| Summary: | 博士 === 國立清華大學 === 化學工程學系 === 89 === Poly(phenylene vinylene) (PPV) and its derivatives are the most popular electroluminescent polymers due to their semi-conductive and good fluorescence properties. However, intermolecular interactions, aggregation of polymer chains, energy transfer processes and effects of these factors on the fluorescence as well as electroluminescence properties of PPV polymers have not been well understood so far. Therefore, in this research, the absorption (Uv-Vis), emission (PL), excitation (PLE) and time-resolved photoluminescence (TR-PL) spectroscopies were used to clarify the topics on blocked PPV (Poly-DSB I ~ VI) with well-defined conjugational length, fully conjugated dialkoxy substituted PPV polymer with different ratios of short- to long-side-chain segments (dROPPV-x/y, BCHAPPV-x/y and OC1C10-PPV) as well as blends composed of Cz-PF (blue, as the host), ROPh-PPV (green) and dROPPV-1/1 (orange). The existence of sequential energy transfer mechanism was proposed and proved. Based on these findings, effects of concentration of solutions, the length and the steric hindrance of side chains as well as the solidification processes on the intermolecular interactions and aggregation and consequently on the photoluminescence and electroluminescence properties were studied.
Poly-DSB I ~ VI are PPV polymers with ethylene, methylene or ether groups as conjugation interrupters. All of them can evolve deep-blue lights in the dilute solutions. However, the fluorescence color shifts to sky-blue to green in concentrated solutions and in films due to trace amount of aggregates formed by intra- and inter-molecular interactions. Aggregates are ground-state species and can be directly excited. The absorption and emission wavelengths of aggregates red shift as the interactions become stronger. Sequential energy transfer mimicking photosynthesis from individual lumophores to the most aligned aggregates, via looser aggregates, is directly observed by TR-PL.
In dilute solutions, dROPPV-1/1 can form aggregates due to the high content of the co-monomer with short side chain; the intermolecular interactions become stronger as the concentration increases. Annealing of its solid film also promotes aggregation. Energy can transfer from single polymer chain to the compact aggregates, via loose aggregates. The intermolecular interactions in aggregates are weak by spin coating and strong by drop casting. Therefore, the tails of absorption and PLE spectra in the long-wavelength region are more elevated in the dROPPV-1/1 drop-cast films.
Though aggregates do form in dROPPV-0/1, 0.2/1 and BCHAPPV-0/1, films of these polymers still show single-polymer-chain properties due to the imperceptible amount of aggregates caused by the higher content of the repeating units with long side chains or the huge steric hindrance imposed by the bulky CHA groups. For OC1C10-PPV, each repeating unit possesses one long side chain and therefore the intermolecular interactions are suppressed. Accordingly, the aggregation of polymer chains is suppressed and the solidification processes do not affect the luminescence properties of films. However, after annealing, same extent of aggregation of OC1C10-PPV is still observed to that of dROPPV-1/1.
Cz-PF contains three emitting species: single chain, aggregates and exciplexes. Because energy can be transferred from Cz-PF to dROPPV-1/1, the PL of the blend with only 2.5% dROPPV-1/1 compromises the guest luminescence in a large portion. In contrast, the blend with 2.5% ROPh-PPV does not show any emission from the guest polymer in the film PL, because ROPh-PPV just functions as an intermediate transfer station and energy is finally transferred to the Cz-PF aggregates from single Cz-PF chains via ROPh-PPV.
For Cz-PF and its blends, the contributions to the emission from aggregates and guest polymer are much higher in EL than in PL, respectively. This is probably due to the quantum well structure formed between Cz-PF and the aggregates in Cz-PF film, and between Cz-PF and the guest polymer in the blends. In light-emitting devices with Cz-PF or its blends, most electrons and holes injected from cathode and anode are likely to be trapped in the LUMO and HOMO of aggregates or guest polymers, respectively. Therefore, most light evolved from the device is from Cz-PF aggregates or from the guest polymers, where most electrons and holes recombine.
From the results of this research, it is concluded that the supramolecular structures and physical properties of conjugated polymers can be highly inhomogeneous even if the chemical composition is uniform and homogeneous. Intermolecular interactions between conjugated segments result in a generation of new ground-state emitting species, aggregates, which function the same in the luminescence processes as the dopant used in OLED. Luminescence properties (PL and EL) and device performance of polymers or blends can be adjusted or significantly changed by a very small amount of aggregates. This thesis would provide very useful information for a molecular design of new electroluminescent polymer and PLED fabrication process.
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