Schottky Tunnel Contacts for Efficient Coupling of Photovoltaics and Catalysts Christopher E. D. Chidsey Department of Chemistry Stanford University Collaborators: Paul C. McIntyre, Y.W. Chen, J.D. Prange, A. Scheuermann, M. Gunji, O. Hendricks Support: Precourt Institute for Energy Seed Grant; CIS Seed Funds; Stanford Graduate Fellowship, NSF Graduate Fellowship, GCEP
Storage: Energy Density Comparison Pumped Hydro Storage e.g. pump 1 L of water up the Hoover Dam Energy density 1.8 kj/l Li Ion Rechargeable Battery Energy density* 0.9-2.2 MJ/L Hydrogen Storage e.g. H 2 in metal hydrides Energy density 5-15 MJ/L Liquid Fuel e.g. 2 C 8 H 18 + 25 O 2 16 CO 2 + 18 H 2 O Energy density 35 MJ/L * http://www.greencarcongress.com/2009/12/panasonic-20091225.html Sorensen, B., Renewable Energy Conversion, Transmission and Storage. Elsevier. (2007) 2
Major Scientific Challenges of Photoelectrochemical Fuel Synthesis Oxidative stability of anode Proper alignment of band edges Optimizing solar absorption Making liquid fuels rather than hydrogen gas (not addressed in this work) 3
Potential, V (vs. NHE) Titanium Pourbaix Diagram 2 Why we want to use Titanium Dioxide Water Oxidation 1 0 Proton Reduction TiO 2-1 Ti 3+ Ti 2 O 3-2 TiO Ti 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 ph 4
Photoelectrochemical Electrolysis of Water Schematic of solar hydrogen synthesis by photolysis of water using a semiconducting photoanode with E g = 3.0 ev (such as TiO 2,* SrTiO 3 ). Basic Research Needs for Solar Energy Utilization, US DOE (2005) * A. Fujishima and K. Honda, Nature 238, 37-38 (1972). 5
Photoanode Selection Stable semiconductor absorbers tend to have large band gaps e.g. TiO 2 with 3.4eV Only able to absorb UV portion of solar spectrum A B Si TiO 2 Fe 2 O 3 M. Grätzel, Nature 414, 338-44 (2001). Use silicon with a smaller band gap but protect it with larger band gap, corrosion-resistant layer of TiO 2. Must also control band offsets and add additional voltage. 6
Background Protecting Si Photoanodes A. Bard et. al. (J. Electrochem. Soc., 1977, 124(2), 225-229.) There does not appear to be any advantage in depositing TiO 2 because of the inability to transfer holes from the substrate through the TiO 2. Conclusion: Don t Use TiO 2 H. Gerisher et. al. (J. Electrochem. Soc., 1983, 130(11), 2173-2179.) We disregard in our treatment the special case of such thin layers (< 50 Å) it is hardly possible to produce such thin layers without pinholes or larger defects. Conclusion: Don t Use Thin Layers Our Work: Use Thin Layers of TiO 2 7
Atomic-Layer-Deposition (ALD) Protection of Silicon Electrode Combine chemical stability of TiO 2 with efficient photoabsorption by Si substrate TDMAT (g) Saturated adsorption Coat thin TiO 2 by ALD as corrosion resistant tunnel oxide TiO 2(s) HNMe 2 (g) H 2 O (g) Deposit thin surface layer of a known water oxidation catalyst (e.g. Ir) Y.W. Chen et al., Nature Mater. 10, 539 (2011). 8
Water Oxidation in the Dark Electrode: Ir/TiO 2 /p+si Electrode: Ir/TiO 2 /p + -Si Anode: 2H 2 O (l) O 2(g) +4H + (aq)+4e - Y.W. Chen et al., Nature Mater. 10, 539 (2011). Use heavily-doped p+si as substrate Sufficient holes for oxidation 2 nm of TiO 2 as corrosion resistant tunnel oxide 3 nm of Ir: water oxidation catalyst and hole transport mediator Operation in acidic, neutral, and basic solutions Overpotentials at 1 ma/cm 2 1M NaOH: 384 mv ph 7: 346 mv 1M H 2 SO 4 : 332 mv Low overpotential Comparable to the best water oxidation anodes reported 9
Water Oxidation in Simulated Solar Light Electrode: Ir/TiO 2 /n-si Use lightly-doped n-si as substrate Holes must be photo-generated for efficient oxidation Without illumination No observable peaks With AM 1.5 illumination Overpotentials at 1 ma/cm 2 1M NaOH: -171 mv ph 7: -219 mv 1M H 2 SO 4 : -200 mv Large water oxidation current density below equilibrium (dark) potential Inferred photovoltage 550 mv Y.W. Chen et al., Nature Mater. 10, 539 (2011). 10
Light Electrolysis n-si Substrates 1 M H 2 SO 4 Current saturation under illumination: 26 ma/cm 2 60% charge collection Theoretical limit for Silicon: 43 ma/cm 2 11
Stability of Anodes n-si with Solar Illumination Hold spot of anode at a constant current of 5 ma/cm 2 1 M Base 1 M Acid Samples with TiO 2 Samples with TiO 2 Samples without TiO 2 Samples without TiO 2 12
Stability of Anodes TEM Before (left) and after (right) images of Ir/TiO 2 /Si anode for 3 hr stability Test Y.W. Chen et al., Nature Mater. 10, 539 (2011). 13
Stability of Anodes XPS Depth Profiling After Stability Test Protection with TiO 2 Y.W. Chen et al., Nature Mater. 10, 539 (2011). 14
Electronic Transport Characterization Ir/2 nm TiO 2 /p + -Si 2 nm TiO 2 /p + -Si Use Fe(CN) 6 3- /Fe(CN) 6 4- redox pair to study charge transfer efficiency in electrodes Fe(CN) 6 3- /Fe(CN) 6 4- redox reaction has low kinetic barrier (Scherer et al., J. Electroanal. Chem., 85, p77, 1977) Importance of Ir layer as carrier transport mediator No Fe(II)/Fe(III) peaks observed for TiO 2 /p + -Si samples without Ir top layer Large Fe(II)/Fe(III) peak with the thin Ir top layer Peak-to-peak splitting similar to conductive electrodes (e.g. ITO) Efficient electron transport explained by band structure Nearly flat band at equilibrium on p + - Si substrates 15
Electronic Transport Characterization Ir/2 nm TiO 2 /n-si light Ir/2 nm TiO 2 /p + -Si Ir/2 nm TiO 2 /n-si No oxidation wave observed without illumination Lack of holes in n-si in the dark Have to generate electron-hole pairs for oxidation reaction to proceed Difficult to do with thick Si depletion layer Oxidation wave recovers with illumination Electron-hole pairs supplied by incident photons E 0 shifts to the lower potential Effective photovoltage ~550mV when compared to the dark CV Similar to the photovoltage observed for water oxidation 16
TiO 2 Thickness Effects Amorphous TiO 2 as-deposited Film thickness can be controlled using ALD cycle number Films have smooth interfaces and uniform thickness A.G. Scheuermann et al., Energy Environ. Sci. DOI: 10.1039/c3ee41178h 17
TiO 2 Thickness Effects Ir/2 nm TiO 2 /p + -Si Ir/10 nm TiO 2 /p + -Si Standard ferri/ferrocyanide redox couple in water Peak-to-peak splitting measures barrier to electron transport from electrode to electrolyte Y.W. Chen et al., Nature Mater. 10, 539 (2011). Thin (2 nm) TiO 2 has a very small peak-to-peak splitting ~130 mv Thick (10 nm) TiO 2 has a much larger peak-to-peak splitting ~610 mv Greater barrier to hole transport through thick TiO 2 ALD enables growth of a thin and pinhole-free TiO 2 tunnel oxide facile carrier transport 18
Overpotential (mv) TiO 2 Thickness Effects: Overpotential Ir/TiO 2 /p + Si 600 500 400 300 200 100 Base ph7 Acid FFC 0 0 2 4 6 8 10 12 TiO 2 thickness (nm) Increasing TiO 2 thickness requires increasing overpotential for the same water oxidation rate; ~ 20 mv/nm TiO 2 At very small thicknesses, the overpotential for water splitting is approximately constant, suggesting it is not limited by electronic conduction A.G. Scheuermann et al., Energy Environ. Sci. DOI: 10.1039/c3ee41178h 19
Conduction Mechanism: Ir Solid State Contact Applied V G = -0.5 V on Ir contact V fb = +0.13 V Y.W. Chen et al., Nature Mater. 10, 539 (2011). Solid-state contact current-voltage measurements No solution here p + Si substrate 50 nm Ir layer as top contact Observe tunneling conduction by varying TiO 2 thickness by varying temperature & applied bias Thin TiO 2 Current almost independent of temperature Direct tunneling through the ALD-grown oxide Thick TiO 2 Current increases (by several orders of magnitude) with temperature Current is thermally activated and bulk limited 20
Capacitance (uf/cm2) Flat band voltage (V) Capacitance (uf/cm2) Solid-State MIS Characterization Good CV behavior of ultrathin films on p-si substrate No V fb shift with thickness Low fixed charge in oxide Predicted shifts with Φ M suggests Fermi-level unpinned 50nm Ir / x TiO 2 / native SiO 2 / p-si / 20nm Pt 2.5 2.0 1.5 1.0 0.5 2 1.5 1 0.5 0 3.0 Gate metal effect on V fb 800kHz 1kHz 2nm TiO 2 0.0-0.5-1.2-0.8-0.4 0.0 0.4 0.8 1.2 2.5 2.0 1.5 Theoretical range Observed 1.0 0.5 0.0 8nm TiO 2 Ir Pt Pd Ni Al -2.0-1.5-1.0-0.5 0.0 0.5 1.0 1.5 2.0 Gate Bias Gate (V) Metal A.G. Scheuermann et al., Energy Environ. Sci. DOI: 10.1039/c3ee41178h / / Gate Bias (V) 1nm TiO 2 800kHz 21
MIS Junction Band Structure 1-3 Bulk-limited electron hopping conduction through the TiO 2 layer and tunneling across the ultra-thin SiO 2 layer. Bulk conduction through TiO 2 contributes ~ 20 mv of overpotential increase per nm of thickness at J = 1 ma/cm 2 Corresponds to a bulk resistivity of 2x10 8 ˑcm 1. M. Perego et al., J. Appl. Phys. 2008, 103, 043509. 2. W. Mönch, J. Appl. Phys. 2010, 107, 013706. 3. M. Houssa et al., J. Appl. Phys. 2000, 87, 8615. The bulk resistivity of TiO 2 will depend on its oxygen stoichiometry, crystallinity, etc. 22
Conclusions Fuels generated from renewable energy can help accommodate intermittency. -high energy density Water is the only readily accessible source of electrons for solar fuel synthesis at large scale. Water oxidation is a kinetically difficult reaction that requires oxidation stable but catalytically active surfaces. Atomic layer deposited tunnel oxides can protect Si and, potentially, other high-quality semiconductor absorbers so they can be used in efficient solar-driven water splitting. Atomic layer deposition of ultrathin and pinhole-free TiO 2 produces a Schottky tunnel junction coupling a high quality semiconductor to a nanoscale oxidation catalyst. ALD-TiO 2 adds only a modest overpotential penalty of 20 mv/nm 23