Phase Transition Mechanism Between Different Phases Of Polyoctylfluorene Thin Films

In the past 20 years,the semiconductor industry has flourished,and the research on organic semiconductor devices has been explored more deeply.Organic light-emitting diode(OLED)is recognized as the most ideal technology for the next-generation flat panel displays and solid-state lighting.However,the stability of the OLED device is still an important index to decide whether OLED can be industrialed.Not only can it be affected by the device structure design,but also the stability of the active layer.The stability of lightemitting layer includes light stability,thermal stability,and solvent vapor stability.In this paper,a classical blue light-emitting polymer,Polyoctylfluorene(PFO)is selected as the research object,and the phase transition mechanism between different phases of PFO is stuied.PFO is a conjugated polymer of polymorphism,beside amorphous,four different phases,i.e.,?,?,??and nematic phase has been identified.Among them,the amorphous phase is most commonly used in device,and ? phase maintains pure blue emission,which can be doped with amorphous phase for improving efficiency of devices significantly.And ? phase is a highly crystallined phase with excellent permformance.In this paper,amorphous phase and ? phase are studied for the thermal stability of PFO,and the microstructure changes of amorphous phase and ? phase PFO in the process of thermal annealing are characterized.In addition,the solvent vapor annealing process of ? phase PFO which has not been studied by scholars has been characterized for the solvent vapor stability of PFO.For polyoctetfluorene film,(1)80-100 °C is a phase transition interval,when the temperature is lower than 80 °C,the energy is not enough for the movement of the main chain of molecules.When the temperature is higher than 100 °C,the molecules in the film will be into metastable phase,and the phase state after annealing is transformed from metastable phase;(2)the PFO side chains are more sensitive to temperature,and the movement of side chains is more active than main chains,which can drive the main chain movement acting on the regularity of the main chain;(3)There are more than one metastable phases during solvent vapor annealing of ? conformation.Based on the analysis of the microstructure of PFO films in different phase states during thermal annealing,it is found that the metastable phase is a common phase state in the phase transition,which is likely to be common in all conjugated polymer?s phase transitions.Besides that,based on X-ray diffraction an in-situ solvent vapor treatment characterization system which can monitor solvent concentrations was built up.Further,It has been confirmed that the metastable phase does exist.It is ? phase during thermal annealing or ?? conformation during solvent vapor annealing,and the existence of the metastable phase has impact on the phase transformation.In addition,we also confirmed the excellent thermal stability of ? phase and the feasible method for obtaining high crystallinity ? phase PFO.This method is of great significance to eliminate Joule heat effect,improving device stability and device efficiency.Totally,we carried out in-depth research on the phase transition mechanism of polyoctetfluorene,developed a characterization system to meet our needs,and improved insitu testing methodology.All of those would have impact on the development of organic semiconductor materials.