on the whole surface of photoanode (TiO2
and ZnO).
39
The low surface coverage of QDs on metal oxide
film electrode will, on the one hand, lead to insufficient
absorption of light and, on the other hand, result in large
portion area of bare TiO2surface contact with the electrolyte
directly, leading to severe charge recombination at the TiO2
oxide/electrolyte interface. Previous work has demonstrated
that when ZnSe was introduced as a passivation layer in CdS/
CdSe quantum dot-sensitized solar cells (QDSCs), compared
with the solar cells without passivation layer and the most
widely used ZnS passivation layer, it presented ∼90% and
∼30% improvement in power conversion efficiency.
40,41
The
present investigation included the design and fabrication of
CdS/CdSe quantum dot-sensitized TiO2solar cells by inserting
two ZnSe layers: one between TiO2and QDs as a buffer layer
and another between QDs and electrolyte as a passivation layer.
Both inner and outer ZnSe layers lead to the enhancement of
current density, open-circuit voltage, and the consequently PCE
of the resultant cell devices. The champion QDSC based on the
CdS/CdSe QDs sensitizer with two ZnSe layers exhibited a
PCE of 7.24% under AM 1.5 G one sun illumination, which is
among the best results of CdS/CdSe cosensitized solar cells so
far reported in literature.
2. EXPERIMENTAL SECTION
2.1. Preparation of TiO2/CdS/CdSe Photoanodes with Two
ZnSe Layers. All the chemicals used in this work were analytical-
grade reagents without further purification. The preparation of
mesoporous TiO2films and deposition of CdS/CdSe QDs all referred
to previous work.
39,40,42
A transparent TiO2mesoporous film was
prepared via doctor blading the TiO2paste on a clean F:SnO2-coated
(FTO, 6−8Ω/square) glass substrate followed by sintering at 500 °C
for 30 min to remove the organic impurities and improve the
crystallinity. CdS QDs were deposited by SILAR method on the
surface of TiO2mesoporous films with five cycles. CdSe QDs were
deposited on the TiO2/CdS film via a chemical bath deposition
(CBD) procedure for 3 h at room temperature.
The ZnSe layers were also deposited by SILAR method according
to our previous publication.
40
Briefly, the films were first immersed in
0.1 M Zn2+ solution for 2 min and then immersed in 0.1 M Se2−for
another 2 min. Following each immersion, the films were thoroughly
rinsed with deionized water to remove the unbound ions and dried. In
this process, the Se2−solution was always purged with N2. According
to previous work,
40
three SILAR cycles of ZnSe outer layer are the
optimized thickness. The optimization of the SILAR cycles of inner
ZnSe layer is shown in Figure S1 (Supporting Information).
Therefore, this immersion was repeated five times to get the optimum
thickness.
2.2. Electrolyte, Counter Electrode, and Device Assembly.
The electrolyte used in this work was freshly prepared polysulfide
solution, which was made by dissolving 1 M sulfur and 1 M sodium
sulfide in 10 mL of deionized water before each test. The counter
electrode is a nanostructured Cu2Sfilm on brass foil, in brief,
immersed a brass foil into 37% HCl at 80 °C for 20 min, taken out to
be rinsed with deionized water and ethanol, dried in air, then
immersed it into the freshly prepared electrolyte for 5 min. This
process was repeated two times, which resulted in the formation of
nanostructured Cu2S on the brass foil. The device was assembled into
sandwich-type with scotch tape placed between the QDs sensitized
photoanode and the counter electrode.
2.3. Characterization. We used the scanning electron microscope
(SEM, JSM-7000) equipped with an energy-dispersive X-ray
spectrometer to characterize the surface morphology and element
composition of the photoanodes. The photocurrent density−voltage
characteristics (J−Vcurves) of the solar cells were recorded by an HP
4155A programmable semiconductor parameter analyzer under AM
1.5 simulated sunlight with a power density of 100 mW cm−2.To
measure the light absorption properties of the photoanodes,
PerkinElmer Lambda 900 UV−vis/IR spectrometer was used.
Electrochemical impedance spectroscopy (EIS) was carried out to
analyze the electronic and ionic processes in the QDSCs using a
Solartron 1287A coupled with a Solartron 1260 FRA/impedance
analyzer. The photoluminescence (PL) spectra and PL decay curves
were recorded with a luminescence spectrometer FLsp920. The
incident photon-to-current conversion efficiency (IPCE) spectra were
obtained using 7-SCSpec response measurement system.
3. RESULTS AND DISCUSSION
Figure 1 is the SEM images showing the surface morphologies
of respective CdS/CdSe QDs based TiO2/QDs, TiO2/ZnSe/
QDs, TiO2/QDs/ZnSe, and TiO2/ZnSe/QDs/ZnSe films. No
appreciable difference was observed in all the four samples; the
mesoporous structure did not exhibit any apparent change.
However, careful observation did reveal a small increase in
particle size and a slightly higher filling density of the pores
with the deposition of inner or outer ZnSe layer. The change is
vague, and it is not possible for us to calculate the specific
enhancement of the particle size and coverage ratio of the QDs
Figure 1. Top-view SEM micrographs of CdS/CdSe QDs based (a, a1) TiO2/QDs, (b, b1) TiO2/ZnSe/QDs, (c, c1) TiO2/QDs/ZnSe, and (d, d1)
TiO2/ZnSe/QDs/ZnSe films.
ACS Applied Materials & Interfaces Research Article
DOI: 10.1021/acsami.6b12842
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To evaluate the influence of inner ZnSe layer on the
subsequent deposition of CdS QDs, we have measured the
UV−vis absorption spectra of the TiO2/CdS and TiO2/ZnSe/
CdS films, as shown in Figure 2. Compared with the TiO2/CdS
film, the light absorption of the TiO2/ZnSe/CdS film shows a
great enhancement not only in absorbance, but also a red-shift
in absorption edge. The light absorption of ZnSe is not longer
than 460 nm because it has a wider bandgap (2.7 eV), so the
noticeable enhancement was not because of the ZnSe layer.
28
This result demonstrates that for the bare TiO2film, the
deposition amount of CdS QDs was low, while with the
presence of inner ZnSe layer, the deposition amount of CdS
QDs was significantly enhanced, which was demonstrated by
the higher light absorbance and the larger red-shift of the
absorption range. Thus, the presence of the ZnSe layer can
enhance the deposition of CdS QDs; the ZnSe layer might have
served as a seeding layer to enhance the nucleation and growth
of CdS QDs with high quality, leading to a high coverage ratio
on the surface of photoanodes.
Figure 3a shows the light absorption spectra of CdS/CdSe
QDs based TiO2/QDs, TiO2/ZnSe/QDs, TiO2/QDs/ZnSe,
and TiO2/ZnSe/QDs/ZnSe films over the wavelength ranging
from 300−800 nm under the same measurement condition.
Obviously, both the deposition of inner ZnSe layer and outer
ZnSe layer have a great and different influence on the optical
absorption of the CdS/CdSe QDs based TiO2/QDs films. With
the deposition of inner ZnSe layer, the photoelectrode shows
an increase in absorbance, while with the deposition of outer
ZnSe layer, the photoelectrode shows not only an enhancement
in absorbance, but also a noticeable red-shift in absorption
range. When two ZnSe layers were deposited, the photo-
electrode shows a further increase in absorbance compared with
the photoanode with single ZnSe layer and a same red-shift in
absorption range.
The higher absorbance indicates that the amount of QDs
deposited on the photoelectrode with inner ZnSe layer is
higher than that without ZnSe layer. The inner ZnSe layer
appears to favor the deposition of CdS QDs, and in turn CdS
layer promotes the growth of CdSe QDs.
33,43
Thus, the
deposition amount of CdSe QDs was increased, which leads to
a higher light absorbance. The introduction of ZnSe outer layer
results in an increased absorbance and a significant red-shift of
the absorption edge, in accordance with the results reported
earlier.
40,41
ZnSe has a bandgap of 2.7 eV, which is wider than
1.74 eV of CdSe QDs, so the change in absorption was not
caused by ZnSe itself.
28
According to the literature, the high
absorption of the photoelectrode might be attributed to (1) the
partial overlap of the exciton wave functions of CdS/CdSe QDs
and the outer ZnSe layer resulting from an interaction between
the two parts
21,44
and (2) increased deposition amount and
particle size of CdSe QDs. The light absorption of TiO2/CdS/
CdSe photoanodes with different SILAR cycles of ZnSe outer
layer and the atom ratios of the elements in TiO2/CdS/CdSe
photoanodes were examined, and the results are shown in
Figure S2 (Supporting Information) and Figure S3 (Supporting
Information). The absorption shows a strong dependence on
the number of SILAR cycles for deposition of ZnSe passivation
layer, and the red-shift of the absorption edge mainly occurs in
the first cycle. The amount of Cd is more than the total amount
of S and Se combined in TiO2/CdS/CdSe photoanode.
Therefore, the excessive cadmium existing on the surface of
TiO2/CdS/CdSe film would react with Se2−during the first
ZnSe SILAR deposition process, leading to a further growth
and formation of the CdSe QDs. Although these explanations
sound reasonable, the oxidization of Se ions appeared to take
place on the surface of ZnSe layer, which could have
contribution to the higher absorption. More detailed experi-
ments are still ongoing to illustrate this issue, separately. As
demonstrated in Figure 3b, the optical band gap of the QDs can
be estimated by extrapolating the linear portion of the (Ahv)2
versus hv plots, according to eq 1:
45−47
=−Ahv c hv E
(
)( )
2g(1)
where Ais the absorbance, cis a constant, vis the photon
frequency, and his Plank constant. Obviously, both the inner
and outer ZnSe layer can contribute to enhanced light
absorption, and the TiO2/CdS/CdSe photoanode with two
ZnSe layers shows a higher light absorption compared to the
photoanode without or with single ZnSe layer.
Figure 2. UV−visible absorption spectra of TiO2/CdS, TiO2/ZnSe/
CdS films.
Figure 3. (a) UV−visible absorption spectra and (b) (Ahv)2versus hv curves of the CdS/CdSe QDs based TiO2/QDs, TiO2/ZnSe/QDs, TiO2/
QDs/ZnSe, and TiO2/ZnSe/QDs/ZnSe films.
ACS Applied Materials & Interfaces Research Article
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To estimate the resistance distribution and charge recombi-
nation processes, Figure 4a gives the comparison of electro-
chemical impedance spectra (EIS) of the TiO2/QDs, TiO2/
ZnSe/QDs, TiO2/QDs/ZnSe, and TiO2/ZnSe/QDs/ZnSe
QDSCs, which were measured under a forward bias (−0.6 V)
and dark condition. The spectra were composed of two
semicircles fitted by an equivalent circuit (inset in Figure 4a).
48
The fitting results are listed in Table 1. The first small
semicircle, almost invisible, indicates the recombination
resistance at the counter electrode/electrolyte interface (R1).
Because the same counter electrode and electrolyte were used
during the measurement, almost no difference was observed in
R1among these four QDSCs. The second large semicircle
indicates the charge transfer resistance at the TiO2/QDs/
electrolyte interface and in the TiO2films (Rct).
23,49
We can see
there is a noticeable difference in Rct; the Rct value for the
QDSCs with two ZnSe layers is 196.3 Ω, while the Rct for the
QDSCs without passivation layer and with single ZnSe
passivation layers is only 68.0 Ω, 95.3 Ω, and 156.2 Ω. The
charge recombination resistance in a QDSC is mainly
determined by Rct.AhighRct value means a reduced
recombination of the electrons and holes.
23
Figure 4b shows
the Bode plots of the TiO2/QDs, TiO2/ZnSe/QDs, TiO2/
QDs/ZnSe, and TiO2/ZnSe/QDs/ZnSe QDSCs. The curve
peak of the Bode plots can be used to estimate the electron
lifetime (τn) according to eq 2:
50
τ
ωπ
==
f
11
2
n
min min (2)
where fmin is the peak frequency at the minimum phase angle in
the bode plots. The corresponding electron lifetime of the
QDSCs is listed in Table 1. The τnof the QDSCs with a two
ZnSe layer is 61.2 ms, which is much longer than that of the
QDSCs without ZnSe layer or just with single ZnSe layer.
Increases in both Rct and τnsuggest a reduced interface charge
recombination in QDSCs. According to the above discussion,
two ZnSe layers have superior ability to suppress interfacial
charge recombination processes compared to the single ZnSe
layer.
Figure 5a shows the PL spectra of TiO2/QDs, TiO2/ZnSe/
QDs, TiO2/QDs/ZnSe, and TiO2/ZnSe/QDs/ZnSe films. In
the PL test, the films are illuminated under light source with
photon energy above the band gap energy of QDs. Under light
illumination, the QDs are excited by photons and generate
electron−hole pairs, which will finite momenta existence in the
conduction and valence bands, respectively. The PL emission
derives from the recombination of the photoinduced electrons
and holes.
51
Therefore, a high emission is an indication of
increased recombination of photoinduced electrons and holes.
As shown in Figure 5a, there are two peaks in each curve
around 500 and 650 nm, which correspond to the emission of
CdS and CdSe QDs, repectively. The PL emission of CdSe
QDs is markedly quenched despite the increase in absorbance
after loading with ZnSe outer layer, which indicates the
recombination of photogenerated electron−hole pairs is
significantly reduced. In addition, the PL emission peak of
CdSe QDs also shifts to a longer wavelength, in a good
accordance with the UV−vis results. However, with ZnSe inner
layer deposition, there’s a slight increase in PL emission
intensity of CdSe QDs. This was due to the fact that with ZnSe
inner layer deposited, more CdSe QDs are deposited and more
Figure 4. (a) Nyquist plot and (b) Bode plot curves of the CdS/CdSe QDs based TiO2/QDs, TiO2/ZnSe/QDs, TiO2/QDs/ZnSe, and TiO2/ZnSe/
QDs/ZnSe solar cells measured under forward bias (−0.6 V) in dark condition.
Table 1. Electrochemical Impedance Results of QDSCs
samples R1(Ω)Rct (Ω)τn(ms)
TiO2/QDs 1.9 68.0 11.5
TiO2/ZnSe/QDs 1.8 95.3 26.5
TiO2/QDs/ZnSe 1.5 156.2 40.0
TiO2/ZnSe/QDs/ZnSe 1.7 196.3 61.2
Figure 5. (a) PL curves and (b) normalized PL decay curves of the CdS/CdSe QDs based TiO2/QDs, TiO2/ZnSe/QDs, TiO2/QDs/ZnSe, and
TiO2/ZnSe/QDs/ZnSe films.
ACS Applied Materials & Interfaces Research Article
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excitons are produced, in accordance with the enhanced
absorbance. For the PL emission of CdS QDs, it is quenched
with the deposition of two ZnSe layers, although the existense
of ZnSe inner layer can lead to high absorbance of CdS QDs as
shown in Figure 2. This phenomenon can be attributed to less
defects existing in the CdS QDs. So the existence of ZnSe inner
layer helps CdS QDs formation with enhanced crystallinity and
better surface quality.
Figure 5b depicts the excited state electron radiative decay
curves of the photoanodes, which are employed to analyze the
influence of inner and outer ZnSe layer coating on the
photogenerated electrons and holes recombination and
electrons injection rate. The PL lifetimes of the QDs are
estimated to be 10.685, 7.909, 1.285, and 0.752 ns for the
TiO2/QDs, TiO2/ZnSe/QDs, TiO2/QDs/ZnSe, and TiO2/
ZnSe/QDs/ZnSe films, respectively. This result shows that the
introduction of inner and outer ZnSe layer can shorten the PL
lifetime of the QDs. The short-lived excited state of the QDs
means the enhanced injection kinetics of the photogenerated
electrons in the QDs.
52,53
Therefore, the introduction of inner
and outer ZnSe layer will benefit the electrons from QDs inject
to TiO2.
Figure 6a present the schematic diagram of TiO2/ZnSe/
QDs/ZnSe solar cells that consist of a TiO2photoanode with
an inner ZnSe buffer layer, an outer ZnSe passivation layer,
CdS/CdSe QDs, a sulfide/polysulfide (S2−/Sx2−) electrolyte,
and a Cu2S counter electrode. According to the literature,
21,28
the band edge structure of all the materials in the photoanode is
shown in Figure 6b. Under light illumination, CdS/CdSe
quantum dots will be excited by photons and generate
electron−hole pairs, and then the electrons will transfer to
the conduction band of TiO2and holes will transfer to the
electrolyte and released by the redox couples in the electrolyte.
Meanwhile, severe charge recombination processes also occur
in the QDSCs and severely decrease the performance.
54,55
In
this structure, the outer ZnSe passivation layer will not only
prevent recombination of electrons in the conduction band of
the QDs and TiO2with the oxidized form of the redox couple
in electrolyte, but also facilitate the holes transfer to the
electrolyte, and the deposition of ZnSe outer layer also helped
to enhance light absorption. Inner ZnSe buffer layer serves as a
seed layer benefit the CdS and CdSe QDs deposition, which
leads to a sufficient and even deposition of QDs with high
quality on the TiO2photoelectrode. This will not only help the
CdS QDs formation with fewer defects and decrease the charge
recombination between TiO2/electrolyte interfaces, but also
enhance light absorption. Herein, we should point out that
although ZnSe has a higher conduction band edge than that of
CdS/CdSe QDs and TiO2, the PL lifetime of the QDs is
shortened by the introduction of inner ZnSe layer. This result
shows that the inner ZnSe layer does not seem to hinder the
electron injection from the conduction band of QDs to the
TiO2; instead, more efficient charge injection occurs at the
TiO2/ZnSe/QDs interfaces.
Figure 7a presents the incident photon-to-current conversion
efficiency (IPCE) spectra to evaluate the light absorption and
electron collection characteristics. As reported in literature,
56
the IPCE values are lower than 80% restricted by the reflection
of the glass subtract. From Figure 7a, we can find the IPCE
values for the solar cell with two ZnSe layers are very close to
80%, which indicate its good performance. The integration of
IPCE over the solar spectrum is the Jsc of the solar cells; as
expected, the IPCE values of the solar cells with two ZnSe
layers are higher than that of the solar cells without ZnSe layer
or with single ZnSe layer, which is in a same trend with the
variation of Jsc of the corresponding cells. The IPCE was
estimated by eq 3:
57
ηη=××IPCE LHE ct c
c
(3)
Figure 6. (a) Schematic diagram and (b) band edges structure for efficient transport of excited electrons and holes of the CdS/CdSe QDs based
TiO2/ZnSe/QDs/ZnSe solar cells.
Figure 7. (a) IPCE spectra and (b) LHE spectra of the CdS/CdSe QDs based TiO2/QDs, TiO2/ZnSe/QDs, TiO2/QDs/ZnSe, and TiO2/ZnSe/
QDs/ZnSe solar cells.
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where LHE is light-harvesting efficiency, ηct is charge-transfer
efficiency, and ηcc is charge-collection efficiency. The light
harvesting efficiency “LHE”is given by eq 4:
58,59
=− −
L
HE 1 10 absorbanc
e
(4)
Figure 7b shows the LHE value of the QDSCs, which is in a
same trend with the light absorption of the photoelectrodes.
The LHE of the photoelectrodes with two ZnSe is higher than
that without ZnSe layer or with single ZnSe layer. ηct is
determined by the energy level difference between the
conduction band edge of QDs and the photoanode, which
was the driving force for the photoexcited electrons transfer.
Theoretically, the energy level difference between the
conduction band edge of CdS/CdSe QDs and TiO2is over
200 mV, which will be sufficient to drive charge transfer
process.
33
To discuss the effect of different ZnSe layers on the
ηct value, we can refer to the PL decay as shown in Figure 5b.
Figure 5b indicates that the injection of electrons from QDs to
TiO2was accelerated by the introduction of ZnSe layer, so it is
helpful for improving ηct.ηcc isrelatedtothecharge
recombination in solar cells, the EIS results, as well as the J−
Vcurves in dark condition indicates the deposition of ZnSe
layers can reduce charge recombination processes. In addition,
the deposition of two ZnSe passivation layer is better,
corresponding with a much higher charge collection efficiency.
Consequently, the TiO2/CdS/CdSe QDSCs with two ZnSe
layers show the highest IPCE.
Figure 8 compares the J−Vcurves of the solar cells measured
under one sun (AM 1.5, 100 mA cm−2) illumination. Table 2
shows the average values of the open circuit voltage (Voc), short
circuit current density (Jsc), fill factor (FF), and power
conversion efficiency (PCE), which are obtained from the J−
Vcurves. The PCE values of the CdS/CdSe cosensitized TiO2
solar cells without ZnSe layer or just with outer ZnSe layer are
3.49%, 5.05%, and 6.16%, respectively. When two ZnSe layers
were deposited, Jsc and Voc increased to 20.95 mA cm−2and
0.62 V, and the PCE reaches 7.03%. Such an enhancement in
PCE can be attributed to the enhanced light absorption and
reduced charge recombination. The champion QDSC based on
the CdS/CdSe QDs sensitizer with two ZnSe layers is
exhibiting a PCE of 7.24%. So far, the highest PCE of CdS/
CdSe cosensitized QDSCs was 7.11% with nanostructured
counter electrode based on nonstoichiometric Cu2−xSe electro-
catalysts.
60
However, with Au or Cu2S counter electrode, the
best PCE value of CdS/CdSe cosensitized QDSCs was ∼6%,
still not easy to get up to 7%.
61,62
To the best of our knowledge,
the PCE of 7.24% is one of the highest values for CdS/CdSe
cosensitized solar cells.
4. CONCLUSIONS
Insertion of two ZnSe layers at the interfaces between QDs and
TiO2and electrolyte has demonstrated to promote the
deposition of QDs and to decrease charge recombination,
which collectively led to high efficiency QDSCs. The inner
ZnSe buffer layer served as a seed layer to increase the
subsequent deposition of CdS and CdSe QDs, which led to a
sufficient and evenly deposition of high quality QDs and
benefitted the efficient photon capturing and suppressed
interfacial charge recombination; the outer ZnSe layer acted
as a passivation layer by not only preventing charge
recombination, but also enhancing light absorption. As a result,
both Voc and Jsc were increased, and an overall power
conversion efficiency of 7.24% has been achieved, which is
considerably higher than the QDSCs without ZnSe layer or
with single ZnSe layer and definitely among the highest
reported data for CdS/CdSe cosensitized QDSCs.
■ASSOCIATED CONTENT
*
SSupporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acsami.6b12842.
Optimization of the SILAR cycles of inner ZnSe layer
(J−Vcurves), UV−vis of TiO2/QDs photoanodes with
various cycles of outer ZnSe layer; EDS of TiO2/QDs
photoanodes (PDF)
■AUTHOR INFORMATION
Corresponding Authors
*E-mail: gzcao@u.washington.edu. Phone: +1-206-616-9084.
*E-mail: jiansheliu@dhu.edu.cn.
ORCID
Guozhong Cao: 0000-0003-1498-4517
Notes
The authors declare no competing financial interest.
Figure 8. Comparison of photocurrent density−voltage (J−V) curves
of the CdS/CdSe QDs based TiO2/QDs, TiO2/ZnSe/QDs, TiO2/
QDs/ZnSe, and TiO2/ZnSe/QDs/ZnSe solar cells.
Table 2. Photovoltaic Properties of Average Values Obtained from the J−VCurves with Different ZnSe Layers
a
samples Voc (V) Jsc (mA cm−2)FF η(%) (average)
TiO2/QDs 0.51 13.26 0.51 3.49
TiO2/ZnSe/QDs 0.54 18.28 0.51 5.05
TiO2/QDs/ZnSe 0.59 19.30 0.54 6.16
TiO2/ZnSe/QDs/ZnSe 0.62 20.95 0.54 7.03
TiO2/ZnSe/QDs/ZnSe 0.61 21.49 0.55 7.24 (Champion)
a
The average value of each piece of data was obtained by testing at least six cells.
ACS Applied Materials & Interfaces Research Article
DOI: 10.1021/acsami.6b12842
ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
F

■ACKNOWLEDGMENTS
This work was financially supported by the National Science
Foundation (NSF, DMR 1505902), and F.H. would also like to
acknowledge the scholarship by China Scholarship Council.
This work was also supported by National Natural Science
Foundations of China (Nos. 21377023 and 21477019).
■REFERENCES
(1) O'Regan, B.; Gratzel, M. A Low-Cost, High-Efficiency Solar-Cell
Based on Dye-Sensitized Colloidal TiO2Films. Nature 1991,353,
737−740.
(2) Zhang, Q.; Dandeneau, C. S.; Zhou, X.; Cao, G. ZnO
Nanostructures for Dye-Sensitized Solar Cells. Adv. Mater. 2009,21,
4087−4108.
(3) Hagfeldt, A.; Boschloo, G.; Sun, L.; Kloo, L.; Pettersson, H. Dye-
Sensitized Solar Cells. Chem. Rev. 2010,110, 6595−6663.
(4) Han, L.; Islam, A.; Chen, H.; Malapaka, C.; Chiranjeevi, B.;
Zhang, S.; Yang, X.; Yanagida, M. High-efficiency dye-sensitized solar
cell with a novel co-adsorbent. Energy Environ. Sci. 2012,5, 6057−
6060.
(5) Huo, Z.; Tao, L.; Dai, S.; Zhu, J.; Zhang, C.; Chen, S.; Zhang, B.
Quasi-solid-state dye sensitized solar cells using supramolecular gel
electrolyte formed from two-component low molecular mass organo-
gelators. Sci. Chin. Mater. 2015,58, 447−454.
(6) Ren, F.; Li, S.; He, C. Electrolyte for quantum dot-sensitized solar
cells assessed with cyclic voltammetry. Sci. Chin. Mater. 2015,58, 490−
495.
(7) Wang, Q.; Chen, H.; Liu, G.; Wang, L. Control of organic-
inorganic halide perovskites in solid-state solar cells: a perspective. Sci.
Bull. 2015,60, 405−418.
(8) Niu, G.; Guo, X.; Wang, L. Review of recent progress in chemical
stability of perovskite solar cells. J. Mater. Chem. A 2015,3, 8970−
8980.
(9) Boix, P. P.; Nonomura, K.; Mathews, N.; Mhaisalkar, S. G.
Current progress and future perspectives for organic/inorganic
perovskite solar cells. Mater. Today 2014,17,16−23.
(10) Green, M. A.; Ho-Baillie, A.; Snaith, H. J. The emergence of
perovskite solar cells. Nat. Photonics 2014,8, 506−514.
(11) Ruhle, S.; Shalom, M.; Zaban, A. Quantum-Dot-Sensitized Solar
Cells. ChemPhysChem 2010,11, 2290−2304.
(12) Gonzalez-Pedro, V.; Xu, X.; Mora-Sero, I.; Bisquert, J. Modeling
High-Efficiency Quantum Dot Sensitized Solar Cells. ACS Nano 2010,
4, 5783−5790.
(13) Kamat, P. V. Quantum Dot Solar Cells. Semiconductor
Nanocrystals as Light Harvesters. J. Phys. Chem. C 2008,112,
18737−18753.
(14) Nozik, A. J.; Beard, M. C.; Luther, J. M.; Law, M.; Ellingson, R.
J.; Johnson, J. C. Semiconductor Quantum Dots and Quantum Dot
Arrays and Applications of Multiple Exciton Generation to Third-
Generation Photovoltaic Solar Cells. Chem. Rev. 2010,110, 6873−
6890.
(15) Hanna, M. C.; Nozik, A. J. Solar conversion efficiency of
photovoltaic and photoelectrolysis cells with carrier multiplication
absorbers. J. Appl. Phys. 2006,100, 074510.
(16) Ren, Z.; Wang, J.; Pan, Z.; Zhao, K.; Zhang, H.; Li, Y.; Zhao, Y.;
Mora-Sero, I.; Bisquert, J.; Zhong, X. Amorphous TiO2 Buffer Layer
Boosts Efficiency of Quantum Dot Sensitized Solar Cells to over 9%.
Chem. Mater. 2015,27, 8398−8405.
(17) Du, J.; Du, Z.; Hu, J. S.; Pan, Z.; Shen, Q.; Sun, J.; Long, D.;
Dong, H.; Sun, L.; Zhong, X.; Wan, L. J. Zn-Cu-In-Se Quantum Dot
Solar Cells with a Certified Power Conversion Efficiency of 11.6. J.
Am. Chem. Soc. 2016,138, 4201−4209.
(18) Zhao, K.; Pan, Z.; Mora-Sero, I.; Canovas, E.; Wang, H.; Song,
Y.; Gong, X.; Wang, J.; Bonn, M.; Bisquert, J.; Zhong, X. Boosting
Power Conversion Efficiencies of Quantum-Dot-Sensitized Solar Cells
Beyond 8% by Recombination Control. J. Am. Chem. Soc. 2015,137,
5602−5609.
(19) Chuang, C. H.; Brown, P. R.; Bulovic, V.; Bawendi, M. G.
Improved performance and stability in quantum dot solar cells through
band alignment engineering. Nat. Mater. 2014,13, 796−801.
(20) Shen, Q.; Kobayashi, J.; Diguna, L. J.; Toyoda, T. Effect of ZnS
coating on the photovoltaic properties of CdSe quantum dot-
sensitized solar cells. J. Appl. Phys. 2008,103, 084304.
(21) Ahmed, R.; Zhao, L.; Mozer, A. J.; Will, G.; Bell, J.; Wang, H.
Enhanced Electron Lifetime of CdSe/CdS Quantum Dot (QD)
Sensitized Solar Cells Using ZnSe Core-Shell Structure with Efficient
Regeneration of Quantum Dots. J. Phys. Chem. C 2015,119, 2297−
2307.
(22) Liu, Z.; Miyauchi, M.; Uemura, Y.; Cui, Y.; Hara, K.; Zhao, Z.;
Sunahara, K.; Furube, A. Enhancing the performance of quantum dots
sensitized solar cell by SiO2 surface coating. Appl. Phys. Lett. 2010,96,
233107.
(23) Tian, J.; Zhang, Q.; Uchaker, E.; Gao, R.; Qu, X.; Zhang, S.;
Cao, G. Architectured ZnO photoelectrode for high efficiency
quantum dot sensitized solar cells. Energy Environ. Sci. 2013,6, 3542.
(24) Robel, I.; Subramanian, V.; Kuno, M.; Kamat, P. V. Quantum
dot solar cells. Harvesting light energy with CdSe nanocrystals
molecularly linked to mesoscopic TiO2 films. J. Am. Chem. Soc. 2006,
128, 2385−2393.
(25) Guijarro, N.; Lana-Villarreal, T.; Mora-Sero, I.; Bisquert, J.;
Gomez, R. CdSe Quantum Dot-Sensitized TiO2 Electrodes: Effect of
Quantum Dot Coverage and Mode of Attachment. J. Phys. Chem. C
2009,113, 4208−4214.
(26) Tubtimtae, A.; Wu, K.-L.; Tung, H.-Y.; Lee, M.-W.; Wang, G. J.
Ag2S quantum dot-sensitized solar cells. Electrochem. Commun. 2010,
12, 1158−1160.
(27) Pan, Z.; Mora-Sero, I.; Shen, Q.; Zhang, H.; Li, Y.; Zhao, K.;
Wang, J.; Zhong, X.; Bisquert, J. High-Efficiency ″Green″Quantum
Dot Solar Cells. J. Am. Chem. Soc. 2014,136, 9203−9210.
(28) Yang, L.; McCue, C.; Zhang, Q.; Uchaker, E.; Mai, Y.; Cao, G.
Highly efficient quantum dot-sensitized TiO2 solar cells based on
multilayered semiconductors (ZnSe/CdS/CdSe). Nanoscale 2015,7,
3173−3180.
(29) Santra, P. K.; Chen, Y.-S. Role of Mn2+ in Doped Quantum
Dot Solar Cell. Electrochim. Acta 2014,146, 654−658.
(30) Wang, G.; Wei, H.; Luo, Y.; Wu, H.; Li, D.; Zhong, X.; Meng, Q.
A strategy to boost the cell performance of CdSexTe1-x quantum dot
sensitized solar cells over 8% by introducing Mn modified CdSe
coating layer. J. Power Sources 2016,302, 266−273.
(31) Santra, P. K.; Kamat, P. V. Mn-doped quantum dot sensitized
solar cells: a strategy to boost efficiency over 5%. J. Am. Chem. Soc.
2012,134, 2508−2511.
(32) Zhang, Y.; Zhu, J.; Yu, X.; Wei, J.; Hu, L.; Dai, S. The optical
and electrochemical properties of CdS/CdSe co-sensitized TiO2 solar
cells prepared by successive ionic layer adsorption and reaction
processes. Sol. Energy 2012,86, 964−971.
(33) Lee, Y.-L.; Lo, Y.-S. Highly Efficient Quantum-Dot-Sensitized
Solar Cell Based on Co-Sensitization of CdS/CdSe. Adv. Funct. Mater.
2009,19, 604−609.
(34) Yu, X.-Y.; Liao, J.-Y.; Qiu, K.-Q.; Kuang, D.-B.; Su, C.-Y.
Dynamic Study of Highly Efficient CdS/CdSe Quantum Dot-
Sensitized Solar Cells Fabricated by Electrodeposition. ACS Nano
2011,5, 9494−9500.
(35) Zhang, R.; Luo, Q.-P.; Chen, H.-Y.; Yu, X.-Y.; Kuang, D.-B.; Su,
C.-Y. CdS/CdSe Quantum Dot Shell Decorated Vertical ZnO
Nanowire Arrays by Spin-Coating-Based SILAR for Photoelectro-
chemical Cells and Quantum-Dot-Sensitized Solar Cells. ChemPhy-
sChem 2012,13, 1435−1439.
(36) Yan, K.; Zhang, L.; Qiu, J.; Qiu, Y.; Zhu, Z.; Wang, J.; Yang, S. A
quasi-quantum well sensitized solar cell with accelerated charge
separation and collection. J. Am. Chem. Soc. 2013,135, 9531−9539.
(37) Zhou, R.; Niu, H.; Zhang, Q.; Uchaker, E.; Guo, Z.; Wan, L.;
Miao, S.; Xu, J.; Cao, G. Influence of Deposition Strategies on CdSe
Quantum Dot-Sensitized Solar Cells: A Comparison between
Successive Ionic Layer Adsorption and Reaction and Chemical Bath
Deposition. J. Mater. Chem. A 2015,3, 12539−12549.
ACS Applied Materials & Interfaces Research Article
DOI: 10.1021/acsami.6b12842
ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
G

(38) Diguna, L. J.; Shen, Q.; Kobayashi, J.; Toyoda, T. High
efficiency of CdSe quantum-dot-sensitized TiO2 inverse opal solar
cells. Appl. Phys. Lett. 2007,91, 023116.
(39) Tian, J.; Gao, R.; Zhang, Q.; Zhang, S.; Li, Y.; Lan, J.; Qu, X.;
Cao, G. Enhanced Performance of CdS/CdSe Quantum Dot
Cosensitized Solar Cells via Homogeneous Distribution of Quantum
Dots in TiO2Film. J. Phys. Chem. C 2012,116, 18655−18662.
(40) Huang, F.; Hou, J.; Zhang, Q.; Wang, Y.; Masse, R. C.; Peng, S.;
Wang, H.; Liu, J.; Cao, G. Doubling the power conversion efficiency in
CdS/CdSe quantum dot sensitized solar cells with a ZnSe passivation
layer. Nano Energy 2016,26, 114−122.
(41) Huang, F.; Zhang, Q.; Xu, B.; Hou, J.; Wang, Y.; Masse, R. C.;
Peng, S.; Liu, J.; Cao, G. A comparison of ZnS and ZnSe passivation
layer on CdS/CdSe co-sensitized quantum dot solar cells. J. Mater.
Chem. A 2016,4, 14773−14780.
(42) Yang, Z.; Zhang, Q.; Xi, J.; Park, K.; Xu, X.; Liang, Z.; Cao, G.
CdS/CdSe Co-Sensitized Solar Cell Prepared by Jointly Using
Successive Ion Layer Absorption and Reaction Method and Chemical
Bath Deposition Process. Sci. Adv. Mater. 2012,4, 1013−1017.
(43) Toyoda, T.; Oshikane, K.; Li, D.; Luo, Y.; Meng, Q.; Shen, Q.
Photoacoustic and photoelectrochemical current spectra of combined
CdS/CdSe quantum dots adsorbed on nanostructured TiO2 electro-
des, together with photovoltaic characteristics. J. Appl. Phys. 2010,108,
114304.
(44) Verma, S.; Kaniyankandy, S.; Ghosh, H. N. Charge Separation
by Indirect Bandgap Transitions in CdS/ZnSe Type-II Core/Shell
Quantum Dots. J. Phys. Chem. C 2013,117, 10901−10908.
(45) Tsunekawa, S.; Fukuda, T.; Kasuya, A. Blue shift in ultraviolet
absorption spectra of monodisperse CeO2-x nanoparticles. J. Appl.
Phys. 2000,87, 1318−1321.
(46) Gao, R.; Wang, L.; Geng, Y.; Ma, B.; Zhu, Y.; Dong, H.; Qiu, Y.
Interface modification effects of 4-tertbutylpyridine interacting with
N3 molecules in quasi-solid dye-sensitized solar cells. Phys. Chem.
Chem. Phys. 2011,13, 10635−10640.
(47) Tian, J.; Zhang, Q.; Zhang, L.; Gao, R.; Shen, L.; Zhang, S.; Qu,
X.; Cao, G. ZnO/TiO2 nanocable structured photoelectrodes for
CdS/CdSe quantum dot co-sensitized solar cells. Nanoscale 2013,5,
936−943.
(48) Koide, N.; Islam, A.; Chiba, Y.; Han, L. Improvement of
efficiency of dye-sensitized solar cells based on analysis of equivalent
circuit. J. Photochem. Photobiol., A 2006,182, 296−305.
(49) Park, K.; Zhang, Q.; Garcia, B. B.; Cao, G. Effect of Annealing
Temperature on TiO2−ZnO Core−Shell Aggregate Photoelectrodes
of Dye-Sensitized Solar Cells. J. Phys. Chem. C 2011,115, 4927−4934.
(50) Kern, R.; Sastrawan, R.; Ferber, J.; Stangl, R.; Luther, J.
Modeling and interpretation of electrical impedance spectra of dye
solar cells operated under open-circuit conditions. Electrochim. Acta
2002,47, 4213−4225.
(51) Li, F. B.; Li, X. Z. The enhancement of photodegradation
efficiency using Pt-TiO2 catalyst. Chemosphere 2002,48, 1103−1111.
(52)Zheng,K.;Karki,K.;Z
ídek, K.; Pullerits, T. Ultrafast
photoinduced dynamics in quantum dot-based systems for light
harvesting. Nano Res. 2015,8, 2125−2142.
(53) Shen, T.; Tian, J.; Lv, L.; Fei, C.; Wang, Y.; Pullerits, T.; Cao, G.
Investigation of the role of Mn dopant in CdS quantum dot sensitized
solar cell. Electrochim. Acta 2016,191,62−69.
(54) Mora-Sero, I.; Gimenez, S.; Fabregat-Santiago, F.; Gomez, R.;
Shen, Q.; Toyoda, T.; Bisquert, J. Recombination in Quantum Dot
Sensitized Solar Cells. Acc. Chem. Res. 2009,42, 1848−1857.
(55) Tian, J.; Cao, G. Control of Nanostructures and Interfaces of
Metal Oxide Semiconductors for Quantum-Dots-Sensitized Solar
Cells. J. Phys. Chem. Lett. 2015,6, 1859−1869.
(56) Wang, J.; Mora-Sero, I.; Pan, Z.; Zhao, K.; Zhang, H.; Feng, Y.;
Yang, G.; Zhong, X.; Bisquert, J. Core/shell colloidal quantum dot
exciplex states for the development of highly efficient quantum-dot-
sensitized solar cells. J. Am. Chem. Soc. 2013,135, 15913−15922.
(57) Shin, S. S.; Kim, J. S.; Suk, J. H.; Lee, K. D.; Kim, D. W.; Park, J.
H.; Cho, I. S.; Hong, K. S.; Kim, J. Y. Improved Quantum Efficiency of
Highly Efficient Perovskite BaSnO3-Based Dye-Sensitized Solar Cells.
ACS Nano 2013,7, 1027−1035.
(58) Choi, Y.; Seol, M.; Kim, W.; Yong, K. Chemical Bath Deposition
of Stoichiometric CdSe Quantum Dots for Efficient Quantum-Dot-
Sensitized Solar Cell Application. J. Phys. Chem. C 2014,118, 5664−
5670.
(59) Wang, K.-C.; Jeng, J.-Y.; Shen, P.-S.; Chang, Y.-C.; Diau, E. W.-
G.; Tsai, C.-H.; Chao, T.-Y.; Hsu, H.-C.; Lin, P.-Y.; Chen, P.; Guo, T.-
F.; Wen, T.-C. p-type Mesoscopic Nickel Oxide/Organometallic
Perovskite Heterojunction Solar Cells. Sci. Rep. 2014,4, 4756.
(60) Bai, Y.; Han, C.; Chen, X.; Yu, H.; Zong, X.; Li, Z.; Wang, L.
Boosting the efficiency of quantum dot sensitized solar cells up to
7.11% through simultaneous engineering of photocathode and
photoanode. Nano Energy 2015,13, 609−619.
(61) Xu, Y.-F.; Wu, W.-Q.; Rao, H.-S.; Chen, H.-Y.; Kuang, D.-B.; Su,
C.-Y. CdS/CdSe co-sensitized TiO2 nanowire-coated hollow Spheres
exceeding 6% photovoltaic performance. Nano Energy 2015,11, 621−
630.
(62) Zhang, B.; Zheng, J.; Li, X.; Fang, Y.; Wang, L. W.; Lin, Y.; Pan,
F. Tuning band alignment by CdS layers using a SILAR method to
enhance TiO/CdS/CdSe quantum-dot solar-cell performance. Chem.
Commun. 2016,52, 5706−5709.
ACS Applied Materials & Interfaces Research Article
DOI: 10.1021/acsami.6b12842
ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
H