A Change in the Shape of the Efficiency Curve

1. 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 Data from Eg = 0.73 eV Sample Radiative Defect-Related Efficiency = = Where A = SRH Coefficient, B = Radiative Coefficient and n = Carrier Density Nominal Epistructure Parameters A Change in the Shape of the Efficiency Curve Lattice-Matched Case Lattice-Mismatched Case m = Total Mismatch (%) Experimental Setup Laser Diode (980 nm) Cryostat @ 77K Photodiode Lowpass Filter Sample ND Filters While the theory fits well in the lattice-matched case, the model does not fit the shape of the efficiency curve in the mismatched samples. This phenomenon is attributed to a change in the distribution of energy levels at defect sites.* (See reference below) : 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. Excitation-dependent transition between defect-related and radiative recombination in lattice-mismatched InGaAs/InAsP heterostructures F.E. Weindruch and T.H. Gfroerer, Davidson College M.W. Wanlass, National Renewable Energy Laboratory Abstract Calibration to Obtain the Absolute Efficiency Derivatives of Best-Fit Curve Lattice-mismatched (Indium-rich) InGaAs heterostructures grown on InP substrates are strong candidates for thermophotovoltaic cells, devices that convert thermal radiation into electricity.  We are studying a set of incrementally lattice-mismatched InGaAs/InAsP double heterostructures by measuring the radiative efficiency as a function of excitation power at 77K and comparing the rates of defect-related (nonradiative) and radiative recombination in these structures. We present preliminary results on how the transition between defect-dominated and radiative recombination depends on lattice-mismatch. Motivation: Thermophotovoltaic (TPV) Power Blackbody Radiation Heat The derivatives of the polynomial fit show where the curvature of the relative efficiency inflects. We scale the relative efficiency curves to obtain 50% absolute efficiency at the infection point. Semiconductor TPV ConverterCells Heat Source Blackbody Radiator TPV Cells are designed to convert infrared blackbody radiation into electricity. A Theoretical Model Lattice-mismatched In-rich InGaAs on InP Bandgap vs. Alloy Composition Blackbody Radiation Absorbed GaAs Substrate Severe Mismatch Bandgap Energy (eV) InAs Atom Spacing (Angstroms) Increasing the Indium concentration in the InGaAs lowers the bandgap and increases the fraction of blackbody radiation that is absorbed in the cell. Comparing the Defect-Related and Radiative Rates @ 50% Radiative Efficiency, n = A/B Total Rate at 50% Efficiency = An + Bn2 = 2A2/B Sample Structure Threshold Active Layer Increasing Lattice Mismatch Exceeding a threshold mismatch of ~1% increases the defect-related rate relative to the radiative rate. Conclusions • Moderate mismatch does not affect the rate of defect-related recombination relative to the radiative rate. • Large mismatch has an appreciable effect on this ratio. • The threshold that distinguishes these two regimes is approximately 1% lattice mismatch. • A change in the shape of the efficiency curve is observed for all mismatched samples relative to the lattice-matched case. The phenomenon is attributed to a change in the distribution of energy levels at defect sites. Further work is needed to test this hypothesis. Acknowledgements and References This project is supported by the Research Corporation and the Petroleum Research Fund. * T. Saitoh, H. Iwadate and H. Hasegawa, Jpn. J. Appl. Phys. 30, 3750 (1991). Corresponding Author: Tim Gfroerer Physics Department, Davidson College Davidson, NC 28036 (tigfroerer@davidson.edu)