Future prospects of semiconductor materials for solar and photoelectrochemical cells

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Future prospects of semiconductor materials for
solar and photoelectrochemical cells
Solar to Fuel Future Challenges and
Solutions LBNL Workshop March 28 29, 2005
W. Walukiewicz Electronic Materials
ProgramMaterials Sciences Division Lawrence
Berkeley National Laboratory
This work was supported by the Director's
Innovation Initiative, National Reconnaissance
Office and by the Office of Science, U.S.
Department of Energy under Contract No.
DE-AC03-76SF00098.
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Collaborators

• J. Wu, K. M. Yu, W. Shan, J. W. Ager, J. Beeman,
  E. E. Haller, M. Scarpulla, O. Dubon, and J.
  Denlinger
• Lawrence Berkeley National Laboratory,
• University of California at Berkeley
• W. Schaff and H. Lu, Cornell University
• A. Ramdas and I. Miotkowski, Purdue University
• P. Becla, Massachusetts Institute of Technology

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Outline

• High Efficiency Solar Cell Concepts
• New semiconductors for multijunction solar cells
• GaxIn1-xN alloys
• Intermediate band solar cell materials
• Highly mismatched alloys (HMAs)
• II-Ox-VI1-x HMAs as intermediate band materials
• Group III-nitrides for photoelectrochemical cells
• Challenges and prospects

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Solar CellsUltimate Efficiency Limits

• Intrinsic efficiency limit for a solar cell using
  a single semiconducting material is 31.
• Light with energy below the bandgap of the
  semiconductor will not be absorbed
• The excess photon energy above the bandgap is
  lost in the form of heat.
• Single crystal GaAs cell 25.1 AM1.5, 1x
• Multijunction (MJ) tandem cell
• Maximum thermodynamically achievable efficiencies
  are increased to 50, 56, and 72 for stacks of
  2, 3, and 36 junctions with appropriately
  optimized energy gaps

Eg1 gt Eg2 gt Eg3
Cell 1 (Eg1)
Cell 2 (Eg2)
Cell 3 (Eg3)
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Multijunction Solar Cells
State-of-the art 3-junction GaInP/Ga(In)As/Ge
solar cell 36 efficient
M. Yamaguchi et. al. Space Power Workshop 2003
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Direct bandgap tuning range of In1-xGaxNPotential
material for MJ cells

• The direct energy gap of In1-xGaxN covers most of
  the solar spectrum
• Multijunction solar cell based on this single
  ternary could be very efficient

LBNL/Cornell work J. Wu et al. APL 80, 3967
(2002)
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InGaN is radiation hardelectron, proton, and He
irradiation
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Surface Electron Accumulation
CB
EFS
VB

• Surface/interface native defects (dangling bonds)
  are similar to radiation-induced defects

• High concentration of defects near surface
  Fermi level pinning

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P-type Doping of InN
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In1-xGaxN alloys as solar materials

• Significant progress in achieving p-type doping
• Exceptional radiation hardness established
• Surface electron accumulation in In-rich alloys
• Quality of InN/GaInN interfaces

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Multijunction vs. Multiband

• Multi-band
• Single junction (no lattice-mismatch)
• N bands ? N(N-1)/2 gaps
• ? N(N-1)/2 absorptions
• Add one band ? add N absorptions

• Multi-junction
• Single gap (two bands) each junction
• N junctions ? N absorptions
• Efficiency30-40

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Theoretical efficiency of Intermediate band solar
cells

• Intermediate Band Solar Cells can be very
  efficient
• Max. efficiency for a 3-band cell63
• Max. efficiency for a 4-band cell72
• In theory, better performance than any other
  ideal structure of similar complexity
• But NO multi-band materials realized to date

Luque et. al. PRL, 78, 5014 (1997)
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Highly Mismatched Alloys for Multiband Cells

• Oxygen in II-VI compounds has the requisite
  electronegativity and atomic radius difference
• XO 3.44 RO 0.073 nm
• XS 2.58 RS 0.11nm
• XSe 2.55 RSe 0.12 nm
• XTe 2.1 RTe 0.14
• Oxygen level in ZnTe is 0.24 eV below the CB edge
• Can this be used to form an intermediate band?
• Synthesis
• Very low solid solubility limits of O in II-VI
  compounds
• Nonequilibrium synthesis required

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Zn1-yMnyOxTe1-x Intermediate Band Material
K. M. Yu et. al., Phys. Rev. Lett., 91, 246403
(2003)
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Zn0.88Mn0.12O0.03Te0.97 Intermediate Band
Semiconductor
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Photovoltaic action
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How efficient can they be?Multi-band ZnMnOTe
alloys

• Calculations based on the detailed balance model
  predict maximum efficiency of more than 55 in
  alloys with 2 of O

• The location and the width of the intermediate
  band in ZnMnOxTe1-x is determined by the O
  content, x
• Can be used to maximize the solar cell efficiency

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Intermediate band semiconductors Challenges an
prospects

• Synthesis of suitable materials with scalable
  epitaxial techniques (MBE growth of ZnOxSe1-x
  achieved)
• N-type doping of intermediate band with group VII
  donors (Cl, Br)
• Control of surface properties of the PLM
  synthesized materials
• Other highly mismatched alloys GaPyNxAs1-x-y
• Fundamentals
• Nature of the intermediate band localized vs.
  extended
• Carrier relaxation processes

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Photoelectrochemical cells for hydrogen
generation
Joel W. Ager, Alexis T. Bell, Miquel Salmeron,
Wladek WalukiewiczElectronic Materials
ProgramMaterials Sciences Division Lawrence
Berkeley National Laboratory Chemical Sciences
Division
InN supportFY03 LDRD, FY04 Director's
Innovation Initiative, National Reconnaissance
Office
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J. A. Turner, Science 285, 687 (1999)
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Photoelectrochemical H2 generation
1.       Absorption of light near the surface of
the semiconductor creates electron-hole pairs.
2.       Holes (minority carriers) drift to the
surface of the semiconductor (the photo anode)
where they react with water to produce oxygen
2h H2O -gt ½ O2 (g) 2H 3.       Electrons
(majority carriers) are conducted to a metal
electrode (typically Pt) where they combine with
H ions in the electrolyte solution to make H2
2e- 2H -gt H2 (g) 4.       Transport of H
from the anode to the cathode through the
electrolyte completes the electrochemical
circuit. The overall reaction 2hn H2O -gt
H2(g) ½ O2 (g)
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Why is it hard to do?

• Oxides
• Stable but efficiency is low (large gap)
• III-Vs
• Efficiency is good but surfaces corrode
• Approaches
• Dye sensitization (lifetime issues)
• Surface catalysis
• No practical PEC H2 production demonstrated
• Efficiency and lifetime

Adapted from M. Grätzel, Nature 414, 388 (2001)
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What are the fundamental issues?

• Band structure engineering
• To match water redox potentials and achieve high
  solar efficiency
• Fundamental understanding of the
  electrode/electrolyte interface
• To accelerate water splitting reaction and reduce
  corrosion

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Why use nitrides?Direct bandgap tuning range of
InGaN

• The direct energy gap of In1-xGaxN covers most of
  the solar spectrum
• Multijunction solar cell based on this single
  ternary could be very efficient

LBNL/Cornell work J. Wu et al. APL 80, 3967
(2002)
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III-Nitrides tuning the band edges

• Their conduction and valence band edges straddle
  the H/H2 and O2/H2O redox potentials.
• They can be made with the optimal bandgap of 2.0
  eV
• Experimentally determined by our group
• They have superior corrosion resistance compared
  to other semiconductors of similar energy gaps.

InGaN J. Wu et al. APL 80, 3967 (2002) GaNAs J.
Wu et al., PRB
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PhotocurrentIn0.37Ga0.63N
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Surface modification conceptsCatalysis and
corrosion inhibition

• Catalysts can facilitate the oxidation of water
  on the anode and reduction of protons on the
  cathode
• Candidate materials
• Anode Pt, Pt/Ru alloys, RuO2, MoO3, ZrO2
• Cathode Porphyrins, phtalocyanins, ferrocenes
• Corrosion can be inhibited by an oxide coating

O2
H2O
hn
H
Catalyst
O
Photoanode
HO
H
H
h e
e
e
To cathode
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Fundamental and Practical Issues

• Synthesis of materials MBE, MOCVD, PLM
• Charge transport and doping
• Evaluate photo cathode (p-type semiconductor
  surface) vs. photo anode (n-type semiconductor
  surface) designs
• Measurements of band offsets
• Fundamental studies in-situ and ex-situ of the
  electrolyte-semiconductor interface
• Surface modification
• Kinetic H2 production and corrosion rates.