Takayuki Chiba

Solution-processed multilayer small-molecule light-emitting devices with high-efficiency white-light emission

Recent developments in the field of π-conjugated polymers have led to considerable improvements in the performance of solution-processed organic light-emitting devices (OLEDs). However, further improving efficiency is still required to compete with other traditional light sources. Here we demonstrate efficient solution-processed multilayer OLEDs using small molecules. On the basis of estimates from a solvent resistance test of small host molecules, we demonstrate that covalent dimerization or trimerization instead of polymerization can afford conventional small host molecules sufficient resistance to alcohols used for processing upper layers. This allows us to construct multilayer OLEDs through subsequent solution-processing steps, achieving record-high power efficiencies of 36, 52 and 34 lm W(-1) at 100 cd m(-2) for solution-processed blue, green and white OLEDs, respectively, with stable electroluminescence spectra under varying current density. We also show that the composition at the resulting interface of solution-processed layers is a critical factor in determining device performance.

Solution‐Processed White Phosphorescent Tandem Organic Light‐Emitting Devices

Solution-processed phosphorescent tandem organic light-emitting devices (OLEDs) exhibit extremely high efficiencies (94 cd A(-1) ) and 26% external quantum efficiency (EQE) at 5000 cd m(-2) for green phosphorescent devices and 69 cd A(-1) and 28% EQE at 5000 cd m(-2) for white phosphorescent devices. Development of these highly efficient solution-processed tandem-OLEDs with inverted device structure paves the way to printable, low-cost, and large-area white lighting.

Fabrication of organic light-emitting devices comprising stacked light-emitting units by solution-based processes.

Multi-organic light-emitting devices comprising two light-emitting units stacked in series through a charge-generation layer are fabricated by solution processes. A zinc oxide nanoparticles/polyethylene-imine bilayer is used as the electron-injection layer and phosphomolybdic acid is used as the charge-generation layer. Appropriate choice of solvents during spin-coating of each layer ensures the nine-layered structure fabricated by solution processes.

Instant Low‐Temperature Cross‐Linking of Poly(N‐vinylcarbazole) for Solution‐Processed Multilayer Blue Phosphorescent Organic Light‐Emitting Devices

Poly(N-vinylcarbazole) undergoes cross-linking to highly solvent-resistant films through an oxidative coupling reaction, for which an annealing process takes only 3 min at 110 °C. This reaction allows the construction of a solution-processed multilayer OLED without a time-consuming annealing process. The maximum external quantum efficiency reaches 18%, and remains at 17%, even at a high brightness of 10 000 cd m(-2) for all-solution-processed blue OLEDs.

Solution-processed inorganic-organic hybrid electron injection layer for polymer light-emitting devices.

A lithium quinolate complex (Liq) has high solubility in polar solvents such as alcohols and can be spin-coated onto emitting polymers, resulting in a smooth surface morphology. A polymer light-emitting device fabricated with spin-coated Liq as an electron injection layer (EIL) exhibited a lower turn-on voltage and a higher efficiency than a device with spin-coated Cs₂CO₃ and a device with thermally evaporated Ca. The mixture of ZnO nanoparticles and Liq served as an efficient EIL, resulting in a lower driving voltage even in thick films (∼10 nm), and it did not require a high-temperature annealing process.

High-Efficiency Perovskite Quantum-Dot Light-Emitting Devices by Effective Washing Process and Interfacial Energy Level Alignment

All inorganic perovskites quantum dots (PeQDs) have attracted much attention for used in thin film display applications and solid-state lighting applications, owing to their narrow band emission with high photoluminescence quantum yields (PLQYs), color tunability, and solution processability. Here, we fabricated low-driving-voltage and high-efficiency CsPbBr3 PeQDs light-emitting devices (PeQD-LEDs) using a PeQDs washing process with an ester solvent containing butyl acetate (AcOBu) to remove excess ligands from the PeQDs. The CsPbBr3 PeQDs film washed with AcOBu exhibited a PLQY of 42%, and a narrow PL emission with a full width at half-maximum of 19 nm. We also demonstrated energy level alignment of the PeQD-LED in order to achieve effective hole injection into PeQDs from the adjacent hole injection layer. The PeQD-LED with AcOBu-washed PeQDs exhibited a maximum power efficiency of 31.7 lm W-1 and EQE of 8.73%. Control of the interfacial PeQDs through ligand removal and energy level alignment in the device structure are promising methods for obtaining high PLQYs in film state and high device efficiency.

Solution-processed organic light-emitting devices with two polymer light-emitting units connected in series by a charge-generation layer

We present a solution-based process to fabricate stacked OLEDs consisting of two polymer light-emitting units (LEUs), connected in series by a charge generation layer (CGL). We used Cs2CO3-doped ZnO nanoparticles as an EIL on the LE-polymer to improve the electron injection from the cathode. The surface morphology of a spin-coated metal oxide nanoparticle appears to be rough, with many gaps due to agglutination of nanoparticles. We chose poly(4-vinyl pyridine) (PVPy) as a binder to improve the film morphology of the ZnO:Cs2CO3 mixture and facilitate the formation of a uniform and dense film to prevent the solvent from soaking into the 1st LEU. The efficient solution-based processing of EILs in the 1st CGL containing MoO3/poly-TPD bilayers was employed for the construction of an MPE device. The device exhibited a sum current efficiency of 10 cd A−1, with 4 cd A−1 contributed by the 1st unit and 6 cd A−1 by the 2nd unit.

Solution-processable electron injection materials for organic light-emitting devices

Solution-processed organic light-emitting devices (OLEDs) have progressed as potential candidates for cost-effective solid-state lighting and flat panel displays. In this highlight, we focus on the recent progress of the state-of-the-art solution-processable electron injection materials: (i) alkali metal-containing compounds, (ii) n-type semiconducting metal oxides, (iii) π-conjugated ionic polymers, and (iv) nonionic polymers. These materials are soluble in water, alcohol, or a water–alcohol mixture solvent and can be formed into a film by a solution process. We discuss the essential characteristics of these electron injection materials and the performance of the solution-processed OLEDs made using them.

Efficient Electron Injection by Size- and Shape-Controlled Zinc Oxide Nanoparticles in Organic Light-Emitting Devices.

Three different sized zinc oxide (ZnO) nanoparticles were synthesized as spherical ZnO (S-ZnO), rodlike ZnO (R-ZnO), and intermediate shape and size ZnO (I-ZnO) by controlling the reaction time. The average sizes of the ZnO nanoparticles were 4.2 nm × 3.4 nm for S-ZnO, 9.8 nm × 4.5 nm for I-ZnO, and 20.6 nm × 6.2 nm for R-ZnO. Organic light-emitting devices (OLEDs) with these ZnO nanoparticles as the electron injection layer (EIL) were fabricated. The device with I-ZnO showed lower driving voltage and higher power efficiency than those with S-ZnO and R-ZnO. The superiority of I-ZnO makes it very effective as an EIL for various types of OLEDs regardless of the deposition order or method of fabricating the organic layer, the ZnO layer, and the electrode.

Influence of Azimuth Anchoring on Bistable Properties of Bistable Nematic Liquid Crystal Cells

A theoretical expression for a range of the d/P0 ratio in which two distinct states exist in the equilibrium condition has been derived as a function of Asa0d/K, where Asa0 is the azimuthal anchoring energy, K the elastic constant and d the cell thickness. An expression for the lower limit of azimuthal anchoring energy exhibiting two distinct states has been derived. Theoretical calculations have been performed on the bi-stable energy curve which has two minimal energy levels corresponding to bi-stable states. It has been shown that the energy barrier between the two states mainly originates from the azimuthal anchoring energy and the height of the energy barrier is affected by the d/P0 ratio and cell thickness.