Photovoltaics and Photoexcited Carrier Dynamics of Double-Layered CdS_CdSe Quantum Dot-Sensitized...

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

DAVID PUBLISHING

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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



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)

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lectrolyte w

M S). The I

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with the s

easurements[

uated under

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2 h.

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and

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ame

33].

one

olar

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an

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set

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Th

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, the open ci

the photovol

Improved-Tr

ig. 1 shows

p of the imp

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ple is broug

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two diffrac

the diffracti

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eneratively

A-1000: Cl

elength of

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elength of

orption in

1 Schema

roved-TG tec

yered CdS/C

luate the s

rcuit voltage

taic conversi

ansient Grat

he schemati

oved-TG sy

ump and pro

dichroic mi

ion grating.

profile has

de of the tra

t near the tr

by the opticdiffracted b

e grating in

tions progre

on intensity i

the improve

amplified

ark-MXR I

75 nm, repe

50 fs. The

. The pump

ave optical

nce (TOP

520 nm s

dS/CdSe Q

tic diagram

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ort circuitcu

(Voc), the fil

on efficiency

ing Techniqu

diagram of

stem. In the i

be laser pul

rror, before b

For the pu

an interfer

smission gr

nsmission gr

l interferencoth by the

ducedon thes

ss in the sa

s detected [2

d-TG experi

titanium/sap

c.) with a

tition rate o

robe pulse (

pulse was ge

arametric a

S) system

uitable for

Ds. The inte

of experime

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

phire lase

fundamental

1 kHz, an

75 nm) was

erated using

plifier of

and set

the optical

nsity of the

tal setup o

,



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

h for the Cd

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

e of IO-Ti

on). Highly

honeycomb

of the film

cross section

onthe wall

that the siz

th adsorpti

electron mi

O2; white li

timal thickn

performanc

it). The effe

has been ca

experiment conversion

rption time

Se QDs.

ows the PA

d those ads

h), and dou

together wi

lueshift was

and Photo

om 2 to

a commerci

s of the bot

mm. The sa

uring TG ex

that carri

the TG tec

rocesses (h

ping) under

mp intensity

nce of the

sity was lin

es overlappe

e normalize

sion

anning elec

2 electrode

porous nan

structure wa

ere measur

al SEM im

of honeyco

e and amou

n time. Fi

croscopy (T

e part: CdS

ess of the

was less th

ct of CBD ti

refully studi

s showed t efficiency

f 0.5 h for th

spectra of t

rbed with

le-layered

h bulk CdS

exhibited ag

xcited CarriDot-Sen

μJ/pulse.

l product wit

h the pump

ples showe

periments. It

rs depopul

nique, shoul

le trapping

our experim

[27]. It was

aximum si

ear and that

d with each

to the TG

tron micros

with CdSe

structure of

s confirmed.

ed as ~3 μ

ges. CdSe

b structure,

nt of CdSe

. 3 shows

M) image (b

QDs). It sh

nS coating

n 1 nm (less

me on the de

d. The resul

hat the higcan beachi

e CdS QDs a

e bare IO-

dS QDs (0.

dS (0.5 h)/

e. With the

ainst bulk C

r DynamicssitizedSolar

The

h 40

and

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has

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d be

and

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also

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the

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the

The

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Ds

and

Ds

the

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ows

for

than

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s of

hestved

nd 6

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h),

dSe

Ds

Se,

Fig.

wit

Fig.

(8h

Fig.

Cd

tog

whi

gro

Cd

of Double-LaCells

2 SEM im

CdSe QDs (8

3 TEM im

adsorption, bl

4 PA spect

QDs (0.5 h),

ther with tho

ch is indicati

wth of QDs.

e QDs in C

yered CdS/C

ge (top view

h adsorption).

ge of IO-Ti

ack line: TiO2

a of the IO-T

dSe QDs (6 h

e of bare IO-

ve of the qua

The averag

S (0.5h)/Cd

dSe Quantu

) of the IO-

2 electrode w

; white line: C

O2 electrodes

), and CdS (0.

iO2 and bulk

ntum confine

e diameters

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

,




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.




607

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