Light-trapping optimization in wet-etched silicon photonic crystal solar cells

Abstract

We demonstrate, by numerical solution of Maxwell's equations, near-perfect solar light-trapping and absorption over the 300-1100 nm wavelength band in silicon photonic crystal (PhC) architectures, amenable to fabrication by wet-etching and requiring less than 10 lm (equivalent bulk thickness) of crystalline silicon. These PhC's consist of square lattices of inverted pyramids with sides comprised of various (111) silicon facets and pyramid center-to-center spacing in the range of 1.3-2.5 lm. For a wet-etched slab with overall height H ¼ 10 lm and lattice constant a ¼ 2.5 lm, we find a maximum achievable photo-current density (MAPD) of 42.5 mA/cm 2 , falling not far from 43.5 mA/cm 2 , corresponding to 100% solar absorption in the range of 300-1100 nm. We also demonstrate a MAPD of 37.8 mA/cm 2 for a thinner silicon PhC slab of overall height H ¼ 5 lm and lattice constant a ¼ 1.9 lm. When H is further reduced to 3 lm, the optimal lattice constant for inverted pyramids reduces to a ¼ 1.3 lm and provides the MAPD of 35.5 mA/cm 2. These wetetched structures require more than double the volume of silicon, in comparison to the overall mathematically optimum PhC structure (consisting of slanted conical pores), to achieve the same degree of solar absorption. It is suggested these 3-10 lm thick structures are valuable alternatives to currently utilized 300 lm-thick textured solar cells and are suitable for large-scale fabrication by wet-etching. V

Towards 30% Power Conversion Efficiency in Thin-Silicon Photonic-Crystal Solar Cells

“…The rear passivation layer is implemented as a 50 nm SiO 2 (n = 1.45) layer. This reduces parasitic absorption losses in real-world back contact such as silver [13] and justifies our modeling of the back-reflector as a PEC. We apply periodic boundary conditions (PBC) along x and y−directions and put perfectly matched layers (PML) at the computation boundaries normal to the z−direction.…”

“…Unlike the classical pyramid arrays with 10µm base lengths [23] that can be described by ray-optics, we consider the largely unexplored regime of micro-pyramid photonic crystals with wavelength-scale features and novel wave-optics phenomena. Using a rigorous FDTD solution of Maxwell's equations, light-trapping and absorption beyond the Lambertian limit was demonstrated [13] in an optimized 10 µm thick c − Si inverted micro-pyramid PhC. The optimum lattice constant in this case is 2500 nm, yielding a MAPD of 42.5 mA/cm 2 .…”

“…Harvesting of solar energy through coupling of sunlight into slow photonic modes of ultra-thin c − Si PhCs has been discussed previously with modulated nanowire [10,11] and slanted conical pore [12] architectures. Arrays of slightly thicker inverted pyramid [13][14][15][16] and parabolic pore [17] photonic crystals have also been shown to support PIR and slow optical modes to increase the optical path length and dwell time of the photons resulting in significantly increased absorption. Wave-interference based strong resonances associated with parallel-to-interface refraction (PIR) [7] are particularly significant to absorption enhancement in the 800 − 1100 nm wavelength range [17] where c−Si has weak intrinsic absorption.…”

Light-trapping in perovskite solar cells

“…It is of interest to explore nano-imprinting methods for the fabrication of thin-film, inverted-cone photonic crystals. It is also of interest to study the possibility of deposition of perovskites onto thin-film Silicon photonic crystals 31 to realized very high-efficiency tandem solar cells.…”

Nanostructured Silicon Used for Flexible and Mobile Electricity Generation

“…A recent simulation revealed that a set of nanostructured silicon (with substrate thickness of 3–8 μm) can achieve a short‐circuit current of 42.5 mA cm −2 by optimizing the inverted nanopyramid structure, as shown in Figure b. Compared with that for currently utilized few‐hundred‐micrometer‐thick‐silicon textured solar cells, where high‐quality Si is required to achieve a similar short‐circuit current density, as shown in Figure c,d, the requirement of silicon quality for high‐performance solar cells with low weight may be relaxed. It has been reported that many forms of nanostructured silicon, such as nanopillars, inverted nanopyramids, and nanoholes, enable light harvesting.…”