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Abstract Efficient charge-carrier injection and transport in organic light-emitting devices (OLEDs) are essential to simultaneously achieving their high efficiency and long-term stability. However, the charge-transporting layers (CTLs) deposited by various vapor or solution processes are usually in amorphous forms, and their low charge-carrier mobilities, defect-induced high trap densities and inhomogeneous thickness with rough surface morphologies have been obstacles towards high-performance devices. Here, organic single-crystalline (SC) films were employed as the hole-transporting layers (HTLs) instead of the conventional amorphous films to fabricate highly efficient and stable OLEDs. The high-mobility and ultrasmooth morphology of the SC-HTLs facilitate superior interfacial characteristics of both HTL/electrode and HTL/emissive layer interfaces, resulting in a high Haacke’s figure of merit (FoM) of the ultrathin top electrode and low series-resistance joule-heat loss ratio of the SC-OLEDs. Moreover, the thick and compact SC-HTL can function as a barrier layer against moisture and oxygen permeation. As a result, the SC-OLEDs show much improved efficiency and stability compared to the OLEDs based on amorphous or polycrystalline HTLs, suggesting a new strategy to developing advanced OLEDs with high efficiency and high stability.

Abstract Highly efficient blue fluorescent materials have recently attracted great interest for organic light-emitting diode (OLED) application. Here, two new pyrene based organic molecules consisting of a highly rigid skeleton, namely SPy and DPy, are developed. These two blue light emitters exhibit excellent thermal stability. The experiment reveals that the full-width at half-maximum (FWHM) of the emission spectrum can be tuned by introducing different amounts of 9,9-diphenyl-N-phenyl-9H-fluoren-2-amine on pyrene units. The FWHM of the emission spectrum is only 37 nm in diluted toluene solution for DPy. Furthermore, highly efficient blue OLEDs are obtained by thermally activated delayed fluorescence (TADF) sensitization strategy. The blue fluorescent OLEDs utilizing DPy as emitters achieve a maximum external quantum efficiency (EQE) of 10.4% with the electroluminescence (EL) peak/FWHM of 480 nm/49 nm. Particularly, the EQE of DPy-based device is boosted from 2.6% in non-doped device to 10.4% in DMAc-DPS TADF sensitized fluorescence (TSF) device, which is a 400% enhancement. Therefore, this work demonstrates that the TSF strategy is promising for highly efficient fluorescent OLEDs application in wide-color-gamut display field. Graphical Abstract

For developing high-performance organic light-emitting diodes (OLEDs) with thermally activated delayed fluorescent (TADF) emitters, the diphenyltriazine (TRZ) unit was introduced onto the 2′- and 3′-positions of xanthene moiety of spiro[fluorene-9,9′-xanthene] (SFX) to construct n-type host molecules, namely 2′-TRZSFX and 3′-TRZSFX. The outward extension of the TRZ unit, induced by the meta-linkage, resulted in a higher planarity between the TRZ unit and xanthene moiety in the corresponding 3′-TRZSFX. Additionally, this extension led to a perched T1 level, as well as a lower unoccupied molecular orbital (LUMO) level when compared with 2′-TRZSFX. Meanwhile, the 3′-TRZSFX molecules in the crystalline state presented coherent packing along with the interaction between TRZ units; the similar packing motif was spaced apart from xanthene moieties in the 2′-TRZSFX crystal. These endowed 3′-TRZSFX superior electron transport capacity in single-carrier devices relative to the 2′-TRZSFX-based device. Hence, the 3′-TRZSFX-based TADF–OLED showed remarkable electroluminescent (EL) performance under the operating luminance from turn-on to ca. 1000 cd·m−2 with a maximum external quantum efficiency (EQEmax) of 23.0%, thanks to its matched LUMO level with 4CzIPN emitter and better electron transport capacity. Interestingly, the 2′-TRZSFX-based device, with an EQEmax of 18.8%, possessed relatively low roll-off and higher efficiency when the operating luminance exceeded 1000 cd·m−2, which was attributed to the more balanced carrier transport under high operating voltage. These results were elucidated by the analysis of single-crystal structures and the measurements of single-carrier devices, combined with EL performance. The revealed position effect of the TRZ unit on xanthene moiety provides a more informed strategy to develop SFX-based hosts for highly efficient TADF–OLEDs.

Since the first development of organic light-emitting diodes (OLEDs) in 1987, imidazole derivatives, mainly including phenanthroimidazole (PI) and benzimidazole (BI), have increasingly attracted attention. Their strong electron-withdrawing properties make them suitable for emitters, hosts, and electron-transporting materials (ETMs). In this review, an overview of the recent developments regarding OLEDs based on imidazole derivatives, especially the relationship between the molecule structure and the device performance as fluorescent and host materials, is given.

Colloidal cesium lead halide (CsPbX3, X = Cl, Br, and I) perovskite nanocrystals (NCs) demonstrate supreme optical properties in the spectra region of infrared, red, and green. High-performance blue-emitting counterparts are still eagerly required for next-generation full-color displays. However, it is challenging to obtain efficient blue perovskite NCs, especially in a deep blue region with an emission wavelength of around 460 nm or shorter. Herein, calcium halide and ammonium ions are applied simultaneously to modify the CsPb(Br/Cl)3 NCs in situ to reduce surface defects, finally remarkably enhancing the photoluminescence quantum yield (PLQY) from 13% to 93% with an emission peak at 455 nm and the Commission Internationale de l’Eclairage (CIE) coordinates at (0.147, 0.030), which is close to the requirement of the Rec.2020 standard and also meets the requirement of blue emission in DCI-P3. Bright white emission and a wide color gamut are also achieved by combining the commercial red-emitting and green-emitting phosphors. The combination of time-resolved PL spectra and femtosecond transient absorption results discloses the reason for PLQY improvement as suppressing the nonradiative recombination.

We systematically studied the influence of weak microcavity effects on the optical properties of tandem white organic light-emitting diodes (WOLEDs) using optical simulation. Based on the simulation results, an image processing method is developed and verified for rational design of high-performance tandem WOLEDs. The results indicate that low operating voltage, good color rendering index, and angular emission properties can be simultaneously obtained in tandem WOLEDs by engineering the device structure. This study provides promising direction for the development of high-performance tandem WOLEDs. [ABSTRACT FROM AUTHOR]

Novel tetraarylsilane-centred compounds (DCzSiPy and DDCzSi) have been introduced in fabricating efficient white organic light-emitting diodes. Owing to its bipolar transportation characteristics of DCzSiPy, more balanced charge carrier injected into the emitting layer is achieved when it is selected as a single host. Here, double-carbazole-based material DDCzSi is used as hole-transporting layer despite that it is previously reported as a host for blue-phosphorescent organic light-emitting diodes. Highly efficient white phosphorescent organic light-emitting diodes with single-host structures are demonstrated. The maximum current efficiency is 41.5 cdA-1 and the maximum power efficiency is 39.4 lm W-1. And the external quantum efficiency roll-off from maximum to that at 5000 cdm-2 is only 19.8%. We adjust the proportion of the blue/orange emitting layer, lower roll-off of 14.7% is obtained. However, the maximum current efficiency and power efficiency are reduced to 36.7 cdA-1 and 33.2 lm W-1, respectively. [ABSTRACT FROM AUTHOR]

The fabrication of planar heterojunction (PHJ) organic photovoltaic (OPV) cells using tetramethyl substituted copper(II) phthalocyanine (CuMePc) as an electron donor and C60 as an acceptor is described. The impact of post-fabrication thermal annealing upon the performance of these cells has been examined. Atomic force microscopy (AFM) images and UV–visible absorption spectra of CuMePc thin films revealed crystallization of CuMePc induced by thermal annealing at 190 °C. The crystallized CuMePc films accounted for improved hole mobility, broadened absorption spectrum, and increased donor/acceptor interface in the as-fabricated cells after thermal annealing. AFM images also revealed that the surface of MoO3 film was smooth and close-packed after thermal annealing, which efficiently blocked the leakage current in the annealed cells, leading to dramatic improvement of performance for the PHJ cells using CuMePc as the electron donor and MoO3 or V2O5 as the anode buffer layer. The power conversion efficiency of the thermal-annealed PHJ cell with a configuration of ITO/MoO3/CuMePc/C60/Bathocuproine/Al was higher than that of the bulk heterojunction (BHJ) cell fabricated by co-depositing CuMePc and C60. It was mainly because the isolated clusters of CuMePc and/or C60 molecules formed during the fabrication of the BHJ cell was avoided in the PHJ cell. [ABSTRACT FROM AUTHOR]