Optical properties of latticemismatched semiconductors for thermophotovoltaic cells

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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

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Outline

• Motivation
• Sample Structure and Experimental technique
• Results and Analysis
• Conclusions and Future Work

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Motivation Thermophotovoltaic (TPV) Power
Heat
Blackbody Radiation
Semiconductor TPV Converter Cells
Heat Source
Blackbody Radiator
TPV Cells are designed to convert infrared
blackbody radiation into electricity.
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Motivation (continued)
Bandgap vs. Alloy Composition
Blackbody Radiation Absorbed
Increasing the Indium concentration in the InGaAs
lowers the bandgap and increases the fraction of
blackbody radiation that is absorbed in the cell.
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Sample Structure
Nominal Epistructure Parameters
Active Layer
Active Layer
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.
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Experimental Setup
Laser Diode 1 Watt _at_ 980 nm
Photodiode
Cryostat _at_ 77K
Lowpass Filter
Sample
ND Filters
Laser Light
Luminescence
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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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Results Data Calibration
Data from Eg 0.73 eV Sample
Derivatives of Best-Fit Curve
The derivatives show where the curvature of the
relative efficiency inflects. We scale the
relative efficiency to 50 absolute efficiency at
the infection point.
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A Simple Theoretical Model
Efficiency
Where A SRH Coefficient, B Radiative
Coefficient and n Carrier Density
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Defect-related vs. Radiative Rate
_at_ 50 Radiative Efficiency, n
A/B ________________ Total Rate _at_ 50 Efficiency
An Bn2 2A2/B
Exceeding a threshold mismatch of 1 increases
the defect-related rate relative to the radiative
rate.
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Shape of the Efficiency Curve
Lattice-matched case
Lattice-mismatched case
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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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
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A Better Theoretical Fit
The addition of band-edge exponential tails to
the density of defect states gives a much better
fit.
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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.

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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.