Multi-resonant light trapping: New paradigm, new limits

We propose a novel approach for light trapping in solar cells. It is based on absorption through multiple resonant modes in the critical coupling regime (A=100%). A simple model is developed for the calculation of the upper limit of optical absorption in a slab of semiconductor. This multi-resonant critical coupling limit exceeds the conventional lambertian limit (4n2) for any absorber thickness. We also provide a numerical example of multi-resonant absorption in a 45nm-thick CIGS layer. It demonstrates that absorption can overcome the conventional limit on a broad spectral range.