High-Radiance Shortwave Infrared Light-Emitting Diodes Based on Highly Stable PbS Colloidal Quantum Dots.

PbS quantum dot light-emitting diodes (QLEDs) emitting around 1550 nm promise important applications in optical communications. However, due to insufficient suppression of surface traps for large-size PbS quantum dots (QDs), their performance under large driving current density was not satisfactory. In this work, octanethiol surfactant was added into a PbS QD solution and adsorbed onto the dot surface. As a result, the surface traps and the continuous oxidation of the unprotected (100) facets in PbS QDs were greatly suppressed. Therefore, the PbS QDs with octanethiol doubled their photoluminescence efficiency and showed outstanding stability. The PbS-based QLEDs with benchmark device structure showed a breakthrough high radiance of 18.3 W sr-1 m-2 with >2000 mA/cm2 driving current density. The efficient passivation of surface traps with octanethiol surfactant and the suppressed coupling between excitons and surface states under large working current were the main reasons for achieving the breakthrough high radiance.

Elaborating the interplay between the detecting unit and emitting unit in infrared quantum dot up-conversion photodetectors.

The quantum dot up-conversion device combines an infrared photodetector (PD) and a visible quantum-dot light-emitting diode (QLED) to directly convert infrared targets to visible images. However, large efficiency loss is usually induced by the integration of the detecting unit and the emitting unit. One of the important reasons is the performances of the PD and QLED units restraining each other. We regulated the equilibrium between infrared absorption and visible emission by changing the thicknesses of infrared active layers in up-conversion devices. A good balance could be achieved between the absorption of 980 nm incident light and the out-coupling of the 634 nm emission when the active layer thickness is 140 nm, leading to the best performance of the up-conversion device. As more photogenerated carriers are produced with the increase of infrared illumination intensity, the external quantum efficiency (EQE) of the QLED unit in the up-conversion device remains little changed. This suggests the limited amount of photogenerated holes in the PD unit does not limit the EQE of the QLED unit. However, a PD unit with a high ratio of photogenerated holes trapped near the interconnection decreased the EQE in the QLED unit. This work provides new insights into the interplay between the PD and QLED units in up-conversion devices, which is crucial for their further improvements.

Quantum dot light-emitting diodes with high efficiency at high brightness via shell engineering.

Quantum dot light-emitting diodes (QD-LEDs) have made great development in the performance. However, the efficiency droop at high brightness limits their applications in daylight displays and outdoor lightings. Herein, we systematically regulate the shell structure and composition, and the results indicate that CdSe-based QDs with ZnSe interlayer and thinner ZnSeS outermost layer as emitting layers (EML) enable high-performance QD-LEDs. Accordingly, the devices exhibit peak external quantum efficiency (EQE) of 22.9% with corresponding brightness of 67,840 cd/m2, and this efficiency can be still maintained > 90% of the maximum value even at 100,000 cd/m2, which satisfies the requirements for high-brightness display and lighting applications. This strong performance is mainly attributed to the ZnSe/ZnSeS graded shell that smooths the injection barrier between QD EML and the adjacent hole transport layers (HTL), and then improves the hole injection and charge injection balance, in particular at the high luminance and/or at high current density.

Improved Efficiency of All-Inorganic Quantum-Dot Light-Emitting Diodes via Interface Engineering.

As the charge transport layer of quantum dot (QD) light-emitting diodes (QLEDs), metal oxides are expected to be more stable compared with organic materials. However, the efficiency of metal oxide-based all-inorganic QLEDs is still far behind that of organic-inorganic hybrid ones. The main reason is the strong interaction between metal oxide and QDs leading to the emission quenching of QDs. Here, we demonstrated nickel oxide (NiOx)-based all-inorganic QLEDs with a maximum current efficiency of 20.4 cd A-1 and external quantum efficiency (EQE) of 5.5%, which is among the most efficient all-inorganic QLEDs. The high efficiency is mainly attributed to the aluminum oxide (Al2O3) deposited at the NiOx/QDs interface to suppress the strong quenching effect of NiOx on the QD emission, together with the molybdenum oxide (MoOx) that reduced the leakage current and facilitated hole injection, more than 300% enhancement was achieved compared with the pristine NiOx-based QLEDs. Our study confirmed the effect of decorating the NiOx/QDs interface on the performance enhancement of the all-inorganic QLEDs.

High-efficiency, deep blue ZnCdS/CdxZn1-xS/ZnS quantum-dot-light-emitting devices with an EQE exceeding 18.

We report a facile and robust synthesis of ZnCdS core/shell quantum dots (QDs) with thick CdxZn1-xS (x = constant) uniform alloys as an intermediate shell which can provide effective confinement of excitons within the ZnCdS cores and ultrathin ZnS outermost shell to improve the stability by epitaxial growth at a relatively high temperature. The resulting nearly monodisperse ZnCdS/CdxZn1-xS/ZnS core/shell QDs have high photoluminescence quantum yield (near to 100%) and high color purity (full width at half maximum (FWHM) < 18 nm). More importantly, the ZnCdS/CdxZn1-xS/ZnS core/shell QDs have good chemical/photochemical stability and more efficient carrier transport performance compared with ZnCdS/ZnS core/shell QDs. Two types of QDs of ZnCdS/ZnS and ZnCdS/CdxZn1-xS/ZnS were incorporated into the solution-processed hybrid QD-based light-emitting device structure as the emissive layer. We find that the presence of the CdxZn1-xS shell makes a profound impact on device performances such as the external quantum efficiency and current efficiency. The corresponding light-emitting diodes exhibited a high EQE exceeding 18%, a peak current efficiency of 3.4 cd A-1 and low efficiency roll-off. Such excellent results of ZnCdS/CdxZn1-xS/ZnS-based QLEDs are likely attributable to the QD's high PL QY and very thin ZnS outermost shell which did not sacrifice the charge injection efficiency in QLEDs.

Combined Effects of Temperature and the Microcystin MC-LR on the Feeding Behavior of the Rotifer Brachionus calyciflorus.

The aim of this study was to investigate the responses in filtration and grazing rates of five rotifer strains of the species Brachionus calyciflorus under different temperatures and MC-LR concentrations. The results showed that strain identity, MC-LR concentration, temperature, and the interactions of these factors significantly affected both response variables, with the exception of the interaction of strain and MC-LR on the grazing rates. At low MC-LR concentrations and for the control group, the filtration and grazing rates increased with increasing temperature. The filtering and grazing rates of B. calyciflorus exposed to higher MC-LR concentrations, however, showed no evident enhancement with increasing of temperature. At high temperatures, the filtration and grazing rates of all rotifer strains decreased significantly with increasing concentration of MC-LR, however B. calyciflorus exhibited a refractory stability in the presence of increased MC-LR levels at lower temperatures.

Crystal structure, photoluminescence and electroluminescence of three bluish green light-emitting iridium complexes.

Three bis-cyclometalated iridium complexes ((TPP)2Ir(acac), (TPP)2Ir(tpip) and (TPP)2Ir(pic)) with 2-(2-trifluoromethyl)pyrimidine-pyridine (TPP) as the main ligand, 2,4-pentanedionate (acac), tetraphenylimidodiphosphinate (tpip) and picolinate (pic) as the ancillary ligands, respectively, were prepared. Their photoluminescence and electrochemistry properties were investigated in detail, and (TPP)2Ir(tpip) was also examined by X-ray crystallography. These complexes show bluish green emission with a quantum efficiency of 11-14%. The organic light emitting diodes (OLEDs) with the structure of ITO/TAPC (1,1-bis[4-(di-p-tolylamino)phenyl]cyclohexane, 40 nm)/mCP (1,3-bis(9H-carbazol-9-yl)benzene, 10 nm)/Ir complex (8 wt%):PPO21 (3-(diphenylphosphoryl)-9-(4-(diphenylphosphoryl)phenyl)-9H-carbazole, 25 nm)/TmPyPB (1,3,5-tri(m-pyrid-3-yl-phenyl)benzene, 50 nm)/LiF (1 nm)/Al (100 nm) were fabricated to evaluate the potential application of these complexes. A (TPP)2Ir(tpip) emitter based device showed the best performance of a maximum current efficiency (ηc) value of 37.61 cd A(-1) and a maximum external quantum efficiency (EQE) of 13.7% with low efficiency roll-off.

Syntheses, photoluminescence, and electroluminescence of a series of iridium complexes with trifluoromethyl-substituted 2-phenylpyridine as the main ligands and tetraphenylimidodiphosphinate as the ancillary ligand.

Five bis-cyclometalated iridium complexes with tifluoromethyl-substituted 2-phenylpyridine (ppy) at different positions of its phenyl group as the main ligands and tetraphenylimidodiphosphinate (tpip) as the ancillary ligand, 2-6 (1 is a trifluoromethyl-free complex), were prepared, and their X-ray crystallography, photoluminescence, and electrochemistry were investigated. The number and positions of trifluoromethyl groups at the phenyl ring of ppy greatly affected the emission spectra of Ir(3+) complexes, and their corresponding emission peaks at 533, 502, 524, 480, and 542 nm were observed at room temperature, respectively. Constructed with complexes 2-6 as the emitters, respectively, the organic light-emitting diodes (OLEDs) with the structure of indium-tin oxide/1,1-bis[4-(di-p-tolylamino)phenyl]cyclohexane (30 nm)/Ir (x wt %):bis[3,5-bis(9H-carbazol-9-yl)phenyl]diphenylsilane (15 nm)/1,3,5-tris(1-phenyl-1H-benzo[d]imidazol-2-yl)phenyl (45 nm)/LiF (1 nm)/Al (100 nm) showed good performances. Particularly, device G4 based on 4-trifluoromethyl-substituted complex 4 with x = 8 wt % obtained a maximum luminance of over 39000 cd m(-2) and maximum luminance efficiency (η(L)) and power efficiency (η(p)) of 50.8 cd A(-1) and 29.0 lm W(-1), respectively. The results suggested that all of the complexes 2-6 would have potential applications in OLEDs.

Synthesis and photoluminescence properties of rhenium(I) complexes based on 2,2':6',2''-terpyridine derivatives with hole-transporting units.

Based on 2,2':6',2''-terpyridine ligands (L1), five terpyridine derivatives, namely 4'-carbazol-9-yl-2,2':6',2''-terpyridine (L2), 4'-diphenylamino-2,2':6',2''-terpyridine (L3), 4'-bis(4-tert-butylphenyl)amino-2,2':6',2''-terpyridine (L4), 4'-[naphthalen-1-yl-(phenyl)amino]-2,2':6',2''-terpyridine (L5), 4'-[naphthalen-2-yl(phenyl)amino]-2,2':6',2''-terpyridine (L6) and their corresponding Re(I) complexes ReL(n)(CO)3Cl (n = 1­6) have been synthesized and characterized by elemental analysis and 1H NMR spectroscopy. The X-ray crystal structure of ReL3(CO)3Cl has also been obtained. The luminescence spectra of ReL2(CO)3Cl­ReL5(CO)3Cl, obtained in CH2Cl2 solution at room temperature, show strong dπ (Re) → π* (diimine) MLCT character (λ(max) 600 nm) and a small red shift relative to ReL1(CO)3Cl. This, confirmed by the study of the triplet energy levels of the L1­L6 ligands at low temperature (77 K rigid matrix), indicates that the introduction of electron-donating moieties on the terpyridine unit decreases the triplet levels of the ligands, leading to a reduction of the energy gap between d and π* orbitals. In the solid state, upon MLCT excitation, all the complexes show an even stronger emission and a blue spectral shift (λ(max) ∼ 550 nm) compared to those obtained in solution.