optical properties of lattice- mismatched semiconductors for thermo-photovoltaic cells tim gfroerer,...
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Optical properties of lattice-mismatched semiconductors for thermo-photovoltaic cells
TIM GFROERER, Davidson CollegeDavidson, NC USA
in collaboration with the National Renewable Energy Laboratory, USA
- Supported by Research Corporation
and the Petroleum Research Fund
OutlineMotivation
Sample Structure and Experimental technique
Results and Analysis
Conclusions and Future Work
Motivation: Thermophotovoltaic(TPV) Power
TPV Cells are designed to convert infrared blackbody radiation into electricity.
Semiconductor TPV Converter Cells
Heat Source Blackbody Radiator
Heat Blackbody Radiation
Motivation (continued)
0.0 0.5 1.0 1.5
0.0
0.2
0.4
0.6
0.8
1.0
T = 1300oC
Nor
mal
ized
inte
nsity
Energy (eV)
Increasing the Indium concentration in the InGaAs lowers the bandgap and increases the fraction of blackbody radiation that is absorbed in the cell.
Blackbody Radiation AbsorbedBandgap vs. Alloy Composition
5.6 5.7 5.8 5.9 6.0 6.10.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
InAs
GaAs
SevereMismatch
Substrate
Ban
dgap
(eV
)
Lattice parameter (Angstroms)
Sample Structure
Active Layer Active
Layer
Nominal Epistructure Parameters
Eg(x) x y m n
0.73 eV 0.47 0 0 0
0.65 eV 0.40 0.14 -0.46 2
0.60 eV 0.34 0.27 -0.87 4
0.55 eV 0.28 0.40 -1.28 6
0.50 eV 0.22 0.53 -1.69 8
m = Total Mismatch (%)
InAsP grading layers above the substrate are used to reduce the density of misfit dislocations at the interfaces of the active layer.
Experimental Setup
: Laser Light : Luminescence
Laser Diode1 Watt @ 980 nm
ND Filters
Cryostat @ 77K
Sample
Photodiode
Lowpass Filter
Experimental Data
Photoluminescence intensity (normalized by the excitation power) vs. the rate of electron-hole pair generation and recombination in steady state.
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0
20
40
60
80
100
A
bsol
ute
Rad
iativ
e E
ffici
ency
e-h Pair Generation and Recombination (cm-3s-1)
Eg= 0.73 eV
Eg= 0.65 eV
Eg= 0.60 eV
Eg= 0.55 eV
Eg= 0.50 eV
Results: Data Calibration
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0.0
0.5
1.0
1.5
2.0
Rel
ativ
e R
adia
tive
Effi
cien
cy (
a.u.
)
e-h Pair Generation and Recombination (cm-3s-1)
Eg= 0.73 eV
3rd Order Polynomial Fit
The derivatives show where the curvature of the relative efficiency inflects. We scale the relative efficiency to 50% absolute efficiency at the infection point.
Data from Eg = 0.73 eV Sample Derivatives of Best-Fit Curve
18 20 22 24-40
-30
-20
-10
0
10
20
30
40
Inflection Point
Der
ivat
ive
(arb
itrar
y un
its)
Log[Generation and Recombination (cm-3s-1)]
First Derivative of Fit Second Derivative
A Simple Theoretical Model
Efficiency =
Where A = SRH Coefficient, B = Radiative Coefficient and n = Carrier Density
2
2
Rate Total
Rate Radiative
BnAn
Bn
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Abs
olut
e R
adia
tive
Effi
cie
ncy
e-h Pair Generation and Recombination (cm-3s-1)
Eg= 0.73 eV
Theoretical Fit
Defect-related vs. Radiative Rate
Exceeding a threshold mismatch of ~1% increases the defect-related rate relative to the radiative rate.
0.50 0.55 0.60 0.65 0.70 0.751020
1021
IncreasingLatticeMismatch
Threshold
A2 /B
(cm
-3s-1
)
Nominal Bandgap Energy (eV)
@ 50% Radiative Efficiency, n = A/B________________
Total Rate @ 50% Efficiency =
An + Bn2 = 2A2/B
Shape of the Efficiency Curve
While the simple theory fits well in the lattice-matched case, the model does not fit the shape of the efficiency curve in the mismatched samples.
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Ab
solu
te R
adi
ativ
e E
ffici
enc
y
e-h Pair Generation and Recombination (cm-3s-1)
Eg= 0.73 eV
Theoretical Fit
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20
40
60
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100
Abs
olut
e R
adia
tive
Effi
cien
cye-h Pair Generation and Recombination (cm-3s-1)
Eg= 0.60 eV
Theoretical Fit
Lattice-matched case Lattice-mismatched case
Defect-related Density of States
Distribution of defect levels in simple theory
Distribution of defect levels in better theory
valence band edge
valence band edge
conduction band edge
conduction band edge
0.0 0.1 0.2 0.3 0.4 0.5 0.610-1
102
105
108
1011
1014
1 x 1014
Den
sity
of s
tate
s (c
m-3eV
-1)
Energy (eV)
0.0 0.1 0.2 0.3 0.4 0.5 0.610-1
102
105
108
1011
1014
3 x 1013
Energy (eV)
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60
80
100
Ab
solu
te R
ad
iativ
e E
ffici
en
cy
e-h Pair Generation and Recombination (cm-3s-1)
Eg= 0.60 eV
Theoretical Fit
A Better Theoretical Fit
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100
Abs
olut
e R
adia
tive
Effi
cien
cy
e-h Pair Generation and Recombination (cm-3s-1)
Eg= 0.60 eV
Theoretical Fit
DOSDOS
The addition of band-edge exponential tails to the density of defect states gives a much better fit.
Conclusions Moderate mismatch does not increase defect-
related recombination relative to the radiative rate in these structures. Large mismatch has an appreciable effect on this ratio.
The threshold that distinguishes these two regimes is approximately 1% lattice mismatch.
The shape of the efficiency curve in all mismatched samples differs from the lattice-matched case.
The change is attributed to a re-distribution of defect levels within the gap.
Future Work Continue fitting low temperature efficiency
curves to more detailed theory accounting for the distribution of energy levels at defects.
Compare results with complementary transport measurements including photoconductivity and DLTS.
Connect defect-related density of states with the microscopic structure of defects.
Measure efficiency curves at higher temperatures to further characterize defect-related, radiative, and Auger recombination.