Directional selectivity and ultra‐light‐trapping in solar cells

Abstract

Directional and energy selective optical surface structures attached to solar cells may result in both, enhancement or deterioration of photovoltaic performance of a solar cell. On the one hand, restricting the cell acceptance to the small incidence angle of direct and circumsolar irradiation enhances the maximum path length of the light in a solar cell with Lambertian surfaces even above the Yablonovitch limit (ultra-light-trapping). On the other hand, restrictions to small acceptance angles imply losses of diffuse sunlight, even for perfectly tracked cells. Using temporally resolved solar irradiation spectra, we simulate the enhancement and loss in the annual energy yield of silicon solar cells of various thicknesses. We assume an idealized angular and energy selective filter on top of the Lambertian surface of the absorber and compare the results to a Lambertian surface only. We find a maximum annual gain in the energy density of 32.5% for 1 mu m, and of similar to 10% for 10 mu m and 100 mu m thick perfectly tracked crystalline silicon solar cells. The simulation implies various tracking modes and two different locations. Finally, we introduce two possible realizations of such a filter; a Ru-gate stack and inverted opals. In experimental measurements, we could verify the absorptance enhancement by such a structure applied on top of a thin silicon wafer

The feasibility of high-efficiency InAs/GaAs quantum dot intermediate band solar cells

“…Consequently, the surface textures used in present solar cells are believed to offer belowLambertian light trapping, with optical path length enhancements of around 10 [46]. It should be observed that the Lambertian Limit is not an upper limit; it can be superseded at some wavelengths through the use of diffraction gratings [47], angle-selective filters [48] and near-field absorption enhancement techniques [49]. However, in this work, we take an optical absorption enhancement of 50 to be optimistic and absorption enhancements of 10 to be realistic, and do not consider the wavelength dependence of the absorption enhancement.…”

Nanostructures for photon management in solar cells

“…Ulbrich and colleagues present a design for front-side photonic crystals which exploit directional selectivity to enhance performance [ 150 ]. The principle of this structure is to place a selective filter at the front of the solar cell over top of a randomized scattering layer.…”

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

“…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] …”