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The use of omnidirectional mirrors (an special case of distributed Bragg reflectors) as cladding for planar waveguides is proposed and analyzed. The proposed structure is an all-porous silicon multilayer consisting of a core layer inserted between two omnidirectional mirrors. The transfer matrix method is applied for the modal analysis. The influence of the parameters of the waveguide structure on the guided modes is analyzed. These parameters are the layer thickness and number of periods of the omnidirectional mirror, and the refractive index and thickness of the core layer. Finally, the confinement of the omnidirectional mirror cladding is analyzed with respect to two other different distributed Bragg reflector claddings. [ABSTRACT FROM AUTHOR]

This paper presents a compact electrical equivalent circuit which describes the dark current-voltage characteristics of nonideal p-n junction solar cells in a wide range of temperatures. The model clearly separates the voltage drop in the junction and bulk regions. It is based on the combination of two exponential mechanisms, shunt and series resistances and space-charge limited current. In order to increase the accuracy of the parameter extraction process, both ln(I-V) and its derivative plots are fitted simultaneously. From the temperature dependence of the extracted parameters, the conduction mechanisms governing the I-V characteristics can be obtained without assuming dominating terms. In addition, the extracted parameters can be related to other electrical magnitudes obtained from such independent measurements as capacitance-voltage measurements (diffusion potential) and illuminated current-voltage characteristics (series resistance and open-circuit voltage). To exemplify the application, a p+ a-SiC:H/n c-Si solar cell is studied and a number of major physical aspects derived from the analysis of the fitting values are discussed. [ABSTRACT FROM AUTHOR]

We have fabricated, electrically characterized and simulated n-type hydrogenated amorphous silicon germanium alloys on p-type crystalline-silicon heterojunction diodes with three different a-SiGe:H layer thicknesses: 37, 86, and 200 nm. The capacitance–voltage results confirm the existence of abrupt heterojunctions. The conduction and valence-band discontinuities of the heterojunctions and the electron affinity of the n-type a-SiGe:H films were obtained. The conduction mechanisms were determined by analyzing the temperature dependence of the current–voltage characteristics. The results show that at low forward bias (V<0.45 V) the diodes with thinner amorphous layers (37 and 86 nm) are dominated by recombination in the a-SiGe:H depletion region, whereas the thicker diode (200 nm) is dominated by multistep tunneling through depletion region. In addition, at high forward bias (V>0.45 V) the space-charge limited effect becomes the main transport mechanism in all the measured devices. The increase in the amorphous layer thickness also causes an increase in the leakage reverse current. Numerical simulations support the proposed transport mechanisms. [ABSTRACT FROM AUTHOR]

Multilayers consisting of the periodic repetition of two layers with different refractive indices (nH and nL) may have an omnidirectional band gap, the width of which depends on the incidence medium and on the refractive index ratio nH/nL. In porous silicon, this ratio is limited by the material and fabrication characteristics. This article makes a theoretical study of two multilayer structures in order to enlarge the omnidirectional band gap with the same refractive index ratio by varying only the thickness of the period. These structures are named balanced and unbalanced mirrors and consist of a few periodic multilayers stacked together. Porous silicon balanced and unbalanced mirrors with enlarged omnidirectional band gap for 1.55-μm applications are proposed. The results obtained with these mirror structures are compared with the results obtained with a chirped structure with the same characteristics. [ABSTRACT FROM AUTHOR]

The performance of the standard hydrogenated amorphous silicon carbon–crystalline silicon solar cell is extensively compared with the performance of a hybrid structure subjected to a high-temperature annealing processing. Our analysis indicates that high-temperature-annealed heterojunctions show more robustness in the presence of energy offsets and defective amorphous-crystalline interfaces. Annealed hybrid cells are also less vulnerable to the negative impact of amorphous silicon carbon doped layers with poor electrical properties. Furthermore, annealed structures have the potential to generate higher efficiencies than conventional heterojunctions regardless of the wafer quality. The presence of boron at the amorphous-crystalline interface and in the wafer front region plays an important role in annealed hybrid structures that are made with low-quality wafers or where there is a highly defective amorphous-crystalline interface. In this scenario, a linear boron profile in the wafer front region is more appropriate, for which there is an optimum thickness. For low defect amorphous-crystalline interfaces and high-quality wafers, a boron exponential profile is more appropriate when boron creates additional defects in the front region of the wafer. The shape of the boron profile becomes less relevant when the boron does not add additional defects to the front region of high-quality wafers and when the amorphous-crystalline interface is low defect or defect-free. [ABSTRACT FROM AUTHOR]

P[SUP+]-type hydrogenated amorphous silicon-carbon (a-Si[SUB1-x]C[SUBx] :H) on n-type crystalline silicon (c-Si) heterojunction diodes were fabricated and characterized electrically. The effects of thermal annealing on the electrical transport properties of a-Si[SUB0.8]C[SUB0.2]:H/c-Si diodes were investigated by measuring their current-voltage characteristics. From the dark current-voltage characteristics measured at different temperatures (298-373 K), transport mechanisms were analyzed in detail. Two carrier transport mechanisms were found to be the origin of forward current. At low bias voltage and temperatures above 320 K as-deposited diodes are dominated by recombination currents on the amorphous side of the space charge region whereas annealed diodes are mainly dominated by diffusion mechanisms. In contrast, at temperatures below 320 K, both types of diodes are mainly dominated by multitunneling capture emission. At higher voltages, the current becomes space charge limited for both diodes throughout the temperature range studied. [ABSTRACT FROM AUTHOR]

We present a compact model based on the Landauer transmission theory for the silicon quantum wire and quantum well metal-oxide-semiconductor field effect transistor (MOSFET) working in the ballistic limit. This model captures the static current-voltage characteristics in all the operation regimes, below and above threshold voltage. The model provides a basic framework to account for the electronic transport in MOSFETs, being easily adaptable to gate structures such as the double-gate or gate-all-around. Numerical simulations based on the proposed model have been compared with experiments and quantum mechanical self-consistent simulations, with good agreement. [ABSTRACT FROM AUTHOR]

Presents information on a study which characterized and fabricated annealed amorphous silicon-type crystalline silicon heterojunction diodes for different doping concentrations. Fabrication; Experimental results and discussion; Conclusions.

Provides information on a study that fabricated and electrically characterized n-type amorphous silicon on p-type crystalline silicon heterojunction diodes. Description on the fabrication technology; Results and discussion on the study.

Presents a study which proposed to determine the conduction band offset in single quantum well (SQW) structures. Overview of the conduction band diagram of a SQW structure with a Schottky contact; Details on the computer curves of the capacitance and conductance over frequency; List of the values of the conduction band offset.

Performance losses and aging mechanisms are investigated in state-of-the-art PTB7:PC70BM solar cells. Inverted devices incorporating a vanadium pentoxide (V2O5) top contact have efficiencies of 8%. After aging the unencapsulated devices, no changes are observed in the open circuit voltage ( Voc) or short circuit current ( Jsc); however, the fill factor (FF) drops from 0.7 to 0.61. An s-shape initially appears in the J- V curve after aging, which can be reduced by cycling through the J- V curve under illumination. This is discussed in context of the redox properties of V2O5. With impedance spectroscopy, it is demonstrated that changes to the contact interfaces are completely reversible and not responsible for the performance loss. Intensity modulated photocurrent spectroscopy combined with device modeling reveals that the loss in FF is due to trap formation in the active layer. Additionally it is observed that the performance of pristine devices is limited by optical absorption in the thin active layer and the build-up of space charge which hinders carrier extraction. [ABSTRACT FROM AUTHOR]

The influence of pore diameter over the optical response of nanoporous anodic alumina (NAA) films is analyzed by reflectance interference spectroscopy. NAA films manufactured by a two-step anodization procedure in oxalic acid exhibiting three well-defined pore diameter distributions with pores of 32 ± 4, 50 ± 3, and 73 ± 2 nm are studied. The optical detection of biomolecules is investigated by serially dosing protein A, human IgG and anti-human IgG into a nanoporous matrix using a custom-made flow cell. The results demonstrate that the transduction signal, the variation of effective optical thickness upon IgG binding to protein A (ΔEOT), depends on the nanopore diameter: for small pore diameter (32 nm) no significant differences in signals are observed for different protein concentrations whereas for larger pore diameters (50 nm and 73 nm) the signals increase for increasing concentrations from 10 to 100 μg mL−1. Our experiments also show that this signal can be further enhanced by amplification with anti-human IgG due to the multiple binding events between the antigen and the antibody. These results will enable the development of more sensitive interferometric biosensors based on NAA. [ABSTRACT FROM AUTHOR]

In this work we study theoretically the tunability of a double-cavity Fabry–Pérot multilayer structure composed of porous silicon mirrors and liquid crystal cavities. By means of the anisotropic Transfer Matrix Method we analyze changes on wavelength and transmission of two resonant frequencies within the stop band. Tuning is achieved by allowing different orientations of the LC optical axes within the cavities, while keeping the optical axes parallel to the layers. Copyright © 2009 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

A fast and cost-effective technique is applied for fabricating cobalt and nickel nanopillars on aluminium substrates. By applying an electrochemical process, the aluminium oxide barrier layer is removed from the pore bottom tips of nanoporous anodic alumina templates. So, cobalt and nickel nanopillars are fabricated into these templates by DC electrodeposition. The resulting nanostructure remains on the aluminium substrate. In this way, this method could be used to fabricate a wide range of nanostructures which could be integrated in new nanodevices. [ABSTRACT FROM AUTHOR]

Bottom-contact organic thin-film transistors with active layers made of n-type small-molecule perylene diimide Polyera ActivinkTM N1400 were fabricated and were compared with those of 6,13-bis (triisopropylsilylethinyl)pentacene and poly(triarylamine) (PTAA). For the three devices, the mobility values and the distribution of the density of states (DOS) were estimated from current–voltage measurements. The mobility value of the N1400 samples increases up to four orders of magnitude within the measured voltage range, reaching the PTAA mobility values at high gate voltages. Finally, we analyze the effect of the distribution of the DOS on the mobility by studying the free-charge sheet concentration. [ABSTRACT FROM AUTHOR]

By means of finite-element numerical modeling, we analyze the influence of the nanostructured dissociation interface geometry on the behavior of interdigitated heterojunction full organic solar cells. A systematic analysis of light absorption, exciton diffusion, and carrier transport, all in the same numerical framework, is carried out to obtain their dependence on the interface geometrical parameters: pillar diameter and height, and nanostructure period. Cells are constituted of poly(3-hexylthiophene) (P3HT) and 1-(3-methoxycarbonyl)-propyl-1-phenyl-(6,6)C61. Results show that light absorption is maximum for pillar heights of 80 nm and 230 nm. However, due to the short exciton diffusion length of organic materials, the analysis of the exciton diffusion process reveals that the 80 nm thickness gives rise to a higher photocurrent, except for the smaller pillar diameters. In terms of efficiency, it has been observed that the charge carrier transport is weakly dependent on the geometric parameters of the nanostructured interface if compared with the exciton diffusion process. The optimal cell is a device with a pillar height of 80 nm, a structure period of 25 nm, and a ratio of the nanopillar diameter to the period of 0.75, with an efficiency 3.6 times higher than the best planar bilayer reference device. This structure is such that it reaches a compromise between having a high proportion of P3HT to increase light absorption but preserving a small pillar diameter and interpillar distance to ensure an extended exciton dissociation interface. [ABSTRACT FROM AUTHOR]

The mechanisms limiting the dark current in pentacene (Pc)/C60 solar cell were determined using the temperature dependence of the current-density-voltage characteristics. Our analysis allowed us to calculate the effective barrier height for electrons and holes at the interface. We then determined the built-in potential of the heterojunction and the effective doping concentration of the active layers, using capacitance-voltage characteristics. These physical parameters were used to simulate a band-energy diagram for a Pc/C60 solar cell in equilibrium. Finally, we determined a relation between the effective barrier height and the built-in potential. [ABSTRACT FROM AUTHOR]

We present a compact physics-based model for the nanoscale gate-all-around MOSFET working in the ballistic limit. The current through the device is obtained by means of the Landauer approach, being the barrier height the key parameter in the model. The exact solution of the Poisson's equation is obtained in order to deal with all the operation regions tracing properly the transitions between them. [ABSTRACT FROM AUTHOR]

The dark current-voltage characteristics of poly (3-hexylthiophene) (P3HT)/n-type crystalline silicon solar cells were analyzed using an electrical equivalent circuit. We found that without illumination transport occurs due to hopping between localized states at the P3HT/silicon interface not only at low voltages, through multitunneling capture emission, but also at medium voltages, through tunneling-enhanced recombination. At high voltages the current is limited by series resistance and space-charge limited mechanisms. At low reverse voltages the current is limited by shunt resistance. From the temperature dependence of the equivalent circuit’s fitting parameters, we were able to estimate some physical parameters of the P3HT layer, namely the electron affinity, the charge carrier concentration and the characteristic temperature of the exponential trap distribution. The extracted P3HT values are in good agreement with previously reported values obtained using different methods but our approach takes into account that the P3HT layer is in a solar cell. [ABSTRACT FROM AUTHOR]