No abstract

Developing the ability to 3D print various classes of materials possessing distinct properties could enable the freeform generation of active electronics in unique functional, interwoven architectures. Achieving seamless integration of diverse materials with 3D printing is a significant challenge that requires overcoming discrepancies in material properties in addition to ensuring that all the materials are compatible with the 3D printing process. To date, 3D printing has been limited to specific plastics, passive conductors, and a few biological materials. Here, we show that diverse classes of materials can be 3D printed and fully integrated into device components with active properties. Specifically, we demonstrate the seamless interweaving of five different materials, including (1) emissive semiconducting inorganic nanoparticles, (2) an elastomeric matrix, (3) organic polymers as charge transport layers, (4) solid and liquid metal leads, and (5) a UV-adhesive transparent substrate layer. As a proof of concept for demonstrating the integrated functionality of these materials, we 3D printed quantum dot-based light-emitting diodes (QD-LEDs) that exhibit pure and tunable color emission properties. By further incorporating the 3D scanning of surface topologies, we demonstrate the ability to conformally print devices onto curvilinear surfaces, such as contact lenses. Finally, we show that novel architectures that are not easily accessed using standard microfabrication techniques can be constructed, by 3D printing a 2 × 2 × 2 cube of encapsulated LEDs, in which every component of the cube and electronics are 3D printed. Overall, these results suggest that 3D printing is more versatile than has been demonstrated to date and is capable of integrating many distinct classes of materials.

In this work, a thermally and mechanically robust, smooth transparent conductor composed of silver nanowires embedded in a colorless polyimide substrate is introduced. The polyimide is exceptionally chemically, mechanically, and thermally stable. While silver nanowire networks tend not to be thermally stable to high temperatures, the addition of a titania coating on the nano­wires dramatically increases their thermal stability. This allows for the polyimide to be thermally imidized at 360 °C with the silver nanowires in place, creating a smooth (<1 nm root mean square roughness), conductive surface. These transparent conducting substrate‐cum‐electrodes exhibit a conductivity ratio figure of merit of 272, significantly outperforming commercially available indium‐tin‐oxide (ITO)‐coated plastics. The conductive polymide is subjected to various mechanical tests and is used as a substrate for a thermally deposited, flexible, organic light‐emitting diode, which shows improved device performance compared to a control device made on ITO coated glass.

Organic light-emitting diodes (OLEDs) are now entering mainstream display markets and are also being explored for next-generation lighting applications. In both types of applications, high external quantum efficiency (EQE) is of premium importance for both low power consumption and long lifetime. It is well known that one of the bottlenecks in achieving high EQE in OLEDs is the low light-extraction efficiency, which is limited to <20%, mostly because total internal reflections occurring at interfaces between optically distinctive layers confine a significant portion of the light within the substrate (¼ ''substrate-confined'' mode) or within the organic/indium tin oxide (ITO) layers (¼ ''wave-guided'' mode). [1][2][3] Hence, many device structures have been proposed to extract light that would not normally be outcoupled: some have attempted to extract the light that is confined in a substrate by introducing structures such as microlens array (MLA) [4][5][6][7] or pyramidal arrays [8,9] on the backside of the substrate, where other research groups have tried to extract the light that is confined within organic/ITO layers by introducing optical structures such as photonic crystals [10,11] or low-index grids [12] that can disrupt the wave-guiding of the light within the organic/ITO layers. The latter may be carried out in a direct way by converting wave-guided modes directly into outcoupled modes or in an indirect way by converting wave-guided modes into substrate-confined modes and then extracting them with structures mentioned in the former approach.Criteria for choosing a specific method or structure over others depend on the target applications: for display applications, methodologies such as MLA and substrate structuring are often avoided due to their optical blurring effect; for lighting applications, such methods are readily accepted, but complex processes that add too much cost are generally not welcomed. In both cases, compatibility with a common fabrication technique and large-area fabrication is strongly preferred, and the Lambertian angular dependence and the absence of spectral dependence are also preferred in most situations. Here we introduce a novel anode structure based on micropatterned ITOs coated with high-conductivity (HC)-grade poly(3,4-ethylenedioxythiophene): poly(styrene sulfonate) (PEDOT:PSS) layers. This proposed electrode structure can improve the outcoupling efficiency of OLEDs in a relatively simple way without severe spectral dependence, blurring (optional), or deviation from the normal angular dependence. Figure 1 illustrates a tilted top-view and cross-section of the proposed anode structure and its working principle. ITO layers are patterned so that the square opening (W o  W o ) repeats in a square lattice layout with a spatial period of W t . For simplicity, we consider a situation where W t ¼ 2 W o . In this case, the width of ITO strips (W ITO (¼ W t -W o )) next to each opening equals W o , and the ITO-less portion is 25% per each unit cell. The spatial period and the dimension...

Despite high internal quantum efficiencies, planar organic light-emitting diodes (OLEDs) typically suffer from limited outcoupling efficiencies. To improve this outcoupling efficiency we develop a new thin (~2 µm) light scattering layer, which employs air voids (lowindex scattering centers) embedded in a high-index polyimide matrix to effectively frustrate the substrate-trapped mode light, increasing the outcoupling efficiency. The porous polyimide scattering layers are created through the simple and scalable fabrication technique of phase inversion. Optical properties of the scattering layers are characterized via microscopy, transmittance/haze measurements and ellipsometry, which demonstrate excellent scattering properties of these scattering layers. We integrate these films into a green OLED stack where they show a 65% enhancement in external quantum efficiency (EQE) and 77% enhancement in power efficiency. Furthermore we integrate these layers into a white OLED where we observe similar enhancements. Both the green and white OLEDs additionally demonstrate excellent color stability over wide viewing angles with the integration of this thin scattering layer.Since the first observation of electroluminescence in organic solids 1 and the demonstration of a bilayer fluorescent organic light-emitting diode (OLED), 2 significant improvements have been realized in this thin film-based photonic device. The development of materials with improved transport properties, 3,4 chemical and thermal stabilities 5,6 and high luminescent quantum yields 7,8 have brought several important breakthroughs in device performance, while a deeper understanding of device physics and interfacial properties 9-11 have allowed for engineering devices to realize internal quantum efficiencies near the theoretical maximum of 100%. 12,13 While device electrical efficiency is approaching its limit, there is still significant room for improving optical efficiency, often referred to as outcoupling efficiency or light extraction efficiency. Outcoupling efficiency can be calculated with the aid of advanced modeling techniques 14-16 which show that only approximately 20-30% of emitted photons escape a noncavity planar OLED fabricated on a conventional glass substrate. [17][18][19] Hence, in order to fully convert the input electrical power into optical power, it is essential to overcome this low outcoupling efficiency.It should be noted that two factors are closely associated with typically low OLED outcoupling efficiency: the planarity of the device and refractive indices of the thin film stack. A planar organic/metal interface leads to evanescently coupled surface plasmonic losses, 20 while an index gradient starting from high-index organic layers to mid-index glass substrate to low-index ambient air leads to laterally travelling waveguided and substrate-trapped loss modes. 21 While the surface plasmonic loss mode can be reduced by spacing oscillating dipoles (i.e. emitters) away from the organic/metal interface or by introducing corrugation, 22 wav...