1. Lecture 18-19 Photoelectrochemical Water Splitting
Reference.
N.S. Lewis et al, Solar water splitting, Chem. Rev ,
Lecture note

2. 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

3. How Much Sunlight We Get?
Power density of sun given by the blackbody radiation
𝐻 𝑠𝑢𝑛 =𝜎 𝑇 4 =5.961 × ( 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

4. 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

5. 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

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

7. Solar Spectrum
Black body radiation
AM0

8. Solar Spectrum
Black body radiation
AM0
AM1.5G

9. Solar Spectrum
Black body radiation
AM0
AM1.5G

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

11. (Nate Lewis)

12. The Need for Solar Fuels
(Nocera, EES 2010)

13. (Nate Lewis)

14. 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

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

16. Energetics of Water Splitting

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

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

19. Department of Energy (DOE) Target

20. PEC Device Configurations

21. Single Photoelectrodes
PEC Configuration 1
Single Photoelectrodes

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

23. Many Assumption Required
Caution!
Many Assumption Required

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

25. No Known Single Materials to Split Water without External Bias

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

27. Operating Principle

28. Theoretical STH Efficiency of p/n PEC Cells

29. Potential Materials

30. Photoanodes

31. Band Structures

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

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

38. 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.

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

40. 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

41. 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.

42. 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

43. 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!

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

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

47. PEC Configuration 3
PEC-PV Cell

48. Theoretical STH Efficiency

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

50. Band Diagram in Dark

51. Band Diagram under Illumination

54. PEC Configuration 4
PV Cell

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

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

58. Role of Co-catalysts

59. HER
OER

60. PEC Co-catalysts Requirement

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

62. 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

63. 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

64. 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 %.

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

66. Department of Energy (DOE) Target

67. 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!