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Lund University Chemical Physics Department
Villy Sundström
Dye sensitized Perovskite e-
Catalyst
Charge Carrier Dynamics of Nanostructured Solar Cell Materials Studied by Time-
Resolved THz Spectroscopy
10 5
10 6
10 7
10 8
10 9
10 10
10 13
10 14
10 15
10 16
10 17
10 18
10 19
Frequency (Hz)
Region Radio & TV Microwaves Infrared UV RTG
10 20
THz
10 1 2
Visible
11 10
Solar energy – no CO2 FUEL - Hydrogen FUEL
HEAT - solar cooker
Solar tower
Heliostats
HEAT
Photosynthesis - biomass ELECTRICITY
PLASTIC SOLAR CELLS
PCBM
NN
S
S Sn
R R'
LBPF7 = DiH, R=R'=C6H13 (hexyl)LBPF1 = HEH, R=C6H13 (hexyl) R'=CH2CH(C2H5)C4H9 (2-ethylhexyl)LBPF5 = DiO, R=R'=C8H17 (octyl)LBPF6 =DiD, R=R'= C12H25 (dodecyl)
APFO3 ITO Al
• LIGHT ABSORPTION
• ENERGY TRANSFER
• CHARGE GENERATION
• CHARGE SEPARATION
• FORMATION OF MOBILE CHARGES
• CHARGE RECOMBINATION
• CHARGE TRANSPORT
• CHARGE EXTRACTION
Viewing electrons at work – Terahertz
spectroscopy 10
5 10
6 10
7 10
8 10
9 10
10 10
13 10
14 10
15 10
16 10
17 10
18 10
19 Frequency
(Hz)
Region Radio & TV Microwaves Infrared UV RTG
10 20
THz
10 1 2
Visible
11 10
1 THz = 1 ps = 0.3 mm = 33 cm–1 = 4.1 meV = 48 K
Carlito Ponseca
Time-domain terahertz spectroscopy
Emitter Sample
THz pulse
Sensor
Optical pump pulse
-3 -2 -1 0 1 2
Time (ps)
1.2
0.8
0.4
0
-0.4
-0.8
-1.2
Ele
ctr
ic f
ield
(a.u
.)
Transmitted THz pulses
Ground state
Excited state
Difference
• Measure shape of the E-field • Amplitude + Phase info • Real + Imaginary part of conductivity • More info!
ZnTe
THz Case Studies
THz
Re
Im
Molecular exciton
THz
Free charges Re
Im
Re
Im
Confined mobile charges
THz
Re
Im
THz
Bound charges, polarons
Charge mobility in
plastic solar cells
Confined charges Mobile charges
Δ = concentration x mobility
H Nemec et al. JPC C 112 (2008) 6558; PRB 79 (2009) 245326; Phys. Rev. Lett. 104, 197401 (2010)
C. Ponseca et al. JACS 134 (2012) 11836
K. Zidek et al. JACS 134 (2012) 11836
Confined mobile charges
THz
Re
Im
NN
S
S Sn
R R'
LBPF7 = DiH, R=R'=C6H13 (hexyl)LBPF1 = HEH, R=C6H13 (hexyl) R'=CH2CH(C2H5)C4H9 (2-ethylhexyl)LBPF5 = DiO, R=R'=C8H17 (octyl)LBPF6 =DiD, R=R'= C12H25 (dodecyl)
Origin of THz response – e- or e+? Monomer/PCBM – electron mobility Polymer/PCBM – hole mobility
C. Ponseca et al. J. Phys. Chem. Lett., 3 (2012) 2442
1.5 ps
0.5 ps
+ ~ 10 x - Hole mobility increases with chain length
-
+
space,time S0
S1 + - -
+ -
h
kCT k*
CS CTn
CT0
kCS
ktransport
kextraction En
erg
y
Electrode
kBR
SC
fs – ns ns – sub-ms
CHARGE SEPARATION IN BHJ SOLAR CELLS - TREFISH
NN
S
S Sn
R R'
LBPF7 = DiH, R=R'=C6H13 (hexyl)LBPF1 = HEH, R=C6H13 (hexyl) R'=CH2CH(C2H5)C4H9 (2-ethylhexyl)LBPF5 = DiO, R=R'=C8H17 (octyl)LBPF6 =DiD, R=R'= C12H25 (dodecyl)
NN
S
S Sn
R R'
LBPF7 = DiH, R=R'=C6H13 (hexyl)LBPF1 = HEH, R=C6H13 (hexyl) R'=CH2CH(C2H5)C4H9 (2-ethylhexyl)LBPF5 = DiO, R=R'=C8H17 (octyl)LBPF6 =DiD, R=R'= C12H25 (dodecyl)
+ + +
Drift distancse
Mobility
Or Monte Carlo Simulation
CHARGE SEPARATION – TREFISH
Free charges at ~ 500 ps
S. De et al. JACS 129 (2007) 8466 S.K. Pal et al. JACS 132 (2010) 12440 D. Vithanage et al. Nature Comm. 4 (2013) 2334 V. Pranculis et al. JACS 136 (2014)11331
D ~
Charge separation in DSC
H Nemec et al PRL 104, 197401 (2010);
J. Photochem. Photobiol. A. 215 (2010) 123
e-
e+
e-
Electron-Hole interaction
Charge separation in DSC
0 0.2 0.4 0.6 0.8 1 1.2
Frequency (THz)
1.2
0.8
0.4
0
-0.4
-0.8
-1.2
Ima
gin
ary
R
ea
l
All delays, 36 % (cut) Real, 1 ps
All delays, 36 % (cut) Real, 3.7 ps
All delays, 36 % (cut) Real, 11.8 ps
All delays, 36 % (cut) Real, 36.1 ps
All delays, 36 % (cut) Real, 109 ps
All delays, 36 % (cut) Real, 328 ps
All delays, 36 % (cut) Real, 984 ps
All delays, 36 % (cut) Imaginary, 1 ps
All delays, 36 % (cut) Imaginary, 3.7 ps
All delays, 36 % (cut) Imaginary, 11.8 ps
All delays, 36 % (cut) Imaginary, 36.1 ps
All delays, 36 % (cut) Imaginary, 109 ps
All delays, 36 % (cut) Imaginary, 328 ps
All delays, 36 % (cut) Imaginary, 984 ps
-spectrum
Confined
charges
t
N
N N
N
Zn
OH
OO
OH
(c)
D
D* EC D + + e – mobile
(2)
(1)
(3)
(4)
(5) (6)
H Nemec et al PRL 104, 197401 (2010)
(t) - Mobile charge formation
0 50 100 150 200
Pump-probe delay (ps)
1
0.8
0.6
0.4
0.2
0
Sin
gal (a
rb.
units)
Ru-N3/ZnO
ZnTPP-Ipa/ZnO
Ru-N3/TiO2
ZnTPP-Ipa/TiO2
(a)
TiO2
ZnO
0 100 200 300 400 500
A (
A.U
.)
time / ps
Optical TA
TiO2, ZnO
< 5 ps Oxidized dye formed
DSSC to Perovskites Historic Evolution
From: HJ Snaith, J. Phys. Chem. Lett. 2013, 4, 3623−3630
CH3NH3
Iodide
CH3NH3PbI3
Pb
Organohalide perovskite solar cells – the issues
Organohalide Perovskite
Pb
• >15 % efficiency • Injection in P/TiO2? • P/Al2O3 higher efficiency than P/TiO2
• Also thin film P • Micrometer carrier diffusion • Exitons or free charges? • Mobility?
CH3NH3
Iodide
CH3NH3PbI3
Pb
Organohalide perovskite solar cells THz conductivity
P/TiO2 Neat P; P/Al2O3
TA
Perovskite SE
e- absorption
PL • Free charges in ps
• Injection in P/TiO2 • s recombination • Very high carrier mobility
• Balanced transport • Slow activated recombination
C. Ponseca et al. JACS 136 (2014) 5189 T.J Savenije et al . JPC Lett. 5 (2014)2189
MW conductivity
Al2O3
Al2O3
-
-
-+
+
TiO2
TiO2
+
-
-
--
-+
+
+
CONCLUSIONS
How to view charge in motion
Plastic solar cells – generation of mobile charges
From DSC to perovskite solar cells
Lund University Chemical Physics Department
Co-workers • Arkady Yartsev • Tonu Pullerits • Swati De • Suman Pal • Torbjörn Pascher • Mattias Andersson • Yingyot Infashaeng • Dimali Vithanage • Carlito Ponseca • Jens Uhlig • Wilfred Fullagar • Ujjwal Mandal • Dharma Kuruntu • Pavel Chabera • Tobias Harlang • Sophie Canton • Yizhu Liu • Kasper Kjaer • Mohamed Abdellah • Kaibo Zheng
Collaborators • Vidas Gulbinas • Olle Inganäs • Mats Andersson • Jenny Nelson • Hiroshi Imahori • Shen YE • Hynek Nemec • Nenad Vukmirović • Stenbjörn Styring • Leif Hammarström • Kenneth Wärnmark • Licheng Sun • Christian Bressler • M.M. Nielsen • Joel Ullom • Tom Savenije • Petter Persson • Lisa Fredin