Light Trapping

In solar cells with a simple geometry, light rays enter the cell through the front surface and, if not absorbed, leave through the rear surface of the cell. More sophisticated arrangements exist that extend the path of light inside the cell, and they are usually referred to as optical confinement or light trapping. In crys­talline or amorphous silicon solar cells, light trapping is used to reduce the thickness of the cell without lowering the light absorption within the cell. Light trapping can also be used to enhance the open-circuit voltage [8,9].

The most common light-trapping features include a textured top surface combined with an optically reflecting back surface (Figure 9). In the ideal case, Yablonovich [10,11] (see also [12]) has shown that a randomly textured (so-called Lambertian) top surface in combination with a perfect back-surface reflector produces a light-trapping scheme that enhances the light intensity inside the cell by a factor of n2c where, as in Section 3.1, nsc is the refractive index of the solar cell material. This arrangement also increases the average path length of light rays inside the cell from 2 W, in the case of single pass through the cell, to 4n2cW in the case of complete light trapping, where W the cell thickness. Schemes have been developed to enhance the operation of practical devices including crystalline, polycrystalline, and amorphous silicon cells (discussed in Chapters Ib-2, -3, and -4, and in Chapter Ic-1). With application to the latter cells, Schropp and Zeman [13] consider the trapping and scattering of light at rough interfaces in some detail. In gallium-arsenide cells, multilayer Bragg reflectors (in place of the back-surface reflector) have been used with success (see Chapter Id-1).


FIGURE 9 The textured top surface reduces reflection from the solar cell and, when combined with a reflecting back surface, helps to confine or ‘trap’ light within the cell.

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