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Synergistic plasmonic and photonic crystal light-trapping: Architectures for optical up-conversion in thin-film solar cells

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Abstract

We demonstrate, numerically, that with a 60 nanometer layer of optical up-conversion material, embedded with plasmonic core-shell nano-rings and placed below a sub-micron silicon conical-pore photonic crystal it is possible to absorb sunlight well above the Lambertian limit in the 300-1100 nm range. With as little as 500 nm, equivalent bulk thickness of silicon, the maximum achievable photo-current density (MAPD) is about 36 mA/cm2, using above-bandgap sunlight. This MAPD increases to about 38 mA/cm2 for one micron of silicon. Our architecture also provides solar intensity enhancement by a factor of at least 1400 at the sub-bandgap wavelength of 1500 nm, due to plasmonic and photonic crystal resonances, enabling a further boost of photo-current density from up-conversion of sub-bandgap sunlight. With an external solar concentrator, providing 100 suns, light intensities sufficient for significant nonlinear up-conversion can be realized. Two-photon absorption of sub-bandgap sunlight is further enhanced by the large electromagnetic density of states in the photonic crystal at the re-emission wavelength near 750 nm. It is suggested that this synergy of plasmonic and photonic crystal resonances can lead to unprecedented power conversion efficiency in ultra-thin-film silicon solar cells.

© 2013 Optical Society of America

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Figures (9)

Fig. 1
Fig. 1 A unit cell of the combined plasmonic and photonic crystal silicon solar cell structure based on a square lattice of slanted conical-pores in silicon and metallic core-shell rings. The up-conversion layer, in which the up-converter is doped, is sandwiched between the slanted cone PC and the Ag back reflector. The plasmonic core-shell ring resonator is integrated inside the up-conversion layer.
Fig. 2
Fig. 2 The MAPD of the cell (from above-bandgap solar absorption) with and without the up-conversion (UC) buffer layer as a function of the buffer layer thickness.
Fig. 3
Fig. 3 (a) The absorption spectra of (i) the bare slanted conical pore photonic crystal-based solar cell without the UC layer (black line) and (ii) the cell with the UC layer (red line), (iii) with both the UC layer and the core-shell ring (blue line), and (iv) the Lambertian statistical ray trapping limit (green line). (b) The absorption loss as a function of wavelength in the back reflector of (i) the bare cell without the UC layer (black line), (ii) the cell with the UC layer only (red line) and (iii) the cell having both the UC layer and the core-shell ring (blue line).
Fig. 4
Fig. 4 Short circuit current optimization for the slanted conical pore PC cell with lattice constant a = 850 nm, cone radius R = 425 nm, and up-conversion layer thickness tuc = 60 nm for various core-shell ring geomeries: (a) MAPD as a function of inner radius for fixed ring thickness tr = 35 nm (b) MAPD as a function of ring thickness for fixed inner ring radius Ri = 120 nm. The maximum short circuit current is obtained for inner ring radius Ri = 120 nm and thickness tr = 35 nm.
Fig. 5
Fig. 5 Normalized electric field intensity at certain wavelengths in the chosen xz plane and yz plane view, respectively. The peak intensity enhancement in the photonic crystal at 750 nm and 1032 nm is about 100 and 200, respectively.
Fig. 6
Fig. 6 The MAPD of slanted concial pore PC’s as function of pore depth with and without plasmonic core-shell ring resonators. Here the ratio of pore height to equivalent bulk thickness of silicon is about 1.6. On the same graph, we plot the Lambertian limit for (nonporous) films of thickness equal to the pore height/F where F~1.6. In other words, the Lambertian films contain an equal amount of silicon to the photonic crystal films to which they are directly compared. At the pore height of 800 nm (500 nm of Si), the MAPD is reduced by only about 5% from the MAPD at the pore height of 1600 nm (1 micron of Si). This suggests the possibility of high-efficiency, thin-film silicon solar cells with equivalent bulk thickness of 500 nm.
Fig. 7
Fig. 7 Angular response in term of the MAPD of the slanted conical pore PC cell with and without plasmonic core-shell ring resonators for TE- and TM-polarized light illuminations.
Fig. 8
Fig. 8 Ratio of the sub-bandgap solar absorption (integrated over the up-conversion volume) in the 60 nm thick up-conversion layer with and without the plasmonic core-shell ring resonator.
Fig. 9
Fig. 9 Normalized electric field intensity in the up-conversion layer caused by the core-shell ring at resonant wavelengths of 1350 nm and 1500 nm. The peak intensity ratio for 1500 nm is about 1400. Here |E0|2 is the incident solar intensity at the chosen wavelength.

Equations (2)

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A E Int = 300nm 1100nm A( λ )×AM1.5G ( λ )dλ 300nm 1100nm AM1.5G ( λ )dλ .
MAPD= 300nm 1100nm eλ hc A( λ )×AM1.5G ( λ )dλ.
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