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Nanophotonic light trapping with patterned transparent conductive oxides

Alok P. Vasudev, Jon A. Schuller, and Mark L. Brongersma

Open Access Open Access

Abstract

Transparent conductive oxides (TCOs) play a crucial role in solar cells by efficiently transmitting sunlight and extracting photo-generated charge. Here, we show how nanophotonics concepts can be used to transform TCO films into effective photon management layers for solar cells. This is accomplished by patterning the TCO layer present on virtually every thin-film solar cell into an array of subwavelength beams that support optical (Mie) resonances. These resonances can be exploited to concentrate randomly polarized sunlight or to effectively couple it to guided and diffracted modes. We first demonstrate these concepts with a model system consisting of a patterned TCO layer on a thin silicon (Si) film and outline a design methodology for high-performance, TCO-based light trapping coatings. We then show that the short circuit current density from a 300 nm thick amorphous silicon (a-Si) cell with an optimized TCO anti-reflection coating can be enhanced from 19.9 mA/cm2 to 21.1 mA/cm2, out of a possible 26.0 mA/cm2, by using an optimized nanobeam array. The key differences and advantages over plasmonic light trapping layers will be discussed.

© 2012 Optical Society of America

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Figures (5)
Fig. 1
Fig. 1 Schematic of our model structure consisting of a periodic array of transparent conductive oxide (ZnO) beams placed on top of a thin crystalline Si layer. The structure is supported by a silica substrate. We characterize the square beams by their width, w, and we define the array by a grating pitch or a reciprocal lattice vector, G.

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Fig. 2
Fig. 2 Spectral dependence of the scattering efficiency, Qsca, for square cross-section ZnO nanobeams in air. The dimensions, w, of the beams are indicated on the right-hand side and the spectra for different sizes are vertically offset in increments of 3 for clarity. The spectral dependence of the AM 1.5 solar irradiance is shown in the background (grey) for reference.

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Fig. 3
Fig. 3 A ZnO nanobeam array significantly enhances the absorption in a silicon layer at almost all wavelengths. (a) The AM 1.5 solar irradiance (orange) provides the spectral power density of sunlight and the spectral response of the silicon/silica stack (black) diminishes with longer wavelength, excepting a Fabry-Perot peak near 450 nm. (b) The absorption enhancement provided by the nanobeam array under both TE (blue) and TM (red) illumination exceeds that of a planar antireflection coating (green). (c) The product of the solar irradiance, the spectral response, and the absorption enhancement, integrated over wavelength, yields the short-circuit current density value. We find that the short-circuit current density generated in the silicon layer by the ZnO nanobeam array enhances that of the bare structure by 87% for unpolarized light, in comparison to the 46% enhancement gained using a planar antireflection coating. The unpolarized light enhancement was calculated by averaging the TE and TM enhancements, 78% and 95% respectively.

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Fig. 4
Fig. 4 Absorption enhancement maps and field intensity plots reveal the physics behind the absorption enhancements. (a,b) Absorption enhancement maps for TE (a) and TM (b) polarized illumination contain two classes of features: broad, G-invariant regions and narrow, G-dependent bands. The former can be assigned to Mie-like scattering resonances associated with individual ZnO nanobeams while the latter derive from coupling of light into diffracted and wave-guided modes by the array of nanobeams. (c) Magnetic field distribution for a Mie-like resonance, taken from the location marked C in (a). (d) Electric field distribution for a diffracted mode, taken from the location marked D in (b). (e) Electric field distribution for a wave-guided mode, taken from the location marked E in (b).

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Fig. 5
Fig. 5 An optimized ZnO nanobeam array improves the optical performance of a realistic amorphous silicon solar cell. (a) An amorphous silicon solar cell stack consisting of an Al reflector, a ZnO spacer layer, a 300 nm a-Si layer and an array of ZnO nanobeams. The square ZnO nanobeams have a width of 240 nm and the array pitch is 408 nm. (b) TE and TM absorption maps for this structure consist of the same features as before, implying Mie-like resonances and the coupling of light into diffracted and wave-guided modes constitute the absorption enhancement physics. (c) The ZnO nanobeam array (blue) improves upon the performance of an optimized planar ZnO film (green) in terms of allowing the a-Si film to reach its absorbing capacity, defined as full absorption of the above-band gap portion of the AM 1.5 spectrum (orange). (d) The ZnO nanobeams improve the short-circuit current density of an optimal solar cell with a planar ZnO layer from 19.9 mA/cm2 to 21.1 mA/cm2. The maximum achievable short-circuit current density is 26.0 mA/cm2, assuming complete absorption of the above-band gap AM 1.5 spectrum.

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