gratings in 1D and 2D

After a short hiatus caused by the end of the semester, I'm back to blogging! And more specifically, back to discussing light trapping in solar cells.

One interesting issue that arises in designing diffraction light trapping is the choice of periodicity. In a 2D world, only one value must be chosen. It is set by the desired diffraction limit -- for a solar cell, this would generally correspond to the upper end of the absorption spectrum (where the greatest gains are possible). However, the real world is 3D, so one must choose two periods. The easiest choice is to have both be the same as in the 2D world. However, it's not necessarily the case that such a choice is the optimum or even really that close to optimum. Ideally, two sets of peaks would be created by the two gratings, and not overlap with each other. However, since the spacing is not constant, that's probably not realistic. So the 2D case will generally entail less than 100% enhancement compared to the 1D grating. Also, keep in mind that only 800-1100 nm is targeted for enhancement, so the second period must be at least 800/1100 = 73% of the smaller period. Right now, I'm looking into what relative periods are optimal, and how that's influenced by various factors such as the natural absorption length and material thickness. Hopefully I'll have some results on this idea soon!