Fotoniikka - valaiseva iltapäivä seminaarin esitys 24.9.2013. Center for Functional Materials, Physics, Åbo Akademi University, Turku, Finland.

1. Plasmon Enhanced Solar Cells
Björn Törngren,1 Simon Sandén,2 Kenta Akitsu,3 Takaya Kubo,3
Hiroshi Segawa,3 Ronald Österbacka,2 Jan-Henrik Smått1
1 Center for Functional Materials, Physical Chemistry, Åbo Akademi University,Turku, Finland.
2 Center for Functional Materials, Physics, Åbo Akademi University,Turku, Finland.
3 Research Center for Advanced Science and Technology (RCAST),
The University of Tokyo,Tokyo, Japan
Fotoniikka – valaiseva iltapäivä – Turku 24.09.2013

2. Joint project with RCAST
 Plasmon Enhanced Organic Hybrid Solar Cells (POHSC) – 3 yr
 Physical Chemistry (Smått) and Physics (Österbacka)
 Research Center for Advanced Science and Technology (RCAST),The
University of Tokyo, Japan (Prof. Segawa)
 Several research visits between Finland and Japan

3. POHSC – general outline
PhysChem (ÅAU):
Synthesis and
modification of core-
shell nanoparticles
RCAST (UT):
Development of novel
dyes (NIR) –
porphyrin-based
Solar cell applications:
Assembly into DSSC and organic SC
Physics (ÅAU):
Modeling of
plasmonic core-shell
particles
Plasmon enhanced NPs
NPs with
tunable
absorption
properties 300 400 500 600 700 800 900
Absorbance(a.u.)
Wavelength (nm)

4.  Surface plasmons (SPs) are coherent oscillations of conduction
electrons on a metal surface excited by electromagnetic radiation
at a metal-dielectric interface
 Resonance at a specific wavelength (Au NPs: ~530 nm)
Willets, K.A.;Van Duyne, R. P. Ann. Rev. Phys. Chem. 2006, 58, 267.
The surface plasmon effect

5. Elongated nanoparticles (nanorods)
 Elongated NPs show two surface plasmon peaks
 By controlling the aspect ratio of gold nanorods, the
position of the longitudinal peak can be tuned
Murphy, et al. J. Phys. Chem. B 2005, 109, 13857.
Transverse mode (T)
Longitudinal mode (L)
T
L

6. Synthesis of Au nanoparticles/rods
 Au NPs with varying size
by controlling the amount
and strength of the reducing
agent (Ø 15–30 nm)
 Seed-mediated growth can
be used to synthesize
elongated Au nanorods with
varying aspect ratios
 Shifting the surface plasmon
resonance from 520 nm up
to 1000 nm
Smith,D; and Korgel, B; Langmuir 2008, 24, 644.

7. 10 nm
Glass
ITO
P3HT
PCBM
Gold
P3HT PCBM
Organic or hybrid solar cellsDye-sensitized solar cells
Surface plasmons in solar cells

8. Current topics of the project
 Stability issues of core/shell nanoparticles in DSSCs
 Plasmon-enhanced polymer-sensitized solar cells
(PSSC)
 Charge generation and charge transport studies
 Gold nanorods to modify the light absorption range
 Plasmonic particles in organic bulk hetero-junction
solar cells

9. Core-shell Au@SiO2 nanoparticles
Why is a thin silica shell needed?
1) It acts as an insulator to avoid charge
recombination within the metal
2) It adjusts the plasmon-dye separation
distance to minimize quenching
3) It protects the metal core from
corrosion when the particles are
subjected to electrolyte solution
4) It gives a better thermal stability that
prevents sintering of the gold cores.
Sheehan et al. J. Phys. Chem. C, 2013, 117, 927−934

10. Silica coating approaches
Turkevich approach:
 ~15 nm gold cores
Na2SiO3

11. Coating thickness and completeness
APS APS +
Stöber
MPTMS Direct Stöber
Gold core diameter: 16 nm

12. Modeling of plasmonic core-shell particles
 Another aspect of the project is to model the plasmonic
behavior of these materials using Mie theory
 Optimal particle structure can be found
λ = 550 nm Qext
SiO2shellthickness(nm)
Au core radius (nm)
X nm
Water
n = 1.333

13. Modeling of silica thickness (Mie theory)
APS: 0.5 nm APS + Stöber: 5 nm MPTMS: 1.3 nm Direct Stöber: ~20 nm

14. Chemical stability in iodide electrolyte
Coating methods:
A: APS B: APS + Stöber
C: Direct Stöber D: MPTMS

15. Temperature stability of Au@SiO2 NPs
Au@SiO2 NPs (MPTMS method)
RT 500 ºC
 Heating required for sintering of TiO2 NPs in a DSSC could
potentially damage the silica shell  No detectable difference!

16. Assembly of (plasmon-enhanced) DSSC
TiO2 nanoparticle paste (w/ or w/o Au@SiO2 NPs)
screen-printed on the substrate, and sintered at 500 °C
Electrolyte injectionPlacing two electrodes
FTO/Glass
substrate
TiO2 nanoporous layer
Pt coated-FTO
TheTiO2 substrate was immersed in the
solution overnight
Solar cell structure
Dye solution
Photoanode
Spacer
Injection hole
Electrolyte
(30 µm)
Pt-coated FTO
Photoanode
Solar cell fabrication
(I3
–/I– redox couple
in acetonitrile)
Slide courtesy of Kenta Akitsu

17. Plasmon-enhanced DSSC performance
Sample Au@SiO2 Thickness
[µm]
VOC
[V]
JSC
[mA/cm2]
FF PCE
[%]
A1 1 wt% 1.726 0.85 4.0 0.68 2.3
A2 1 wt% 1.899 0.85 3.9 0.68 2.3
T1 – 1.844 0.85 3.7 0.68 2.2
T2 – 1.868 0.85 3.4 0.69 2.0
B.Törngren et al. J. Colloid Interface Sci., submitted

18. Polymer-sensitized solar cell (PSSC)
 Polythiophene derivative polymers where
COOH units are attached to the polymer
backbone to facilitate sensitization to TiO2
 Uses a low molecular weight polymer as
dye (here: PT-C 85, i.e. HR 85%)
Electrolyte
solution
Pt/FTO
TiO2/Polymer FTO
R=H, Me
PT derivative with
carboxylic acid groups
MW: 1100, 2700
Hydrolysis ratio (H:Me ratio)
can be varied 0%-95%
Akitsu et al. Jpn. J.Appl. Phys. 51 (2012) 10NE04

19. Plasmon-enhanced PSSCs
 TiO2 paste incorporating 1 wt% Au@SiO2 NPs was manufactured
 Photoanode was made by screen-printing on FTO
 PT-C 85 was adsorbed onto the TiO2 layer overnight
Akitsu et al., in preparation.

20. I-V and IPCE of Plasmon Enhanced PSSCs
7
6
5
4
3
2
1
0
Currentdensiy,mA/cm
2
0.60.50.40.30.20.10
Voltage, V
70
60
50
40
30
20
10
0
IPCE,%
700600500400300
Voltage, V
3 layers
2 layers
1 layer
Sample Voc Jsc FF 
Au 1 0.52 2.2 0.55 0.6
Ref 1 0.52 1.1 0.52 0.3
Au 2 0.53 4.6 0.55 1.3
Ref 2 0.50 3.7 0.55 1.0
Au 3 0.53 5.9 0.51 1.6
Ref 3 0.50 5.0 0.54 1.4
Dashed: with
Au@SiO2 NPs
Solid: without
Au@SiO2 NPs

21. IPCE enhancement factor: IPCE(Au)/IPCE(Ref)
 IPCE enhancement over 400-650 nm
 Peak slightly red-shifted compared to absorption of Au@SiO2 NPs
in EtOH  larger dielectric constant of TiO2
3.0
2.5
2.0
1.5
1.0
0.5
Au/Ref.,-
650600550500450400350300
Wavelength, nm
1.4
1.2
1.0
0.8
0.6
Au/Ref.,- 650600550500450400350300
Wavelength, nm
0.14
0.12
0.10
0.08
0.06
0.04
0.02
0.00
Absorbance,-
Au@SiO2 NPs in EtOH

22. Outlook of other possibilities
Au nanorods to enhance
absorption in NIR region
Plasmonic particles in organic
BHJ solar cells
0
0.2
0.4
0.6
0.8
1
1.2
1.4
300 400 500 600 700 800 900
Absorbance(a.u.)
Wavelength (nm)
Gold nanoparticles
Gold nanorods
Longitudinal
mode
Aspect ratio ~3
Transverse
mode
10 nm
Glass
ITO
P3HT
PCBM
Gold
Au NPs

23. Summary
 Plasmonic particles (e.g. gold nanoparticles) have proven
useful for improving the efficiency of photovoltaic devices
 A thin but complete silica shell is needed to protect the Au
NPs in the harsh electrolyte solutions used in DSSCs
 Plasmon-enhanced polymer-senisitized solar cells (PSSCs)
have been manufactured using novel polythiophene
derivative polymers
 In both DSSCs and PSSCs the efficiency can be improved by
incorporating Au@SiO2 nanoparticles