Fraction scattered into substrate is more cylindrical and hemispherical nano structures than spherical...
When a metal particle is placed on the rear of the cell, long wavelengths are scattered and the radiation can be trapped more efficiently. The particle's geometry (or size) leads to a better/worse scattering efficiency (see Fig. 4b for instance). The more the amount of light scattered (it depends on the particle's geometry and size) , the less the amount of light reflected (that is, light that follows the "straigth path").
A more graphic explanation might be this: Spherical particles have a single contact point with the substrate and show a dipolar scattering pattern. Only a small part of the scattered light provides the needed high parallel momentum values (in-plane wave vector needed for coupling to the substrate), since the wave vector values show a broad distribution. This becomes worse for increasing the particle size, only a point dipole can perfectly couple to the substrate.
Now for both cylinders and hemispherical particles (which naturally occur in fabrication) show a large contact area. Forward scattering into the substrate is favoured, since the correct wave vector ("index matching method") is provided from many points of the geometry.
You might find it instructive to read Catchpole and Polman's work on this topic [APL93, 191113 (2008)].
http://dx.doi.org/10.1063/1.3021072
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