Thermally activated radiative efficiency enhancement in a GaAs/GaInP heterostructure* Brant West and Tim Gfroerer, Davidson College Mark Wanlass, National Renewable Energy Laboratory, Golden, CO * Supported by the American Chemical Society – Petroleum Research Fund Abstract When electron-hole pairs are generated in a semiconductor, recombination proceeds via radiative and nonradiative events. We measure the radiative efficiency as a function of laser excitation intensity and temperature to explore recombination mechanisms in alloys that may be useful for multi-junction solar cells. In a 1.65 eV bandgap GaAs 0.86 P 0.14 /GaInP heterostructure, we observe a systematic decrease in efficiency with increasing temperature as predicted by a simple model. Assuming a temperature- independent rate of nonradiative defect-related recombination, the decrease in radiative efficiency is attributed to the theoretical decrease in the band-to-band (B-B) radiative rate. In contrast, we observe an increase in radiative efficiency with temperature between 77K and 120K in a 1.43 eV bandgap GaAs/GaInP heterostructure. Above 120K, the efficiency levels off and then slowly decreases as the temperature is raised to 300K. We hypothesize that a defect level lies close in proximity to one of the bands, such that the thermal energy at low temperatures is insufficient to activate trapped carriers to the band where they can participate in B-B recombination. Above 120K, the thermal energy is sufficient to facilitate these transitions. Low-temperature, sub-bandgap spectra reveal a weak, radiative defect-related transition approximately 0.15 eV below the B-B emission, which subsides with increasing temperature. An Arrhenius plot of the escape rate yields an activation energy of approximately 0.09 eV. These energies are comparable, but the magnitude of the difference suggests that a more sophisticated model may be required to fully explain our results. Conclusions and Future Work - We observe an unexpected increase in radiative efficiency with increasing temperature. - We propose thermal depletion of nonradiative defect levels as a possible explanation. - Temperature-dependent sub-bandgap transitions seem to support this hypothesis. -A more sophisticated model may be required to fully explain the results … (See SESAPS abstract CB.00008: Modeling defect level occupation for recombination statistics by Topaz, et al. for more information.) Efficiency Results: Band Gap Energy = 1.65 eV Efficiency Results: Band Gap Energy = 1.43 eV Luminescence Spectra of Low- Band Gap Sample Integrated SBG Intensity vs. 1/kT Some Basic Semiconductor Theory Motivation: Lattice-Mismatched Multi-Junction Solar Cells Absorption of Light in Multilayer Cell Experimental Setup A Possible Explanation Any photon energy exceeding the band-gap energy of the semiconductor is lost in the form of heat, decreasing the conversion efficiency. Stacking several different semiconductors on top of one another allows for more efficient conversion of the broad incident spectrum. The laser light is incident upon the semiconductor sample, producing luminescence. We collect this emitted light and focus it onto a photodiode for efficiency measurements, or into the spectrometer for spectral analysis. In general, the efficiency should increase with increasing carrier density and decrease with increasing temperature. In the 1.65 eV band-gap energy sample, the downward shift in radiative efficiency with increasing temperature is readily observed. The solid curves are fits using the theory described above. In the 1.43 eV structure an increase in radiative efficiency is observed from 77K-120K before the expected decrease in efficiency ensues. A possible explanation for this increase in radiative efficiency with temperature is thermal excitation from a nonradiative defect level. The presence of this level may be evident in the luminescence spectrum. As hypothesized, a sub-bandgap (SBG) peak approximately 0.15 eV below the band-to-band (B-B) recombination is present. This Arrhenius plot of the thermal quenching of the SBG emission indicates that the defect level is approximately 0.09 eV below the band edge. Deviation from the spectral analysis suggests that a more sophisticated model may be required.