Full-color Display Applications Research Articles

Phosphonic Chloride Assisted Fabrication of Highly Emissive Mixed Halide Perovskite Films in Ambient Air for Blue Light-Emitting Diodes.

Blue perovskite light-emitting diodes (LEDs) have emerged as promising candidates for full-color display and lighting applications. However, the fabrication of blue-emitting perovskite films typically requires an inert environment, leading to increased complexity and cost in the manufacturing process, which is undesirable for applications of perovskite LEDs. Herein, we report a strategy to fabricate bright blue-emitting perovskite films in ambient air by incorporating phosphonic chlorides in a perovskite precursor solution. We used two different phosphonic chlorides, diphenylphosphonic chloride (DPPC) and phenylphosphonic dichloride (PPDC), and comparatively studied their effects on the properties of perovskite films and the blue LEDs. It is found that PPDC possesses a stronger chlorination ability due to higher hydrolysis reactivity; meanwhile, it has a stronger interaction with the perovskite compared to DPPC, resulting in an improved film quality and enhanced blue emission with a photoluminescence quantum yield of 45%, which represents the record value for the air-processed blue perovskite films. Blue perovskite LEDs are fabricated, and the emission wavelengths are effectively tuned by controlling the concentration of phosphonic chlorides. Benefiting from the optimized perovskite films with reduced nonradiative recombination and promoted charge injection and transport, the PPDC-derived blue perovskite LEDs exhibit improved performance with an external quantum efficiency of 3.3% and 1.2% for the 490 and 480 nm emission wavelength, respectively.

Enhancing energy transfer via inhibition defect density of coupled quasi-2D perovskite layers for efficient sky-blue light-emitting diodes

Metal halide perovskite materials have emerged as unique and promising candidates for full-color display and solid-state lighting applications. While substantial progress has been made in developing green, red and near-infrared perovskite light-emitting diodes (PeLEDs) with external quantum efficiency (EQE) approaching their theoretical upper limit, the performance of blue PeLEDs is still lagging far behind. Here, synergistic long and short hydrobromide chains are strategically introduced into the perovskite to regulate composition and dimensional engineering for promoting coupled quasi-two-dimensional perovskite thin films with enhanced photoluminescence quantum yield (PLQY) and reduced trap state density. The inclusion of the hydrobromide can also simultaneously passivate the perovskite defects by reducing nonradiative recombination loss and increasing the formation of n >1 phases for efficient exciton energy transfer. Encouraged by these findings, we finally demonstrate the most stable sky-blue PeLEDs to date with a half-lifetime of 515 min at high external quantum efficiency (EQE＞10 %), emitting at 486 nm with a peak EQE of up to 15.6 %.

Elastic Organic Crystals Exhibiting Amplified Spontaneous Emission Waveguides with Standard Red Chromaticity of the Rec.2020 Gamut.

The use of mechanically flexible molecular crystals as optical transuding media is demonstrated for a plethora of applications; however, the spectral peaks of optical outputs located mainly in the range of 400-600nm are insufficient for practical telecommunication and full-color display applications. Herein, two elastically bendable organic crystals are reported that show red emission of the rec.709 gamut under 365nm UV light irradiation yet generate rec.2020 gamut red optical waveguides and amplified spontaneous emissions when irradiated by a 355nm laser. Capitalizing on the extended π-conjugation and donor-acceptor character, as well as mechanical elasticity, these organic crystals exhibit flexible optical waveguides with Commission Internationale de L'Eclairage (CIE) coordinates of (0.70, 0.29), nearly identical to the red chromaticity of the rec.2020 gamut required for ultrahigh-definition (UHD) displays. Notably, one of the elastic crystals functions as a soft resonance cavity, resulting in amplified spontaneous emission waveguides with CIE coordinates of (0.71, 0.29) and the standard red chromaticity of the rec.2020 gamut, both in straight and bent states. This study presents a new avenue for the development of high-purity red-emissive crystalline materials to create all-organic, lightweight, and mechanically compliant optical telecommunication and UHD display devices.

Achromatic diffractive liquid-crystal optics for virtual reality displays

Diffractive liquid-crystal optics is a promising optical element for virtual reality (VR) and mixed reality as it provides an ultrathin formfactor and lightweight for human factors and ergonomics. However, its severe chromatic aberrations impose a big challenge for full-color display applications. In this study, we demonstrate an achromatic diffractive liquid-crystal device to overcome this longstanding chromatic aberration issue. The proposed device consists of three stacked diffractive liquid crystal optical elements with specifically designed spectral response and polarization selectivity. The concept is validated by both simulations and experiments. Our experimental results show a significant improvement in imaging performance with two types of light engines: a laser projector and an organic light-emitting diode display panel. In addition, our simulation results indicate that the lateral color shift is reduced by ~100 times in comparison with conventional broadband diffractive liquid-crystal lens. Potential applications for VR-enabled metaverse, spatial computing, and digital twins that have found widespread applications in smart tourism, smart education, smart healthcare, smart manufacturing, and smart construction are foreseeable.

Water assisted chlorination enables ambient air fabrication of mixed halide perovskite films for blue light emitting diodes

Blue perovskite light emitting diodes (LEDs) have attracted remarkable attention in recent years owing to their potential in full-color display and lighting applications. However, blue-emitting perovskite films usually require to be fabricated in inert condition, which increases the fabrication complexity and cost of perovskite LEDs, being unfavorable for practical applications. Herein, we report a strategy to fabricate blue perovskite films in ambient air with high humidity for LED applications. Diphenylphosphinic chloride (DPPOCl) is introduced into perovskite film, which can react with the water in air and release chlorine ion, thus leading to the chlorination of perovskite and the consequent blueshifted luminescence. Meanwhile, the PO groups can passivate the perovskite film and suppress nonradiative recombination. Based on the ambient air fabricated blue-emitting perovskite films, perovskite LEDs are constructed, which exhibit bright and spectrally stable electroluminescence (EL). The emission wavelength of the LEDs can be effectively tuned from cyan to pure blue by varying the concentration of DPPOCl. Our work demonstrates a feasible approach to fabricate blue perovskite films in high humidity air, which could contribute to lowering down the fabrication cost of blue perovskite LEDs and further boosting their applications in display and lighting.

Azaborine as a Versatile Weak Donor for Thermally Activated Delayed Fluorescence

Extensive research has been devoted to the development of thermally activated delayed fluorescence emitters, especially those showing pure-blue emission for use in lighting and full-color display applications. Toward that goal, herein we report a novel weak donor, 1,4-azaborine (AZB), with complementary electronic and structural properties compared to the widely used dimethylacridan (DMAC) or carbazole (Cz) donors. Coupled with a triazine acceptor, AZB-Ph-TRZ is the direct structural analogue of the high-performance and well-studied green TADF emitter DMAC-TRZ and has ΔEST = 0.39 eV, a photoluminescence quantum yield (ΦPL) of 27%, and λPL = 415 nm in 10 wt % doped mCP films. The shortened analogue AZB-TRZ possesses red-shifted emission with a reduced singlet-triplet gap (ΔEST = 0.01 eV) and fast reverse intersystem crossing (kRISC of 5 × 106 s-1) in mCP. Despite a moderate ΦPL of 34%, OLEDs with AZB-TRZ in mCP showed sky-blue emission with CIE1931(x,y) of (0.22,0.39) and a maximum external quantum efficiency (EQEmax) of 10.5%. Expanding the chemist's toolkit for the design of blue donor-acceptor TADF materials will enable yet further advances in the future, as AZB is paired with a wider range of acceptor groups.

Ultrasmall water-stable CsPbBr3 quantum dots with high intensity blue emission enabled by zeolite confinement engineering.

Ultrasmall CsPbBr3 perovskite quantum dots (PQDs) as promising blue-emitting materials are highly desired for full-color display and lighting applications, but their inferior efficiency and poor ambient stability hinder extensive applications. Herein, a "break-and-repair" strategy has been developed to tightly confine monodispersed ultrasmall CsPbBr3 PQDs in a zeolite. In this strategy, the CsPbBr3 PQDs are introduced into the zeolite via a high temperature evaporation method, wherein the perovskite precursors break the zeolite framework, and amino acids and silane are then used to fix the damaged framework and lock the perovskite QDs within the matrix. By modulating the synthetic conditions to control the growth of CsPbBr3, PQDs with ultrasmall size of 2 nm have been obtained in the zeolite, giving emission centered at 460 nm with a high quantum yield of 76.93%. Strikingly, the PQDs@zeolite composite exhibits water-induced reversible photoluminescence promoted by the coordination between the amino acids and PQDs in a dynamic manner, achieving enhanced water stability (14 days in aqueous solution). This work provides a new perspective for the synthesis of water-stable blue-emitting perovskite composites for potential applications in lighting fields.

Large-Area Blade-Coated Deep-Blue Polymer Light-Emitting Diodes with a Narrowband and Uniform Emission.

Large-area polymer light-emitting diodes (PLEDs) manufactured by printing are required for flat-panel lighting and displays. Nevertheless, it remains challenging to fabricate large-area and stable deep-blue PLEDs with narrowband emission due to the difficulties in precisely tuning film uniformity and obtaining single-exciton emission. Herein, efficient and stable large-area deep-blue PLEDs with narrowband emission are prepared from encapsulated polydiarylfluorene. Encapsulated polydiarylfluorenes presented an efficient and stable deep-blue emission (peak: 439nm; full width at half maximum (FWHM): 39nm) in the solid state due to their single-chain emission behavior without inter-backbone chain aggregation. Large-area uniform blade-coated films (16 cm2 ) are also fabricated with excellent smoothness and morphology. Benefitting from efficient emission and excellent printed capacity, the blade-coated PLEDs with a device area of 9 mm2 realized uniform deep-blue emission (FWHM: 38nm; CIE: 0.153, 0.067), with a corresponding maximum external quantum efficiency and the brightness comparable to those of devices based on spin-coated films. Finally, considering the essential role of deep-blue LEDs, a preliminary patterned PLED array with a pixel size of 800 × 1000 µm2 and a monochrome display is fabricated, highlighting potential full-color display applications.

Pushing the Emission Envelope for Full-Color Realization of Colloidal Semiconductor Core/Shell Nanoplatelets

Colloidal semiconductor nanoplatelets (NPLs) have been known to exhibit favorable optical characteristics, such as a narrow emission bandwidth, giant oscillator strength, suppressed Auger recombination, and polarized emission, which can merit their use in light-emitting diodes and lasing devices. However, to date, NPLs have shown a relatively limited range of emission energy because of their thickness-dependent discrete band gap and restricted choice of chemical composition (mostly CdSe-based), impeding the realization of R, G, and B full-color display based on NPLs. CdSe-based core/shell NPLs can only cover the emission range from yellowish green (∼550 nm) to red (∼620 nm). In this study, we synthesize CdZnSe/ZnS core/shell NPLs with significantly enhanced spectral tunability over a broad spectral range (425–570 nm). Particularly, to overcome the difficulty in the synthesis of composition-controlled CdZnSe NPLs, we carried out a Cd-to-Zn direct cation exchange reaction on CdSe/ZnS core/shell NPLs using ZnI2 and an oleylamine (or trioctylphosphine oxide) ligand, which allows for the composition change in NPL emitting core while maintaining the original shape of NPLs. With the control of the reaction temperature and time as well as the ligands, the exchange reaction enables us to demonstrate an unprecedentedly wide emission range and spectral tunability of NPLs with the emission wavelength reaching 425 nm. Furthermore, as the core alloying proceeds, NPLs maintain excellent color purity and exhibit linearly polarized emission. Our light-emitting devices using the cation-exchanged CdZnSe/ZnS NPLs hinted at the possibility of using the NPLs in full-color display applications. Based on the elemental mapping analysis and experimental results, we proposed the temperature-dependent cation exchange reaction process in core/shell NPLs.

Replacing cesium with formamidinium cation in colloidal perovskite nanoplatelets: Synthetic alloying and post-synthetic cation exchange towards efficient blue emission

Blue-emitting perovskite materials have attracted remarkable attention in recent years owing to their potential in full-color display and lighting applications. Two-dimensional (2D) perovskite nanoplatelets (NPls) are considered as competitive blue emitters due to high carrier recombination probability and good spectrum stability compared with their mixed-halide counterparts. However, the 2D nature and high defect density of perovskite NPls generally result in nonradiative carrier recombination, making it problematic for achieving efficient blue emission. Herein, we reported a novel way to enhance the blue emission efficiency of perovskite NPls. We found that by using formamidinium cation (FA+) to replace the cesium cation (Cs+) during NPls synthesis, the photoluminescence (PL) of the NPls can be significantly enhanced. The PL quantum yield (PLQY) of NPls exhibited a monoclinic rising tendency with increased FA+ content, and the FAPbBr3 NPls finally show efficient deep blue emission at 433 nm with PLQY up to 70% as well as narrow emission bandwidth of only 12 nm. We further demonstrated that efficient blue emission can be realized not only by FA+ alloying but also by post-synthetic treatment of the as-synthesized CsPbBr3 NPls using FA+, where the FA+ was capable to replace the Cs+ in perovskite NPls through cation exchange. Resultantly, PLQY as high as 90% was achieved at a pure blue wavelength of 457 nm. Our work provides a promising strategy for achieving efficient and color-pure blue-emitting perovskite materials, which could potentially further promote their light-emitting applications.

Unravelling Alkali‐Metal‐Assisted Domain Distribution of Quasi‐2D Perovskites for Cascade Energy Transfer toward Efficient Blue Light‐Emitting Diodes

Solution processable quasi‐2D (Q‐2D) perovskite materials are emerging as a promising candidate for blue light source in full‐color display applications due to their good color saturation property, high brightness, and spectral tunability. Herein, an efficient energy cascade channel is developed by introducing sodium bromide (NaBr) in phenyl‐butylammonium (PBA)‐containing mixed‐halide Q‐2D perovskites for a blue perovskite light‐emitting diode (PeLED). The incorporation of alkali metal contributes to the nucleation and growth of Q‐2D perovskites into graded distribution of domains with different layer number <n>. The study of excitation dynamics by transient absorption (TA) spectroscopy confirms that NaBr induces more Q‐2D perovskite phases with small n number, providing a graded energy cascade pathway to facilitate more efficient energy transfer processes. In addition, the nonradiative recombination within the Q‐2D perovskites is significantly suppressed upon Na+ incorporation, as validated by the trap density estimation. Consequently, the optimized blue PeLEDs manifest a peak external quantum efficiency (EQE) of 7.0% emitting at 486 nm with a maximum luminance of 1699 cd m−2. It is anticipated that these findings will improve the understanding of alkali‐metal‐assisted optimization of Q‐2D perovskites and pave the way toward high‐performance blue PeLEDs.

Angle robust reflected plasmonic color palettes with expanded color gamut

By investigating the relationship between the color saturation and the square nanoblock period, the plasmonic color filters based on a two-dimensional metal–insulator–metal square nanoblock array structure with the optimum period are designed to generate highly saturated subtractive colors. Then, the realizable color gamut is broadly extended to 70% of the sRGB color space by introducing the supercell array consisted of centrosymmetric T-shaped dimers and the reflective full-color palette can provide a high angular tolerance of ∼30°. Moreover, the smooth transitions between two different colors can be achieved by mixing two types of supercell arrays with different dimensions. The supercell array structure also exhibits high sensitivities and good color responses when exposed on the ambient environments. Such excellent performances open the avenue for full-color display, colorimetric sensing, and filtering applications.

Extending the color response range of Yb3+ concentration-dependent multimodal luminescence in Yb/Er doped fluoride microrods by annealing treatment

For anti-counterfeiting and full-color display applications, adjusting the Yb3+ concentrations or excitation conditions is the basic and simple way to control luminescence color and intensity. However, this approach is significantly limited because the luminescence color is not very sensitive to the response of doped Yb3+ concentrations or excitation conditions. Herein, we have demonstrated that the as-synthesized NaLuF4:Yb/Er and their annealed counterpart LuOF:Yb/Er microrods exhibit upconversion/downshifting double-mode luminescence with complementary and rich colors from green, yellow to red under 980/365/488 nm excitation. The anti-counterfeiting patterns printed with these microrods inks have the ability of the color response to the excitation wavelength, excitation power and laser scanning speed, which make them be hard to duplicate. This study indicates that Yb/Er doping NaLuF4 and their complementary LuOF microrods have great potential in multilevel anti-counterfeiting application and display. In addition, heat treatment way also brings novel ideas to the synthesis of high efficient luminescence materials.

Deep-blue high-efficiency triplet-triplet annihilation organic light-emitting diodes using donor- and acceptor-modified anthracene fluorescent emitters

Deep-blue organic electroluminescence (EL) emitters are important for full-color display and solid-state lighting applications of organic light-emitting diode (OLED) technology. Triplet-triplet annihilation (TTA) is one efficient approach for realizing efficient deep-blue EL. In this study, a PPI donor and a BI acceptor are connected via an anthracene (AN) bridge to obtain a bipolar deep-blue TTA emitter, PPI-AN-BI. The effects of the donor and the acceptor moieties are studied by comparing photophysical properties of molecules obtained by replacing either the PPI or the BI group with a phenyl (P) group to obtain BI-AN-P or PPI-AN-P. Non-doped OLEDs based on the three molecules display high-performance deep-blue EL. Using PPI-AN-BI as a non-doped emitter, a trilayer device exhibits a maximum external quantum efficiency (EQE) of 8.4% with a CIE index of (0.15, 0.15), and a bilayer device exhibits a maximum EQE of 6.9% with a CIE index of (0.15, 0.15). It is experimentally and theoretically demonstrated that triplet pair transition and enhanced oscillator strength can be the main reasons contributing to high performances. This work would provide an efficient method for designing deep-blue non-doped EL emitters.

Efficient and Bright Pure-Blue All-Inorganic Perovskite Light-Emitting Diodes from an Ecofriendly Alloy.

Metal halide perovskite light-emitting diodes (PeLEDs) have been regarded as alternative candidates for full-color display applications with rapid progress to surge the external quantum efficiencies (EQEs) over 20%. However, in contrast to the high efficiencies of green, red, and near-infrared PeLEDs, the performance of their blue cousins is still lagging behind, especially the pure-blue one. Obtaining blue perovskite films with negligible nonradiative recombination loss and high stability is of great importance to realize efficient and spectrally stable blue PeLEDs. In this work, through partially replacing the toxic lead ions (Pb2+) with ecofriendly strontium ions (Sr2+) to tune the emission wavelength along with using passivation strategies, all-inorganic pure-blue perovskite films with a high photoluminescence quantum yield of 60.7% were achieved, which then delivered PeLEDs with a luminance of 510 cd m-2 and an EQE of 1.43%. The device yields a record radiance among the most efficient PeLEDs at 467 nm. In addition, the resultant PeLEDs displayed exceptional spectral stability during the electrical bias operation. Our work provides a promising avenue to develop environmentally friendly perovskite materials for efficient and spectrally stable pure-blue PeLEDs and beyond.

All-inorganic encapsulation for remarkably stable cesium lead halide perovskite nanocrystals: toward full-color display applications

High-temperature solid-phase synthesis is able to yield full-color metal–halide perovskite nanocrystals with remarkable stabilities.

Tuning the Excited State of Tetradentate Pd(II) Complexes for Highly Efficient Deep-Blue Phosphorescent Materials.

Deep-blue-light-emitting materials are urgently desired in high-performance organic light-emitting diodes (OLEDs) for full-color display and solid-state lighting applications. However, the development of stable and efficient deep-blue emitters remains a great challenge. Herein, a series of stable and efficient tetradentate Pd(II)-complex-based deep-blue emitters with rigid 5/6/6 metallocycles and no F atom were designed and synthesized. These deep-blue emitters employ various isoelectronic five-membered heteroaryl-ring-containing ligands to exhibit extremely narrow emission spectra peaking at 439-443 nm with a full width at half-maximum (fwhm) of only 22-38 nm in 2-methyltetrahydrofuran at room temperature. In particular, the design of an intramolecular hydrogen bond enabled the 1-phenyl-1,2,3-trazole-based Pd(II) complexes to achieve CIEy < 0.1 (0.069-0.078; CIE is Commission Internationale de L'Eclairage). Theoretical calculation and natural transition orbital analysis reveal that these deep-blue materials emit light exclusively from their ligand (carbazole)-centered (3LC) states. Moreover, the triplet excited-state property can be efficiently regulated through ligand modification with isoelectronic oxazole and thiazole rings or pyridine rings, resulting in sky-blue-to-yellow materials, which emit light originating from an admixture of metal-to-ligand charge-transfer (3MLCT) and intraligand charge-transfer states. The newly developed Pd(II) complexes are strongly emissive in various matrixes with a quantum efficiency of up to 51% and also highly thermally stable with a 5% weight-reduction temperature (ΔT5%) of up to 400 °C. Deep-blue OLEDs with CIEy < 0.1 employing Pd(II) complexes as emitters were successfully fabricated for the first time. This study demonstrates that the Pd(II) complexes can act as excellent phosphorescent light-emitting materials through rational molecular design and also provide a valuable method for the development of Pd(II)-complex-based efficient and stable deep-blue emitters.

Tuning State Energies for Narrow Blue Emission in Tetradentate Pyridyl-Carbazole Platinum Complexes.

Narrow, deep blue emitters are highly desired in the field of organic light emitting diodes for high quality full color display and solid-state lighting applications. PtNON is reported as a deep blue emitting phosphor but is limited by its broad emission spectrum, making it unsuitable for high quality full color display applications. In this work, we report a strategy to fine-tune the color and the emission line shape of PtNON derivatives by incorporating electron donating (methyl or methoxy) or withdrawing (trifluoromethyl) substituent groups at the positions para to the nitrogen of the pyridines in PtNON. These substitutions resulted in destabilization or stabilization of the charge transfer state (CT) relative to the ligand centered (LC) state, resulting in complexes with narrow or broad emission spectra in various media. PtNON-OMe emits predominantly from the LC state, giving a narrow emission spectrum with fwhm = 48 nm in any media. PtNON-Me emits largely from the LC state in nonpolar media (fwhm = 54 nm) and predominantly from the CT state in polar media (fwhm = 83 nm). Last, PtNON-CF3 emits solely from the CT state in any media, giving it a broad emission spectrum (fwhm = 98 nm). The photoluminescence quantum yields of PtNON-OMe, PtNON-Me, and PtNON-CF3 in 1% doped PMMA films are 89, 95 and 20% with emission lifetimes of 27.1, 7.17, and 0.96 μs, respectively.

Enhanced Performance of Red Perovskite Light-Emitting Diodes through the Dimensional Tailoring of Perovskite Multiple Quantum Wells.

Halide perovskite multiple quantum wells (MQWs) have recently shown great potential in the field of light-emitting diodes. We report a facile solution-based approach to fabricate dimensionality-tunable perovskite MQWs by introducing 1-naphthylmethylammonium (NMA) cations into CsPbI3 perovskites. Through the dimensional tailoring of (NMA)2Csn-1PbnI3n+1 perovskite MQWs, the crystallinity and photoluminescence quantum efficiencies (PLQEs) are significantly improved. We have obtained high-performance red perovskite light-emitting diodes (PeLEDs) with a luminance of 732 cd m-2 and a maximum external quantum efficiency of 7.3%, which are among the best-performing red PeLEDs. Significantly, the maximum luminance of our PeLEDs is obtained at a low applied voltage of 3.4 V, with a turn-on voltage close to the perovskite band gap (Vturn-on ≈ 1.9 V). These outstanding performance characteristics demonstrate that dimensional tailoring of perovskite MQWs is a feasible and effective strategy to achieve high-performance PeLEDs, which is attractive for full-color display applications of perovskites.

Development and Fabrication of AlGaInP-Based Flip-Chip Micro-LEDs

The fabrication of AlGaInP-based flip-chip micro light-emitting-diodes (LED; emitting area: 4.5 mil $\times5$ mil) with horizontal electrodes is reported in this paper. The thickness of the epitaxial layer of the thin LED structure was reduced to 50% of that of the traditional thick LED, whereas carrier concentration in the n-type GaAs contact layer was increased to $5 \times 10^{18}$ cm−3 to meet the Ohmic contact requirement. At a current injection of 5 mA, the thin LED exhibited a forward voltage, output power, and external quantum efficiency of 1.8 V, 1.9 mW, and 19%, respectively. The optoelectronic performance of the thin LED was as good as that of the traditional thick red LED. The technique proposed by this paper can be used to integrate AlGaInP-based LEDs with nitride LEDs for full-color display applications.