Sang Il Park

Omnidirectional printing of flexible, stretchable, and spanning silver microelectrodes.

Flexible, stretchable, and spanning microelectrodes that carry signals from one circuit element to another are needed for many emerging forms of electronic and optoelectronic devices. We have patterned silver microelectrodes by omnidirectional printing of concentrated nanoparticle inks in both uniform and high–aspect ratio motifs with minimum widths of approximately 2 micrometers onto semiconductor, plastic, and glass substrates. The patterned microelectrodes can withstand repeated bending and stretching to large levels of strain with minimal degradation of their electrical properties. With this approach, wire bonding to fragile three-dimensional devices and spanning interconnects for solar cell and light-emitting diode arrays are demonstrated.

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.

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.

Waterproof AlInGaP optoelectronics on stretchable substrates with applications in biomedicine and robotics

Inorganic light-emitting diodes and photodetectors represent important, established technologies for solid-state lighting, digital imaging and many other applications. Eliminating mechanical and geometrical design constraints imposed by the supporting semiconductor wafers can enable alternative uses in areas such as biomedicine and robotics. Here we describe systems that consist of arrays of interconnected, ultrathin inorganic light-emitting diodes and photodetectors configured in mechanically optimized layouts on unusual substrates. Light-emitting sutures, implantable sheets and illuminated plasmonic crystals that are compatible with complete immersion in biofluids illustrate the suitability of these technologies for use in biomedicine. Waterproof optical-proximity-sensor tapes capable of conformal integration on curved surfaces of gloves and thin, refractive-index monitors wrapped on tubing for intravenous delivery systems demonstrate possibilities in robotics and clinical medicine. These and related systems may create important, unconventional opportunities for optoelectronic devices.

Stretchable GaAs Photovoltaics with Designs That Enable High Areal Coverage

Recent research in advanced materials and mechanics demonstrates the possibility for integrating inorganic semiconductors with soft, elastomeric substrates to yield systems with linear elastic mechanical responses to strains that signifi cantly exceed those associated with fracture limits of the constituent materials (e.g. ∼ 1% for many inorganics). This outcome can provide stretching to strain levels of tens of percent (in extreme cases, more than 100%), for diverse, reversible modes of deformation, including bending, twisting, stretching or compressing. [ 1–7 ]

Light emission characteristics and mechanics of foldable inorganic light-emitting diodes.

Organic light emitting diodes (OLEDs) offer attractive alternatives to conventional inorganic devices due to their ability to be deposited over large areas on amorphous, thin fl exible substrates, for systems with lightweight, durable, and compact designs. [ 1 , 2 ] Recent work suggests that inorganic light-emitting diode (ILED) technologies can be adapted to reproduce many of these characteristics, [ 3 ] while at the same time providing performance advantages in brightness, robust operation and effi ciency. [ 3–5 ] Here we report advances in this type of ILED approach designed to enable degrees of bendability than signifi cantly exceed those previously achieved, [ 3 ] by exploiting concepts of neutral mechanical plane designs that we successfully developed for use in other fl exible inorganic device technologies in electronics [ 6 ] and photovoltaics. [ 7 ] The studies include quantitative analysis of the underlying mechanics, with the fi rst direct connections between such calculations and device performance through experimental measurements of bending induced shifts in the emission wavelength. The results provide strategies to achieve bending to radii as small as 0.7 mm with

Electrically interconnected assemblies of microscale device components by printing and molding

This letter presents approaches for assembly and electrical interconnection of micro/nanoscale devices into functional systems with useful characteristics. Transfer printing techniques provide deterministic control over an assembly process that occurs prior to or simultaneously with a soft lithographic molding step that defines relief features in a receiving polymer. Filling these features with conducting materials that are processable in the form of liquids or pastes yields integrated interconnects and contacts aligned to the devices. Studies of the underlying aspects and application to representative systems in photovoltaics and solid state lighting indicators provide insights into the process and its practical use.