Characterization of silicon heterojunctions for solar - BioMed Central
Center for Interface Science: Solar Electric MaterialsOrganic/organic’ heterojunctions: Macromol....
Transcript of Center for Interface Science: Solar Electric MaterialsOrganic/organic’ heterojunctions: Macromol....
Center for Interface Science:Solar Electr ic Materials
Research supported as part of the Center for Interface Science: Solar Electric Materials (CISSEM), an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Award Number DE‐SC0001084
“The Interface Science of Emerging Thin Film Solar Energy Conversion Technologies: Learning to Understand, Deal With and (Occasionally) Love
Recombination and All That It Implies”
Scialog 2012 – Biosphere IINeal Armstrong
Center for Interface Science: Solar Electric Materialswww.solarinterface.org
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“Generation III” PVs – What are they? Where are they headed?
New Thin Film PV Technologies
e.g. Solarmer, Polyera, Heliatek…..
Nearly 7% module efficiency
http://www2.imec.be
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Roll‐to‐roll vacuum processing??
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Where are solar cell efficiencies going?
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December 2011
Heliatek
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DOE SunShot Forum, June 2012
Can we make PV competitive without
subsidy?
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nano‐laminate barrier layers/substrate
bottom contact
charge selective interlayer
active layer
top contact
light management
nano‐laminate barrier layers
charge selective interlayer
The motivation for interface science
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contact
active layer
Interlayer ca. 10‐30 nm)
contact
contact
Interlayer
Ratcliff, Zacher et al. JPC Letters Perspective 2011
Images: Kai‐Lin Ou, Xerxes Steirer, Delvin Tadytin
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The motivation for interface science
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Ratcliff, Zacher et al. JPC Letters Perspective 2011
Layer‐by‐layer assembly
SemiconductorNanocrystalActive Layers
NC‐polymer hybrids
Selective interlayer
ITO
What makes a good contact?Transparency, conductivity, low‐cost, earth abundant, scalable
Other issues: Heterogeneity in electrical propertiesInterfacial compatibility with organic or inorganic active layers
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CISSEM Identity – Principal Investigators
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http://www.nrel.gov/gis/solar.html
DavidGinger
JosephBerry
DanaOlson
DavidGinley
AssociateDirector
Neal ArmstrongDirector
Jeanne PembertonAssociateDirector
S. ScottSaavedraAssociateDirector
OliverMonti
DominicMcGrath
SethMarderAssociateDirector
Jean‐LucBrédas
SamuelGraham, Jr.
BernardKippelenAssociateDirector
AntoineKahn
Chemistry, Electrical & Mechanical Engineering, Materials Science, Optical Sciences, Physics
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(A)
PTH
VOC
JSC PMAX
(B)
J
phJ oJ n PR ASR A
V
+
-
(C)
exp 1S So ph
o B P
V JR V JRJ J Jn k T e R
1ln
o
phBoOC J
Je
TknV SC OC
SOLAR
J V FFP
**
(D)
(E) (F)
=FF =
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Component Materials and Interfaces in OPVs and Thin Film PVs
• Contacts (oxides and metals)• Charge selective interlayers (both oxide
and molecular materials)• Substrates and barrier layers (often
materials which are complementary to the contacts and interlayers)
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Facile and selectivecharge harvesting
• Contacts (oxides and metals)• Charge selective interlayers (both oxide
and molecular materials)• Substrates and barrier layers (often
materials which are complementary to the contacts and interlayers)
Component Materials and Interfaces in OPVs and Thin Film PVs
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• Contacts (oxides and metals)• Charge selective interlayers (both oxide
and molecular materials)• Substrates and barrier layers (often
materials which are complementary to the contacts and interlayers)
Oxide and metal contacts, and oxide interlayers
Component Materials and Interfaces in OPVs and Thin Film PVs
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• Contacts (oxides and metals)• Charge selective interlayers (both oxide
and molecular materials)• Substrates and barrier layers (often
materials which are complementary to the contacts and interlayers)
Dipolar and redox‐active interface modifiers
Component Materials and Interfaces in OPVs and Thin Film PVs
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• Contacts (oxides and metals)• Charge selective interlayers (both oxide
and molecular materials)• Substrates and barrier layers (often
materials which are complementary to the contacts and interlayers)
Unique approaches to interface characterization
Component Materials and Interfaces in OPVs and Thin Film PVs
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The Tools of CISSEM – Waveguide Spectroscopy
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Potential modulated ATR Waveguide SpectroscopyW.M. Keck Center, University of ArizonaSaavedra Laboratory, University of Arizona
Transient Waveguide Absorbance SpectroscopySaavedra Laboratory, University of Arizona
New tools have been developed in CISSEM to characterize electron transfer at interfaces on multiple time and length scales in solution (and are being developed for condensed phase environments).
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The Tools of CISSEM – trEFM
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trEFMGinger Laboratory, University of Washington
New implementations of Atomic Force Microscopy (AFM) to characterize heterogeneous, nano‐scale electrical properties at interfaces: Time Resolved Electrostatic Force Microscopy (trEFM). Non‐contact trEFM can recover sub‐microsecond transients to characterize formation and migration of photo‐generated charges.
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The Tools of CISSEM – IPES and TPPE
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fs Angle‐Resolved Two‐Photon Photoemission (TPPE) SpectroscopyMonti Laboratory, University of Arizona
IPESKahn Laboratory, Princeton University
CISSEM uniquely combines ultrasensitive, high‐resolution photoemission and inverse‐photoemission spectroscopies to map electronic structure of excited state levels of interfacial regions of contact and interlayer materials.
TPPE spectroscopy characterizes electronic structure of excited state levels of interfaces, and with fsec time resolution, can provide direct insight into electron transfer at interfaces.
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The Tools of CISSEM – UHV Surface Raman
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Surface Raman SpectroscopyPemberton Laboratory, University of Arizona
CISSEM uses unique combinations of ultra‐sensitive optical and x‐ray surface spectroscopies to probe molecular composition and orientation at oxide/organic and metal/organic interfaces: vibrational spectroscopies, NEXAFS, and X‐ray reflectivity
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REALLY BAD SOLAR CELLS!!Albery, Archer – Nature 1977Albery, Accounts of Chemical Research 1982
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Example redox couples: Ru(bipy)3+2 and Fe+2/Fe+3 IF all electrochemical processes
completely optimized: η ≈ 18%
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Albery, Archer – Nature 1977Albery, Accounts of Chemical Research 1982
Albery, Archer – Nature 1977Albery, Accounts of Chemical Research 1982Need for “kinetically selective contacts!”
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Grätzel, M., Recent Advances in Sensitized Mesoscopic Solar Cells. Accounts of Chemical Research 2009, 42 (11), 1788‐1798.
see also Science Nov. 2011
Real compositional and energetic asymmetry
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ee‐‐hh++
≈
Energy
(eV)
‐4.0
‐3.0
‐5.0
‐6.0
EF,TCO
EHOMOD
ELUMOA
ELUMOD
EF,M
TCO
Metal
eVAC = 0
Don
or
Acceptor
VOC = f(Vbi)≈ EHOMO
D – ELUMOA
load
EHOMOA
JSC = f(ELUMOD – ELUMO
A
Tang, et al. Appl. Phys. Lett 1986, 1987 Two‐layer OLED and OPVs
Type II Organic Heterojunction Devices
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Factors controlling OPV
efficiency
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“Interlayer films” are needed to provide both kinetic and thermodynamic selectivity for charge harvesting
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E vs. (NHE
) (volts)
(+)
(‐)
Energy (e
V)
≈
evac
Transparen
t con
tact
“hole‐selective”interlayer
+ Top contact
‐
+
X
‐X
“electron‐selective”interlayer
Active layer(s)
EF,e
EF,h
IP EA
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Energetics (ECB, EVB of organic & oxide interlayers
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•Active layer materials
•“Electron selective interlayers”
•“Hole‐selecitveinterlayers”
•“Dopants” and high Φinterlayers
•“Tunable “interlayers
•Notable absences: interface dipole effects!!Brabec et al. J. Mater. Chem 2010
Erin Ratcliff, Brian Zacher J. Phys. Chem. Lett. Perspective (2011)
e‐C60‐.
C60
HOMOD
LUMOA
Energy
+
e‐
1
2
+
3
)()()()(
2
TkEEq
CBsnTk
EEq
VBsp
isspnsr
B
TCB
B
VBT
eNnSeNpS
npnSqSJ
Photocurrent (extraction)limited by Surface Recombination – a comparable event occurs for hole extraction at the opposite electrode
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10-5
10-4
10-3
10-2
10-1
100
101
102
Cur
rent
den
sity
(mA
/cm
2 )
-2 -1 0 1 2
Voltage (V)
10% O2 Dark10% O2 Light 0% O2 Dark 0% O2 Light
20
15
10
5
0
-5
-10Cur
rent
den
sity
(mA
/cm
2 )
0.80.40.0-0.4Voltage (V)
10% O2 Dark 10% O2 Light 0% O2 Dark 0% O2 Light
0, 10% O2 sp ZnO interlayers in BHJ devices(TFD ITO/sp ZnO/BHJ/MoOx/Ag)
ETL VOC (V) Jsc (mA/cm2) F.F. PCE (%)
0% O2 ZnO 0.51±0.01 9.2±0.6 0.50±0.01 2.3±0.210% O2 ZnO 0.49±0.01 5.6±0.7 0.20±0.02 0.6±0.1
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These same issues are relevant in describing the photoelectrochemical conversion of sunlight to fuels, using
planar or nanowire array electrodes
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Intrinsic defects in Würtzite ZnO
O Zn
Zinc vacancy
Oxygen vacancy
Zinc interstitialZinc antisite
A. Janotti, C.G. Van de Walle , PHYSICAL REVIEW B 76, 165202 2007
Ideal würtzite ZnO
Oxygen antisite
10%
‐partially filled band gap states due to broken bonds of O atoms ‐acceptor (V0
Zn, V‐1Zn ,V‐2
Zn)
Oxygen interstitial‐ O‐Oi bond (Oi split)‐ Oi
‐2(octahedral)
‐ acceptor
12% 23%
‐ most probable donor for n‐ZnO
‐ donor‐ very unstable
‐ Zn in the position of O
‐ ZnO‐ O distances 8% loner vs. quilibrium bonds
‐ O in the position of Zn
‐ O ‐OZn bond
De
e-
e-
e-
e-primary e-
secondary e-
e-
incident radiation
e-
e-
e-
primary e-
secondary e-
(a) (b)
10 15 20 25 30
0
50000
100000
150000
Inte
nsity
Kinetic Energy (eV)
AuAu with C16 thiolsource energy, 21.2 eV
EF
Au spectrum width (w)
Evac
Determination of:
Ionization potentials (IP), EVB
Local shifts in vacuum level (interface dipoles)
Frontier orbital energy offsets
Organic/organic’ heterojunctions:Macromol. Rapid Commun. 2009, 30, 717–731Appl. Phys. A., 95, 209‐218 (2009)
Self‐assembled monolayers:Journal of Physical Chemistry C, 113, 20328‐20334 (2009
Tethered monolayers of SC‐NCsACS Applied Materials and Interfaces, 2, 863‐869, (2010)
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UPS/XPS ++ layer‐by‐layer deposition (vacuum and glove box) of organic semiconductors, semiconductor nanocrystals, interlayers, etc.
New Capabilities Layer‐by‐Layer OPV and interlayer formation (vacuum deposited small
molecules)
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Inverse photoemission spectroscopy (IPES)
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e‐selective oxide interlayers: sol‐gel (printed) versus CVD or ALD (nanometer control of
thickness and electrical properties
Kai‐Lin Ou/Delvin Tadytin/Xerxes SteirerACS Applied Materials & Interfaces
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Energetics (ECB, EVB of organic & oxide interlayers
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•Active layer materials
•“Electron selective interlayers”
•“Hole‐selecitveinterlayers”
•“Dopants” and high Φinterlayers
•“Tunable “interlayers
•Notable absences: interface dipole effects!!Brabec et al. J. Mater. Chem 2010
Erin Ratcliff, Brian Zacher J. Phys. Chem. Lett. Perspective (2011)
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New forms of solution‐processed NiOx interlayers:
K. Xerxes Steirer, Paul Ndione, N. Edwin Widjonarko, Matthew T. Lloyd, Jens Meyer, Erin L. Ratcliff, Antoine Kahn, Neal R. Armstrong, Calvin J. Curtis, David S. Ginley, Joseph J. Berry, and Dana C. Olson
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K. Xerxes Steirer, et al., Advanced Energy Materials (2011)
Performance, scalability and lifetimes are enhanced!
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Brian Zacher et al. JPC C 2011
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Trajectories of some of the charges
emanating from the D/A interface:
500 successful transits from one
D/A site
“Fast pathways”
d
1 2
-1 -0.5 0 0.5 1-10
-5
0
5
10
Bias (V)
Cur
rent
(nA
)
AmplifierV
CuPc
ITO/glass
MacDonald, Veneman, et al. in preparation
Conducting tip AFM: Mapping of electrical properties for contacts and interlayers
-2 -1 0 1 2
-4
-2
0
2
4
Bias (V)
Cur
rent
(nA
)
Gold OP-ITO HCL+FeCl3 ODPA-ITO Increasing φ
Less ohmic
Current through CuPc thin filmsdominated by V2 dependence ifcontact is Ohmic (Mott‐Guerny)
Conducting tip AFM: Mapping of electrical properties for contacts
and interlayers
MacDonald, Veneman, ACS Nano – this week!
060
120
060
120
060
1200
60120
2 4 6
0 500nm
Gold
DetergentSolvent CleanedITO
O2‐Plasma Cleaned ITO
10-5
10-4
10-3
10-2
10-1
100
101
102
103
Cur
rent
Den
sity
(m
A c
m-2
)
2.01.00.0-1.0
Bias (V)
10
8
6
4
2
0
-2
-4Cur
rent
Den
sity
(m
A c
m-2
)
2.01.51.00.50.0-0.5-1.0
Bias (V)
C6 C8 C14 C18 DSC
a) b)
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“Generation III” PVs – What are they? Where are they headed?
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http://energysciencegroup.ning.com/
CISSEM Interface‐to‐Face Research Conference, 2010
Energy Science Group
http://energysciencegroup.ning.com/