layers, which limited the injection of charge carriers, particularly electrons, into the emissive QD layer. Thus, replacement of the organic charge transport layers with inorganic materials seemed to be the solution. As it was reported 12 , the use of zinc tin oxide for an n-type charge transport layer and nickel oxide for a p-type one led to a higher current density in QLEDs, reaching 4 A/cm 2. However, because of the considerable energy barrier between NiO and QDs, the device suffered from an insufficient hole injection rate and, hence, had an EQE lower than 0.1%. Then, engineering of hybrid devices with an inorganic ETL and an organic HTL became the main direction of QLED evolution, which continues to the present day 4,13,14. Although early studies reported the use of various inorganic oxides and chalcogenides as electron-transport materials 15 , ZnO has been proved to be the most favorable ETL material due to its high transparency, low work function, and high electron mobility 4,5. Formation of ZnO ETLs for QLEDs could be done using different deposition techniques, such as the sol-gel method 16,17 , spray pyrolysis 18,19 , sputtering 20 , etc. Nevertheless, colloidal ZnO NPs have become the most widely used material for QLED ETL due to their optimal electronic structure, simple synthesis, and the possibility of using them as a wet-process-compatible conductive ink 13,21-24. However, in the QLED structure, electron transfer from the QDs to ZnO induced by the energy difference between the conduction band minima (CBM) of these materials often leads to exciton dissociation and significant reduction of the EL efficiency. This mechanism is known as the cause of QD luminescence quenching in various QD-based systems 25-27. In this regard, the possibility to control the CBM energy level of a ZnO-based ETL by doping can be a promising way to optimize the electronic structure and performance of the device. One of the first dopants that was used for this purpose was magnesium (Mg) 8,28,29. Zhang et al. demonstrated a peak EQE of 18.2% and 18.1% for red and green QLEDs employing ZnMgO ETLs, respectively 30. Here, a single-layer ETL consisting of Zn 0.9 Mg 0.1 O NPs, which efficiently suppressed interfacial quenching of QD luminescence, has been used. However, because it was found that the charge transfer rate and QD blinking in the multilayer structures are mostly related to the Fermi levels of metal oxide 31,32 , group-III elements, such as aluminum (Al) 33-35 and gallium (Ga) 7 , became widely used as n-type dopants 36 for fabrication of ZnO ETLs. For instance, the use of ZnO doped with Al (AZO) as the ETL material in solution-processed QLEDs led to a 1.8-fold enhancement of device performance compared with ZnO-based QLEDs 35. The reason for this was effective suppression of spontaneous electron transfer at the interface of QDs and ETL, which was attributed to a decreased work function (WF) and the proper CBM alignment of the electron-transport layer and QDs, both induced by the presence of Al dopants inside the ZnO mat...

Efficient electroluminescence was revealed in single-layer light-emitting diodes based on electron-hole conducting polymers containing the nanocrystalline phase of cyanine dyes (J-aggregates). These species exhibit a very narrow emission band with a maximum in the red to infrared spectrum range. The J-aggregates play an active role in the charge carrier transport of the composites.

To obtain luminescent lanthanide complexes with a low energy LMCT state the 2-(2'-mercaptophenyl)benzothiazolates, Ln(SSN), and 2-(2'-mercaptophenyl)benzoxazolates, Ln(OSN) (Ln = Gd, Yb), were synthesized by the reaction of amides Ln[N(SiMe)] with respective thiophenols. Ytterbium complexes were structurally characterized by X-ray diffraction analysis. Cyclic voltammetry revealed that the deprotonated mercaptophenyl ligands have significantly lower oxidation potentials than their phenoxy analogues and some β-diketones. The photophysical properties of Gd and Yb compounds were studied both in solution and in the solid state. The fluorescence spectra of the compounds in solution display the bands of the keto and enol forms of the ligands. No energy transfer from the organic part to Yb has been detected in solutions of both Yb complexes, whereas in solids an intense metal-centered emission in the near infrared region was observed. The solid Gd compounds exhibited room temperature phosphorescence caused by unusually efficient intersystem crossing facilitated by the essentially reducing properties of OSN and SSN ligands. To explain the sensitization process occurring in solids Yb(OSN) and Yb(SSN) a specific non-resonant energy transfer mechanism via a ligand to metal charge transfer state has been proposed. Based on the Yb derivatives, NIR-emitting OLEDs with 860 μW cm maximal irradiance were obtained. Their Gd counterparts showed bright electrophosphorescence (up to 1350 cd m) in the devices containing doped emission layers.

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