Raphael Thys
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A new experimental study conducted by researchers at the University of Minnesota produced a fully 3D printed OLED display using a multi-technology and multimaterial 3D printing method. The proof-of-concept demonstration produced a highly flexible and fully functional 8 × 8 OLED display with all pixels turning on successfully.
This ability to fully 3D print active electronic and optoelectronic devices could enable unique device form factors via strategies untethered from conventional microfabrication facilities. Organic light-emitting diode (OLED) displays are competitive alternatives to liquid crystal displays (LCDs) due to their characteristics of self-emission, high contrast ratio, full viewing angle, power efficiency, and mechanical flexibility. Typically, in commercial OLED displays, the active layers (or emitting layers) are thermally evaporated to achieve high uniformity and resolution. Printing methods are being actively investigated because of the potential for scaling up to large panel displays and the reduction of material waste. Extrusion-based 3D printing in particular, both thermal and pneumatic, has emerged as a method to assemble a wide palette of materials with varying viscosities, with the possibility of transcending the planar limitations of conventional microfabrication and catalyzing the emergence of truly 3D active electronic devices. 3D printed electronics have features such as spatially structured architectures, direct side-by-side assembly of hybrid devices, the seamless interweaving of diverse materials, and printability on moving, free-form, and deformable surfaces.
Schematic of the fully 3D-printed OLED display and printing methodology.(A) Exploded view of the OLED display demonstrating its layer-by-layer structure. Layers 1 through 6 are 3D-printed components. The OLED display was printed on PET films that were mounted with electrical connection pins and encapsulated with PDMS. (B) Schematic demonstrating the methods for printing and reconfiguring each component of the OLED display. (C) Energy band diagram of the OLED showing the transport and recombination of the charge carriers under a constant or pulsed external voltage. From left to right, the four materials are AgNPs, PEDOT:PSS, MDMO-PPV, and EGaIn. The inset image displays the molecular structure of MDMO-PPV. hv, absorbed photons. (D) Electroluminescence (EL) spectrum of the 3D-printed OLED. a.u., arbitrary units.
Recent progress in 3D printing electronic materials has moved beyond passive conductors toward active electronic materials, including semiconducting quantum dots and conjugated polymers for optoelectronic devices such as LEDs and photodetectors. The ability to formulate optoelectronic devices entirely on 3D printing systems enables an unconventional design space for displays and image sensors. However, further, development is required in layer-stacking mechanisms and printing methodologies for interconnected optoelectronic arrays with individually addressable pixels.
A Fully 3D printed OLED display in which all functional components are fabricated by printing methods could lead to futuristic concepts, such as higher-dimensional form factors, displays interwoven with soft robotics for electroluminescent body parts and three-dimensionally structured pixel matrices for holography.
Yet, the researchers warned, methodologies to fully print OLED displays require overcoming several challenges to transfer the materials and processes to printing platforms. Innovations in the material systems, device configurations, printing processes, and design modalities are required to comprehensively print next-generation displays in a manner that is completely untethered from conventional microfabrication manufacturing facilities.
One obstacle lies in the nonuniformity of extrusion-printed active layers, owing to the directional mass transport within the printed droplets that is driven by the capillary flow during solvent evaporation. A second challenge is the creation of repeatable and stable polymer-metal junctions between the active layer and the cathode using the 3D printing approach at room temperature. Last, the printed cathode structures should present a uniform array of conductors so that electrical interfaces can be established between the individual pixels and spatially structured interconnects.
To solve the printability issue for the electrodes and encapsulation layer, the researchers extrusion-printed functional inks of a wide range of viscosities, in the form of solutions, liquids, pastes, and resins. Specifically, inks based on metallic nanoparticles were extrusion-printed as ring-shaped bottom interconnects to define the pixel positions and sizes, followed by coating the active areas with a conductive polymer to form a composite anode structure. The top cathode array was extrusion-printed with a eutectic liquid metal stabilized by an oxide shell that formed at room temperature via contact with air. Last, a composite paste material formed the top conductive interconnects, creating an individually addressable OLED matrix via intimate extrusion printing of traces along with the cathode array.
In addition to extrusion printing, spray painting and mechanical reconfiguration were used to optimize the fabrication of active layers and polymer-metal junctions, respectively. To improve the uniformity of printed active layers, a spray nozzle was integrated on the 3D printer to atomize the inks into droplets on the scale of tens of micrometers. The reduced droplet size translated into a suppressed directional mass transport within the active layer, leading to a more uniform distribution of the electroluminescent polymer. A controllable thickness of the active layer was realized by tuning the ink concentration and spray time for each pixel.
In summary, the 3D printed OLED display was realized by seamlessly integrating materials with disparate rheological and electrical properties entirely on a “one-pot” 3D printing platform that united multiple processing modalities including extrusion, spray, and mechanical reconfiguration. This novel device structure and 3D printing approach enabled a proof-of-concept demonstration of a highly flexible and fully functional 8 × 8 OLED display with all pixels turning on successfully.