Transcript Document

Novel Materials for Heat-Based Solar Cells
L.P. Priestley and T.H. Gfroerer (Davidson College)
M.F. Fairley (Spelman College) and M.W. Wanlass (National Renewable Energy Lab)
Abstract
We are studying a set of materials that may be useful for a promising new technology called thermophotovoltaics (TPV). TPV cells are similar
to solar cells, but they convert radiant heat (rather than light) into electricity. To optimize the efficiency of these devices, we seek to decrease
the threshold energy for absorption so that more radiant heat is absorbed in the cell. In the particular material that we are investigating, Indium
Gallium Arsenide (InGaAs), the threshold energy can be reduced by increasing the ratio of Indium atoms to Gallium atoms. However,
changing this ratio produces a difference in atomic spacing between the InGaAs and the underlying substrate material, which is required for
crystal growth. This difference (called lattice mismatch) leads to the formation of defects, which usually has a deleterious effect on the fate of
heat-generated excitations in the cell. Ideally, all heat-generated excitations are swept out of the cell by an internal electric field, producing
electricity. In reality, the excitations can be lost by a variety of recombination mechanisms during their brief residence in the cell. We have
studied the optical properties of InGaAs as a function of the Indium-to-Gallium ratio in order to gauge how lattice mismatch and other
fundamental properties associated with the InGaAs material affect these mechanisms. Ultimately, our work should facilitate the design of more
efficient thermophotovoltaic cells.
Recombination (Loss) Mechanisms
Motivation: Thermophotovoltaic (TPV) Power
Heat
Conduction Band
Blackbody Radiation
PHOTON
(LIGHT)
Heat Source
Semiconductor TPV
Converter Cells
Blackbody Radiator
Defect
Level
PHONONS
(HEAT)
Valence Band
Radiative Recombination
(produces light)
TPV Cells are designed to convert infrared blackbody radiation into electricity.
Defect-Related Recombination
(produces heat)
How TPV Cells Generate Electricity
P~10-1
P~(0.5)10~10-3
P~(0.5)16~10-5
-
E-Field
- -
Conduction Band
P~10-5
P~10-3
-
P~(0.5)4~10-1
ELECTRON
Bandgap
PHOTON
(LIGHT)
Probable Transition
Improbable Transitions
HOLE
Valence Band
+
+
E-Field
+
+
When a blackbody photon (with energy exceeding the bandgap) is absorbed, an electron is excited into the
conduction band, leaving a hole behind in the valence band. If they do not recombine, an internal electric field
sweeps the electrons and holes away, creating electricity.
Electrons can recombine with holes in the valence band by releasing a photon (light) or many phonons (heat).
These processes reduce the efficiency of a TPV cell. The probability P of transitions involving phonon
emission depends on the number of phonons required, which is determined by the position of the defect level
in the gap.
Density of States
16
10
Density of States (cm-3eV-1)
Efficiency of TPV Cells
Recycling
12
10
8
10
4
10
0
10
0.0
Reflector
100
0.4
0.2
0.4
0.2
5.6
InAs
InAs
5.7
5.8
5.9
6.0
6.1
spacing
(Angstroms)
AtomAtom
Spacing
(Angstroms)
0.0
0.0
0.5
1.0
1.5
2.0
0.6
Conduction Band
2.5
Energy (eV)
Energy
(eV)
60
40
20
0
19
Increasing the Indium concentration in the InGaAs lowers the bandgap Eg and increases the fraction of
blackbody radiation that is absorbed in the cell.
EV
heat
10
21
Energy
23
10
10
EC
EV Energy EC
60
40
EV Energy EC
20
Eg = 0.68 eV
0
25
10
Recombination Rate (cm-3s-1)
80
Log(DOS)
0.6
Radiative Efficiency (%)
0.8
80
Log(DOS)
0.6
Radiative Efficiency (%)
0.8
light
T = 1300C
C
T=1300
Substrate
Severe
Severe Mismatch
Mismatch
1.0
100
Eg = 0.80 eV
1.0
Substrate (InP)
Energy (eV)
0.5
Log(DOS)
Blackbody Radiation Absorbed
Normalized Intensity
Intensity
Normalized
Bandgap
energy(eV)
(eV)
Energy
Bandgap
1.2
0.4
Theoretical Fits
1.6
1.4
0.3
The distribution of defect levels within the bandgap can be represented by a density of states (DOS) function
as shown above.
Lattice-mismatched In-rich InGaAs on InP
GaAs
GaAs
0.2
Valence Band
The photons that are not absorbed by the TPV cell can be recycled back to the blackbody radiator, minimizing
the loss of heat energy. For this reason, TPV technology can be more efficient than ordinary solar cell
technologies.
Bandgap vs. Alloy Composition
0.1
18
10
20
10
22
10
24
10
Recombination Rate (cm-3s-1)
Assuming a discrete DOS concentrated near the center of the bandgap, we get a good fit for our latticematched structure (Eg = 0.80 eV), but a similar DOS is incompatible with the lattice-mismatched
(Eg = 0.68 eV) results. In this case, a DOS function with defect levels concentrated near the band edges gives
a much better fit.
InGaAs
Conclusions and Acknowledgements
• In contrast to the lattice-matched material, defect levels in the lattice-mismatched structures appear to be
concentrated near the band edges.
• This location makes them less likely to facilitate recombination so the efficiency of InGaAs-based TPV cells
is not compromised by the presence of defects.
• Lattice-mismatched InGaAs appears to be a promising candidate for TPV technologies.
Substrate
(InP)
DEFECT
Increasing the Indium concentration also produces lattice-mismatch between the InGaAs and the substrate
(InP) because the atomic spacing in the Indium-rich InGaAs is different from that of InP. The atoms without
bonds constitute defects in the crystal structure that produce additional energy levels within the bandgap.
* This work is supported by Research Corporation and the American Chemical Society –
Petroleum Research Fund