Wei-Kai Lee

Sky‐Blue Organic Light Emitting Diode with 37% External Quantum Efficiency Using Thermally Activated Delayed Fluorescence from Spiroacridine‐Triazine Hybrid

Extremely efficient sky-blue organic electroluminescence with external quantum efficiency of ≈37% is achieved in a conventional planar device structure, using a highly efficient thermally activated delayed fluorescence emitter based on the spiroacridine-triazine hybrid and simultaneously possessing nearly unitary (100%) photoluminescence quantum yield, excellent thermal stability, and strongly horizontally oriented emitting dipoles (with a horizontal dipole ratio of 83%).

Bis-Tridentate Ir(III) Complexes with Nearly Unitary RGB Phosphorescence and Organic Light-Emitting Diodes with External Quantum Efficiency Exceeding 31%.

A new class of neutral bis-tridentate Ir(III) metal complexes that show nearly unitary red, green, and blue emissions in solution is prepared and employed for the fabrication of both monochrome and white-emitting organic light-emitting diodes, among which a green device gives external quantum efficiency exceeding 31%.

Enhancing Optical Out-Coupling of Organic Light-Emitting Devices with Nanostructured Composite Electrodes Consisting of Indium Tin Oxide Nanomesh and Conducting Polymer.

A nanostructured composite electrode consisting of a high-index indium-tin-oxide nanomesh and low-index high-conductivity conducting polymer effectively enhances coupling of internal radiation of organic light-emitting devices into their substrates. When combining this internal extraction structure and the external extraction scheme, a very high external quantum efficiency of nearly 62% is achieved with a green phosphorescent device.

Achieving Nearly 30% External Quantum Efficiency for Orange–Red Organic Light Emitting Diodes by Employing Thermally Activated Delayed Fluorescence Emitters Composed of 1,8‐Naphthalimide‐Acridine Hybrids

The combination of rigid acridine donor and 1,8-naphthalimide acceptor has afforded two orange-red emitters of NAI-DMAC and NAI-DPAC with high rigidity in molecular structure and strongly pretwisted charge transfer state. Endowed with high photoluminescence quantum yields (ΦPL ), distinct thermally activated delayed fluorescence (TADF) characteristics, and preferentially horizontal emitting dipole orientations, these emitters afford record-high orange-red TADF organic light-emitting diodes (OLEDs) with external quantum efficiencies of up to 21-29.2%, significantly surpassing all previously reported orange-to-red TADF OLEDs. Notably, the influence of microcavity effect is verified to support the record-high efficiency. This finding relaxes the usually stringent material requirements for effective TADF emitters by comprising smaller radiative transition rates and less than ideal ΦPL s.

Enhancing light out-coupling of organic light-emitting devices using indium tin oxide-free low-index transparent electrodes

With its increasing and sufficient conductivity, the conducting polymer poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) has been capable of replacing the widely used but less cost-effective indium tin oxides (ITOs) as alternative transparent electrodes for organic light-emitting devices (OLEDs). Intriguingly, PEDOT:PSS also possesses an optical refractive index significantly lower than those of ITO and typical organic layers in OLEDs and well matching those of typical OLED substrates. Optical simulation reveals that by replacing ITO with such a low-index transparent electrode, the guided modes trapped within the organic/ITO layers in conventional OLEDs can be substantially suppressed, leading to more light coupled into the substrate than the conventional ITO device. By applying light out-coupling structures onto outer surfaces of substrates to effectively extract radiation into substrates, OLEDs using such low-index transparent electrodes achieve enhanced optical out-coupling and external quantum efficiencies in comparison with conventional OLEDs using ITO.

Unlocking the Full Potential of Conducting Polymers for High‐Efficiency Organic Light‐Emitting Devices

By carefully tuning the thicknesses of low-optical index PEDOT:PSS and high-index ITO layers in organic light-emitting devices (OLEDs), very high optical coupling efficiencies can be obtained through the generation of appropriate microcavity effects. These experiments result in an external quantum efficiency (EQE) of 33.7% for green phosphorescent OLEDs and even higher EQEs of 54.3% can be obtained by adopting an external out-coupling lens.

Room-temperature-processed flexible n-InGaZnO/p-Cu2O heterojunction diodes and high-frequency diode rectifiers

In this work, we report successful implementation of room-temperature-processed flexible n-InGaZnO/p-Cu2O heterojunction diodes on polyethylene naphthalate (PEN) plastic substrates using the sputtering technique. Using n-type InGaZnO and p-type Cu2O films deposited by sputtering at room temperature, flexible n-InGaZnO/p-Cu2O heterojunction diodes were successfully fabricated on PEN plastic substrates. The didoes on PEN substrates exhibited a low apparent turn-on voltage of 0.44?V, a high rectification ratio of up to 3.4???104 at ?1.2?V, a high forward current of 1?A?cm?2 around 1?V and a decent ideality factor of 1.4, similar to the characteristics of n-InGaZnO/p-Cu2O diodes fabricated on glass substrates. The characterization of the frequency response of the room-temperature-processed flexible n-InGaZnO/p-Cu2O heterojunction diode rectifiers indicated that they are capable of high-frequency operation up to 27?MHz, sufficient for high-frequency (13.56?MHz) applications. Preliminary bending tests on diode characteristics and rectifier frequency responses indicate their promise for applications in flexible electronics.

Extraordinary transmittance in three-dimensional crater, pyramid, and hole-array structures prepared through reversal imprinting of metal films

We used a reversal imprinting-in-metal (RIM) process to fabricate various three-dimensional (3D) metal structures under low pressure. Molds featuring different shapes were used to pattern various subwavelength metal structures, including pyramidal, hole-array, and crater-like structures. Refractive index matching and cavity effects both enhanced the degree of transmission of these structured metal films. The crater-like structure appears to be a promising material because of the unique properties imparted by the elongated and gradually tapering spacing of its cavities. From both near-field simulations and experimentally obtained optical spectra, we found that the cavity effect in the crater-like structure led to significantly enhanced transmission of the optical intensity. Thus, this RIM process allows the ready fabrication of various two- and three-dimensional metallic structures for use in surface plasmon-based devices.

Efficient thermally activated delayed fluorescence of functional phenylpyridinato boron complexes and high performance organic light-emitting diodes

A new series of functional phenylpyridinato boron complexes possessing thermally activated delayed fluorescence (TADF) has been strategically designed and synthesized. These boron complexes utilize phenylpyridine as the electron acceptor (A) that links to carbazole or triphenyl amine as the electron donor (D) via a core boron atom, forming four-coordinate neutral boron complexes. The selection of boron to spatially separate the donor and acceptor takes advantage of facile functionalization. The TADF properties of the resulting D–A functional materials in various solvents have been investigated via their emission spectra and associated relaxation dynamics. The results show that the operation of TADF is strongly solvent polarity dependent in fluid states, and several underlying mechanisms are discussed. OLEDs fabricated by fppyBTPA and dfppyBTPA show EL efficiencies of up to (20.2%, 63.9 cd A−1, 66.9 lm W−1) and (26.6%, 88.2 cd A−1, 81.5 lm W−1), respectively, in which a nearly 27% EQE for the dfppyBTPA device is among the most efficient TADF OLEDs so far.

Analyses of optical out-coupling of organic light-emitting devices having micromesh indium tin oxide and conducting polymer as composite transparent electrode.

UNLABELLED We report the characterization and analyses of organic light-emitting devices (OLEDs) using microstructured composite transparent electrodes consisting of the high-index ITO (indium tin oxide) micromesh and the low-index conducting polymer PEDOT PSS [poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate)], that are fabricated by the facile and convenient microsphere lithography and are useful for enhancing light extraction. The rigorous electromagnetic simulation based on the three-dimensional finite-difference time-domain (FDTD) method was conducted to study optical properties and mechanisms in such devices. It provides a different but consistent viewpoint/insight of how this microstructured electrode enhances optical out-coupling of OLEDs, compared to that provided by ray optics simulation in previous works. Both experimental and simulation studies indicate such a microstructured electrode effectively enhances coupling of internal radiation into the substrate, compared to devices with the typical planar ITO electrode. By combining this internal extraction structure and the external extraction scheme (e.g. by attaching extraction lens) to further extract radiation into the substrate, a rather high external quantum efficiency of 46.8% was achieved with green phosphorescent OLEDs, clearly manifesting its high potential.