| Conjugated polymers have good flexibility and solution processability,low manufacturing cost and large-area production,and have been widely used in optoelectronics,such as organic light-emitting diodes(OLED),organic solar cells(OSC)and organic field effect transistor(OFET).Poly(9,9-dioctylfluorene-co-dithiophene)(F8T2),poly(9,9-dioctylfluorene)(PFO)and poly(3-hexylthiophene)(P3HT)are the extensively studied typical conjugated polymers.F8T2 is a typical conjugated polymer with crystalline and liquid crystalline properties.It has good thermal stability,high crystallinity and hole transport properties and is widely used to prepare ordered photovoltaic films.PFO is a classic blue-emitting polymer possessing excellent thermal stability,high photoluminescence quantum efficiency and good charge transport properties.P3 HT is a highly crystalline optoelectronic material with excellent charge transport properties and has been extensively studied in field effect transistors and solar cells.Studies have shown that the device properties of conjugated polymers are closely related to the chain conformation and alignment structure in the film.Different chain order and arrangement will cause changes in the photophysical and electronic properties of the polymer,thus affecting its charge transport properties.For a particular conjugated polymer,ordered molecular chains and closely packed crystal structures tend to favor charge transport along the chain backbone and interchains.However,during film-forming process,the macromolecular chains of conjugated polymers usually formed with high conformational freedom,irregular chain arrangements,and a lot of disordered structures,which limit their charge transport properties.Therefore,the aggregate structure in the film must be appropriately optimized to further improve the device performance of the conjugated polymer.As a soft substance,conjugated polymers are easily oriented under external stimuli.Based on that,this paper uses the external field(thermal annealing,solvent vapor,electric field)to regulate the F8T2,PFO,P3 HT molecules chain to form an ordered structure so as to improve the device carrier mobility based on the condensed polymer condensed matter physics.The main research content is divided into four parts:In the first part of the work,we studied the effect of thermal annealing on the F8T2 aggregate structure.First,by comparing the chain behavior of F8T2 in dilute solutions,thin films and annealed films,we found that annealing can rearrange the F8T2 molecules and produce red-shifted absorption peaks.Using theoretical calculations,we performed a spectral simulation of F8T2 and proved that it is a J-aggregation generated by molecular chain slip stacking.By tuning the annealing temperature,solvent,molecular weight,concentration and other parameters,we explored the formation conditions of F8T2 J-aggregation.It is found that the J-aggregation of F8T2 is formed during the annealing process at the crystallization and liquid crystal temperature,and depends on some parameters including solvent,molecular weight and concentration.Through further structural characterization,we obtained the microstructure characteristics of J-aggregation and analyzed its formation process and mechanism.We found that J-aggregation exhibits a long-range ordered compact ?-stacked structure,J-aggregated crystals are nanorod structures,and J-aggregated liquid crystals are fibrous structures.It is a long-range ordered closely packed aggregate structure formed by the slip stacking of molecular chains caused by the increase of the degree of order in the chain during the annealing process,which leads to the decrease of side chain steric hindrance and the increase of molecular interaction.In this part of the study,the J-aggregation structure of the block conjugated polymer F8T2 with crystal and liquid crystal properties was reported for the first time,and the relationship between its spectrum and structure was studied in detail,which has certain guiding significance for the further investigation of condensed structure of the block conjugated polymerin the future and expanding its application range in the field of optoelectronics.In the second part of the work,we applied the external electric field to the thermal annealing process of F8T2,and studied the effect of external electric field on the ordered structure and carrier mobility of F8T2 annealing at different phase transition temperatures,and the ability of external electric field to control the orientation and microstructure of F8T2 different phase polymer chains as well as revealed related processes and mechanisms.It is found that the external electric field can induce the F8T2 molecules in different temperature annealed films to orient along the direction of the external electric field and form an ordered structure.However,different temperature annealed F8T2 chains exhibit different responses to the external electric field,the molecular chains annealed above glass transition temperature(130°C)are arranged in the direction of the external electric field to form a needle-like crystal structure;the molecular chain annealed at crystallization temperature(200°C)further improve the orderness and size of crystallization,and promote the formation of sub-growth cluster crystals;the molecular chain annealed at liquid crystal temperature(280°C)form an ordered nanorod structure.The hole mobility of the three annealed films was also increased by 127%,367% and 320%,respectively.The main reason for the difference in F8T2 structure and mobility growth after external electric field is that the F8T2 molecular chains annealed at different temperatures have different move ability,chain interaction and self-assembly tendency,which makes the external electric field have different regulation mechanisms on the molecular chain,and then form different ordered structures that possess different impact on carrier migration.This part of the study shows that the influence of external electric field on molecular order structure and carrier mobility is closely related to the nature of molecular chain,which has important guiding significance for the future regulation of condensed structure of conjugated polymer molecules by external electric field.In the third part of the work,we applied the external electric field to the solvent vapor annealing process of weak polar polymer PFO,and studied the effect of the external electric field on the ordered structure and carrier mobility of the PFO molecule with the help of polar solvents.First,we explored the effect of solvent polarity on the PFO of external electric field orientation and revealed its discipline via using different solvents.The study found that the external electric field can effectively induce the conformational transition and orientation of the PFO chain with the help of soluble polar solvent,forming ordered ?-phase crystals and improving the carrier mobility of the device.The external electric field and the polar solvent show a synergistic effect in the orientation of the PFO chain.The stronger the solvent polarity,the more obvious the ? phase content increases,and the more ordered the aggregate structure,the greater the contribution to the carrier mobility.Through further analysis,we reveal the process and mechanism of PFO orientation and alignment in an external electric field and a polar solvent atmosphere,namely solvent penetration,solvent polarization,PFO induced orientation,and solvent removal.This study used an external electric field and a polar solvent to orient the weak polar conjugated polymer for the first time,and successfully proved its feasibility,which provides a new method for the preparation of ordered photovoltaic films in the future.In the fourth part of the work,we applied electric field and solvent vapor annealing to the polar polymer P3 HT,and studied the effect of electric field on molecular chain regulation,film structure and carrier mobility of the polar conjugated polymer under solvent vapor annealing.It was found that the orientation of P3 HT in the solvent vapor annealing and electric filed treated process depends on the polarity of the solvent.Under the action of strong polar chloroform vapor annealing,the crystallinity and size of P3 HT increased by 102.8% and 31.9%,respectively,and the self-assembly of the molecular chains formed a highly ordered nanowire structure,as a result,the carrier mobility also increased by 378%.At the same time,we have discovered the process and mechanism of the external field to orient and pack P3 HT during polar solvent vapors annealing.Compared with the PFO in the third part,we considered that there is a strong interaction between the polar polymer P3 HT dipole and the polar solvent molecule dipole.After the polar solvent molecules are oriented,the dipole interaction tends to make P3 HT to form a highly ordered aggregate structure.This study not only expands the application range of the electric field,but also proves that the external electric field and polar solvent atmosphere treatment is an effective method for preparing ordered photovoltaic films regulating the order of the polymer and the morphology of the film and improving the photoelectric properties of the materials.In this paper,the conditions,processes and mechanisms of effective regulation of conjugated polymers from disorder to order are revealed by studying the changes of chain conformation,aggregation structure and morphology of conjugated polymers under external field.It is of great significance to prepare thin film devices with high order and excellent photoelectric properties through the external field,and has theoretical guiding significance for studying the ordered films of other conjugated polymer systems in the future. |
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