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8/12/2019 Photovoltaics and Photoexcited Carrier Dynamics of Double-Layered CdS_CdSe Quantum Dot-Sensitized Solar Cells
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Journal of Materials Science and Engineering A 3 (9) (2013) 601-608
Photovoltaics and Photoexcited Carrier Dynamics of
Double-Layered CdS/CdSe Quantum Dot-Sensitized
Solar Cells
Taro Toyoda1, 2, Yohei Onishi1, Kenji Katayama3, Tsuguo Sawada4, Shuzi Hayase2, 5 and Qing Shen1, 2
1. Department of Engineering Science, Faculty of Informatics and Engineering, The University of Electro-Communications, Chofu,
Tokyo 182-8585, Japan
2. CREST, Japan Science and Technology Agency (JST), Kawaguchi, Saitama 332-012, Japan
3. Department of Applied Chemistry, Faculty of Science and Engineering, Chuo University, Bunkyo-ku, Tokyo 112-8551, Japan
4. Japan Science and Technology Agency (JST), Kawaguchi, Saitama 332-012, Japan
5. Graduate School of Life Science and Systems Engineering, Kyushu Institute of Technology, Kitakyushu 808-0196, Japan
Received: July 03, 2013 / Accepted: August 01, 2013 / Published: September 10, 2013.
Abstract: The photovoltaics of double-layered CdS/CdSe quantum dot (QD)-sensitizedsolar cells (QDSCs) were investigated.
TheCdS/CdSe quantum dot was adsorbed on inverse opal (IO)-TiO2. IO-TiO2 electrodehas a honeycomb structure with
largeinterconnected pores that lead to better infiltration despite thesmaller surface area than nanoparticulate electrode.Animprovement
in the photovoltaic conversion efficiency (~ 3.8%) wasachieved compared with single-layered CdSe-QDSCs (~ 2.9%).We
investigated the ultrafast photoexcited carrier dynamics (the electron and hole relaxation processes) ofCdS/CdSe-QDSCs
byimproved-transient grating (TG) technique.TG technique basically depends on the refractive index changes duetophotoexcited
carriers. The ultrafast carrier dynamics of CdS/CdSe- and CdSe-QDSCsshow fast (hole) and slow (electron) relaxation processeswithdecay times of a fewpicoseconds and a few tens of picoseconds, respectively. The electron relaxation timewas shorter in
theCdS/CdSe-QDSCs than in theCdSe-QDSCs, indicating a reduction in the numbers ofrecombination centers due to the
pre-adsorbed CdS QDs layer.
Key words: Semiconductor quantum dot, sensitized solar cell, photovoltaic property, carrier dynamics, transient grating technique.
1. Introduction
Recently, semiconductor quantum dot (QD)-
sensitized solar cells (QDSCs) have been attracting a
lot of interest [1-5]. These cells are considered to be promising alternatives to dye-sensitized solar cells
(DSCs) due to their tunable energy gap by changing
their size [6], higher extinction coefficient due to
quantum confinement effect [1], greater intrinsic
Corresponding authors: Taro Toyoda, D.Sc., professor,research fields: applied physics, materials science. E-mail:[email protected]. Qing Shen, Ph.D., associate professor,research field: applied physics and laser spectroscopy. E-mail:[email protected].
dipole moment [7], and the possibility of multiple
excitongeneration and/or nonthermalised charge
carrier [8-10]. However, the photovoltaic conversion
efficiencies of QDSCs lag behind those of DSCs. The
crucial factor is their relatively low electron
injection [11]. The poor performance of QDSCs is
ascribed to the difficulty of assembling QDs into
mesoporous TiO2 electrodes, indicating the importance
of the electrode morphology on achieving reliable
assembly [5, 12]. In sensitized solar cells, the
working-electrode should have a high surface area to
increase the amount of sensitizer loading in order to
enhance light harvesting. However, the recombination
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Photovoltaics and Photoexcited Carrier Dynamics of Double-Layered CdS/CdSe QuantumDot-Sensit izedSolar Cells
602
process is proportional to the surface area. The surface
area in QDSCs may not need to be increased by as
much as in DSCs, because of their higher extinction
coefficient.A balance between recombination and light
harvesting is needed to maximize the sensitized solar
cell performance [13, 14]. To address the penetration
of both sensitizers and redox couples, an approach
using inverse opal (IO)-TiO2 electrode has been
proposed [15, 16]. IO-TiO2 electrodes have a
honeycomb structure with large interconnected pores
that lead to better infiltration despite the smaller
surface area compared to nanoparticulate electrodes [5],
together with a photonic band gap which has a
possibility to enhance light-harvesting efficiency [17].In addition to modification of the electrode
morphology, the use of QDs requires the development
of new strategies. One of the current key issues to
pursue higher photovoltaic conversion efficiency is to
search for suitable nanocomposite sensitizers to
enhance the harvest of solar light [1]. Here, we
describe the performance of double-layered
sensitizers in QDSCs based on a CdSe QD sensitizer
with pre-adsorbed CdS QDs (termed double-layered
CdS/CdSe QDs) [18]. The pre-adsorbed CdS layer
deposited prior to the adsorption of the CdSe
improved the QDSC performance [18]. Recent
developments in double-layered CdS/CdSe-QDSCs
have shown high electron injection efficiency and fast
electron transfer [19-24]. Hence, a detailed
investigation into the function of double-layered
CdS/CdSe QD sensitizers on the performance of
QDSCs is of great interest and would benefit the
research in the QDSC area.In this study,
double-layered CdS/CdSe QDs coupled with IO-TiO2
electrodes were used as working electrodes for QDSC.
The studies on photoexcited carrier dynamics are
important not only for basic science but also for
improving the photovoltaic properties. To date, there
is a few information on the ultrafast carrier dynamics
of double-layered semiconductor QD sensitizers on
TiO2 electrodes. Here, we have investigated the
ultrafast photoexcited carrier dynamics of CdS/CdSe-
QDSCs by the improved-transient grating (TG)
technique [25]together with optical absorption and
photovoltaic characteristics. In general, TG method is
a time-resolved optical technique by which various
kinds of dynamics can be measured [25]. Although the
TG measurement provides valuable information, it is
not widely used because of the complicated apparatus
requirements. The improved-TG technique features:
(1) very simple optical alignment because no lenses
are used to focus beams on the samples; (2) easy
control of the phase difference between the prove and
references; (3) high stability of phase due to the short
optical path length of the probe reference beams; (4)high sensitivity; and (5) evaluation of both electron
and hole relaxation processes, under low pump light
intensity [25-28]. It basically depends on the refractive
index changes due to photoexcited carriers (electron
and hole) [5, 9, 26-28].
2. Experiments
2.1 Sample Preparation
IO-TiO2 electrodes were prepared on FTO-coatedglass by the replication of self-organizing materials [15].
The synthetic opal samples were assembled by
immersing the FTO substrate vertically in a
monodispersive polystyrene (474 nm in diameter)
suspension and heating the solvent in an oven at 40 °C
until it had completely evaporated, leaving behind a
colloidal crystal film on the substrate. A 10 μL drop of
2% TiCl4 in methanol was added onto the colloidal
surface. After hydrolysis for 30 min, the sample was
subsequently heated to 80°C in air. This process was
repeated several times to ensure the filling of all voids.
The sample was subsequently heated at 450 °C for 1 h
to calcinate the template and anneal the TiO2 [29, 30].
CdS and CdSe QDs were adsorbed onto the IO-TiO2
electrodes using a chemical bath deposition
(CBD)method [19, 29-32]. The TiO2 electrodes were
immersed in a container filled with the final solution.
First, adsorption of CdS QDs on the IO-TiO2 electrode
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was carried
Then, CdSe
from 0.5 t
coated with
and reaction
passivate th
of the QDs [
a potential
between th
indicating t
into the elec
2.2 Optical
The optigas-microp
which is a p
has proved
semi-transp
nonradiativ
300 W xen
PA spectru
wavelength
frequency o
intensity is
coefficient
setting. The
the microph
into a lock-i
by the calib
black sheet
The inc
efficiency(I
the photovosandwich st
(active are
polysulfide
value was
measured
conditions a
The photov
sun illumin
hotovoltaic
out at 10°C
QDs were
12 h. Sub
ZnS by suc
(SILAR) m
surface in o
33-35]. Oute
barrier due
interface
at the leaka
trolyte can b
bsorption
al absorptioone photoa
hotothermal
to be usef
rent or op
deexcitatio
n lamp was
measurem
range of 30
33 Hz at roo
proportion
[19] in the
PA signal
one output th
n amplifier.
ration to the
hat was prop
ident phot
CE: i.e., ext
ltaic measur ucture cell
: 0.25 cm2)
solution (1
found from
y a zero
s those used
oltaic proper
tion (AM 1.
and Photo
in the dark
repared at
sequently, t
essive ionic
thod for 3 ti
der to preve
r ZnS can be
o large ban
f QDs and
e of electro
e inhibited.
Photocurre
n spectra wcoustic (P
detection me
l for optic
que sample
processes
used as the
ents were c
0-800 nm w
m temperatu
l to the o
adjustments
as monitore
rough a prea
he PA spec
PA signal in
ortional to t
n to cur
ernal quantu
ements wereith a Cu2S
[37].The e
Na2S + 1
the short-ci
shuntmeter
for the PA m
ies wereeva
: 100 mW/c
xcited CarriDot-Sen
rom 0.5 to 1
0°C in the
e samples
-layer adsor
mes for 1 mi
t photocorro
considered t
gap (~3.8
the electro
ns from the
t Measurem
re measure) spectrosc
thod. PA me
al absorptio
s by measu
[19, 33, 36
light source.
rried out in
ith a modul
re. The PA si
tical absor
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by first pas
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ra were obta
tensity of ca
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ent conver
efficiency)
performedcounter-elect
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M S). The I
rcuitphotocu
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easurements[
uated under
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]. A
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then
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and
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rent
ame
33].
one
olar
sim
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an
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on
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it c pro
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Th
an
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Fig.
imp
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Improved-Tr
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luate the s
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profile has
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the improve
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75 nm, repe
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. The pump
ave optical
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dS/CdSe Q
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nsmission gr
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robe pulse (
pulse was ge
arametric a
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uitable for
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603
rrent density
factor (FF),
(η).
e
experimental
mproved-TG
e beams are
eing incident
p beam, the
ence pattern
ting. When
ating surface
pattern. Thetransmission
ample (TG).
e direction,
5]. The laser
ents was
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75 nm) was
erated using
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and set
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,
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604
pump bea
transmissio
lines/mm.
probe laser
apparent ph
been previ
mechanism,
ascribed to
electron inj
condition o
found that
intensity on
waveforms
very well wsignal inten
3. Results
Fig. 2 s
(SEM) ima
(8h adsorpt
film on the
thicknesses
examining
are adsorbe
we can fin
increase wi
transmissio
line part: Ti
that the op
photovoltai
detection li
performanc
consecutive photovoltai
with an ads
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Fig. 4 sh
electrode a
CdSe QD (
(6 h), QDs
adsorption,
hotovoltaic
ranged fr
grating was
he diameter
eams were 5
otodamage d
usly show
monitored b
one body
ction or trap
very low pu
the depende
pump inte
f the respon
hen they weity.
and Discu
ows the sc
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of the film
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that the siz
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ows the PA
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that carri
the TG tec
rocesses (h
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mp intensity
nce of the
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es overlappe
e normalize
sion
anning elec
2 electrode
porous nan
structure wa
ere measur
al SEM im
of honeyco
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n time. Fi
croscopy (T
e part: CdS
ess of the
was less th
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refully studi
s showed t efficiency
f 0.5 h for th
spectra of t
rbed with
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h bulk CdS
exhibited ag
xcited CarriDot-Sen
μJ/pulse.
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The averag
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2 electrode w
; white line: C
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), and CdS (0.
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ntum confine
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Se (6 h) QDs
iO2 electrod
th CdSe QDs
dSe QDs).
adsorbed with
h)/CdSe (6 h)
CdSe.
ment and the
of CdS an
were 4.4 nm
,
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Photovoltaics and Photoexcited Carrier Dynamics of Double-Layered CdS/CdSe QuantumDot-Sensit izedSolar Cells
606
and0.44 m0(m0: electron rest mass), respectively [39],
both the photoexcited electron and hole carrier
densities in the CdSe QDs contribute to S(t). Hence, we
can characterize the electron and hole relaxation
processes simultaneously. S(t) can be fitted with a two
exponential relaxation processes and an offset S0, as
shown in Eq. (1) using a least-squares fit and
convoluting with a 1 ps Gaussian distribution
representing the laserpulse,
S (t ) = A1exp (-
1
) + A2exp (
2
) + S 0 (1)
where, A1, A2 and S0 are constants, and τ 1 (fast) and τ 2
(slow) are the time constants of the
tworelaxationprocesses. The constant term S0
corresponds to the longer relaxation process of the orderof ns. τ1 and τ2correspond to the decrease in the
photoexcited hole carrier density (trapping by the CdSe
QD surface states) and the photoexcited electron carrier
density (trapping and transfer to the TiO2 conduction
band), respectively [25-27, 40-46]. τ1 and τ2 are 9.0 ps
and 68 ps for CdS/CdSe QDs and 8.0 ps and 98 ps for
CdSe QDs. τ1 is similar in the CdS/CdSe QDSC and the
CdSe QDSC, indicating the smooth transfer of holes to
the surfaces of the QDs. τ2 in the CdS/CdSe-QDSCs isfaster than in the CdSe-QDSCs, indicating reduced
numbers of recombination centers and interface states
due to the pre-adsorbed CdS QDs [20]. The electron
rate constants derived from the TG measurements are
roughly estimated to be 1.5 × 1010 s-1 for CdS/CdSe
QDs and 1.0 × 1010 s-1 for CdSe QDs, indicating that
the electron-injection constant of CdS/CdSe QDs is
50% larger than that of CdSe QDs. Also, S0 is smaller
in CdS/CdSe QDs than in CdSe QDs, indicating that
there are a fewer recombination centers in CdS/CdSe
QDs. The high-resolution TEM measurements by Chenget
al. showed that the {100} planes of CdS (lattice
spacing: 0.359 nm) were stacked on the {101} planesof
the TiO2 (lattice spacing: 0.351 nm) at a certain angle,
while the {101} planes of CdSe (lattice spacing: 0.329
nm) were connected to the {100} planes of CdS [21].
As a result, it indicates the reduce of interfacial lattice
misfit, leading to a less-defectiveinterface between the
interfaces [21]. These results agree with our
photovoltaic and TG characteristics, confirming that
the pre-adsorbed CdS QD layer enables a smoother
transfer of photoexcited electrons from the CdSe QDs
to the TiO2 electrode. In addition, it suggests the
possibility of a large distribution of the electron
wavefunction in the CdS/CdSe QDs [11]. It also
shows the possibility that the pre-adsorbed CdS QD
layer enables a smoother transfer of photoexcited
electrons from the CdSe QDs to the IO-TiO2
electrode.
4. Conclusions
In conclusion, we have demonstrated improvements
in the photovoltaic performance of QDSCs with
IO-TiO2electrodes by using double-layered CdS/CdSe
QDs sensitizer (maximum IPCE = 72%, η = 3.8%) in
place of single-layered CdSe QDs sensitizer
(maximum IPCE = 56%, η = 2.9%). A 30% increase in
η was achieved with the application of double-layered
CdS/CdSe QDs. Measurements of the ultrafast
photoexcited carrier dynamics by the improved-TG
technique showed a faster electron transfer rate
constant in CdS/CdSe QDs than CdSe QDs, indicating
reduced numbers of recombination centers and
interface states due to less defective interfaces
between both TiO2/CdS and CdS/CdSe. Inaddition, this
suggests that the energy levels in the CdS and CdSe
QDs are aligned [20] and that there is a large
distribution of the electron wavefunction in the ground
state in CdS/CdSe QDs [11].
Acknowledgments
This research was supported by the
StrategicJapan-Spain Cooperation Program of the
Japan Science and Technology Agency (JST). Part of
this work was supported by PRESTO (JST), CREST
(JST), and a Grant-in Aid for Scientific Research (No.
21310073) from the Ministry of Education, Sports,
Science and Technology of the Japanese Government.
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Photovoltaics and Photoexcited Carrier Dynamics of Double-Layered CdS/CdSe QuantumDot-Sensit izedSolar Cells
607
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8/12/2019 Photovoltaics and Photoexcited Carrier Dynamics of Double-Layered CdS_CdSe Quantum Dot-Sensitized Solar Cells
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Photovoltaics and Photoexcited Carrier Dynamics of Double-Layered CdS/CdSe QuantumDot-Sensit izedSolar Cells
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