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Optical properties of lattice-mismatched semiconductors for thermo-photovoltaic cells

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## Optical properties of lattice-mismatched semiconductors for thermo-photovoltaic 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**Outline**• Motivation • Sample Structure and Experimental technique • Results and Analysis • Conclusions and Future Work**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.**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.**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 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.**Experimental Setup**Laser Diode 1 Watt @ 980 nm Photodiode Cryostat @ 77K Lowpass Filter Sample ND Filters : Laser Light : Luminescence**Experimental Data**Photoluminescence intensity (normalized by the excitation power) vs. the rate of electron-hole pair generation and recombination in steady state.**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.**A Simple Theoretical Model**Efficiency = Where A = SRH Coefficient, B = Radiative Coefficient and n = Carrier Density**Defect-related vs. Radiative Rate**@ 50% Radiative Efficiency, n = A/B ________________ Total Rate @ 50% Efficiency = An + Bn2 = 2A2/B Exceeding a threshold mismatch of ~1% increases the defect-related rate relative to the radiative rate.**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.**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**A Better Theoretical Fit**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.