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46713-B10
Exploring Defect Level Position and Occupation in Metamorphic Heterostructures for Multi-junction Photovoltaic Converters

Timothy H. Gfroerer, Davidson College

Technological improvement of the conversion efficiency in state-of-the-art high efficiency solar cells depends, in part, on the development and refinement of high-bandgap alloys that can be grown on GaAs substrates.� The bandgap in alloys like GaAsP and GaInP can be tuned to match design parameters that capitalize on the power available in the high-energy portion of the solar spectrum.� However, the atomic spacing in these crystals differs from that of the underlying GaAs, so a higher concentration of crystalline defects is expected in GaAsP and GaInP epilayers.� These defects impair solar cell performance by providing new pathways for photo-excited charge carriers to recombine rather than generate electricity.

At Davidson College, we are using three different experimental techniques to characterize defects in GaAsP and GaInP.� In the first, we use spatially-integrated measurements of photoluminescence spectra and radiative efficiency to investigate collective properties of defect-related transitions.� In the second, we use transient capacitance measurements to explore transport into and out of defect-related states.� And in the third, which is a new experiment for us in 2008, we use a digital CCD camera to obtain photoluminescence images showing the defect distribution and carrier depletion via recombination at defects.

I.� Photoluminescence spectra from 1.75eV GaAsP are shown in Figure 1.� The broad defect-related (D-R) peak below the band-to-band (B-B) emission indicates that radiative recombination is occurring through relatively shallow defect levels in this alloy.� At higher temperatures these transitions become nonradiative and, with increasing temperature, the defect states become thermally depleted.� Analysis of the radiative efficiency (the radiative divided by the total recombination rate) provides further evidence that a broad band of shallow states exists in this material.� Running a rigorous computational algorithm to minimize the error between experimental measurements and theoretical predictions of radiative efficiency vs. photo-excitation produces the results shown in Figure 2.� The inset graph shows the computer-generated density of states (DOS) distribution between the valence and conduction band edges (E_v and E_c).� We find that a high density of defect levels just below E_c is required to obtain the best fit.

II. Transient capacitance measurements on diodes during and after the application of a bias pulse can be used to monitor the capture and emission of carriers into and out of defect-related traps.� Representative transients on devices incorporating 1.75eV GaAsP are shown in Figure 3.� The capture and escape response deviates strongly from conventional exponential behavior.� Such non-exponential behavior is often attributed to the presence of a broad band of defect energies, with shallow levels emptying quickly at early times and slower deeper levels contributing at later times.� However, with non-exponential behavior occurring during both the introduction and exclusion of carriers from the depletion region, we assign the behavior to anomalous transport rather than the filling and thermally-activated emptying of defect states.� For example, if carriers are hopping between defect sites, quick motion would be expected between nearby sites, while slower motion would occur during more distant hops.� We fit the temperature-dependent response to the stretched exponential function shown in the graph, holding the stretching parameter d and amplitude A fixed.� Arrhenius plots of the resulting capture and emission rates (k) are linear and yield comparable capture and escape activation energies (see Figure 4), providing further evidence for the transport-limited interpretation noted above.� Indeed, we also observe a thermally-activated, stretched-exponential response in 2.0eV GaInP, suggesting that the simultaneous incorporation of Gallium and Phosphorous may be responsible for this effect.

III. This summer, we undertook a new experiment to image the photoluminescence generated by these materials using a high-sensitivity CCD camera.� We observed striking spatial inhomogeneity in the luminescence brightness, even in nominally lattice-matched GaAs.� We also noted that the size of the defect-related features depends strongly on the laser excitation intensity (see Figure 5). ��At low density, carriers diffuse more readily to defective regions rather than recombining radiatively, producing larger effective �dead� areas.� We model the behavior by assigning a large defect recombination rate to the defect position and allowing diffusion to control the density of carriers in adjacent regions (see Figure 6).� The results are consistent with our observations, but a more sophisticated model will be required to achieve better quantitative agreement with experiment.� At this stage, we can conclude that (1) if the defect recombination rate is non-uniform throughout the sample, diffusion will contribute to a reduction in net efficiency and (2) even for high-quality semiconductors with few defects, diffusion can lead to significant defect recombination at low excitation intensity.� Preliminary images from GaAsP grown lattice-mismatched on GaAs suggest that a hypothetical network of defective pixels will be required to reconcile our diffusion-limited model with experiment.� Since the defect concentration in these structures is much higher, we expect photo-excited carriers to find defects more readily, raising the photo-excitation intensity threshold for good solar cell performance.

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