[en] Highlights: • A chromium oxide layer is formed as diffusion barrier by thermal oxidation process on STS substrate. • A Cr_2O_3 layer effectively reduces impurities diffusion into the CIGS absorber layer. • The Cr_2O_3 layer plays an important role in increasing the efficiency by reduction of impurity diffusion. - Abstract: It is known that the efficiency of flexible Cu(In,Ga)Se_2 (CIGS) solar cells fabricated on stainless-steel (STS) substrates deteriorates due to iron (Fe) and Cr impurities diffusing into the CIGS absorber layer. To overcome this problem, a nanoscale homomorphic chromium oxide layer was formed as a diffusion barrier by thermal oxidation on the surface of STS substrates for 1 min at 600 °C in oxygen atmosphere. By TEM and grazing-incidence X-ray diffraction (GIXRD), it was confirmed that the formed oxide layer on surface of STS substrates was a Cr_2O_3 layer. It was found that the formed homomorphic Cr_2O_3 thin layer of about 15 nm thickness was an effective diffusion barrier to reduce impurity diffusion into the CIGS layer by secondary ion mass spectroscopy (SIMS). In contrast to the efficiency of CIGS solar cell without homomorphic Cr_2O_3 diffusion layer is 8.6%, whereas with diffusion barrier it increases to 10.6% because of impurities such as Fe and Cr from the STS substrate into the CIGS layer. It reveals that the layer formed on the surface of STS substrate by thermal oxidation process plays an important role in increasing the performance of CIGS solar cells.

[en] We reported the first example of the green lyotropic chromonic liquid crystalline compound based on perylene bisimide. The color tuning of perylene bisimide was achieved by introducing electron-donating substituents at the bay area. The syntheses proceeded in good yields via a short and convenient synthetic route. This dye exhibits good thermal stability, solubility in water and organic solvents as well. The structure was characterized by ¹H-NMR, ¹³C-NMR, mass, and UV/Vis spectroscopies. Aqueous solution of hydrochloride salt exhibited lyotropic chromonic liquid crystalline columnar phase at room temperature examined by polarized optical microscopy. Perylene bisimides (PBIs) are widely used as dyes and pigments since they show outstanding chemical, thermal and photochemical stability. Compared to other dyes and pigments, PBIs have advantages such as better solubility, synthetic simplicity, and less trouble in purification. Beside the conventional uses, PBIs are key chromophores for high-tech applications such as re-prographic processes, fluorescent solar collectors, photovoltaic cells, optical switches, and lasers. In this context, PBIs are generally referred to as functional dyes. Especially, PBIs having ionic periphery have attracted much attention because they can form lyotropic chromonic liquid crystal (LCLC) in aqueous solution. The anisotropic orientation of molecules with π-π stacking of aromatic core can exploit their intrinsic properties most efficiently in the potential applications such as coatable polarizers, oriented optical films, and biosensors

[en] A novel tetrathiafulvalene (TTF)-based main-chain polymer (6TTF-polymer) was successfully synthesized via a condensation polymerization between a newly synthesized dihydroxy TTF derivative and a malonyl chloride, and its chemical structure was characterized by spectroscopic techniques. Molecular weight of the 6TTF-polymer (9,030 g/mol by gel permeation chromatography) was large enough to form the ductile film. The electrochemical and optical properties of the 6TTF-polymer were further estimated by cyclic voltammetry, ultraviolet and photoluminescence spectroscopes. The highest occupied molecular orbital level (E_H_O_M_O = .4.79 eV) and band-gap energy (E_g = 1.91 eV) of the 6TTF-polymer suggested that TTF-based polymer could act as a good electron donating material for the optoelectronic applications

[en] This paper describes a new method for the dispersing and surface-functionalization of metal oxide magnetic nanoparticles (10 nm) with poly(allylamine) (PAA). In this approach, Fe₃O₄ nanoparticles, prepared with diethanolamine (DEA) as the surface capping agent in diethyleneglycol (DEG) and methanol, are ligand exchanged with PAA. This method allows the dispersing of magnetic nanoparticles into individual or small clusters of 2-5 nanoparticles in aqueous solutions. The resulting nanoparticles are water soluble and stable for months. The PAA stabilized Fe₃O₄ nanoparticles are characterized by TEM, TGA, and FT-IR. The PAA-coated Fe₃O₄ nanoparticles will allow further chemical tailoring and engineering of their surfaces for biomedical applications

[en] This paper reports the formation of GaN and InN quantum dots (QDs) with symmetric spherical shapes, grown on SiN/Si(111). Spherical QDs are grown by modulating initial growth behavior via gallium and indium droplets functioning as nucleation sites for QDs. Field-emission scanning electron microscope (FE-SEM) images show that GaN and InN QDs are formed on curved SiN/Si(111) instead of on a flat surface similar to balls on a latex mattress. This is considerably different from the structural properties of In(Ga)As QDs grown on GaAs or InP. In addition, considering the shape of the other III–V semiconductor QDs, the QDs in this study are very close to the ideal shape of zero-dimensional nanostructures. Transmission-electron microscope images show the formation of symmetric GaN and InN QDs with a round shape, agreeing well with the FE-SEM results. Compared to other III–V semiconductor QDs, the unique structural properties of Si-based GaN and InN QDs are strongly related to the modulation in the initial nucleation characteristics due to the presence of droplets, the degree of lattice mismatch between GaN or InN and SiN/Si(111), and the melt-back etching phenomenon. (paper)

[en] The electro-optic response of a carbon nanotube (CNT) cluster has been investigated. The cluster absorbs incident light before stretching. In the presence of an electric field, the cluster starts stretching along the field direction and contracts back to its original stage when the applied voltage is removed. The stretched cluster absorbs and transmits incident light with its electric vector propagating parallel and perpendicular to the long axis of the stretched cluster, respectively. Utilizing this selective light absorption property of a CNT cluster, a tunable polarizer or non-emissive light modulator can be realized.

[en] We proposed a method to reduce the operating voltage and the grey-to-grey switching time of a vertically aligned liquid crystal display using a mixture of planar and vertical polyimide alignment materials. The surface anchoring energy of the two-polyimide mixture was smaller than that of the pure vertical polyimide and consequently, liquid crystal molecules were easily switched to a planar state with an electric field, resulting in a greater maximum retardation than that of the pure polyimide at the same applied voltage. Rising time was also significantly reduced due to the suppressed optical bouncing effect in the mixed planar polyimide, and the decaying time showed negligible change. With the proposed approach, we can reduce the cell gap to obtain half-wave retardation allowing for faster response time while keeping a low operating voltage. (paper)

[en] Cu(In, Ga) Se₂ (CIGS) thin-film solar cells are currently the fastest growing photovoltaic technology due to the higher performance ratio and lower energy payback time compared to the silicon, and good stability. Despite of these interesting properties, CIGS solar cells suffer from poor spectral response at short wavelengths particularly in the range of 300–400 nm. In the present work, we have introduced GaN nanowires in CIGS solar cells in order to improve the performance of photovoltaic device in the short wavelength region (300–400 nm). To evaluate the influence of GaN nanowires, the morphological, structural and electrical properties have been studied. These studies have shown the introduction of GaN nanowires not only improve the cell efficiency but also enhance the spectral response in the wavelength range 300–400 nm. The CIGS solar cells fabricated with GaN nanowires displayed power conversion efficiency of 10.15% with a fill factor of 65%, which are substantially higher than that of reference cell (without GaN nanowires).

[en] A polymer-stabilized vertical alignment (PS-VA) mode using a new type of photoreactive monomer for polymer stabilization of the liquid crystal (LC) director was developed. Conventional reactive mesogens having a higher molecular weight than those of the host LC tend to aggregate and form large-sized polymer grains when exposed to ultraviolet (UV) light, subsequently deteriorating the quality of the dark state. To address these problems, bis(4-hydroxyphenyl) diacrylates were synthesized with four different linking groups as stabilizing monomers (SMs) which have molecular weights similar to that of the host LC. Their stabilizing characteristics with respect to the molecular size and polarity of SMs were evaluated by examining the electro-optic characteristics of LC cells after UV irradiation. The results showed that the SM containing a small linking group in size between biphenyls with high polarity was favored to achieve excellent polymer stabilization. The SM containing an ether linkage showed excellent electro-optic characteristics with no large-sized polymer grains even in the absence of a photo-initiator. Consequently, we anticipate that SMs, polar and smaller in size, can improve the electro-optic characteristics in PS-VA mode. (paper)

[en] The current study concerns about the large band gap of TiO₂ for its use as photocatalysts. The photocatalytic activity of core-shell structured Au@TiO₂ nanoparticles were enhanced by the doping of nitrogen. The nitrogen doping has been done by simple hydrothermal method taking ethylenediamine as the precursor for nitrogen. The crystals structure of TiO₂ shell remained unaltered even with the introduction of nitrogen. The photocatalytic activity of the prepared samples were evaluated towards the hydrogen evolution from photocatalytic water splitting under solar light irradiation. It was found that nitrogen doped core-shell structured Au@TiO₂ nanoparticles (Au@N-TiO₂) showed higher photocatalytic activity with an average H₂ evolution rate of 4880 μmol h⁻¹g⁻¹, which is 3.79 times more than that of bare TiO₂ in 4 h under xenon light irradiation. The relationship among the other samples was in order of Au@N-TiO₂ > Au@TiO₂ > N-TiO₂ > TiO₂. This enhanced photocatalytic activity of Au@N-TiO₂ can be responsible for the formation of an plasmonic photocatalyst and the formation of an impurity band between the conduction band (CB) and the valence band (VB) of TiO₂.