InGaP single-junction solar cells are grown on lattice-matched Ge-on-Si virtual substrates using metal-organic chemical vapor deposition. Optoelectronic simulation results indicate that the optimal collection length for InGaP single-junction solar cells with a carrier lifetime range of 2-5 ns is wider than approximately 1 lm. Electron beam-induced current measurements reveal that the threading dislocation density (TDD) of InGaP solar cells fabricated on Ge and Ge-on-Si substrates is in the range of 10 4 -3 Â 10 7 cm À2 . We demonstrate that the open circuit voltage (V oc ) of InGaP solar cells is not significantly influenced by TDDs less than 2 Â 10 6 cm À2 . Fabricated InGaP solar cells grown on a Ge-on-Si virtual substrate and a Ge substrate exhibit V oc in the range of 0.96 to 1.43 V under an equivalent illumination in the range of $0.5 Sun. The estimated efficiency of the InGaP solar cell fabricated on the Ge-on-Si virtual substrate (Ge substrate) at room temperature for the limited incident spectrum spanning the photon energy range of 1.9-2.4 eV varies from 16.6% to 34.3%.
A compact, single element concentrator comprising a near linear array of prisms has been designed to simultaneously split and concentrate the solar spectrum. Laterally aligned solar cells with different bandgaps are devised to be fabricated on a common Si substrate, with each cell absorbing a different spectral band optimized for highest overall power conversion efficiency. Epitaxial Ge on Si is used as a low cost virtual substrate for III-V materials growth. Assuming no optical loss for the prism concentrator, no shadowing and perfect carrier collection for the solar cells, simulations show that 39% efficiency can be achieved for a parallel four-junction (4PJ) InGaP-GaAs-Si-Ge cell under 200X concentration, and higher efficiency is possible with more junctions.
A comparative study is performed to quantify the difference in efficiency and spectral sensitivity between a tandem junction and its spectrum splitting parallel junction counterpart. Direct normal solar spectra in a representative sunny site, Tucson, Arizona are calculated using the SPCTRAL2 model at 15-minute intervals throughout a year with real-time meteorological data input. The corresponding efficiencies of the two junctions under 500X concentration at cell temperatures deduced from thermal modeling with real-time ambient temperatures are computed. Both junction structures comprise the same materials, InGaP, GaAs and Ge, and are each optimized to the AM1.5D standard spectrum and cell temperature of 25 ο C, under which the parallel junction achieves a 1.0% absolute (and 2.5% relative) higher efficiency than the tandem junction. The two junctions are compared for their hourly, daily, and yearly average efficiencies. It is found that the yearly average efficiency of the parallel junction is 2.65% absolute (and 7.31% relative) higher than that of the tandem junction.
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