Yong-Jin Pu

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.

Tuning Energy Levels of Electron-Transport Materials by Nitrogen Orientation for Electrophosphorescent Devices with an ‘Ideal’ Operating Voltage

In the last two decades, many studies have focused on improving external electroluminescence effi ciency ( η ext ) of organic lightemitting devices (OLEDs). [ 1 ] Lowering operating voltages is also crucially important to improve their power conversion effi ciency especially for high power-effi ciency applications such as in mobile devices. [ 2 ] OLEDs with p-doped hole-transport layers (HTLs) and n-doped electron-transport layers (ETLs), called p-i-n OLEDs, have been developed as low-voltage effi cient lighting sources. [ 3 ] However, a hole/exciton-block buffer layer is crucially important to prevent exciton quenching in the emissive layer (EML) by the alkaline metal dopants in the ETL, and insertion of the hole/exciton-block buffer layer and usage of n -doped ETL may induce more complexity of device structure and thus higher cost. In addition, the approach of using alkaline metals might be hampered by stability issues and leads to manufacturing diffi culties. We report here on a series of 1,3,5-triazine-core-containing electron-transport materials (ETMs) as an undoped ETL, whose energy levels can be tuned by introducing pyridine rings on the periphery of the molecule and also orientation of nitrogen. An unprecedented low operating voltage of 2.42 V, corresponding to the emitting photon energy ( hv ), was achieved for the fac -tris(2-phenylpyridine) iridium (Ir(PPy) 3 ) based green phosphorescent OLEDs at 100 cd m − 2 without consumption of their effi ciency. Moreover, the threshold voltage for electroluminescence can be even 0.1 ∼ 0.2 V lower than the minimum value of hv /e. As an ETM that can act as an ETL as well as a hole/excitonblock layer, it should posses a low-lying lowest unoccupied molecular orbital (LUMO) energy level to give a low electron injection barrier, a low-lying highest occupied molecular orbital (HOMO) energy level to block hole leakage from EML, and a

High‐Performance Blue Phosphorescent OLEDs Using Energy Transfer from Exciplex

An efficient energy transfer from an exciplex between a sulfone and an arylamine derivatives to a blue phosphorescent emitter enables OLED performances among the best, of over 50 lm W(-1) at 100 cd m(-2) . The formation of the exciplex realizes a barrier-free hole-electron recombination pathway, thereby leading to high OLED performances with an extremely low driving voltage of 2.9 V at 100 cd m(-2) .

High-Performance Green OLEDs Using Thermally Activated Delayed Fluorescence with a Power Efficiency of over 100 lm W(-1).

A green organic light-emitting device (OLED) with an extremely high power efficiency of over 100 lm W(-1) is realized through energy transfer from an exciplex. An optimized OLED showed a maximum external efficiency of 25.7%, and a power efficiency of 79.4 lm W(-1) at 1000 cd m(-2) , which is 1.6-times higher than that of state-of-the-art green thermally activated delayed fluorescence (TADF) OLEDs.

Electronegative oligothiophenes based on difluorodioxocyclopentene-annelated thiophenes: synthesis, properties, and n-type FET performances.

A series of oligothiophenes containing difluorodioxocyclopentene-annelated thiophene units was synthesized, and their electronic properties and structures were investigated by spectroscopic and electrochemical measurements and X-ray analyses. The oligothiophenes having the terminal difluorodioxocyclopentene annelations showed n-type semiconducting behavior on FET devices, and the quaterthiophene revealed field-effect electron mobility as high as 1.3 x 10(-2) cm2 V(-1) s(-1).

Dual efficiency enhancement by delayed fluorescence and dipole orientation in high-efficiency fluorescent organic light-emitting diodes

To explain the origin of extremely high efficiencies of deep-blue fluorescent organic light-emitting diodes(OLEDs) with anisotropic-shapedanthracene derivatives, the enhancements of singlet-exciton generation efficiency and outcoupling efficiency were investigated by transient electroluminescence measurement and variable angle spectroscopic ellipsometry, respectively. Both the delayed fluorescence from singlet excitons generated via triplet-triplet annihilation and the outcoupling enhancement by dipole orientation of emitters were found to contribute to the high external quantum efficiencies of the devices. This dual efficiency enhancement is important for understanding and further improving high-performance fluorescent OLEDs.

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.

Solution-processable organic fluorescent dyes for multicolor emission in organic light emitting diodes

Four novel fluorescent dyes, bis(difluorenyl)amino-substituted carbazole 1, pyrene 2, perylene 3, and benzothiadiazole 4, were synthesized by C–N cross-coupling with a palladium catalyst. These dyes are soluble in common organic solvents, and their uniform films were formed by spin-coating from their solutions. Their glass transition temperatures were sufficiently high (120–181 °C) to form amorphous films for organic light emitting diodes. These solution processable dyes exhibited strong photoluminescence (PL) in the film form (1: sky blue, 2: blue-green, 3: yellow, and 4: deep red). Optical and electrochemical properties of the compounds were investigated with photoelectron spectroscopy and cyclic voltammetry. The energy levels obtained from both measurements were in good agreement, and those levels were related to the electronic properties of the central core; the electron-donating carbazole compound showed the lowest ionization potential and the electron-withdrawing benzothiadiazole compound showed the largest electron affinity. Simple double layer devices were prepared with these fluorescent dyes as emitting layer and bis(2-methyl-8-quinolinolato)(4-phenylphenolato)aluminium(III) (BAlq) as a common hole blocking layer for each color. Electroluminescence colors were the same as those of the PL spectra in each compound. These multicolor electroluminescences show that these conjugated oligomers can be candidates for solution processable light emitting materials for OLEDs as well as conjugated polymers or dendrimers.

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.

Single Benzene Green Fluorophore: Solid‐State Emissive, Water‐Soluble, and Solvent‐ and pH‐Independent Fluorescence with Large Stokes Shifts

Benzene is the simplest aromatic hydrocarbon with a six-membered ring. It is one of the most basic structural units for the construction of π conjugated systems, which are widely used as fluorescent dyes and other luminescent materials for imaging applications and displays because of their enhanced spectroscopic signal. Presented herein is 2,5-bis(methylsulfonyl)-1,4-diaminobenzene as a novel architecture for green fluorophores, established based on an effective push-pull system supported by intramolecular hydrogen bonding. This compound demonstrates high fluorescence emission and photostability and is solid-state emissive, water-soluble, and solvent- and pH-independent with quantum yields of Φ=0.67 and Stokes shift of 140 nm (in water). This architecture is a significant departure from conventional extended π-conjugated systems based on a flat and rigid molecular design and provides a minimum requirement for green fluorophores comprising a single benzene ring.