Integration of Si Heterojunction Solar Cells with III-V Solar Cells by the Pd Nanoparticle Array-Mediated "Smart Stack" Approach.

This paper describes the way to fabricate two-terminal tandem solar cells using Si heterojunction (SHJ) bottom cells and GaAs-relevant III-V top cells by "smart stack", an approach enabling the series connection of dissimilar solar cells through Pd nanoparticle (NP) arrays. It was suggested that placing the Pd NP arrays directly on typical SHJ cells results in poor tandem performance because of the insufficient electrical contacts and/or deteriorated passivation quality of the SHJ cells. Therefore, hydrogenated nanocrystalline Si (nc-Si:H) layers were introduced between Pd NPs and SHJ cells to improve the electrical contacts and preserve the passivation quality. Such nc-Si:H-capped SHJ cells were integrated with InGaP/AlGaAs double-junction cells, and a certified efficiency of 27.4% (under AM 1.5 G) was achieved. In addition, this paper addresses detailed analyses of the 27.4% cell. It was revealed that the cell had a relatively large gap at the smart stack interface, which limited the short-circuit current density (thereby the efficiency) of the cell. Therefore, higher efficiency would be expected by reducing the interfacial gap distance, which is governed by the height of the Pd NPs.

Application of polydimethylsiloxane surface texturing on III-V//Si tandem achieving more than 2 % absolute efficiency improvement.

Silicon based multi-junction solar cells are a promising approach for achieving high power conversion efficiencies using relatively low-cost substrates. In recent years, 2-terminal triple-junction solar cells using GaInP/GaAs as top cells and Si bottom cell have achieved excellent efficiencies. Epitaxial growth or wafer bonding has been used for the integration of the cells. This requires the top surface of the Si cell to be polished for effective integration, sacrificing the light trapping in the Si cell. The poor long wavelength light absorption in silicon limits the tandem cell efficiency as it is limited by current mismatch. In this work, for the first time, an external surface texturing is attached onto a GaInP/GaAs//Si wafer bonded triple-junction solar cell, using polydimethylsiloxane (PDMS) layers with surface geometries replicated from various pyramidally-textured silicon wafers. With reduced reflection, the short circuit current density is increased by 0.95 mA/cm2, while the overall cell efficiency is boosted by more than 2 % absolute.

Impact of nanometer air gaps on photon recycling in mechanically stacked multi-junction solar cells.

We investigate photon recycling at the top subcell in mechanically stacked multi-junction solar cells with nanometer air gaps between the subcells. We determine the incident-angle-dependence of the reflectivity from the rear surface of the top subcell. The results show that more than 30% of the luminescence at the top subcell is reflected at the air gap even for an air gap thickness of 10 nm. In addition, we demonstrate enhanced luminescence extraction in GaAs//InGaAsP dual-junction devices with nanometer air gaps compared to a device with no gap between the subcells. Our findings indicate that an efficient photon recycling can be realized even for air gaps of a few tens of nanometers.

Effects of white rice containing enriched gamma-aminobutyric acid on blood pressure.

Gamma-aminobutyric acid (GABA) is an inhibitory neurotransmitter with beneficial effects including antihypertension and antistress properties. In this study, we examined the effects of GABA-enriched white rice (GABA rice) on blood pressure (BP) in 39 mildly hypertensive adults in a randomized, double-blind, placebo-controlled study. The participants were divided into a test group (n = 22) who consumed rice with 11.2 mg GABA/100 g of rice and a placebo group (n = 17) who consumed rice with 2.7 mg GABA/100 g of rice. For 8 weeks, the participants took 150 g of either the GABA rice or the placebo rice. Hematological examinations were performed on both groups at 0, 4, and 8 weeks after the start of rice consumption. Home BP was self-measured two times daily, morning and evening, from 1 weeks before to 2 weeks after the intervention. Although the hospital BP and evening BP measurements of the participants showed no significant change, consumption of the GABA rice improved the morning BP compared with the placebo rice after the 1(st) week and during the 6(th) and 8(th) weeks. These results showed the possibility that the GABA rice improves morning hypertension.

Integration of Light Trapping Silver Nanostructures in Hydrogenated Microcrystalline Silicon Solar Cells by Transfer Printing.

One of the potential applications of metal nanostructures is light trapping in solar cells, where unique optical properties of nanosized metals, commonly known as plasmonic effects, play an important role. Research in this field has, however, been impeded owing to the difficulty of fabricating devices containing the desired functional metal nanostructures. In order to provide a viable strategy to this issue, we herein show a transfer printing-based approach that allows the quick and low-cost integration of designed metal nanostructures with a variety of device architectures, including solar cells. Nanopillar poly(dimethylsiloxane) (PDMS) stamps were fabricated from a commercially available nanohole plastic film as a master mold. On this nanopatterned PDMS stamps, Ag films were deposited, which were then transfer-printed onto block copolymer (binding layer)-coated hydrogenated microcrystalline Si (µc-Si:H) surface to afford ordered Ag nanodisk structures. It was confirmed that the resulting Ag nanodisk-incorporated µc-Si:H solar cells show higher performances compared to a cell without the transfer-printed Ag nanodisks, thanks to plasmonic light trapping effect derived from the Ag nanodisks. Because of the simplicity and versatility, further device application would also be feasible thorough this approach.

Capturing by self-assembled block copolymer thin films: transfer printing of metal nanostructures on textured surfaces.

A method to fabricate metal nanostructures by transfer printing, applicable to textured surfaces, is described. The key is the use of self-assembled polystyrene-block-poly-2-vinylpyridine thin films as binding layers. The plasmonic properties of the obtained metal (Ag) nanostructures showed the potential of this method in the design of novel devices.

Building upon patterned organic monolayers produced via catalytic stamp lithography.

Soft lithographic sub-100 nm chemical patterning was demonstrated on organic monolayer surfaces using poly(dimethylsiloxane)-based stamps decorated with Pd nanostructures, structures termed "catalytic stamps". Chemically reactive azide or alkene functionalities were incorporated on oxide-capped silicon surfaces and utilized for patterning via Pd-catalyzed hydrogenation or Heck reactions. The catalytic stamps were soft lithographic stamps based on PDMS with embedded nanoscale palladium catalysts, prepared via block copolymer-based templating. Nanoscale chemical patterns were readily generated on the azide or alkene precursor surfaces simply by applying the Pd catalytic stamps and the reactive molecule, the molecular ink, to the surface, thanks to the highly localized catalytic transformations induced by the patterned, immobilized solid Pd catalysts. A series of successful postfunctionalization reactions on the resulting patterned surfaces further demonstrated the utility of this approach to construct novel designs of nanoarchitectures, with potentially unique and innovative properties.

Nanoscale patterning of organic monolayers by catalytic stamp lithography: scope and limitations.

Developing a method to pattern organic molecules, particularly on the sub-100-nm scale, is of wide interest in current nanoscience for a broad range of technological applications. Because of the efficiency and simplicity of soft lithography, here we describe in detail the process for synthesizing and applying catalytic stamp lithography, a process that can easily produce sub-100-nm patterns on surfaces; in this work, the approach is demonstrated on silicon. Catalytic stamps were fabricated through a two-step procedure in which the nanoscale pattern of catalysts is produced via a self-assembled block-copolymer-templated synthesis of metallic nanostructures on SiO(x)/Si supports, followed by the production of the poly(dimethylsiloxane) (PDMS) stamp on top of the as-patterned metals. Simply peeling off the as-formed PDMS stamp removes the metallic nanostructures, leading to the functional stamp. Two different patterns, pseudohexagonal and linear Pt nanoarrays, were produced from a single block copolymer, PS(125000)-b-P2VP(58500), by controlling the morphology of thin-film templates through tetrahydrofuran vapor annealing. When terminal alkenes, alkynes, or aldehydes with different functionalities were used as molecular inks, these Pt nanopatterns on catalytic stamps were translated into corresponding molecular arrays on Si(111)-H and Si(100)-H(x) surfaces because catalytic hydrosilylation took place exclusively underneath patterned Pt nanostructures. With this straightforward approach, the resolution limit of conventional microcontact printing (approximately 100 nm) could be downsized to a sub-20-nm scale, while maintaining the advantages of stamp-based patterning (i.e., large-area, high-throughput capabilities and low cost).

Catalytic stamp lithography for sub-100 nm patterning of organic monolayers.

A stamp-based nanoscale patterning technique of organic monolayers, termed catalytic stamp lithography, is described. The surface of poly(dimethylsiloxane) was patterned with catalytic Pd nanoparticles (NPs) via the use of self-assembled block copolymers. Using this catalytic stamp, catalytic hydrosilylations of terminal alkenes/alkynes were performed on H-terminated Si(111) or Si(100) surfaces to create nanoscale patterns of organic monolayers. Since the reaction takes place exclusively underneath the patterned Pd NPs (localized catalysis), the pattern formation is less susceptible to ink diffusion and stamp deformation, even at this sub-100 nm scale. A range of different molecular inks can be utilized to produce monolayer patterns of different chemical functionalities, and the stamps can be reused multiple times. The potential utility of this kind of chemically patterned surfaces as the basis for more complicated nanoarchitectures was demonstrated through gold nanoparticle capture, with a thiol-terminated nanopatterned silicon surface.

Oxovanadium(v)-catalyzed oxidative biaryl synthesis from organoborate under O2.

Oxidative ligand coupling of organoborates was catalyzed by VO(OEt)Cl(2) under oxygen atmosphere, which provides a versatile method for the selective synthesis of symmetrical or unsymmetrical biaryls.