Conclusions and Acknowledgements Theoretical Fits Novel Materials for Heat-Based Solar Cells We are studying a set of materials that may be useful for a promising new technology called thermophotovoltaics (TPV). TPV cells are similar to solar cells, but they convert radiant heat (rather than light) into electricity. To optimize the efficiency of these devices, we seek to decrease the threshold energy for absorption so that more radiant heat is absorbed in the cell. In the particular material that we are investigating, Indium Gallium Arsenide (InGaAs), the threshold energy can be reduced by increasing the ratio of Indium atoms to Gallium atoms. However, changing this ratio produces a difference in atomic spacing between the InGaAs and the underlying substrate material, which is required for crystal growth. This difference (called lattice mismatch) leads to the formation of defects, which usually has a deleterious effect on the fate of heat-generated excitations in the cell. Ideally, all heat-generated excitations are swept out of the cell by an internal electric field, producing electricity. In reality, the excitations can be lost by a variety of recombination mechanisms during their brief residence in the cell. We have studied the optical properties of InGaAs as a function of the Indium-to-Gallium ratio in order to gauge how lattice mismatch and other fundamental properties associated with the InGaAs material affect these mechanisms. Ultimately, our work should facilitate the design of more efficient thermophotovoltaic cells. TPV Cells are designed to convert infrared blackbody radiation into electricity. Semiconductor TPV Converter Cells Heat SourceBlackbody Radiator Heat Blackbody Radiation Increasing the Indium concentration in the InGaAs lowers the bandgap E g and increases the fraction of blackbody radiation that is absorbed in the cell. Conduction Band Valence Band PHOTON (LIGHT) Bandgap ENERGY ELECTRON HOLE E-Field When a blackbody photon (with energy exceeding the bandgap) is absorbed, an electron is excited into the conduction band, leaving a hole behind in the valence band. If they do not recombine, an internal electric field sweeps the electrons and holes away, creating electricity. Substrate (InP) InGaAs DEFECT The photons that are not absorbed by the TPV cell can be recycled back to the blackbody radiator, minimizing the loss of heat energy. For this reason, TPV technology can be more efficient than ordinary solar cell technologies. Increasing the Indium concentration also produces lattice-mismatch between the InGaAs and the substrate (InP) because the atomic spacing in the Indium-rich InGaAs is different from that of InP. The atoms without bonds constitute defects in the crystal structure that produce additional energy levels within the bandgap. Electrons can recombine with holes in the valence band by releasing a photon (light) or many phonons (heat). These processes reduce the efficiency of a TPV cell. The probability P of transitions involving phonon emission depends on the number of phonons required, which is determined by the position of the defect level in the gap. The distribution of defect levels within the bandgap can be represented by a density of states (DOS) function as shown above. Assuming a discrete DOS concentrated near the center of the bandgap, we get a good fit for our lattice- matched structure (E g = 0.80 eV), but a similar DOS is incompatible with the lattice-mismatched (E g = 0.68 eV) results. In this case, a DOS function with defect levels concentrated near the band edges gives a much better fit. Density of States Motivation: Thermophotovoltaic (TPV) Power How TPV Cells Generate Electricity Abstract Efficiency of TPV Cells Lattice-mismatched In-rich InGaAs on InP Reflector Recycling PHONONS (HEAT) Defect Level PHOTON (LIGHT) Conduction Band Valence Band P~10 -3 P~(0.5) 10 ~10 -3 P~10 -5 P~10 -1 P~(0.5) 16 ~10 -5 P~(0.5) 4 ~10 -1 Recombination (Loss) Mechanisms Radiative Recombination Defect-Related Recombination (produces light) (produces heat) Probable Transition Improbable Transitions In contrast to the lattice-matched material, defect levels in the lattice-mismatched structures appear to be concentrated near the band edges. This location makes them less likely to facilitate recombination so the efficiency of InGaAs-based TPV cells is not compromised by the presence of defects. Lattice-mismatched InGaAs appears to be a promising candidate for TPV technologies. * This work is supported by Research Corporation and the American Chemical Society – Petroleum Research Fund Valence Band Conduction Band ENERGY Density of States (cm -3 eV -1 ) Energy (eV) L.P. Priestley and T.H. Gfroerer (Davidson College) M.F. Fairley (Spelman College) and M.W. Wanlass (National Renewable Energy Lab) Blackbody Radiation Absorbed T = 1300 C Normalized Intensity Energy (eV) Bandgap vs. Alloy Composition GaAs Substrate InAs Bandgap Energy (eV) Atom Spacing (Angstroms) Severe Mismatch Normalized Intensity Energy (eV) T=1300 C Radiative Efficiency (%) Recombination Rate (cm -3 s -1 ) Radiative Efficiency (%) Recombination Rate (cm -3 s -1 ) heat light