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Multijunction solar cells for conversion of concentrated sunlight to electricity

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Abstract

Solar-cell efficiencies have exceeded 40% in recent years. The keys to achieving these high efficiencies include: 1) use of multiple materials that span the solar spectrum, 2) growth of these materials with near-perfect quality by using epitaxial growth on single-crystal substrates, and 3) use of concentration. Growth of near-perfect semiconductor materials is possible when the lattice constants of the materials are matched or nearly matched to that of a single-crystal substrate. Multiple material combinations have now demonstrated efficiencies exceeding 40%, motivating incorporation of these cells into concentrator systems for electricity generation. The use of concentration confers several key advantages.

©2010 Optical Society of America

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Figures (5)

Fig. 1
Fig. 1 Historic summary of champion solar-cell efficiencies.
Fig. 2
Fig. 2 Schematic showing how each layer of a multijunction solar cell absorbs a portion of the solar spectrum. The rectangles underneath represent semiconductor materials with band gaps that absorb the indicated portion of the solar spectrum. The semiconductor materials transmit light with energy less than the band gap of the material, providing a natural way to filter the light.
Fig. 3
Fig. 3 Band gap as a function of lattice constant for selected semiconductors. The symbols represent the elements or binaries, with circles indicating direct-gap materials and triangles and squares representing indirect transitions to the X and L bands, respectively. The lines indicate ternary alloys.
Fig. 4
Fig. 4 Schematic of a solar cell. The solid white lines indicate the conduction and valence bands of the semiconductor layers; the dotted white lines indicate the Fermi level in the dark. As shown, the light is incident from the left; absorption of the light creates electron-hole pairs. These minority carriers diffuse through the n- and p-type layers. They may be separated at the junction (within the width of the depleted layer) and then used to do work in an outside circuit. However, carriers that recombine before reaching the junction are lost. The window and passivating layers help to prevent loss of the minority carriers, increasing the efficiency of the solar cell.
Fig. 5
Fig. 5 Effect of concentration on the open-circuit voltage (Voc), fill factor (measure of the squareness of the current-voltage curve), and efficiency of the cell described in reference [11].

Tables (1)

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Table 1 Summary of Recent Three-Junction Champion Efficiencies

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