Electron transfer optimisation in organic solar cellsgcep.stanford.edu › pdfs ›...
Transcript of Electron transfer optimisation in organic solar cellsgcep.stanford.edu › pdfs ›...
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Electron transfer optimisation in Electron transfer optimisation in organic solar cellsorganic solar cells
James DurrantCentre for Electronic Materials and Devices
Departments of ChemistryImperial College London
• Introductory remarks• Charge recombination vs. charge separation and transport• Interface engineering• Inhomogeneity
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Why organic PV now?• Political: global warming• Commercial: perception that Si based PV
may not have the potential for mass PV production
• Scientific: building up recent advances in– Organic electronics – LED’s and FET’s– Molecular electronics: supermolecular
photochemistry– Materials control and measurement on the
nanometer scale
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Organic photovoltaic technologiesGlass substrate
ITOMixed Layer
Glass substrateITO
Mixed Layer
h+e-
Dense TiO2 (~ 40 nm)(Hole blocking layer)
Porous TiO2 (~100 nm)
Polymer (~50nm)
Device structure
ITO substrate
+ -
LightTiO2 nanoparticles
PEDOT
Au electrode
Silver paint
Dense TiO2 (~ 40 nm)(Hole blocking layer)Dense TiO2 (~ 40 nm)(Hole blocking layer)
Porous TiO2 (~100 nm)Porous TiO2 (~100 nm)
Polymer (~50nm)Polymer (~50nm)
Device structure
ITO substrate
+ -
LightTiO2 nanoparticles
PEDOT
Au electrode
Silver paint
Device structure
ITO substrateITO substrate
+ -+ -
LightLightTiO2 nanoparticlesTiO2 nanoparticles
PEDOTPEDOT
Au electrodeAu electrode
Silver paint Silver paint
dye sensitisedphotoelectrochemical Molecular thin film
Polymer/C60 blend
Organic/inorganic hybrid
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Challenges• Stability
– liquid versus solid state, O2, water….
• Processibility– low temp processing on flexible substrates
• Efficiency– Improved red spectral response– Improved voltage and FF whilst maintaining high IQE– Efficiency versus processibility / stability issues
Haque et al. Chem Comm 2003
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Molecular donor/acceptor dyads
0.1 1.0 10.0 100.0 1000.00.0
1.0
2.0
3.0
4.0
0.0 20.0 40.0 60.0 80.00.0
1.0
2.0
3.0
4.0
m∆
OD
Time (µs)
m∆
OD
Time (µs)
SS
SS
OC12H25
C12H25ON
C8H17
τ50% = 20 µs
SS
S SOC6H13
C6H13O
SS
NC8H17
τ50% = 0.8 µs
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Kinetics in organic solar cellsPolymer
hυ
e-
Charge separation
Charge recombination
h+
C60
AlITO
electron collection
holecollection
e-
h+
transport
e- transport
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Light driven charge separation
Time / ps-5 0 5 10 15 20 25
Elec
tron
inje
ctio
n yi
eld
0.0
0.5
1.0
10-6 10-5 10-4 10-3 10-2 10-1
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
m∆O
D
Log10time / seconds
Ultrafastinjection Millisecond
recombination
Tachibana et al. J. Phys Chem 1996
TiO2
hν
e-
Electroninjection
Charge recombination
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Model of Reaction dynamics
CB
Transport S / S+
Injection
Trapping
S* / S+
Charge recombination
TiO2 Dye
Charge recombination dynamics controlled upon electron transport and interfacial electron transfer kinetics depending
upon metal oxide and sensitiser dye employed.
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Molecular Control of Recombination
k ∝ exp(-βr) where β = 0.95 ± 0.2 Å-1
Ti
Ti
RuN
N
C
C
O
OO
O
r
Clifford et al JACS 2004
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Signatures of transport in recombination dynamics
2.2%50
−∝ ntn = A t-0.25
1
10
100
1000
0.001 1 1000 1000000
t50% / nse
per D
ye+
Ethanol triflate
Non-linear dependence on electron density:
t50% ∝ n-1/α
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
1.E+01 1.E+02 1.E+03 1.E+04 1.E+05 1.E+06 1.E+07 1.E+08
T ime (ns)
Rel
ativ
e de
nsity
of e
xcite
d dy
es S
(t)/S
0
0 mV
100mV
200mV
300mV 400mV
Dispersive (Stretched exponential) decaysStrong dependence on TiO2 EF
Haque et al. J Phys Chem B 1998, 2000, Nelson et al. Phys Rev B 1999, 2001
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Recombination in MDMO-PPV/PCBM blends
polymer PCBM
π
Charge separation
Charge recombination
π∗
• Recombination kinetics dominated by slow, thermally activated power law decay resulting from
10-6 10-5 10-4 10-3 10-2 10-1
10-7
10-6
10-5
positive polaron trapping in polymer
Montanari et al. APL 2002 Nogueira et al. J Phys Chem B 2003
T = 220 K T = 298 K
∆O
D
time (s)
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Recombination versus Transport in polymer / C60 devices
g(E)
10-8 10-7 10-6 10-5 10-4 10-3 10-210-7
10-6
10-5
10-4
10-3
∆ A
bsor
banc
e
75µJ data 4µJ data 0.22µJ data
0.25µJ
4µJ
75µJ
Time (s)
10-6 10-5 10-4 10-3
10-7
10-6
10-5
10-4
10-3
30V
60V
Cur
rent
Den
sity
/ ar
b. u
nits
Time / s
TAS studies of recombination TOF studies of transport
• Smooth lines from trapping/detrapping model with same dos• Same microscopic model explains both recombination and
transport• Open question of benefit of traps
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Recombination versus transport in dye sensitised solar cells
Transport dynamics
Recombination to redox couple
Recombination to dye cations
TiO2 200µs 10ms 600µs
SnO2 300ns 9µs ~ 600ns
Cur
rent
Den
sity
/ A
cm-2
0.0 0.2 0.4 0.6 0.8-0.002
0.000
0.002
0.004
0.006
J sc
VocVoltage/V
TiO2
SnO2
SnO2/MgO
Cur
rent
Den
sity
/ A
cm-2
0.0 0.2 0.4 0.6 0.8-0.002
0.000
0.002
0.004
0.006
J sc
VocVoltage/V
TiO2
SnO2
SnO2/MgO
CB
Transport S / S+
Injection
Trapping
S* / S+
Recombination
TiO2 Dye
I-/I3-
Regeneration
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Charge separation versus recombination
102 104 106 108 1010 10120.0
0.2
0.4
0.6
0.8
Mon
ochr
omat
ic E
ffic
ienc
yCharge separation rate / s-1
Two level system numerical model of organic solar cellBased on assumption that electronic coupling for charge separation and recombination scale proportionally.
J.Nelson et al.Phys.Rev.B 2004, Appl.Phys.A 2004
J
J
V/2 Jav
Jav
Jca
V/2
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Charge separation in dye sensitised solar cells
100 101 102 103 104 105
0.0
0.5
1.0
(ii)
(i)
(ii)
(i)
Inje
ctio
n Yi
eld
time / picoseconds
Dye sensitised film
Solar cell
Haque et al. JACS 2004
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Dynamics versus Device function
Electrolyte Jsc/mAcm-2 Voc /Volts η / % τ50%(inj) τinit(rec)+Li+ 16.8 0.51 5.5 ~10 ps 20 ms+Li+/tBP 16.3 0.63 7.25 ~150 ps 100 ms+tBP 7 0.73 3.75 ~ 1 ns 400 ms
Optimised device:• Injection just sufficient to compete with excited state decay to ground• Allows minimisation of recombination losses
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CB /
trap
states
2
1
TiO2 Dye
I- / I3-
hv
3
D*/D+
D/D+
2
1
TiO2 Dye
I- / I3-
hv
3
D*/D+
D/D+
Influence of electrolyte composition upon density of conduction band / trap states in TiO2
Electrolyte B: No Li+
• Slow Electron Injection (1)
• Slow Charge Recombinationrates (2) & (3)
Electrolyte A: Both Li+ and 4-tert-butyl pyridine
• IntermediateElectron Injection rate (1)
• Intermediate Charge Recombination rates (2) & (3)
E
D/D
2
1
TiO2
CB /
trap
states
Dye
hv
3
I- / I3-
D*/D+
D/D++
Electrolyte C: No 4-tert-butyl pyridine
• Fast Electron Injection rate (1)
• Fast Charge Recombination rates (2) & (3)
Electrolyte control of interfacial dynamics
Optimum device performance: injection half-time ~ 150 ps
Electrolyte B+ tert-butyl pyridine
Electrolyte A+ tert-butyl pyridine and Li+
Electrolyte C+ Li+
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Materials approaches to control of interfacial electron transfer dynamics
10-6 10-5 10-4 10-3 10-2 10-1
0.00
0.05
0.10
0.15
0.20
0.25
m∆O
.D.
Time / Seconds
N NN N
N NN N
CH3
CH3
H3C
H3C
N NH3CO
SO3Na
HOA
B
a b
c d
a b
c d
Al2O3coated
Uncoated
10-6 10-5 10-4 10-3 10-2
0
1
2
3
m∆O
D
Time / Seconds
TiO 2 MFHTM
DFHTM
Dye
TiO 2 MFHTM
Li + - DFHTM
Dye
+ Li+- Li+
10-6 10-5 10-4 10-3 10-2
0
1
2
3
m∆O
D
Time / Seconds
TiO 2 MFHTM
DFHTM
DyeTiO 2 MFHTM
DFHTM
Dye
TiO 2 MFHTM
Li + - DFHTM
Dye
+ Li+- Li+ N N
OCH3 OCH3
n
O
OO
O
O
OO
O
Li+ Li+
Li+- DFHTM
Haque et al.Adv Mat 2004
Haque et al. Adv Func Mat2004
Palomares et al. JACS 2003
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HeterosupramolecularPhotochemistry
TiO2
hν
picoseconds
nanoseconds Supramolecularcontrol of recombination dynamics
~ 1 s
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Distance control: supersensitiserfunction
1E-6 1E-5 1E-4 1E-3 0.01 0.1 10
1
N719 Pump:550nm, Probe:800nm N845 Pump:516nm, Probe:850nm
∆O.D
. (no
rmal
ized
)
Time [s]
N
N
N
NRu
NN
CSC
S
HOOC
HOOC
COOH
COOH
NN
N
NRu
NN
CSC
S
HOOC
HOOC
O
N
OCH3
OCH3
HOMO calcs:Increase in distance ~ 4 Å
e-
Hirata et al.Chem. Eur. J.2004
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Influence of inhomogeneityWide bandgapsemiconductor
Adsorbed Sensitiser Dye
Electrolyte
S0 / S +
S* / S +
e-
Charge recombination
Electron injection
I-
e-
Dye re-reduction
/ I 3-
Inhomogeneous energetics result in non-exponential dynamics and make device optimisation much harder
∆inhomo
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1D* / D+
E
g(E)
<di>=0
d1
d2
g1g0g2
TiO2 Dye
Modelling electron injection energetics
Monte Carlo Simulation as detailed in:Tachibana et al. (2002) J. Photochem Photobiol A: ChemistryOnly fit parameters k(0) and ratio ∆/E0
100 101 102 103 104 105
0.0
0.5
1.0
(ii)
(i)
(ii)
(i)
Inje
ctio
n Yi
eld
time / picoseconds
( ) ( ) ( )( ) ( ) ⎟⎟
⎠
⎞⎜⎜⎝
⎛==
02
2 2exp00
0Edk
VdV
kdk iii Inhomogeneous broadening
∆inhomo ~ 0.15 eV film∆inhomo ~ 0.3 eV DSSC
g(E)∝ exp(E/E0)
Excited state decay
to ground
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Hole transfer in solid state DSSC’s:
Valence Band
Conduction Band
S0 / S+
S* / S+
e-
Wide bandgap semiconductor
Adsorbed Sensitiser Dye
HTM/HTM+
e-
Hole transporting material
Dye re-reduction
Hole transfer ~ 300 ps(Another example of kinetic redundency!)Hole transfer controlled by thermodynamics not kinetics
N N
OCH3
H3CO
OCH3
OCH3
NN
OCH3
OCH3
OCH3
H3CO
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Hole transfer yield as function of mean reaction free energy
-0.4 -0.2 0.0 0.2 0.40
20
40
60
80
100
Yie
ld o
f hol
e tra
nsfe
r / %
∆G(Dye-HTM) / eV
Homogeneous Model
Inhomogeneous Model
Distribution of D/D+ states
Vacuum Level
IP
+Em(HTM/HTM+)
Em(D/D+)
∆G(Dye-HTM) = Em(HTM+/ HTM) – Em(D+/ D)
Experimental data Inhomogeneous Model
N N
R2
R3
R1
R4
Haque et al . Chem Phys Chem (2003)
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Minimisation of energetic inhomogeneity
-0.4 -0.2 0.0 0.2 0.40
20
40
60
80
100
Dye
rege
nera
tion
effic
ienc
y / %
∆G(dye-HTM)
ITO
/ eV
TiO2 Dye HTM / Li+
TiO2 Dye HTM
Li+
Li+
Li+Li+
Li+
Li+
Li+ Li+
Li+
Li+
ITO
- Li+
+ Li+
-0.4 -0.2 0.0 0.2 0.40
20
40
60
80
100
Dye
rege
nera
tion
effic
ienc
y / %
∆G(dye-HTM)
ITO
/ eV
TiO2 Dye HTM / Li+
TiO2 Dye HTM
Li+
Li+
Li+Li+
Li+
Li+
Li+ Li+
Li+
Li+
ITO
- Li+
+ Li+
ITO
/ eV
TiO2 Dye HTM / Li+TiO2 Dye HTM / Li+
TiO2 Dye HTM
Li+Li+
Li+Li+
Li+Li+Li+Li+
Li+Li+
Li+Li+
Li+Li+ Li+Li+
Li+Li+
Li+Li+
ITO
- Li+
+ Li+
Ionic screening by Li+ ions reduces inhomogeneity of hole transfer energetics
N N
O
OO
OO
OO
O
R2
R3
R1
R4Li+ Li+
2[(CF3SO2)2]-
N N
R2
R3
R1
R4
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Conclusions
• Exciting times for organic PV• Optimisation of electron transfer dynamics
in organic PV requires consideration of:– Recombination versus transport, and the role of
traps – Charge separation versus recombination and the
potential for interface engineering– Energetic inhomogeneities
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AcknowledgementsColleagues at Imperial College:Colleagues at Imperial College:Jenny Nelson, Jenny Nelson, DonalDonal Bradley, David Bradley, David KlugKlugSteffan Cook, Ana Flavia Nogueira, Ivan Montanari, Samantha HandaEmilio Palomares, Saif Haque, Narukuni Hirata, Alex Green, Hari Upadahyaya, John Clifford
Collaborations:Michael Gratzel (EPFL), Jan Kroon (ECN)Andrew Holmes (Cambridge / ICL), Serdar Sariciftci (Linz)Christoph Brabec (Siemens/Konarka), Nazario Martin (Madrid), Kees Hummelen (Groningen), Merck Chemicals, Dow Chemicals, Covion GmbH, Johnson Matthey Ltd.
FundingFunding::EPSRC, DTI, EU, BP