future prospects of semiconductor materials for solar and photoelectrochemical cells
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Future prospects of semiconductor materials for solar and
photoelectrochemical cells
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.
Solar to Fuel – Future Challenges and Solutions
LBNL Workshop March 28 – 29, 2005
LBNL solar workshop
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 UniversityA. Ramdas and I. Miotkowski, Purdue UniversityP. Becla, Massachusetts Institute of Technology
LBNL solar workshop
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
LBNL solar workshop
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
AM
1.5
sol
ar fl
ux(1
021 p
hoto
ns/s
ec/m
2 /m
)
1 2 3 4Energy (eV)
1
2
3
4
5
Cell 1 (Eg1)
Cell 2 (Eg2)
Cell 3 (Eg3)
Eg1 > Eg2 > Eg3
LBNL solar workshop
Multijunction Solar CellsState-of-the art 3-junction GaInP/Ga(In)As/Ge solar cell: 36 % efficient
M. Yamaguchi et. al. – Space Power Workshop 2003
LBNL solar workshop
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)
LBNL solar workshop
InGaN is radiation hardelectron, proton, and He+ irradiation
LBNL solar workshop
Surface Electron Accumulation
Surface/interface native defects (dangling bonds) are similar to radiation-induced defects
VB
CBEg = 0.7eV
Surface Bulk
EF
~1eV
EFS
High concentration of defects near surface – Fermi level pinning
-7.5
-7.0
-6.5
-6.0
-5.5
-5.0
-4.5
-4.0
0 0.2 0.4 0.6 0.8 1
ECBE
EVBE
Ban
d E
dge
(eV
, rel
ativ
e to
vac
uum
leve
l)
GaNE
g=3.4eV
InNE
g=0.7eV
GaAsE
g=1.4eV
GaInPE
g=1.9eV
X (Ga)
In1-x
GaxN
EFS
LBNL solar workshop
P-type Doping of InN
-1000
-500
0
500
1000
1500
0 50 100 150 200 250
Ene
rgy
(meV
)
Depth (Å)
CB
VB
EF
NA = 1019
ND = 1017
Eb(acc)
= 0
LBNL solar workshop
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
LBNL solar workshop
Multijunction vs. Multiband
junction1
junction2
junction3
I
Multi-junction• Single gap (two bands) each junction• N junctions N absorptions• Efficiency~30-40%
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
LBNL solar workshop
0
Ei
Eg
Ec
iv
cicv
VB
IB
CB
qV
Luque et. al. PRL, 78, 5014 (1997)
Theoretical efficiency of Intermediate band solar cells
Intermediate Band Solar Cells can be very efficient Max. efficiency for a 3-band cell=63% Max. efficiency for a 4-band cell=72% In theory, better performance than any other ideal structure of similar complexity
But NO multi-band materials realized to date
LBNL solar workshop
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 nmXS = 2.58; RS = 0.11nmXSe = 2.55; RSe = 0.12 nmXTe = 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
-2.0
0.0
2.0
5.2 5.6 6.0 6.4 6.8
VBCB
Ene
rgy
(eV
)
Lattice parameter (Å)
EFS
EO
CdTe
CdTe
MnTe
MnTe
ZnTe
ZnTeZnSe
ZnSe
MgTe
MgTe
LBNL solar workshop
Zn1-yMnyOxTe1-x: Intermediate Band Material
K. M. Yu et. al., Phys. Rev. Lett., 91, 246403 (2003)
LBNL solar workshop
Zn0.88Mn0.12O0.03Te0.97: Intermediate Band Semiconductor
LBNL solar workshop
Photovoltaic action
LBNL solar workshop
How efficient can they be?Multi-band ZnMnOTe alloys
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
30
35
40
45
50
55
60
2.1
2.2
2.3
2.4
2.5
2.6
2.7
0 0.01 0.02 0.03
max
imum
effi
cien
cy (%
)
optimal operation voltage (V
)
oxygen mole fraction
Zn0.88
Mn0.12
OxTe
1-x
EO=2.06eV
EM=2.32eV
C=3.5eV
Calculations based on the detailed balance model predict maximum efficiency of more than 55% in alloys with 2% of O
1.6
1.8
2.0
2.2
2.4
2.6
2.8
0.00 0.05 0.10 0.15 0.20
Zn0.88
Mn0.12
OxTe
1-x
E_E
+
0 0.01 0.02 0.03 0.04
ener
gy (e
V)
CLM
2x (eV2)
EM
=2.32eV
EO=2.06eV
O mole fraction, x
E_
E+
LBNL solar workshop
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
LBNL solar workshop
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 support:FY03 LDRD, FY04 Director's Innovation Initiative, National Reconnaissance Office
LBNL solar workshopJ. A. Turner, Science 285, 687 (1999)
LBNL solar workshop
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 -> 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+ -> H2 (g)
4. Transport of H+ from the anode to the cathode through the electrolyte completes the electrochemical circuit.
The overall reaction : 2h + H2O -> H2(g) + ½ O2 (g)
LBNL solar workshop
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 lifetimeAdapted from M. Grätzel, Nature 414, 388 (2001)
LBNL solar workshop
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
LBNL solar workshop
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)
LBNL solar workshop
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
LBNL solar workshop
PhotocurrentIn0.37Ga0.63N
LBNL solar workshop
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
H2OHO O H H
e
H+ h
h + e
e
Catalyst Photoanode
O2
To cathode
LBNL solar workshop
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.
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