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

Abstract

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.

Single-Molecule Spectroscopy of Polyfluorene Chains Reveals β-Phase Content and Phase Reversibility in Organic Solvents

“…11 This b phase is characterized by a red-shifted luminescence spectrum 7 and an increased exciton diffusion length and results in devices with improved performance. 3,5,12,13 Therefore, PFO has high commercial potential in addition to research value to understand CP polymorphism. 14,15 As a result, significant efforts have been devoted to understanding and controlling the chemical and solution-processing parameters that induce chain planarization and promote b-phase content in PFO polymers.…”

Discovery and structure characteristics of the intermediate-state conformation of poly(9,9-dioctylfluorene) (PFO) in the dynamic process of conformation transformation and its effects on carrier mobility

“…As is known, thin films used for photo-electron devices not only need ordered chain packing as the channels of the carrier transportation but also require smooth surfaces in order to decrease the interface obstacles between the electrodes. 34 …”

“…33 However, few studies have been devoted to quantitative research on the PFO b conformation; deeper and more detailed study is crucial to basic and application studies of PFO. [34][35][36][37] It was found that a relatively low content of PFO b conformation can contribute to ordered aggregation of PFO and thus enhance its carrier mobility. Conversely, a remarkably high content of b conformation will prompt PFO chains to form macroscopic aggregations, which can result in a rough lm surface and is unsuitable for device fabrication and device performance optimization.…”

Efficiency enhancement of polyfluorene: Polystyrene blend light-emitting diodes by simultaneous trap dilution and β-phase formation

“…1) since it has been shown that the hole mobility is significantly higher for b-phase PFO than for the amorphous phase. 17,38 We reason that this mobility-enhancing effect of the b-phase may compensate for the suppression of the current density due to spatial separation of transport sites.…”