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

The Effect of Acceptor Structure on Emission Color Tuning in Organic Semiconductors with D–π–A–π–D Structures

A series of novel donor–acceptor D–π–A–π–D compounds were synthesized and characterized in order to determine the influence of different acceptor units on their properties. The introduction of acceptor moieties had a direct impact on the HOMO and LUMO energy levels. Fluorescence spectra of compounds can be changed by the choice of an appropriate acceptor and were shifted from the green to the near-infrared part of spectra. Due to observed concentration induced emission quenching, the green exciplex type host was used to evaluate the potential of synthesized molecules as emitters in organic light emitting diodes (OLEDs)

Realizing 20% External Quantum Efficiency in Electroluminescence with Efficient Thermally Activated Delayed Fluorescence from an Exciplex

Herein we report the investigation of non-doped exciplex formed in blends of 2,4,6-Tris[3-(diphenylphosphinyl)phenyl]-1,3,5-triazine (PO-T2T), working as the one-electron acceptor molecule, with different electron donors. The emission of these exciplexes spans from blue to orange-red, and shows clear contribution from thermally activated delayed fluorescence (TADF). We focus our studies on the properties of TADF in these systems, covering the physical meaning of the different transient components observed in the luminescence decays of these blends. Our results unravel the intricate role of reverse intersystem crossing due to spin-orbit coupling and hyperfine interactions and internal conversion, which affect the efficiency of the TADF mechanism. Organic light-emitting diodes fabricated with these blends show in some cases remarkable performances. Green exciplex blends, in particular, exhibited the current efficiency of 60 cd A-1, power efficiency of 71 lm W-1 and external quantum efficiency of 20%. We believe our results will contribute significantly to highlight the potential advantages of intermolecular exciplexes in the area of organic light-emitting diodes

MAPb(Br1–xClx)3 Hybrid Perovskite Materials for Direct X-ray Detection

International audienceThe study and development in recent years of hybrid (organic−inorganic) halide perovskite materials have given them an unprecedented opportunity for direct ionizing radiation detection, given their large attenuation coefficient and sufficient charge carrier mobility lifetime product. The use of single crystals, considered as model materials, allows us to investigate their intrinsic properties. Characterizations under X-ray illumination of detector devices based on methylammonium lead tribromide (MAPbBr 3) single crystals, obtained by optimized growths, show good sensitivity but high dark current density. To improve this critical parameter, while using MAPbBr 3 as the base material, we employ anion engineering within the halide elements. We present here mixed halide perovskite crystals, with bromide partially replaced with chloride, obtained through optimized growths using modified inverse temperature crystallization in dimethylformamide, leading to high-quality single crystals of the general formula MAPb(Br 1−x Cl x) 3. Six chlorine contents are targeted and carefully determined experimentally via energy-dispersive X-ray analysis and X-ray powder diffraction. For each composition, several crystals are synthesized and used to prepare X-ray detection devices. Their optoelectronic properties are determined under standard X-ray medical conditions and hint at the existence of an optimal composition. MAPb(Br 0.85 Cl 0.15) 3 exhibits the best sensitivity with a value of S ≈ 3 μC mGy air −1 cm −2 for RQA5 spectral quality and the lowest dark current density with a value of J dark ≈ 22 nA mm −2 , both recorded at a 50 V mm −1 electric field. This sensitivity value doubles our own MAPbBr 3 single crystal device and is higher than that of CsI(Tl)-or a-Se-based flat panels. The present work broadens the benefits and drawbacks of employing halide engineering in perovskite materials to improve the optoelectronic performance under high-energy radiation

Electronic Structure of Exciplexes and the Role of Local Triplet States on Efficiency of Thermally Activated Delayed Fluorescence

In this work, we present an investigation of the electronic states in a series of thermally activated delayed fluorescence (TADF) exciplexes formed with the popular electron-transport compound TpBpTa and hole-transporting TCTA, TAPC, TPD10, TPD, and NPB. We rationalize the photophysical behavior of exciplexes by using computational methods and demonstrate that the reason for the commonly observed temporal red shift in the time-resolved spectra is related to the distribution of molecular conformations, thus CT energy, in film. We also use spectrally resolved thermoluminescence (SRTL) measurements to give insight into the trapping phenomena in exciplex blends. The results demonstrate that trapped charge carriers in the majority of studied exciplexes recombine through the luminescent intermolecular CT state. In addition, we report OLED devices using the said exciplexes in the emissive layer. The best performance is obtained with the TCTA:TpBpTa and TAPC:TpBpTa exciplexes showing maximum external quantum efficiencies (EQEs) of 8.8% and 7.2%, respectively

Coupling between Ion Drift and Kinetics of Electronic Current Transients in MAPbBr 3 Single Crystals

International audienceThe optoelectronic properties of halide perovskite materials have fostered their utilization in many applications. Unravelling their working mechanisms remains challenging because of their mixed ionic–electronic conductive nature. By registering, with high reproducibility, the long-time current transients of a set of single-crystal methylammonium lead tribromide samples, the ion migration process was proved. Sample biasing experiments (ionic drift), with characteristic times exhibiting voltage dependence as ∝ V–3/2, is interpreted with an ionic migration model obeying a ballistic-like voltage-dependent mobility (BVM) regime of space-charge-limited current. Ionic kinetics effectively modify the long-time electronic current, while the steady-state electronic currents’ behavior is nearly ohmic. Using the ionic dynamic doping model (IDD) for the recovering current at zero bias (ion diffusion), the ionic mobility is estimated to be ∼10–6 cm2 V–1 s–1. Our findings suggest that ionic currents are negligible in comparison to the electronic currents; however, they influence them via changes in the charge density profile