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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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Summary Evidence is mounting that the major histocompatibility complex (MHC) molecule HLA-F regulates the immune system in pregnancy, infection, and autoimmunity by signaling through NK-cell receptors (NKRs). We present structural, biochemical and evolutionary analyses demonstrating that HLA-F presents peptides of unconventional length dictated by a newly arisen mutation (R62W) that has produced an open-ended groove accommodating particularly long peptides. Compared to empty HLA-F open conformers (OC), HLA-F tetramers bound with human-derived peptides differentially stained leukocytes suggesting peptide-dependent engagement. Our in vitro studies confirm that NKRs differentiate between peptide-bound and peptide-free HLA-F. The complex structure of peptide-loaded β2m-HLA-F bound to the inhibitory LIR1 revealed similarities to high-affinity recognition of the viral MHC-I mimic UL18 and a docking strategy that relies on contacts with HLA-F as well as β2m, thus precluding binding to HLA-F OC. These findings provide a biochemical framework to understand how HLA-F could regulate immunity via interactions with NKRs.

Iron acquisition is critical for virulence of the human pathogenic fungus Cryptococcus neoformans. The cryptococcal transcript for the extracellular mannoprotein Cig1 is highly regulated by iron and abundant in iron-starved cells, suggesting a role in iron acquisition. Indeed, loss of Cig1 resulted in delayed growth on heme at physiological pH. Expression of CIG1 is regulated by the pH-responsive transcription factor Rim101, and loss of Rim101 also impaired growth on heme. A cig1Δ mutant was less susceptible than the wild-type strain to noniron metalloporphyrins, further indicating a role for Cig1 in heme uptake. Recombinant Cig1 exhibited the absorbance spectrum of a heme-binding protein upon heme titration, and Cig1 may therefore function as a hemophore at the cell surface. Cig1 contributed to virulence in a mouse model of cryptococcosis but only in a mutant that also lacked the high-affinity iron uptake system. Overall, Cig1-mediated heme uptake is a potential therapeutic target in C. neoformans.

FIG. 4. (a) Absorption in the vicinity of an anti-crossing of two resonances with the same symmetry, as a function of frequency and photonic crystal slab thickness. (b) Absorption spectra for three photonic crystal slab thicknesses. The sections of the spectra containing the anti-crossing resonances are shown in red. The exchange of line shapes in the spectra for the thickest and the thinnest slab is a signature of anti-crossing. Panels (c) and (d) show the distribution of jE jj j 2 along transverse cuts through the centerline of the unit cell. Both resonances have odd symmetry.