Polyfluorenes

This chapter reviews the synthesis of polymers based on bridged phenylenes for use in electronic applications such as blue-light-emitting diodes and polymer lasers. We show how the optoelectronic properties may be tuned by varying the amount of bridging between one bridge per two phenylenes in polyfluorenes through to completely bridged ladder-type polyphenylenes, and by changing the nature of the substituents at the bridgeheads. Of particular importance is the suppression of undesirable long wavelength emission by controlling interchain interactions and choosing synthetic methods so as to hinder or prevent the formation of emissive defects. The electroluminescence efficiency of these materials can also be enhanced by the use of charge-transporting substituents. Copolymerisation with lower band-gap units enables tuning of the emission colour across the entire visible range.

This article mainly reviews the approaches that have been proposed to improve the device performance of polyfluorene (PF)-based polymer light-emitting diodes (PLEDs). Chemical modifications on main chains, side chains, and chain ends of PFs via the incorporation of charge-transport moieties can reduce the hole- and electron-injection barriers; while physical manipulation on main-chain structures of PFs, poly(9,9-di-n-octylfluorene) (PFO), after dipping in mixed solvent/non-solvent (tetrahydrofuran/methanol) can generate β-phase with extended conjugation length, leading to a promoted charge balance and stable pure blue emission. Hole- and electron-injection barriers can be effectively lowered by the insertion of hole-transport and electron-injection layers, respectively. The recent development of PFs with various emission colors, via physical blending or chemical modification, are presented for a comprehensive understanding of PFs for device applications. The deliberate choice of cathode material with work function matching the lowest unoccupied molecular orbital (LUMO) levels of PFs is another efficient method for increasing electron flux, and is also discussed in this review.

Poly(dibenzosilole)s are an emerging class of polymers with similar optoelectronic properties to polyfluorenes. With increased stability towards oxidation, several poly(dibenzosilole)-based devices, such as light emitting diodes, have shown improved performance over their polyfluorene counterparts. As a consequence of reduced conjugation in the polymer chain, some poly(dibenzosilole)s have high triplet excited state energies, which make them suitable hosts for blue triplet emitters in electrophosphorescent devices.

This chapter reviews the recent progress made in the synthesis and characterization of carbazole-based oligomers and polymers. In fact, four classes of those p-type semiconducting materials will be presented: oligomers and polymers derived from carbazoles, indolo[3,2-b]carbazoles, diindolo[3,2-b:2′,3′-h]carbazoles, and bisindenocarbazoles. Synthetic pathways used to obtain ladder-type polymers with rod-like structure will also be reported. The characterization of the optical and electrical properties will be discussed with a strong emphasis on the structure–property relationships. Great attention will also be paid to the performances obtained with these materials used as active layers in different types of devices such as field-effect transistors, light-emitting diodes, photovoltaic cells, and thermoelectric devices.

Polyfluorenes containing metal complexes, especially phosphorescent heavy-metal complexes, are a type of important optoelectronic materials. The present review summarizes the synthesis and optoelectronic properties of polyfluorenes with phosphorescent heavy-metal complexes on the chain. Efficient energy transfer from polymer main-chain to metal centers occurs in these host–guest systems. The promise of strong emission and the easiness for processing in organic electronic devices provide incentives to develop these materials intensively. The range of applications of these materials spans the whole field of interaction between light and electricity. Especially, attention is given to the interesting optoelectronic properties and promising applications in organic light-emitting diodes (OLEDs), memory devices, and sensors.

Recent advances in fluorene-based conjugated oligomers are surveyed, including molecular design, material synthesis and characterization, and potential application to organic photonics and electronics, such as light-emitting diodes, solid-state lasers, field effect transistors, and solar cells.

The fundamental optical properties and processes that occur in the polyfluorenes are discussed. Details are given on the production of singlet and triplet excitons, their emissive decay channels via fluorescence and phosphorescence and non emissive decay via quenching, together with exciton lifetime and quantum yields of production. The interaction of excitons in these polymers is then discussed at length describing exciton migration dynamics, exciton–exciton annihilation processes and delayed fluorescence and the trapping of excitons at defect sites. Photogenerated charge state production is described along with intra-chain charge transfer states in fluorene co-polymers. Finally, sections describing the optical properties of the beta-phase of polyfluorene are given along with amplified spontaneous emission which occurs at high beta phase exciton populations.

This account provides a state of the art overview of polyfluorene structure and phase behaviour in solutions and the solid state. This review covers key aspects of the hierarchical intra- and intermolecular self-assembly starting at the molecular level and extentding up to larger length scale structures. This includes crystallization, alignment on surfaces and texture. Many Central ideas are highlighted via structural archetypes. Recent theoretical treatments for understanding these structural properties are discussed and the implications for opto-electronics and photophysics are described.

Polyfluorenes and polyfluorene-type polymers have emerged as a promising class of emitter materials for realizing blue polymeric light-emitting devices. However, during device operation material degradation leads to the occurrence of an unwanted green emission band at 2.2–2.3 eV. This chapter reviews the latest scientific investigations on the origin of this low energy emission band, putting special focus on chemical (i.e., keto defect sites, hydroxy-terminated polyfluorenes, etc.) and interface defects. Along this line, the formation of defect sites during polymer synthesis, their enhancement in the solid state upon, e.g., thermal stress, and the defect emission under device operation are discussed. Finally, novel approaches for realizing stable blue emitter materials are included, demonstrating that a thorough consideration of the herein presented results in materials design paves the way to polyfluorene-type polymers suitable for practical applications.

Single molecule spectroscopy resolves some of the crucial issues in the photophysics of polyfluorenes. After an introduction on single molecule spectroscopy we present the ability of this technique in revealing the intramolecular nature of the photophysical phenomena encountered in the different phases of polyfluorene and in fluorenone–fluorene copolymers. First, we correlate the peculiar emission properties of the phases to their chain extension, probed by using polarisation sensitive studies. Moreover, by looking at single chromophores in the β phase, we demonstrate how a slight bending in this one-dimensional structure does not disrupt the π conjugation while impacting on the exciton linewidth, a crucial parameter for the comprehension of energy and charge transfer. As a definitive answer to the colour degradation, we report the single chain spectroscopy of fluorenone–fluorene copolymers, demonstrating the monomolecular origin of the green emission. More generally, the presented experiments illustrate the unique possibilities offered by single molecule spectroscopy in correlating the structure and the resulting electronic properties in conjugated polymers.