The Investigation Of Chain Conformation And Photoelectric Properties Based On Conjugated Polymer Poly(9,9-dioctylfluorene)

As a important kind of polyfluorene, poly(9,9-dioctylfluorene)(PFO) has attracted much attention because of its distinguished optical and electrical characteristics, also is known as a kind of blue light material. Through varying degrees external field stimulation, PFO main chain adopts different torsional angle, especially the β conformation shows a coplanar conformation, which results in an extended PFO chain conjugation length and an increase degree of order. In this paper, we use external field stimulation(temperature and solvent) to induce different chain conformation of PFO, further study the chain conformation and optical properties. The results are as followings:A quantitative relationship on the heredity from solution to films based on poly(9, 9-dioctylfluorene)(PFO) β conformation and its formed mechanism are revealed, and the effect of β the conformation contents on hole mobility is investigated. We find the β conformation can completely heredity from solution to films during the process of films formed. It indicates that PFO β conformation is enough stable so that it isn’t impacted by the dynamic transition process from liquid state to solid state. Besides, the aggregation of molecular chains isn’t a necessary condition to form the β conformation, but the formed β conformation must be with aggregated structure, which is a new understanding to the mechanism of PFO β conformation forming. In addition, we find that the hole mobility is obviously enhanced by an order of magnitude when the β conformation increases from 0% to 5.4%. It is infered that formed ordered structure of PFO(i.e., β conformation) is the main reason for the obviously enhancement of the hole mobility.Different concentrations of PFO solutions are selected to investigate the effect of β conformation contents. With the increasing concentration of PFO, β conformation content is gradually increased. Due to the different ratio of growth in different concentration ranges, we find that the aggregate size of β conformation is about 100 nm by dynamic light scattering(DLS). Meanwhile, we find that the concentration changes will not affect the content of β conformation when the β conformation reaches a maximum value, even if adding a poor solvent does not alter the content of β conformation. Accordingly, we make a point about PFO β conformation saturation: the β conformation content will be stable at the maximum value around 45%, no matter how much field stimulation cannot increase it.The changing of chain conformation based on PFO is investigated when we use different content of poor solvent(ethanol). Surprisingly, except the α conformation and β conformation we find another conformation, named it transition state conformation, its torsional angle between 135°and 165°. Meanwhile, the transition state conformation can enhance the charge density and hole mobility of PFO. Its hole mobility is almost equivalent to the PFO device containing 5.4% β conformation, it indicates that the transition state conformation also shows a relatively coplanar conformation, results in an extended PFO chain conjugation length and an increase degree of order, which in favor of the carrier mobility transport. More meaningful, compare to β conformation, the transition state conformation shows good film forming characteristics and the photoelectric properties is as good as β conformation.