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Photoelectrochemical Water Systems for H2 Production
2007 DOE Hydrogen, Fuel Cells, and Infrastructure Technologies Program Review
May 17, 2007
John A. Turner, Todd Deutsch, Jeff Head, and Paul VallettNational Renewable Energy Laboratory
[email protected] 303-275-4270
This presentation does not contain any proprietary or confidential information.
PD 10
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Overview
• Project start date: 1991• Project end date: tbd• Percent complete: tbd
Barriers addressedM. Materials durabilityO. Materials efficiencyN. Device configuration designs
• Total project funding to date– DOE share: $5.9M (~0.75 FTE
+ postdoc, average)• Funding received in FY 2006:
$140k• Funding for FY 2007: $800k
Budget
Timeline Barriers
Interactions/collaborations– UNLV-SHGR– University of Nevada, Reno– Colorado School of Mines– University of Colorado– Program production solicitation
• MVSystems, Inc• Midwest Optoelectronics
Partners
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Photoelectrochemical Conversion Goals and Objectives
1. Identify and characterize new semiconductor materials that have appropriate bandgaps and are stable in aqueous solutions.
2. Study multijunction semiconductor systems for higher efficiency water splitting.
3. Develop techniques for the energetic control of the semiconductor electrolyte interface and for the preparation of transparent catalytic coatings and their application to semiconductor surfaces.
4. Identify environmental factors (e.g., pH, ionic strength, solution composition, etc.) that affect the energetics of the semiconductor, the properties of the catalysts, and the stability of the semiconductor.
5. Develop database to house a library of the material properties discovered by the DOE program.
The goal of this research is to develop a stable, cost effective, photoelectrochemical-based system that will split water using sunlight as the only energy input. Our objectives are:
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Efficiency – band gap (Eg) must be at least 1.6-1.7 eV, but not over 2.2 eV; must have high photon to electron conversion efficiency
Material Durability –semiconductor must be stable in aqueous solution
Energetics – band edges must straddle H2O redox potentials (Grand (Grand Challenge)Challenge)
1.23 eV1.6-1.7 eV
p-typeSemiconductor
Eg
CounterElectrode
H2O/H2
H2O/O2
All must be satisfied simultaneously.
Electron Energy
Material Challenges (the big three)Characteristics for Ideal Photoelectrochemical Hydrogen
Production Material
i
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Approach: High Efficiency Materials & Low-Cost Manufacturing
PEC devices must have the same internal photon-to-electron conversion efficiency as commercial PV devices.
• III-V materials have the highest solar conversion efficiency of any semiconductor material– Large range of available bandgaps– ….but
• Stability an issue – nitrides show promise for increased lifetime• Band-edge mismatch with known materials – tandems an answer
• I-III-VI materials offer high photon conversion efficiency and possible low-cost manufacturing– Synthesis procedures for desired bandgap unknown– ….but
• Stability in aqueous solution?• Band-edge mismatch?
• Other thin-film materials with good characteristics– SiC: low-cost synthesis, stability– SiN: emerging material
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Approach: Materials Summary
GaPN - NREL (high efficiency, stability)CuInGa(Se,S)2 - UNAM (Mexico), NREL (low cost)
Silicon Nitride - NREL (protective coating, new material)GaInP2 - NREL (fundamental materials understanding)Energetics
Band edge controlCatalysisSurface studies
The primary task is to synthesize the semiconducting material or the semiconductor structure with the necessary properties. This involves material research issues (material discovery), multi-layer design and fabrication, and surface chemistry. Activities are divided into the task areas below – focus areas in black:
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III-V Nitrides: GaP1-xNxPrevious work with III-V nitrides for PEC water splitting
showed improved stability with very small amounts of nitrogen, but problems with morphology at higher nitrogen content
• GaP1-xNx
– Addition of small amounts of N causes GaP band gap to narrow (bowing) and transition to become direct
– Nitrogen enhances stability
02 2.1 2.2 2.3 2.4
0.06% N0.11% N0.15% N0.20% N1.10% N1.60% N
Nor
mal
ized
Pho
tocu
rrent
2(A
U)
Photon Energy (eV)
GaP1-xNx Epilayer Direct Band Gap
Goal: Stable III-V Nitride Material
Dr. Todd Deutsch
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substrate
epilayer
Lattice mismatch from nitrogen addition
Lattice matched
Increasing the Nitrogen Content:Lattice Matching Using Indium as a Lattice Expander
Results: Latticed matched with more than 2% nitrogen
Ga.96In.04N.024P.976
Ga.95In.05N.025P.975
Jeff Head, Paul Vallett
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R2 = 0.9996
0.0E+00
2.0E-03
4.0E-03
6.0E-03
8.0E-03
1.0E-02
1.2E-02
1.4E-02
1.60 1.70 1.80 1.90 2.00 2.10 2.20
Energy (eV)
Nor
mal
ized
Pho
tocu
rren
t Squ
ared
(A
rbitr
ary)
Band Gap = 1.96 eV
Photocurrent SpectroscopyTo Determine the GaInPN Band Gap
GaInPN
Silicon
1 µm
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Durability Analysis Showed Improved Corrosion Resistance
• 24-hour corrosion test– Constant 5 mA/cm2 applied current– Sample illuminated at 1 sun
• Profilometry etch depth– 0.1 µm average GaInPN:Si GaInP2:GaAs
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GaInPN on Silicon Tandem Cell
• GaInPN top cell– Visible light conversion– Corrosion protection
• Si-bottom cell– Absorbs IR radiation passing
through the top cell– Provides energy to lower h+
below O2 redox potential
• Also applied back surface field for field-aided charge collection
GaInPN
Silicon
Nea
r IRhν
Visi
blehν
Zn-doped GaInPN
GaInPN
Silicon cell
1 µm
0.1 µm
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PEC Energy DiagramE
nerg
y
Aqueous SolutionGaInPNSilicon
H2O/H2
H2O/O2
e-
h+
I
Paul Vallett
h+
e-
h+
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-2.0
-1.5
-1.0
-0.5
0.0
0.5
1.0
1.5
0 2 4 6 8 10 12 14
pH
Pote
ntia
l (V)
Open Circuit PotentialMott SchottkyPhotocurrent Onset
Band Edge Energetics
To vacuum level
vs. A
g+/A
gCl
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Water Splitting
0.00
0.05
0.10
0.15
0.20
0.25
0.30
0.35
Zn -GaInPN/Si
GaInPN/Si
Sola
r to
Hyd
roge
n Ef
ficie
ncy
(%)
.• Zn-doped layer provides back surface field and improves efficiency
• Nitrogen negatively impacts the electronic properties of material
• Photon to chemical energy conversion efficiency– GaInPN: ~25% – GaInP2: ~60%
Goal: Stable III-V Nitride Material
Jeff Head, Paul Vallett
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0
10
20
30
40
50
60
70
300 400 500 600 700
2.4%N2.5%NGaPGaInP2
IPC
E %
Wavelength (nm)
Integrated Photon-to-Electron Current Efficiency (IPCE)
PEC devices must have the same internal photon-to-electron conversion efficiency as commercial PV devices.
Todd Deutsch
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CuGaSe2 Tandem Cell Configuration:Possible High Efficiency, But a New
Deposition Approach is Required
CGS: 1.3μmITO: 150nm
Dr. Jennifer E. Leisch
Maximum theoretical water-splitting efficiency = 28%
Goal: Thin-Film Based PEC Tandem Cell
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Electrodeposited CGS - Deposition of Ga DifficultDeposition Bath Compositions Have Been Developed To
Produce Good Quality Films
CuGaSe2 (CGS)
http://staff.aist.go.jp/paul-fons/chalcopyrite.html#
Chalcopyrite Structure
Good quality electrodeposited CIGS film on Mo substrate
Fair quality electrodeposited CGS film
on Mo substrate
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Conclusions• III-V nitrides
– Higher nitrogen content realized with In as lattice expander– PEC water splitting demonstrated on GaInPN:Si tandem cell– Nitrogen enhanced stability, but nitrides grown by MOCVD have
poor performance– p+ layer field aided charge collection, increased efficiency– Nitride epilayer needs improvement
• CIGSSe thin films– Theoretical 23% water-splitting efficiency with Si-CuGaSe2
tandem cell, but configuration difficult to realize– Possible to electrodeposit good quality films with high Ga– Annealing system is being constructed and trials will soon be
underway
Goal: High-Efficiency PEC Devices
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Future Plans
• Remainder of FY 2007:– Continue understanding and improving nitride-based
material: III-V nitrides and SiN– Develop new electrosynthesis approaches and low-
temperature annealing processes for CIGSSe films– Improve tandem cell design with thin-film CIGSSe
• For FY 2008:– Look at possible new materials with UNLV, CSM …– Coatings: SiN, SiC …– Multijunction structures