Photocurrent enhancement in thin film amorphous silicon solar cells with silver nanoparticles

Silver nanoparticles embedded in a dielectric material have strong scattering properties under light illumination, due to localized surface plasmons. This effect is a potential way to achieve light trapping in thin‐film solar cells. In this paper we study light scattering properties of nanoparticles on glass and ZnO, and on silver coated with ZnO, which represent the back reflector of a solar cell. We find that large nanoparticles embedded in the dielectric at the back contact of amorphous silicon solar cells lead to a remarkable increase in short circuit current of 20% compared to co‐deposited cells without nanoparticles. This increase is strongly correlated with the enhanced cell absorption in the long wavelengths and is attributed to localized surface plasmons. We also discuss the electrical properties of the cells. Copyright © 2010 John Wiley & Sons, Ltd.     

H2 plasma treatment at the p/i interface of a hydrogenated amorphous Si absorption layer for high‐performance Si thin film solar cells

Plasma treatment (PT) of the buffer layer for highly H₂‐diluted hydrogenated amorphous silicon (a‐Si:H) absorption layers is proposed as a technique to improve efficiency and mitigate light‐induced degradation (LID) in a‐Si:H thin film solar modules. The method was verified for a‐Si:H single‐junction and a‐Si:H/microcrystalline silicon (µc‐Si:H) tandem modules with a size of 200 × 200 mm² (aperture area of 382.5 cm²) under long‐term light exposure. H₂ PT at the p/i interface was found to eliminate non‐radiative recombination centers in the buffer layer, and plasma‐enhanced chemical vapor deposition at low radio‐frequency power was found to suppress the generation of defects during the growth of a‐Si:H absorption layers on the treated buffer layers. With optimized H₂ PT of the a‐Si:H single‐junction module, the stabilized short circuit current and fill factor increased, and the stabilized open circuit voltage moves beyond its initial value. The results demonstrate 7.7% stabilized efficiency and 10.5% LID for the a‐Si:H single‐junction module and 10.82% stabilized efficiency and 7.76% LID for the a‐Si:H/µc‐Si:H tandem module. Thus, the growth of an a‐Si:H absorption layer on a H₂ PT buffer layer can be considered as a practical method for producing high‐performance Si thin film modules. Copyright © 2012 John Wiley & Sons, Ltd.     

Characteristics of large‐scale nanohole arrays for thin‐silicon photovoltaics

Nanostructured crystalline silicon is promising for thin‐silicon photovoltaic devices because of reduced material usage and wafer quality constraint. This paper presents the optical and photovoltaic characteristics of silicon nanohole (SiNH) arrays fabricated using polystyrene nanosphere lithography and reactive‐ion etching (RIE) techniques for large‐area processes. A post‐RIE damage removal etching is subsequently introduced to mitigate the surface recombination issues and also suppress the surface reflection due to modifications in the nanohole sidewall profile, resulting in a 19% increase in the power conversion efficiency. We show that the damage removal etching treatment can effectively recover the carrier lifetime and dark current–voltage characteristics of SiNH solar cells to resemble the planar counterpart without RIE damages. Furthermore, the reflectance spectra exhibit broadband and omnidirectional anti‐reflective properties, where an AM1.5 G spectrum‐weighted reflectance achieves 4.7% for SiNH arrays. Finally, a three‐dimensional optical modeling has also been established to investigate the dimension and wafer thickness dependence of light absorption. We conclude that the SiNH arrays reveal great potential for efficient light harvesting in thin‐silicon photovoltaics with a 95% material reduction compared to a typical cell thickness of 200 µm. Copyright © 2012 John Wiley & Sons, Ltd.     

Channel‐power profile estimation for orthogonal frequency‐division multiplexing systems

In this work, a channel‐power profile estimation for orthogonal frequency‐division multiplexing systems, based on the cyclic prefix (CP), is introduced. By knowing the delay of each path, the time‐dispersion information can be derived. The proposed method, considering long intersymbol interference (ISI) fading channels, requires only the coarse symbol timing information. More specifically, quasi‐stationary fading channels are considered. The basic contribution is to obtain the maximum‐likelihood estimation of the correlation coefficient based on the CP. Subsequently, the relationship between the correlation coefficient and the channel‐tap powers is explored. With the estimate of correlation coefficient, the least‐square solution of the channel‐tap powers can be determined. The proposed method is suitable for both short and long ISI channels. Furthermore, the Cramér–Rao lower bound of the channel‐power profile estimation is analyzed, and simulations confirm the advantages of the proposed estimator. Copyright © 2011 John Wiley & Sons, Ltd.     

Morphological Stabilization by Supramolecular Perfluorophenyl‐C60 Interactions Leading to Efficient and Thermally Stable Organic Photovoltaics

A new PC₆₁BM‐based fullerene, [6,6]‐phenyl‐C₆₁ butyric acid pentafluorophenyl ester (PC₆₁BP^F) is designed and synthesized. This new n‐type material can replace PC₆₁BM to form a P3HT:PC₆₁BP^F binary blend or serve as an additive to form a P3HT:PC₆₁BM:PC₆₁BP^F ternary blend. Supramolecular attraction between the pentafluorophenyl group of PC₆₁BP^F and the C₆₀ cores of PC₆₁BP^F/PC₆₁BM can effectively suppress the PC₆₁BP^F/PC₆₁BM materials from severe aggregation. By doping only 8.3 wt% PC₆₁BP^F, device PC₆₁BP^F651 exhibits a PCE of 3.88% and decreases slightly to 3.68% after heating for 25 h, preserving 95% of its original value. When PC₆₁BP with non‐fluorinated phenyl group is used to substitute PC₆₁BP^F, the stabilizing ability disappears completely. The efficiencies of PC₆₁BP651 and PC₆₁BP321 devices significantly decay to 0.44% and 0.11%, respectively, after 25 h isothermal heating. Most significantly, this strategy is demonstrated to be effective for a blend system incorporating a low band‐gap polymer. By adding only 10 wt% PC₆₁BP^F, the PDTBCDTB: PC₇₁BM‐based device exhibits thermally stable morphology and device characteristics. These findings demonstrate that smart utilization of supramolecular interactions is an effective and practical strategy to control morphological evolution.     

Facile Generation of L10‐FePt Nanodot Arrays from a Nanopatterned Metallopolymer Blend of Iron and Platinum Homopolymers

Hard ferromagnetic (L1₀ phase) FePt alloy nanoparticles (NPs) with extremely high magnetocrystalline anisotropy are considered to be one of the most promising candidates for the next generation of ultrahigh‐density data storage system. The question of how to generate ordered patterns of L1₀‐FePt NPs and how to transform the technology for practical applications represents a key current challenge. Here the direct synthesis of L1₀ phase FePt NPs by pyrolysis of Fe‐containing and Pt‐containing metallopolymer blend without post‐annealing treatment is reported. Rapid single‐step fabrication of large‐area nanodot arrays (periodicity of 500 nm) of L1₀‐ordered FePt NPs can also be achieved by employing the metallopolymer blend, which possesses excellent solubility in most organic solvents and good solution processability, as the precursor through nanoimprint lithography (NIL). Magnetic force microscopy (MFM) imaging of the nanodot pattern indicates that the patterned L1₀ phase FePt NPs are capable of exhibiting decent magnetic response, which suggests a great potential to be utilized directly in the fabrication of bit patterned media (BPM) for the next generation of magnetic recording technology.     

Iron Oxide‐Labeled Collagen Scaffolds for Non‐Invasive MR Imaging in Tissue Engineering

Non‐invasive imaging holds significant potential for implementation in tissue engineering. It can be used to monitor the localization and function of tissue‐engineered implants, as well as their resorption and remodelling. Thus far, however, the vast majority of effort in this area of research have focused on the use of ultrasmall super‐paramagnetic iron oxide (USPIO) nanoparticle‐labeled cells, colonizing the scaffolds, to indirectly image the implant material. Reasoning that directly labeling scaffold materials might be more beneficial (enabling imaging also in the case of non‐cellularized implants), more informative (enabling the non‐invasive visualization and quantification of scaffold degradation), and easier to translate into the clinic (cell‐free materials are less complex from a regulatory point‐of‐view), three different types of USPIO nanoparticles are prepared and incorporated both passively and actively (via chemical conjugation; during collagen crosslinking) into collagen‐based scaffold materials. The amount of USPIO incorporated into the scaffolds is optimized, and correlated with MR signal intensity, showing that the labeled scaffolds are highly biocompatible, and that scaffold degradation can be visualized using MRI. This provides an initial proof‐of‐principle for the in vivo visualization of the scaffolds. Consequently, USPIO‐labeled scaffold materials seem to be highly suitable for image‐guided tissue engineering applications.     

Photo‐Stable Organic Thin‐Film Transistor Utilizing a New Indolocarbazole Derivative for Image Pixel and Logic Applications

Small molecule pentacene layer has been a representative among many organic thin‐film transistor (OTFT) channels with decent p‐type mobilities, but it is certainly light‐sensitive due to its relatively small highest occupied molecular orbital‐lowest unoccupied molecular orbital (HOMO‐LUMO) gap (1.85 eV). Although a few other small molecule‐based layers have been reported later, their photo‐stabilities or related device applications have hardly been addressed. Here, a new photostable organic layer is reported, heptazole (C₂₆H₁₆N₂), which has almost the same HOMO level as that of pentacene but with a higher HOMO‐LUMO gap (≈2.95 eV). This heptazole OTFT displays a decent mobility comparable to that of conventional amorphous Si TFTs, showing good photostability unlike pentacene OTFTs. An image pixel driving the photostable heptazole OTFT connected to a pentacene/Al Schottky photodiode is demonstrated. This heptazole OTFT also conveniently forms a logic inverter coupled with a pentacene OTFT, sharing Au for source/drain.     

Relaxor/Ferroelectric Composites: A Solution in the Quest for Practically Viable Lead‐Free Incipient Piezoceramics

Recently developed lead‐free incipient piezoceramics are promising candidates for off‐resonance actuator applications with their exceptionally large electromechanical strains. Their commercialization currently faces two major challenges: high electric field required for activating the large strains and large strain hysteresis. It is demonstrated that design of a relaxor/ferroelectric composite provides a highly effective way to resolve both challenges. Experimental results in conjunction with numerical simulations provide key parameters for the development of viable incipient piezoceramics.     

Carbon‐Based Anodes for Lithium Sulfur Full Cells with High Cycle Stability

The lithium sulfur battery system has been studied since the late 1970s and has seen renewed interest in recent years. However, even after three decades of intensive research, prolonged cycling can only be achieved when a large excess of electrolyte and lithium is used. Here, for the first time, a balanced and stable lithium sulfur full cell is demonstrated with silicon–carbon as well as all‐carbon anodes. More than 1000 cycles, a specific capacity up to 1470 mAh g^?1 _sulfur (720 mAh g^?1 _cathode), and a high coulombic efficiency of over 99% even with a low amount of electrolyte are achieved. The alternative anodes do not suffer from electrolyte depletion, which is found to be the main cause of cell failure when using metallic lithium anodes.