Matthew Meitl

Pattern Transfer Printing by Kinetic Control of Adhesion to an Elastomeric Stamp

An increasing number of technologies require large-scale integration of disparate classes of separately fabricated objects into spatially organized, functional systems1,2,3,4,5,6,7,8,9. Here we introduce an approach for heterogeneous integration based on kinetically controlled switching between adhesion and release of solid objects to and from an elastomeric stamp. We describe the physics of soft adhesion that govern this process and demonstrate the method by printing objects with a wide range of sizes and shapes, made of single-crystal silicon and GaN, mica, highly ordered pyrolytic graphite, silica and pollen, onto a variety of substrates without specially designed surface chemistries or separate adhesive layers. Printed p–n junctions and photodiodes fixed directly on highly curved surfaces illustrate some unique device-level capabilities of this approach.

Printed Assemblies of Inorganic Light-Emitting Diodes for Deformable and Semitransparent Displays

Bend Me, Stretch Me In the push toward flexible electronics, much research has focused on using organic conducting materials, including light-emitting diodes (LEDs), because they are more readily processed using scalable techniques. Park et al. (p. 977) have developed a series of techniques for depositing and assembling inorganic LEDs onto glass, plastic, or rubber. Conventional processing techniques are used to connect the LEDs in order to create flexible, stretchable displays, which, because the active diode material only covers a small part of the substrate, are mostly transparent. Methods to fabricate and assemble inorganic light-emitting diodes provide a route toward transparent, flexible, or stretchable display devices. We have developed methods for creating microscale inorganic light-emitting diodes (LEDs) and for assembling and interconnecting them into unusual display and lighting systems. The LEDs use specialized epitaxial semiconductor layers that allow delineation and release of large collections of ultrathin devices. Diverse shapes are possible, with dimensions from micrometers to millimeters, in either flat or “wavy” configurations. Printing-based assembly methods can deposit these devices on substrates of glass, plastic, or rubber, in arbitrary spatial layouts and over areas that can be much larger than those of the growth wafer. The thin geometries of these LEDs enable them to be interconnected by conventional planar processing techniques. Displays, lighting elements, and related systems formed in this manner can offer interesting mechanical and optical properties.

GaAs photovoltaics and optoelectronics using releasable multilayer epitaxial assemblies

Compound semiconductors like gallium arsenide (GaAs) provide advantages over silicon for many applications, owing to their direct bandgaps and high electron mobilities. Examples range from efficient photovoltaic devices to radio-frequency electronics and most forms of optoelectronics. However, growing large, high quality wafers of these materials, and intimately integrating them on silicon or amorphous substrates (such as glass or plastic) is expensive, which restricts their use. Here we describe materials and fabrication concepts that address many of these challenges, through the use of films of GaAs or AlGaAs grown in thick, multilayer epitaxial assemblies, then separated from each other and distributed on foreign substrates by printing. This method yields large quantities of high quality semiconductor material capable of device integration in large area formats, in a manner that also allows the wafer to be reused for additional growths. We demonstrate some capabilities of this approach with three different applications: GaAs-based metal semiconductor field effect transistors and logic gates on plates of glass, near-infrared imaging devices on wafers of silicon, and photovoltaic modules on sheets of plastic. These results illustrate the implementation of compound semiconductors such as GaAs in applications whose cost structures, formats, area coverages or modes of use are incompatible with conventional growth or integration strategies.

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.

Bendable GaN high electron mobility transistors on plastic substrates

A procedure for fabricating flexible forms of high electron mobility transistors (HEMTs) supported on plastic substrates is described. The process uses a combination of conventional top-down, wafer scale fabrication protocols to define a printable form of ultrathin, device quality multilayer AlGaN∕GaN single crystalline microstructures—a so-called microstructured semiconductor ink—and soft-lithographic printing methods to effect their registered transfer to a plastic substrate. These procedures yield high performance, bendable HEMT arrays that are mechanically durable—ones with effective transconductances exceeding nearly all reported forms of printed thin-film transistors.

Printing-based assembly of quadruple-junction four-terminal microscale solar cells and their use in high-efficiency modules

Expenses associated with shipping, installation, land, regulatory compliance and on-going maintenance and operations of utility-scale photovoltaics can be significantly reduced by increasing the power conversion efficiency of solar modules through improved materials, device designs and strategies for light management. Single-junction cells have performance constraints defined by their Shockley-Queisser limits. Multi-junction cells can achieve higher efficiencies, but epitaxial and current matching requirements between the single junctions in the devices hinder progress. Mechanical stacking of independent multi-junction cells circumvents these disadvantages. Here we present a fabrication approach for the realization of mechanically assembled multi-junction cells using materials and techniques compatible with large-scale manufacturing. The strategy involves printing-based stacking of microscale solar cells, sol-gel processes for interlayers with advanced optical, electrical and thermal properties, together with unusual packaging techniques, electrical matching networks, and compact ultrahigh-concentration optics. We demonstrate quadruple-junction, four-terminal solar cells with measured efficiencies of 43.9% at concentrations exceeding 1,000 suns, and modules with efficiencies of 36.5%.

Stretchable Semiconductor Technologies with High Areal Coverages and Strain‐Limiting Behavior: Demonstration in High‐Efficiency Dual‐Junction GaInP/GaAs Photovoltaics

Notched islands on a thin elastomeric substrate serve as a platform for dual-junction GaInP/GaAs solar cells with microscale dimensions and ultrathin forms for stretchable photovoltaic modules. These designs allow for a high degree of stretchability and areal coverage, and they provide a natural form of strain-limiting behavior, helping to avoid destructive effects of extreme deformations.

Stress focusing for controlled fracture in microelectromechanical systems

This letter describes a strategy for controlling fracture in microelectromechanical systems (MEMSs) based on the control of corner sharpness. Studies of model MEMS structures with round (radius of approximately microns), intermediate, and sharp (<10nm) corners demonstrate the effects of corner sharpness on the concentration of applied stress. Finite-element analysis reveals that stress distributions intensify and localize as sharpness increases, and transfer printing experiments demonstrate the influence of stress concentration on breakability.

A high concentration photovoltaic module utilizing micro-transfer printing and surface mount technology

We describe a high concentration photovoltaic (CPV) module utilizing micro-transfer printed (µ-TP) dual-junction GaInP/GaAs solar cells and an ELO (Epitaxial Lift-Off) process used to fabricate very small cells (<0.5 mm2) using 1st use and reused GaAs substrates. The benefits of this technology include high efficiency, simple distributed heat transfer at high concentration ratios, and short optical paths. This approach enables the use of low cost, high reliability surface mount assembly of large backplanes for integration into CPV modules. To minimize compound semiconductor use and maximize cell efficiency, we combine plano-convex primary and spherical secondary optics to concentrate sunlight 1000X over a +/−0.8 degree angle of acceptance. Receiver efficiencies of ELO dual-junction GaInP/GaAs cells of >30% at 1,000 sun concentration are reported. Coupled with a >80% efficient optical train, module efficiencies greater than 24% have been achieved with dual-junction µ-TP solar cells.