Organic solution-processible electroluminescent molecular glasses for non-doped standard red OLEDs with electrically stable chromaticity
Graphical abstract
Introduction
Ink-jet printing is an effective approach to mass-product organic light-emitting diodes (OLEDs) for panel displays, which have been recognized as next-generation leading panel displays replacing liquid crystal displays (LCDs), due to its fast speed, high precision, low noise, low cost, environmental protection and high utilization ratio of electroluminescent materials [1], [2]. Up to now, it is a unique method to have developed full-color OLED panel displays by drop-on-demand (DOD) mode. However, ink-jet printing is mostly applied to polymer light-emitting diodes (PLEDs) at present [3], [4], [5], [6]. In this case, the polymeric viscoelastic behavior causes many disadvantages for polymer ink-jet printing, such as the generation of filament and satellite droplet from high polymer viscosity, the printability decrease from polymeric interchain winding, the difficult position fixing of high viscous droplets, and so on. In order to solve these problems and further to reduce the fabrication cost of OLED panel displays, the ink-jet printing of small molecular OLEDs (sm-OLEDs) has attracted much more attention due to simple synthetic route and low viscosity of organic small molecular electroluminescent materials [7], even phosphorescent sm-OLEDs. Accordingly, organic solution-processible electroluminescent molecular glasses would be paid much more attention for ink-jet printing of sm-OLEDs owing to their good film-forming property, the phase stability of solid film, the synthetic accessibility, the device fabrication without vacuum deposition and the possibility to accelerate the printing speed.
Organic molecular glasses, used for emissive layer [8], [9], [10], [11], [12], electron-injecting layer [13] and hole-transporting layer [14], [15], have been studied in the last decade. One of them, the development of organic electroluminescent molecular glasses (OEMGs) is a key issue to realize the mass-production of sm-OLEDs for full color plat displays, especially organic glassy emissive molecules for three primary colors (RGB). To date, solution-processible OEMGs are quite rare although a few of red [16], [17], green [8] and blue [9], [10], [11], [12] OEMGs have been reported. For example, carbazole-triphenylamine end-capped di(thiphen-2-yl)benzothiadiazole (CAPTB) for non-doped solution-processed red OEMG with Commission Internationale De L’Eclairage (CIE) coordinate of (0.66, 0.33) [16], [17]; pyrene-functioned diarylfluorenes for solution-processible green OEMG with voltage-stable CIE coordinate of (0.28, 0.54); asymmetrically 9,10-disubsituted anthracenes for solution-processible blue OEMG with voltage-stable CIE coordinate of (0.16, 0.12) [8]; 9,9′-spirobifluorene bearing carbazole groups at the molecular periphery for solution-processible deep-blue OEMG with CIE coordinate of (0.16, 0.05) [10]. One of them, the CIE coordinates of red OEMG CAPTB are basically in accord with the National Television System Committee (NTSC) standard (0.67, 0.33) for pure red color and very closed to the Phase Alternating Line (PAL) standard (0.64, 0.33) for pure red color. Unfortunately, its voltage chromatic stability is not studied, which is a very important issue for practical application of voltage/current-driven OLEDs, especially for red electroluminescent emitters. The voltage chromatic stability of red emitters, not only red electroluminescent polymers but also red electroluminescent organic small molecules [18], [19], [20], [21], [22], [23], is difficultly achieved especially for non-doped red emitters [24], [25], [26], [27], [28], [29], [30], [31], [32] since their molecular structures usually bear a large conjugated system or push–pull characteristics derived from the donor and acceptor moieties, which will result in a strong π–π stacking or intermolecular static interaction. Out of question, it is more difficult to achieve non-doped standard red OLEDs with voltage chromatic stability from solution-processible OEMGs.
Herein, three novel OEMGs bearing a donor–acceptor–donor molecular structure are synthesized with benzothiadiazole as an acceptor and with carbazole/phenothiazine/triphenylamine as a donor. Their chemical structures are identified in detail by 1H NMR, 13C NMR, IR and elemental analysis. The thermal, optical and electrical properties are respectively measured by thermogravimetric analysis (TG), differential scanning calorimeter (DSC), UV–vis spectra, fluorescent spectra and cyclic voltammetry (CV). The non-doped electroluminescent layers are spin-coated from their tetrahydrofuran solutions through a solution-processible approach and applied as active layers of sm-OLEDs with a device configuration ITO/PEDOT:PSS/OEMG/Alq3/Al. The current–voltage (I–V) and luminance–voltage (L–V) characteristics are investigated in detail. Especially, the electrically chromatic stability is carefully evaluated through the measurement of electroluminescent spectra and CIE coordinates at different operating voltages.
Section snippets
Instruments and characterization
1H/13C NMR spectra are recorded on Bruker DPX-400 nuclear magnetic resonance spectrometer using d-chloroform or d-dimethylsulphoxide solution at room temperature. Infrared (IR) spectra are measured using Nicolet Protégé 460 Infrared Spectrometer. Elemental analysis is carried out on EA 1110CHNSO Elemental Analyzer. Thermogravimetric (TG) property is investigated on NETZSCH TG-209 Thermogravimetric Analyzer at heating and cooling rates of 10 °C/min under N2 atmosphere. Differential scanning
Synthesis
The donor–acceptor (D–A) structure is usually adopt in the molecular design of red electroluminescent chromophores because the push–pull molecular character is an effective and convenient approach to lower the band gap. Up to now, this kind of electroluminescent devices with D–A emitters as an active layer is generally fabricated by thermal vacuum deposition. In addition, the instability of light-emitting performance often takes place owing to the static interaction of donor and acceptor. To
Conclusion
Organic electroluminescent molecular glasses (OEMGs) are synthesized for non-doped red OLEDs through the introduction of nonplanar donor and branched aliphatic chain into D–A–D electroluminescent emitters. The target OEMGs are characterized in detail by 1H NMR, 13C NMR, IR, elemental analysis, TG, DSC, UV–vis, fluorescent spectra, etc. The results indicate that the resulting OEMGs are in accord with the chemical structures as we expected. Through the replacement of electron-donating group in
Acknowledgement
The authors greatly acknowledge the financial supports from the National Natural Science Foundation of China (NSFC, U1304212 and 21274133).
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These authors equally contributed to this work.