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!

solar cells #2

I've been thinking some more about the solar cell problem. The most recent question asked by one of my colleagues was, what happens if you have a conventional light-trapping scheme? It seems reasonable to compare that with our problem in the context of the same situation if possible. One challenge is that in the geometrical optics picture, any direction of light propagation should be permitted; whereas in the crystalline picture, there are only certain wavevectors allowed by conservation of crystal momentum. If one starts with wavevector k, one can only add and subtract reciprocal lattice vectors: i.e., k->k+G. The solution is to start off with a large cell in the direction of periodicity, and increase it until the solution converges for a large enough period. So, I recently performed this test, and found that texturing for the optimal angle (16 degrees for normally incident light) actually yields similar performance as the photonic crystal. BUT, the good news is, that it seems that the two can be combined to yield a greater performance. For a 8 micron thin silicon crystal, I found that 1D texturing and a 2D photonic crystal both yield an enhancement of 10% -- but combined, they yield a 15% improvement. This could have important implications for the implementation of this system in solar cells.

I gave a talk today at the MIT Center for Integrated Photonic Systems meeting in which I discussed my results for several different cases, which you can download here.