High-Efficiency and Emission-Tunable Inorganic Blue Perovskite Light-Emitting Diodes Based on Vacuum Deposition.

The fabrication of perovskite light-emitting diodes (PeLEDs) with vacuum deposition shows great potential and commercial value in realizing large-area display panel manufacturing. However, the electroluminescence (EL) performance of vacuum-deposited PeLEDs still lags behind the counterparts fabricated by solution process, especially in the field of blue PeLEDs. Here, the fabrication of high-quality CsPbBr3- x Clx film through tri-source co-evaporation is reported to achieve high photoluminescence quantum yield (PLQY). Compared with the conventional traditional dual-source co-evaporation, the tri-source co-evaporation method allows for freely adjustable elemental ratios, enabling the introduction of the lattice-matched Cs4 Pb(Br/Cl)6 phase with the quantum-limited effect into the inorganic CsPb(Br/Cl)3 emitter. By adjusting the phase distribution, the surface defects of the emitter can be effectively reduced, leading to better blue emission and film quality. Further, the effects of Cs/Pb ratio and Br/Cl ratio on the PLQY and carrier recombination dynamics of perovskite films are investigated. By optimizing the deposition rate of each precursor source, spectrally stable blue PeLEDs are achieved with tunable emission ranging from 468 to 488 nm. Particularly, the PeLEDs with an EL peak at 488 nm show an external quantum efficiency (EQE) of 4.56%, which is the highest EQE value for mixed-halide PeLEDs fabricated by vacuum deposition.

A Yellow-Emitting Homoleptic Iridium(III) Complex Constructed from a Multifunctional Spiro Ligand for Highly Efficient Phosphorescent Organic Light-Emitting Diodes.

To suppress concentration quenching and to improve charge-carrier injection/transport in the emission layer (EML) of phosphorescent organic light-emitting diodes (PhOLEDs), a facial homoleptic iridium(III) complex emitter with amorphous characteristics was designed and prepared in one step from a multifunctional spiro ligand containing spiro[fluorene-9,9'-xanthene] (SFX) unit. Single-crystal X-ray analysis of the resulting fac-Ir(SFXpy)3 complex revealed an enlarged Ir···Ir distance and negligible intermolecular π-π interactions between the spiro ligands. The emitter exhibits yellow emission and almost equal energy levels compared to the commercial phosphor iridium(III) bis(4-phenylthieno[3,2-c]pyridinato-N,C2')acetylacetonate (PO-01). Dry-processed devices using a common host, 4,4'-bis(N-carbazolyl)-1,1'-biphenyl, and the fac-Ir(SFXpy)3 emitter at a doping concentration of 15 wt % exhibited a peak performance of 46.2 cd A-1, 36.3 lm W-1, and 12.1% for the current efficiency (CE), power efficiency (PE), and external quantum efficiency (EQE), respectively. Compared to control devices using PO-01 as the dopant, the fac-Ir(SFXpy)3-based devices remained superior in the doping range between 8 and 15 wt %. The current densities went up with increasing doping concentration at the same driving voltage, while the roll-offs remain relatively low even at high doping levels. The superior performance of the new emitter-based devices was ascribed to key roles of the spiro ligand for suppressing aggregation and assisting charge-carrier injection/transport. Benefiting from the amorphous stability of the emitter, the wet-processed device also exhibited respectful CE, PE, and EQE of 32.2 cd A-1, 22.1 lm W-1, and 11.3%, respectively, while the EQE roll-off was as low as 1.7% at the luminance of 1000 cd m-2. The three-dimensional geometry and binary-conjugation features render SFX the ideal multifunctional module for suppressing concentration quenching, facilitating charge-carrier injection/transport, and improving the amorphous stability of iridium(III)-based phosphorescent emitters.

Evaluating Fluorinated-Aniline Units with Functionalized Spiro[Fluorene-9,9'-Xanthene] as Hole-Transporting Materials in Perovskite Solar Cells and Light-Emitting Diodes.

In the field of perovskite optoelectronics, developing hole-transporting materials (HTMs) on the spiro[fluorene-9,9'-xanthene] (SFX) platform is one of the current research focuses. The SFX inherits the merits of spirobifluorene in terms of the configuration and property, but it is more easily derivatized and regulated by virtue of its binary structure. In this work, we design and synthesize four isomeric SFX-based HTMs, namely m-SFX-mF, p-SFX-mF, m-SFX-oF, and p-SFX-oF, through varying the positions of fluorination on the peripheral aniline units and their substitutions on the SFX core, and the optoelectronic performance of the resulting HTMs is evaluated in both perovskite solar cells (PSCs) and light-emitting diodes (PeLEDs) by the vacuum thermal evaporating hole-transporting layers (HTLs). The HTM p-SFX-oF exhibits an improved power conversion efficiency of 15.21% in an inverted PSC using CH3NH3PbI3 as an absorber, benefiting from the deep HOMO level and good HTL/perovskite interface contact. Meanwhile, the HTM m-SFX-mF provides a maximum external quantum efficiency of 3.15% in CsPb(Br/Cl)3-based PeLEDs, which is attributed to its perched HOMO level and shrunken band-gap for facilitating charge carrier injection and then exciton combination. Through elucidating the synergistic position effect of fluorination on aniline units and their substitutions on the SFX core, this work lays the foundation for developing low-cost and efficient HTMs in the future.

Single-crystalline hole-transporting layers for efficient and stable organic light-emitting devices.

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.

The Synergy of the Buried Interface Surface Energy and Temperature for Thermal Evaporated Perovskite Light-Emitting Diodes.

Multisource coevaporation is such a promising method for the preparation of perovskite films. However, there is limited research about the effects of the buried interface on thermal-evaporated perovskite light-emitting diodes (PeLEDs). In this study, the effects of buried interfaces on thermal-evaporated all-inorganic perovskite films are systematically investigated. It is found that the low-surface-energy buried interface promotes the formation of columnar grain by suppressing heterogeneous nucleation, and functional groups on the high-surface-energy interface have a significant effect on the actual element ratio of the film. The substrate temperature can affect the nucleation and film-formation kinetics of the columnar grains. As a result of the synergistic strategy, a peak external quantum efficiency (EQE) of 8.6% is achieved in the green PeLEDs with a stable emission peak at 516 nm, which is among the best thermal-evaporated PeLEDs reported. This work provides an insight into the preparation of perovskites by thermal evaporation and builds the groundwork for future studies.

Unraveling the Position Effect of Spiroxanthene-Based n-Type Hosts for High-Performance TADF-OLEDs.

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.

A Demulsification-Crystallization Model for High-Quality Perovskite Nanocrystals.

A room-temperature technique with all-nonpolar-solvent, which circumvents the sensitivity of ionic perovskite to polar solvent, has become attractive for the synthesis of metal halide perovskite nanocrystals (PNCs). However, the lack of understanding of the inner mechanism, especially for the state of the precursor and the crystallization process of the PNCs, hinders further development of this technique. Here, through systematic study of the Pb precursor and in situ characterization of the PNCs, it is revealed that the reverse micelle nature of the Pb precursor exactly creates a novel demulsification-crystallization (D-C) model, namely, a two-stage nucleation is divided by a demulsification process for the PNCs. On this basis, a top efficiency for green light-emitting diodes based on PNCs is obtained with a maximum external quantum efficiency of 22.5% through tailoring the D-C model using a multiple-acid-anion synergistic assisted strategy to obtain high-quality PNCs. Beyond the high efficiency, the work paves the way for diverse ideas in PNC synthesis.

A sensitization strategy for highly efficient blue fluorescent organic light-emitting diodes.

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.

CsPb(Br/Cl)3 Perovskite Nanocrystals with Bright Blue Emission Synergistically Modified by Calcium Halide and Ammonium Ion.

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.

Exceptionally efficient deep blue anthracene-based luminogens: design, synthesis, photophysical, and electroluminescent mechanisms.

Achieving high-efficiency deep blue emitter with CIEy < 0.06 (CIE, Commission Internationale de L'Eclairage) and external quantum efficiency (EQE) >10% has been a long-standing challenge for traditional fluorescent materials in organic light-emitting diodes (OLEDs). Here, we report the rational design and synthesis of two new deep blue luminogens: 4-(10-(4'-(9H-carbazol-9-yl)-2,5-dimethyl-[1,1'-biphenyl]-4-yl)anthracen-9-yl)benzonitrile (2M-ph-pCzAnBzt) and 4-(10-(4-(9H-carbazol-9-yl)-2,5-dimethylphenyl)anthracen-9-yl)benzonitrile (2M-pCzAnBzt). In particular, 2M-ph-pCzAnBzt produces saturated deep blue emissions in a non-doped electroluminescent device with an exceptionally high EQE of 10.44% and CIEx,y (0.151, 0.057). The unprecedented electroluminescent efficiency is attributed to the combined effects of higher-order reversed intersystem crossing and triplet-triplet up-conversion, which are supported by analysis of theoretical calculation, triplet sensitization experiments, as well as nanosecond transient absorption spectroscopy. This research offers a new approach to resolve the shortage of high efficiency deep blue fluorescent emitters.