We have fabricated large-area, thin-film multijunction solar cells based on hydrogenated amorphous silicon (a-Si:H) and nanocrystalline silicon (nc-Si:H) made in a large area batch reactor. The device structure consisted of an a-Si:H/nc-Si:H/nc-Si:H stack on Ag/ZnO back reflector coated stainless steel substrate, deposited using our proprietary High Frequency (HF) glow discharge technique. For the nc-Si:H films, we investigated two deposition rate regimes: (i) low rate <1 nm/s and (ii) high rate >1 nm/s. We optimized the deposition parameters, such as pressure, gas flow, dilution, and power. We did SIMS analysis on the optimized films, and found the impurity concentrations were one order of magnitude lower than the films made with the conventional RF process. In particular, the oxygen concentration is reduced to ~1018 cm-3. This value is among the lowest oxygen concentration reported in literature. The low impurity content is attributed to proprietary cathode hardware and the optimized deposition process. During the initial optimization and investigative phase, we fabricated small-area (0.25 cm2 and 1.1 cm2) cells. The information obtained from the initial phase was used to fabricate large-area (aperture area 400 cm2) cells, and encapsulated the cells using the same flexible encapsulants that are used in our commercial product. We have light soaked the low-rate and high-rate encapsulated modules. The highest initial efficiency of the low-rate modules is 12.0% as confirmed by NREL. The highest corresponding stable efficiency attained for the low-rate samples cells is 11.35%. For the high-rate small-area (1.1 cm2) cells, the highest initial active-area efficiency and corresponding stable efficiency attained are 13.97% and 12.9%, respectively. We present the details of the research conducted to develop the low- and high-rate cells and modules.

We analyze the effect of charged defects on the electrical domains, phase transition characteristics and electrical properties of ferroelectric thin films with thin dead layers using a non-linear thermodynamic model. Depending on their density and field strength, defects can pin and couple to electrical domains in the film. For ultrathin films, depolarizing effects dominate and the transition from the paraelectric state is into the multidomain ferroelectric state during cooling and is strongly smeared. The competition between defect induced extrinsic effects and the dead layer related limit is demonstrated.

Metal-insulator-metal (MIM) resistive switching devices are being pursued for a number of applications, including non-volatile memory and high density/low power computing. Reported resistive switching devices vary greatly in the choice of metal oxide and electrode material. Importantly, the choice of both the metal oxide and electrode material can have significant impact on device performance, their ability to switch, and the mode of switching (unipolar, bipolar, nonpolar) that results. In this study, three metal oxides (Cu2O, HfOx, and TiOx) were deposited onto copper bottom electrodes (BEs). Four different top electrode (TE) materials (Ni, Au, Al, and Pt) were then fabricated on the various metal oxides to form MIM structures. Devices were then characterized electrically to determine switching performance and behavior. Our results show that the metal TE plays a large role in determining whether or not the MIM structure will switch resistively and what mode of switching (unipolar, bipolar, or non-polar) is observed.

Being a dedicated and enthusiastic high school science teacher is not enough to successfully prepare our children to take on the challenges of the 21st century and live up to its potential. We need high quality professional development opportunities in order to enrich our subject knowledge and teaching skills and reflect these skills in our craft. The Glenn Commission report, released ten years ago, details goals and associated action strategies included addressing professional development needs in order to deliver high-quality teaching as well as providing for teachers to engage in common study. We typically must scrutinize long lists of potential development opportunities to weigh the value of the program against the commitment of time and likelihood that intent of the training can be implemented. Beyond the training comes the quest for resources necessary for implementation and support to sustain the intent once new ideas and skills are brought back to school. Too often do teachers get their batteries charged from a professional development experience only to return to school where they become challenged to employ new skills or ideas and become further discouraged if there is no sustained support from the professional development sponsor. The best value-added programs that I have experienced are those where professional relationships can be forged through a significant and meaningful experience. Through these relationships, support networks can be established to help sustain knowledge and initiatives to provide a world-class education for our children.

I have had the excellent fortune to experience a top quality professional development program at the Princeton Center for Complex Materials (PCCM), a Materials Research Science and Engineering Center (MRSEC). My experience with the PCCM programs has demonstrated to me how a truly effective program can change lives. Over the past six consecutive summers I have gained invaluable experience starting with the Research Experience for Teachers (RET) program and subsequent involvement with PUMA and other PCCM programs that have provided me with the necessary resources to improve my teaching skills, depth of knowledge in my discipline and enable me to sustain a higher quality science program at my school. Through the RET program, I engaged directly with professors for two consecutive summers who were enthusiastic about helping improve my teaching skills and supportive of my pursuit to improve the science program at my school. This experience has led to the development of two new courses I have been able to offer for the past four years in Chemistry and Materials Science designed to engage students through hands on experiences. It was this experience that became the catalyst for me to further collaborate with local industry professionals who joined my cause and also helped in the development of one of the two new courses. Through this short paper, I will expand on my professional development experiences over the past six years to demonstrate how others can maximize opportunities provided by MRSEC educational outreach programs.

Nanoporous MgAl2O4 particulates with high porosities were successfully prepared from sol-gel reactions, solvent exchange with castor oil and subsequent combustion and calcination at 700 °C. The products were crystalline and semitransparent. Changes in the metal precursor concentrations allowed control of pore volumes from 0.7 to 1.1 cm3/g and average pore sizes from 14 to 19 nm. The specific surface areas are about 200 m2/g regardless of the precursor concentrations. After heating at 1000 °C for 10 hours, the products kept about 70% of their original pore volume and about 60% of the original surface area. Heating at 1100 °C caused a drastic reduction of pore volume and surface area to 40 and 36%, respectively, as the average particle size increased to 23 nm.

Polycrystalline samples of the single-layered cobaltate La2-x Cax CoO4 were prepared in a wide doping range of 0 ≤ x ≤ 1.5. Structural properties were characterized at room temperature. The orthorhombic distorted structure of the mother compound La2CoO4 changes to a tetragonal structure for x = 0.5 and then becomes orthorhombic again for x > 0.5. The magnetic properties were investigated in the temperature range from 5 K ≤ T ≤ 300 K. With increasing hole-doping a successive decrease of antiferromagnetic exchange is observed for x ≤ 0.5 whereas an increase of ferromagnetic exchange evolves for x ≥ 0.5.

In molecular solar energy harvesting systems, quantum mechanical features may be apparent in the physical processes involved in the acquisition and migration of photon energy. With a sharply declining distance-dependence in transfer efficiency, the excitation energy generally takes a large number of steps en route to the site of its utilization; quantum features are rapidly dissipated in an essentially stochastic process. In the case of engineered dendrimeric polymers, each such step usually takes the form of an inward hop between chromophores in neighboring generation shells. A physically intuitive, structure-determined adjacency matrix formulation of the energy flow affords insights into the key harvesting and inward funneling processes. A numerical method based on this analytic approach has now been developed and is able to deliver results on significantly larger dendrimeric polymers, with the help of large multi-processor computers. Central to this study is the interpretation of key features such as the relevance of a spectroscopic gradient and the presence of traps or irregularities due to conformational changes and folding. With the objective of fine-tune the funneling process, this model now allows the incorporation of parameters derived from quantum chemical calculations, affording new insights into the detailed operation of the harvesting process in a variety of dendrimer systems.

We describe how students explore materials science concepts using animated interactive spreadsheets. An engaging pedagogy is created in the classroom using spreadsheets in a way that initially camouflages mathematical complexity, which can later be revealed and taught. Use of off-the-shelf spreadsheet software, including freeware makes these spreadsheets universally available.

The length-scales at which thermal transport crosses from the diffusive to ballistic regime are of much interest particularly in the design and improvement of nano-structured materials. In this work, we demonstrate that the departure from diffusive transport has been observed in Si and GaAs using an optical transient thermal grating technique where an arbitrary, experimentally set length scale can be imposed on a material. In a transient thermal grating experiment, crossed laser pulses interfere creating a well-defined periodic absorption and temperature profile. A probe beam is diffracted from this transient grating and length-scale dependent thermal transport properties can be determined from the signal decay. As the length scale is decreased to lengths shorter than the mean free paths of heat carrying phonons, quasi-ballistic heat transport effects become apparent allowing us to map out length scales and mean free paths relevant to nondiffusive thermal transport in Si and GaAs.

Increasing solar cell efficiency by using spectral conversion is addressed in this article. To that purpose rare-earth doped YAG nanoparticles exhibiting down-conversion and quantum cutting properties have been prepared. These nanoparticles have been synthesized with different concentrations of dopants in order to optimize the luminescence and the quantum cutting efficiency. Results on the incorporation of selected material into the encapsulating layer of c-Si based PV-modules are also presented. The effect of down-conversion has been demonstrated through the increase of photocurrent of encapsulated silicon solar cells.

Shape Memory Alloys (SMA) undergo reversible martensitic transformation in response to changes in temperature or applied stress, exhibiting specific properties of superelasticity and shape memory. At present there is a high scientific and technological interest to develop these properties at small scale, to apply SMA as sensors and actuators in MEMS technologies. In order to study the thermo-mechanical properties of SMA at micro and nano scale, instrumented nano indentation is being widely used for nano compression tests. By using this technique, superelasticity and shape memory at the nano-scale has been demonstrated in micro and nano pillars of Cu-Al-Ni SMA. However the martensitic transformation seems to exhibit a different behavior at small scale than in bulk materials and a size effect on superelasticity has been recently reported. In the present work we will overview the thermo-mechanical properties of Cu-Al-Ni SMA at the nano-scale, with special emphasis on size effects. Finally, the above commented size effects will be discussed on the light of the microscopic mechanisms controlling the martensitic transformation at nano scale.

The high surface to volume ratio of nanoparticles allows a detailed experimental study of the surface phenomena associated with solid bridging. Besides bulk analyses, the local view on the structure and composition via HRTEM is particularly essential. 50 nm core shell particles consisting of a silicon (Si) core and a SiO2 shell were used as model system to understand surface phenomena appearing for Si-based nanostructures. Evaporative drying from de-ionized water shows the most significant bridging effect based on SiO2. There is only a localized deposition of oxides between the particles during the drying process and no overall oxidation. For the deposition material, silicates are the most likely candidates.

Locating cobalt promoters on catalytically MoS2 structures is a challenging task to achieve; this is due to the size on those MoS2 nanostructures. Previous reports in the literature indicate that specific locations for Co in MoS2 slabs are (1010)-plane creating either a sulfur-Co or Molybdenum-Co termination edge, due to lower energy required for the permutation Mo, S and Co to occur. We present results obtained from Density Functional Theory study done on the interface between MoS2 and Co9S8 crystal structures; the interface show an interesting thiocubane cluster and it is suspected to be the responsible for Mo-S-Co bonding to exist, along with HDS reaction. In order to understand electronic properties on thiocubane Density of States and Mulliken Population Analysis calculations were implemented using Cambridge Serial Total Energy Package (CASTEP). Results indicate a strong electron donation from Co to Mo through intermediate sulfur atom bonded to both metals while an enhanced metallic character is also found.

Over the past few years, thermoelectric (TE) materials have been receiving an increasing amount of attention owing to their promising potential for energy conversion and thermal management applications. Thermal characterisation techniques offer a powerful tool in investigating and optimizing the TE device performance. In addition, they can provide a better understanding of the underlying fundamental principles such as Peltier effects at the interfaces of the active medium. In this paper, we present the design and thermal characterisation of integrated highspeed microcoolers based on SiGe superlattices. The electrode metalisation is laid out as a coplanar waveguide, enabling to supply electrical pulses with short rise times to the coolers. We employ a variety of CCD-based transient thermoreflectance imaging methods to perform an extensive dynamic thermal analysis. These techniques provide 2-D temperature maps of the chip surface with ∼100ns temporal and submicron spatial resolution without the need to scan the sample. Net cooling in the 2 degree range is observed, with response times well below 1μs. This is almost two orders of magnitude faster compared to the best in the literature. The obtained images also confirm the previous observations that the Peltier cooling term responds faster than the Joule heating term, in agreement with their expected locality and associated thermal mass. This provides potential to study ultrafast electron-phonon interactions during Peltier effects.

Field emission from as-grown carbon nanotube (CNTs) films often suffered from high threshold electric field, and low emission site density due to screening effects. These problems can be resolved by patterned growth of CNTs on lithographically prepared catalyst films. However, these approaches are expensive and not applicable for future emitting devices with large display areas. Here we show that as-grown CNTs films can have low emission threshold field and high emission density without using any lithography processes. We have reduced screening effects and work function of as-grown CNTs films and created the novel CNT matrices by addition of vapor- and/or liquid- phase deposition. Furthermore, these CNT matrices can continuous emit electrons for 40 hours without significant degradation. The fabrication of our CNT matrices is described as follows. First, CNT films were grown by plasma-enhanced chemical vapor deposition. These vertically-aligned multiwalled carbon nanotubes (VA-MWCNTs) are having typical length and diameter of 4 microns and 40 nm, respectively. Spacing between these CNTs is ~80 nm in average, leading to poor emission properties due to the screening effect. These as-grown samples were then subjected to the deposition of strontium titanate (SrTiO3) by pulsed-laser deposition to reduce both the work function and screening effect of CNTs. The emission properties of these coated samples can be further improved by fully filled the spaces between VA-MWCNTs by poly-methyl metha acrylate (PMMA). The field emission threshold electric field was decreased from 4.22 V/μm for as-grown VA-MWCNTs to 1.7 V/μm for SrTiO3 coated VA-MWCNTs. The addition filling with PMMA and mechanical polishing can further reduce the threshold to 0.78V/μm for the so called PMMA-STO-CNT matrices. Long term emission stability and emission site density were also enhanced.

The charge transport properties critically depend on the degree of ordering of the chains in the solid state as well as on the density of chemical or structural defects. In general, goodelectronic performance requires strong electronic coupling between adjace nt molecules in the solid-state that yield strong intermolecular π-overlap. Herein, we newly designed and synthesized organic semiconducting materials having both aryl (Ar) and perfluoroaryl (FAr) as substituents for organic electronics along with molecular packing control. Regarding this molecular design, we hypothesized and expected that the Ar and FAr substituents would induce well-defined π-π stacking structure of charge transport units for high performance organic electronics devices.

This article presents the first man-made material based on the structure ofnacre that successfully duplicates the mechanism of tablet sliding. Thismaterial was made of millimeter size PMMA tablets arranged in columns andheld by fasteners. Strain hardening was provided by tablet waviness,delaying localization and leading to strains at failure 3-5 times greaterthan bulk PMMA. Analytical and finite element models successfully capturedthe locking mechanisms, enabling a rigorous design and optimization ofsimilar composites based on different materials or at different lengthscales. This work demonstrates how key features and mechanisms in naturalnacre can be successfully harnessed in engineering materials. Interestingly,the development of this model material and of its associated models alsounveiled two new mechanisms, the effect of free surfaces and “unzipping”.Both mechanisms may be relevant to natural materials such as nacre orbone.

The roles of hydrogen plasma radicals on passivation of several kinds of crystallized poly-Si thin films were investigated using optical emission spectroscopy (OES) combined with Hall mobility, Raman spectra, and absorption coefficient spectra. It was found that different kinds of hydrogen plasma radicals are responsible for passivation of dissimilar poly-Si crystallized by different method. Radicals Hα with lower energy are mainly responsible for passivating the poly-Si crystallized by solid phase crystallization (SPC) whose crystallization precursor was made by plasma enhanced chemical vapor deposition (PECVD). Higher energy radicals H* are more effective in passivating defects left over by Ni in poly-Si crystallized by Metal Induced Crystallization (MIC). The highest energy radicals Hβ and Hγ are needed to passivate the defects in poly-Si crystallized by SPC but whose precursor was made by low pressure CVD (LPCVD).

Two-Photon initiated polymerization (TPIP) has shown great promise for fabrication of complex micro- and nano-structures. The method has been used to fabricate such structures over small areas (< 1 mm2) because of slow fabrication speeds and resulting long fabrication times. In order for TPIP to reach practical application in a commercial setting fabrication times need to be reduced by orders of magnitude. We report results on a highly photosensitive initiation system for photoresists based on free radical and cationic polymerization, where photosensitivity is increased 102- to 103-fold compared to previously reported photoinitiation systems. Threshold writing speeds are determined for critical exposure conditions, including laser power, type and concentration of photoinitiation system, and photoresist type. Surface roughness, a critical parameter in applications such as optics and microfluidics, for example, is also used to determine threshold writing speed. The utility of the approach is demonstrated by making a cell phone keypad light guide from a microreplication tool fabricated using the highly photosensitive photoresist.