Enhancing the light absorption in ultrathin-film silicon solar cells is important for improving efficiency and reducing cost. We introduce a double-sided grating design, where the front and back surfaces of the cell are separately optimized for antireflection and light trapping, respectively. The optimized structure yields a photocurrent of 34.6 mA/cm 2 at an equivalent thickness of 2 μm, close to the Yablonovitch limit. This approach is applicable to various thicknesses and is robust against metallic loss in the back reflector. KEYWORDS: Solar cells, light trapping, antireflection, crystalline silicon, absorption enhancement, nanocone gratings T here is significant recent interest in designing ultrathin crystalline silicon solar cells with active layer thickness of a few micrometers.1−17 Efficient light absorption in such thin films requires both broadband antireflection coatings and effective light trapping techniques, which often have different design considerations. In this Letter, we show that by employing a double-sided grating design, we can separately optimize the geometries for antireflection and light trapping purposes to achieve broadband light absorption enhancement. The photocurrent generated by the proposed thin film absorber is able to reach the Yablonovitch limit. 18−20We use nanocones as the basic building elements for the grating geometry because of their unique optical properties and compatibility with inexpensive fabrication techniques. 21−23 The structure we consider, as shown in Figure 1a, contains a crystalline silicon thin film with nanocone gratings also made of silicon. The circular nanocones form two-dimensional square lattices on both the front and the back surfaces. The film is placed on a mirror. As a starting point, we assume the mirror is made of a perfect electric conductor (PEC). We will consider the more realistic silver mirror with metal loss toward the end of the paper.The optimization process is as follows: For a given structure with two-dimensional nanocone gratings, using the rigorous coupled wave analysis (RCWA), 24−26 we calculate the absorption spectrum from which we determine the short circuit current assuming an air mass 1.5 (AM1.5) incident solar irradiance. In a supercell of period 1000 nm, we optimize the geometry over six parameters, the numbers of primitive cells and the base radii and heights of the nanocones on both sides, for the greatest photocurrent generated from the structure. In the optimization, we adjust those geometrical parameters, as well as the thickness of the uniform layer sandwiched between the top and bottom gratings, while ensuring that the structures always consist of the same amount of silicon as a flat thin film structure with a predetermined thickness. We refer to this thickness as the equivalent thickness of our nanostructured thin film.Our optimized structure for an equivalent thickness of 2 μm is shown in Figure 1a. For the top nanocones, the period is 500 nm, the base radius is 250 nm, and the height is 710 nm; for the bottom...

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Miniaturized spectrometers have significant potential for portable applications such as consumer electronics, health care, and manufacturing. These applications demand low cost and high spectral resolution, and are best enabled by single-shot free-space-coupled spectrometers that also have sufficient spatial resolution. Here, we demonstrate an on-chip spectrometer that can satisfy all of these requirements. Our device uses arrays of photodetectors, each of which has a unique responsivity with rich spectral features. These responsivities are created by complex optical interference in photonic-crystal slabs positioned immediately on top of the photodetector pixels. The spectrometer is completely complementary metal–oxide–semiconductor (CMOS) compatible and can be mass produced at low cost.

Silicon has been driving the great success of semiconductor industry, and emerging forms of silicon have generated new opportunities in electronics, biotechnology, and energy applications. Here we demonstrate large-area free-standing ultrathin single-crystalline Si at the wafer scale as new Si materials with processability. We fabricated them by KOH etching of the Si wafer and show their uniform thickness from 10 to sub-2 μm. These ultrathin Si exhibits excellent mechanical flexibility and bendability more than those with 20-30 μm thickness in previous study. Unexpectedly, these ultrathin Si materials can be cut with scissors like a piece of paper, and they are robust during various regular fabrication processings including tweezer handling, spin coating, patterning, doping, wet and dry etching, annealing, and metal deposition. We demonstrate the fabrication of planar and double-sided nanocone solar cells and highlight that the processability on both sides of surface together with the interesting property of these free-standing ultrathin Si materials opens up exciting opportunities to generate novel functional devices different from the existing approaches.

We derive tight upper and lower bounds of the ratio between decay rates to two ports from a single resonance exhibiting Fano interference, based on a general temporal coupled-mode theory formalism. The photon transport between these two ports involves both direct and resonance-assisted contributions, and the bounds depend only on the direct process. The bounds imply that, in a lossless system, full reflection is always achievable at Fano resonance, even for structures lacking mirror symmetries, while full transmission can only be seen in a symmetric configuration where the two decay rates are equal. The analytic predictions are verified against full-field electromagnetic simulations.