Alfred J. Baca

Ultrathin silicon solar microcells for semitransparent, mechanically flexible and microconcentrator module designs

The high natural abundance of silicon, together with its excellent reliability and good efficiency in solar cells, suggest its continued use in production of solar energy, on massive scales, for the foreseeable future. Although organics, nanocrystals, nanowires and other new materials hold significant promise, many opportunities continue to exist for research into unconventional means of exploiting silicon in advanced photovoltaic systems. Here, we describe modules that use large-scale arrays of silicon solar microcells created from bulk wafers and integrated in diverse spatial layouts on foreign substrates by transfer printing. The resulting devices can offer useful features, including high degrees of mechanical flexibility, user-definable transparency and ultrathin-form-factor microconcentrator designs. Detailed studies of the processes for creating and manipulating such microcells, together with theoretical and experimental investigations of the electrical, mechanical and optical characteristics of several types of module that incorporate them, illuminate the key aspects.

Semiconductor Wires and Ribbons for High- Performance Flexible Electronics

This article reviews the properties, fabrication and assembly of inorganic semiconductor materials that can be used as active building blocks to form high-performance transistors and circuits for flexible and bendable large-area electronics. Obtaining high performance on low temperature polymeric substrates represents a technical challenge for macroelectronics. Therefore, the fabrication of high quality inorganic materials in the form of wires, ribbons, membranes, sheets, and bars formed by bottom-up and top-down approaches, and the assembly strategies used to deposit these thin films onto plastic substrates will be emphasized. Substantial progress has been made in creating inorganic semiconducting materials that are stretchable and bendable, and the description of the mechanics of these form factors will be presented, including circuits in three-dimensional layouts. Finally, future directions and promising areas of research will be described.

Mechanically flexible thin-film transistors that use ultrathin ribbons of silicon derived from bulk wafers

This letter introduces a type of thin-film transistor that uses aligned arrays of thin (submicron) ribbons of single-crystal silicon created by lithographic patterning and anisotropic etching of bulk silicon (111) wafers. Devices that incorporate such ribbons printed onto thin plastic substrates show good electrical properties and mechanical flexibility. Effective device mobilities, as evaluated in the linear regime, were as high as 360cm2V−1s−1, and on/off ratios were >103. These results may represent important steps toward a low-cost approach to large-area, high-performance, mechanically flexible electronic systems for structural health monitors, sensors, displays, and other applications.

Multidimensional architectures for functional optical devices.

Materials exhibiting multidimensional structure with characteristic lengths ranging from the nanometer to the micrometer scale have extraordinary potential for emerging optical applications based on the regulation of light-matter interactions via the mesoscale organization of matter. As the structural dimensionality increases, the opportunities for controlling light-matter interactions become increasingly diverse and powerful. Recent advances in multidimensional structures have been demonstrated that serve as the basis for three-dimensional photonic-bandgap materials, metamaterials, optical cloaks, highly efficient low-cost solar cells, and chemical and biological sensors. In this Review, the state-of-the-art design and fabrication of multidimensional architectures for functional optical devices are covered and the next steps for this important field are described.

Molded plasmonic crystals for detecting and spatially imaging surface bound species by surface-enhanced Raman scattering

This report introduces a type of plasmonic crystal that consists of metal coated nanostructures of relief molded on a polymer film as a substrate for surface-enhanced Raman scattering (SERS). Such crystals exhibit SERS enhancement factors of ∼105, over large areas and with sufficiently high levels of uniformity for precise two-dimensional Raman mapping of surface bound monolayers. The ease of fabrication together with the high sensitivities and spatial resolution that can be achieved suggests an attractive route to SERS substrates for portable chemical warfare agent detection, environmental monitors, noninvasive imaging of biomolecules, and other applications.

Selective solvent-free chromium detection using cadmium-free quantum dots

Abstract. Currently, the method of choice to test for the presence of chromium in water is to submit samples to a lab for testing. We present a simple field-ready test that is selective for the presence of chromium at concentrations of 100 ppb or greater. The Environmental Protection Agency maximum contaminant level (MCL) for total chromium is 100 ppb. This test uses a simple on/off fluorescent screening employing the use of silver indium sulfide (AgInS2) quantum dots (QDs). These QDs were impregnated into cotton pads to simplify field testing without the need for solvents or other liquid chemicals to be present. The change in fluorescence is instant and can be readily observed by eye with the use of a UV flashlight.

Si solar microcells for modules with reduced purity requirements, high voltage outputs and mechanically stretchable designs

We recently reported a strategy, in which modules consist of large-scale arrays of small, interconnected ultrathin (i.e. 1–20 µm) Si microcells (μ-cells) formed by anisotropic etching of bulk wafers and integrated with a soft printing technique. Here we report three new advances in this type of printed, μ-cell technology. First, we show that μ-cells formed with low purity, solar grade wafers (Dow Corning ® 101 SOG Si metal), can achieve efficiencies much higher than those possible with corresponding bulk cells formed with the same material. Second, we demonstrate high voltage mini-modules that incorporate these μ-cells and lead to high voltage outputs. Finally, we demonstrate the fabrication of mechanically stretchable solar cell modules which are non-coplanar (i.e. arch shaped). The results show that these materials and designs yield a stretchable layout that can undergo strains of up to 30 % without failure.