An ambipolar BODIPY derivative for a white exciplex OLED and cholesteric liquid crystal laser toward multifunctional devices

A new interface engineering method is demonstrated for the preparation of an efficient white organic light-emitting diode (WOLED) by embedding an ultrathin layer of the novel ambipolar red emissive compound 4,4-difluoro-2,6-di(4-hexylthiopen-2-yl)-1,3,5,7,8-pentamethyl-4-bora-3a,4a-diaza-s-indacene (bThBODIPY) in the exciplex formation region. The compound shows a hole and electron mobility of 3.3 × 10–4 and 2 × 10–4 cm2 V–1 s–1, respectively, at electric fields higher than 5.3 × 105 V cm–1. The resulting WOLED exhibited a maximum luminance of 6579 cd m–2 with CIE 1931 color coordinates (0.39; 0.35). The bThBODIPY dye is also demonstrated to be an effective laser dye for a cholesteric liquid crystal (ChLC) laser. New construction of the ChLC laser, by which a flat capillary with an optically isotropic dye solution is sandwiched between two dye-free ChLC cells, provides photonic lasing at a wavelength well matched with that of a dye-doped planar ChLC cell

Spin- and Voltage-dependent emission from Intra- and Intermolecular TADF OLEDs

Organic light emitting diodes (OLEDs) based on thermally activated delayed fluorescence (TADF) utilize molecular systems with a small energy splitting between singlet and triplet states. This can either be realized in intramolecular charge transfer states of molecules with near-orthogonal donor and acceptor moieties or in intermolecular exciplex states formed between a suitable combination of individual donor and acceptor materials. Here, we investigate 4,4'-(9H,9'H-[3,3'-bicarbazole]-9,9'-diyl)bis(3-(trifluoromethyl) benzonitrile) (pCNBCzoCF3), which shows intramolecular TADF but can also form exciplex states in combination with 4,4',4''-tris[phenyl(m-tolyl)amino]triphenylamine (m-MTDATA). Orange emitting exciplex-based OLEDs additionally generate a sky-blue emission from the intramolecular emitter with an intensity that can be voltage-controlled. We apply electroluminescence detected magnetic resonance (ELDMR) to study the thermally activated spin-dependent triplet to singlet up-conversion in operating devices. Thereby, we can investigate intermediate excited states involved in OLED operation and derive the corresponding activation energy for both, intra- and intermolecular based TADF. Furthermore, we give a lower estimate for the extent of the triplet wavefunction to be >1.2 nm. Photoluminescence detected magnetic resonance (PLDMR) reveals the population of molecular triplets in optically excited thin films. Overall, our findings allow us to draw a comprehensive picture of the spin-dependent emission from intra- and intermolecular TADF OLEDs.Comment: 9 pages, 5 figure

Thermally Activated Delayed Fluorescence in Organic Semiconductors and Its Application in Light-Emitting Diodes

The presence of the effect of thermally activated delayed fluorescence (TADF) in organic light-emitting materials (emitters), manifested in the "collecting" of triplet excitons in organic semiconductor complexes that do not contain noble metals, creates excellent prerequisites for the application of TADF materials in the technology of manufacturing organic light-emitting diodes (OLED). The significant progress in solving theoretical and technical problems, achieved in the process of development of highly efficient TADF materials, paves the way for the formation of the future of organic electronics. This review presents the analyses of the nature of the long-term fluorescence generation mechanism at the molecular level and the up-to-date strategies for designing TADF donor-acceptor materials, as well as exciplex intermolecular complexes. Special attention is focused on the analysis of TADF emitter ambipolar materials with a highly twisted, rigid molecular structure, which reveal a tendency towards the multi-channel emission mechanisms and their implementation in a variety of OLED structure architectures

InP/ZnS quantum dots synthesis and photovoltaic application

In the present paper hybrid core-shell InP/ZnS quantum dots were prepared by the one pot synthesis method which does not require additional component injections and which complies more with cost requirements. The synthesized quantum dots were characterized by X-ray diffraction and optical spectroscopy methods. The applicability of the synthesized InP/ZnS core-shell particles in inverted solar cells fabricated with a step-by-step procedure which combines thermal vacuum deposition and spin-coating techniques was investigated. The resulting efficiency of the fabricated inverted solar cell is comparable to that of quantum-dot sensitized TiO2 based solar cells. Therefore, hybrid core-shell InP/ZnS particles can be considered as multifunctional light-harvesting materials useful for implementation in different types of photovoltaic devices, such as quantum dot sensitized solar cells and inverted solar cells.Funding Agencies|Ministry of Education and Science of Ukraine [0121U109506, 0121U107533]; Swedish Research Council [2020-04600]</p

A novel donor-acceptor carbazole and benzothiadiazole material for deep red and infrared emitting applications

A novel organic material (C1) with the structure D-π-A-π-D was synthesised and characterised. Carbazole was utilised as the electron donor and benzothiadiazole as the electron acceptor unit. The electrochemical, optical and electronic properties of the synthesised compound were studied. Compound C1 exhibits absorption in the visible and ultraviolet range with a high molar absorption coefficient. A strong solvatochromic effect was observed in its emission spectra. Electrochemical and spectroelectrochemical measurements were performed in order to estimate the properties of the molecule in different redox states. Electron paramagnetic resonance (EPR) measurements indicate the delocalisation of radical cations and radical anions over different moieties. Interpretations of the electrochemical and optical results are supported by DFT calculations. OLEDs based on C1 present efficient emission in red and infrared spectral ranges, with a quantum efficiency of 3.13% and a current efficiency of 6.8 cd A-1. The performance is considerably better than what has been reported for analogous devices, based on other carbazole and benzothiadiazole units

Derivatives of 2-Pyridone Exhibiting Hot-Exciton TADF for Sky-Blue and White OLEDs

Development of emissivematerials for utilization inorganic light-emittingdiodes (OLEDs) remains a highly relevant research field. One of themost important aspects in the development of efficient emitters forOLEDs is the efficiency of triplet-to-singlet exciton conversion.There are many concepts proposed for the transformation of tripletexcitons to singlet excitons, among which thermally activated delayedfluorescence (TADF) is the most efficient and widespread. One of thevariations of the TADF concept is the hot exciton approach accordingto which the process of exciton relaxation into the lowest energyelectronic state (internal conversion as usual) is slower than intersystemcrossing between high-lying singlets and triplets. In this paper,we present the donor-acceptor materials based on 2-pyridoneacceptor coupled to the different donor moieties through the phenyllinker demonstrating good performance as components of sky-blue, green-yellow,and white OLEDs. Despite relatively low photoluminescence quantumyields, the compound containing 9,9-dimethyl-9,10-dihydroacridinedonor demonstrated very good efficiency in sky-blue OLED with thesingle emissive layer, which showed an external quantum efficiency(EQE) of 3.7%. It also forms a green-yellow-emitting exciplex with4,4 &amp; PRIME;,4 &amp; DPRIME;-tris[phenyl(m-tolyl)amino]triphenylamine.The corresponding OLED showed an EQE of 6.9%. The white OLED combiningboth exciplex and single emitter layers demonstrated an EQE of 9.8%together with excellent current and power efficiencies of 16.1 cdA(-1) and 6.9 lm W-1, respectively.Quantum-chemical calculations together with the analysis of photoluminescencedecay curves confirm the ability of all of the studied compounds toexhibit TADF through the hot exciton pathway, but the limiting factorreducing the efficiency of OLEDs is the low photoluminescence quantumyields caused mainly by nonradiative intersystem crossing dominatingover the radiative fluorescence pathway.Funding Agencies|Ministry of Education and Science of Ukraine [0123U101690, 0121U107533]; Swedish Research Council [2022-06725]; Swedish Research Council [2020-04600]; European Social Fund [09.3.3-LMT-K-712-23-0125]; Research Council of Lithuania (LMTLT)</p

Three-terminal light-emitting device with adjustable emission color

A three-terminal organic light-emitting device with a periodic interrupted middle electrode is developed to allow for an adjustable emission color. The emission results from three independent light-emitting diodes with one diode utilizing exciplex emission. An equivalent electrical circuit is suggested taking the current–voltage characteristics and the direction of current flow through the organic structure into account. Two diodes are formed between the embedded middle electrode and the LiF/Al top and ITO bottom electrode, respectively, and the third diode utilizes that part of the device without the middle-electrode exhibiting exciplex emission. It will be shown that the spectrum of the emitted light can be tuned from blue to orange by controlling the applied potentials to the device terminals

Deep-Blue High-Efficiency TTA OLED Using <i>Para</i>- and <i>Meta</i>-Conjugated Cyanotriphenylbenzene and Carbazole Derivatives as Emitter and Host

Elaboration of the appropriate host materials proved to be not less important for the fabrication of a highly efficient OLED than the design of emitters. In the present work, we show how by simple variation of molecular structure both blue emitters exhibiting delayed fluorescence and ambipolar high triplet energy hosts can be obtained. The compounds with a <i>para</i>-junction revealed higher thermal stability (<i>T</i>_ID up to 480 °C), lower ionization potentials (5.51–5.60 eV), exclusively hole transport, and higher photoluminescence quantum efficiencies (0.90–0.97). <i>Meta</i>-linkage leads to ambipolar charge transport and higher triplet energies (2.82 eV). Introduction of the accepting nitrile groups in the <i>para</i>-position induces intensive delayed fluorescence via a triplet–triplet annihilation up-conversion mechanism. By utilization of the <i>para</i>-substituted derivative as an emitter and the <i>meta</i>-substituted isomer as the host, a deep-blue OLED with the external quantum efficiency of 14.1% was fabricated