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Title: Novel star-shaped bi-polar blue emitting materials for solution-processed undoped OLEDs
Authors: He, Xuehan
Keywords: DRNTU::Engineering::Materials::Photonics and optoelectronics materials
DRNTU::Science::Chemistry::Organic chemistry::Organic synthesis
DRNTU::Engineering::Materials::Organic/Polymer electronics
Issue Date: 2016
Source: He, X. (2016). Novel star-shaped bi-polar blue emitting materials for solution-processed undoped OLEDs. Doctoral thesis, Nanyang Technological University, Singapore.
Abstract: Since the first organic light-emitting diodes (OLEDs) device was fabricated in 1987, great efforts have been put into this research field and produced fruitful achievements. Today, OLEDs have partly entered the commercial marketplace. Full-color displays require highly and equally performance of primary RGB OLEDs. However, blue OLEDs usually show inferior device performance (including color purity, efficiency, stability and longevity) compared with green and red OLED devices. This is because blue emitting materials usually possess intrinsic wide band gaps that cause difficulties in molecular design and injection of charges. Especially, undoped blue emitting materials which could host phosphors with lower energy have attracted much interest for their application in effective white hybrid OLEDs. On the other hand, solution processability of an emitting material enables a low-cost large-area flexible display and is beneficial for the industrialization of OLEDs. Motivated by the above, this project focuses on developing novel undoped blue emitters. So far, the study of solution processed blue OLEDs based on undoped emitting materials is quite limited. Most reported emitting materials that exhibited good performance are star-shaped oligofluorenes. Inspired by them, a series of star-shaped molecules Sn-CzOXD (n = 1~5) were designed because their highly branched structure could afford high solid quantum efficiency by reducing the molecular aggregation and thus the fluorescence quenching. L1-Cz-OXD, a linear counterpart of S1-Cz-OXD was synthesized for comparison study. Sn-Cz-OXD incorporated an electron-deficient tris(1,3,4-oxadiazole)phenylene (t-OXD) ring as core and three electron-rich 3,6-di-tertbutylcarbazole (t-BCz) as terminal groups bridged by different amount of π-conjugated spacers. Considering the intrinsically unstable emission spectrum of fluorene units, a benzene unit was selected as the conjugated spacer. Owing to the donor-π-acceptor structure, improved balance of hole and electron transport properties was expected. With longer spacers, the quantum efficiency of the emitter was expected to increase due to the better suppressing of intermolecular interactions by longer branches. Besides, the emission spectrum of molecules should be slightly blue shifted because of the reduced intramolecular charge transfer ability. In addition, to improve the solubility of emitting materials, alkoxy chains were introduced in S4-Cz-OXD and S5-Cz-OXD. As a result, target star-shaped molecules were successfully synthesized, characterized and applied in undoped OLEDs. They showed high quantum efficiency, excellent spectra thermal stability, good device performance and expected change trend with the extended conjugated length of the branches. Among them (Sn-Cz-OXD, n = 1~5), solution processed undoped OLED devices based on S4-Cz-OXD with configuration of ITO/PEDOT:PSS/EML/TPBI/LiF/Al exhibited the best device performance. They achieved a maximum luminance of 7804 cd m-2, a maximum current efficiency (CE) of 4.69 cd A-1, a maximum external quantum efficiency (EQE) of 4.20% and a maximum power efficiency (PE) of 2.08 lm W-1 with CIE coordination of (0.157, 0.146). In addition, it is worth to mention that L1-Cz-OXD emitted deep blue emission at about 430 nm with high solid quantum efficiency and excellent spectrum thermal stability. An optimized undoped device with configuration of ITO/MoO3/TCTA/L1-CzOXD/TPBI/LiF/Al exhibited good performance. A maximum EQE of 3.09% with CIE coordination of (0.16, 0.07) was achieved. Besides, L1-Cz-OXD exhibited crystallization properties and it could be grown as a single crystal, which endows it the potential to be employed in other applications.
DOI: 10.32657/10356/69088
Fulltext Permission: open
Fulltext Availability: With Fulltext
Appears in Collections:MSE Theses

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