Satoru Ohisa

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.

Highly efficient, deep-red organic light-emitting devices using energy transfer from exciplexes

We developed a highly efficient, deep-red organic light-emitting device (OLED) with an external quantum efficiency of nearly 18% with a very low turn-on voltage of 2.41 V and an electroluminescence emission wavelength (λEL) of 670 nm using energy transfer from an exciplex host to a deep-red phosphorescent emitter, bis(2,3-diphenylquinoxaline)iridium(dipivaloylmethane)[(DPQ)2Ir(dpm)].

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.

Molecular Interdiffusion between Stacked Layers by Solution and Thermal Annealing Processes in Organic Light Emitting Devices

In organic light emitting devices (OLEDs), interfacial structures between multilayers have large impacts on the characteristics of OLEDs. Herein, we succeeded in revealing the interdiffusion in solution processed and thermal annealed OLEDs by neutron reflectometry. We investigated interfaces between a polymer under layer and small molecules upper layer. The small molecules diffused into the swollen polymer layer during the interfacial formation by the solution process, but the polymer did not diffuse into the small molecules layer. At temperatures close to the glass transition temperatures of the materials, asymmetric molecular diffusion was observed. We elucidated the effects of the interdiffusion on the characteristics of OLEDs. Partially mixing the interface improved the current efficiencies due to suppressed triplet-polaron quenching at the interface. Controlling and understanding the interfacial structures of the miultilayers will be more important to improve the OLED characteristics.

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.

Poly(pyridinium iodide ionic liquid)-based electron injection layers for solution-processed organic light-emitting devices

The development of electron injection materials is one of the most important steps for obtaining highly efficient low-driving-voltage organic light-emitting devices (OLEDs). In this paper, we report poly(ionic liquid) (PIL)-based electron injection layers (EILs) for solution-processed OLEDs. We synthesized poly(N-alkyl-4-vinyl-pyridinium iodide) containing three different alkyl chains (propyl, butyl, and hexyl chains) and investigated the charge-transfer interactions between the pyridinium cations and the iodide anions along with the electronic structures and thermal properties of the PILs. We also fabricated and evaluated OLEDs containing PILs as EILs. The fabricated OLEDs exhibited lower driving voltages than the conventional device containing an alkali metal complex (8-quinolinolato lithium) as the EIL. The longer alkyl chain of the PILs was more effective in reducing the driving voltage.

Addition of Lithium 8-Quinolate into Polyethylenimine Electron-Injection Layer in OLEDs: Not Only Reducing Driving Voltage but Also Improving Device Lifetime

Solution-processed electron injection layers (EILs) comprising lithium 8-quinolate (Liq) and polyethylenimine ethoxylated (PEIE) are highly effective for enhancing electron injection from ZnO to organic layers and improving device lifetime in organic light-emitting devices (OLEDs). Doping of Liq into PEIE further reduces the work function of zinc oxide (ZnO) by enhancing dipole formation. The intermolecular interaction between Liq and PEIE was elucidated by UV-vis absorption measurement and quantum chemical calculation. The OLEDs with ZnO covered with PEIE:Liq mixture exhibited lower driving voltage than that of the device without Liq. Furthermore, as doping concentration of Liq into PEIE increased, the device lifetime and voltage stability during constant current operation was successively improved.

Purification of Perovskite Quantum Dots Using Low-Dielectric-Constant Washing Solvent “Diglyme” for Highly Efficient Light-Emitting Devices

Cesium lead halide (CsPbX3, X = Cl, Br, or I) perovskite quantum dots (QDs) are known as ionic nanocrystals, and their optical properties are greatly affected by the washing solvent used during the purification process. Here, we demonstrate the purification process of CsPbBr3 perovskite QDs using low-dielectric-constant solvents to completely remove impurities, such as the reaction solvent and desorbed ligands. The use of the ether solvent diethylene glycol dimethyl ether (diglyme), having a low dielectric constant of ε = 7.23, as a poor solvent for reprecipitation allowed for multiple wash cycles, which led to high purity and high photoluminescence quantum yield for CsPbBr3 QDs. The light-emitting device constructed with the CsPbBr3 QDs and washed twice with diglyme (two-wash) showed a low turn-on voltage of 2.7 V and a peak external quantum efficiency of over 8%. Thus, the purification of perovskite QDs with multiple wash cycles using a low-dielectric-constant solvent is an effective approach for enhancing not only the optical properties but also the efficiency of perovskite quantum dot light-emitting devices.

Air‐Stable and High‐Performance Solution‐Processed Organic Light‐Emitting Devices Based on Hydrophobic Polymeric Ionic Liquid Carrier‐Injection Layers

A lot of research, mostly using electron-injection layers (EILs) composed of alkali-metal compounds has been reported with a view to increase the efficiency of solution-processed organic light-emitting devices (OLEDs). However, these materials have intractable properties, such as a strong affinity for moisture, which cause the degradation of OLEDs. Consequently, optimal EIL materials should exhibit high electron-injection efficiency as well as be stable in air. In this study, polymer light-emitting devices (PLEDs) based on the commonly used yellow-fluorescence-emitting polymer F8BT, which utilize poly(diallyldimethylammonium)-based polymeric ionic liquids, are experimentally and analytically investigated. As a result, the optimized PLED employing an EIL comprising poly(diallyldimethylammonium) bis(trifluoromethanesulfonyl)imide (poly(DDA)TFSI), which is expected to display good moisture resistance because of water repellency of fluorocarbon groups, exhibits excellent storage stability in air and electroluminescence performance with a low turn-on voltage of 2.01 V, maximum external quantum efficiency of 9.00%, current efficiency of 30.1 cd A-1 , and power efficiency of 32.4 lm W-1 . The devices with poly(DDA)TFSI show one of the highest efficiencies as compared to the reported standard PLEDs. Moreover, poly(DDA)TFSI is applied as a hole-injection layer (HIL). The optimized PLED using poly(DDA)TFSI as the HIL exhibits performances comparable to those of a device that uses a conventional poly(3,4-ethylenedioxy-thiophene):poly(4-styrenesulfonate) HIL.

Colorful Squaraines Dyes for Efficient Solution-Processed All Small-Molecule Semitransparent Organic Solar Cells

Semitransparent organic solar cells (ST-OSCs) exhibit highly promising applications to develop integrated photovoltaics and power-generating windows. However, the development of ST-OSCs is significantly lagging behind opaque OSCs, especially for all small-molecule ST-OSCs. Here, four unique squaraines dyes (IDPSQ, SQ-BP, D-BDT-SQ, and AzUSQ) were successfully used as donors in ST-OSCs, whose colors can be tuned from purple to blue, green, and dark green, respectively. While using ultrathin Ag as a transparent electrode, the ST-OSCs fabricated using IDPSQ:PC71BM, SQ-BP:PC71BM, D-BDT-SQ:PC71BM, and AzUSQ:PC71BM yield power conversion efficiencies (PCEs) of 2.96, 4.36, 4.91, and 1.71%, respectively, and their colors are purple, cyan, brown, and light brown, respectively. Compared to their opaque OSCs (PCEs of 3.95, 5.45, 5.84, and 1.91%, respectively), the reduction in the PCEs are as low as 25, 20, 16, and 10%, respectively. Furthermore, each of these ST-OSCs exhibit good average visible transmittance (AVT) of 25-30%, favorable CIE color coordinates, and a color rendering index (CRI) of 71-97%. Finally, by changing the thickness of the Ag electrode, an impressive PCE of 4.9% along with an AVT of 25% and a CRI of 97% can be obtained in D-BDT-SQ:PC71BM-based ST-OSCs, which is the highest PCE among all small-molecule ST-OSCs.