Millimeter Thin and Rubber‐Like Solid‐State Lighting Modules Fabricated Using Roll‐to‐Roll Fluidic Self‐Assembly and Lamination

Abstract

A millimeter thin rubber-like solid-state lighting module is reported. The fabrication of the lighting module incorporates assembly and electrical connection of light-emitting diodes (LEDs). The assembly is achieved using a roll-to-roll fluidic self-assembly. The LEDs are sandwiched in-between a stretchable top and bottom electrode to relieve the mechanical stress. The top contact is realized using a lamination technique that eliminates wire-bonding. The field of stretchable electronics has seen a rapid increase in research activities in recent years. It enables new kinds of applications such as smart clothing,1 conformable photovoltaics,2 stretchable batteries,3, 4 artificial electronic skins,5, 6 or mechanically soft and conformable health monitoring devices7, 8 to give a few recent examples. The realization typically requires integration of functional semiconductor materials on a low-temperature stretchable polymeric rubber-like support that is in stark contrast to traditional semiconductor manufacturing which requires the use of high-temperature processing and rigid semiconductor wafers. From a technical point of view, a classification into two groups can be made: Class 1 – “polymeric substrate first” uses the low-temperature polymeric substrates right at the outset of the processing sequence. Commonly direct-write techniques are subsequently used to deposit the required functional materials. This approach may provide certain advantages from a cost savings point of view; however, any subsequent processing step is limited in terms of processing temperature. Class 1 methods are often challenged when it comes to the performance of the devices and the complexity of the circuits or systems they can produce. While new materials9, 10 including nanomaterials are sought to be the solution, many of the challenges due to the low-temperature processing requirements remain. For example, the field of printable electronics that applies electronic inks in the form of nanoparticles using direct-write inkjet-like printing methods has not yet achieved the breakthroughs since the high-temperature processing and annealing steps required from a device point of view cannot be carried out.11-13 Class 2 – “polymeric substrate last” refers to methods that delay the use of the low-temperature substrate as long as possible. This class of methods shares the advantage that they can be combined with well-established semiconductor device technologies; the use of nanomaterials is not required. While this may look as a capitulation going back to the old and established method, it has been proven to be a very successful methodology. A pioneer in the field has been Menard et al.14 Today, established semiconductor device technologies and segmentation (etching) methods are frequently used to produce microscopic functional device segments, which are subsequently transferred onto low-temperature substrates. Various forms of stretchable semiconductor device layers and stretchable metallic interconnects have been reported. The first examples were based on semiconductor materials such as GaAs,2, 15 InGaN,16 and Si15, 17, 18 which can be stretched and compressed (demonstrated 50%) if the thin film is transferred onto a polymeric substrate in a way to yield buckled 3D wave-like topologies19 similar to an accordion fold. The required fabrication of very thin device layers is perhaps a slight disadvantage. However, it is also possible to use more rigid device elements. Here, segmented devices are distributed on rubber-like substrate and electrically connected using stretchable meander shaped metallic interconnects. This approach has been particularly successful and various new applications were demonstrated; conformable optoelectronic modules,20-22 stretchable sensor arrays,7 and rubber-like electrocardiographic electrode arrays23 are some examples. In contrast with the first method, this methodology enables, in principle, direct use of commercial devices and circuit e