Quantifying light-trapping in silicon solar cells and wafers via photoluminescence spectra

We demonstrate the use of photoluminescence spectra measured on silicon wafers and solar cells to quantify light-trapping properties. Photoluminescence spectra are used to calculate the absorptance, from which the optical path-length enhancement factor Z is determined as a function of wavelength. We show that this contactless method yields path-length enhancement values that are consistent with conventional methods based on quantum efficiency measurements on complete cells. We also demonstrate the method on passivated planar and textured silicon wafers, both with and without rear-side reflectors, yielding consistent results. Furthermore, the use of a simple parameterization for the measured wavelength-dependent Z values allows determination of the long-wavelength path-length enhancement factor Z 0 , which is, in principle, independent of wavelength and wafer thickness, in the absence of parasitic absorption in the wafer bulk. This allows a direct comparison of the light-trapping properties of different surface morphologies, demonstrating a powerful, contactless method for measuring and optimizing light-trapping properties at the wafer level. We also demonstrate and discuss the sensitivity of the technique to small changes in temperature, which cause steep changes in the absorption coefficient near the phonon edges in the luminescence spectra.