Metal halide perovskites (MHPs) are increasingly recognized as promising materials for future display technologies due to their exceptionally high color purity. In this talk, we will explore the unique benefits and approaches in utilizing MHPs for display technologies, focusing on innovative nanostructures and material strategy in precisely engineered colloidal perovskite nanocrystals (PNCs) for high luminous efficiency in perovskite light-emitting diodes (PeLEDs). This includes the addition of zero-dipole cations and bromide-incorporated molecules for effective mitigation of defect sites in PNCs. In terms of scalable manufacturing, we have developed a modified bar-coating technique that yields PeLEDs with emission efficiency comparable to those of PeLEDs with small emission area. Additionally, we'll discuss an advanced core/shell PNC synthesis method, leading to realization of simultaneously bright, efficient, and stable PeLEDs. Further, we developed a novel hybrid tandem PeLEDs with an ideal optical structure that emits light much efficiently with narrow bandwidth. These developments highlight the potential of MHPs as leading materials for self-emissive displays.
We report that highly-efficient large-area PeLEDs with high uniformity can be realized by the use of colloidal perovskite nanocrystals (PNCs), which decouples the crystallization of perovskites from the film-formation process. PNCs are pre-crystallized and surrounded by organic ligands, and thus are not affected by the film formation process, so simple modified-barcoating which facilitates the evaporation of residual solvent provides uniform large-area films. PeLEDs that incorporated the uniform barcoated PNC films achieved external quantum efficiency (EQE) of 23.26% and EQE of 22.5% in a large pixel area of 102 mm2 with high reproducibility.
Metal halide perovskites (MHPs) have emerged as promising candidates for next-generation display because of their advantages on luminescent properties and external quantum efficiency exceeding 20 % in less than 5 years since the first efficient EL operation. However, the short operational lifetime of PeLEDs is limiting their practical application. Especially, the ion migration in perovskite under intense electric field is known to destroy the crystal structure and cause device failure.
Here, we suggest new strategies to overcome the lifetime limitation of PeLEDs. First, we introduced the proton-transfer-induced 3D/2D hybrid structure with extremely suppressed ion migration and low defect density, showing extremely suppressed luminance overshoot and >20 times longer operational lifetime. Also, we could further prolong the lifetime of PeLEDs by using ideal mixed-cation system and suppressing the electric-field-induced catastrophic failure by inducing self-assembled core/shell structure.
Multilayered small-molecule organic light-emitting diodes (OLEDs) have been commonly used for balanced charge transport and exciton confinement. However, standard method that uses high-vacuum deposition for multilayered OLEDs entails high material and fabrication cost, and it has been a critical impediment to low-cost production. Solution-processing of small-molecule OLEDs has been considered as a promising method, but limited luminous efficiency of solution-processed OLEDs have also been a hurdle for practical use. Therefore, high-efficiency in solution-processed small-molecule OLEDs with a simple device structure should be developed. Here, we report highly efficient solution-processed simplified OLEDs using novel electron-transporting host materials based on tetraphenylsilane with pyridine moieties. These host materials have high triplet energy levels (> 2.8 eV), wide band gaps (> 4.0 eV), and high glass transition temperature. We additionally used multifunctional polymeric hole injection layer and mixed-host emitting layer to achieve simple device architecture without hole transporting or electron blocking layer. Our novel electron transporting host materials which have higher electron transporting ability and triplet energy levels than that of conventional electron transporting host material (2,2′,2"-(1,3,5-Benzinetriyl)-tris(1-phenyl-1-H-benzimidazole)) provided more balanced charge transport and efficient energy transfer preventing backward energy transfer from phosphorescent dopants to host. Orange-red (~97.5 cd/A), green (~101.5 cd/A), and white (~74.2 cd/A) solution-processed phosphorescent OLEDs with simple device structure showed the highest recorded electroluminescent efficiencies of solution-processed OLEDs without additional light outcoupling structure reported to date. We also demonstrated a solution-processed flexible solid-state-lighting device as a potential application.
Resistive random-access memory (RRAM) is a candidate next generation nonvolatile memory due to its high access speed, high density and ease of fabrication. Especially, cross-point-access allows cross-bar arrays that lead to high-density cells in a two-dimensional planar structure. Use of such designs could be compatible with the aggressive scaling down of memory devices, but existing methods such as optical or e-beam lithographic approaches are too complicated. One-dimensional inorganic nanowires (i-NWs) are regarded as ideal components of nanoelectronics to circumvent the limitations of conventional lithographic approaches. However, post-growth alignment of these i-NWs precisely on a large area with individual control is still a difficult challenge.
Here, we report a simple, inexpensive, and rapid method to fabricate two-dimensional arrays of perpendicularly-aligned, individually-conductive Cu-NWs with a nanometer-scale CuxO layer sandwiched at each cross point, by using an inorganic-nanowire-digital-alignment technique (INDAT) and a one-step reduction process. In this approach, the oxide layer is self-formed and patterned, so conventional deposition and lithography are not necessary. INDAT eliminates the difficulties of alignment and scalable fabrication that are encountered when using currently-available techniques that use inorganic nanowires. This simple process facilitates fabrication of cross-point nonvolatile memristor arrays. Fabricated arrays had reproducible resistive switching behavior, high on/off current ratio (Ion/Ioff) ~10 6 and extensive cycling endurance. This is the first report of memristors with the resistive switching oxide layer self-formed, self-patterned and self-positioned; we envision that the new features of the technique will provide great opportunities for future nano-electronic circuits.
Flexible organic light-emitting diodes (OLEDs) are candidates for next-generation solid-state lighting because they have merits such as low driving voltage, various color tuning, designable form, and large-area light emission. Although OLEDs’ efficiency, luminance, and lifetime have been improved enough to be commercialized, they are still inflexible despite being based on organic materials. To achieve efficient and reliable flexible OLEDs for solid-state lighting, flexible substrates for OLEDs should be developed. For this purpose, progress must be made in developing good flexible substrates, electrode materials, and encapsulation techniques compatible with these flexible substrates. Here, we review and discuss progress made in these three technologies for solid-state lighting using flexible OLEDs. Addressing the technical challenges associated with the development of high performing flexible substrates, electrode materials compatible with these substrates and good encapsulation techniques would lead to efficient and reliable flexible OLEDs and make flexible solid-state lighting commercially feasible.
In order to realize the high-performance solution-processed transistors (OTFTs) on plastic substrate, it is essential to have a solution-processible organic gate insulator which can give high field-effect mobility and on/off ratio in the devices and should endure sever photolithography process. Our crosslinked gate insulator film has a good chemical resistance to electrode etchants. However, the etchant exposure of the gate insulators resulted in an increase of the off-current while keeping the on-current the same. We also demonstrate solution processed n-type OTFTs with high mobility based on the soluble derivatives of fullerene (C60) as n-type channel materials. We obtained high electron mobilities of 0.02-0.1 cm2/V.s depending on the workfunction of the source and drain metals, demonstrating that the electron injection is contact-limited. Furthermore, we fabricated n-type OTFTs by all solution deposition process including source and drain metals as well as gate insulators and organic semiconductors. These types of OTFTs can be well suited for a wide range of existing and future flexible circuits and display applications which require a simplified process and low-weight and low-cost products.
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