Light absorption enhancement in thin-film GaAs solar cells with flattened light scattering substrates

摘要

A flattened light scattering substrate (FLiSS) was investigated for enhancing the light absorption in thin-film GaAs solar cells. The FLiSS investigated in this work was limited to those composed of periodic refractive index distribution, although its concept is not necessarily limited to such a structure. The following guidelines were found via optical simulation: (i) the morphological distribution of refractive indices in a FLiSS plays a key role, and an inverted pyramid-like shape is very efficient in light scattering. (ii) There are an optimum period and a depth in a FLiSS, although efficient light scattering is achievable in a wide parameter space. However, periods less than 0.4 μm result in poor light scattering effect. (iii) The contrast in the refractive indices of the two materials in the FLiSS should be large enough, typically Δn > 1.5. At the same time, parasitic absorption loss in the FLiSS must be minimized. An optimized FLiSS, which satisfies the requirements mentioned above, can increase the absorption in thin GaAs cells more efficiently than a flat reflector, and a high current density of approximately 30 mA/cm~2 is potentially achievable with a 1-μm-thick absorber. For experimental verification, a 2D grating FLiSS with InZnO and amorphous Si was developed and applied to thin film GaAs solar cells. As a result, a significant increase in the current density as well as in the spectral response in a long wavelength region was demonstrated, as expected from the optical simulation.