In Situ Real-time Characterization Of Structure Change In The Thin Film Of Poly Octylfluorene When It Is Under Annealing

The organic electroluminescence light emitting device, in order to achievefull-color display, it must have a stable red, green, and blue color emitting. Currently, onlyred and green organic light emitting material has value for commercial development for itslight-emitting efficiency and service life, As for blue light, to achieve high stability, highefficiency emission is still a big problem. Poly （9,9-di-n-octyl-2,7-fluorene）（PFO） has beenextensively studied due to its high hole mobility, high photoluminescence quantumefficiency, and polymorphism. Beside amorphous, four different phases,α, α’, β and nematicphase, have been identified using X-ray diffraction （XRD）, transmission electron microscope,UV-Vis absorption spectrum and photoluminescence spectrum. Among these phase, the βphase of PFO has been highly focused because of its pure blue emission and specificphysical properties.The PFO thin film achieved by normal spin-coating method isamorphous. To generate beta phase in the thin film, the thin film need to be treated bymethods such as solvent vapor annealing or dipping into solvent/non-solvent mixture. Inparticular, the photoluminescence and electroluminescence can be dramatically enhanceddue to self-dopant effect when trace content beta phase PFO generate. Alpha phase PFO alsohas excellent performance and it can be more easily achieved by thermal annealing, amethod frequently used to remove the residue solvent after spin-coating. Our previous workdemonstrates that properly controlling thermal annealing temperature can lead to low contentof alpha phase PFO, which can also dramatically enhance the photoluminescence andelectroluminescence due to self-dopant.Although the crystalline structure of α phase has been clearly determined asorthorhombic （a=2.56nm, b=2.34nm, c=3.32nm,8chains, with space group P2₁2₁2₁anddensity1.041g/mL）, the mechanism of phase transition of thin film from amorphous toalpha phase is still unclear. This is due to that the past studies used thick PFO film （ca.50μm） in beta phase and thus the phenomenon in phase transition of thin film from amorphousto alpha phase, such as the critical phase transition temperature of80℃and increase of alphaphase content with increasing the thermal annealing temperature, had not been observed.Therefore it is necessary to in-suit study the PFO thin film during the phase transition process from amorphous to alpha phase to study the mechanism. In this paper, wecharacterized the change of nanostructure of PFO thin film in-suit during thermal annealingby using grazing incidence X-ray diffraction, UV-Vis absorption spectrum and infraredspectrum to reveal the mechanism of phase transition and discover a novel metastable liquidcrystalline phase. And successfully explained the mechanism of phase transition of PFOduring the thermal annealing process. During the heating process, when the temperature wasbelow80℃, movement of main chain of PFO amorphous film did not appeared, but sidechain began to move, resulting in the high regularity of the whole system. Due to the limitedimpact of the side chain on the entire system, in this period the diffraction peak of relativevalue is very low. When the temperature arrived at80℃, movement of the main chainhappened as well as the fusion of side chain, and the state of system changed fromamorphous phase to a more orderly metastable k phase. With the increasing of temperature,more and more amorphous phase transferred into k phase, at the same time the regularity ofthe system is higher and higher. Once the temperature reached160℃, melting began. In thecooling process, rearrangement of main chain happened, leading to formation of k phase andmore regularity of the system. Without limitation of prior entanglement of molecular chainsand space, the rearrangement of main chain when cooling was far more easier than theheating process. When the temperature cooled to80℃, the metastable k phase changed intoa stable alpha phase. With the decreasing of temperature, side chain started rearranging, andthe internal stress of the side chain in the limited space due to activities on the main chaintangles, made the structure of the main chain experiencing a certain amount of fine-tuning,together with a small decline of the overall regularity. While the high crystalline alphafluorene film is heating up again below80℃, the main chain keeps still and the side chainstart moving, but little impact happened to the system. As soon as the temperature raised to80℃, tangles of side chain on main chain gradually eliminate in the melting process, mainchain released, then the neat arrangement of alpha phase appeared at80℃. Therefore, thereis a slightly rising of overall regularity. Above80℃, alpha phase begin to transfer intometastable phase until melting. Following cooling process repeat.