Supramolecular Sky‐Blue Phosphorescent Polymer Iridium Complexes for Single‐Emissive‐Layer Organic Light‐Emitting Diodes

Novel supramolecular phosphorescent polymers (SPPs) are synthesized as a new class of solution‐processable electroluminescent emitters. The formation of these SPPs takes advantage of the efficient non‐bonding assembly between bis(dibenzo‐24‐crown‐8)‐functionalized iridium complex monomer and bis(dibenzylammonium)‐tethered co‐monomer, which is monitored by ¹H NMR spectroscopy and viscosity measurements. These SPPs show good film morphology and an intrinsic glass transition with a T_g of 94–116 °C. Noticeably, they are highly photoluminescent in solid state with quantum efficiency up to ca. 78%. The photophysical and electroluminescent properties are strongly dependent on the molecular structures of the iridium complex monomers.

     

Enhancement of Photostability in Blue‐Light‐Emitting Polymers Doped with Gold Nanoparticles

A dramatic increase in the photostability of a blue‐light‐emitting polymer, poly(9,9‐dioctylfluorene), was achieved by the addition of 5–10 nm gold nanoparticles. The optical absorption band of the gold nanoparticles was tuned to resonate the triplet exciton ground state bandgap energy of the polymer. Photo‐oxidation rate of poly(9,9‐dioctylfluorene) was effectively reduced by doping the polymer with very small amounts (≈10⁻⁶–10⁻⁵ volume fraction) of the gold nanoparticles.

Retarded photo‐oxidation in PDOF nanocomposite films with various doped gold nanoparticles.     

Organic Light‐Emitting Diodes Using a Neutral π Radical as Emitter: The Emission from a Doublet

Triplet harvesting is a main challenge in organic light‐emitting devices (OLEDs), because the radiative decay of the triplet is spin‐forbidden. Here, we propose a new kind of OLED, in which an organic open‐shell molecule, (4‐N‐carbazolyl‐2,6‐dichlorophenyl)bis(2,4,6‐trichlorophenyl)methyl (TTM‐1Cz) radical, is used as an emitter, to circumvent the transition problem of triplet. For TTM‐1Cz, there is only one unpaired electron in the highest singly occupied molecular orbital (SOMO). When this electron is excited to the lowest singly unoccupied molecular orbital (SUMO), the SOMO is empty. Thus, transition back of the excited electron to the SOMO is totally spin‐allowed. Spectral analysis showed that electroluminescence of the OLED originated from the electron transition between SUMO and SOMO. The magneto‐electroluminescence measurements revealed that the spin configuration of the excited state of TTM‐1Cz is a doublet. Our results pave a new way to obtain 100 % internal quantum efficiency of OLEDs.     

Efficient Solid‐State Light‐Emitting CuCdS Nanocrystals Synthesized in Air

Semiconductor nanocrystals (NCs) possess high photoluminescence (PL) typically in the solution phase. In contrary, PL rapidly quenches in the solid state. Efficient solid state luminescence can be achieved by inducing a large Stokes shift. Here we report on a novel synthesis of compositionally controlled CuCdS NCs in air avoiding the usual complexity of using inert atmosphere. These NCs show long‐range color tunability over the entire visible range with a remarkable Stokes shift up to about 1.25 eV. Overcoating the NCs leads to a high solid‐state PL quantum yield (QY) of ca. 55 % measured by using an integrating sphere. Unique charge carrier recombination mechanisms have been recognized from the NCs, which are correlated to the internal NC structure probed by using extended X‐ray absorption fine structure (EXAFS) spectroscopy. EXAFS measurements show a Cu‐rich surface and Cd‐rich interior with 46 % Cu^I being randomly distributed within 84 % of the NC volume creating additional transition states for PL. Color‐tunable solid‐state luminescence remains stable in air enabling fabrication of light‐emitting diodes (LEDs).     

Controllable Red,Green, Blue (RGB) and Bright White Upconversion Luminescence of Lu2O3:Yb3+/Er3+/Tm3+ Nanocrystals through Single Laser Excitation at 980 nm

Light fantastic! Lu₂O₃:Yb³⁺/Er³⁺/Tm³⁺ nanocrystals with controllable red, green, blue (RGB) and bright white upconversion luminescence by a single laser excitation of 980 nm have been successfully synthesized (see picture). Due to abundant UC PL colors, it can potentially be used as fluorophores in the field of color displays, back light, UC lasers, photonics, and biomedicine.

     

Bipolar Host Materials for Organic Light‐Emitting Diodes

It is important to balance holes and electrons in the emitting layer of organic light‐emitting diodes to maximize recombination efficiency and the accompanying external quantum efficiency. Therefore, the host materials of the emitting layer should transport both holes and electrons for the charge balance. From this perspective, bipolar hosts have been popular as the host materials of thermally activated delayed fluorescent devices and phosphorescent organic light‐emitting diodes. In this review, we have summarized recent developments of bipolar hosts and suggested perspectives of host materials for organic light‐emitting diodes.     

Electrophosphorescent Heterobimetallic Oligometallaynes and Their Applications in Solution‐Processed Organic Light‐Emitting Devices

By combining the iridium(III) ppy‐type complex (Hppy=2‐phenylpyridine) with a square‐planar platinum(II) unit, some novel phosphorescent oligometallaynes bearing dual metal centers (viz. Ir^III and Pt^II) were developed by combining trans‐[Pt(PBu₃)₂Cl₂] with metalloligands of iridium possessing bifunctional pendant acetylene groups. Photophysical and computational studies indicated that the phosphorescent excited states arising from these oligometallaynes can be ascribed to the triplet emissive Ir^III ppy‐type chromophore, owing to the obvious trait (such as the longer phosphorescent lifetime at 77 K) also conferred by the Pt^II center. So, the two different metal centers show a synergistic effect in governing the photophysical behavior of these heterometallic oligometallaynes. The inherent nature of these amorphous materials renders the fabrication of simple solution‐processed doped phosphorescent organic light‐emitting diodes (PHOLEDs) feasible by effectively blocking the close‐packing of the host molecules. Saliently, such a synergistic effect is also important in affording decent device performance for the solution‐processed PHOLEDs. A maximum brightness of 3 356 cd m^?2 (or 2 708 cd m^?2), external quantum efficiency of 0.50 % (or 0.67 %), luminance efficiency of 1.59 cd A^?1 (or 1.55 cd A^?1), and power efficiency of 0.60 Lm W^?1 (or 0.55 Lm W^?1) for the yellow (or orange) phosphorescent PHOLEDs can be obtained. These results show the great potential of these bimetallic emitters for organic light‐emitting diodes.     

Luminescence Properties of Ca2Ga2SiO7:RE Phosphors for UV White‐Light‐Emitting Diodes

A series of Eu²⁺‐, Ce³⁺‐, and Tb³⁺‐doped Ca₂Ga₂SiO₇ phosphors is synthesized by using a high‐temperature solid‐state reaction. The powder X‐ray diffraction and structure refinement data indicate that our prepared phosphors are single phased and the phosphor crystalizes in a tetrahedral system with the ${P\bar 42m}$ (113) space group. The Eu²⁺‐ and Ce³⁺‐doped phosphors both have broad excitation bands, which match well with the UV light‐emitting diodes chips. Under irradiation of λ=350 nm, Ca₂Ga₂SiO₇:Eu²⁺ and Ca₂Ga₂SiO₇:Ce³⁺, Li⁺ have green and blue emissions, respectively. Luminescence of Ca₂Ga₂SiO₇:Tb³⁺, Li⁺ phosphor varies with the different Tb³⁺ contents. The thermal stability and energy‐migration mechanism of Ca₂Ga₂SiO₇:Eu²⁺ are also studied. The investigation results indicate that the prepared Ca₂Ga₂SiO₇:Eu²⁺ and Ca₂Ga₂SiO₇:Ce³⁺, Li⁺ samples show potential as green and blue phosphors, respectively, for UV‐excited white‐light‐emitting diodes.     

Blue‐Light‐Emitting Conjugated Polymers Derived From 2,7‐Carbazoles

Blue‐light‐emitting 2,7‐carbazole‐based conjugated copolymers have been prepared by Yamamoto or Suzuki cross‐coupling reactions. By introducing highly substituted aromatic comonomers, fully soluble high‐molecular‐weight copolymers have been obtained. Moreover, these amorphous polymeric materials exhibit good thermal stability and interesting redox properties. All these features make these new conjugated polymers highly promising for the development of single‐polymer‐layer blue‐light‐emitting diodes.

     

2,5‐Difluorenyl‐Substituted Siloles for the Fabrication of High‐Performance Yellow Organic Light‐Emitting Diodes

2,3,4,5‐Tetraarylsiloles are a class of important luminogenic materials with efficient solid‐state emission and excellent electron‐transport capacity. However, those exhibiting outstanding electroluminescence properties are still rare. In this work, bulky 9,9‐dimethylfluorenyl, 9,9‐diphenylfluorenyl, and 9,9′‐spirobifluorenyl substituents were introduced into the 2,5‐positions of silole rings. The resulting 2,5‐difluorenyl‐substituted siloles are thermally stable and have low‐lying LUMO energy levels. Crystallographic analysis revealed that intramolecular π–π interactions are prone to form between 9,9′‐spirobifluorene units and phenyl rings at the 3,4‐positions of the silole ring. In the solution state, these new siloles show weak blue and green emission bands, arising from the fluorenyl groups and silole rings with a certain extension of π conjugation, respectively. With increasing substituent volume, intramolecular rotation is decreased, and thus the emissions of the present siloles gradually improved and they showed higher fluorescence quantum yields (Φ_F=2.5–5.4 %) than 2,3,4,5‐tetraphenylsiloles. They are highly emissive in solid films, with dominant green to yellow emissions and good solid‐state Φ_F values (75–88 %). Efficient organic light‐emitting diodes were fabricated by adopting them as host emitters and gave high luminance, current efficiency, and power efficiency of up to 44 100 cd m^?2, 18.3 cd A^?1, and 15.7 lm W^?1, respectively. Notably, a maximum external quantum efficiency of 5.5 % was achieved in an optimized device.     

Phosphorescent Cyclometalated Iridium(III) Complexes That Contain Substituted 2‐Acetylbenzo[b]thiophen‐3‐olate Ligand for Red Organic Light‐Emitting Devices

We report the synthesis of a new class of thermally stable and strongly luminescent cyclometalated iridium(III) complexes 1 – 6 , which contain the 2‐acetylbenzo[b]thiophene‐3‐olate (bt) ligand, and their application in organic light‐emitting diodes (OLEDs). These heteroleptic iridium(III) complexes with bt as the ancillary ligand have a decomposition temperature that is 10–20 % higher and lower emission self‐quenching constants than those of their corresponding complexes with acetylacetonate (acac). The luminescent color of these iridium(III) complexes could be fine‐tuned from orange (e.g., 2‐phenyl‐6‐(trifluoromethyl)benzo[d]thiazole (cf₃bta) for 4 ) to pure red (e.g., lpt (Hlpt=4‐methyl‐2‐(thiophen‐2‐yl)quinolone) for 6 ) by varying the cyclometalating ligands (C‐deprotonated C^N). In particular, highly efficient OLEDs based on 6 as dopant (emitter) and 1,3‐bis(carbazol‐9‐yl)benzene (mCP) as host that exhibit stable red emission over a wide range of brightness with CIE chromaticity coordinates of (0.67, 0.33) well matched to the National Television System Committee (NTSC) standard have been fabricated along with an external quantum efficiency (EQE) and current efficiency of 9 % and 10 cd A^?1, respectively. A further 50 % increase in EQE (>13 %) by replacing mCP with bis[4‐(6H‐indolo[2,3‐b]quinoxalin‐6‐yl)phenyl]diphenylsilane (BIQS) as host for 6 in the red OLED is demonstrated. The performance of OLEDs fabricated with 6 (i.e., [(lpt)₂Ir(bt)]) was comparable to that of the analogous iridium(III) complex that bore acac (i.e., [(lpt)₂Ir(acac)]; 6 a in this work) [Adv. Mater.­ 2011 , 23, 2981] fabricated under similar conditions. By using ntt (Hnnt=3‐hydroxynaphtho[2,3‐b]thiophen‐2‐yl)(thiophen‐2‐yl)methanone) ligand, a substituted derivative of bt, the [(cf₃bta)₂Ir(ntt)] was prepared and found to display deep red emission at around 700 nm with a quantum yield of 12 % in mCP thin film.     

Hydrogen‐Bonded Supramolecular Conjugated Polymer Nanoparticles for White Light‐Emitting Devices

Supramolecular polyfluorenol enable assembly into conjugated polymer nanoparticles (CPNs). Poly{9‐[4‐(octyloxy)phenyl]fluoren‐9‐ol‐2,7‐diyl} (PPFOH)‐based supramolecular nanoparticles are prepared via reprecipitation. PPFOH nanoparticles with diameters ranging from 40 to 200 nm are obtained by adding different amounts of water into DMF solution. Size‐dependent luminescence is observed in PPFOH‐based hydrogen‐bonded nanoparticles that is different from that of poly(9,9‐dioctylfluorenes). Finally, white light‐emitting devices using CPNs with a size of 80 nm exhibit white emission with the CIE coordinates (0.31, 0.34). Amphiphilic conjugated polymer nanoparticles are potential organic nano‐inks for the fabrication of organic devices in printed electronics.

     

Triarylboron‐Based Fluorescent Organic Light‐Emitting Diodes with External Quantum Efficiencies Exceeding 20 %

Triarylboron compounds have attracted much attention, and found wide use as functional materials because of their electron‐accepting properties arising from the vacant p orbitals on the boron atoms. In this study, we design and synthesize new donor–acceptor triarylboron emitters that show thermally activated delayed fluorescence. These emitters display sky‐blue to green emission and high photoluminescence quantum yields of 87–100 % in host matrices. Organic light‐emitting diodes using these emitting molecules as dopants exhibit high external quantum efficiencies of 14.0–22.8 %, which originate from efficient up‐conversion from triplet to singlet states and subsequent efficient radiative decay from singlet to ground states.