the potential of a-SiC films for PEC-2010

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www.mvsystemsinc.com NHA conference May3-6, 2010 Potential of using a-SiC:H photoelectrode for water splitting Feng Zhua, Jian Hua, Ilvydas Matulionisa, Josh Gallona, 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 Contract # DE-FC36-07GO17105

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Focus of this talk PEC - photoelectrochemical a-SiC – hydrogenated amorphous silicon carbide hybrid device- a-SiC integrated with a-Si/a-Si tandem solar cell STH- solar-to-hydrogen Introduction A-SiC:H photoelectrode PEC characteristics Challenges of a-SiC:H for water splitting ---Corrosion resistance in aqueous electrolyte ---The band-edge alignment of the a-SiC:H with electrolyte ---Charge carrier extraction at the a-SiC:H/electrolyte interface Pathway towards STH conversion efficiency>10% Conclusions

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Hydrogen production by conventional methods greenhouse gases consume huge energy, i.g. electrolytical, thermochemical PEC : split water using sunlight Why hydrogen production by photoelectrochemical (PEC) technique?

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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 catalyst Stainless steel foil a-Si nip/nip tandem solar cells ITO WO3 light electrolyte a-Si/a-Si tandem solar cell * MVS publication in Proc. SPIE, Vol. 6340, (2006)63400K.

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Deposition system –cluster tool system 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 chamber ZnO, ITO, Al, Mo….. PECVD chambers Load Lock 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. www.mvsystemsinc.com

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a-SiC material and its solar cell performance Device configuration and performance: a-SiC(i) (~300 nm, Eg~ 2 eV) a-Si(n+) a-SiC(p+) SnO2 (Asahi U-type) Light For devices with thickness of ~ 100 nm, Jsc ~ 8.45 mA/cm2 achieved As CH4 flow increases, the bandgap, Eg, increases Eg vs. CH4/(CH4+SiH4) * 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" Philosophical Magazine, 89:28,（2009）2723-2739

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Performance of a-Si tandem device Current vs. Voltage Configuration of a hybrid PEC cell a-SiC(i), ~100nm SnO2 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 A-Si/a-Si tandem device performance using same bandgap, 1.75eV A-Si tandem solar cell

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Behaves like a p-type photoelectrode Photocurrent: >8 mA/cm2 at -1.5v, vs. SCE Flatband voltage: +0.26V (vs Ag/AgCl) The flatband voltage of the a-SiC 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 a-SiC photoelectrode a-SiC photoelectrode The challenges of a-SiC:H for water splitting - The band-edge alignment of the a-SiC:H with electrolyte [ Data measured by NREL and HENI] extra voltage H2/H2O O2/H2O 1.23eV Pt SCE-saturate calomel electrode

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In the hybrid PEC device, the Vfb shifts by ~+1.6V or +0.97V below H2O/O2 half-reaction potential at pH2 Flatband voltage vs. pH for hybrid PEC device [ Data measured by NREL and HENI] The challenges of a-SiC:H for water splitting - The band-edge alignment of the a-SiC:H with electrolyte H2/H2O O2/H2O 1.23eV Pt

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Photocurrent is low charge carrier extraction problem at a-SiC/electrolyte interface The challenges of a-SiC:H for water splitting - Charge carrier extraction 3-electrode setup 2-electrode setup 0.33 mA/cm2

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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 Effects of SiOx on a-SiC photoelectrode * HF etch time: 30 sec [ Data measured by NREL] * Y. Zhang, K. George, M. Bär, C. Heske, J. Hu, F. Zhu, and A. Madan, “Chemical and Electronic Structure of a-SiC Thin Films for Photoelectrochemical Water Splitting”,2009 Materials Research Society Spring Meeting, San Francisco, CA, April 13 – 17, 2009 . 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" Philosophical Magazine, 89:28,（2009）2723-2739 H2O/H2 H2O/O2 1.23eV 2.0eV H2O/H2 H2O/O2 2.0eV With SiOx layer Without SiOx layer

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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 1.3 mA/cm2 achieved by removal of SiOx layer from a-SiC surface. solar-to-hydrogen efficiency 1.6% [ Data measured by HNEI ] Increase of photocurrent by removal of surface SiOx layer

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Hybrid PEC versus solid state configuration Jsc~5mA/cm2 Possible STH efficiency ~6% Jsc=1.3mA/cm2 STH efficiency ~1.6% solid state configuration Hybrid PEC cell 2.0eV Surface modification* charge carrier extraction problem ITO *The progress will be reported in SPIE conference @ San Diego, August, 2010

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Sample tested: hybrid PEC cell Counter electrode: Pt Electrolyte: buffer pH2 (sulphamic acid solution with added potassium biphthalate) Current bias: 1.6 mA/cm2 Before testing After 200-hr testing H2 production throughout the test No degradation after 200 hours of corrosion test [ Data measured by NREL ] Good durability of a-SiC photoelectrode

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Road map towards STH efficiency > 10% a-Si/a-Si nc-Si/a-Si

Summary : 

Summary Photocurrent of the hybrid PEC device has been improved to ~1.3 mA/cm2, STH of ~1.6% The hybrid PEC cell exhibits excellent durability in pH2 electrolyte for ~200 hours (so far tested). In solid state version, the device has achieved a current density of > 5 mA/cm2 (possible STH efficiency >6%). Surface modification is underway to improve charge carrier extraction problem at the a-SiC/electrolyte interface* Simulation has shown the pathway toward STH>10% *The progress will be reported in SPIE conference @ San Diego, August, 2010

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And thanks Ed Valentich for assistance in sample preparation. Thanks