Light trapping in thin-film solar cells: the role of guided modes

Abstract

This paper studies theoretically light trapping in a solar cell configuration consisting of a 50-500 nanometer-thin planar silicon (aSi:H) film with a planar silver back-reflector, and scatterer(s) placed directly on the silicon surface. The usual picture for thicker films is that part of the light incident on the scatterer(s) can be coupled into the silicon film at a continuum of angles above the critical angle for the silicon-air interface, in which case light will be trapped and subsequently absorbed. However, for thin films a more appropriate picture is that of light being coupled into the guided modes of the air-silicon-silver geometry corresponding to discrete angles. The scattering of light into each guided mode, and out-of-plane scattering, will be quantified by the related scattering cross section. It will be shown that scatteringcross- section spectra have sharp resonances near cut-off wavelengths of guided modes, with more closely spaced resonances for thicker films. Total resonant cross sections can easily exceed physical cross sections by a factor 10. This study also includes light trapping due to coupling into the Surface-Plasmon-Polariton mode that exists due to the silver surface. It will be shown that peaks in scattering cross sections can be tuned via the geometry to the appropriate wavelength range where light trapping is advantageous due to weak absorption in the silicon, resulting in an optimum thickness around 250 nanometers. We consider both theoretical calculations with and without material losses, and both dielectric and metal scatterers are considered. The calculations were carried out with Green’s function integral equation methods.