Solar light trapping in slanted conical-pore photonic crystals: Beyond statistical ray trapping

Abstract

We demonstrate that with only 1 lm, equivalent bulk thickness, of crystalline silicon, sculpted into the form of a slanted conical-pore photonic crystal and placed on a silver back-reflector, it is possible to attain a maximum achievable photocurrent density (MAPD) of 35.5 mA/cm 2 from impinging sunlight. This corresponds to absorbing roughly 85% of all available sunlight in the wavelength range of 300-1100 nm and exceeds the limits suggested by previous "statistical ray trapping" arguments. Given the AM 1.5 solar spectrum and the intrinsic absorption characteristics of silicon, the optimum carrier generation occurs for a photonic crystal square lattice constant of 850 nm and slightly overlapping inverted cones with upper (base) radius of 500 nm. This provides a graded refractive index profile with good anti-reflection behavior. Light trapping is enhanced by tilting each inverted cone such that one side of each cone is tangent to the plane defining the side of the elementary cell. When the solar cell is packaged with silica (each pore filled with SiO 2), the MAPD in the wavelength range of 400-1100 nm becomes 32.6 mA/cm 2 still higher than the Lambertian 4n 2 benchmark of 31.2 mA/cm 2. In the near infrared regime from 800 to 1100 nm, our structure traps and absorbs light within slow group velocity modes, which propagate nearly parallel to the solar cell interface and exhibit localized high intensity vortex-like flow in the Poynting vector-field. In this near infrared range, our partial MAPD is 10.9 mA/cm 2 compared to a partial MAPD of 7 mA/cm 2 based on "4n 2 statistical ray trapping." These results suggest silicon solar cell efficiencies exceeding 20% with just 1 lm of silicon. V

Plasmonic photo-thermoelectric energy converter with black-Si absorber

“…It was demonstrated [19][20][21] by numerical simulations that slanted conical holes arranged in a photonic crystal (PhC) lattice in an active 1600-nm-thick Si-layer on top of a silver reflector would increase solar cell absorbance up to 85% of the solar light in a spectral window of 300-1100 nm. This exceeds the statistical ray trapping limit [22], which for bulk cells is n 4 /sin 2 2 θ ( ) where n is the refractive index of solar cell and θ is the angle of the emission cone in the medium surrounding the cell [23].…”

Light-trapping in perovskite solar cells

“…15,16 Light-trapping has proved beneficial in Silicon, Gallium Arsenide and organic solar cells. [17][18][19][20][21][22] Light absorption enhancement and PCE improvement have been demonstrated both experimentally and theoretically. However, light-trapping enhancements have not yet been fully exploited in perovskite solar cells.…”

“…We identify an inverted-vertical-cone photonic crystal with MAPD of 25.1 mA/cm 2 . This is 92% of the total available photocurrent and exceeds the Lambertian limit (23.7 mA/cm 2 ) 19 for an equivalent thickness of 180 nm. For the CH(NH 2 ) 2 PbI 3 perovskite, the total available photocurrent from sunlight from 300 nm to 850 nm is 30.5 mA/cm 2 .…”

“…With this anti-reflection coating, the absorption is enhanced for wavelengths longer than 500 nm, leading to an improved MAPD of 24.3 mA/cm 2 . For an equivalent bulk thickness of L = 180 nm of CH 3 NH 3 PbI 3 , the absorption spectrum in the "Lambertian limit" 19,28 is given by: Here α = 4πk λ is the absorption coefficient, n and k are the real and imaginary parts of the refractive index of CH 3 NH 3 PbI 3 . The Lambertian absorption benchmark is shown in Fig.…”

“…Our light-trapping strategy is based on parallel-to-interface refraction of incident sunlight into slow-light modes within the active range. 19,20 The efficient capture of this additional sunlight requires a modified structure. As in the case of CH 3 NH 3 PbI 3 , we now optimize the MAPD of CH(NH 2 ) 2 PbI 3 using our inverted cone photonic crystal architecture.…”