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Micro transfer printing (µTP) is a useful method for heterogenous integration of micro-scale devices but typically requires additional electrical interconnection of devices following print. In this paper, the concept of interconnect at print is explored and a stacked configuration demonstrating electrical interconnection of 3-D micrometer scale electrical devices is shown. Structures that are 45µm by 25µm consisting of four metallized spikes on the bottom and recessed pyramids at the top are stacked, probed, and shown to illuminate light demonstrating their electrical interconnection from print.

The incumbent flat-panel technologies, liquid crystal display (LCD) and organic light-emitting diode display (OLED), are ill-suited to produce compact, efficient, and robust high-brightness displays. LCDs are very inefficient, only a small fraction (~5%) of the generated light exits the display. To achieve highbrightness LCDs, practitioners create extremely bright back-light units using inorganic LEDs which require expensive and unreliable active cooling solutions. OLEDs use organic molecules to form light emitting diodes within each display pixel. The lifetime of the organic light emitters is inversely proportional with the display brightness; therefore, OLEDs are not suitable for highbrightness applications. In sharp contrast, inorganic LEDs made using wafer-level semiconductor technology are long-lived, even when operating at high luminance. Displays that use inorganic LEDs as the light-emitters within each display pixel already dominate the giant video walls that increasingly decorate our highways and streetscapes. Today, there are many efforts around the world aimed at making highly miniaturized inorganic LEDs, called microLEDs, and developing methods to transfer those microLEDs from their native substrate to the destination display substrate. Effective techniques to produce microLED displays must have the capability to quickly and accurately transfer millions of microscale devices and are called "mass transfer" technologies. Micro-transfer-printing using elastomer stamps is one such "mass transfer" technology that has been used to produce prototype microLED displays. Here, we will describe how micro-transfer-printing combined with wafer-level packaging techniques can produce highbrightness displays. We will provide fabrication details and characterization results of various 5.1" 70 PPI microLED displays. In one example, we produced a monochrome green display using 8 μm x 15 μm flip-chip InGaN microLEDs with a maximum brightness in excess of 30,000 nits. We will highlight application opportunities and remaining challenges for high-brightness displays using microLEDs.

Flat panel displays are ubiquitous and dominated today by liquid crystal and OLED technologies. Increasingly, there is an expectation that microLED will exhibit superior performance metrics and become a new mainstream category of flat panel displays. They have the potential to be very bright, to be power efficient, and to enable new within-panel capabilities. High-throughput, high-yield, mass transfer technologies that accurately and cost-effectively integrate large arrays of wafer-fabricated microdevices onto non-native display substrates are key enablers for microLED displays. Transfer-printing with elastomer stamps is a candidate mass transfer technology for making next generation displays. A variety of microLED displays, including displays controlled with transfer-printed microICs, have been designed and fabricated using elastomer stamp transfer-printing.

We have developed multijunction solar cells on both GaAs and InP platforms to split the solar spectrum efficiently among subcells that maintain lattice-match to their respective source substrates. The GaAs-based dual-junction solar cell has achieved 31% AM1.5D concentrator efficiency. The InP-based triple junction solar cell has achieved 8.7% AM1.5D efficiency under concentration. Using micro-transfer printing, the wide-bandgap GaAs-based subcells will be stacked on top of the narrow-gap InP-based subcells and characterized to determine 4-terminal AM1.5D efficiency of the five-junction device. The microcells, less than 200 μm on a side, are assembled on transparent glass substrates, permitting diffuse light transmission to be collected by a silicon backplane. The approach facilitates efficient collection of both direct and diffuse sunlight for potential applications in a wide variety of climates. Major milestones in III-V device development are detailed.

Displays that use direct light emission from microscale inorganic light emitting diodes (ILEDs) have the potential to be very bright and also very power efficient. High-throughput assembly technologies that accurately and cost-effectively deposit large arrays of ILEDs onto display substrates with high yield are key enablers for ILED displays. Transfer-printing with elastomer stamps is a candidate assembly technology for making ILED displays. A variety of passive matrix ILED displays have been designed and fabricated using transfer-printed microscale ILEDs.

Transfer printing is a materials assembly technique that uses elastomeric stamps for heterogeneous integration of various classes of micro- and nanostructured materials into two- and three-dimensionally organized layouts on virtually any type of substrate. Work over the past decade demonstrates that the capabilities of this approach create opportunities for a wide range of device platforms, including component- and system-level embodiments in unusual optoelectronic technologies with characteristics that cannot be replicated easily using conventional manufacturing or growth techniques. This review presents recent progress in functional materials and advanced transfer printing methods, with a focus on active components that emit, absorb, and/or transport light, ranging from solar cells to light-emitting diodes, lasers, photodetectors, and integrated collections of these in functional systems, where the key ideas provide unique solutions that address limitations in performance and/or functionality associated with traditional technologies. High-concentration photovoltaic modules based on multijunction, micro- and millimeter-scale solar cells and high-resolution emissive displays based on microscale inorganic light-emitting diodes provide examples of some of the most sophisticated systems, geared toward commercialization.

We propose a novel methodology for III-V substrate cost reduction that does not rely on chemical-mechanical polishing (CMP) or low-throughput release layer etching. The details of this process are provided, and results of an economic model for GaAs and InP virtual substrates dictate 20x and 34x reductions to substrate cost, respectively. The impact of modeled assumptions are quantified in order to better understand the potential range of end-goal virtual substrate cost.

Elastomer stamp micro-transfer-printing is a highly scalable method for the assembly of microscale components onto non-native substrates. One of the key value propositions of micro-transfer-printing is that the transfer stamp can be scaled to wafer-dimensions and can transfer tens to thousands of micro-devices in a single step, equating to multiple millions of units per hour. Here, we report on the results of systematically scaling the stamp from 12.8 mm × 12.8 mm to a full 150 mm stamp, capable of transferring all the required devices to a 150 mm receiving wafer in one operation. The 150 mm stamp is designed to transfer more than 80,000 chips in one print cycle. This study was carried out using silicon nitride test vehicles that were specially designed for this project. We will discuss how stamp scaling impacts transfer yield and the implications for ultra-high throughput assembly of micro-devices. In addition, we will explore the capability to transfer very small devices down to 3 µm × 3 µm.

High-throughput assembly of miniature wafer-fabricated packages onto panel substrates provides a manufacturing framework for high-performance multi-functional displays and other large-format systems. Control circuits, light emitters, sensors, and other micro-components formed in high-density arrays on wafers use a variety of processes and materials that do not easily translate to large-format panel processing. Systems assembled from some or all of those components can therefore exhibit combinations of properties and performance characteristics that are difficult to achieve by panel processes only. Here, we demonstrate hierarchical assembly strategies for fabricating high-performance systems using elastomer stamp micro-transfer-printing. In this work, red, green and blue microscale inorganic LEDs (µILEDs) are fabricated on their respective native wafer substrates and then assembled onto non-native intermediate silicon wafers. The intermediate silicon wafer, populated with heterogeneous µILEDs, then undergoes conventional wafer-level processes, such a dielectric depositions and thin-film metallization, to form miniature fan-out packages. Here, we will demonstrate three heterogeneous µILEDs integrated within a 75 µm × 35 µm fan-out package. We will present how this microscale package can be undercut and then micro-transfer-printed directly onto large-format application substrates. The print-compatible packages also include sharp pressure-concentrating conductor structures which allow the heterogeneous fan-out packages to be electrically interconnected to large-format substrates during the printing operation. We will present functional µILED displays that have been fabricated using these assembly techniques. We will report on the benefits of using intermediate packaging substrates for manufacturing of high-performance large-format systems, such as displays. We will also demonstrate strategies for repairing large multi-functional systems.

Inorganic light emitting diodes (LEDs) serve as bright pixel-level emitters in displays, from indoor/outdoor video walls with pixel sizes ranging from one to thirty millimeters to micro displays with more than one thousand pixels per inch. Pixel sizes that fall between those ranges, roughly 50 to 500 microns, are some of the most commercially significant ones, including flat panel displays used in smart phones, tablets, and televisions. Flat panel displays that use inorganic LEDs as pixel level emitters (μILED displays) can offer levels of brightness, transparency, and functionality that are difficult to achieve with other flat panel technologies. Cost-effective production of μILED displays requires techniques for precisely arranging sparse arrays of extremely miniaturized devices on a panel substrate, such as transfer printing with an elastomer stamp. Here we present lab-scale demonstrations of transfer printed μILED displays and the processes used to make them. Demonstrations include passive matrix μILED displays that use conventional off-the shelf drive ASICs and active matrix μILED displays that use miniaturized pixel-level control circuits from CMOS wafers. We present a discussion of key considerations in the design and fabrication of highly miniaturized emitters for μILED displays.

© 2016, National Academy of Sciences. All rights reserved. Emerging classes ofconcentrator photovoltaic (CPV) modules reach efficiencies that are far greater than those of even the highest performance flat-plate PV technologies, with architectures that have the potential to provide the lowest cost of energy in locations with high direct normal irradiance (DNI). A disadvantage is their inability to effectively use diffuse sunlight, thereby constraining widespread geographic deployment and limiting performance even under the most favorable DNI conditions. This study introduces a module design that integrates capabilities in flat-plate PV directly with the most sophisticated CPV technologies, for capture of both direct and diffuse sunlight, thereby achieving efficiency in PV conversion of the global solar radiation. Specific examples of this scheme exploit commodity silicon (Si) cells integrated with two different CPV module designs, where they capture light that is not efficiently directed by the concentrator optics onto large-scale arrays of miniature multijunction (MJ) solar cells that use advanced III-V semiconductor technologies. In this CPV+scheme ("+" denotes the addition of diffuse collector), the Si and MJ cells operate independently on indirect and direct solar radiation, respectively. On-sun experimental studies of CPV+modules at latitudes of 35.9886° N (Durham, NC), 40.1125° N (Bondville, IL), and 38.9072° N (Washington, DC) show improvements in absolute module efficiencies of between 1.02% and 8.45% over values obtained using otherwise similar CPV modules, depending on weather conditions. These concepts have the potential to expand the geographic reach and improve the cost-effectiveness of the highest efficiency forms of PV power generation.

Discrete gallium nitride high electron mobility transistors (HEMTs) are fabricated on oriented silicon, then undercut and assembled onto non-native silicon CMOS wafers by elastomer stamp micro-transfer-printing. The thin, less than 5 µm thick, gallium nitride transistors were then electrically interconnected using conventional thin-film metallization processes. Electrical measurements reveal that the heterogeneous integration process is benign to the underlying silicon transistors, and that the heterogeneous wide bandgap GaN transistors maintain their characteristic high voltage performance after being undercut and transferred to the non-native CMOS wafer.

Sharp electrically conductive structures integrated into micro-transfer-print compatible components provide an approach to forming electrically interconnected systems during the assembly procedure. Silicon micromachining techniques are used to fabricate print-compatible components with integrated, electrically conductive, pressure-concentrating structures. The geometry of the structures allow them to penetrate a polymer receiving layer during the elastomer stamp printing operation, and reflow of the polymer following the transfer completes the electrical interconnection when capillary action forces the gold-coated pressure-concentrator into a metal landing site. Experimental results and finite element simulations support a discussion of the mechanics of the interconnection.

Elastomer stamp based micro assembly or micro-transfer printing is a practical method for heterogeneous integration of micro-scale devices onto non-native substrates. In this paper, we evaluate the effect of stamp lifetime on performance and assess the useful lifetime of a stamp, both key metrics for using this technology in a manufacturing environment. We also review the performance of micro transfer-printing in several applications where >99% print yields and precise placement has been demonstrated.

The realization of GaAs lasers on a silicon substrate using a print transfer process offers an alternative wafer-bonding technique for the hybrid integration of optoelectronics.

Although compound semiconductors like gallium arsenide have a substantial performance advantage over silicon in photovoltaic and optoelectronic applications, these do not outweigh the costly process of growing large, high-quality layers of these materials and transferring them to flexible or transparent substrates for use in devices such as solar cells, night vision cameras and wireless communication systems. But now John Rogers and his team demonstrate a new fabrication approach that may remove this disadvantage. They grow films of GaAs and AlGaAs in thick, multilayered assemblies in a single deposition sequence, then release the individual layers and distribute them over foreign substrates by printing. The technological potential of this strategy to large-area applications is illustrated with the fabrication of GaAs devices such as field-effect transistors on glass and photovoltaic modules on sheets of plastic. Although compound semiconductors like gallium arsenide (GaAs) offer advantages over silicon for photovoltaic and optoelectronic applications, these do not outweigh the costly process of growing large layers of these materials and transferring them to appropriate substrates. However, a new fabrication approach is now demonstrated: films of GaAs and AlGaAs are grown in thick, multilayered assemblies in a single sequence; the individual layers are then released and distributed over foreign substrates by printing. 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 devices1,2 to radio-frequency electronics3,4 and most forms of optoelectronics5,6. 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.

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.

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.

Thin-film diamond has many potential applications in electronics and optoelectronics, microelectromechanical systems (MEMS), wear-resistant coatings, thermal management, and other areas owing to its exceptional electronic, optical, mechanical, chemical/tribological, and thermal properties, respectively. However, challenges in the integration of thin-film diamond with other materials continue to limit its widespread use. Thin-film diamond is most commonly implemented in these systems by directly growing the material on the surfaces of device substrates, where it is used as uniform or lithographically patterned films. This approach places restrictions on the range of applications because all known growth techniques involve relatively high temperatures (>400 8C), vacuum or low pressures, and often other demanding conditions. Integrating thin-film diamond on low-temperature plastics, for example, is not possible. Large-area substrates are also not well-matched to the capabilities of existing deposition techniques, being particularly cost-ineffective when the required diamond coverage is

This article demonstrates a method for fabricating high quality single-crystal silicon ribbons, platelets and bars with dimensions between ∼100 nm and ∼5 cm from bulk (111) wafers by using phase shift and amplitude photolithographic methods in conjunction with anisotropic chemical etching procedures. This “top-down” approach affords excellent control over the thicknesses, lengths, and widths of these structures and yields almost defect-free, monodisperse elements with well defined doping levels, surface morphologies and crystalline orientations. Dry transfer printing these elements from the source wafers to target substrates by use of soft, elastomeric stamps enables high yield integration onto wafers, glass plates, plastic sheets, rubber slabs or other surfaces. As one application example, bottom gate thin-film transistors that use aligned arrays of ribbons as the channel material exhibit good electrical properties, with mobilites as high as ∼200 cm 2 V –1 s –1 and on/off ratios >10 4 .

We report the use of networks of single-walled carbon nanotubes (SWNTs) with high and moderate coverages (measured as number of tubes per unit area) for all of the conducting (i.e., source, drain, and gate electrodes) and semiconducting layers, respectively, of a type of transparent, mechanically flexible, thin-film transistor (TFT). The devices are fabricated on plastic substrates using layer-by-layer transfer printing of SWNT networks grown using optimized chemical vapor deposition (CVD) procedures. The unique properties of the SWNT networks lead to electrical (e.g., good performance on plastic), optical (e.g., transparent at visible wavelengths), and mechanical (e.g., extremely bendable) characteristics in this “all-tube” TFT that would be difficult, or impossible, to achieve with conventional materials.

This work was supported by the DARPA-funded and AFRL-managed Macroelectronics Program, and in part by the National Science Foundation (DMI-0355532 and CHE-0402420) using the facilities at the Frederick Seitz Materials Research Laboratory, supported by the Department of Energy (DEFG02-96ER45439). M. A. M. thanks the Fannie and John Hertz Foundation for their support via a graduate fellowship.

We show that small diameter, single-walled carbon nanotubes can serve as templates for performing polymer imprint lithography with feature sizes as small as 2 nm − comparable to the size of an individual molecule. The angstrom level uniformity in the critical dimensions of the features provided by this unusual type of template provides a unique ability to investigate systematically the resolution of imprint lithography at this molecular scale. Collective results of experiments with several polymer formulations for the molds and the molded materials suggest that the density of cross-links is an important molecular parameter that influences the ultimate resolution in this process. Optimized materials enable reliable, repetitive patterning in this single nanometer range.

This paper presents results on the selective ablation of individual single-walled carbon nanotubes by use of intense picosecond laser pulses. Linearly polarized pulses ablate only those tubes that are oriented substantially along the polarization direction. When applied to random submonolayer networks of tubes on solid supports, this procedure can produce collections of aligned tubes oriented perpendicular to the polarization direction. Detailed examples highlight essential aspects of the approach and some features of the underlying physics that governs the process. Thin film transistors that use networks of tubes that have been laser processed in this manner show extremely high orientational anisotropy in the effective mobility. The results could be important for transistors that have potential applications as sensors or logic elements in macroelectronic systems or for optical elements with highly anisotropic properties.

This paper demonstrates the use of arrays of networks of single wall carbon nanotubes (SWNTs) and electrical breakdown procedures for building thin film transistors (TFTs) that have good, reproducible performance and high current output. Channel length scaling analysis of these TFTs indicates that the resistance at the source/drain contacts is a small fraction of the device resistance, in the linear regime. When measured with the channel exposed to air or coated by poly(methyl methacrylate) (PMMA), these transistors operate in the unipolar p mode. By spin-coating the polymer polyethylenimine (PEI) on the channel region, these transistors can be switched to operate in the unipolar n mode. Patterning the exposure of a single channel to PMMA and PEI yields p -n diodes. These results indicate that SWNT-TFTs can provide the building blocks of complex complementary circuits for a range of applications in macroelectronics, sensors, and other systems.

This article briefly describes two recently developed soft-lithographic techniques that can be used to fabricate complex, well-defined three-dimensional (3D) nanustructures. The first relies one the single or multilayer transfer of thin solid 'ink' coatings from high-resolution rubber stamsp. The second uses these stamps as conformable phase masks for proximity field nanopatterning of thin layers of transparent photopolymers. Although both techniques use the same pattern-transfer elements, they rely on completely different physical principles and they provide complementary patterning capabilities. The operational simplicity of the techniques, their ability to pattern large areas quickly, and the flexibility in the geometry of structures that can be formed with them suggest general utility for 3D nanomanufacturing.

This paper presents methods for solution casting and transfer printing collections of individual single-walled carbon nanotubes (SWNTs) onto a wide range of substrates, including plastic sheets. The deposition involves introduction of a solvent that removes surfactant from a suspension of SWNTs as it is applied to a substrate. The subsequent controlled flocculation (cF) produces films of SWNTs with densities that can be varied between a few tubes per square micron to thick multilayers in a single deposition step and with orientation determined by the direction of solution flow. High-resolution rubber stamps inked in this manner can be used to print patterns of tubes with geometries defined by the relief structure on the surface of the stamp. Thin film transistors fabricated with these techniques demonstrate their potential use in flexible “macroelectronic” systems.

Electrically conducting nanocomposites of bismuth metal and insulating ceramic phases of SiO2 and MgO were generated via high-energy ball milling for 24 h using zirconia milling media. The resulting nanocomposites contain Bi nanoparticles with sizes down to 5 nm in diameter. The morphology is a strong function of the oxide phase: specifically, the Bi appears to wet MgO while it forms spherical nanoparticles on the SiO2. X-ray diffraction measurements indicate a nominal bismuth grain size of 50 nm, and peak fitting to a simple bidisperse model yields a mixture of approximately 57 % bulk bismuth and 43 % 27 nm diameter crystallites. Nanoparticles as small as 5 nm are observed in transmission electron microscopy (TEM), but may not constitute a significant volume fraction of the sample. Differential scanning calorimetry reveals dramatic broadening in the temperatures over which melting and freezing occur and a surprising persistence of nanostructure after thermal cycling above the melting point of the Bi phase.

This letter describes the use of nanotransfer printing (nTP) for forming three-dimensional (3D) structures with feature sizes between tens of nanometers and tens of microns over areas of several square millimeters. We demonstrate three different approaches-deep etching through printed hard masks, direct transfer of three-dimensional structures, and purely additive fabrication of multilayer stacks-for using nTP to fabricate a range of complex 3D nanostructures, including closed channels, suspended beams, and nanochannel stacks, that would be difficult or impossible to build with other methods.

Transfer-printing is an important, commercial technology for manufacturing state of the art CPV modules, and has emerged recently as a key enabling technology for the realization of ultra-high-efficiency, mechanically stacked III-V solar cells with low cost. This paper presents the latest results for microscale CPV cells grown on GaAs, InP and GaSb substrates for ultra-high-efficiency, four-terminal, mechanically stacked architectures. The latest findings from a combination of modeling, growth, processing and characterization of single and multijunction solar cells are described, and the roadmap to the long-term goal of using transfer-printing to produce the first solar cell with 50% conversion efficiency is outlined.

In this work, a multijunction solar cell is developed on a GaSb substrate that can efficiently convert the long-wavelength photons typically lost in a multijunction solar cell into electricity. A combination of modeling and experimental device development is used to optimize the performance of a dual junction GaSb/InGaAsSb concentrator solar cell. Using transfer printing, a commercially available GaAs-based triple junction cell is stacked mechanically with the GaSb-based materials to create a four-terminal, five junction cell with a spectral response range covering the region containing >99% of the available direct-beam power from the Sun reaching the surface of the Earth. The cell is assembled in a mini-module with a geometric concentration ratio of 744 suns on a two-axis tracking system and demonstrated a combined module efficiency of 41.2%, measured outdoors in Durham, NC. Taking into account the measured transmission of the optics gives an implied cell efficiency of 44.5%.

Transfer-printing is a key enabling technology for the realization of ultra-high-efficiency, mechanically stacked II–IV solar cells with low cost. In this work, we describe the development of InGaAs solar cells, designed to harvest long wavelength photons when stacked in tandem with a high efficiency InGaP/GaAs/InGaAsNSb triple junction solar cell. High performance InGaAs solar cells, grown on InP by MOCVD, were achieved through a combination of detailed modeling, material development and device characterization. The transfer printing apparatus of Semprius Inc. was used to create a four-terminal device with an uncertified conversion efficiency of 44.1% at 690 suns.

Conventional multi-junction (MJ) cells are limited by requirements in epitaxial growth and current-matching. Mechanically stacked MJ cells circumvent these disadvantages, but existing approaches lack scalable manufacturing processes and suitable interfaces between the stacked cells. Here we present materials and strategies designed to bypass these limitations. The schemes involve (1) printing of microscale solar cells, (2) advanced optical/electrical/thermal interface materials and (3) 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 1000 suns, and modules with efficiencies of 36.5%.

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%.

Displays using direct light emission from microscale inorganic light-emitting diodes (μILEDs) have the potential to be very bright and also very power efficient. High-throughput technologies that accurately and cost-effectively assemble microscale devices on display substrates with high yield are key enablers for μILED displays. Elastomer stamp transfer printing is such a candidate assembly technology. A variety of μILED displays have been designed and fabricated by transfer printing, including passive-matrix and active-matrix displays on glass and plastic substrates.

Micro transfer printing and other forms of micro assembly deterministically produce heterogeneously integrated systems of miniaturized components on non-native substrates. Most micro assembled systems include electrical interconnections to the miniaturized components, typically accomplished by metal wires formed on the non-native substrate after the assembly operation. An alternative scheme establishing interconnections during the assembly operation is a cost-effective manufacturing method for producing heterogeneous microsystems, and facilitates the repair of integrated microsystems, such as displays, by ex post facto addition of components to correct defects after system-level tests. This letter describes pressure-concentrating conductor structures formed on silicon (1 0 0) wafers to establish connections to preexisting conductive traces on glass and plastic substrates during micro transfer printing with an elastomer stamp. The pressure concentrators penetrate a polymer layer to form the connection, and reflow of the polymer layer bonds the components securely to the target substrate. The experimental yield of series-connected test systems with >1000 electrical connections demonstrates the suitability of the process for manufacturing, and robustness of the test systems against exposure to thermal shock, damp heat, and mechanical flexure shows reliability of the resulting bonds.

Semprius' two-stage pupil imaging concentrated photovoltaic (CPV) module design incorporates extremely costeffective glass ball secondary lenses in addition to plano-convex primary lens arrays. Optimization of the optical concentrator design involves modeling the illumination uniformity of the primary aperture (the 'pupil') on the multijunction solar cell in response to the secondary lens index, diameter, surface quality, location, and tolerance offsets. We reconcile our theoretical model with experimental results from a single fully adjustable 'concentrating unit cell', and we thereby create a robust model for design updates, for tolerance and sensitivity modeling, and for prediction of full module and on-sun tracker performance based on receiver placement relative to our primary lens array. In this paper, we discuss the rationale behind our optics approach, our criteria for optimizing our optics, and our tolerancing approach. Then we discuss our experimental approach, including our universally adjustable 'concentrating unit' fixture, our light source, and our primary and secondary optics. We show sensitivity curves of our 'concentrating unit' performance to receiver placement, and ball lens size. We reconcile these with our ray-traced model, and, finally, we show predicted module performance based on receiver tolerance data and receiver wiring in the module using a parameter driven high level circuit model.

Advanced empirical models have been developed to analyze or predict the performance of flat-plate or concentrator photovoltaic modules/systems in the outdoor environment [1]. These models typically rely on the use of empirical equations driven by several coefficients which are empirically determined through regression analysis of large sets of experimental data collected in the field. For instance, the performance model developed by Sandia Laboratories can be used to predict the performance of a photovoltaic module/system under varying (direct normal) irradiance, air mass, ambient temperature or wind speed. As the model empirically-determined coefficients are derived from averaged data sets (filtering out most transient effects), it is not always possible to link these coefficients to specific module design parameters. Moreover, since the detailed internal module wiring configuration is typically neglected, these empirical models cannot be used to accurately predict the performance of modules/systems when modules are partially shadowed. To overcome this limitation, specific models have been developed for predicting the non-linear effect of partial shading on PV systems [2,3]. This paper presents a generalized multi-physics performance model relying on the use of physical equations and elementary electrical circuit network models. This model can be used for predicting, comparing or analyzing the performance of concentrated photovoltaic modules or systems. The model is particularly useful for predicting the impact of a design change in the module/system materials, wiring configuration, solar cell type, or of the concentrator optics. Following a presentation of the model architecture, a first example presents how this performance model can be effectively used to optimize the module internal wiring configuration in order to minimize the impact of receiver current mismatch an reduce string losses at the system level. The model can also be used to determine the impact of shorts/opens defects on module performance. This performance model can also be used to determine the optimum method for binning and placing an array of individual receivers onto the backplane of micro-cell based concentrator photovoltaic modules [4,5]. A second example illustrates how the model can be easily extended through the use of high level analytical equations in order to perform multi-physics simulations. The impact of thermal expansion on the performance of a CPV module is studied using semi-empirical optical throughput equations of the CPV module optics coupled to thermal equations. Finally, a last example highlights the intrinsic capability of the model to accurately predict the non-linear effect of partial shading. Experimental data are presented to support these analyses.

Semprius micro transfer-prints small high efficiency solarcells to produce very low weight, flexible, and weatherized solar panels. The micro-cell design is based on a conventional lattice matched GaInP/GaAs dual junction cell epitaxially grown over a sacrificial release layer suitable for post-release transfer-printing. Solar panels with 12.8% efficiency and 280 W/Kg specific power have been fabricated. We anticipate higher performance panels with specific power over 500 W/Kg can be achieved by printing higher efficiency three junction solar cells, optimizing the cell size and process improvements to reduce lamination thickness.

Semprius has developed a novel microcell based, highly scalable HCPV module that addresses performance, cost and reliability requirements for utility scale solar installations. Semprius has fabricated dual junction cell based engineering prototype modules with 1000X concentration based on this technology. A 1 kW HCPV system using these modules was installed in Tucson to validate the technology and acquire on‐sun data. Eight months of on‐sun results from this system are presented.

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 ( 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.

Semprius is applying a novel massively parallel, automated production process to address CPV’s reliability, performance, cost, and scalability requirements. The new design approach utilizing patented micro‐transfer printing technology enables the use of many very small cells (0.36 mm2) with benefits including high efficiency, simple distributed heat transfer, high concentration ratio, and small thin concentrating optical elements. We briefly describe the design approach and provide detailed supporting on‐sun measurements.