From Starting Formation to the Saturation Content of the β‑Phase in Poly(9,9-dioctylfluorene) Toluene Solutions

In this research, the relationship between chain aggregation and poly­(9,9-dioctylfluorene) (PFO) β-phase content with a change of the PFO solution concentration is investigated. Photoluminescence and UV–vis absorption spectra are used to explore the PFO β-phase and its content. Chain aggregation is explored by dynamic light scattering (DLS). It is found that the chain aggregation size of the starting to form PFO β-phase is approximately 100 nm. The β-phase content increases with the chain aggregation degree. When the β-phase content is in the range of 44% ± 2%, the maximum value of the content (saturation content) of the β-phase is reached and cannot be changed by any external field that could induce β-phase formation. Therefore, the concept of the saturation content of the PFO β-phase is first proposed. The mechanism from PFO β-phase formation to saturation content is explored by DLS and atomic force microscopy. This research is significant to understand and control the molecular chain condensed-state process as well as phase transition from solution to film to achieve a photoelectric device with high charge carrier mobility, stability, and efficiency

Quantitative Study on β‑Phase Heredity Based on Poly(9,9-dioctylfluorene) from Solutions to Films and the Effect on Hole Mobility

In this work, the quantitative relationship in the heredity of β-phase from a solution to a thin film based on poly­(9,9-dioctylfluorene) (PFO), the mechanism of β-phase formation, and the effects of β-phase contents on hole mobility were investigated. The heredity based on PFO β-phase from the solution to the thin film was characterized through UV–vis absorption. Results indicated that β-phase can be completely transferred from solutions to films during drying to form films. PFO β-phase was stable and could manage the dynamic changes from a liquid state to a thin-film state. The β-phase content was higher in the diluted solutions, and the reason was revealed through dynamic light scattering. Thus, a new structure model was constructed, and polymer chain aggregation was rendered unnecessary during PFO β-phase formation. The energy status of the β-phase was lower than that of the α-phase. Consequently, PFO chains were autonomously assembled to become orderly. The chemical environment of the low-concentration solution was more suitable than that of the high-concentration solution. The polymer chains in the former could more freely adapt to a flat geometry than those in the latter to facilitate interchain stacking. Chain aggregation was then observed through transmission electron microscopy. Photoinduced charge extraction with a linear increase in voltage was also performed to examine the charge density and hole mobility of PFO. Hole mobility could be enhanced by an order of magnitude when β-phase was increased from 0% to 5.4%. Thus, the presence of a small amount of ordered domains that can form interconnected channels could strongly enhance the carrier transport of materials in poorly ordered organic thin films, such as PFO. This condition is possibly beneficial for photoelectronic devices, and the adaptive nature of PFO chains in solutions to form a flat geometry is the main factor that promotes the order of the system