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Excitonic Solar Cells: The Challenges of Efficiency and Durability
Garry Rumbles Chemical and Biosciences Center
NREL Golden, Colorado
NREL/PR-270-43347 Presented at the 33rd IEEE Photovoltaic Specialist Conference held May 11-16, 2008 in San Diego, California
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Or. “Convince an old skeptic like me that OPV has a future.”
• Do not use anecdotal evidence. – e.g. Organic Light-Emitting Diodes, Liquid Crystal Displays, Conventional
Photographs, Photocopiers, Car paint.
• No comparison with other thin-film technologies.
• From the viewpoint of basic science .
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The questions to ask of an OPV device:
• How efficient is it? • How long does it last?
• How big is it? • How many can we make? • How much do they cost? • Can they be made sustainable? • Do they contain toxic components? • How much energy required to build? • How do they work?
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What is an OPV cell?
• Examples of Excitonic Solar Cells (XSC) – Dye-sensitized (Grätzel) solar cell (DSSC). – Two-layer, planar (Tang) solar cell. – Bulk heterojunction (Sariciftci and Heeger) solar cell. – Hybrids.
• Why ‘Excitonic’? Coupled/correlated electron-hole pairs. – Absorbed photon produces a neutral excitation, not free carriers.
Therefore, a dissociation interface is required.
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Excitonic Solar Cells
Dye-sensitised TiO2 solar cell Blended (bulk heterojunction) OPV cell O’Regan and Grätzel, Nature, 353, 737 (1991) G. Yu, J. Gao, J. C. Hummelen, F. Wudl, and A. J. Heeger,
Two-layer OPV cell Science 270, 1789 (1995) C.W.Tang, APL, 48(2), 183 (1986)
• Large titania surface area to • Solution processible. increase absorption of adsorbed • Planar and simple. • Short exciton diffusion lengths. dye.
• Vacuum deposited molecular films. • Ultrafast and efficient exciton • Ultrafast and efficient electron dissociation. injection from dye to titania. • Absorption by both layers.
• Redox couple to transport holes. N
N
Ru
N
N
N
C
S
N C
S
HOOC
COOTBA
COOH
COOTBA
NN N
N
N
N
N
N
Cu
N
O
N
N
O
N
n
O
O
O
O
N719
PV
CuPc MEH-PPV
PCBM
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The Bulk Heterojunction (BH)
• Solution processible. – Both the PEDOT:PSS and Active layer can be deposited from solution.
(e.g. Spin, Spray, Ultrasonic, Ink-jet, Doctor blade)
• Short exciton diffusion lengths. – Requires two components of the Active layer, the Donor and Acceptor,
to phase separate and excitons not have to travel more than 10 nm.
• Ultrafast and efficient exciton dissociation. – Interface between Donor and Acceptor must efficiently dissociate
exciton and inhibit recombination.
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The Prototypical Poly(3-hexylthiophene): PCBM BH device
S
C6H13
n S
C6H13
S
C6H13
P3HT
O
O
PCBM
PEDOT
PSS
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How it works
• P3HT absorbs photon. • Exciton diffuses to interface. • Exciton dissociates. • Carriers diffuse, driven by chemical potential, to electrodes.
– Electrons through PCBM network to cathode. – Holes through P3HT phase to anode.
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Why P3HT and PCBM?
• P3HT absorbs further to the red, with optical bandgap at < 2 eV. – But Short Exciton lifetime, τ, ~ 300 - 500 ps.
• Exciton migrates to interface – But Diffusion length < 10 nm. Hence blend.
• Exciton dissociates to electron and hole at interface with PCBM. – Ultrafast, 100% yield, minimal recombination.
• Carrier recombination is minimal. – Why?
• Holes transport through P3HT. – Hole mobility, µh, > 10-3 cm2/Vs
• Electrons transport (hopping) through PCBM. – Electron mobility, µe, > 10-3 cm2/Vs
• Summary: – P3HT is a good absorber, it is ordered and therefore exhibits good hole
(and possibly exciton) mobility. – PCBM forms an ideal donor-acceptor interface with P3HT.
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Now Realistic How?
Voc (V) 0.62 0.65
Jsc (mA/cm2) 10 14
Increase absorption (OD 2), reduce recombination
FF (%) 67 75
Reduce series resistance from
50 to 15 Ω
Efficiency (%)
(4.2)* 6.8
Champion P3HT:PCBM devices
* Yang Yang UCLA
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Red absorber
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How Stable?
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IPCE
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How Stable?
Look for ‘report from Degradation Summit’. Defining standards for OPV
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Top contact and reproducibilty
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Source of instability?
• Oxygen, Water and UV light. – Reduction – Sensitization
• Contacts – Acidity of PSS – Delamination – Reducing metals
• Oxidising species – Polaron+
• Reducing species – Polaron-, PCBM-
• Adventious dopants!
• Thermal – Internal conversion
Extrinsic effects Intrinsic effects
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Now Realistic Plan* How?
Voc (V) 0.62 0.65 1.1 Lower LUMO (and HOMO)
Jsc (mA/cm2) 10 14 25 Shift absorption to red
FF (%) 67 75 80 Improve transparent
electrode
Efficiency (%)
(4.2) 6.8 22 Larger carrier mobilities
Champion devices
* Sean Shaheen. Department of Physics, DU and NREL.
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Voc
So this is what it looks like?
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Improve stability
Voc
5.5 eV
3.2 eV
Polymer1
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Improve stability and absorb more red
Voc
5.5 eV
4.5 eV
Polymer2
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But. Everything else remains the same?
• Exciton binding energy • Still exciton dissociation? • Still minimal recombination (Marcus theory)? • Same morphology!
– Mobilities same?
• Any successes?
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Changing the polymer…..
S
S
S
SC16H33
C16H33
n
T0T0TT16 N
SN
SS n
ZZ50 (PCPDTBT)
S
S
S
S
n
C12H25
C12H25T12T12TT0
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Plextronics - Plexcore® PV NREL-Certified Performance
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Introducing States
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Triplets!
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Biased by the skeptics:
• How efficient is it? • How long does it last?
• How big is it? • How many can we make? • How much do they cost? • Can they be made sustainable? • Do they contain toxic components? • How much energy required to build? • How do they work?
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Spray-deposited devices
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Devices on NREL doped-SWNT Networks
• Ultrasonic spray deposition • Several ~ 3% devices • Thick active layers - spun at
200 rpm • Reducing electrode roughness
is key • PEDOT can be eliminated
40
20
0
Cur
rent
Den
sity
(mA
/cm2
)
-1.0 -0.5 0.0 0.5 1.0Voltage (V)
ITO Referenceη = 3.5%Jsc = 11.09Voc = 570.6FF = 55%
60
40
20
0
Cur
rent
Den
sity
(mA
/cm2
)
-1.0 -0.5 0.0 0.5 1.0Voltage (V)
SWCNTη = 3.5%Jsc = 10.65Voc = 571.8FF = 57.6%
15
10
5
0
-5C
urre
nt D
ensi
ty (m
A/c
m2)
-1.0 -0.5 0.0 0.5 1.0Voltage (V)
NO PEDOTη = 1.05%Jsc = 5.4Voc = 491.1FF = 39.2%
40
30
20
10
IPC
E
800700600500400Wavelength (nm)
CNT ITO ref
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TiOx optical spacer and stabilizer
Kim et al., Adv. Mat. 18, 572 (2006)
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A path forward?
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Summary
• Increase efficiencies incrementally for P3HT:PCBM up to a limit of ~7%.
• Provides a foundation on to which new concepts can be tested.
• Learn how bulk heterojunction works. Understand role of interface states.
• Test new electrodes. • Test new acceptors. (Remember
unique property of C60).
• Chemistry is the tool to pave the way forward.
• New red absorbers with more tolerance to attack by oxygen/water.
• Gain a fundamental understanding of how devices work.
• Foundation for third-generation concepts.
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Acknowledgements
• Nikos Kopidakis • Sean Shaheen • David Ginley • Jao van de Lagemaat • Teresa Barnes • Jeff Blackburn • Rob Tenent • Brian Gregg • Dana Olson
• Andrew Ferguson • Matt Reese • Tom Reilly • Anthony Morfa • Matt White • Xerxes Steirer • Alex Nardes • Ben Rupert • Erkan Kose • Will Rance
Funding: US DOE. Office of EERE. Office of Science.