Design, Synthesis And Optoelectronical Properties Of Solution-Processable Deep-Blue Fluorescent Materials

Electroluminescent materials are the critical component of organic light emitting diodes (OLEDs). Full-color displays require primary RGB emission of relatively equal stability, efficiency, and color purity. At present, red and green light emitting materials have basically met the requirements of practical application. However, the performance of blue emitting devices is unsatisfactory for the intrinsic wide band-gap of blue light emitting materials, which makes it hard to inject charge into an emitting layer. Solution-processed OLEDs, which can realize low-cost and the fabrication of large-area thin film, have been attracting considerable interests in the past decades due to their broad applications in the solid-state illumination sources and large-area flat-panel displays. In this thesis, a series of solution processable multifunctional blue fluorescent materials have been designed and synthesized. The structure-property relationships have been investigated.The main contents and results are described as follows:In Chapter1, a brief introduction of the concept of OLEDs including the device structures, functional materials and operating mechanism therein is given at first. Then, strategies and approaches of developing highly efficient blue fluorescent materials for use in non-doped OLEDs are illustrated. Finally, the design ideas and main contents of this thesis are outlined.In Chapter2, a series of starburst oligofluorenes are designed and synthesized by convergent approach. The intermolecular aggregation and crystallizing tendency have been effectively suppressed by the bulky star-shaped architecture of the compound. The introduction of the electron-donating bridged-triphenylamine improved the hole transporting property of the materials. Simple double-layer devices using the starburst oligofluorenes as solution-processable emitters achieved the maximum current efficiency of3.83cd A-1, and maximum external quantum efficiency of4.19%with Commission International de l’Eclairage (CIE) coordinates of (0.16,0.09), which is comparable to the best performances of the non-doped solution-processed deep-blue OLEDs based on starburst oligofluorenes.In Chapter3, two star-shaped oligofluorenes with hexakis(fluoren-2-yl)benzene as core and hole-transporting diphenylamine group as end-capper are designed and synthesized by a modified convergent core-creating approach. The two compounds show good thermal and morphological stability, suitable energy level, and deep-blue emission with high quantum efficiency. Solution-processed OLEDs featuring them as emitters displayed deep-blue emission with excellent color stability. The device based on HFB-terF-Dpa showed a maximum brightness of3861cd m-2, a maximum current efficiency of2.28cd A-1, and a maximum external quantum efficiency of1.99%with CIE coordinates of (0.15,0.11).In Chapter4, a series of multi-branched oligofluorenes with a phosphine oxide center are designed and synthesized. The existence of the phosphine oxide results in a trigonal-pyramidal structure, which effectively prevent the intermolecular aggregation and fluorescence quenching in the solid state. The electron-withdrawing characteristic of phosphine oxide group improved the electron transporting property of the compounds. Devices based on these materials displayed deep-blue emission and the device performances are significantly enhanced with the extension of the oligofluorene arms. The best performance is achieved by PPO-TF3, with a turn-on voltage of4.2V, a maximum current efficiency of1.88cd A-1, a maximum external quantum efficiency of3.39%, and CIE coordinates of (0.16,0.09).In Chapter5, two bipolar blue fluorescent materials are designed and synthesized by linking electron-withdrawing phenyl phosphine oxide and electron-donating N-phenylnaphthalen-1-amine with terphenyl or quaterphenyl bridges. The hole-and electron-transporting properties are improved by the arylamine and phosphine oxide moieties. Simple non-doped deep-blue OLEDs based on the two compounds showed high efficiency, excellent deep-blue electroluminescent color stability and extremely low efficiency roll-off. The device with PPO-TFNPA as the emitter showed a turn-on voltage of4.1V, a maximum current efficiency of1.78cd A-1and a maximum external quantum efficiency of1.79%with CIE coordinates of (0.15,0.09). When the brightness is increasing to1000cd m-2, the current efficiency still remains a high value of1.76cd A-1.In Chapter6, the molecule structure is optimized on the basis of chapter5, and four bipolar blue fluorescent materials are designed and synthesized by linking electron-withdrawing phenyl phosphine oxide and electron-donating arylamine with fluorene bridges. The alkyl chains endow these compounds with good solution processability and film-forming property. Solution-processed devices based on these materials show high efficiency, excellent color stability and extremely low efficiency roll-off. PPOF2-based device showed a maximum current efficiency of2.36cd A-1and a maximum external quantum efficiency of2.06%with CIE coordinates of (0.15,0.11). With the brightness of1000cd m-2, the current efficiency still remains at2.15cdA-1.In Chapter7, two oligofluorene chains are linked to the3-and5-position of a phenyl ring with an electron-withdrawing benzimidazole group, and end-caped with electron-donating diphenylamine groups to obtain a series of bipolar blue emitters. All compounds show good thermal stability, matched energy level, and high efficient deep-blue emission. The meta-linkage of benzimidazole group and the two diphenylamine substituted oligofluorene chains effectively suppresses the conjugation between two oligofluorenes, and intramolecular interaction of the electron donor diphenylamine and electron acceptor benzimidazole. Solution-processed device based on BIF2obtained a maximum brightness of3861cd m-2at9.2V, a maximum current efficiency of1.49cd A-1and a maximum external quantum efficiency of1.30%with CIE coordinates of (0.16,0.12).In Chapter8, a series of bipolar blue emitters are designed and synthesized by linking two hole-transporting diphenylamine groups to electron-transporting1,3,4-oxadiazole center with oligofluorene bridges. The center oxadiazole group interrupts the conjugation between two oligofluorene chains, while the oligofluorene bridges effectively suppresses the intramolecular interaction of diphenylamine and oxadiazole moieties. Solution-processed device using OXDF3as the emitter displayed deep-blue emission with CIE coordinates of (0.15,0.12), and achieved a maximum brightness of4984cd m-2, a maximum current efficiency of2.61cd A-1and a maximum external quantum efficiency of2.12%, which is among the highest values of the non-doped deep-blue OLEDs based on solution-processable bipolar materials.