Effects of Spectral Error in Efficiency Measurements of GaInAs-Based Concentrator Solar Cells

Abstract

This technical report documents a particular error in efficiency measurements of triple-absorber concentrator solar cells caused by incorrect spectral irradiance-specifically, one that occurs when the irradiance from unfiltered, pulsed xenon solar simulators into a GaInAs bottom subcell is too high. For cells designed so that the light-generated photocurrents in the three subcells are nearly equal, this condition can cause a large increase in the measured fill factor, which, in turn, causes a significant artificial increase in the efficiency. The error is readily apparent when the data under concentration are compared to measurements with correctly balanced photocurrents, and manifests itself as discontinuities in plots of fill factor and efficiency versus concentration ratio. In this work, we simulate the magnitudes and effects of this error with a device-level model of two concentrator cell designs, and demonstrate how a new Spectrolab, Inc., Model 460 Tunable-High Intensity Pulsed Solar Simulator (T-HIPSS) can mitigate the error. Although in this work the error has been identified in GaInAs bottom subcells, increasing the photocurrent in any subcell will produce similar errors, regardless of the chemical composition.

Device characterization for design optimization of 4 junction inverted metamorphic concentrator solar cells

“…This sensitivity extends to high-concentration measurements 14 . While spectral adjustability of flash simulators has been less important for multijunction devices with only two current-matched junctions, this limitation has become a critical issue for current matched 3J and 4J devices.…”

Concentrator cell efficiency measurement errors caused by unfiltered xenon flash solar simulators

“…We have also verified that excess 1-eV bottom subcell photocurrents cause artificial increases in FF with carefully controlled experiments in a 1-sun multisource simulator [12,13]. The irradiance into the bottom subcell of a GaInP/GaAs/GaInAs triple-bandgap cell was varied from the balanced condition while the middle-top photocurrent ratio was held at 1.00.…”

“…Spectral adjustment is accomplished with computer-controlled shutters that cover the mirrors, thus giving the ability to subtract light in six different wavelength bands [12]. Thus, it is now possible to correctly balance all three photocurrents under concentration.…”

“…FIGURE 6. FF and I sc for a GaInP/GaAs/GaInAs tripleband-gap cell vs GaInAs bottom subcell photocurrent ratios, measured in a 1-sun spectrally adjustable simulator [12].…”

EMCORE four-junction inverted metamorphic solar cell development

“…Due to this limitation, the spectrum could be adjusted such that J1, J2, and J4 were within a few % of the same concentration ratio, but J3 was completely unknown and is expected to be overfilled. Overfilling of subcells can affect Voc somewhat, and fill factor (FF) significantly [8][9][10], leading to measured FF as high as 90%. It is expected that the FF of this device as measured could be somewhat inflated, but the hero device shown in Fig.…”