Lecture 18-19 Photoelectrochemical Water Splitting Reference. N.S. Lewis et al, Solar water splitting, Chem. Rev. 2010 110, 6646-6473. Lecture note http://les.kaist.ac.kr/B_Lecture

Sun Powered by nuclear fusion Surface temperature ~ 6000K Behaves ideal black body radiation Total power emitted by sun is 9.5  1025 W 150  109 m away from Earth Life on Earth based on Sun’s energy, i.e., photosynthesis

How Much Sunlight We Get? Power density of sun given by the blackbody radiation 𝐻 𝑠𝑢𝑛 =𝜎 𝑇 4 =5.961 × 10 7 ( 𝑊 𝑚 2 ) 𝐻 0 = 𝑅 𝑠𝑢𝑛 2 𝐷 2 𝐻 𝑠𝑢𝑛 Power density at a body D away from sun We get ~1366 W/m2 at Earth’s outer atmosphere

Atmospheric Effects Atmospheric effects have several impacts on the solar radiation at the Earth's surface. The major effects for photovoltaic applications are: a reduction in the power of the solar radiation due to absorption, scattering and reflection in the atmosphere a change in the spectral content of the solar radiation due to greater absorption or scattering of some wavelengths the introduction of a diffuse or indirect component into the solar radiation local variations in the atmosphere (such as water vapor, clouds and pollution) which have additional effects on the incident power, spectrum and directionality. * Scattered light = diffused light = ~ 10% of direct light From: PV CD-ROM

Air Mass The Air Mass is the path length which light takes through the atmosphere normalized to the shortest possible path length (that is, when the sun is directly overhead). The Air Mass quantifies the reduction in the power of light as it passes through the atmosphere and is absorbed by air and dust. 𝐴𝑀=1/cos⁡(𝜃) AM 1: Sun directly overhead AM 1.5 G: Standard, 1 kW/m2 G (Global): Direct and diffused sunlight D (Direct): Direct sunlight only AM0: just above atmosphere (space application) Zenith angle From: PV CD-ROM

Solar Spectrum 𝐻 𝑠𝑢𝑛 =𝜎 𝑇 4 𝜆 𝑝 =2900/𝑇 Black body radiation

Solar Spectrum Black body radiation AM0

Solar Spectrum Black body radiation AM0 AM1.5G

Solar Spectrum Black body radiation AM0 AM1.5G

Solar Energy Potential Worldwide solar energy Theoretical: 120,000 TW Energy in 1 hour of sunlight 430 EJ http://en.wikipedia.org/wiki/Photovoltaics (using 8% efficient solar cells) 1 hour of sunlight = 1 year of human energy consumed (2002)

(Nate Lewis)

The Need for Solar Fuels (Nocera, EES 2010)

(Nate Lewis)

Photoelectrochemical (PEC) cells Solar H2 Production : Photoelectrochemical (PEC) cells 2H2O O2 4H+ + 4e- 2H2 H2 p-Semiconductor Counter Electrode 4H+ 4e- As we’ve seen this week, there are many interesting talks about producing Hydrogen fuels using sunlight and water at semiconductor interfaces. Briefly in ideal photoelectrochemical cells as schematically shown here, a semiconductor with band-gap over 1.7 eV can split water to produce H2 at semiconductor/water interface with sunlight. But unfortunately, there are very few photoelectrochemical materials and systems satisfies reliable and efficient PEC water splitting. So there are intense research on searching materials and systems, such as III-V and metal oxide semiconductors. The ideal fuel: 2H2O + 4e- + 4h+  2H2 + O2 Photons provide needed ~1.7 eV (1.23 eV + overpotential) per e-h pair

Fujishima and Honda - Illuminated n-TiO2 - Nature 1972, cited > 12000 times

Energetics of Water Splitting

Earth abundant and environmentally benign J. Turner, DOE review (2007)

Water Splitting Efficiency Solar-to-hydrogen (STH) efficiency Applied bias STH efficiency

Department of Energy (DOE) Target

PEC Device Configurations

Single Photoelectrodes PEC Configuration 1 Single Photoelectrodes

Theoretical STH Efficiency of Single Photoelectrodes Jaramillo et al, EES 2013

Many Assumption Required Caution! Many Assumption Required

Materials for PEC Water Splitting NHE Bandgap too large Corrosion Si 1.12 eV Grätzel, Nature 414, 338 (2001).

No Known Single Materials to Split Water without External Bias

PEC Configuration 2 p/n PEC Cell Originally proposed and demonstrated by Dr. Arthur J. Nozik at NREL in 1977!

Operating Principle

Theoretical STH Efficiency of p/n PEC Cells

Potential Materials

Photoanodes

Band Structures

State of the Art Fe2O3 Photoanode J. Lee et al, Sci. Rep. 2013

State of the Art BiVO4 Photoanode Krol et al, Nat Comm 2013

State of the Art Si Photocathode J. Oh et al., Energy & Envir Science. 2011 500 nm 2H+ H2 70 mV black Si polished Si 200 mV Also, we observe that nanoporous black Si reduced overpotential for H2 production. For example, black Si shows anodic shift of on-set voltage about 70 mV and about 200 mV when it’s fully operating. Comparing the dotted line and red solid line clearly show that observed anodic shift originates from nanostructure of black Si itself. We believe that increased surface area of black Si facilitate H2 production at low overpotential by providing more reaction sites. However, we can not completely rule out catalytic effects of residual Au atoms which exists about ~ 1 ppm level on Si.

Effect of Pt Nanoparticles polished Si 160 mV, 35% current density loss mention Jsc at the reversible voltage black Si E0 (2H+/H2)

Buried pn Junction for Additional Anodic Shift 2H+/H2 Vph Light Si without pn junction water p-Si Dark 2H+/H2 Vph Light water n+-Si p-Si Dark Si with pn junction Buried pn junction boosts the photovoltage for PEC water splitting Boettcher et al., JACS 2011

Buried Black Si Photocathode Jihun Oh et al, Nature Nanotechnology 2012 500 ~ 600 nm deep nanoporous layer Process compatible with standard PV manufacturing lines (POCl3, Al BSF) Achieved 18.2% efficiency Liquid etch baths would replace $M silicon nitride tool no dielectric AR coating Evaporated Pt So we fabricated black silicon solar cells using a conventional PV manufacturing lines using POCl3 diffusion and oxidation surface passivation. And we achieved 16.8% efficiency. While that is about absolute 3 % more efficient than previous results, the efficiency is still lower than what we expect from the low reflectance.

Pt-decorated Black Si with Buried pn Junction ~ 490 mV say that Pt deposition is different. E0 (2H+/H2) Buried pn junction boosts the photovoltage for PEC water splitting

Pt-decorated Black Si with Buried pn Junction - Effect of Counter Electrode - Pt CE (Dark) RuO2 CE (Dark) h=0.16% Can be applicable to oxide photoanodes!

State of the Art InP Photocathode Javey, Angew. Chemie 2012

Very Few Examples of p/n PEC Diodes Why? 1 2

PEC Configuration 3 PEC-PV Cell

Theoretical STH Efficiency

GaInP2 on GaAs PEC/PV Cells (Turner et al, Science 1998)

Band Diagram in Dark

Band Diagram under Illumination

PEC Configuration 4 PV Cell

Triple Junction a-Si PV Cells (Bockris et al, APL 1989)

Nocera’s Artificial Leaf (Nocera et al, Science 2011)

Role of Co-catalysts

HER OER

PEC Co-catalysts Requirement

Need for Nanostructures - New AR Technology - l d An effective medium if d < l

Black Si for Solar Energy Conversion and Storage Nanostructured black Si for broadband AR, - Provide density-graded layer to eliminate the air-Si interface n h n1 = 3.42 (Si) n0 = 1 PV cells with black Si Excellent AR on off-normal light mean better angular performance No SiNx AR coatings  low cost J.Oh et al., EES 2011 62

Scaling Law in d/l over 2 Orders of Magnitude! Branz et al, APL 2009 R~5% d~l/4 with C~8 Need > 250 nm grade in Si PV Anti-reflection with l/4 for 1000 nm near-IR photons

Broadband Anti-reflection Now, reflection measurement confirms that 3 min black etch can reduce average reflection lower than 2 % over entire above bandgap solar spectrum. 3 min black etch can reduce solar spectrum weighted RAVE < 2 %.

Needs for Nanostructures 2 Many examples from nanostructured metal oxide photoanodes!

Department of Energy (DOE) Target

Despites more than 40 years of research, there are still many challenges left to achieve > 15% STH. This field is called “Holy Grail of Chemistry” and needs ambitious Templar Knights like YOU!