Large Crystalline Grains Research Articles

A Fast Deposition‐Crystallization Procedure for Highly Efficient Lead Iodide Perovskite Thin‐Film Solar Cells

AbstractThin‐film photovoltaics based on alkylammonium lead iodide perovskite light absorbers have recently emerged as a promising low‐cost solar energy harvesting technology. To date, the perovskite layer in these efficient solar cells has generally been fabricated by either vapor deposition or a two‐step sequential deposition process. We report that flat, uniform thin films of this material can be deposited by a one‐step, solvent‐induced, fast crystallization method involving spin‐coating of a DMF solution of CH3NH3PbI3 followed immediately by exposure to chlorobenzene to induce crystallization. Analysis of the devices and films revealed that the perovskite films consist of large crystalline grains with sizes up to microns. Planar heterojunction solar cells constructed with these solution‐processed thin films yielded an average power conversion efficiency of 13.9±0.7 % and a steady state efficiency of 13 % under standard AM 1.5 conditions.

A fast deposition-crystallization procedure for highly efficient lead iodide perovskite thin-film solar cells.

Thin-film photovoltaics based on alkylammonium lead iodide perovskite light absorbers have recently emerged as a promising low-cost solar energy harvesting technology. To date, the perovskite layer in these efficient solar cells has generally been fabricated by either vapor deposition or a two-step sequential deposition process. We report that flat, uniform thin films of this material can be deposited by a one-step, solvent-induced, fast crystallization method involving spin-coating of a DMF solution of CH3NH3PbI3 followed immediately by exposure to chlorobenzene to induce crystallization. Analysis of the devices and films revealed that the perovskite films consist of large crystalline grains with sizes up to microns. Planar heterojunction solar cells constructed with these solution-processed thin films yielded an average power conversion efficiency of 13.9±0.7% and a steady state efficiency of 13% under standard AM 1.5 conditions.

Cu2ZnSn(S,Se)4 solar cells based on chemical bath deposited precursors

A low-cost method has been developed to fabricate Cu2ZnSn(S,Se)4 solar cells. By this method, firstly SnS, CuS, and ZnS layers are successively deposited on a molybdenum/soda lime glass (Mo/SLG) substrate by chemical bath deposition. The Cu2ZnSn(S,Se)4 thin films are obtained by annealing the precursor in a selenium atmosphere utilizing a graphite box in the furnace. The obtained Cu2ZnSn(S,Se)4 thin films show large crystalline grains. By optimizing the preparation process, Cu2ZnSn(S,Se)4 solar cells with efficiencies up to 4.5% are obtained. The results imply that the Cu2ZnSn(S,Se)4/CdS interface and the back contact may be limiting factors for solar cell efficiency.

In插入层对硅衬底外延InN晶体质量和光学特性的影响

Indium interlayers with thickness of 0,0.1,0.5,1 nm were deposited respectively before wurtzite InN was grown on silicon substrate by plasma-assisted molecular beam epitaxy(PAMBE).X-ray diffraction(XRD) spectra,scanning electron microscope(SEM),absorption and photoluminescence(PL) spectra were adopted to analyze the influence of indium interlayer on the crystal and optical properties of InN.XRD and SEM results indicate that indium interlayer with the thickness of 0.5 nm can improve the morphology of InN epitaxial with larger crystalline grains and have a better crystal quality.Absorption and PL spectra show that the sample with 0.5 nm indium interlayer exhibits the smallest blue-shift of absorption edge,the narrowest FWHM of PL spectra and the best near-band-edge radiative recombination efficiency.In conclusion,indium interlayer with appropriate thickness does have a positive influence on the crystal and optical properties of InN grown on silicon substrate.

A New Route for Growing Large Grains in Metals

Controlled thermal processing can create alloys with large crystalline grains that can lead to improvements in materials performance. [Also see Report by Omori et al. ]

Growth, characterization, and magnetic properties of a Li(Mn,Ni)PO4 single crystal

LiMn0.95Ni0.05PO4 single crystals have been grown for the first time by the travelling-solvent floating-zone method at low argon pressure. The grown sample exhibits large single crystalline grains as revealed by means of polarization microscopy, X-ray Laue back scattering, and single-crystal X-ray diffraction. The composition of the crystal was determined by Energy-dispersive X-ray (EDX) spectroscopy. LiMn0.95Ni0.05PO4 orders in an orthorhombic olivine-like structure as expected and phase purity was confirmed by powder X-ray diffraction. An oriented cuboid with size of 2.4×2.5×2.7mm3 along a, b, and c crystalline directions, respectively, was used for anisotropic magnetic measurements.

Preparation of Monolithic Cu(In<SUB>0.7</SUB>Ga<SUB>0.3</SUB>)Se<SUB>2</SUB> Nanopowders and Subsequent Fabrication of Sintered CIGS Films

Cu(In,Ga)Se2 (CIGS) is a compound semiconductor and is one of the most attractive light-absorbing materials for use in thin film solar cells. Among the various approaches to prepare CIGS thin films, the powder process offers an extremely simple and materials-efficient method. Here, we report the mechano-chemical preparation of CIGS compound powders suitable for fabrication of CIGS films by a powder process. We found that the CIGS phase was formed from the elemental powders of Cu, In, and Se and liquid Ga using high energy milling process with a milling time as short as 40 min at 200 rpm due to a self-accelerating exothermic reaction. The morphology and size of the CIGS powders changed with a function of the milling speed (100-300 rpm), leading to an optimal condition of milling at 200 rpm for 120 min. We also found that it was difficult to obtain a monolithic phase of the CIGS powders without severe particle aggregation by mechano-chemical milling alone. Therefore, in combination with the milling, subsequent heat-treatment at 300 degrees C was performed, which successfully provided monolithic CIGS nanopowders suitable for powder process. When a thin film was fabricated from the monolithic CIGS nanopowders, a highly dense film with large crystalline grains was obtained. The CIGS film preserved its chemical composition of CuIn0.7Ga0.3Se2 after sintering as evidenced by Raman spectroscopy, EDS and SAED pattern of transmission electron microscopy. The film was also found suitable for a light absorbing layer of CIGS solar cells with its band gap energy of 1.14 eV evaluated by transmittance spectroscopy.

Porosities and Fatigue Property of Two A356 Cast Aluminum Alloy

To investigate the fatigue property of A356 cast aluminum alloy, the pore size distribution in two kinds of A356 alloy were statistically tested, and the S-N curves were plotted by conducting four-point bend fatigue test, then the fractography of the fatigue specimens were examined under scan electron microscope (SEM). It was indicated that the fatigue strength of the A356.2 alloy contenting more large pores and rougher crystalline grain was lower than that of A356.1 alloy, though the yield strength of A356.2 alloy was higher. Most of the fatigue cracks initiated from the pores that located on or close to the surface of the specimens, and the fracture of the specimens were induced from the growth of multi-sites initiated fatigue crack.

Rapid melt crystallization of amorphous-silicon thin films

We investigated the potential to melt crystallize amorphous Si thin films on glass with electromagnetic irradiation at 13.56 MHz, 2.45 GHz, and 110 GHz, respectively. We showed that the melt crystallization of millimeter-wave annealing could be an ideal choice for achieving high-quality semiconductor thin films with thickness in the micron range. In particular, silicon pillars with large ultra-high crystalline quality grains were experimentally realized by millimeter-wave annealing amorphous silicon on glass. Our simulation results were consistent with the experimental ones. Such understandings may enable us to prepare high-quality thin films on inexpensive substrates for solar cells and other solid-state devices.

Giant texturing effect in multiferroic MnWO4 polycrystals

Different methods of texturing polycrystalline materials have been developed over the years to use/probe anisotropic material properties with relative ease, where complicated and expensive single crystal growth processes could be avoided. In this paper, particle morphology assisted texturing in multiferroic MnWO4 has been discussed. Detailed powder x-ray diffraction vis-a-vis scanning electron microscopic studies on differently annealed and processed samples have been employed to probe the giant texturing effect in polycrystalline MnWO4. A quantitative measure of the texturing has been carried out by means of the Rietveld analysis technique. Qualitative presentation of magnetic and dielectric data on textured pellet demonstrated the development of clear anisotropic physical properties in polycrystalline pellets. Finally, we established that the highly anisotropic plate-like particles are formed due to easy cleavage of the significantly large crystalline grains.

Graphene Induced Surface Reconstruction of Cu

An atomic-scale study utilizing scanning tunneling microscopy (STM) in ultrahigh vacuum (UHV) is performed on large single crystalline graphene grains synthesized on Cu foil by a chemical vapor deposition (CVD) method. After thermal annealing, we observe the presence of periodic surface depressions (stripe patterns) that exhibit long-range order formed in the area of Cu covered by graphene. We suggest that the observed stripe pattern is a Cu surface reconstruction formed by partial dislocations (which appeared to be stair-rod-like) resulting from the strain induced by the graphene overlayer. In addition, these graphene grains are shown to be more decoupled from the Cu substrate compared to previously studied grains that exhibited Moiré patterns.

Mineralogy and petrology of Stardust particles encased in the bulb of track 80: TEM investigation of the Wild 2 fine-grained material

We have characterized by transmission electron microscopy the mineralogy of samples extracted from the walls of the Stardust track 80. More than 500 fragments were studied using conventional imaging, electron diffraction and EDX microanalysis. Two categories of particles are distinguishable in equal proportions (wt%). The first one is comprised of relatively large crystalline grains (≈1μm on average), dominated by silicates (olivine and pyroxene). They display a wide range of compositions and microstructures comparable to those found in terminal particles. Minor phases including magnetite and apatite are also present. Their occurrence suggests that the Wild 2 material underwent aqueous alteration to some extent. The second type of particle, called GEMS-like material, is made of silica-rich glassy clumps embedding iron sulfide beads and vesicles. Their microstructure is characteristic of thermally modified particles that have suffered strong interaction with the silica aerogel during the hypervelocity impact. This melted material may form by shedding of melted and vaporized material, but given the shape of the impact track and high diversity of surviving mineral compositions, much of it originated from fine-grained aggregates that disaggregated during the collection. Chemical mapping at the nano-scale allowed the localization of individual components within the silica-rich glass. They are dominated by Mg-rich components with a size less than 300nm. The average composition of this thermally modified material is close to the solar abundance for the major elements Fe, Mg and S. The fine-grained material has probably not been chemically fractionated in the protoplanetary disk before its incorporation in comet Wild 2 unlike the sulfur depleted matrix of chondrites. From these two categories of particles, we deduce that Wild 2 is likely made of an assemblage of relatively large evolved grains (first category) cemented by a fine-grained material with primitive chemistry (second category). The pre-impact configuration of the incident material deduced from this study seems comparable to the matrix of the most primitive chondrites (3.0) or to chondritic porous interplanetary dust particles.

Direct Low‐Temperature Integration of Nanocrystalline Diamond with GaN Substrates for Improved Thermal Management of High‐Power Electronics

AbstractA novel approach for the direct synthetic diamond–GaN integration via deposition of the high‐quality nanocrystalline diamond films directly on GaN substrates at temperatures as low as 450–500 °C is reported. The low deposition temperature allows one to avoid degradation of the GaN quality, which is essential for electronic applications The specially tuned growth conditions resulted in the large crystalline diamond grain size of 100–200 nm without coarsening. Using the transient “hot disk” measurements it is demonstrated that the effective thermal conductivity of the resulting diamond/GaN composite wafers is higher than that of the original GaN substrates at elevated temperatures. The thermal crossover point is reached at ≈95–125 °C depending on the thickness of the deposited films. The developed deposition technique and obtained thermal characterization data can lead to a new method of thermal management of the high power GaN electronic and optoelectronic devices.

TEM, chemical etching and FTIR characterization of ZnTe grown by physical vapor transport

AbstractZnTe ingots were obtained by the physical transport method, using an in‐house designed and built tubular furnace. The growth of ZnTe subsequently to the growth of a seed of this material allowed obtaining an ingot formed by only one large and single crystalline grain. TEM was used for the characterization of the as‐grown ZnTe single crystal ingots and commercial single‐crystalline wafers of the same material but grown by a higher temperature and more expensive technique, the Bridgman method. Both materials show very good crystalline microstructure, although some stacking faults were found in the commercial one. The infrared transmittance spectra of both materials were measured by FTIR and some differences, most likely due to differences in raw materials and growth methods, were found. The effectiveness and convenience of several chemical etchants to obtain the dislocation density and the minimal misorientation between adjacent subgrains in the as‐grown ZnTe wafers were checked. It has been found as the most advantageous the chemical solution that does not produce over‐etching. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Stress Dependence of Magnetic Properties in Non-oriented Electrical Steel Sheets

This paper discusses the stress dependence of magnetizing properties in non-oriented electrical steel sheets (NO). Compression stresses deteriorate the magnetic properties at low flux density, but improve them at high flux density, where magnetization rotations are induced. Tensile stresses reduce magnetizing fields at low flux density, but increase them at high flux density. The stress dependence of NO is affected by the exciting direction, as the rolling direction of NO is easy to magnetize and sensitive to stresses. High-grade types of NO (50A290, etc), which have high Si content and large crystalline grains, are more sensitive to stresses than low grades (50A1300, etc), and the magnetizing fields of high grades at high compression are higher than those of low grades. This stress dependence may depend on magnetization behaviors (domain wall displacement and magnetization rotation) and magnetostriction. Domain wall displacement mainly occurs at lower flux density, and in this case tensile stresses improve magnetic properties and compression deteriorate them because magnetic domains orient the magnetization to <100> and λ100 is positive. However, magnetizations rotate to exciting directions at high flux densities, with the result that the <111> component of magnetization is induced, and in this case compression improves the magnetizing properties, because λ111 is negative, whereas tensile stress deteriorates them. Therefore, stress dependences may be modeled by magnetization behaviors and magnetostriction, and analysis of the magnetic properties using this model of stress dependence may improve NO steels and their applications.

Fabrication of thin-film-type Bi2Sr2CaCu2O8+δ intrinsic Josephson junctions by pulsed-laser-deposition

Intrinsic Josephson junctions were fabricated onBi2Sr2CaCu2O8+δ thinfilms. The thin films were prepared by a pulsed-laser-deposition technique at the substrate temperature of500 °C. The deposited filmswere annealed at 884–890 °C for crystallization. Bi2Sr2CaCu2O8+δ thin films were successfully obtained with the stoichiometric composition. At the annealing temperatureof 888 °C,a film consisting of large crystalline grains has been obtained; a flat terrace area reaches several tensof µm and is larger than the typical mesa size for the intrinsic Josephson junction. The mesastructure patterned on the annealed film has exhibited clear multiple branches inthe current–voltage characteristic peculiar to the intrinsic Josephson junctions.

Electron microscopy of bulk metallic glass machining chips

Machined Zr 52.5Ti 5Cu 17.9Ni 14.6Al 10 bulk metallic glass (BMG) chips were characterized using high resolution electron microscopy. Compared to conventional processing techniques, machining produces very high heating/cooling, and deformation rates. It is therefore of interest to compare structural changes in machining chips with those produced by conventional processing. Large (∼1 μm) crystalline grain, residual amorphous region, and phase separation in the amorphous–crystalline transition region were detected in bright field TEM images. Three equilibrium phases, Zr 2Cu, ZrAl 2, and Zr 2Ni, which have been identified in samples undergoing conventional annealing, were revealed from selected area electron diffraction patterns of the chips. High magnification TEM micrographs showed nanocrystallites, about 10 nm in size, in the amorphous–crystalline transition region.

DUST PROCESSING AND GRAIN GROWTH IN PROTOPLANETARY DISKS IN THE TAURUS-AURIGA STAR-FORMING REGION

Mid-infrared spectra of 65 T Tauri stars (TTS) taken with the Infrared Spectrograph (IRS) on board the Spitzer Space Telescope are modeled using dust at two temperatures to probe the radial variation in dust composition in the uppermost layers of protoplanetary disks. Most spectra indicating crystalline silicates require Mg-rich minerals and silica, but a few suggest otherwise. Spectra indicating abundant enstatite at higher temperatures also require crystalline silicates at temperatures lower than those required for spectra showing high abundance of other crystalline silicates. A few spectra show 10 micron complexes of very small equivalent width. They are fit well using abundant crystalline silicates but very few large grains, inconsistent with the expectation that low peak-to-continuum ratio of the 10 micron complex always indicates grain growth. Most spectra in our sample are fit well without using the opacities of large crystalline silicate grains. If large grains grow by agglomeration of submicron grains of all dust types, the amorphous silicate components of these aggregates must typically be more abundant than the crystalline silicate components. Crystalline silicate abundances correlate positively with other such abundances, suggesting that crystalline silicates are processed directly from amorphous silicates and that neither forsterite, enstatite, nor silica are intermediate steps when producing either of the other two. Disks with more dust settling typically have greater crystalline abundances. Large-grain abundance is somewhat correlated with greater settling of disks. The lack of strong correlation is interpreted to mean that settling of large grains is sensitive to individual disk properties. Lower-mass stars have higher abundances of large grains in their inner regions.

Radiation-pressure mixing of large dust grains in protoplanetary disks

Dusty disks around young stars are formed out of interstellar dust that consists of amorphous, submicrometre grains. Yet the grains found in comets and meteorites, and traced in the spectra of young stars, include large crystalline grains that must have undergone annealing or condensation at temperatures in excess of 1,000 K, even though they are mixed with surrounding material that never experienced temperatures as high as that. This prompted theories of large-scale mixing capable of transporting thermally altered grains from the inner, hot part of accretion disks to outer, colder disk regions, but all have assumptions that may be problematic. Here I report that infrared radiation arising from the dusty disk can loft grains bigger than one micrometre out of the inner disk, whereupon they are pushed outwards by stellar radiation pressure while gliding above the disk. Grains re-enter the disk at radii where it is too cold to produce sufficient infrared radiation-pressure support for a given grain size and solid density. Properties of the observed disks suggest that this process might be active in almost all young stellar objects and young brown dwarfs.

Laser crystallization for large-area electronics

Laser crystallization is reviewed for the purpose of fabrication of polycrystalline silicon thin film transistors (poly-Si TFTs). Laser-induced rapid heating is important for formation of crystalline films with a low thermal budget. Reduction of electrically active defects located at grain boundaries is essential for improving electrical properties of poly-Si films and achieving poly-Si TFTs with high performances. The internal film stress is attractive to increase the carrier mobility. Recent developments in laser crystallization methods with pulsed and continuous-wave lasers are also reviewed. Control of heat flow results in crystalline grain growth in the lateral direction, which is important for fabrication of large crystalline grains. We also report an annealing method using a high-power infrared semiconductor laser. High-power lasers will be attractive for rapid formation of crystalline films over a large area and activation of silicon with impurity atoms.