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.