| Conjugated polymers are very attractive in electroluminescent (EL) diodes due totheir solution process, easy device fabrication and possibility for realizing large areadisplays. Polyfluorene (PF) is a promising blue emitting material with high luminanceand efficiency, which attracts attention of many groups at home and abroad. And itcan be synthesized through Suzuki cross-coupling reaction, with identified structureand broad room for molecular design. The color and opto-electronic properties can beadjusted by introducing functional groups such as thiadiazole to the backbone of PF;to the side chain of PF, it is also convenient to introduce some functional units such ascarbazole to improve the opto-electronic properties, without affecting the conjugatedstructure of the main chain. This thesis focuses on high-quality and high-emittingproperty polyfluorene derivatives with modified main chain and side chain; this isactualized by balancing the carriers transport and enhancing the solubility of polymers,and the properties of EL devices are carefully researched. The model polymer isPDHCF, a PF derivative with carbazole functionalized side chain. Carbazole has thefollowing advantages: First, the oxidation potential of carbazole is low, which canimprove HOMO level of the polymer, and facilitate the hole injection and transport inOLEDs. Second, the thermal properties can be improved by the rigid carbazole. Third,carbazole is an electroactive group, which can realize the fabrication of electrochemical deposition (ED) devices. We began the research in two aspects basedon the PDHCF model.Firstly, we research the EL properties of PDHCF, and the luminous efficienciesof single layer device (ITO/PEDOT(40nm)/PDHCF(10mg/mL, CHCl3)/CsF(1.5nm)/Al(120nm)) and double layer device (ITO/PEDOT(40nm)/PDHCF(10mg/mL,CHCl3)/TPBI(40nm)/CsF(1.5nm)/Al(120nm)) are0.57cd/A and1.1cd/A,respectively. The opto-electronic properties are improved by carbazolefunctionalization, compared with PDHF (0.01cd/A in single layer device) withoutcarbazole groups. Then the electron transport units (benzothiadiazole,naphthothiadiazole and benzoselenadiazole) are introduced to the backbone ofPDHCF and polymer PCFBTs, PCFSe and PCFNT0.1are gotten, to adjust theemitting color and enhance the electron transport ability; and the polymers withdifferent radios of hole and electron transport units are synthesized, in order tobalance the hole and electron carriers transport better. Among the green polymers,PCFBT0.2(BTz:fluorene=2:8) exhibits the best EL properties, with maximumluminescence of8620cd/m2and maximum luminous efficiency of10.8cd/A in thedouble layer device; compared with F8BT, the efficiency is improved by13times.The experimental results demonstrate that the complementation of functionalized sidechain with the polymer backbone in electronic and optical properties is an effectiveway to design high performance electroluminescent polymers.In the Chapter3, the polymers with other functional units (cyano andphosphonate) on side chain are synthesized based on PDHCF. The advantages are asfollows: First, the strong polarity of cyano and phosphonate units can enhance thesolubilities of polymers, which are reduced by carbazole groups, so it is morebeneficial for fabricating the opto-electrical devices on solution-process. Second,cyano and phosphonate are electron transport units, so the carriers transport balance inside chain of polymers can be realized with carbazole units, which is very beneficialfor improving the opto-electronic properties of EL devices. Among the polymers,PFCzPO has a good solubility in common organic solvent and EL properties, and wecan fabricate a thick film by electrochemical deposition approach. The single layer ED device based on the ED film of PFCzPO (ITO/ED film(~100nm)/LiF(0.5nm)/Al(100nm)) exhibits a maximum luminous efficiency of3.8cd/A with CIE (0.16,0.11), which is among the top level in the blue polymer EL devices. |
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