Organic Light-emitting Diodes Research Articles

Deciphering TADF Mechanisms in Metal-Free Organic Emitters BrA-HBI: Utilizing Optimally Tuned Range-Separated Functionals for Insight

Abstract Metal-free organic emitters, characterized by their thermally activated delayed fluorescence (TADF) properties, offer considerable promise for the creation of highly efficient organic light-emitting diodes (OLEDs). Recently, Shao et al. presented a novel excited state intramolecular proton transfer (ESIPT) system BrA-HBI, demonstrating an emission quantum yield of up to 50% [Adv. Funct. Mater. 32, 2201256 (2022)]. However, many open issues cannot be answered solely by experimental means only and require detailed theoretical investigations. For instance, what causes the activation of TADF from the Keto* tautomer and leads to fluorescence quenching in the Enol* form? Herein, we provide a theoretical investigation on the TADF mechanism of the BrA-HBI molecule by optimally tuned range-separated functionals. Our findings reveal that ESIPT occurs in the BrA-HBI molecule. Moreover, we have disclosed the reason for the fluorescence quenching of the Enol* form and determined that the T 2 state plays a dominant role in the TADF phenomenon. In addition, double hybrid density functionals method was utilized to verify the reliability of optimally tuned range separation functionals on the calculation of the TADF mechanism in BrA-HBI. These findings not only provide a theoretical reference for development of highly efficient organic light-emitting diodes, but also demonstrate the effectiveness of the optimally tuned range-separated functionals in predicting the luminescence properties of TADF molecules.

Efficient solution-processed near-infrared organic light-emitting diodes with a binary-mixed electron transport layer

A binary-mixed electron transport layer (ETL) has been reported for constructing solution-processable near-infrared organic light-emitting diodes (NIR OLEDs). Relative to the single-component ETL, the binary-mixed ETL composed of PDINN:TPBi can enhance the carrier transport capacity, reduce device impedance, and weaken fluorescence quenching of the emitting layer. By carefully selecting an appropriate luminescent material Y5 (a nonfullerene electron acceptor in organic solar cells) and precisely fine-tuning the molecular aggregation in active layer using a mixed solvent, the morphology is optimized and luminescence performance is enhanced, resulting in efficient NIR OLEDs with an emission peak at 890 nm. The experiment showcases a Y5-based near-infrared OLED with a maximum radiance of 34.9 W sr-1 m-2 and a maximum external quantum efficiency of 0.50%, which is among the highest values reported for non-doped fluorescent NIR OLEDs with an emission peak over 850 nm.

Design and performance of sulfur and selenium-substituted triarylboron D3-A TADF emitters for OLED applications

This study presents the design, synthesis, and comprehensive theoretical and photophysical analysis of two new D3-A type thermally activated delayed fluorescence (TADF) emitters for organic light-emitting diode (OLED) applications. Utilizing a triarylboron core as the electron-accepting group and phenothiazine (PTZ) or phenoselenazine (PSZ) as electron-donating units, the molecules BTP-S and BTP-Se were developed. The D3-A structure supports the separation of frontier molecular orbitals (FMOs), leading to minimized singlet-triplet energy gaps (ΔEST), which are crucial for the TADF mechanism. Density functional theory (DFT) calculations presented that BTP-S and BTP-Se exhibit band gaps (Eg) of 2.52 and 3.23 eV, respectively, with BTP-S showing an ΔEST value as low as 0.007 eV for the S1-T1 transition at the lowest energy conformation. Photophysical studies revealed high photoluminescence quantum yields (PLQYs) for both compounds, with BTP-S achieving up to 85 % in mCP films and BTP-Se up to 59 %. In vacuum-processed OLEDs, BTP-S achieved a maximum external quantum efficiency (EQE) of 25.3 %, a current efficiency (ηc) of 195.8 cd/A, and a maximum luminance (Lmax) of 17356 cd/m2, while BTP-Se reached an EQE of 7.5 %, an ηc of 132.19 cd/A, and an Lmax of 16826 cd/m2 likely limited by the contributions of a folded-donor conformer enabled by the Se substitution. These findings underscore the impact of donor unit selection and conformation on the TADF characteristics, and provide valuable insights for designing high-performance OLED materials.

Dual Multiresonance Core Strategy Enable Efficient Pure Blue Organic Light‐Emitting Devices Based on Fluorene Linkages

AbstractMultiresonance thermally activated delayed fluorescence (MR‐TADF) emitters with high color purity in virtue of their inherent narrowband emission have received great interest in organic light‐emitting diodes (OLEDs). However, it remains a big challenge to develop the ultrapure blue MR‐TADF emitters with high efficiency. In this work, a novel “dual‐MR‐core” strategy is proposed by connecting two parent N‐B‐O‐skeletons with non‐conjugate 9‐position substituted fluorene linkages for high efficient deep‐blue MR‐TADF emitters, namely H‐FOBN and Me‐FOBN, which possess the highly twisted structure with suppressed aggregation. Finally, the vacuum‐deposited deep‐blue OLED exhibits excellent external quantum efficiency (EQE) of 25.1% with small full width at half maximum (FWHM) of 28 nm, as well as CIE of (0.14, 0.08). Furthermore, owing to enhanced solubility, the solution‐processed deep‐blue OLED based on Me‐FOBN shows EQE of 11.3%, with small FWHM of 32 nm and CIE of (0.14,0.09). These outstanding performances confirm that this “dual‐MR‐core” strategy provides a feasible approach to develop high efficient ultrapure blue MR‐TADF emitters.

Suppression of dead zones in slot-coated organic thin films by monitoring of meniscus formation for OLEDs

There inevitably appear dead zones in organic thin films fabricated by sheet-to-sheet slot-die coating because a coating start varies and the recovery time of internal pressure in slot-die head differs for each coating, resulting in poor coating repeatability. Slot-coated thin films within dead zones are thinner or thicker, possibly causing non-uniform light emission from solution-processable organic light-emitting diodes (OLEDs). To tackle it, we have devised an automatic coating start method based on the monitoring of meniscus formation. It automatically starts coatings when the cross-sectional area of meniscus reaches a certain optimal value in such a way that the start of each coating is kept unchanged. It is found that high-viscosity (4800 cPs) polydimethylsiloxane (PDMS) requires a longer time for the internal pressure of slot-die head to reach a steady state than low-viscosity (80 cPs) poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS). It can be reduced to a great extent by applying a method of discharging the solution in advance (pre-discharging scheme). Compared with high-viscosity PDMS films, low-viscosity PEDOT:PSS films are shown to have longer dead zones and poorer repeatability due to the fact that the dead zone of relatively thin films varies sensitively to a small change in the coating start timing. With this scheme, we have successfully fabricated a highly uniform OLED device with no dead zones in the emission area.

Photophysical properties of donor (D)-acceptor (A)-donor (D) diketopyrrolopyrrole (A) systems as donors for applications to organic electronic devices.

Fourteen substituted diketopyrrolopyrrole (DPP) molecules in a donor (D)-acceptor (DPP)-donor (D) arrangement were designed. We employed density functional theory, time-dependent DFT, DFT-MRCI and the ab initio wave function second-order algebraic diagrammatic construction (ADC(2)) methods to investigate theoretically these systems. The examined aromatic substituents have one, two, or three hetero- and non-hetero rings. We comprehensively investigated their optical, electronic, and charge transport properties to evaluate potential applications in organic electronic devices. We found that the donor substituents based on one, two, or three aromatic rings bonded to the DPP core can improve the efficiency of an organic solar cell by fine-tuning the highest occupied molecular orbital/lowest unoccupied molecular orbital levels to match acceptors in typical bulk heterojunctions acceptors. Several properties of interest for organic photovoltaic devices were computed. We show that the investigated molecules are promising for applications as donor materials when combined with typical acceptors in bulk heterojunctions because they have appreciable energy conversion efficiencies resulting from their low ionization potentials and high electron affinities. This scenario allows a more effective charge separation and reduces the recombination rates. A comprehensive charge transfer analysis shows that D-A (DDP)-D systems have significant intramolecular charge transfer, further confirming their promise as candidates for donor materials in solar cells. The significant photophysical properties of DPP derivatives, including the high fluorescence emission, also allow these materials to be used in organic light-emitting diodes.

Gold Coordination-Accelerated Multi-Resonance TADF Emission for Efficient Solution-Processible Ultrapure Deep-Blue OLEDs.

Multi-resonance (MR) type emitters have emerged as highly promising candidates for high-resolution organic light-emitting diodes (OLEDs). However, thermally activated delayed fluorescence (TADF) emissions with simultaneous short excited state lifetimes and ultrapure blue color (a CIEy close to 0.046 and an emission peak > 440 nm) have rarely been obtained for MR emitters. Herein, we report a design of dual gold-coordinated MR molecules to achieve efficient and short-lived ultrapure blue TADF emission. The dinuclear Au(I) complex, namely iPrAuBN, shows a narrowband deep-blue emission with a peak maximum of 448 nm and a full width at half maximum (FWHM) of 29 nm in doped film. The coordination with two Au atoms significantly shortens the delayed fluorescence lifetime to 7.8 μs in comparing with 60.6 μs for the organic analogue. Solution-processed OLED doped with iPrAuBN demonstrates an ultrapure blue electroluminescence with a peak maximum of 442 nm, a FWHM of 19 nm, Commission International de l'Éclairage (CIE) coordinates of (0.154, 0.036), and a maximum external quantum efficiency of 14.8%.

Influence of Process Parameters on Properties of Non-Reactive RF Magnetron-Sputtered Indium Tin Oxide Thin Films Used as Electrodes for Organic Light-Emitting Diodes

Influence of Process Parameters on Properties of Non-Reactive RF Magnetron-Sputtered Indium Tin Oxide Thin Films Used as Electrodes for Organic Light-Emitting Diodes

Blue hyperphosphorescence based on green Ir(III) sensitizer with dual CF3 substituted imidazo[4,5-c]pyridin-2-ylidene cyclometalates

Hyperphosphorescent organic light-emitting diodes (HPOLEDs) are drawing increased attention, as the efficient Förster resonance energy transfer (FRET) from phosphorescent sensitizer to the narrowband fluorescent terminal emitter may give improved performances. In this work, we reported a class of Ir(III) phosphors based on the di-trifluoromethyl (CF3) substituted imidazo[4,5-c]pyridin-2-ylidene chelates; i.e., (L6F) and (L6FB). The f-isomers exhibited efficient blue emission with peak max. 443 − 462 nm, while m-counterparts exhibited green emission between 501 – 512 nm in degassed toluene solution. The theoretical calculation indicates divergent MLCT, LLCT and ILCT contributions for distinctive isomers. OLEDs based on dopant m-Ir(L6FB)3 exhibited green luminescence at 505 nm, EQEmax of 20.3 % and CIEx,y of (0.277, 0.462), while respective HPOLEDs with terminal emitter ν-DABNA showed blue hyperphosphorescence peaking at 468 nm, EQEmax of 25.2 %, CIEx,y of (0.174, 0.204) and EQE of 22.7 % at 1000 cd·m-2. Therefore, our finding demonstrates the effective conversion of the green electrophosphorescence to blue hyperphosphorescence via the rapid FRET process.

External electric field induced emission behavior for ESIPT-based 2-(benzo[d]thiazol-2-yl)-4-(pyren-1-yl)phenol towards near-infrared region

Organic light-emitting diodes (OLEDs) for low energy transfer and double emission, but the current methods for regulating ESIPT processes are mostly solvent and substituent effects. Here, utilizing the density theory functional (DFT) and time-dependent density functional theory (TD-DFT) methods, the ESIPT process controlled by an external electric field (EEF) is proposed, and the changes in photophysical properties of 2-(benzo[d]thiazol-2-yl)-4-(pyren-1-yl)phenol (PyHBT) are investigated. Structural parameter variations and IR vibrational spectra measure the prerequisite for the ESIPT process, namely, intramolecular hydrogen bond (IHB) strength, and the scanned potential energy curves (PECs) demonstrate that the ESIPT process of PyHBT is harder to execute as the positive EEF increases, and the opposite is true for the negative EEF. The absorption and fluorescence spectra show shifts under the distinct EEFs, and even the emission wavelength reaches the short-wave near-infrared (SW-NIR) region (780–1100 nm), such as 815.2 nm for a positive EEF of + 30 × 10-4 a.u. in the keto form. Additionally, the fluorescence intensity of PyHBT is strongly influenced by the positive EEF, especially in the enol form, and the investigation of the mechanism by hole-electron analysis demonstrates that under the positive EEF, the twisted intramolecular charge transfer (TICT) process is induced, which triggers the weakening of the fluorescence intensity. In summary, our work not only complements the theoretical approach to modulate the ESIPT process, but also reveals that the photophysical properties of materials affected by the external electric field are even expected to reach the NIR region.

Spin-Flip-Restricted Multiple-Resonance Emitters for Extended Device Lifetime in Indolocarbazole-Based Blue Organic Light-Emitting Diodes.

In this study, a multiple-resonance (MR) core structure is developed with a spin-flip-restricted emission mechanism based on a fused indolo[3,2,1-jk]carbazole (ICz) framework as emitters to improve the lifetime of blue organic light-emitting diodes. The molecular skeleton modulation approach applied to the conjugated π-system effectively stabilizes the triplet energy of the fused ICz emitters and narrows the full-width-at-half maximum (<20nm). In addition, the emitters exhibit higher exciton stability than conventional boron-based MR emitters. The fused ICz-based blue fluorescent device exhibits a high external quantum efficiency of 7.2%, a blue index of 68.6cd A-1 at a Commission internationale de l'éclairage y coordinate (CIEy) of 0.075, and a device lifetime 1.8 times longer than that of a boron-based emitter. In addition, a phosphor-sensitized fluorescent device based on the ICz emitter exhibited an improved external quantum efficiency of 20.6% with a CIEy coordinate of 0.076.

Chiral electron transporting material for circularly polarized organic light-emitting diode

Chiral electron transporting material for circularly polarized organic light-emitting diode

Recent Advances in Fluorescent Polyimides.

Polyimide (PI) refers to a type of high-performance polymer containing imide rings in the main chain, which has been widely used in fields of aerospace, microelectronic and photonic devices, gas separation technology, and so on. However, traditional aromatic PIs are, in general, the inefficient fluorescence or even no fluorescence, due to the strong inter- and intramolecular charge transfer (CT) interactions causing unavoidable fluorescence quenching, which greatly restricts their applications as light-emitting functional layers in the fabrication of organic light-emitting diode (OLED) devices. As such, the development of fluorescent PIs with high fluorescence quantum efficiency for their application fields in the OLED is an important research direction in the near future. In this review, we provide a comprehensive overview of fluorescent PIs as well as the methods to improve the fluorescence quantum efficiency of PIs. It is anticipated that this review will serve as a valuable reference and offer guidance for the design and development of fluorescent PIs with high fluorescence quantum efficiency, ultimately fostering further progress in OLED research.

Robust Sandwich-Structured Thermally Activated Delayed Fluorescence Molecules Utilizing 11,12-Dihydroindolo[2,3-a]carbazole as Bridge.

Constructing folded molecular structures is emerging as a promising strategy to develop efficient thermally activated delayed fluorescence (TADF) materials. Most folded TADF materials have V-shaped configurations formed by donors and acceptors linked on carbazole or fluorene bridges. In this work, a facile molecular design strategy is proposed for exploring sandwich-structured molecules, and a series of novel and robust TADF materials with regular U-shaped sandwich conformations are constructed by using 11,12-dihydroindolo[2,3-a]carbazole as bridge, xanthone as acceptor, and dibenzothiophene, dibenzofuran, 9-phenylcarbazole and indolo[3,2,1-JK]carbazole as donors. They hold outstanding thermal stability with ultrahigh decomposition temperatures (556-563 oC), and exhibit fast delayed fluorescence and excellent photoluminescence quantum efficiencies (86%-97%). The regular and close stacking of acceptor and donors results in rigidified molecular structures with efficient through-space interaction, which are conducive to suppressing intramolecular motion and reducing reorganized excited-state energy. The organic light-emitting diodes (OLEDs) using them as emitters exhibit excellent electroluminescence performances, with maximum external quantum efficiencies of up to 30.6%, which is a leading value for the OLEDs based on folded TADF emitters. These results demonstrate the proposed strategy of employing 11,12-dihydroindolo[2,3-a]carbazole as bridge for planar donors and acceptors to construct efficient folded TADF materials is applicable.

Optical Properties and Growth Characteristics of 8-Quinolinolato Lithium (Liq) Nano-Layers Deposited by Gas Transport Deposition.

Organometallic complexes containing reactive alkali metals, such as lithium (Li), represent a promising material approach for electron injection layers and electron transport layers (EILs and ETLs) to enhance the performance of Organic Light-Emitting Diodes (OLEDs). 8-Quinolinolato Lithium (Liq) has shown remarkable potential as an EIL and ETL when conveyed in very thin films. Nevertheless, the deposition of nano-layers requires precise control over both thickness and morphology. In this work, we investigate the optical properties and morphological characteristics of Liq thin films deposited via Organic Vapor Phase Deposition (OVPD). Specifically, we present our methodology for analyzing the measured pseudodielectric function <ε(ω)> using Spectroscopic Ellipsometry (SE), alongside the nano-topography of evaporated Liq nano-layers using Atomic Force Microscopy (AFM). This information can contribute to the understanding of the functionality of this material, since ultra-thin Liq interlayers can significantly increase the operational stability of OLED architectures.

A temperature sensor with a wide spectral range based on a dual-emissive TADF dendrimer system

Dual emission from thermally activated delayed fluorescence (TADF) emitters is often difficult to observe, especially in solution, limited by Kasha’s rule. Two TADF dendrimers containing N-doped polycyclic aromatic hydrocarbons as acceptors are designed and synthesized. Compound 2GCzBPN, having a strongly twisted geometry, exhibits TADF, while 2GCzBPPZ, possessing a less twisted geometry, shows dual emission associated with the monomer and aggregate that is TADF. The demonstration reveals that 2GCzBPPZ can serve as a temperature sensor with excellent temperature sensitivity and remarkably wide emission color response in solution. By embedding 2GCzBPPZ in paraffin we demonstrate a spatial-temperature sensor that shows a noticeable emission shift from yellow to green and ultimately to blue as the temperature increases from 20 to 200 °C. We finally demonstrate the utility of these TADF dendrimers in solution-processed organic light-emitting diodes.

Surface Passivation of the Cu2–xSe Electrode During the Chemical Bath Deposition

Herein, p‐type flexible and transparent electrodes of Cu2–xSe are produced at different conversion times at 20, 25, and 30 s of polyester/Cu thin films via chemical bath deposition. To study the charge transport properties across the Cu2–xSe layer, the organic light‐emitting diodes (OLEDs) are produced according to the following configuration: polyester/Cu2–xSe/poly(3,4‐ethylenedioxythiophene):poly(styrene sulfonate)/poly[2‐methoxy‐5‐(2‐ethylhexyloxy)‐1,4‐phenylenevinylene]/aluminum (polyester/Cu2–xSe/PEDOT:PSS/MEH‐PPV/Al), resulting in a direct tunneling in the transport of holes. The control of the barrier's energy between the Cu2–xSe electrode and the MEH‐PPV polymer allows it to tune selectively of the OLED’ charge transport mechanism. The morphological analysis of the Cu2–xSe electrode, carried out using atomic force microscopy, as well as the temperature dependence of the current–voltage measurements in the OLED (50–300 K) shows the ideal deposition time in the chemical bath. In contrast, impedance spectroscopy results confirm the inexistence of the Cu2–xSe/PEDOT:PSS interface using 30 s to Cu2–xSe synthesis. As a result, the control of the injection mechanism of charges can be obtained by reducing the barrier energy to hole transport during the synthesis process of the Cu2–xSe layer via chemical bath deposition, thus simplifying and reducing the costs of the device's processing.

Chemical reactions as a means of installing adlayers on electron transport layers

Adlayers are often placed at metal-on-organic interfaces as a common strategy to alleviate damage during metal deposition by thermal evaporation. Methods of chemically installing adlayers have been recently demonstrated on organic semiconductors that address these interfacial issues while providing many secondary benefits. Chemical installation has yet to be attempted at the cathode-electron transport layer (ETL) interface within organic light-emitting devices (OLEDs), offering a powerful option to optimize electron injection, improve surface wetting, and reduce metal penetration. Here, a reaction between TPBi (2,2′,2′’-(1,2,5-benzinetriyl)-tris(1-phenyl-1-H-benzimidazole) and propylene oxide results in a controllable 1–3 nm thick layer of propylene oxide as shown by high-resolution X-ray photoelectron spectroscopy (XPS) and energy dispersive X-ray spectroscopy (EDX). The reactive addition of the adlayer at temperatures below 40℃ does not affect the morphology of the thin film and reaches a high degree of coverage within 3 h. Integration of this layer into a phosphorescent OLED does not introduce any significant negative impact on device function. This result opens up the possibility of introducing further specific functionality into the adlayer to engineer OLED performance.

Manipulating excited states for constructing ultraviolet hybridized local and charge transfer fluorophores and non-doped OLEDs with high color purity

The development of efficient ultraviolet (UV) organic light-emitting materials and devices holds paramount significance for their practical applications. Nonetheless, attaining a short-wavelength electroluminescence peak of less than 390 nm while maintaining high color purity in non-doped devices remains a formidable challenge. In this work, we have successfully designed and synthesized two UV hybridized local and charge transfer fluorophores, specifically 4DBF-CZCN and 2DBF-CZCN, which are comprised of cyanide-modified phenylcarbazole and a weak electron donor dibenzofuran. And the properties of excited state are well regulated through the different linking site of dibenzofuran. The resulting non-doped devices exhibit not only emission peak wavelengths of 390 and 388 nm, accompanied by narrow full-width-at-half-maximums of 42 and 38 nm, respectively, corresponding to color coordinates of (0.164, 0.034) and (0.164, 0.036), but also admirable external quantum efficiencies of 3.00 % and 2.86 % at high luminance levels of 1910 and 1252 cd m−2, respectively.

Structural and Electronic Factors Controlling the Efficiency and Rate of Intersystem Crossing to the Triplet State in Thiophene Polycyclic Derivatives.

Thiophene polycyclic derivatives are widely used in organic light-emitting diodes, photovoltaics, and medicinal chemistry applications. Understanding the electronic and structural factors controlling their intersystem crossing rates is paramount for these applications to be successful. This study investigates the photophysical, electronic structure, and excited state dynamics of 1,2-benzodiphenylene sulfide, benzo[b]naphtho[1,2-d]thiophene, and benzo[b]naphtho[2,3-d]thiophene in polar aprotic and non-polar solvents. Steady-state absorption and emission spectroscopy, femtosecond transient absorption spectroscopy, and DFT and TD-DFT calculations are employed. Low fluorescence quantum yields of 1.2 to 2.7% are measured in acetonitrile and cyclohexene, evidencing that the primary relaxation pathways in these thiophene derivatives are nonradiative. Linear interpolation of internal coordinates calculations predict that an S-C bond elongation reaction coordinate facilitates the efficient intersystem crossing to the T1 state. Excitation of 1,2-benzodiphenylene sulfide and benzo[b]naphtho[1,2-d]thiophene at 350 nm or benzo[b]naphtho[2,3-d]thiophene at 365 nm, populates the lowest-energy 1ππ* state, which relaxes to the 1ππ* minimum in tens of picoseconds or intersystem crosses to the triplet manifold in ca. 500 ps to 1.1 ns depending on the position at which the benzene rings are added. Excitation at 266 nm does not affect the intersystem crossing rates. Laser photodegradation experiments demonstrate that the thiophene polycyclic derivatives are highly photostable.