Molecules In Organic Light Emitting Diodes Research Articles

Role of Spin Polarization and Dynamic Correlation in Singlet-Triplet Gap Inversion of Heptazine Derivatives.

The new generation of proposed light-emitting molecules for organic light-emitting diodes (OLEDs) has raised considerable research interest due to its exceptional feature─a negative singlet-triplet (ST) gap violating Hund's multiplicity rule in the excited S1 and T1 states. We investigate the role of spin polarization in the mechanism of ST gap inversion. Spin polarization is associated with doubly excited determinants of certain types, whose presence in the wave function expansion favors the energy of the singlet state more than that of the triplet. Using a perturbation theory-based model for spin polarization, we propose a simple descriptor for prescreening of candidate molecules with negative ST gaps and prove its usefulness for heptazine-type molecules. Numerical results show that the quantitative effect of spin polarization decreases linearly with the increasing highest occupied molecular orbital-lowest unoccupied molecular orbital (HOMO-LUMO) exchange integral. Comparison of single- and multireference coupled-cluster predictions of ST gaps shows that the former methods provide good accuracy by correctly balancing the effects of doubly excited determinants and dynamic correlation. We also show that accurate ST gaps may be obtained using a complete active space model supplemented with dynamic correlation from multireference adiabatic connection theory.

Orientation distributions of vacuum-deposited organic emitters revealed by single-molecule microscopy

The orientation of luminescent molecules in organic light-emitting diodes strongly influences device performance. However, our understanding of the factors controlling emitter orientation is limited as current measurements only provide ensemble-averaged orientation values. Here, we use single-molecule imaging to measure the transition dipole orientation of individual emitter molecules in a state-of-the-art thermally evaporated host and thereby obtain complete orientation distributions of the hyperfluorescence-terminal emitter C545T. We achieve this by realizing ultra-low doping concentrations (10−6 wt%) of C545T and minimising background levels to reliably measure its photoluminescence. This approach yields the orientation distributions of >1000 individual emitter molecules in a system relevant to vacuum-processed devices. Analysis of solution- and vacuum-processed systems reveals that the orientation distributions strongly depend on the nanoscale environment of the emitter. This work opens the door to attaining unprecedented information on the factors that determine emitter orientation in current and future material systems for organic light-emitting devices.

Theoretical Insights into the Optical and Excited State Properties of Donor–Phenyl Bridge–Acceptor Containing Through-Space Charge Transfer Molecules

A comparative new strategy to enhance thermally activated delayed fluorescence (TADF) of through-space charge transfer (CT) molecules in organic light-emitting diodes (OLEDs) is investigated. Generally, TADF molecules adopt a twisted donor and acceptor structure to get a sufficiently small ΔEST and a higher value of the spin-orbit coupling matrix element (SOCME). However, molecules containing donor-phenyl bridge-acceptor (D-p-A) units and featuring π-stacked architectures have intramolecular CT contribution through space and exhibit high TADF efficiency. We have explored the insights into the TADF mechanism in D-p-A molecules using the density functional theory (DFT) and time-dependent DFT methods. The calculated optical absorption and ΔEST values are found to be in good agreement with available experimental data. Interestingly, we found the origin of the SOCME to be the twisted orientation of the donor and bridge moieties. Also, we predicted similar molecules with enhanced OLED efficiency with different substitutions.

Deep learning for development of organic optoelectronic devices: efficient prescreening of hosts and emitters in deep-blue fluorescent OLEDs

The highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energies, which are key factors in optoelectronic devices, must be accurately estimated for newly designed materials. Here, we developed a deep learning (DL) model that was trained with an experimental database containing the HOMO and LUMO energies of 3026 organic molecules in solvents or solids and was capable of predicting the HOMO and LUMO energies of molecules with the mean absolute errors of 0.058 eV. Additionally, we demonstrated that our DL model was efficiently used to virtually screen optimal host and emitter molecules for organic light-emitting diodes (OLEDs). Deep-blue fluorescent OLEDs, which were fabricated with emitter and host molecules selected via DL prediction, exhibited narrow emission (bandwidth = 36 nm) at 412 nm and an external quantum efficiency of 6.58%. Our DL-assisted virtual screening method can be further applied to the development of component materials in optoelectronics.

The optical spectra of DMAC-based molecules for organic light-emitting diodes: Hybrid-exchange density functional theory study.

Organic light‐emitting diodes (OLED) have considerable advantages over the conventional counterpart. Molecular design by simulations is important for the discovery of new material candidate to improve the performance of OLED. Recently, thermally assisted delayed fluorescence OLED based on DMAC (9,9‐dimethyl‐9,10‐dihydroacridine)‐related molecules have been found to have superior performance. In this work, a series of first‐principles calculations are performed on DMAC‐DPS (diphenylsulfone, emission of blue‐color light), DMAC‐BP (benzophenone, green), DMAC‐DCPP (dicyclohexylphosphonium, red), and the newly designed DMAC‐BF (enaminone difluoroboron complexes, red) molecules, based on time‐dependent density‐functional theory, the hybrid‐exchange density functional, and the long‐range corrected hybrid‐exchange density functional. By varying the percentage of Hartree–Fock (HF) exchange in the hybrid‐exchange functional, the emission spectra can be over 97% fitted to the experimental results. We found that the fitted proportion of HF will increase as the wavelengths of the molecules decrease (30% for DPS, 20% for BP, and 10% for DCPP). By contrast, the long‐range corrected hybrid‐exchange density functional can lead to a good estimate on the absorption spectra. In addition, we have also applied our fitting computational procedure to the newly designed molecule. The molecular orbitals involved in the related excited states have also been investigated for these molecules, which show a common charge‐transfer characteristic between the acceptor part (DPS/BP/DCPP/BF) and the donor (DMAC).

Persistent Dimer Emission in Thermally Activated Delayed Fluorescence Materials.

We expose significant changes in the emission color of carbazole-based thermally activated delayed fluorescence (TADF) emitters that arise from the presence of persistent dimer states in thin films and organic light-emitting diodes (OLEDs). Direct photoexcitation of this dimer state in 1,2,3,5-tetrakis(carbazol-9-yl)-4,6-dicyanobenzene (4CzIPN) reveals the significant influence of dimer species on the color purity of its photoluminescence and electroluminescence. The dimer species is sensitive to the sample preparation method, and its enduring presence contributes to the widely reported concentration-mediated red shift in the photoluminescence and electroluminescence of evaporated thin films. This discovery has implications on the usability of these, and similar, molecules for OLEDs and explains disparate electroluminescence spectra presented in the literature for these compounds. The dimerization-controlled changes observed in the TADF process and photoluminescence efficiency mean that careful consideration of dimer states is imperative in the design of future TADF emitters and the interpretation of previously reported studies of carbazole-based TADF materials.

Investigating the molecular orientation of Ir(ppy)3 and Ir(ppy)2(acac) emitter complexes by X-ray diffraction

We study thermally evaporated thin films of Ir(ppy)3 and Ir(ppy)2(acac) by means of grazing incidence X-ray diffraction (GIXRD) and grazing incidence wide-angle X-ray scattering (GIWAXS). Ir(ppy)3 and Ir(ppy)2(acac) are both widely used as phosphorescent green emitter molecules in organic light-emitting diodes (OLEDs) and it was previously found that differences in their average transition dipole orientation affect the light extraction efficiency in OLEDs. Here we show that in pure films both materials form crystalline grains and that these grains exhibit a preferred orientation with respect to the substrate. When doped into an amorphous host, both the orientation and formation of the crystallites remain nearly unchanged for the concentration range accessible with GIXRD and GIWAXS. This is remarkable given that the transition dipole moments have found to be oriented only for Ir(ppy)2(acac) but isotropic for Ir(ppy)3. Analysis of the crystallite size indicates that the tendency to form crystallites is stronger for Ir(ppy)3 than for Ir(ppy)2(acac). From a comparison of the thin-film diffraction data of Ir(ppy)3 to its powder pattern, we infer that Ir(ppy)3 molecules are oriented with their permanent dipole moment roughly parallel to the substrate. Our findings will guide the further understanding of the mechanisms controlling transition dipole orientation and may thus lead to further improvements in device efficiency.

Efficient emitting molecules in organic light-emitting diodes on the basis of the control of vibronic couplings

A high external quantum efficiency observed in organic light-emitting diodes using PTZ-BZP (PTZ: 10-hexyl-phenothiazin, and BZP: 4-phenyl-2,1,3-benzothiadiazole) as an emitting molecule is studied based on the vibronic coupling density analyses employing the time-dependent density functional theory. The high efficiency can be attributed to a high efficiency of exciton generation which originates from reverse intersystem crossings from the higher T3 and T2 states than T1 to the emitting S1 state. The nonradiative transitions from T3 and T2 to the lower triplet states are suppressed because of small offdiagonal vibronic coupling constants. The small vibronic couplings originate from the multi-configurationality of these triplet states. A molecular design principle for efficient emitting molecules is discussed from the view of the control of vibronic couplings.

Phosphorescence lifetimes of organic light-emitting diodes from two-component time-dependent density functional theory.

"Spin-forbidden" transitions are calculated for an eight-membered set of iridium-containing candidate molecules for organic light-emitting diodes (OLEDs) using two-component time-dependent density functional theory. Phosphorescence lifetimes (obtained from averaging over relevant excitations) are compared to experimental data. Assessment of parameters like non-distorted and distorted geometric structures, density functionals, relativistic Hamiltonians, and basis sets was done by a thorough study for Ir(ppy)3 focussing not only on averaged phosphorescence lifetimes, but also on the agreement of the triplet substate structure with experimental data. The most favorable methods were applied to an eight-membered test set of OLED candidate molecules; Boltzmann-averaged phosphorescence lifetimes were investigated concerning the convergence with the number of excited states and the changes when including solvent effects. Finally, a simple model for sorting out molecules with long averaged phosphorescence lifetimes is developed by visual inspection of computationally easily achievable one-component frontier orbitals.

Different orientation of the transition dipole moments of two similar Pt(II) complexes and their potential for high efficiency organic light-emitting diodes

The efficiency of organic light-emitting diodes (OLEDs) is especially limited by their low light outcoupling efficiency. An approach for its enhancement is the use of horizontally oriented emitter molecules with respect to the substrate. In this study we quantitatively determine the orientation of the optical transition dipole moments in doped films of two similar phosphorescent Pt(II) complexes having a linear molecular structure. These emitters are employed in OLED devices and their efficiency is analyzed by optical simulations. For an OLED with slightly more horizontally oriented emitter molecules an external quantum efficiency (ηEQE) of 15.8% at low current-density is realized, indicating a relative improvement of outcoupling efficiency of 5.3% compared to the isotropic case. However, a very similar complex adopting isotropic molecular orientation yields ηEQE of only 11.5% implying an imperfect charge carrier balance in the OLED device and a shift of the recombination zone. Furthermore, we highlight the enormous potential of horizontal molecular orientation of emitting molecules in OLEDs.

Preparation of Organic Light-Emitting Diode Using Coal Tar Pitch, a Low-Cost Material, for Printable Devices

We have identified coal tar pitch, a very cheap organic material made from coal during the iron-making process, as a source from which could be obtained emissive molecules for organic light-emitting diodes. Coal tar pitch was separated by simple dissolution in organic solvent, and subsequent separation by preparative thin-layer chromatography was used to obtain emissive organic molecules. The retardation factor of preparative thin-layer chromatography played a major role in deciding the emission characteristics of the solution as photoluminescence spectra and emission-excitation matrix spectra could be controlled by modifying the solution preparation method. In addition, the device characteristics could be improved by modifying the solution preparation method. Two rounds of preparative thin-layer chromatography separation could improve the luminance of organic light-emitting diodes with coal tar pitch, indicating that less polar components are favorable for enhancing the luminance and device performance. By appropriate choice of the solvent, the photoluminescence peak wavelength of separated coal tar pitch could be shifted from 429 nm (cyclohexane) to 550 nm (chloroform), and consequently, the optical properties of the coal tar pitch solution could be easily tuned. Hence, the use of such multicomponent materials is advantageous for fine-tuning the net properties at a low cost. Furthermore, an indium tin oxide/poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate)/coal tar pitch/LiF/Al system, in which the emissive layer was formed by spin-coating a tetrahydrofuran solution of coal tar pitch on the substrate, showed a luminance of 176 cd/m2. In addition, the emission spectrum of coal tar pitch was narrowed after the preparative thin-layer chromatography process by removing the excess emissive molecules.

Storage of charge carriers on emitter molecules in organic light-emitting diodes

Organic light-emitting diodes (OLEDs) using the red phosphorescent emitter iridium(III)bis(2-methyldibenzo[f,h]quinoxaline) (acetylacetonate) [$\mathrm{Ir}{(\mathrm{MDQ})}_{2}(\mathrm{acac})$] are studied by time-resolved electroluminescence measurements. A transient overshoot after voltage turn-off is found, which is attributed to electron accumulation on $\mathrm{Ir}{(\mathrm{MDQ})}_{2}(\mathrm{acac})$ molecules. The mechanism is verified via impedance spectroscopy and by application of positive and negative off-voltages. We calculate the density of accumulated electrons and find that it scales linearly with the doping concentration of the emitter. Using thin quenching layers, we locate the position of the emission zone during normal OLED operation and after voltage turn-off. In addition, the transient overshoot is also observed in three-color white-emitting OLEDs. By time- and spectrally resolved measurements using a streak camera, we directly attribute the overshoot to electron accumulation on $\mathrm{Ir}{(\mathrm{MDQ})}_{2}(\mathrm{acac})$. We propose that similar processes are present in many state-of-the-art OLEDs and believe that the quantification of charge carrier storage will help to improve the efficiency of OLEDs.

Synthesis of new light-emitting donor–acceptor materials: Isomers of phenothiazine–quinoline molecules

Highly fluorescent isomeric donor–acceptor molecules, 2-(4-phenyl-2-quinolyl)-10-methylphenothiazine (2PQMPT) and 3-(4-phenyl-2-quinolyl)-10-methylphenothiazine (3PQMPT), were synthesized in high yields (>85%) and found to exhibit a factor of 2.4 disparity in electroluminescence efficiency. Bright (10,000–25,000 cd/m 2) and efficient (3.6–8.7 cd/A) green organic light-emitting diodes were realized from the two donor–acceptor molecules. The new molecules are good model systems for the study of structure–property relationships of donor–acceptor molecules for organic light-emitting diodes.

High-luminance and high-efficiency organic electroluminescent devices with a doped co-host emitter system

TDCJTB, BMDIN and TDIN have been synthesized for using as red fluorescent dye molecules in organic light-emitting diodes (OLEDs). The Commission International d'Eclairage (CIE) coordinates and PL peaks of the devices are [0.61, 0.39], [0.65, 0.35], [0.62, 0.38] and 579 nm, 619 nm, 597 nm, respectively. Compared with traditional DCM ([0.58, 0.41], 575 nm) the three devices have better brightness and CIE coordinates. When TBQ is doped in the three materials the EL efficiency can all be improved at different levels. BMDIN as a high efficient red organic light-emitting material based on this series of fluorescent dyes doped in co-host emitter (CHE) systems of TBQ/poly( N-vinylcarbazole) (PVK) can achieve a higher luminescent efficiency and brightness than the other materials. Present research work shows that when TBQ's concentration is 30%, the maximum EL efficiency is 6.1 cd/A at 40 mA/cm 2.

Study of the characteristics of new red dopants in OLED

New red dopants TDCJTB and TDIN have been synthesized for using as red fluorescent dye molecules in organic light-emitting diodes (OLEDs). Red-light-emitting devices have been fabricated with a structure of ITO/PVK: dopant/2, 9-dimethyl-4, 7 diphenyl-1, 10-phenanthroline (BCP)/Alq 3/LiF/Al. The Commission International d’Eclairage (CIE) coordinates and PL peaks of the devices are [0.61, 0.39], [0.62, 0.38] and 579, 597 nm, respectively. The maximum EL brightness of TDCJTB and TDIN reached 1311 cd/m 2 (18 V) and 2497 cd/m 2 (16 V), respectively. Compared with traditional DCM ([0.58, 0.41], 1032 cd/m 2 (18 V)) the two devices have better CIE coordinates and brightness. When TBQ is doped in these materials, the EL efficiency can both be improved at different level. Present research work shows that when TBQs concentration is 30%, the maximum EL efficiency TDCJTB and TDIN is 2.57 and 2.54 cd/A at 40 mA/cm 2, respectively.

Synthesis and characterization of high molecular weight metaloquinolate‐containing polymers

AbstractIn this article, a new method to synthesize novel metaloquinolate (AlQ3, ZnQ2)‐containing polymers is reported. A model polymer with 8‐hydroxyquinoline ligands can be obtained by free‐radical copolymerization with methyl methacrylate (MMA), then metaloquinolate (AlQ3, ZnQ2)‐containing polymers are prepared by coordinating reaction with di(8‐hydroxyquinoline) aluminum (AlQ2) chelates or mono (8‐hydroxyquinoline) zinc (ZnQ) chelates without crosslinking. The structures of products are confirmed by NMR, FTIR, ultraviolet‐visible, elementary analysis, photoluminescence spectrum, and gel permeation chromatography analysis. They are soluble in common solvents and suitable to form films. The use of AlQ2 and ZnQ avoided the crosslinking caused by the AlQ3, ZnQ2 formation between different polymer chains. Different from the traditional small organic molecules in organic light‐emitting diodes (OLEDs) fabrication, the polymer can be processed by spin coating without phase separation. Compared to the PMMA or MMA‐co‐HEMA‐CH2‐Hq, the Tg of the metaloquinolate‐containing polymers was much higher. It should be of interest for OLED applications. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 99: 1945–1952, 2006

Spatially selective flash sublimation of small organic molecules for organic light-emitting diodes and display applications

A flash-sublimation technique for the spatially selective deposition of small organic molecules is presented. Single-pulse electrically heated copper stripes (width 100 μm) serve as heating elements. The relevant time scale of our technique is on the order of milliseconds. Under high vacuum conditions, the heating elements are used to locally flash-sublimate small-molecule material from a previously coated polyimide foil onto a (ITO-)glass substrate, positioned at a vertical distance of 60 μm. The spatial resolution in our nonoptimized experiments was 250 μm. Organic light-emitting diode (OLED) devices with flash-deposited tris-(8-hydroxyquinoline)aluminum (Alq3) as the active material are demonstrated with satisfying electrical and optical properties. Flash sublimation of a stacked nonintermixed Alq3 (30 nm) [2-methyl-6-[2-(2,3,6,7-tetrahydro-1H,5Hbenzo[ij]quinolizin-9-yl)ethenyl]-4H-pyran-4-ylidene]propane-dinitrile (DCM2) (1 nm)/Alq3 (30 nm) layer is shown to yield a red-emitting (λ=624 nm) DCM2-doped Alq3 layer. Our technique presents a simple alternative to the use of shadow masks for full-color small-molecule OLED displays.