Physics and chemistry of CdTe/CdS thin film heterojunction photovoltaic devices: fundamental and critical aspects

Among the armoury of photovoltaic materials, thin film heterojunction photovoltaics continue to be a promising candidate for solar energy conversion delivering a vast scope in terms of device design and fabrication. Their production does not require expensive semiconductor substrates and high temperature device processing, which allows reduced cost per unit area while maintaining reasonable efficiency. In this regard, superstrate CdTe/CdS solar cells are extensively investigated because of their suitable bandgap alignments, cost effective methods of production at large scales and stability against proton/electron irradiation. The conversion efficiencies in the range of 6-20% are achieved by structuring the device by varying the absorber/window layer thickness, junction activation/annealing steps, with more suitable front/back contacts, preparation techniques, doping with foreign ions, etc. This review focuses on fundamental and critical aspects like: (a) choice of CdS window layer and CdTe absorber layer; (b) drawbacks associated with the device including environmental problems, optical absorption losses and back contact barriers; (c) structural dynamics at CdS-CdTe interface; (d) influence of junction activation process by CdCl2 or HCF2Cl treatment; (e) interface and grain boundary passivation effects; (f) device degradation due to impurity diffusion and stress; (g) fabrication with suitable front and back contacts; (h) chemical processes occurring at various interfaces; (i) strategies and modifications developed to improve their efficiency. The complexity involved in understanding the multiple aspects of tuning the solar cell efficiency is reviewed in detail by considering the individual contribution from each component of the device. It is expected that this review article will enrich the materials aspects of CdTe/CdS devices for solar energy conversion and stimulate further innovative research interest on this intriguing topic.

Simulations and experiments in punching spring-steel devices with sub-millimeter features

This work demonstrates the feasibility of mesoscale (100 μm to mm) punching of multiple holes of intricate shapes in metals. Analytical modeling, finite element (FE)simulation, and experimentations are used in this work. Two dimensional FE simulations in ABAQUS were done with an assumed material modeling and plane-strain condition. A known analytical model was used and compared with the ABAQUS simulation results to understand the effects of clearance between the punch and the die. FE simulation in ABAQUS was done for different clearances and corner radii at punch, die, and holder. A set of punches and dies were used to punch out a miniature spring-steel gripper. Comparison of compliant grippers manufactured by wire-cut electro discharge machining(EDM) and punching shows that realizing sharp interior and re-entrant corners by punching is not easy to achieve. Punching of circular holes with 5 mm and 2.5 mm diameter is achieved. The possibility of realizing meso-scale parts with complicated shapes through punching is demonstrated in this work; and some strategies are suggested for improvement.

Impedance spectroscopy of novel hybrid composite films of polyvinylbutyral (PVB)/functionalized mesoporous silica

Polyvinyl butyral/functionalized mesoporous silica hybrid composite films have been fabricated by solution casting technique with various weight percentages of functionalized silica. A polyol (tripentaerythritol-electron rich component), which acts as an electron donor to the polymer backbone, was added to enhance the conductivity. The prepared composites were characterized by Fourier transformed infrared spectroscopy and the morphology was evaluated by scanning electron microscopy. Dielectric properties of these freestanding composites were studied using the two-probe method. The dielectric constant and impedance value decreased with the increase in applied frequency as well as with the increase in functionalized silica content in the polyvinyl butyral matrix. An increase in conductivity of the PVB/functionalized silica composites was also observed. (C) 2013 Elsevier Ltd. All rights reserved.

Performance metrics in a hybrid MPI-OpenMP based molecular dynamics simulation with short-range interactions

We discuss the computational bottlenecks in molecular dynamics (MD) and describe the challenges in parallelizing the computation-intensive tasks. We present a hybrid algorithm using MPI (Message Passing Interface) with OpenMP threads for parallelizing a generalized MD computation scheme for systems with short range interatomic interactions. The algorithm is discussed in the context of nano-indentation of Chromium films with carbon indenters using the Embedded Atom Method potential for Cr-Cr interaction and the Morse potential for Cr-C interactions. We study the performance of our algorithm for a range of MPI-thread combinations and find the performance to depend strongly on the computational task and load sharing in the multi-core processor. The algorithm scaled poorly with MPI and our hybrid schemes were observed to outperform the pure message passing scheme, despite utilizing the same number of processors or cores in the cluster. Speed-up achieved by our algorithm compared favorably with that achieved by standard MD packages. (C) 2013 Elsevier Inc. All rights reserved.

Optical, Electrochemical, and Structural Properties of Spray Coated Dihexyl Substituted Poly (3,4 propylene dioxythiophene) Film for Optoelectronics Devices

The dihexyl substituted poly (3,4-propylenedioxythiophene) (PProDOT-Hx(2)) thin films uniformly deposited by cost effective spray coating technique on transparent conducting oxide coated substrates. The electro-optical properties of PProDOT-Hx(2) films were studied by UV-Vis spectroscopy that shows the color contrast about 45% with coloration efficiency of approximate to 185cm(2)/C. The electrochemical properties of PProDOT-Hx(2) films were studied by cyclic voltammetry and AC impedance techniques. The cyclic voltammogram shows that redox reaction of films are diffusion controlled and ions transportation will be faster on the polymer film at higher scan rate. Impedance spectra indicate that polymer films are showing interface charge transfer process as well as capacitive behavior between the electrode and electrolyte. The XRD of the PProDOT-Hx(2) thin films revealed that the films are in amorphous nature, which accelerates the transportation of ions during redox process.

Transfer free suspended graphene devices on silicon using electrodeposited copper

Transfer free processes using Cu films greatly simplify the fabrication of reliable suspended graphene devices. In this paper, the authors report on the use of electrodeposited Cu films on Si for transfer free fabrication of suspended graphene devices. The quality of graphene layers on optimized electrodeposited Cu and Cu foil are found to be the same. By selectively etching the underlying Cu, the authors have realized by a transfer free process metal contacted, suspended graphene beams up to 50 mu m in length directly on Si. The suspended graphene beams do not show any increase in defect levels over the as grown state indicating the efficiency of the transfer free process. Measured room temperature electronic mobilities of up to 5200 cm(2)/V.s show that this simpler and CMOS compatible route has the potential to replace the foil based route for such suspended nano and micro electromechanical device arrays. (C) 2014 American Vacuum Society.

Optical bio-sensing devices based on etched fiber Bragg gratings coated with carbon nanotubes and graphene oxide along with a specific dendrimer

We demonstrate that etched fiber Bragg gratings (eFBGs) coated with single walled carbon nanotubes (SWNTs) and graphene oxide (GO) are highly sensitive and accurate biochemical sensors. Here, for detecting protein concanavalin A (Con A), mannose-functionalized poly(propyl ether imine) (PETIM) dendrimers (DMs) have been attached to the SWNTs (or GO) coated on the surface modified eFBG. The dendrimers act as multivalent ligands, having specificity to detect lectin Con A. The specificity of the sensor is shown by a much weaker response (factor of similar to 2500 for the SWNT and similar to 2000 for the GO coated eFBG) to detect non specific lectin peanut agglutinin. DM molecules functionalized GO coated eFBG sensors showed excellent specificity to Con A even in the presence of excess amount of an interfering protein bovine serum albumin. The shift in the Bragg wavelength (Delta lambda(B)) with respect to the lambda(B) values of SWNT (or GO)-DM coated eFBG for various concentrations of lectin follows Langmuir type adsorption isotherm, giving an affinity constant of similar to 4 x 10(7) M-1 for SWNTs coated eFBG and similar to 3 x 10(8) M-1 for the GO coated eFBG. (C) 2014 Elsevier B.V. All rights reserved.

Performance of an ionomer blend-nanocomposite as an effective gas barrier material for organic devices

A new, flexible, gas barrier material has been synthesized by exfoliating organically modified nano-clays (MMT) in the blends of Surlyn (PEMA) using a copolymer of vinyl alcohol (EVOH) and demonstrated as a gas barrier material. The materials were characterized by Fourier transform infra red (FTIR) and UV-visible spectroscopy, differential scanning calorimetry (DSC), thermo-gravimetric analysis (TGA) and tensile studies. The oxygen and water-vapor permeabilities of the fabricated films were determined by calcium degradation test and a novel permeability setup based on cavity ring down spectroscopy, respectively. Hierarchical simulations of these materials helped us to understand the effect of intermolecular interactions on diffusivities of oxygen and water molecules in these materials. Schottky structured poly(3-hexylthiophene) based organic devices were encapsulated with the fabricated films and aging studies were carried under accelerated conditions. Based on permeability test results and accelerated aging studies, the fabricated PEMA/EVOH/MMT composites were found to be effective in decreasing the permeabilities for gases by about two orders of magnitude and maintaining the lifetime of organic devices.

C-12 Helices in Long Hybrid (alpha gamma)(n) Peptides Composed Entirely of Unconstrained Residues with Proteinogenic Side Chains

Unconstrained gamma(4) amino acid residues derived by homologation of proteinogenic amino acids facilitate helical folding in hybrid (alpha gamma)(n) sequences. The C-12 helical conformation for the decapeptide, Boc-Leu-gamma(4)(R)Val](5)-OMe, is established in crystals by X-ray diffraction. A regular C-12 helix is demonstrated by NMR studies of the 18 residue peptide, Boc-Leu-gamma(4)(AR)Val](9)-OMe, and a designed 16 residue (alpha gamma)(n) peptide, incorporating variable side chains. Unconstrained (alpha gamma)(n) peptides show an unexpectedly high propensity for helical folding in long polypeptide sequences.

Stable and low resistive zinc contacts for SnS based optoelectronic devices

The contact behavior of tin mono sulfide (SnS) nanocrystalline thin films with zinc (Zn) and silver (Ag) contacts was studied. SnS films have been deposited on glass substrates by thermal evaporation technique at a growth temperature of 300 degrees C. The as-grown SnS films composed of vertically aligned nanocrystallites with a preferential orientation along the < 010 > direction. SnS films exhibited excellent chemical stoichiometry and direct optical band gap of 1.96 eV. These films also exhibited excellent Ohmic characteristics and low electrical resistivity with Zn contacts. The observed electrical resistivity of SnS films with Zn contacts is 22 times lower than that of the resistivity with Ag contacts. The interfacing analysis reveals the formation of conductive Zn-S layer between SnS and Zn as interfacial layer. (C) 2014 Elsevier B. V. All rights reserved.

Space-Vector-Based Hybrid Pulsewidth Modulation Techniques for a Three-Level Inverter

Novel switching sequences have been proposed recently for a neutral-point-clamped three-level inverter, controlled effectively as an equivalent two-level inverter. It is shown that the four novel sequences can be grouped into two pairs of sequences. Using each pair of sequences, a hybrid pulsewidth modulation (PWM) technique is proposed, which deploys the two sequences in appropriate spatial regions to reduce the current ripple. Further, a third hybrid PWM technique is proposed which uses all the five sequences (including the conventional sequence) in appropriate spatial regions. Each proposed hybrid PWM is shown, both analytically and experimentally, to outperform its constituent PWM methods in terms of harmonic distortion. In particular, the third proposed hybrid PWM reduces the total harmonic distortion considerably at low- and high-speed ranges of a constant volts-per-hertz induction motor drive, compared to centered space vector PWM.

Aminosilane Functionalized Cenosphere in Poly(vinyl butyral) Composite Films: Moisture Resistant Encapsulated Schottky Devices

Functionalized cenosphere in PVB composite films were fabricated by melt processing. The composites show higher tensile strength with lower failure strain with increased filler ratio in the matrix. Fractographic images of the samples and DMA studies indicate brittle failure of the matrix. Moisture permeation and water contact angle studies reveal improved hydrophobicity of the matrix, while the factor of surface roughness increases the wettability at higher filler content. Schottky-structured devices encapsulated with functionalized cenosphere indicate enhanced resistance to moisture and increased life time for the devices.

Antimicrobial and conducting polymer substrate derived from hybrid structures of silver nanoparticles and multiwall carbon nanotubes

Hybrid nanocomposites of polycaprolactone (PCL) with multiwall carbon nanotubes (MWNTs) and silver nanoparticles (nAg) were prepared by melt mixing. Synergetic effect of the two nanofillers (MWNT and nAg) in PCL matrix was evaluated for dielectric and antibacterial properties. Dielectric results showed that the addition of nAg as filler in PCL matrix (PCL/nAg) had no effect on conductivity, whereas addition of MWNT in PCL matrix (PCL/MWNT) caused a sharp increase in conductivity of PCL. Interestingly, the hybrid nanocomposite (PCL/MWNT/nAg) incorporating MWNT and nAg also exhibited high electrical conductivity. The hybrid composite was found to have antibacterial property similar to that of PCL/nAg composite for lower loading of nAg. This study demonstrates that the synergetic interaction of the nanofillers in the hybrid nanocomposite improves both electrical conductivity and antibacterial properties of PCL.

In vitro osteogenic cell proliferation, mineralization, and in vivo osseointegration of injection molded high-density polyethylene-based hybrid composites in rabbit animal model

The present work reports the biocompatibility property of injection molded HDPE-HA-Al2O3 hybrid composites. In vitro cytocompatibility results reveal that osteogenic cell viability and bone mineralization are favorably supported in a statistically significant manner on HDPE-20% HA-20% Al2O3 composite, in comparison to HDPE-40 wt.% HA or HDPE-40 wt.% Al2O3. The difference in cytocompatibility property is explained in terms of difference in substrate wettability/surface energy and importantly, both the cell proliferation at 7 days or bone mineralization at 21 days on HDPE-20% HA-20% Al2O3 composite are either comparable or better than sintered HA. The progressive healing of cylindrical femoral bone defects in rabbit animal model was assessed by implantation experiments over 1, 4 and 12 weeks. Based on the histological analysis as well as histomorphometrical evaluation, a better efficacy of HDPE-20% HA-20% Al2O3 over high-density polyethylene (HDPE) for bone regeneration and neobone formation at host bone-implant interface was established. Taken together, the present study unequivocally establishes that despite the presence of 20% Al2O3, HDPE-based hybrid composites are as biocompatible as HA in vitro or better than HDPE in vivo.

Effect of silane functionalized alumina on poly(vinyl butyral) nanocomposite films: Thermal, mechanical, and moisture barrier studies

In this work, a hybrid-polymer nanocomposite film, based on polyvinyl butyral/amino-silane functionalized nano alumina, was fabricated by melt processing. The calcium degradation measurements suggest the functionalized nanocomposite films exhibit higher resistance towards moisture penetration as compared to the neat alumina loaded films. Thermal stability, mechanical strength, and contact angle studies of the composites were also conducted to evaluate the performance of the functionalized alumina loaded films. These nanocomposite films were encapsulated over Al/P3HT/ITO Schottky structured device. The changes observed in the current density of the devices to the applied voltage before and after accelerated aging conditions are presented. The nanocomposite with functionalized alumina films exhibits 50% change in current density, which is superior to that attained with neat and non-functionalized films. POLYM. COMPOS., 35:1426-1435, 2014. (c) 2013 Society of Plastics Engineers