16 research outputs found

    Engineering Colloidal Perovskite Nanocrystals and Devices for Efficient and Large-Area Light-Emitting Diodes

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    ConspectusColloidal metal halide perovskite nanocrystals (PNCs) have high color purity, solution processability, high luminescence efficiency, and facile color tunability in visible wavelengths and therefore show promise as light emitters in next-generation displays. The external quantum efficiency (EQE) of PNC light-emitting diodes (LEDs) has been rapidly increased to reach 24.96% by using colloidal PNCs and 28.9% using on-substrate in situ synthesized PNCs. However, high operating stability and a further increase of EQE in PNC-LEDs have been impeded for three reasons: (1) Colloidal PNCs consist of ionic crystal structures in which ligands bind dynamically and therefore easily agglomerate in colloidal solution and films; (2) Long-alkyl-chain organic ligands that adhere to the PNC surface improve the photoluminescence quantum efficiency and colloidal stability of PNCs in solution but impede charge transport in PNC films and limit their electroluminescence efficiency in LEDs; (3) Unoptimized device structure and nonuniform PNC films limit the charge balance and reduce the device efficiency in PNC-LEDs.In this Account, we summarize strategies to solve the limitations in PNCs and PNC-LEDs as consequences of photoluminescence quantum efficiency in PNCs and the charge-balance factor and out-coupling factor in LEDs, which together determine the EQE of PNC-LEDs. We introduce the fundamental photophysical properties of colloidal PNCs related to effective mass of charge carriers and surface stoichiometry, requirements for PNC surface stabilization, and subsequent research strategies to demonstrate highly efficient colloidal PNCs and PNC-LEDs with high operating stability.First, we present various ligand-engineering strategies that have been used to achieve both efficient carrier injection and radiative recombination in PNC films. In situ ligand engineering reduces ligand length and concentration during synthesis of colloidal PNCs, and it can achieve size-independent high color purity and high luminescent efficiency in PNCs. Postsynthesis ligand engineering such as optimized purification, replacement of organic ligands with inorganic ligands or strongly bound ligands can increase charge transport and coupling between PNC dots in films. The luminescence efficiency of PNCs and PNC-LEDs can be further increased by various postsynthesis ligand-engineering methods or by sequential treatment with different ligands. Second, we present methods to modify the crystal structure in PNCs to have alloy- or core/shell-like structure. Such crystal engineering is performed by the correlation between entropy and enthalpy in PNCs and result in increased carrier confinement (increased radiative recombination) and reduced defects (decreased nonradiative recombination). Third, we present strategies to boost the charge-balance factor and out-coupling factor in PNC-LEDs such as modification of thickness of each layer and insertion of additional interlayers, and out-coupling hemispherical lens are discussed. Finally, we present the advantages, potential, and remaining challenges to be solved to enable use of colloidal PNCs in commercialized industrial displays and solid-state lighting. We hope this Account will help its readers to grasp the progresses and perspectives of colloidal PNCs and PNC-LEDs, and that our insights will guide future research to achieve efficient PNC-LEDs that have high stability and low toxicity

    Synergetic Influences of Mixed-Host Emitting Layer Structures and Hole Injection Layers on Efficiency and Lifetime of Simplified Phosphorescent Organic Light-Emitting Diodes

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    We used various nondestructive analyses to investigate various host material systems in the emitting layer (EML) of simple-structured, green phosphorescent organic light-emitting diodes (OLEDs) to clarify how the host systems affect its luminous efficiency (LE) and operational stability. An OLED that has a unipolar single-host EML with conventional poly­(3,4-ethylenedioxythiophene):poly­(styrenesulfonate) (PEDOT:PSS) showed high operating voltage, low LE (∼26.6 cd/A, 13.7 lm/W), and short lifetime (∼4.4 h @ 1000 cd/m<sup>2</sup>). However, the combined use of a gradient mixed-host EML and a molecularly controlled HIL that has increased surface work function (WF) remarkably decreased operating voltage and improved LE (∼68.7 cd/A, 77.0 lm/W) and lifetime (∼70.7 h @ 1000 cd/m<sup>2</sup>). Accumulated charges at the injecting interfaces and formation of a narrow recombination zone close to the interfaces are the major factors that accelerate degradation of charge injection/transport and electroluminescent properties of OLEDs, so achievement of simple-structured OLEDs with high efficiency and long lifetime requires facilitating charge injection and balanced transport into the EML and distributing charge carriers and excitons in EML

    Room-Temperature-Processable Wire-Templated Nanoelectrodes for Flexible and Transparent All-Wire Electronics

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    Sophisticated preparation of arbitrarily long conducting nanowire electrodes on a large area is a significant requirement for development of transparent nanoelectronics. We report a position-customizable and room-temperature-processable metallic nanowire (NW) electrode array using aligned NW templates and a demonstration of transparent all-NW-based electronic applications by simple direct-printing. Well-controlled electroless-plating chemistry on a polymer NW template provided a highly conducting Au NW array with a very low resistivity of 7.5 μΩ cm (only 3.4 times higher than that of bulk Au), high optical transmittance (>90%), and mechanical bending stability. This method enables fabrication of all-NW-based electronic devices on various nonplanar surfaces and flexible plastic substrates. Our approach facilitates realization of advanced future electronics

    Electroluminescence from Graphene Quantum Dots Prepared by Amidative Cutting of Tattered Graphite

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    Size-controlled graphene quantum dots (GQDs) are prepared via amidative cutting of tattered graphite. The power of this method is that the size of the GQDs could be varied from 2 to over 10 nm by simply regulating the amine concentration. The energy gaps in such GQDs are narrowed down with increasing their size, showing colorful photoluminescence from blue to brown. We also reveal the roles of defect sites in photoluminescence, developing long-wavelength emission and reducing exciton lifetime. To assess the viability of the present method, organic light-emitting diodes employing our GQDs as a dopant are first demonstrated with the thorough studies in their energy levels. This is to our best knowledge the first meaningful report on the electroluminescence of GQDs, successfully rendering white light with the external quantum efficiency of ca. 0.1%

    Color Purifying Optical Nanothin Film for Three Primary Colors in Optoelectronics

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    Numerous optical films have been developed to implement optoelectronics with advanced performance. In this study, we propose a color purifying optical nanothin film that improves the performance of optoelectronics by filtering the white light to have a spectrum composed of pure three primary colors of red, green, and blue. It was experimentally confirmed that a wider color gamut that covers 176.33% of the sRGB could be expressed when the suggested optical nanothin film was applied to a display system consisting of a white back light unit and color filters. Furthermore, a full width half-maximum value of 20 nm on average was observed in the three primary colors. When this film was applied to light recording optoelectronics, such as cameras, it acts as a multiband-pass filter that increases the sensitivity of the three primary colors. The principle of this optical nanothin film is based on multiple light resonance inside the film. A theoretical analysis and simulations were conducted to design the structure of the nanothin film and optical characteristics were verified by both experiment and simulation. Because it is fabricated by in situ thermal evaporation it provides advantages in terms of fabrication time and cost, and it also has potential to be fabricated with well-established deposition equipment such as a sputter apparatus. The results of this paper show that nanoscaled thin films sufficiently control the optical phenomenon with a simple structure, implementing advanced optoelectronics

    Ultra-High-Resolution Organic Light-Emitting Diodes with Color Conversion Electrode

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    The implementation of ultra-high-resolution displays is one of the important technologies for advanced displays. In this paper, an ultra-high-resolution organic light-emitting diode display is implemented without the fine metal mask method, but via a color conversion electrode. A red and green color ultra-high-resolution organic light-emitting diode display with a pixel size of 5 μm was experimentally realized without changing any aspects of the structure of the OLED display except for the precisely fine-patterned color conversion electrode. Furthermore, nanometer-scale pixel size can be expected through this method. The color conversion electrode is a multilayer structured nanometer thick conductive optical film, and its applicability was confirmed based on a theoretical analysis and optical simulation. The ultra-high-resolution display with color conversion electrode could be a basic technology for the development of advanced high-resolution displays

    Highly Conductive Transparent and Flexible Electrodes Including Double-Stacked Thin Metal Films for Transparent Flexible Electronics

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    To keep pace with the era of transparent and deformable electronics, electrode functions should be improved. In this paper, an innovative structure is suggested to overcome the trade-off between optical and electrical properties that commonly arises with transparent electrodes. The structure of double-stacked metal films showed high conductivity (<3 Ω/sq) and high transparency (∼90%) simultaneously. A proper space between two metal films led to high transmittance by an optical phenomenon. The principle of parallel connection allowed the electrode to have high conductivity. In situ fabrication was possible because the only materials composing the electrode were silver and WO<sub>3</sub>, which can be deposited by thermal evaporation. The electrode was flexible enough to withstand 10 000 bending cycles with a 1 mm bending radius. Furthermore, a few μm scale patterning of the electrode was easily implemented by using photolithography, which is widely employed industrially for patterning. Flexible organic light-emitting diodes and a transparent flexible thin-film transistor were successfully fabricated with the proposed electrode. Various practical applications of this electrode to new transparent flexible electronics are expected

    Laminated Graphene Films for Flexible Transparent Thin Film Encapsulation

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    We introduce a simple, inexpensive, and large-area flexible transparent lamination encapsulation method that uses graphene films with polydimethylsiloxane (PDMS) buffer on polyethylene terephthalate (PET) substrate. The number of stacked graphene layers (<i>n</i><sub>G</sub>) was increased from 2 to 6, and 6-layered graphene-encapsulation showed high impermeability to moisture and air. The graphene-encapsulated polymer light emitting diodes (PLEDs) had stable operating characteristics, and the operational lifetime of encapsulated PLEDs increased as <i>n</i><sub>G</sub> increased. Calcium oxidation test data confirmed the improved impermeability of graphene-encapsulation with increased <i>n</i><sub>G</sub>. As a practical application, we demonstrated large-area flexible organic light emitting diodes (FOLEDs) and transparent FOLEDs that were encapsulated by our polymer/graphene encapsulant

    Graphenes Converted from Polymers

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    Because the direct formation of large, patterned graphene layers on active electronic devices without any physical transfer process is an ultimate important research goal for practical applications, we first developed a cost-effective, scalable, and sustainable process to form graphene films from solution-processed common polymers directly on a SiO<sub>2</sub>/Si substrate. We obtained few-layer graphene by heating the thin polymer films covered with a metal capping layer in a high-temperature furnace under low vacuum in an Ar/H<sub>2</sub> atmosphere. We find that the metal capping layer appears to have two functions: prevention of vaporization of dissociated molecules and catalysis of graphene formation. We suggest that polymer-derived graphene growth directly on inert substrates in active electronic devices will have great advantages because of its simple, inexpensive, and safer process

    Flexible and Transparent Metallic Grid Electrodes Prepared by Evaporative Assembly

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    We propose a novel approach to fabricating flexible transparent metallic grid electrodes via evaporative deposition involving flow-coating. A transparent flexible metal grid electrode was fabricated through four essential steps including: (i) polymer line pattern formation on the thermally evaporated metal layer onto a plastic substrate; (ii) rotation of the stage by 90° and the formation of the second polymer line pattern; (iii) etching of the unprotected metal region; and (iv) removal of the residual polymer from the metal grid pattern. Both the metal grid width and the spacing were systematically controlled by varying the concentration of the polymer solution and the moving distance between intermittent stop times of the polymer blade. The optimized Au grid electrodes exhibited an optical transmittance of 92% at 550 nm and a sheet resistance of 97 Ω/sq. The resulting metallic grid electrodes were successfully applied to various organic electronic devices, such as organic field-effect transistors (OFETs), organic light-emitting diodes (OLEDs), and organic solar cells (OSCs)
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