1. Optical properties of lattice-mismatched semiconductors for thermo-photovoltaic cells
TIM GFROERER, Davidson College Davidson, NC USA
in collaboration with the National Renewable Energy Laboratory, USA
Supported by Research Corporation
and the Petroleum Research Fund

2. Outline Motivation Sample Structure and Experimental technique
Results and Analysis
Conclusions and Future Work

3. 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.

4. 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.

5. Nominal Epistructure Parameters
Sample Structure
Nominal Epistructure Parameters
Eg(x) x y m n
0.73 eV
0.47
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
-1.28 6
0.50 eV
0.22
0.53
-1.69 8
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.

6. Experimental Setup Laser Diode 1 Watt @ 980 nm Photodiode
77K
Lowpass Filter
Sample
ND Filters
: Laser Light
: Luminescence

7. Experimental Data
Photoluminescence intensity (normalized by the excitation power) vs. the rate of electron-hole pair generation and recombination in steady state.

8. 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.

9. A Simple Theoretical Model
Efficiency =
Where A = SRH Coefficient, B = Radiative Coefficient and n = Carrier Density

10. Defect-related vs. Radiative Rate
@ 50% Radiative Efficiency, n = A/B
________________
Total 50% Efficiency =
An + Bn2 = 2A2/B
Exceeding a threshold mismatch of ~1% increases the defect-related rate relative to the radiative rate.

11. 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.

12. Defect-related Density of States
Distribution of defect levels in simple theory
Distribution of defect levels in better theory
valence band edge
conduction band edge
valence band edge
conduction band edge

13. A Better Theoretical Fit
The addition of band-edge exponential tails to the density of defect states gives a much better fit.

14. 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.

15. 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.