New concepts and materials for solar power conversion

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New concepts and materials for solar power
conversion
W. Walukiewicz Lawrence Berkeley National
Laboratory, Berkeley CA Rose Street Labs Energy,
Phoenix AZ In collaboration with EMAT-Solar
group http//emat-solar.lbl.gov/
This work was supported by the Director's
Innovation Initiative, U.S. Department of
Energy under Contract No. DE-AC03-76SF00098 Rose
Street Labs Energy
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Collaborators
J. Ager, K. M. Yu, L. Reichertz, Z.
Liliental-Weber, V. Kao, J. Denlinger, O. Dubon,
E. E. Haller, N. Lopez, J. Wu LBNL and UC
Berkeley R. Jones, K. Alberi, X. Li, M. Mayer,
R. Broesler, N. Miller, G. Brown Students, UC
Berkeley W Schaff (Cornell University), P. Becla
(MIT), C. Tu (UCSD), A. Ramdas (Purdue
University), J. Geisz (NREL), M. Hoffbauer
(LANL), S. Novikov and T. Foxon (University of
Nottingham)
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Outline

• Introduction
• Group III-nitride semiconductors
• Hybrid and single p/n junction tandem cells
• Highly mismatched alloys
• Intermediate band cells
• Photoelectrochemical cells
• Summary

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The Energy Challenge

• ? With a projected global population of 12
  billion by 2050 coupled with moderate economic
  growth, the total global energy consumption is
  estimated to be 28 TW. Current global use is
  14 TW.
• ? To cap CO2 at 550 ppm (twice the
  pre-industrial level), most of this additional
  energy needs to come from carbon-free sources.
• ? With the exception of nuclear power and
  radioactive decay
• sun is the origin of all renewable and
  non-renewable energy sources on Earth
• ? Solar energy is the largest non-carbon-based
  energy source (100,000 TW).

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US Department of Energy Goals
Adapted from P. Alivisatos
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(No Transcript)
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Solar Energy Potential

• Theoretical 1.2x105 TW solar energy potential
• Energy in 1 hr of sunlight ? 14 TW for a year
• Practical 600 TW solar energy potential
  (50 TW - 1500 TW depending on land fraction etc.
  WEA 2000) Onshore electricity generation
  potential of 60 TW (10 conversion
  efficiency)
• Photosynthesis 90 TW

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Solar Energy Utilization
Fuel
Light
Electricity
Fuels
Electricity
e
sc
M
Semiconductor/Liquid Junctions
Photosynthesis
Photovoltaics
Adapted from Nathan S. Lewis, 1998
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For PV or PEC to provide the full level of C-free
energy required for electricity and fuelsolar
power cost needs to be 2 cents/kWh (0.40/Wp)
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Current Technologies

• Bulk silicon, single crystal, multicrystalline
  (first generation)
• Power conversion efficiencies 14 to 23
  (2.5/Wp)
• Advantages mature technology abundant, nontoxic
  material
• Disadvantages close to the efficiency limit
• Thin films (second generation)
• Amorphous silicon (inexpensive but low
  efficiency, 8 10 )
• CdTe (inexpensive synthesis, low efficiency,
  toxic element Cd) (1/Wp)
• CuInGaSe2 (inexpensive synthesis, rare expensive
  elements, In)
• Multijunction cells (potential 3rd generation)
• Triple junction, Ge/GaInAs/GaInP
• Very efficient (more than 40 record), Very
  expensive (space applications, concentrating
  systems for terrestrial applications).

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How to improve the efficiency?Multijunction
approach vs. Multiband (Intermediate Band)
The intermediate band serves as a stepping
stone to transfer electrons from the valence to
conduction band. Photons from broad energy
range are absorbed and participate in generation
of current.

• Each of the cells efficiently converts photons
  from a narrow energy range.
• Band gaps are selected for optimum coverage of
  the solar energy spectrum.
• Strict materials requirements
• Complex, expensive technology

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Multijuction solar cells

• A stack of single gap solar cells.
• Each of the cells uses different part of solar
  spectrum.
• The open circuit voltage is the sum of the VOCs
  of individual cells.
• Requires current matching.

Larger open circuit voltage
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Three-Junction Solar Cells

• Efficiencies up to 41
• Six different elements
• Three different dopants
• Practically used
• 3-junction cells
• Research
• 4 to 5 junctions

Could this be simplified?
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Full solar spectrum nitrides

• The direct energy gap of In1-xGaxN covers most
  of the solar spectrum

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What is unusual about InN?

• InN has electron affinity of 5.8 eV, larger than
  any other semiconductor
• Extreme propensity for native n-type conduction
  and surface electron accumulation for InN and
  In-rich InxGa1-xN

average energy of native defects
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Integration of InGaN with Si

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N-InGaN/p-Si heterojunction
Typically 60 to 100 nm AlN or GaN buffer is grown
on (111) Si
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InGaN/Si heterojunctions
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InGaN-Si tandem cell
Potential for a high efficiency tandem with
In0.45Ga0.55N on Si double junction cell
Low resistance contact experimentally confirmed
for n-In0.38Ga0.62N on p-Si
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Demonstrated GaN/Si tandem
High quality GaN p-n junction grown on Si p-n
junction using MBE
VOC Si junction only 0.55 V GaN junction
only 1.6 V GaNSi junction 2.4 V No current
matching
Nitride-Si tandem action demonstrated
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External Quantum Efficiency
Clearly observable tandem action
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Highly Mismatched Alloys forIntermediate Band
Cells
The intermediate band serves as a stepping
stone to transfer electrons from the valence to
conduction band. Photons from broad energy
range are absorbed and participate in generation
of current.
Major technological advantage requires single
p/n junction only
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Band Anticrossing in Highly Mismatched Alloys
(HMAs)
Electronegativities, X and atomic radii, R
W. Shan etl al., Phys. Rev. Lett. 82, 1221-1224
(1999) J. Wu et al. Semicon. Sci. Technol. 17,
862 (2002).
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Intermediate Band Solar Cell GaNAs
Blocked Intermediate Band (BIB)
Unblocked Intermediate Band (UIB)
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Intermediate band cell proof of principle
Two thresholds
Two External Quantum Efficiency (EQE) thresholds
VB to IB and VB to CB
Phys. Rev. Lett. in print
Clear evidence for an operational intermediate
band photovoltaic device
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Band Anticrossing in HMAs

• Localized level above CBE and interaction with CB
  
• GaAs(N), ZnSe(O), CdTe(O)
• Localized level below CBE and interaction with CB
• GaAsP(N), ZnTe(O)
• Localized level above VBE and interaction with VB
  
• GaN(As), GaN(Bi), ZnSe(Te), ZnS(Te), ZnO(Se),
  GaAs(Mn)
• Localized level below VBE and interaction with VB
  
• GaAs(Bi), GaAs(Sb), Ge(Sn)

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Photoelectrochemical Cells (PECs)

• Material requirements
• Band gap must be at least 1.8-2.0 eV but small
  enough to absorb most sunlight
• Band edges must straddle Redox potentials
• Fast charge transfer
• Stable in aqueous solution

2hn H2O -gt H2(g) ½ O2 (g)
Counter electrode
H2O/H2

H2O/O2
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Are there any simple materials suitable for solar
PECs?
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GaNAs in the whole composition range

• GaN1-xAsx alloys over the entire composition
  range were grown by plasma-assisted MBE at low
  temperatures
• Alloys are amorphous for 0.15ltxlt0.8, and
  crystalline for other compositions
• Sharp optical absorption gives well-defined
  bandgaps
• Bandgap and band edge tunable in a broad range ?
  potential applications in energy technologies

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Engineering of the Band Offsets for Optimal PECs

• In both GaN1-xAsx and GaN1-xSbx the valence band
  edge moves upward providing better match to
  O2/H2O potential. Additional alloying with In
  reduces the band gap for better utilization of
  the solar spectrum
• CBM remains nearly unchanged as a function of x

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Summary

• New solar concepts are developed based on the
  progress in understanding of the electronic
  structure of complex semiconductor systems
• Highly semiconductor alloys allow for electronic
  band structure engineering through an independent
  control of the conduction and the valence band
  offsets.
• Better understanding of the properties of
  surfaces and interfaces of the dissimilar
  materials essential for the new concepts of high
  efficiency solar and photoelectrochemical cells.

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Intermediate band cell proof of principle
Two thresholds
Clear evidence for an operational intermediate
band photovoltaic device