1
The 4th US-Korea NanoForum
Material and Structural Design
for High Efficiency Dye-Sensitized Solar Cell
HO C O
O
O
C
N
N
Ru
N
O
NCS
NCS
N
C
O
HO
C O
Hyungjun Koo, Beomjin Yoo, Kicheon Yoo, Kyungkon Kim and Nam-Gyu Park*
Center for Energy Materials
Korea Institute of Science and Technology
(KIST)
2
DSSC: structure and operation principle
TiO2
TCO
τinj
CB
HO C O
C
N
N
Ru
N
O
D*/D+
τr
NCS
O
HO
C O
I3¯/I¯
D/D+
NCS
N
C
Voc
surface
states
O
O
τcc
VB
TiO2 Dye
3
issues
* efficiency as high as 11% has been achieved.
* higher efficiency is required in order to be in competition with Si solar cell
Approaches for improving efficiency
Efficiency (η) = (JscⅹVocⅹFF)/Pin
13%
η (%)
Jsc
Voc
FF
Pin
cf.
10 →
13
18 → 23.4
0.75
0.74
100
Case1: JSC increased by
30%
Efficiency (η) = (JscⅹVocⅹFF)/Pin
13%
η (%)
Jsc
Voc
FF
Pin
cf.
10 →
13
18
0.75 → 0.98
0.74
100
Case2: VOC increased by
30%
4
strategy for improving JSC
structural and material
design
light confinement by scattering:
size-dependent scattering efficiency &
bi-functional material
5
nanoparticle films for DSSC
t = 10-12 µm
9.0
9.0
8.5
8.5
Efficiency (%)
Efficiency (%)
t = 5-6 µm
8.0
7.5
7.0
180 190 200 210 220 230 240
o
Autoclave Temp. ( C)
8.0
7.5
7.0
210
220
230
240
o
Autoclave Temp. ( C)
Simple increase of nanoparticle film may not improve the efficiency to great extent!
6
size-dependent scattering efficiency
(a) G1
(b) G2
20
25
30
35
R(220)
R(211)
R(210)
R(200)
R(111)
R(101)
A
*
(a)
A
*
(b)
*
Intensity (a.u.)
R(110)
SL
40
45
2θ (degree)
50
55
60
7
PV properties
Jsc
Increasing
rate
(mA/cm2)
Voc (mV)
FF (%)
Eff. (%)
1L (~7 µm)
12.2 ± 0.1
868 ± 2
71.1 ± 0.1
7.55 ± 0.12
1L+G1
14.9 ± 0.0
852 ± 2
70.4 ± 0.3
8.94 ± 0.08
+18.4%
1L+G2
14.4 ± 0.1
866 ± 1
70.7 ± 0.4
8.78 ± 0.09
+16.3%
80
IPCE
60
40
20
0
400
1L
1 L+G1
1 L+G2
500
600
Wavelength
700
800
8
reflectance spectra
Reflectance (%)
100
80
60
nano-TiO2 film
40
G1 film
G2 film
20
0
400
500
600
700
800
Wavelength (nm)
scattering
overlayer
nano-TiO2
thin film
underlayer
9
bi-functional material
* spherical or flat surface large particles are normally used for light scattering
* except light scattering, such materials
do not contribute to photocurrent
generation due to
low surface area
* motivation: design of bi-functional
material exhibiting
both light scattering and
photocurrent generation
efficiently
Nano-Embossing
Hollow Sphere TiO2
(prepared by Prof. W.-I. Lee)
300 nm
10
IPCE & reflectance comparison
80
NeHS
40
20
0
400
1L
2L
1L-NeHS
1L-CCIC
100
NeHS TiO2 Film
80
500
600
700
Wavelength (nm)
(b)
CCIC TiO2 Particle Film
nanocrystalline TiO2 Film
800
%R
IPCE (%)
60
60
40
20
0
400
500
600
700
Wavelength (nm)
800
900
11
2
Photocurrent density (mA/cm )
conversion efficiency
without long-term post-treatment of TiCl4
20
15
10
5
0
0.0
2
JSC : 17.8 mA/cm
VOC : 0.845 V
FF : 0.69
η : 10.34%
0.2
0.4
0.6
Voltage (V)
0.8
12
interfacial nano-engineering
13
locus of recombination
bulk
recombination
near substrate
recombination
14
methods for suppression of recombination
Mesoporous electrode
TiO2
FTO
core-shell
(to protect bulk recombination)
FTO
TiO2
blocking layer
(to protect recombination
at near substrate)
15
FTO/BL/TiO2: SEM
Chemically formed from Ti precursor solution
Bare
FTO
TiO2
300nm
Blocking layer
0.15 M
0.4 M
300nm
16
8.4
13.2
8.2
13.0
12.8
12.6
0.88
VOC (V)
η (%)
13.4
8.0
7.8
7.6
0.0
0.2
0.4
0.6
0.8
0.76
0.86
0.0
0.2
0.4
0.6
0.8
0.0
0.2
0.4
0.6
0.8
0.74
0.84
0.72
0.82
0.80
measured
under mask with aperture
FF
2
JSC (mA/cm )
effect of Ti-precursor concentration on PV property
0.0
0.2
0.4
0.6
0.8
0.70
Ti precursor concentration (M)
17
Bode
-10
0 -2
-1
10
10
0
10
1
10
2
10
3
10
Hz Frequency, Hz
5
10
10
6
Cole-Cole
0
Hz
0.8M
0.4M
0.2M
0.15M
0.1M
0.05M
bare
B A
C
0
10
1
10
10
2
10
3
4
10
Frequency, Hz
Inverse frequency (1/Hz)
A: pt/electrolyte; B: FTO/electrolyte; C: TiO2/dye/electrolyte
10
10
-3
A
10
-4
10
-5
0.0
0.5
1.0
Precursor concentration (M)
-2
B
10
-3
10
-4
0.0
0.5
1.0
Precursor concentration (M)
Inverse frequency (1/Hz)
-5
4
10
Inverse frequency (1/Hz)
Theta
-20
2nd derivative theta (arbt. unit)
EIS (bode plot)
10
-1
10
-2
C
0.0
0.5
1.0
Precursor concentration (M)
18
Surface modification of bulk TiO2 film:
fast electron transport, slow recombination
photovoltage transient
Time (S)
10
-1
10
-2
10
-3
bare TiO2
Photocurrent
Photovoltage
TiO2 treated with TiCl4(9hr)
Photocurrent
Photovoltage
Transient photocurrent
τTiCl4 treatment < τTiO2
⇒ Enhanced electron transport
by TiCl4 treatment
photocurrent transient
10
-2
10
-1
10
Transient photovoltage
τTiCl4 treatment > τTiO2
0
2
Photocurrent Density(mA/cm )
⇒ Reduced recombination from
TiO2 to electrolyte by TiCl4
treatment
19
DSSC best efficiency
Photocurremt density ( mA/cm2)
22
20
18
16
2
14
Jsc = 19.38 mA/cm
12
Voc = 0.877 V
10
Eff. = 11%
8
6
4
2
0
0.0
FF = 0.647
@ AM 1.5G-1 sun
Reference cell:
NREL-calibrated mono Si
with KG-5 filter
0.2
0.4
0.6
0.8
1.0
Voltage (V)
close to the world best efficiency of 11.1% from EPFL
20
KIST Activity: Organic Photovoltaics
Max. Eff. of 6.1% was obtained from D2
P3HT
PCBM
Thickness: D1<D2<D3
Kim et. al., APL 90, 163511 (2007)
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Acknowledgments
SY. Bang, MS
HJ. Koo
NJ. Myung
KC. Yoo
KT. Lee, staff
BJ. Yoo, Ph.D.
KW. Lee, MS
Dr. K Kim,
Senior Scientist
MA. Lee, MS
TR. Lee
KIST Specialists Laboratory for Dye-Sensitized & Organic Solar Cells
$$$: KIST, MOST and MOCIE