Solution-processed Blue Organic Light-emitting Diodes Research Articles

Donor-σ-acceptor molecules with alkyl σ-linkers of different lengths: Exploration on the exciplex emission and their use for efficient organic light-emitting diodes

Donor-σ-acceptor molecules featuring exciplex emission have been extensively researched for developing thermally-activated delayed fluorescence; however, varying the length of alkyl σ-linker to explore the exciplex emission and to develop efficient exciplex emitters has not been fully conducted. Herein, we report donor-σ-acceptor exciplex emitters (1–3) developed with 9,9-dimethyl-10-phenyl-9,10-dihydroacridine donor, 2,4,6-triphenyl-1,3,5-triazine acceptor, and alkyl σ-linkers of different lengths (−CH2− for 1, –CH2CH2− for 2, and –CH2CH2CH2− for 3). The chemical structures of 1–3 have been verified by X-ray crystallography. Theoretical calculations reveal that intramolecular through-space charge transfer is the dominant charge-transfer mechanism within the molecules. In dilute solution and the films with low concentration of the dopant, intramolecular exciplexes are formed, whereas in the films with high concentration of the dopant, both intra- and inter-molecular exciplexes are formed. 1–3 all show aggregation-induced emission properties. In heavily doped or neat films, 1–3 show sky-blue emission with photoluminescent efficiency of up to 0.70 and high reverse intersystem crossing rates of up to 5.1 × 106 s−1. It is shown that varying the length of alkyl σ-linker regulates the thermally-activated delayed fluorescence of the exciplex formed in neat film. Vacuum-evaporated and solution-processed organic light-emitting diodes using 1–3 as the emitters show sky-blue and blue electroluminescence with peak external quantum efficiencies of up to 14.5% and 13.0%, respectively. The efficiency is among the highest for solution-processed blue organic light-emitting diodes based on small-molecule exciplex emitters reported so far.

Highly efficient solution-processed blue organic light-emitting diodes based on thermally activated delayed fluorescence emitters with spiroacridine donor

High-efficiency solution-processed blue organic light-emitting diodes (OLEDs) were developed using two thermally activated delayed fluorescence (TADF) aromatic molecules, 10-(4-(4,6-diphenyl-1,3,5-triazin-2-yl)-2,5-dimethylphenyl)-10H-spiro[acridine-9,9′-fluorene] (TXSA) and 10-(4-(4,6-diphenyl-1,3,5-triazin-2-yl)-2-methylphenyl)-10H-spiro[acridine-9,9′-fluorene] (TTSA), composed of spiroacridine donor and triazine acceptor units. As a result, the blue devices based on two novel TADF molecules exhibited remarkable electroluminescence with a high quantum efficiency of 14.94% and current efficiency of 29.29cd/A by optimization of emitter doping concentration and properties of the electron transporting layer. Our results demonstrate that TXSA and TTSA TADF molecules are prospective materials to fabricate high-performance solution-processed blue OLEDs with a simple device structure.

Efficient solution-processed blue and white OLEDs based on a high-triplet bipolar host and a blue TADF emitter

Abstract In this study, solution-processed blue organic light-emitting diodes (OLEDs) were constructed by employing POBPCz as a bipolar host material with a high triplet of 3.01 eV and 2CzPN as the blue TADF emitter. A maximum EQE of 12.9% was realized in the blue TADF OLEDs, along with a satisfactory CIE coordinate of (0.19, 0.37). These results were among the best ones for the 2CzPN-based solution processed blue OLEDs. Moreover, solution-processed white OLEDs (WOLEDs) with a single-emitting layer were fabricated by Ir(MDQ)2 (acac) as the orange-red iridium complex. The optimized WOLEDs could achieve a maximum EQE of 12.4%, as well as the stable electroluminescence spectra. Notably, the white-light emission was achieved by the incomplete energy transfer from the blue TADF emitter of 2CzPN to the orange-red emitter of Ir (MDQ)2 (acac) via Forster energy transfer (FRET) process.

Fully solution-processed and multilayer blue organic light-emitting diodes based on efficient small molecule emissive layer and intergrated interlayer optimization

Efficient and fully solution-processed blue organic light-emitting diodes (OLEDs) based on fluorescent small-molecule and methanol/water soluble conjugated polymer as electron-injection material are reported. The emitting layer is 3,6-bis(9,9,9′,9′-tetrakis (6-(9H-carbazol-9-yl)hexyl)-9H,9′H-[2,2′-bifluoren]-7-yl)dib-nzo[b, d]thiophene 5, 5-dioxide (OCSoC) with a blue-fluorescent small-molecule, and a methanol/water soluble polymer poly[(9,9-bis(30-(N,N-dimethylamino)propyl)-2,7-fluorene)-alt-2,7-(9,9-dioctyl-fluorene)] (PFN) acted as electron-injection layer (EIL). All the organic layers are spin-coated from solution. The multilayer device structure with emitting layer/electron-injection layer is achieved by solution-processed method without the dissolution problem between layers. The performances of the devices show that the maximum luminous efficiency of the multilayer device is increased about 43%, compared to the single-layer device. PFN acting as the EIL material plays a key role in the improvement of the device performance when used in solution-processed small-molecule OLEDs.

Synthesis and Photoelectrical Properties of Two Potential Solution-Processed Blue Fluorescent Emitters Based on Fluorene-Arylamine Derivatives End-Capped with Anthracene/Pyrene Molecules

Two novel potential solution-processed blue fluorescent emitters composed of a core fluorenediphenylamine unit capped with either anthracene(FAn) or pyrene(FPy) were synthesized and characterized.They were both soluble in common organic solvents and solutions gave smooth films after spin coating. Their optical properties in solution and in the film were investigated by UV-visible and photoluminescence(PL)spectroscopy. The PL emission maximum of FAn and FPy in the film state were found to be 449 and 465 nm,respectively. The electrochemical properties of the as-prepared samples were studied by cyclic voltammetry.The estimated highest occupied molecular orbital(HOMO) energy levels were-5.37 and-5.36 eV for FAn and FPy, respectively. These results indicate that the introduction of diphenylamine effectively prevents plane stacking of the molecules in the solid state, which suppresses the formation of long-wavelength aggregates,and the high HOMO levels enhance the hole-injection ability of the compounds. The results of differential scanning calorimetry(DSC) and thermogravimetric analysis(TGA) indicate that the two materials have excellent thermal stability with the glass transition temperature of FAn reaching 207 ℃ and the thermal decomposition temperature as high as 439 ℃. The good performance of the fluorescent emitters makes them promising candidates as solution-processed blue organic light-emitting diodes.

Spectral variation of solution-processed blue bis[(4,6-difluorophenyl)-pyridinato-N,C2′] (picolinate) iridium(III) phosphor

Spectral properties of a blue phosphorescent bis[(4,6-difluorophenyl)-pyridinato-N,C2′] (picolinate) iridium(III) (FIrpic) complex were systematically investigated. The electroluminescent spectra of FIrpic in solution-processed organic light-emitting diodes (OLEDs) were focused on, and the effects of doping ratio and solvent solution on spectral characterization were explored in solution-processed blue OLEDs. The results showed that there was an unusual spectral variation between shoulder and peak intensities in the electroluminescent spectra of FIrpic, and the intensity ratio of the shoulder to the peak (the Huang-Rhys factor) was obviously increased when FIrpic was at high doping ratios and dissolved in polar solvents. The theoretical mechanism for this phenomenon was analyzed, indicating that the spectral variation was due to solvation effect induced by strong intermolecular interaction between the solvent and FIrpic molecules. Moreover, the spectral behavior of FIrpic in solution-processed white OLEDs with complementary colors was studied, providing further evidence for the spectral variation.