Photoluminescence Quantum Efficiency Research Articles

Circularly Polarized Thermally Activated Delayed Fluorescence Emitters in Through-Space Charge Transfer on Asymmetric Spiro Skeletons.

This work describes a strategy to produce circularly polarized thermally activated delayed fluorescence (CP-TADF). A set of two structurally similar organic emitters SFST and SFOT are constructed, whose spiro architectures containing asymmetric donors result in chirality. Upon grafting within the spiro frameworks, the donor and acceptor are fixed proximally in a face-to-face manner. This orientation allows intramolecular through-space charge transfer (TSCT) to occur in both emitters, leading to TADF properties. The donor units in SFST and SFOT have a sulfur and oxygen atom, respectively; such a subtle difference has great impacts on their photophysical, chiroptical, and electroluminescence (EL) properties. SFOT exhibits greatly enhanced EL performance in doped organic light-emitting diodes, with external quantum efficiency (EQE) up to 23.1%, owing to the concurrent manipulation of highly photoluminescent quantum efficiency (PLQY, ∼90%) and high exciton utilization. As a comparison, the relatively larger sulfur atom in SFST introduces heavy atom effects and leads to distortion of the molecular backbone that lengthens the donor-acceptor distance. SFST thus has lower PLQY and faster nonradiative decay rate. The collective consequence is that the EQE value of SFST, i.e., 12.5%, is much lower than that of SFOT. The chirality of these two spiro emitters results in circularly polarized luminescence. Because SFST has a more distorted molecular architecture than SFOT, the luminescence dissymmetry factor (|glum|) of circularly polarized luminescence of one enantiomer of the former, namely, either (S)-SFST or (R)-SFST, is almost twice that of (S)-SFOT/(R)-SFOT. Moreover, the CP organic light-emitting diodes (CP-OLEDs) show obvious circularly polarized electroluminescence (CPEL) signals with gEL of 1.30 × 10-3 and 1.0 × 10-3 for (S)-SFST and (S)-SFOT, respectively.

Intermediate-phase-assisted low-temperature formation of \u03b3-CsPbI3 films for high-efficiency deep-red light-emitting devices

Black phase CsPbI3 is attractive for optoelectronic devices, while usually it has a high formation energy and requires an annealing temperature of above 300 °C. The formation energy can be significantly reduced by adding HI in the precursor. However, the resulting films are not suitable for light-emitting applications due to the high trap densities and low photoluminescence quantum efficiencies, and the low temperature formation mechanism is not well understood yet. Here, we demonstrate a general approach for deposition of γ-CsPbI3 films at 100 °C with high photoluminescence quantum efficiencies by adding organic ammonium cations, and the resulting light-emitting diode exhibits an external quantum efficiency of 10.4% with suppressed efficiency roll-off. We reveal that the low-temperature crystallization process is due to the formation of low-dimensional intermediate states, and followed by interionic exchange. This work provides perspectives to tune phase transition pathway at low temperature for CsPbI3 device applications.

A blue and Red Bicolor Emission Phosphor Excited by a Single n-UV and Its Self-reduction of Eu3+ → Eu2+ in Na2Al2B2O7

It has great significance to find a path on generating multicolor with one n-UV chip coated with single phosphor. Herein, Eu doped Na2Al2B2O7 (NAB) phosphor is synthesized by solid state reaction in air atmosphere. Both 4f65d1 → 4f7 transitions of Eu2+ (445 nm) and f–f transitions as 5D0 → 7FJ (J = 0, 1, and 2) of Eu3+ (570–650 nm) have been observed. When increasing Eu doping concentration, both the luminescence intensity and lifetime of Eu3+ are increased and no obvious concentration quenching is observed, while the emission strength of Eu2+ does not change monotonically but with two remarkable peak values. The optimum doping concentration of Eu is 16%. Furthermore, Eu3+ is determined to be from Eu(BO3) matrix itself, and Eu2+ testified by ESR and XPS is created by self-reduction of Eu3+ to Eu2+ in NAB due to charge compensation. When x ≥ 0.16, the Eu3+ ion would rather bond with BO3 triangles in Eu(BO3) than stay as Eu2+ in AlO4 tetrahedral in NAB. Furthermore, the as prepared sample shows an excellent thermal stability and the PL quantum efficiencies of NAB: 0.16Eu sample excited by 341 nm and 393 nm are 11.99% and 9.39%, respectively. Additionally, the calculated CIE chromaticity coordinates signify that a couple of colors can be generated by adjusting the excitation wavelength owing to the strength changes between blue and red emissions. Therefore, this novel bicolor phosphor can serve as a potential candidate for w-LEDs applications.

Phase Control and In Situ Passivation of Quasi-2D Metal Halide Perovskites for Spectrally Stable Blue Light-Emitting Diodes.

The fabrication of efficient and spectrally stable pure-blue perovskite light-emitting diodes (LEDs) has been elusive and remains of great interest. Herein, we incorporate diammonium salts into quasi-2D perovskite precursors for phase control of multiple quantum well structures to yield tunable and efficient emission in the blue region. With detailed characterizations and computational studies, we show that in situ passivation by the diammonium salts effectively modifies the surface energies of quasi-2D phases and inhibits the growth of low-band gap quasi-2D and 3D phases. Such phase control and in situ passivation could afford blue light-emitting perovskite thin films with high photoluminescence quantum efficiencies of, for instance, 75% for the emission peak at 471 nm. Using this perovskite thin film as an emitting layer, spectrally stable pure-blue LEDs with an emission peak at 474 nm and a full width at half-maximum of 26 nm could be fabricated to exhibit a brightness of 290 cd m-2 at 8 V and an external quantum efficiency of 2.17%.

Thiazol-2-thiolate-Bridged Binuclear Platinum(II) Complexes with High Photoluminescence Quantum Efficiencies of up to Near Unity.

Binuclear platinum(II) complexes with strong Pt-Pt interactions are an interesting class of luminescent materials, of which photophysical properties could be controlled via multiple ways through organic ligands and Pt-Pt distance. While a number of binuclear platinum(II) complexes have been developed with tunable emissions, achieving high photoluminescence quantum efficiency (PLQE) remains challenging and of great interest. Here we report the synthesis and characterization of a series of binuclear 2,4-difluorophenylpyridine platinum(II) complexes bridged by thiazol-2-thiolate ligands with different bulkiness. The three complexes were found to have short Pt-Pt distances ranging from 2.916 to 2.945 Å. The strong Pt-Pt interactions lead to pronounced metal-metal-to-ligand charge transfer (MMLCT) absorptions between 450 and 500 nm, and strong 3MMLCT emissions in the orange/red region. The PLQEs of the new complexes are in the ranges of 2-31% in solution and 26-100% in solid state. These complexes also exhibit excellent stability in halogenated solvents.

Highly efficient eco-friendly X-ray scintillators based on an organic manganese halide

Scintillation based X-ray detection has received great attention for its application in a wide range of areas from security to healthcare. Here, we report highly efficient X-ray scintillators with state-of-the-art performance based on an organic metal halide, ethylenebis-triphenylphosphonium manganese (II) bromide ((C38H34P2)MnBr4), which can be prepared using a facile solution growth method at room temperature to form inch sized single crystals. This zero-dimensional organic metal halide hybrid exhibits green emission peaked at 517 nm with a photoluminescence quantum efficiency of ~ 95%. Its X-ray scintillation properties are characterized with an excellent linear response to X-ray dose rate, a high light yield of ~ 80,000 photon MeV−1, and a low detection limit of 72.8 nGy s−1. X-ray imaging tests show that scintillators based on (C38H34P2)MnBr4 powders provide an excellent visualization tool for X-ray radiography, and high resolution flexible scintillators can be fabricated by blending (C38H34P2)MnBr4 powders with polydimethylsiloxane.

High-efficiency red thermally activated delayed fluorescence emitters based on benzothiophene-fused spiro-acridine donor

Red thermally activated delayed fluorescence (TADF) emitters underperform their blue and green counterparts, largely due to their low photoluminescence quantum efficiency and isotropic orientation. Here, we designed two rigid and strong electron-rich donor moieties, 3-methyl-13H-spiro[benzo[4,5]thieno[3,2-c]acridine-5,9′-fluorene] (MSTA) and 3-phenyl-13H-spiro[benzo[4,5]thieno[3,2-c]acridine-5,9′-fluorene] (PSTA), by fusing benzothiophene unit on a spiro-acridine framework. Combined with a rigid, linear, and planar acenaphtho[1,2-b]quinoxaline-9,10-dicarbonitrile (ANQDC) acceptor, two red emitters, ANQDC-MSTA and ANQDC-PSTA, were synthesized and characterized. In virtue of high rigidity and linear donor–acceptor (D-A) molecular skeleton, both emitters showed distinct TADF nature, high photoluminescence quantum efficiencies and high horizontal ratios of emitting dipole orientation of ≈85%. Consequently, the ANQDC-PSTA-based OLEDs realized high external quantum efficiency (EQE) of 24.7% at an electroluminescence peak (ELpeak) of 622 nm, together with Commission Internationale de l’Eclairage 1931 (CIE1931) coordinates of (0.61, 0.38). Furthermore, with the modulation of the microcavity effect on the device architecture, we delivered a 19.1% EQE in a deep-red EL region, with an ELpeak of 646 nm and favorable CIE1931 coordinates of (0.64, 0.36). These efficiencies are on par with the highest EL performance for red TADF OLEDs in the similar color gamut.

Whispering-Gallery Mode Lasing in Perovskite Nanocrystals Chemically Bound to Silicon Dioxide Microspheres.

Cesium lead halide perovskite nanocrystals exhibit high photoluminescence quantum efficiencies and tunability across the visible spectrum. This makes these crystals ideal candidates for solar panels, light-emitting diodes, lasers, and especially nanolasers. Due to the versatility of cation substitution in perovskite nanocrystals, they can be grown on amine-functionalized silicon dioxide nanoparticles, where the amine linker replaces the standard cation structure. Selectively growing luminescent nanocrystals on spherical silicon dioxide microspheres results in the opportunity to populate whispering-gallery modes in these spherical silica microspheres. In this case, the nanocrystal halide composition can be used to selectively tune the emission wavelength mode, and microsphere radius to tune the mode spacing. This silicon dioxide attachment also adds to the overall stability of the system. Through photoluminescence microscopy measurements, we show whispering gallery modes in individual perovskite-coated microspheres for CsPbBr3 and CsPbI3 nanocrystals on 9.2 μm diameter silica spheres and compare these to theoretically predicted optical modes. In CsPbBr3, we provide evidence that these modes will lase under optical excitation, with a threshold of 750 μJ/cm2. This study presents a novel system that, through optimization, could be a promising pathway to achieve facile and stable perovskite nanolasers.

Stable Luminescent Radicals and Radical-Based LEDs with Doublet Emission

Radical-based light-emitting diodes are an innovative type of organic light-emitting diodes (OLEDs), which adopt luminescent radicals as emitters, aiming at improving the external quantum efficienc...

High-Efficiency Narrow-Band Electro-Fluorescent Devices with Thermally Activated Delayed Fluorescence Sensitizers Combined Through-Bond and Through-Space Charge Transfers

Organic light-emitting diodes utilizing thermally activated delayed fluorescence sensitizers and multiple-resonance (MR) dopants may simultaneously offer high efficiencies and narrow-band emissions...

34‐1: Invited Paper: Towards Efficient and Stable Printed Single‐layer OLEDs

The efficiency and stability of single‐layer polymer light‐emitting diodes is compromised by unbalanced charge transport, absence of triplet‐exciton harvesting and low photoluminescence quantum efficiencies. We demonstrate that tuning the energy levels of the organic semiconductor can prevent charge trapping, which is the main cause of unbalanced charge transport. By additionally using a novel strategy to enhance charge injection, efficient single‐layer organic light‐emitting diodes are presented based on a trap‐free organic semiconductor exhibiting thermally activated delayed fluorescence.

Aliovalent Doping and Surface Grafting Enable Efficient and Stable Lead‐Free Blue‐Emitting Perovskite Derivative

AbstractLead halide perovskites have emerged as superstar semiconductors owing to their superior optoelectronic properties. However, the issues of chemical and thermodynamic instability and toxicity are yet to be resolved. Here, the non‐ and Bi3+‐doped all‐inorganic lead‐free perovskite derivatives are reported. Most remarkable is the successful extending of excitation of Cs2ZrCl6 to match with the commercial near ultraviolet light‐emitting diode chips via deliberate Bi3+ aliovalent doping. The blue emission, contrary to self‐trapped exciton (STE) emissions amply reported previously, originates from Bi3+ ionoluminescence with a high photoluminescence quantum efficiency of 50%. The competition for harvesting electrons between STEs and Bi3+ is studied in detail by steady‐ and transient‐state fluorescence spectroscopy in combination with theoretical calculations. Surface grafting endows Cs2ZrCl6:Bi3+ with a robust water‐resistant core–shell‐like structure and abiding emission. Surprisingly, the emission intensity even increases to 115.94% of the initial level after immersing in water for 2 h. The as‐obtained phosphor enables the fabrication of a white light‐emitting diode (w‐LED), achieving CCT = 4179 K and Ra = 81.9. This work not only promotes the step toward development of lead‐free, stable, and high‐efficiency perovskite derivatives for the next‐generation warm w‐LEDs, but also reveals the structure–PL relationship.

Rapid ultrasonic-microwave assisted synthesis of Eu3+ doped Y2O3 nanophosphors with enhanced luminescence properties

A series of Eu3+ doped Y2O3 nanophosphors have been synthesized via an ultra-fast ultrasonic-microwave method followed by thermal annealing. The emission of Y2O3:Eu3+ nanophosphors depends tremendously on the concentration of Eu3+ ions and maximum emission intensity is achieved at 612 nm (red luminescence) with a doping concentration of 3 mol%. Moreover, the Y2O3:Eu3+ nanophosphors in this work have much higher photoluminescence lifetime (1.570 ms) and quantum efficiency (80.88%) than that synthesized by other methods. The results of chromaticity coordinates indicate that the Y2O3:Eu3+ as red-emitting phosphor is promising candidate for white light-emitting diodes (LEDs).

Improving Broadband White-Light Emission Performances of 2D Perovskites by Subtly Regulating Organic Cations.

Recently, 2D organic-inorganic hybrid lead halide perovskites have attracted intensive attention in solid-state luminescence fields such as single-component white-light emitters, and rational optimization of the photoluminescence (PL) performance through accurate structural-design strategies is still significant. Herein, by carefully choosing homologous aliphatic amines as templates, isotypical perovskites [DMEDA]PbCl4 (1, DMEDA=N,N-dimethylethylenediamine) and [DMPDA]PbCl4 (2, DMPDA=N,N-dimethyl-1,3-diaminopropane) having tunable and stable broadband bluish white emission properties were rationally designed. The subtle regulation of organic cations leads to a higher degree of distortion of the 2D [PbCl4 ]2- layers and enhanced photoluminescence quantum efficiencies (<1 % for 1 and 4.9 % for 2). The broadband light emissions could be ascribed to self-trapped excitons on the basis of structural characterization, time-resolved PL, temperature-dependent PL emission, and theoretical calculations. This work gives a new guidance to rationally optimize the PL properties of low-dimensional halide perovskites and affords a platform to probe the structure-property relationship.

Size dependence of quantum efficiency of red emission from GaN:Eu structures for application in micro-LEDs.

GaN-based micro-LEDs typically suffer from a size-dependent efficiency due to the relatively long carrier lifetime and sidewall-related recombination effects. We demonstrate that for red-emitting Eu-doped GaN, sidewall-related recombination is hardly an issue for emission efficiency. We determine the photoluminescence quantum efficiency (PL QE) of Eu-related emission as a function of the size of square structures ranging from 3 to 192 µm. With the support of finite-difference time-domain simulations, we show that the light extraction efficiency and material losses are responsible for the decrease in PL QE for large sizes. For sizes smaller than 24 µm, there is an influence of the sidewall-related non-radiative recombination of carriers on the PL QE; however, it is only minor as a result of the limited carrier diffusion lengths in the Eu-doped material. These properties combined with the high efficiency of luminescence indicate the potential of this material for micro-LED applications.

Strategies to Achieve High Circularly Polarized Luminescence from Colloidal Organic-Inorganic Hybrid Perovskite Nanocrystals.

Colloidal metal halide perovskite nanocrystals (NCs) with chiral ligands are outstanding candidates as a circularly polarized luminescence (CPL) light source due to many advantages such as high photoluminescence quantum efficiency, large spin-orbit coupling, and extensive tunability via composition and choice of organic ligands. However, achieving pronounced and controllable polarized light emission remains challenging. Here, we develop strategies to achieve high CPL responses from colloidal formamidinium lead bromide (FAPbBr3) NCs at room temperature using chiral surface ligands. First, we show that replacing a portion of typical ligands (oleylamine) with short chiral ligands ((R)-2-octylamine) during FAPbBr3 NC synthesis results in small and monodisperse NCs that yield high CPL with average luminescence dissymmetry g-factor, glum = 6.8 × 10-2. To the best of our knowledge, this is the highest among reported perovskite materials at room temperature to date and represents around 10-fold improvement over the previously reported colloidal CsPbClxBryI3-x-y NCs. In order to incorporate NCs into any optoelectronic or spintronic application, the NCs necessitate purification, which removes a substantial amount of the chiral ligands and extinguishes the CPL signals. To circumvent this issue, we also developed a postsynthetic ligand treatment using a different chiral ligand, (R-/S-)methylbenzylammonium bromide, which also induces a CPL with an average glum = ±1.18 × 10-2. This postsynthetic method is also amenable for long-range charge transport since methylbenzylammonium is quite compact in relation to other surface ligands. Our demonstrations of high CPL and glum from both as-synthesized and purified perovskite NCs at room temperature suggest a route to demonstrate colloidal NC-based spintronics.

Tunable Dual‐Emission in Monodispersed Sb3+/Mn2+ Codoped Cs2NaInCl6 Perovskite Nanocrystals through an Energy Transfer Process

Double halide perovskites are a class of promising semiconductors applied in photocatalysis, photovoltaic devices, and emitters to replace lead halide perovskites, owing to their nontoxicity and chemical stability. However, most double perovskites always exhibit low photoluminescence quantum efficiency (PLQE) due to the indirect bandgap structure or parity-forbidden transition problem, limiting their further applications. Herein, the self-trapped excitons emission of Cs2 NaInCl6 by Sb-doping, showing a blue emission with high PLQE of 84%, is improved. Further, Sb/Mn codoped Cs2 NaInCl6 nanocrystals are successfully synthesized by the hot-injection method, showing a tunable dual-emission covering the white-light spectrum. The studies of PL properties and dynamics reveal that an energy transfer process can occur between the self-trapped excitons and dopants (Mn2+ ). The work provides a new perspective to design novel lead-free double perovskites for realizing a unique white-light emission.

Correlating the Structural and Photophysical Properties of Ortho, Meta, and Para‐Carboranyl–Anthracene Dyads

AbstractThe role of the carborane isomer is investigated on the structural and photophysical properties of molecules comprising a carborane cluster and a conjugated organic moiety is investigated by synthesizing isomeric o‐, m‐, and p‐carboranyl‐anthracene donor–acceptor dyads. While appending a carborane leads to emission from a low energy intramolecular charge transfer state for the o‐isomer, as well as emission from an excited state localized on the anthracene, this is not the case for the m‐ and p‐carborane derivatives. This difference is attributed to a lower electron affinity for the latter two isomers. However, adding both m‐ and p‐ deforms the aromatic backbone and increases its structural rigidity, reducing non‐radiative decay pathways and hence enhancing photoluminescence quantum efficiency relative to anthracene.

Synthesis and Chiroptical Properties of †- and ‡-Shaped Molecules Based on Planar Chiral [2.2]Paracyclophane

Abstract Two types of optically active molecules based on planar chiral 4,7,12,15-tetrasubstituted [2.2]paracyclophane were synthesized through chemoselective Sonogashira-Hagihara coupling. One is a †-shaped molecule consisting of two π-electron systems stacked, and the other is a ‡-shaped molecule consisting of three π-electron systems stacked. Their optical and chiroptical properties were investigated, which exhibited large molar absorption coefficient, excellent photoluminescence quantum efficiency, and intense circularly polarized luminescence (CPL).

Highly efficient luminescence from space-confined charge-transfer emitters.

Charge-transfer (CT) complexes, formed by electron transfer from a donor to an acceptor, play a crucial role in organic semiconductors. Excited-state CT complexes, termed exciplexes, harness both singlet and triplet excitons for light emission, and are thus useful for organic light-emitting diodes (OLEDs). However, present exciplex emitters often suffer from low photoluminescence quantum efficiencies (PLQEs), due to limited control over the relative orientation, electronic coupling and non-radiative recombination channels of the donor and acceptor subunits. Here, we use a rigid linker to control the spacing and relative orientation of the donor and acceptor subunits, as demonstrated with a series of intramolecular exciplex emitters based on 10-phenyl-9,10-dihydroacridine and 2,4,6-triphenyl-1,3,5-triazine. Sky-blue OLEDs employing one of these emitters achieve an external quantum efficiency (EQE) of 27.4% at 67 cd m-2 with only minor efficiency roll-off (EQE = 24.4%) at a higher luminous intensity of 1,000 cd m-2. As a control experiment, devices using chemically and structurally related but less rigid emitters reach substantially lower EQEs. These design rules are transferrable to other donor/acceptor combinations, which will allow further tuning of emission colour and other key optoelectronic properties.