approaching or exceeding the limit in organic photovoltaics
TRANSCRIPT
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Approaching or exceeding the
limit in organic photovoltaics
Xiaoyang Zhu
$$ NSF, DOE
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How to escape the Coulomb trap?
• Energetic alignment
• Hot CT excitons
• Gradients
• ……. 1.0
0.5
0.0
6004002000-200
1.0
0.5
0.0
1.0
0.5
0.0
1.0
0.5
0.0
150100500
hn1 =
4.17 eV
4.38 eV
4.59 eV
4.77 eV
Bin
din
g e
nerg
y
1s
1s
2s
3s
IPS
Pump-probe delay (fs)
MRS Bulletin (2010)
Acct. Chem. Res. (2009)
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Model: phthalocyanine/fullerene
Loren Kaake
(UCSB)
A. Jailaubekov
(Cymer)
Nature Mater.
TR-SHG
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Direct & indirect formation of interfacial
CT excitons
• For initiation exciton in CuPc,
interfacial CT exciton forms in 80-
170 fs.
• Direct exciton in the optical gap
(hv ~ 1.5 eV) leads to the
instantaneous formation of
interfacial CT excitons.
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Hot CT exciton cooling sets the time
limit in charge separation at interfaces
e Wai-lun Chan
(U. Kansas)
John Tritsch
Energ
y
Distance to
interface
TR-2PPE
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How to escape the Coulomb trap?
• Hot CT excitons
• Gradient, gradient, gradient
• ……
http://echwaluphotography.wordpress.com/
“Mind the gap!”
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Exceeding the Shockley-Queisser Limit
The SQ limit:
1 photon in,
1 e-h pair out.
J. Appl. Phys. 32
(1961) 510.
Multiple excitons: 1
photon in, 2 or more
e-h pairs out
Science 334 (2011) 1541
Nature Chem. 4 (2012)
Hot carrier cell: Share
the hot electron or hot
hole energy
Science 328 (2010) 1543
Nano Lett. 12 (2012) 1588
h
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Implementation of singlet fission in solar cells
hv
So
S1
T1
hv2So
S1 +
Chromophore 2 Chromophore 1
Leo et al. JAP 106, 064511 (2009).
Baldo et al. Nano Lett. 11, 1495 (2011).
Greenham, Friend, et al. Nano Lett. 12, 1053 (2012);
Nature Comm. 3, 1019 (2012).
Hanna & Nozik, JAP 100, 074510 (2006)
Paci, et al. JACS 128, 16546 (2006).
So + S1
1TT( ) T1 +T1
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Singlet Fission & multi-electron transfer
hv
Wai-lun Chan
(U. Kansas)
John Tritsch
Science 334 (2011) 1541; Nature Chem. 4 (2012); JACS (2012) pending
So + S1
1TT( ) T1 +T1
??
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Seeing the multiexciton state in pentacene
1.83 eV T1
0.86 eV S0
T1 2x ET1 - ES1
= -0.11 eV
S1
Time-resolved two-photon photoemission Science 334 (2011) 1541
S1 ÛME[ ] ME* T1 +T1
hv1 = 2.15 eV; hv2 = 4.65 eV;
100 fs
lifetime
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The coherent mechanism is also responsible
for endothermic singlet fission in tetracene
hv = 2.32
2.3 eV S1 T1 1.25 eV
How is the energy barrier
overcome in singlet fission?
D 2T1 - S1( ) » 0.2 eV
Nature Chem. 4 (2012) S1 ÛME[ ] lives for 7 ps.
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There is no energy relaxation from ME to 2T1 in
tetracene
S1
2xT1
ME
Nature Chem. 4 (2012)
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The higher energy ME state is accessed by coherent
electronic coupling on an ultrafast time scale.
S1
2T1 ME
Tetracene
Pentacene
S1 2T1
ME
Interaction with the
bath: dephasing &
entropic driving force
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Harvesting singlet fission for solar energy
conversion: one vs. two electron transfer
JACS pending
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Singlet fission occurs faster than electron
transfer at the pentacene/C60 interface
Singlet Multiexciton
+ triplets
Science 334 (2011) 1541
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Singlet fission is not competitive with electron
transfer at the tetracene/C60 interface
SF ET
2 × T1
7 ps C60
0.5 ps
26 ps
[S1 ME]
JACS pending
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Simultaneous ET to LUMO & LUMO+1 of C60
CT2 (LUMO+1), 60%
CT1 (LUMO), 40%
JACS pending
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Ultrafast harvesting from the multiexciton state in the
quantum superposition may increase fission yield
p12
Singlet exciton
cooling, decay,
1 e transfer
...
Multiple exciton
localization,
cooling,
2 e transfer,
…
Nbi
Nex= P1®2
g 2
g1
Shabaev, Efros, Nozik
Nano Lett. 6, 2856 (2006)
g 1 g 2
2xET
S1 ÛME[ ] ME* T1 +T1
2xET 2xET 1xET
g 2g 1
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Design principles for harvesting singlet fission in OPVs
• Singlet fission faster than electron
transfer, as is the case for
pentacene but not tetracene.
• For slow SF, limiting interfacial
charge transfer, but recombination
and T - T annihilation is a concern.
• Energy selective electron transfer
from the ME state: need designer
electron acceptors!
• Enhancing the carrier multiplication
yield by electron transfer from the
ME state.
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• Escaping the Coulomb trap with hot CT excitons –
mind the gap!
Phys. Rev. Lett. 101 (2008) 196403; Acct. Chem. Res. 42 (2009) 1779;
Nature Mater. pending.
• Exceeding the limit in OPVs with singlet fission –
harvesting triplets or the quantum coherence
Science 334 (2011) 1541; Nature Chem. 4 (2012); JACS pending
Conclusions