Depleted uranium plasma reduction system study

A system life-cycle cost study was conducted of a preliminary design concept for a plasma reduction process for converting depleted uranium to uranium metal and anhydrous HF. The plasma-based process is expected to offer significant economic and environmental advantages over present technology. Depleted Uranium is currently stored in the form of solid UF{sub 6}, of which approximately 575,000 metric tons is stored at three locations in the U.S. The proposed system is preconceptual in nature, but includes all necessary processing equipment and facilities to perform the process. The study has identified total processing cost of approximately $3.00/kg of UF{sub 6} processed. Based on the results of this study, the development of a laboratory-scale system (1 kg/h throughput of UF6) is warranted. Further scaling of the process to pilot scale will be determined after laboratory testing is complete

Silicon-in-silica spheres via axial thermal gradient in-fibre capillary instabilities

The ability to produce small scale, crystalline silicon spheres is of significant technological and scientific importance, yet scalable methods for doing so have remained elusive. Here we demonstrate a silicon nanosphere fabrication process based on an optical fibre drawing technique. A silica-cladded silicon-core fibre with diameters down to 340 nm is continuously fed into a flame defining an axial thermal gradient and the continuous formation of spheres whose size is controlled by the feed speed is demonstrated. In particular, spheres of diameter \u3c 500 nm smaller than those produced under isothermal heating conditions are shown and analysed. A fibre with dual cores, p-type and n-type silicon, is drawn and processed into spheres. Spatially coherent break-up leads to the joining of the spheres into a bispherical silicon \u27p-n molecule\u27. The resulting device is measured to reveal a rectifying I-V curve consistent with the formation of a p-n junction

Charge Transport Modulation of a Flexible Quantum Dot Solar Cell Using a Piezoelectric Effect

© 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Colloidal quantum dots are promising materials for flexible solar cells, as they have a large absorption coefficient at visible and infrared wavelengths, a band gap that can be tuned across the solar spectrum, and compatibility with solution processing. However, the performance of flexible solar cells can be degraded by the loss of charge carriers due to recombination pathways that exist at a junction interface as well as the strained interface of the semiconducting layers. The modulation of the charge carrier transport by the piezoelectric effect is an effective way of resolving and improving the inherent material and structural defects. By inserting a porous piezoelectric poly(vinylidenefluoride-trifluoroethylene) layer so as to generate a converging electric field, it is possible to modulate the junction properties and consequently enhance the charge carrier behavior at the junction. This study shows that due to a reduction in the recombination and an improvement in the carrier extraction, a 38% increase in the current density along with a concomitant increase of 37% in the power conversion efficiency of flexible quantum dots solar cells can be achieved by modulating the junction properties using the piezoelectric effect

A Two-Step Absorber Deposition Approach To Overcome Shunt Losses in Thin-Film Solar Cells: Using Tin Sulfide as a Proof-of-Concept Material System

As novel absorber materials are developed and screened for their photovoltaic (PV) properties, the challenge remains to reproducibly test promising candidates for high-performing PV devices. Many early-stage devices are prone to device shunting due to pinholes in the absorber layer, producing “false negative” results. Here, we demonstrate a device engineering solution towards a robust device architecture, using a two-step absorber deposition approach. We use tin sulfide (SnS) as a test absorber material. The SnS bulk is processed at high temperature (400˚C) to stimulate grain growth, followed by a much thinner, low-temperature (200˚C) absorber deposition. At lower process temperature, the thin absorber overlayer contains significantly smaller, densely packed grains, which are likely to provide a continuous coating and fill pinholes in the underlying absorber bulk. We compare this two-step approach to the more standard approach of using a semi-insulating buffer layer directly on top of the annealed absorber bulk, and demonstrate a more than 3.5x superior shunt resistance Rsh with smaller standard error σRsh. Electron-beam induced current (EBIC) measurements indicate a lower density of pinholes in the SnS absorber bulk when using the two-step absorber deposition approach. We correlate those findings to improvements in the device performance and device performance reproducibility.Chemistry and Chemical Biolog

Nanostructured photovoltaics : improving device efficiency and measuring carrier transport

Thesis: Ph. D., Massachusetts Institute of Technology, Department of Materials Science and Engineering, 2017.This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.Cataloged from student-submitted PDF version of thesis.Includes bibliographical references (pages 153-163).Photovoltaics (PV) offer a promising route to combat climate change. However, the growth rate of the dominant commercial photovoltaic (PV) technology is limited by large capital expenditure requirements. This motivates fundamental research into thin-film materials, such as lead sulfide (PbS) quantum dots (QDs), that are composed of earth-abundant elements, can be produced through low-cost deposition techniques, and are stable under operating conditions. In this thesis, a device architecture that combines a zinc oxide (ZnO) nanowire ordered bulk heterojunction (OBHJ) architecture with band alignment engineering of the PbS QD film to enhance charge extraction is demonstrated. This approach results in PV devices with photocurrent density greater than 30 mA/cm2, which represents a 15% improvement compared to planar devices and enables solar cells with power conversion efficiency up to 9.6%. This photocurrent density is the highest achieved for QDs with a 1.3 eV band gap, which is the optimal band gap in the detailed balance limit. The enhanced photocurrent in the nanowire devices is shown to be a result of both improved light harvesting due to improved in-coupling of light after the addition of the ZnO nanowire array and improved carrier collection due to the bulk heterojunction effect. Furthermore, electron beam-induced current (EBIC) was used to study charge transport in PbS QD films. It is shown that holes are the minority carrier in PbS QD films treated with tetrabutylammonium iodide (TBAI). This finding indicates that the thickness of OBHJ devices composed of a PbS-TBAI film paired with an n-type nanowire array are constrained by minority carrier transport. Moreover, quantitative EBIC was applied for the first time on PbS QD diodes to measure the bulk minority carrier diffusion length (Lbulk). Lbulk was extrapolated by comparing the effective diffusion length measured at different beam energies. EBIC injection leads to high-level injection conditions, therefore a lower bound for the hole diffusion length in PbS-TBAI QD films is established, with Lbulk e 110 nm. This provides a critical design parameter for OBHJ solar cells. This thesis motivates further work on optimization of ZnO nanowire arrays for PbS QD OBHJ solar cells through array patterning, acceptor-doping, and passivation of the nanowire surface. Furthermore, the EBIC technique developed in this work can be applied to quantitatively measure nanoscale carrier diffusion lengths in other thin-film PV materials.by Paul Harlan Rekemeyer.Ph. D

Improved efficiency in organic/inorganic hybrid solar cells by interfacial modification of ZnO nanowires with small molecules

We demonstrate improved photovoltaic performance of ZnO nanowire/poly(3-hexylthiophene) (P3HT) nanofiber hybrid devices using an interfacial modification of ZnO nanowires. Formation of cascade energy levels between the ZnO nanowire and P3HT nanofiber was achieved by interfacial modification of ZnO nanowires using small molecules tetraphenyldibenzoperiflanthene (DBP) and 3,4,9,10-perylenetetracarboxylic bisbenzimidazole (PTCBI). The successful demonstration of improved device performance owing to the cascade energy levels by small molecule modification is a promising approach toward highly efficient organic/inorganic hybrid solar cellsclose1