Doping of Tetraalkylammonium Salts in Polyethylenimine Ethoxylated for Efficient Electron Injection Layers in Solution-Processed Organic Light-Emitting Devices

For efficient electron injection, a method to control the work functions (WFs) of ZnO electrodes in organic light-emitting devices (OLEDs) is reported in this study. First, ZnO was modified by doping of tetraalkylammonium salts (TRAX) into polyethylenimine ethoxylated (PEIE) for the WF control. Tetrabutylammonium salts (TBAX), where X = chloride, bromide, iodide, acetate, thiocyanate, and tetrafluoroborate anions, were doped into PEIE. A WF of nondoped PEIE-modified ZnO was 3.65 eV, whereas TBAX-doped PEIE-modified ZnO exhibited WFs ranging from 3.52 to 3.00 eV depending on the anion. TBAX salts exhibited different electron-donating capabilities depending on the anion, and the doping of TBAX with a large electron-donating capability exhibited a large WF reduction effect. In addition, tetraethyl- and tetrahexylammonium chlorides were doped into PEIE. PEIE doped with TRACl containing long alkyl chains exhibited a large WF reduction effect due to its low electron-accepting capabilities. In addition, the WF reduction mechanism was considered by the depth direction analysis of the PEIE:TBAX films. Finally, the ZnO/PEIE:TRAX bilayers were applied as electron injection layers in poly­[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] emissive-layer-based OLEDs with an inverted structure. The ZnO/PEIE:TBAX devices with low WFs exhibited low driving voltages

A Solution-Processed Heteropoly Acid Containing MoO₃ Units as a Hole-Injection Material for Highly Stable Organic Light-Emitting Devices

We report hole-injection layers (HILs) comprising a heteropoly acid containing MoO₃ units, phosphomolybdic acid (PMA), in organic light-emitting devices (OLEDs). PMA possesses outstanding properties, such as high solubility in organic solvents, very low surface roughness in the film state, high transparency in the visible region, and an appropriate work function (WF), that make it suitable for HILs. We also found that these properties were dependent on the postbaking atmosphere and temperature after film formation. When the PMA film was baked in N₂, the Mo in the PMA was reduced to Mo­(V), whereas baking in air had no influence on the Mo valence state. Consequently, different baking atmospheres yielded different WF values. OLEDs with PMA HILs were fabricated and evaluated. OLEDs with PMA baked under appropriate conditions exhibited comparably low driving voltages and higher driving stability compared with OLEDs employing conventional hole-injection materials (HIMs), poly­(3,4-ethylene­dioxy­thiophene):poly­(4-styrene­sulfonate), and evaporated MoO₃, which clearly shows the high suitability of PMA HILs for OLEDs. PMA is also a commercially available and very cheap material, leading to the widespread use of PMA as a standard HIM

Two-Dimensional Ca₂Nb₃O₁₀ Perovskite Nanosheets for Electron Injection Layers in Organic Light-Emitting Devices

We report in this article the application of calcium niobate (CNO) perovskite nanosheets for electron injection layers (EILs) in organic light-emitting devices (OLEDs). Four kinds of tetraalkylammonium hydroxides having different alkyl lengths were utilized as the exfoliation agents of a layered compound precursor HCa₂Nb₃O₁₀ to synthesize CNO nanosheets, including tetramethylammonium hydroxide, tetraethylammonium hydroxide, tetrapropylammonium hydroxide (TPAOH), and tetrabutylammonium hydroxide. CNO nanosheet EILs were applied in fluorescent poly­[(9,9-di-<i>n</i>-octylfluorenyl-2,7-diyl)-<i>alt</i>-(benzo­[2,1,3]­thiadiazol-4,8-diyl)] (F8BT) organic light-emitting polymer-based devices. The effects of dispersion concentrations and alkyl chain length on the devices’ performances were investigated. The results demonstrated that OLEDs’ performances were related to the coverage ratio of the CNO nanosheets, their thicknesses, and their work function values. Among the four exfoliation agents, the device with CNO nanosheets exfoliated by TPAOH showed the lowest driving voltage. The OLEDs with the CNO nanosheet EILs showed lower driving voltages compared with the devices with conventional EIL material lithium 8-quinolate

Energy Transfer from Blue-Emitting CsPbBr₃ Perovskite Nanocrystals to Green-Emitting CsPbBr₃ Perovskite Nanocrystals

Cesium lead halide (CsPbBr3) perovskite nanocrystals (NCs) have great potential for optoelectronic applications. However, their photoluminescence quantum yield (PLQY) in the film state is lower than that in the solution state due to concentration quenching; thus, PLQY is an issue in light-emitting device applications. Here, we demonstrate that energy donor–acceptor systems of CsPbBr3 NCs, composed of the same halogen composition but with different sizes, realize efficient suppression of concentration quenching and high optical stability. The energy band gap of CsPbBr3 NCs depends on their size due to the quantum confinement effect and behaves as quantum dots (QDs). The PLQY of a green-emitting CsPbBr3 QD (GPeQD) film mixed with smaller blue-emitting CsPbBr3 QDs (BPeQDs) was approximately 2 times higher than that of the neat film (20%). PL, photoluminescence excitation (PLE), and transient PL (TRPL) spectroscopies revealed that this improvement of PLQY in the film state has roots in FRET from BPeQD to GPeQD, resulting in efficient suppression of concentration quenching. The emission color in this energy donor–acceptor system was stable in the solution and film states, although this could not be achieved by cesium lead halide NCs with different halogens due to the halogen exchange reaction and resulting emission color change

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

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

Precision Synthesis and Atomistic Analysis of Deep-Blue Cubic Quantum Dots Made via Self-Organization

As a crystal approaches a few nanometers in size, atoms become nonequivalent, bonds vibrate, and quantum effects emerge. To study quantum dots (QDs) with structural control common in molecular science, we need atomic precision synthesis and analysis. We describe here the synthesis of lead bromide perovskite magic-sized nanoclusters via self-organization of a lead malate chelate complex and PbBr3– under ambient conditions. Millisecond and angstrom resolution electron microscopic analysis revealed the structure and the dynamic behavior of individual QDsstructurally uniform cubes made of 64 lead atoms, where eight malate molecules are located on the eight corners of the cubes, and oleylammonium cations lipophilize and stabilize the edges and faces. Lacking translational symmetry, the cube is to be viewed as a molecule rather than a nanocrystal. The QD exhibits quantitative photoluminescence and stable electroluminescence at ≈460 nm with a narrow half-maximum linewidth below 15 nm, reflecting minimum structural defects. This controlled synthesis and precise analysis demonstrate the potential of cinematic chemistry for the characterization of nanomaterials beyond the conventional limit

Precision Synthesis and Atomistic Analysis of Deep-Blue Cubic Quantum Dots Made via Self-Organization

As a crystal approaches a few nanometers in size, atoms become nonequivalent, bonds vibrate, and quantum effects emerge. To study quantum dots (QDs) with structural control common in molecular science, we need atomic precision synthesis and analysis. We describe here the synthesis of lead bromide perovskite magic-sized nanoclusters via self-organization of a lead malate chelate complex and PbBr3– under ambient conditions. Millisecond and angstrom resolution electron microscopic analysis revealed the structure and the dynamic behavior of individual QDsstructurally uniform cubes made of 64 lead atoms, where eight malate molecules are located on the eight corners of the cubes, and oleylammonium cations lipophilize and stabilize the edges and faces. Lacking translational symmetry, the cube is to be viewed as a molecule rather than a nanocrystal. The QD exhibits quantitative photoluminescence and stable electroluminescence at ≈460 nm with a narrow half-maximum linewidth below 15 nm, reflecting minimum structural defects. This controlled synthesis and precise analysis demonstrate the potential of cinematic chemistry for the characterization of nanomaterials beyond the conventional limit

Precision Synthesis and Atomistic Analysis of Deep-Blue Cubic Quantum Dots Made via Self-Organization

As a crystal approaches a few nanometers in size, atoms become nonequivalent, bonds vibrate, and quantum effects emerge. To study quantum dots (QDs) with structural control common in molecular science, we need atomic precision synthesis and analysis. We describe here the synthesis of lead bromide perovskite magic-sized nanoclusters via self-organization of a lead malate chelate complex and PbBr3– under ambient conditions. Millisecond and angstrom resolution electron microscopic analysis revealed the structure and the dynamic behavior of individual QDsstructurally uniform cubes made of 64 lead atoms, where eight malate molecules are located on the eight corners of the cubes, and oleylammonium cations lipophilize and stabilize the edges and faces. Lacking translational symmetry, the cube is to be viewed as a molecule rather than a nanocrystal. The QD exhibits quantitative photoluminescence and stable electroluminescence at ≈460 nm with a narrow half-maximum linewidth below 15 nm, reflecting minimum structural defects. This controlled synthesis and precise analysis demonstrate the potential of cinematic chemistry for the characterization of nanomaterials beyond the conventional limit

Precision Synthesis and Atomistic Analysis of Deep-Blue Cubic Quantum Dots Made via Self-Organization

As a crystal approaches a few nanometers in size, atoms become nonequivalent, bonds vibrate, and quantum effects emerge. To study quantum dots (QDs) with structural control common in molecular science, we need atomic precision synthesis and analysis. We describe here the synthesis of lead bromide perovskite magic-sized nanoclusters via self-organization of a lead malate chelate complex and PbBr3– under ambient conditions. Millisecond and angstrom resolution electron microscopic analysis revealed the structure and the dynamic behavior of individual QDsstructurally uniform cubes made of 64 lead atoms, where eight malate molecules are located on the eight corners of the cubes, and oleylammonium cations lipophilize and stabilize the edges and faces. Lacking translational symmetry, the cube is to be viewed as a molecule rather than a nanocrystal. The QD exhibits quantitative photoluminescence and stable electroluminescence at ≈460 nm with a narrow half-maximum linewidth below 15 nm, reflecting minimum structural defects. This controlled synthesis and precise analysis demonstrate the potential of cinematic chemistry for the characterization of nanomaterials beyond the conventional limit