Can three-dimensional printing enable the mass customization of electronic devices? A study that exploits this method to create light-emitting diodes based on 'quantum dots' provides a step towards this goal.
The ability to rapidly print three-dimensional (3D) electronic devices would enable myriad applications, including displays, solid-state lighting, wearable electronics and biomedical devices with embedded circuitry. Writing in Nano Letters, Kong et al.1 report an intriguing route to this goal by creating fully 3D-printed light-emitting diodes (LEDs) based on quantum dots. Quantum dots are semiconducting nanocrystals that exhibit tunable colour emission2,3,4. Using a 3D-printing method based on extruding multiple materials, the researchers patterned quantum-dot-based LEDs (QD-LEDs) on curved surfaces and integrated arrays of the diodes in 3D matrices.
3D printers transform the output files from computer-aided design tools into tangible objects using pattern-generating devices that move along multiple directions in space4. These devices can be light sources that harden resins or fuse powders, or nozzles that directly deposit materials. Since their introduction nearly three decades ago, 3D-printing methods have been used to build myriad objects, primarily prototypes, in a sequential, layer-by-layer fashion.
To create 3D objects of arbitrary form and specific function, a broad palette of materials and multi-material printing platforms are required. One promising approach is 3D extrusion printing5, in which functional inks are deposited through fine cylindrical nozzles under an applied pressure at ambient conditions. Unlike 3D printers that use inkjet print heads, which are suitable only for inks with a narrow range of viscosities (about ten times that of pure water), extrusion-based printing enables materials of widely varying composition and viscosity to be patterned6.
QD-LEDs are multilayer devices built around an active (light-emitting) layer composed of quantum dots4. Multiple QD-LED layer architectures have been explored with the aim of optimizing their external quantum efficiency, that is, the ratio of the number of photons emitted by the QD-LED relative to the number of electrons injected into the device when an electric field is applied between the device's outer metallic electrodes (cathode and anode layers). In the typical embodiment of a QD-LED, the active layer is sandwiched between layers of electron- and hole-transporting materials, where holes are positively charged carriers. The applied electric field causes electrons and holes to move into the active layer, where they recombine to emit photons.
Solution-based processing routes have recently emerged for patterning QD-LEDs with the aim of lowering fabrication costs7,8. Central to this approach is the ability to maintain highly uniform layers between dissimilar materials. To create their QD-LED devices, Kong and colleagues sequentially printed several materials (Fig. 1). First, a conductive silver ring that surrounds a transparent anode layer followed by a hole-transport layer were printed, and annealed at 200 °C (silver ring) and 150 °C (other layers). Next, the active layer was formed by printing quantum dots suspended in a solvent mixture in a drop-wise manner.As solvent evaporation ensued, recirculating fluid flow suppressed quantum-dot migration to the drop edge, yielding relatively uniform, active layers9. Notably, each layer was patterned using immiscible solvents to minimize interlayer mixing. Finally, a cathode layer composed of liquid metal was printed10 and the devices were packaged in a silicone sealant.
To highlight the flexibility of their approach, the authors printed QD-LEDs in multiple formats, including green and orange-red light emitters, 2 × 2 × 2 arrays embedded in a silicone matrix, and QD-LEDs on the surface of contact lenses and other substrates of interest. The printed devices exhibit brightness, an essential metric of device performance, that is 10- to 100-fold below that of the best solution-processed QD-LEDs3,8. However, substantial improvements in device performance are likely to be possible by introducing an electron-transport layer (which was absent in the current architecture), such as one composed of zinc-oxide nanoparticles, and further optimizing the printing process.
The 3D-printing method used by the authors represents a simple, but sophisticated, approach for patterning functional materials. Demonstrated applications of this technique include printing electrodes that interconnect solar-cell and LED arrays11, 3D antennas12 and rechargeable microbatteries13. Although microbatteries rely on multi-material 3D printing of interdigitated cathode and anode layers, Kong and colleagues' study is much more impressive, because up to six, as opposed to two, different materials must be printed sequentially to create their devices.
One intriguing question that arises is whether fully 3D-printing electronic devices is the best approach for creating mass-customized electronics. Another viable strategy would be to combine 3D printing with automated pick-and-place machinery that places electronic components accurately and repeatably to generate objects with embedded circuitry and devices11. LEDs are commercially available that have lateral dimensions akin to those demonstrated by Kong et al., and could be integrated into 3D-printed objects by this hybrid approach.
To vastly expand the capabilities of 3D printing, new functional inks and multi-nozzle print heads and printing platforms must be designed for rapidly and accurately patterning materials over a broad range of compositions and ink-flow behaviour. As these advances are realized, it may be possible to print customized 3D electronic devices in a highly scalable manner. We are becoming increasingly reliant on electronics in our daily lives, and so successful outcomes should be of great benefit to society.Footnote 1
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J.A.L. is the founder and key shareholder of Voxel8 Inc., a start-up company that spun out of her lab at Harvard University in 2014. This company is commercializing new materials and 3D-printing platforms for desktop electronics prototyping. B.Y.A. has no competing financial interests.
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