Time‐resolved investigation of Cu(In,Ga)Se2 growth and Ga gradient formation during fast selenisation of metallic precursors

Ga segregation at the backside of Cu(In,Ga)Se₂ solar cell absorbers is a commonly observed phenomenon for a large variety of sequential fabrication processes. Here, we investigate the correlation between Se incorporation, phase formation and Ga segregation during fast selenisation of Cu–In–Ga precursor films in elemental selenium vapour. Se incorporation and phase formation are analysed by real‐time synchrotron‐based X‐ray diffraction and fluorescence analysis. Correlations between phase formation and depth distributions are gained by interrupting the process at several points and by subsequent ex situ cross‐sectional electron microscopy and Raman spectroscopy. The presented results reveal that the main share of Se incorporation takes place within a few seconds during formation of In–Se at the top part of the film, accompanied by outdiffusion of In out of a ternary Cu–In–Ga phase. Surprisingly, CuInSe₂ starts to form at the surface on top of the In–Se layer, leading to an intermediate double graded Cu depth distribution. The remaining Ga‐rich metal phase at the back is finally selenised by indiffusion of Se. On the basis of a proposed growth model, we discuss possible strategies and limitations for the avoidance of Ga segregation during fast selenisation of metallic precursors. Solar cells made from samples selenised with a total annealing time of 6.5 min reached conversion efficiencies of up to 14.2 % (total area, without anti‐reflective coating). The evolution of the Cu(In,Ga)Se₂ diffraction signals reveals that the minimum process time for high‐quality Cu(In,Ga)Se₂ absorbers is limited by cation ordering rather than Se incorporation. Copyright © 2014 John Wiley & Sons, Ltd.     

Synthesis of Microporous Carbon Nanofibers and Nanotubes from Conjugated Polymer Network and Evaluation in Electrochemical Capacitor

One‐dimensional fibers and tubes are constructed through the oriented carbon‐carbon cross‐linking reactions towards rigid conjugated polymer networks. As the result, a template‐free and one‐step synthesis of CNTs and CNFs is achieved through a simple carbonization of the as‐formed carbon‐rich tubular and fiberlike polyphenylene precursors under argon. Microporous CNTs and CNFs with a surface area up to 900 m² g^–1 are obtained, together with HR‐TEM characterizations indicating the formation of intrinsic microporous structure in these rigid carbon‐rich networks. The primary electrochemical experiments reveal their promising applications as advanced electrodes in electrochemical double‐layered capacitor (EDLC).     

Enhanced Osteoblast Adhesion to Epoxide‐Functionalized Surfaces

As an alternative to expensive extracellular matrix (ECM) proteins generally applied as coatings in Petri dishes used for cell binding, an innovative system based on epoxide‐functionalized monolayers capable of protein binding is proposed. Since cells bind to material surfaces through proteins, protein‐binding surfaces should also promote cell binding. Here we investigate how the cell‐binding properties of an epoxide‐functionalized surface compares with ECM protein gel coated surfaces and tissue culture polystyrene control surfaces. Glass surfaces are functionalized with glycidoxypropyltriethoxysilane (GOPS), which results in an epoxide‐functionalized surface capable of binding proteins through an epoxide–amine reaction. Advancing contact angle measurements and atomic force microscopy measurements confirm the formation of a homogeneous GOPS monolayer. This monolayer is micropatterned with fluorescein‐labeled ECM protein gel by microcontact printing (µCP). Confocal laser scanning microscopy (CLSM) shows accurately transferred ECM protein gel micropatterns. Osteoblasts that are seeded on these micropatterned substrates show a clear preference for adhering to the epoxide‐functionalized areas. The morphology of these cultured osteoblasts is needle‐like with high aspect ratios. As controls, osteoblasts are cultured on GOPS‐functionalized surfaces, unstructured ECM protein gel surfaces, and tissue culture polystyrene (TCPS). The GOPS surfaces demonstrate a drastic increase in cell adhesion after 2 h, whilst the other tests show no adverse effects of this surface on the osteoblasts as compared to ECM and TCPS. CLSM shows healthy cell morphologies on each surface. It is demonstrated for the first time that epoxide groups outperform ECM protein gel in cell adhesion, thereby providing new routes for cost‐effective coatings that improve biocompatibility as well as exciting, new methodologies to control and direct cell adhesion.     

Ultranarrow Bandwidth Organic Photodiodes by Exchange Narrowing in Merocyanine H‐ and J‐Aggregate Excitonic Systems

Ultranarrowband organic photodiodes (OPDs) are demonstrated for thin film solid state materials composed of tightly packed dipolar merocyanine dyes. For these dyes the packing arrangement can be controlled by the bulkiness of the donor substituent, leading to either strong H‐ or strong J‐type exciton coupling in the interesting blue (H‐aggregate) and NIR (J‐aggregate) spectral ranges. Both bands are shown to arise from one single exciton band according to fluorescence measurements and are not just a mere consequence of different polymorphs within the same thin film. By fabrication of organic thin‐film transistors, these dyes are demonstrated to exhibit hole transport behavior in spin‐coated thin films. Moreover, when used as organic photodiodes in planar heterojunctions with C₆₀ fullerene, they show wavelength‐selective photocurrents in the solid state with maximum external quantum efficiencies of up to 11% and ultranarrow bandwidths down to 30 nm. Thereby, narrowing the linewidths of optoelectronic functional materials by exciton coupling provides a powerful approach to produce ultranarrowband organic photodiodes.     

On the Design of Cauer Filters

Simple formulas are given which relate the parameters of Cauer Filters.     

Rationally Engineered Electrodes for a High‐Performance Solid‐State Cable‐Type Supercapacitor

Wire‐shaped electrodes for solid‐state cable‐type supercapacitors (SSCTS) with high device capacitance and ultrahigh rate capability are prepared by depositing poly(3,4‐ethylenedioxythiophene) onto self‐doped TiO₂ nanotubes (D‐TiO₂) aligned on Ti wire via a well‐controlled electrochemical process. The large surface area, short ion diffusion path, and high electrical conductivity of these rationally engineered electrodes all contribute to the energy storage performance of SSCTS. The cyclic voltammetric studies show the good energy storage ability of the SSCTS even at an ultrahigh scan rate of 1000 V s^?1, which reveals the excellent instantaneous power characteristics of the device. The capacitance of 1.1 V SSCTS obtained from the charge–discharge measurements is 208.36 µF cm^?1 at a discharge current of 100 µA cm^?1 and 152.36 µF cm^?1 at a discharge current of 2000 µA cm^?1, respectively, indicating the ultrahigh rate capability. Furthermore, the SSCTS shows superior cyclic stability during long‐term (20 000 cycles) cycling, and also maintains excellent performance when it is subjected to bending and succeeding straightening process.     

Multifunctional Optical Terminals for Microsatellite Clusters – Design Tradeoffs

We study optical terminals with both communication and ranging capabilities for their potential use in short range free-space applications, typically encountered within a cluster of microsatellites. For our considerations we assume satellites with a unit mass between 10 kg and 100 kg, with mutual distances on the order of a few km, and per link data rates range from a few kbits/s to some 100 Mbit/s. We discuss possible network topologies and multiple access methods, andcompare the performance characteristics of different intensity modulation formats, taking into account not only the task of communication but also that of accurate intra-cluster distance measurement. Our considerations revealbasic tradeoffs to be observed when designing such multifunctional optical terminals.     

Polyphenylene Dendrimer‐Templated In Situ Construction of Inorganic–Organic Hybrid Rice‐Shaped Architectures

A novel dendrimer‐templating method for the synthesis of CuO nanoparticles and the in situ construction of ordered inorganic–organic CuO–G2Td(COOH)₁₆rice‐shaped architectures (RSAs) with analogous monocrystalline structures are reported. The primary CuO nanoparticles are linked by the G2Td(COOH)₁₆ dendrimer. This method provides a way to preserve the original properties of primary CuO nanoparticles in the ordered hybrid nanomaterials by using the 3D rigid polyphenylene dendrimer (G2Td(COOH)₁₆) as a space isolation. The primary CuO nanoparticles with diameter of (6.3 ± 0.4) nm are synthesized via four successive reaction steps starting from the rapid reduction of Cu(NO₃)₂ by using NaBH₄ as reducer and G2Td(COOH)₁₆ as surfactant. The obtained hybrid CuO–G2Td(COOH)₁₆ RSA, formed in the last reaction step, possesses a crystal structure analogous to a monocrystal as observed by transmission electron microscopy(TEM). In particular, the formation process of the RSA is monitored by UV–vis, TEM, and X‐ray diffraction. Small angle X‐ray scattering and Fourier transform infrared spectroscopy are used to investigate the role of the dendrimer in the RSA formation process. The obtained results illuminate that Cu²⁺? COO^? coordination bonds play an indispensable role in bridging and dispersing the primary CuO nanoparticles to induce and maintain the hybrid RSA. More importantly, the RSA is retained through the Cu²⁺? COO^?coordination bonds even with HCl treatment, suggesting that the dendrimers and Cu²⁺ ions may form rice‐shaped polymeric complexes which could template the assembly of CuO nanoparticles towards RSAs. This study highlights the feasibility and flexibility of employing the peculiar dendrimers to in‐situ build up hybrid architectures which could further serve as templates, containers or nanoreactors for the synthesis of other nanomaterials.     

An integrated 8-12 GHz fractional-N frequency synthesizer in SiGe BiCMOS for satellite communications

We present an integrated fractional-N low-noise frequency synthesizer for satellite applications. By using two integrated VCOs and combining digital and analog tuning techniques, a PLL lock range from 8 to 12 GHz is achieved. Due to a small VCO fine tuning gain and optimized charge pump output biasing, the phase noise is low and almost constant over the tuning range. All 16 sub-bands show a tuning range above 900 MHz each, allowing temperature compensation without sub-band switching. This makes the synthesizer robust against variations of the device parameters with process, supply voltage, temperature and aging. The measured phase noise is ?87 dBc/Hz and ?106 dBc/Hz at 10 kHz and 1 MHz offset, respectively. In integer-N mode, phase noise values down to ?98 dBc/Hz at 10 kHz and ?111 dBc/Hz at 1 MHz offset, respectively, were measured.