Preparation And Properties Of Oriented Polyfluorene Derivatives Films

Compared with traditional inorganic small molecules,organic conjugated polymers have the advantages of low cost,easy processing,and flexibility.They have been widely used in organic field effect transistors,organic light-emitting diodes,and organic lasers.Among them,polyfluorene-based conjugated polymers are a new type of materials with wide application and excellent performance.The 2,7 and 9 positions of fluorene in polyfluorene-based conjugated polymers can be substituted by different groups to achieve different functions.Incorporation of different co-polymer units with narrow band gaps can endow them full-color emission covering the entire visible light region.However,the performance of the optoelectronic devices based based on thin films of polyfluorene derivatives is largely influenced by the randomly arranged molecular chains and crystals,which makes it difficult to achieve the ideal effect.Therefore,it is of great significance to control the condensed structures of polyfluorene-based conjugated polymer films for achieving the ideal performance of the devices.In this thesis,high-density polyethylene（PE）and isotatic polypropylene（iPP）oriented films were prepared by a melt-drawn method,and their influences on the crystallization of polyfluorene derivatives were studied.Through surface induced epitaxial crystallization on both substrates,highly oriented ultrathin films of polyfluorene derivatives were obtained and the performances of related field effect transistor devices were thus explored.The results obtained in this work illustrate the feasibility and universality of epitaxial crystallization for regulating the structure of polyfluorene derivatives in condensed state,which is benefit for expanding the application of polyfluorene derivatives.The main research contents are as follows:1.The melt-drawn oriented PE ultrathin films were used as substrates for poly[2,7-（9,9-di-octyl-fluorene）-alt-4,7-bis（thiophen-2-yl）benzo-2,1,3-thiadiazole]（PFO-DBT）to induce epitaxial crystallization of PFO-DBT.It is found that epitaxial crystallization of PFO-DBT on PE surface takes place with the molecular chains of both polymers arranged parallel and a high dichroic ratio of 5.79 is achieved.The property studies of field effect transistor devices based on oriented PFO-DBT films show that the devices have high switching ratios and low threshold voltages.The hole mobility measured along molecular chain is up to 1.46×10^-2cm²V^-1s^-1,which is an order of magnitude higher than that measured perpendicular to the molecular chain direction.This value is the highest among those reported in the literature for this material.2.The epitaxial crystallization of a polyfluorene derivative,i.e.,poly[（9,9-di-n-octylfluorenyl-2,7-diyl）-alt-（benzo[2,1,3]thiadia-zol-4,8-diyl）]（F8BT）,on oriented PE and iPP substrates was compared.It is confirmed that both PE and iPP can induce the oriented epitaxial crystallization of F8BT with a parallel chain alignment,but F8BT on PE exhibits a better orientation.The orientation status of F8BT can be further improved by solvent annealing or/and thermal annealing.The property studies of the field effect transistor devices based on oriented F8BT films show that the performance of the devices based on F8BT/PE thin films is much better than the F8BT/iPP ones.While the hole mobility of the device based on F8BT/PE measured along molecular chain direction is up to 2.14×10^-3cm²V^-1s^-1,that of the F8BT device based on F8BT/iPP reaches only 5.43×10^-4cm²V^-1s^-1.