www.mvsystemsinc.com NHA conference May3-6, 2010 The Potential of Using a-SiC:H as the Photoelectrode for Water Splitting Feng Zhu a , Jian Hu a , Ilvydas Matulionis a , Josh Gallon a , Todd Deutsch b , Nicolas Gaillard c , Eric Miller c and Arun Madan a a. MVSystems, Inc., Golden, CO. USA b. National Renewable Energy Laboratory, Golden, CO, USA. c: Hawaii Natural Energy Institute, University of Hawaii at Manoa, HI, USA Supported by the U.S. Department of Energy Program # DE-FC36- 07GO17105
Transcript
www.mvsystemsinc.com NHA conference May3-6, 2010
The Potential of Using a-SiC:H as the Photoelectrode for Water Splitting
Todd Deutschb, Nicolas Gaillardc, Eric Millerc and Arun Madana
a. MVSystems, Inc., Golden, CO. USA
b. National Renewable Energy Laboratory, Golden, CO, USA.
c: Hawaii Natural Energy Institute, University of Hawaii at Manoa, HI, USA
Supported by the U.S. Department of Energy Program # DE-FC36-07GO17105
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PEC - photoelectrochemicala-SiC – hydrogenated amorphous silicon carbide hybrid device- a-SiC integrated with a-Si/a-Si tandem solar cellSTH- solar-to-hydrogen
Introduction
A-SiC:H photoelectrode PEC characteristics
The challenges of a-SiC:H for water splitting
---Corrosion resistance in aqueous electrolyte
---The band-edge of the a-SiC:H
---Charge carrier extraction at the a-SiC:H/electrolyte interface
Pathway towards STH conversion efficiency>10%
Conclusion
Hydrogen production by conventional methods Greenhouse gases
PEC water splitting: Use electricity from sunlight
Why hydrogen production by PEC ?
Goals*:
* R. Garland et al, DOE Hydrogen Production Review Meeting, Arlington, VA, June 12, 2008.
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Advantages of a-SiC:H*Lower Eg (band gap) allows more photocurrent
*Eg 1.9eV-2.3eV with varying carbon incorporation
*Uses the same deposition technique as the a-Si tandem junction, i.g. PECVD
Why amorphous SiC photoelectrode ?
Previously, 3.1% Solar-to-Hydrogen (STH) efficiency achieved in WO3+a-Si tandem PEC hybrid devices (from load-line analysis by HNEI) *
HER catalystStainless steel foil
a-Si nip/nip tandem solar cellsITO
WO3
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electrolyte
a-Si/a-Si tandem solar cell 0
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Voltage (V)
Cur
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STH
effi
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)
a-Si/a-Si tandem
a-Si/a-Si tandem, filtered by WO3 film
WO3 film
integrated HPE performance level
* MVS publication in Proc. SPIE, Vol. 6340, (2006)63400K.
Deposition system –cluster tool system M
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RF power: 10-20 W
Excitation frequency: 13.56 MHz
Pressure: 300-550 mTorrr
SiH4 flow rate: 20 sccm
CH4 flow rate: 0-20 sccm
H2 flow rate 0-100 sccm
Substrate temperature 200°C
Sputtering chamberZnO, ITO, Al, Mo…..
PECVD chambers
LoadLock
Main deposition parameters:
All a-SiC:H films, photoelectrodes, solar cells and the PEC hybrid devices were fabricated in the cluster tool PECVD/Sputtering System, designed and manufactured by MVSystems, Inc.
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a-SiC material and its pin Solar Cell M
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Device configuration and performance:
a-SiC(i) (~300 nm, Eg~ 2 eV)
a-Si(n+)
a-SiC(p+)SnO2 (Asahi U-type)
Ag
Glass
Light
For devices with thickness of ~ 100 nm, Jsc ~ 8.45 mA/cm2 achieved
* F. Zhu, J. Hu, I. Matulionis, Todd Deutsch， Nicolas Gaillard, A. Kunrath, E. Miller and A. Madan, "Amorphous Silicon Carbide Photoelectrode For Hydrogen Production Directly From Water Using Sunlight" Philosophic Magazine, 89:28,（ 2009） 2723-2739
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Voltage (V)
Cu
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(m
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Performance of a-Si tandem device
Current vs. Voltage
Configuration of a hybrid PEC cell
a-SiC(i), ~100nm
SnO2Glass
a-SiC(p)
a-Si p-i-n(top cell)
a-Si p-i-n(bottom cell)
a-SiC(p)
a-Si(n)
a-SiC(p)
a-Si(n)
a-Si(i), 150nm
a-SiC p-i
a-Si(i), 500nm
SN Jsc Voc FF Eff. (mA/cm2) (V) (%)
#A 6.795 1.6 0.717 7.79 #B 8.70 1.66 0.67 9.62
#A
#BComparison of PV performance
#A -used in the first hybrid PEC cell.• #B- improved device. Shall incorporate into the hybrid PEC device.
A-Si tandem solar cell
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Main Features of a-SiC p/i Photoelectrode: Behaves like a p-type photoelectrode
Saturated photocurrent: up to 8 mA/cm2
Flatband voltage: +0.26V (vs Ag/AgCl)
The flatband voltage of the a-SiC p/i photoelectrode is above the water oxidation half-reaction potential which means external voltage is required to initiate water splitting.
Flatband voltage Vfb vs. pH for an a-SiC p/i photoelectrode and a hybrid PEC device
a-SiC(p) (20 nm)
Glass
a-SiC(i) (200nm, 2.0eV)
textured SnO2
light
a-SiC p/i photoelectrode
The challenges of a-SiC:H for water splitting -the band-edge M
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[ Data measured by NREL and HENI]
• Vfb determined by the illuminated open-circuit voltage (IOCP).
• For the p-type photoelectrode, Vfb needs to be below H2O/O2
redox potential for water splitting.
• In the hybrid PEC device, the Vfb shifts by ~+1.6V or +0.97V
below H2O/O2 half-reaction potential at pH2
Flatband potential vs. pH
[ Data measured by NREL and HENI]
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pH
V v
s. A
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hydrogen
oxygen
a-SiC p-i
hybriddevice
textured SnO2
hybrid PEC cell
a-SiC (i), 100nm
Glass
a-SiC(p)
a-Si p-i-n(top cell)
a-Si p-i-n(bottom cell)
a-SiC(p)
a-Si(n)
a-SiC(p)
a-Si(n)
a-Si (i), 150nm
a-SiC p-i
a-Si (i), 500nm
light
The challenges of a-SiC:H for water splitting -the band-edge
charge carrier extraction problem at a-SiC/electrolyte interface
a-SiC(p) (20 nm)
Glass
a-SiC(i) (200nm, 2.0eV)
textured SnO2
light
textured SnO2
hybrid PEC cell
a-SiC (i), 100nm
Glass
a-SiC(p)
a-Si p-i-n(top cell)
a-Si p-i-n(bottom cell)
a-SiC(p)
a-Si(n)
a-SiC(p)
a-Si(n)
a-Si (i), 150nm
a-SiC p-i
a-Si (i), 500nm
light
The challenges of a-SiC:H for water splitting - Charge carrier extraction
3-electrode setup 2-electrode setup
0.33 mA/cm2
After HF etch for 30s, photocurrent increases and its onset shift anodically Photocurrent degrades and reverts to its initial value when the a-SiC photoelectrode is exposed to the air XPS measurement also verify this, please see the paper published in 2009*
Effect of surface SiOx on photocurrent M
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Effects of SiOx on a-SiC photoelectrode
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Potential (V-Ag/AgCl)
Cu
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m2)
pre - HF dip
post - HF dip
67hr later (in air)
* HF etch time: 30 sec
[ Data measured by NREL]
* F. Zhu, J. Hu, I. Matulionis, Todd Deutsch， Nicolas Gaillard, A. Kunrath, E. Miller and A. Madan, "Amorphous Silicon Carbide Photoelectrode For Hydrogen Production Directly From Water Using Sunlight" Philosophic Magazine, 89:28,（ 2009） 2723-2739
Test conditions:
Sample tested: hybrid PEC cell with ZnO/Ag back reflector Setup: 2-Electrode Counter electrode: RuO2
Electrolyte: buffer pH2 HF dip: 5% HF for 30 sec
Textured SnO2 coated with silver and ZnO
1.3 mA/cm2 achieved by removal of silicon oxide from a-SiC with HF etch.
This current density corresponds to 1.6% solar-to-hydrogen efficiency
Effect of surface SiOx on photocurrent of the hybrid PEC device
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Comparison with a Solid-State Configuration
a-SiC (i), 100nm
SnO2Glass
a-SiC(p)
a-Si p-i-n(top cell)
a-Si p-i-n(bottom cell)
a-SiC(p)
a-Si(n)
a-SiC(p)
a-Si(n)
a-Si (i), 150nm
a-SiC p-i
a-Si (i), 500nm
a-SiC (i), 100nm
Glass
a-Si (i), 150nm
a-Si (i), 500nm
ITOExposed to electrolyte
-4
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Potential (V)
Ph
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cu
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(m
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two-electrode test
1.3 mA/cm2
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Cu
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(mA
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Voltage (V)
glass/SnO2/p-i-n/p-i-n/p/a-SiC/ITO
Jsc>5.0 mA/cm2
light
light
The solid state version (right) shows the STH efficiency of hybrid PEC cell should be >6% Low current in hybrid PEC cell (left), indicating the charge carrier extraction problem at the a-SiC/electrolyte interface Surface modification is underway to improve the interface (The progress will be reported in SPIE conference @ San Diego, August, 2010 )
Test conditions: Sample tested: hybrid PEC cell Counter electrode: Pt Electrolyte: buffer pH2 (sulphamic acid solution with added potassium biphthalate) Current bias: 1.6 mA/cm2
Current vs. potential (before and after test)
Before testing
After 200-hr testing
H2 production throughout the test No degradation during durability/corrosion test for 200 hours (So far)
[ Data measured by NREL ]
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Potential (V vs. Ag/AgCl)
Ph
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(mA
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Initial
148 hrs
172 hrs
200 hrs
three-electrode test
textured SnO2
hybrid PEC cell
a-SiC (i), 100nm
Glass
a-SiC(p)
a-Si p-i-n(top cell)
a-Si p-i-n(bottom cell)
a-SiC(p)
a-Si(n)
a-SiC(p)
a-Si(n)
a-Si (i), 150nm
a-SiC p-i
a-Si (i), 500nm
light
The challenges of a-SiC for water splitting –corrosion resistance
