Efficient OLEDs Based on Slot-Die-Coated Multicomponent Emissive Layer

The optimization of multicomponent emissive layer (EML) deposition by slot-die coating for organic light-emitting diodes (OLEDs) is presented. In the investigated EMLs, the yellow-green iridium complex (Ir) was doped in two types of host: a commonly used mixture of poly(N-vinylcarbazole) (PVK) with oxadiazole derivative (PBD) or PVK with thermally activated delayed fluorescence-assisted dopant (10-(4-(4,6-diphenyl-1,3,5-triazin-2-yl)phenyl)-10H-spiro[acridine-9,9′-fluorene], SpiroAC-TRZ). In this article, OLEDs with EML prepared in air by slot-die coating, facilitating industrial manufacturing, are confronted with those with spin-coated EML in nitrogen. OLEDs based on PVK:PBD + 2 wt.% Ir-dopant exhibit comparable performance: ~13 cd A−1, regardless of the used method. The highest current efficiency (21 cd A−1) is shown by OLEDs based on spin-coated PVK with 25 wt.% SpiroAC-TRZ and 2 wt.% Ir-dopant. It is three times higher than the efficiency of OLEDs with slot-die-coated EML in air. The performance reduction, connected with the adverse oxygen effect on the energy transfer from TADF to emitter molecules, is minimized by the rapid EML annealing in a nitrogen atmosphere. This post-treatment causes more than a doubling of the OLED efficiency, from 7 cd A−1 to over 15 cd A−1. Such an approach may be easily implemented in other printing techniques and result in a yield enhancement

Effect of TADF Assistance on Performance Enhancement in Solution Processed Green Phosphorescent OLEDs

Many methods have been proposed to increase the efficiency of organic electroluminescent materials applied as an emissive layer in organic light emitting diodes (OLEDs). Herein, we demonstrate enhancement of electroluminescence efficiency and operational stability solution processed OLEDs by employing thermally activated delayed fluorescence (TADF) molecules as assistant dopants in host-guest systems. The TADF assistant dopant (SpiroAC–TRZ) is used to facilitate efficient energy transfer from host material poly(N–vinylcarbazole) (PVK) to a phosphorescent Ir(III) emitter. We present the analysis of energy transfer and charge trapping—two main processes playing a crucial role in light generation in host–guest structure OLEDs. The investigation of photo-, electro- and thermoluminescence for the double-dopant layer revealed that assistant dopant does not only harvest and transfer the electrically generated excitons to phosphorescent emitter molecules but also creates exciplexes. The triplet states of formed PVK:SpiroAC–TRZ exciplexes are involved in the transport process of charge carriers and promote long–range exciton energy transfer to the emitter, improving the efficiency of electroluminescence in a single emissive layer OLED, resulting in devices with luminance exceeding 18 000 cd/m2 with a luminous efficiency of 23 cd/A and external quantum efficiency (EQE) of 7.4%

Photogeneration and transport in thin films of p- and n-type discotic liquid crystals

Thin layers of discotic liquid crystals, p-type: HBC-C8,2, HBC-C12 and n-type: HATNA 4D, RDlSCl and perylene derivative were prepared by drop- and zone-casting. It is shown, that zone-casting yields aligned, anisotropic films with "edge-on" orientation suitable for FET construction. In the isotropic, obtained by drop-casting films the charge-carrier photogeneration, recombination and transport were investigated by means of xerographic discharge. For the first time it is demonstrated experimentally that in the nominally n-type discotics the majority charge carriers are electrons.SCOPUS: cp.jinfo:eu-repo/semantics/publishe

Charge Carrier Transport in Layers of Discotic Liquid Crystals as Studied by transient photocurrents

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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

Structural, Spectroscopic, Electrochemical, and Electroluminescent Properties of Tetraalkoxydinaphthophenazines: New Solution-Processable Nonlinear Azaacenes

A series of solution-processable tetraalkoxy-substituted dinaphtho­[2,3-<i>a</i>:2′,3′-<i>h</i>]­phenazines were synthesized by reductive functionalization of indanthrone (6,15-dihydrodinaphtho­[2,3-<i>a</i>:2′,3′-<i>h</i>]­phenazine-5,9,14,18-tetraone), an old intractable dye. The melting point of these new compounds was found to decrease from 204 °C to 98 °C upon extension of the number of carbons from 4 to 12 in the alkoxy substituent. All derivatives show a strong tendency to self-organize in 2D as evidenced by STM investigations of monolayers deposited on HOPG. The 2D structure is less dense and shows different alkoxy group interdigitation pattern as compared to the 3D structure determined from the X-ray diffraction data obtained for the corresponding single crystals. Electrochemical, absorption, and emission properties of tetraalkoxy-substituted dinaphtho­[2,3-<i>a</i>:2′,3′-<i>h</i>]­phenazines, studied in solution, are essentially independent of the length of the alkoxy substituents. All derivatives exhibit high photoluminescence quantum yield, approaching 60%. When molecularly dispersed in a solid matrix consisting of poly­(9-vinylcarbazole) (PVK) (60 wt %) and (2-<i>tert</i>-butylphenyl-5-biphenyl-1,3,4-oxadiazole) (PBD) (40 wt %) (so-called “guest/host configuration”), they show green electroluminescence due to an effective energy transfer from the matrix to the luminophore. The best light-emitting diodes were obtained for the butoxy derivative showing a luminance approaching 1500 cd/m² and a luminous efficiency over 0.8 cd/A