PLED 소자의 전자수송능력 향상을 위한 공중합체의 합성과 광-전기적 특성에 관한 연구

Abstract

Polymer light emitting diodes (PLED) have made unexpected dramatic progress in comparison to classical semiconductor light-emitting diodes over the past years. The active layer of PLEDs is prepared using a simple procedure, therefore these device have the potential to provide an innovative low cost technology for backlighting, illumination, and display applications. Conjugated polymers, with alternating single and double bonds which from a π-π* extension in the molecular backbone, combine the optical and electrical properties with the mechanical advantage for preparation of opto-electronic device. Charge carriers (holes and electrons) are injected separately from anode and cathode, recombine in the emitting layer and thus emit light. Balanced rates of injection and transport for both carriers are essential to achieve high (EQE) in a PLED. A straight forward approach to improve the EQE of a PLED is to integrate the functions of carriers transport and light emission into a single polymer, thereby creating an bipolar (donor/acceptor) emissive polymer. However, for polymers with (donor？acceptor)n type backbones, the rather strong intra-molecular charge transfer (ICT) along the main chains is prone to decrease their emissive efficiency in spite of the enhancement of both carriers transport. An alternative strategy to overcome this problem is to prepare polymers with p-type backbones and n-type side chains. Based on this consideration, a series of phenothiazine (PT) and polyfluorene (PF) derivatives with peripheral 1,3,4-oxadiazole groups were successfully prepared and exhibited promising electroluminescent properties. But the application of these materials was restrained by the difficulties in their syntheses. The preparation of other types of polymers with p-type backbones and n-type side chains by relatively simpler synthesis is highly desirable. Triphenylamine (TPA) and PT derivatives, small molecules or polymers containing TPA and PT units are one type of the most widely used hole-transport materials because they are easily oxidized to form stable radical cations. The incorporation of TPA and PT segments into polymers can also improve their solubility and formability. On the other hand, one type of n-building block, 1,3,4-oxadiazole, was aroused special interest in recent years due to its high electron affinity and good thermal stability. It has been successfully incorporated into polymers to apply as the electron-transport materials in multilayer PLEDs. The attempts to simultaneously incorporate PT, TPA and 1,3,4-oxadiazole into small molecules have been proven to be an effective way to obtain materials with high luminescent efficiency. This study reports the preparation and characterization of a series of PTPF, TPAPT and TPAPF segment in polymers backbones with 1,3,4-oxadiazole group as pendant. The 1,3,4-oxadiazole moieties were linked to the PT and TPA so that the electron affinity of the polymer could be improved. The optical, electrochemical, and electroluminescent properties of these copolymer Poly((10-(4-(5-(4-tert-butylphenyl)-(1,3,4)oxadiazol-2- yl)phenyl)-10H-phenothiazine- 3,7-diyl)-alt-(9,9-dihexyl-2,7-fluorene)) (PTOXDPF), Poly (4-(5-(4-tert-butylphenyl)-1,3,4- oxadiazol-2-yl)-N-(4-(10-hexyl-3-methyl-10H-pheno-thiazin-7-yl) phenyl)-N-p-tolylbenzenamine) (TPAOXDPT) and poly(N-(4-(5-(4-tert-butylphenyl)-1,3,4-oxadiazol-2-yl)phenyl)-4-(9,9-dihexyl- 9H-fluoren-2-yl)-N-phenyl-benzenamine (TPAOXDPF) are systematically investigated here. We also synthesized poly(10-hexyl-10H-phenothiazine-3,7-diyl)-alt-(9,9-dihexyl-2,7-fluorene) (PTPF), poly(4-butyl-N-(4-(10-hexyl-3-methyl-10H-phenothiazin-7-yl)-phenyl)-N-p-tolylbenzen-amine) (TPAPT) and poly(N-(4-butylphenyl)-4-(9,9-dihexyl-9H-fluoren-2-yl)-N-phenylbenzen-amine) (TPAPF) to compare the optical, electrochemical, and electroluminescent properties with PTOXDPF, TPAOXDPT and TPAOXDPF. We investigated the fundamental optical, electrochemical and electroluminescent properties as well as the role of 1,3,4-oxadiazole in the improvement of EL efficiency of the polymer.