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

Experimental demonstration of enhanced photon recycling in angle-restricted GaAs solar cells

“…(As in the ESI, † this reecting band can be eliminated with a rugate lter optical design, but for the initial coupler we did not pursue these structures as they are more difficult to fabricate. 17,18 ) As shown in Fig. 3a, when we directly compare the control and angle restrictor on the same cell, current losses of 3.5 to 5.3% are observed with angle restriction, consistent with the measured normal incidence reectivity of the angle restrictor.…”

“…Fortunately, these current losses are not intrinsic, and result from the simplicity of our initial angle restrictor design 17,18 (see ESI †). To isolate the photon recycling effect, we adjust the solar simulator to equalize the currents between the control and angle restrictor, as in Fig.…”

Experimental demonstration of enhanced photon recycling in angle-restricted GaAs solar cells

“…(As in the ESI, † this reecting band can be eliminated with a rugate lter optical design, but for the initial coupler we did not pursue these structures as they are more difficult to fabricate. 17,18 ) As shown in Fig. 3a, when we directly compare the control and angle restrictor on the same cell, current losses of 3.5 to 5.3% are observed with angle restriction, consistent with the measured normal incidence reectivity of the angle restrictor.…”

“…Fortunately, these current losses are not intrinsic, and result from the simplicity of our initial angle restrictor design 17,18 (see ESI †). To isolate the photon recycling effect, we adjust the solar simulator to equalize the currents between the control and angle restrictor, as in Fig.…”

Light Trapping Concepts for Photon Management in Solar Cells

“…For ultra-light trapping devices only theoretical work has been done so far. Rugate stacks have theoretically proven to be more effective than inverted opals grown in the Γ-X directions [33]. However, 3D photonic crystals still could offer a higher performance if an optimum design can be identified.…”

“…This is due to the fact that very few filters exist that exhibit the desired characteristics. A comprehensive overview on the effects of different angular selective structures on silicon solar cells has been given by Ulbrich et al [33,38]. In addition to ideal filters, especially manufacturable rugate stacks [39] and 3D photonic crystals are investigated in this work.…”

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.…”

Experimental demonstration of enhanced photon recycling in angle-restricted GaAs solar cells

“…(As in the ESI, † this reecting band can be eliminated with a rugate lter optical design, but for the initial coupler we did not pursue these structures as they are more difficult to fabricate. 17,18 ) As shown in Fig. 3a, when we directly compare the control and angle restrictor on the same cell, current losses of 3.5 to 5.3% are observed with angle restriction, consistent with the measured normal incidence reectivity of the angle restrictor.…”

“…Fortunately, these current losses are not intrinsic, and result from the simplicity of our initial angle restrictor design 17,18 (see ESI †). To isolate the photon recycling effect, we adjust the solar simulator to equalize the currents between the control and angle restrictor, as in Fig.…”

Light Trapping Concepts for Photon Management in Solar Cells

“…For ultra-light trapping devices only theoretical work has been done so far. Rugate stacks have theoretically proven to be more effective than inverted opals grown in the Γ-X directions [33]. However, 3D photonic crystals still could offer a higher performance if an optimum design can be identified.…”

“…This is due to the fact that very few filters exist that exhibit the desired characteristics. A comprehensive overview on the effects of different angular selective structures on silicon solar cells has been given by Ulbrich et al [33,38]. In addition to ideal filters, especially manufacturable rugate stacks [39] and 3D photonic crystals are investigated in this work.…”

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.…”

Experimental demonstration of enhanced photon recycling in angle-restricted GaAs solar cells

“…(As in the ESI, † this reecting band can be eliminated with a rugate lter optical design, but for the initial coupler we did not pursue these structures as they are more difficult to fabricate. 17,18 ) As shown in Fig. 3a, when we directly compare the control and angle restrictor on the same cell, current losses of 3.5 to 5.3% are observed with angle restriction, consistent with the measured normal incidence reectivity of the angle restrictor.…”

“…Fortunately, these current losses are not intrinsic, and result from the simplicity of our initial angle restrictor design 17,18 (see ESI †). To isolate the photon recycling effect, we adjust the solar simulator to equalize the currents between the control and angle restrictor, as in Fig.…”

Light Trapping Concepts for Photon Management in Solar Cells

“…For ultra-light trapping devices only theoretical work has been done so far. Rugate stacks have theoretically proven to be more effective than inverted opals grown in the Γ-X directions [33]. However, 3D photonic crystals still could offer a higher performance if an optimum design can be identified.…”

“…This is due to the fact that very few filters exist that exhibit the desired characteristics. A comprehensive overview on the effects of different angular selective structures on silicon solar cells has been given by Ulbrich et al [33,38]. In addition to ideal filters, especially manufacturable rugate stacks [39] and 3D photonic crystals are investigated in this work.…”

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.…”