Rugate filter for light-trapping in solar cells

Abstract

We suggest a design for a coating that could be applied on top of any solar cell having at least one diffusing surface. This coating acts as an angle and wavelength selective filter, which increases the average path length and absorptance at long wavelengths without altering the solar cell performance at short wavelengths. The filter design is based on a continuous variation of the refractive index in order to minimize undesired reflection losses. Numerical procedures are used to optimize the filter for a 10 microm thick monocrystalline silicon solar cell, which lifts the efficiency above the Auger limit for unconcentrated illumination. The feasibility to fabricate such filters is also discussed, considering a finite available refractive index range.

Current Concepts for Optical Path Enhancement in Solar Cells

“…For the 3D photonic crystal the inverted opal structure was chosen due to its well-known properties. On first sight it seems feasible to use the inverted opal in the Γ − L direction with the stop gap pointing to the sun and using the spectral shift of the stop gap similar to the mechanism of the rugate filter [51]. But it turns out that the use of the inverted opal in Γ − L direction results in unwanted strong reflections from higher-order stop gaps.…”

Light‐Trapping in Solar Cells by Directionally Selective Filters

“…One way for producing a directionally and spectrally selective reflectance profile is to use a coating system with a continuously varying refractive index [58]. The solid line in Figure 7.3 shows schematically the refractive index n as a function of the optical thickness T opt = ∫ n(z)dz.…”

“…Analytical formulas describe the maximum (upper dash-dotted line), minimum (lower dash-dotted), and mean (dotted) refractive indices, as well as the refractive index n(z) itself; the derivation is given in Ref. [58]. Since these values are governed by only a small number of parameters and the calculation of the angle and wavelength resolved reflectivity requires only a reasonable amount of computational resources, a complete optimization of these parameters is feasible.…”

Efficiency limits of Si/SiO2 quantum well solar cells from first-principles calculations

“…61 The absorptance for Lambertian scattering is the perfect case for isotropic illumination; only for tracked systems, where the light comes always from the same solid angle, better light trapping and higher absorptances are possible. 62 To calculate the absorptance, we assume a front surface reflectance R f = 0 and a back surface reflectance R b =1 as well as one Lambertian diffuser on top of the absorber layer. Then the absorptance follows as, [63][64][65] …”