Synthesis And Properties Of Hyperbranched White Polymer Light-emitting Materials Based On Poly (Octylfluorene)s With Spiro[3.3]Heptane-2,6-Dispirofluorene Branching Point

White organic light emitting devices(WOLEDs) have gradually become the research focus with the development of OLED. WOLED generates efficient saturated white light and is considered as a new type of green semiconductor lighting source. It can be applied to the general and decorative lighting of all kinds of architectures with the advantages of high brightness, low energy consumption, strong adaptability to environment, and long lifetime etc.. White polymer light emitting materials based on poly(octylfluorene)s have high fluorescence quantum yield, good solubility and good thermal stability because of their rigid biphenyl type structures. However, linear polymers suffer from chain winding and crystallization because of the interchain interactions, resulting in poor spectral stability, lowered luminous efficiency, and shortened lifetime. To resolve the problem, the dissertation centers on the research on the synthesis, characterization and properties of hyperbranched white polymer light-emitting materials based on poly(octylfluorene)s with spiro[3.3]heptane-2,6-dispirofluorene as the branching point. The relationship between the molecular structure and the properties was systematically studied by changing the contents of branching point, the structure of light-adjusting groups and the structure of polymer main chains.In detail, the dissertation is divided into six chapters.In Chapter 1, the development of white organic light-emitting materials in lighting applications was introduced briefly. Then, the implementation methods and research status of white polymer light-emitting materials were emphatically discussed. Meanwhile, the structure and performance advantage of hyperbranched polymers were elaborated, and the application and research progress of hyperbranched polymers in OLEDs developments were summarized. Finally, the design ideas and main research contents of this dissertation were put forward thereupon.In Chapter 2, starting from linear copolymer poly(9,9-dioctylfluorene-4,7-dithienyl-2,1,3-benzothiadiazole)(PF-DBT), we synthesized two dumbbell-shaped white polymer lighting emitting materials PF-DBT-SDF and PF-DBT-P through introducing three dimensionally structured 2’,2”,7’,7”-spirofluorene(SDF) and coplanarly structured 1,3,6,8-pyrene at the end of the polymer chains, respectively. The photophysical properties, thermostability, and film forming property of two copolymers were investigated, and single-layered electroluminescent devices based on these polymer materials were prepared. Both of the copolymers showed characteristic spectra of polyfluorene(PF), good film forming ability, relatively high T g s, and achieved white electroluminescence under high voltage. Even more importantly, the maximum brightnesses of copolymers PF-DBT-SDF and PF-DBT-P based device increased by 27% and 14% compared with that of PF-DBT based device, respectively. This indicates that it is a potential method to improve the performance of polymers by introducing groups with large steric hindrance such as SDF to the polymers.In Chapter 3, a series of white light-emitting hyperbranched copolymers consisting of polyfluorene(PF)/4,7-dithienyl-2,1,3-benzothiadiazole(DBT) branches and SDF conjugated branching point were synthesized by adjusting the SDF contents from 1 mol% to 20 mol% and fully characterized. The photophysical, thermal, film-forming, and electroluminescent properties of the copolymers were investigated systematically. All of the copolymers showed characteristic spectra of PF because the introduction of SDF did not interrupt the conjugation of the polymer chain. Meanwhile, no obvious spectral red-shift was observed in films, indicating that the interchain interactions were effectively suppressed by the hyperbranched structure. Moreover, the hyperbranched copolymers exhibited good thermal stability and film-forming ability. The F?rster resonance energy transfer(FRET) from fluorene to DBT in the hyperbranched copolymers remained, so white electroluminescence of single layer devices was achieved. The maximum luminance and maximum current efficiency of copolymer PF-SDF10-DBT(SDF content 10 mol%) based device were 2 and 5.5 times those of the linear copolymer PF-DBT based device. The results lay a good theoretical foundation for the subsequent experiments and further studies of hyperbranched white polymer lighting emitting materials.In Chapter 4, in order to improve the luminous efficiency of the hyperbranched copolymers, red phosphorescent group Ir(piq)2acac as the adjusting group was introduced to replace the orange fluorescent group DBT in the hyperbranched copolymer with 10 mol% SDF. We synthesized a series of fluorescence/phosphorescence hybrid hyperbranched polymer lighting emitting materials PF-SDF10-Ir x by adjusting the content of Ir(piq)2acac from 0.02 mol% to 0.05 mol%. The photophysical, thermal, film forming and electroluminescence properties of the copolymers were investigated. In the PL spectra, no obvious red-shift was observed in films owing to the suppressed interchain interactions. The T d s and T g s of the copolymers were over 400 °C and 150 °C, respectively. Bes ides, the copolymers formed amorphous thin films because of the hyperbranched structure. When the content of Ir(piq)2acac was up to 0.04 mol%, copolymer PF-SDF10-Ir4 achieved white electroluminescence through both intra- and interchain FRET from fluorene unit to Ir(piq)2acac and charge trap on Ir(piq)2acac with a CIE coordinate at(0.30, 0.34). A maximum luminance of 6777.3 cd/m2(18.3 V) and a maximum current efficiency of 4.0 cd/A were obtained in the single-layered device. It is a very promising method to achieve high efficiency white emission by introducing the phosphorescent red groups with high internal quantum efficiency in hyperbranched copolymers.In Chapter 5, in order to improve the performance of the hyperbranched copolymer, for example, to decrease the barrier between the HOMO energy level of the polymer light-emitting layer and the work function of the hole injection layer PEDOT:PSS in devices, to increase the triplet energy level of polymers, and to improve the thermostability of the materials, we introduced the rigid group 3,6-carbazole(Cz) with high HOMO energy level and triplet energy level to the main chain of the hyperbranched copolymers and synthesized a series of hyperbranched polymer lighting emitting materials with poly(fluorene-alt-carbazole) branches PFCz-SDF10-DBTx(x: 0.05–0.1 mol%). The photophysical, thermal, film-forming and electroluminescence properties of the copolymers were investigated. The copolymers exhibited enhanced thermal stabilities with T d s and T g s over 400 °C and 180 °C, respectively, and increased HOMO energy level of-5.1 e V. The single-layered electroluminescent devices based on these polymer materials showed satisfying performances with a low turn-on voltage of about 5 V, a maximum luminance of 7409.5 cd/m2 at 13.5 V and a maximum current efficiency of 4.38 cd/A. PFCz SDF10DBT8 and PFCz SDF10DBT10 devices all achieved white emission with CIE coordinates of(0.28, 0.31) and(0.32, 0.26), respectively. They could be applied into the display with cool white light emitting and lighting with warm white light emitting.The introduction of carbazole groups in hyperbranched polymers with fluorene-alt-carbazole branches is a kind of promising way to improve polymer electroluminescence performance.In Chapter 6, with the purpose of improving the solubility of Ir(III) complexes for a better match in fluorescence/phosphorescence hybrid hyperbranched polymer chains, the N-hexyl carbazole group was introduced to the main ligand. Two kinds of heterotropic Ir(III) complexes(Czhpi)2Ir(fpptz) and(Czh BTZ)2Ir(fpptz) were synthes ized with 1,2,4-triazole as the ancillary ligand and 2-phenylimidazole and 2-(2-hydroxyphenyl)benzothiazole as the main ligands, respectively. They were well soluble in common organic solvents such as toluene, CHCl3, THF, etc..(Czhpi)2Ir(fpptz) and(Czh BTZ)2Ir(fpptz) can be used as green and yellow phosphorescent luminophors for white-light-emitting with three primary colors and complement colors, respectively. The N-hexyl carbazole group with relatively large steric hindrance could effectively suppress the intermolecular interactions and help form amorphous films. The doped electroluminescent devices achieved a maximum luminance of 2696.3 cd/m2, a maximum current efficiency of 3.94 cd/A for(Czhpi)2Ir(fpptz), and a maximum luminance of 9617.2 cd/m2, a maximum current efficiency of 9.43 cd/A, and maximum power efficiency of 3.29 lm/W for(Czh BTZ)2Ir(fpptz). In addition, their absorption peak around 430 nm could well overlap with the emission peak of fluorene unit, which is suitable for introducing these structures into polymer chains to synthesize fluorescence/phosphorescence hybrid hyperbranched white polymer lighting emitting materials. The results lay a solid theoretical foundation for the subsequent experiments and further application.