To move beyond the efficiency limits of single-junction solar cells, junctions of different bandgaps must be used to avoid losses from lack of absorption of low energy photons and energy lost as excited carriers thermalize to the semiconductor band edge. Traditional tandem multijunction solar cells are limited, however, by lattice-matching and current-matching constraints. As an alternative we propose a lateral multijunction design in which a compound holographic optic splits the solar spectrum into four frequency bands each incident on a dual-junction, III-V tandem cell with bandgaps matched to the spectral band. The compound splitting element is composed of four stacks of three volume phase holographic diffraction gratings. Each stack of three diffracts three bands and allows a fourth to pass straight through to a cell placed beneath the stack, with each of the three gratings in the stack responsible for diffracting one frequency band.
Generalized coupled wave analysis is used to model the holographic splitting. Concentration is achieved using compound parabolic trough concentrators. An iterative design process includes updating the ideal bandgaps of the four dual-junction cells to account for photon misallocation after design of the optic. Simulation predicts a two-terminal efficiency of 36.14% with 380x concentration including realistic losses.
In this paper we investigate the use of holographic filters in solar spectrum splitting applications. Photovoltaic (PV)
systems utilizing spectrum splitting have higher theoretical conversion efficiency than single bandgap cell modules.
Dichroic band-rejection filters have been used for spectrum splitting applications with some success however these
filters are limited to spectral control at fixed reflection angles. Reflection holographic filters are fabricated by recording
interference pattern of two coherent beams at arbitrary construction angles. This feature can be used to control the angles over which spectral selectivity is obtained. In addition focusing wavefronts can also be used to increase functionality in the filter. Holograms fabricated in dichromated gelatin (DCG) have the benefit of light weight, low scattering and absorption losses. In addition, reflection holograms recorded in the Lippmann configuration have been shown to produce strong chirping as a result of wet processing. Chirping broadens the filter rejection bandwidth both spectrally and angularly. It can be tuned to achieve spectral bandwidth suitable for spectrum splitting applications. We explore different DCG film fabrication and processing parameters to improve the optical performance of the filter. The diffraction efficiency bandwidth and scattering losses are optimized by changing the exposure energy, isopropanol dehydration bath temperature and hardening bath duration. A holographic spectrum-splitting PV module is proposed with Gallium Arsenide (GaAs) and silicon (Si) PV cells with efficiency of 25.1% and 19.7% respectively. The calculated conversion efficiency with a prototype hologram is 27.94% which is 93.94% compared to the ideal spectrum-splitting efficiency of 29.74%.
Quantum Cascade (QC) wafer quality testing requires intensive processing and characterization. Here, we demonstrate a
minimally invasive technique that gives rapid feedback on wafer quality. A mesa is fabricated using only a single etch
and metallization step. The device is electrically pumped and optically and electrically characterized. The peak
wavelength position and the full width at half maximum (FWHM) as a function of applied electric field, turn-on voltage,
maximum operating current density and threshold current density of the mesas are measured. Results of the mesa and
lasers processed from the same wafer are compared and differed by less than 10 %.
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