Designing pi-conjugated polymers for organic electronics

White polymer light-emitting devices (WPLEDs) have become a field of immense interest in both scientific and industrial communities. They have unique advantages such as low cost, light weight, ease of device fabrication, and large area manufacturing. Applications of WPLEDs for solid-state lighting are of special interest because about 20% of the generated electricity on the earth is consumed by lighting. To date, incandescent light bulbs (with a typical power efficiency of 12-17 lm W(-1) ) and fluorescent lamps (about 40-70 lm W(-1) ) are the most widely used lighting sources. However, incandescent light bulbs convert 90% of their consumed power into heat while fluorescent lamps contain a small but significant amount of toxic mercury in the tube, which complicates an environmentally friendly disposal. Remarkably, the device performances of WPLEDs have recently been demonstrated to be as efficient as those of fluorescent lamps. Here, we summarize the recent advances in WPLEDs with special attention paid to the design of novel luminescent dopants and device structures. Such advancements minimize the gap (for both efficiency and stability) from other lighting sources such as fluorescent lamps, light-emitting diodes based on inorganic semiconductors, and vacuum-deposited small-molecular devices, thus rendering WPLEDs equally competitive as these counterparts currently in use for illumination purposes.

White Polymer Light-Emitting Devices for Solid-State Lighting: Materials, Devices, and Recent Progress

Excimers or Emissive On-Chain Defects?

A series of four new statistical copolymers of 9,9-dihexylfluorene and 9-fluorenone with well-defined structures and a new fluorene−fluorenone−fluorene trimer model compound were synthesized and used to investigate the photophysics, origin of the green emission, and electroluminescence of this class of light-emitting materials. We show that the new oligofluorene trimer with a central fluorenone moiety is an excellent model of the green-emitting chromophore in polyfluorenes. From systematic studies of the steady-state photoluminescence (PL) and PL decay dynamics of solutions of the fluorenone-containing copolymers and oligomer and thin films of the copolymers, we show that the controversial 535-nm green emission band originates from the fluorenone defects in single-chain polyfluorenes and not from intermolecular aggregates or excimers. The green emission, centered at 535 nm, was observed in dilute toluene solutions of all fluorenone-containing copolymers and oligomer; it was long-lived with a single-expone...

Fluorenone-Containing Polyfluorenes and Oligofluorenes: Photophysics, Origin of the Green Emission and Efficient Green Electroluminescence†

A new route to ladder-type pentaphenylenes has been developed in which both good hole-accepting p-type and electron-accepting n-type materials can be prepared from a common intermediate. This key intermediate is a pentaphenylene diester 5 obtained in high yield by Suzuki coupling of 2 equiv of fluorene boronates with 2,5-dibromoterephthalate. Addition of aryllithium followed by ring closure with boron trifluoride produced a blue-emitting ladder-type pentaphenylene. Bromination followed by reductive polymerization with nickel(0) gave new high molecular mass polymers, which show efficient blue emission with a very small Stokes shift. These polymers bridge the gap in emission between polyfluorenes and fully ladder-type polyphenylenes. An alternative ring closure of the dibromopentaphenylene diester 14 with acid made a diketone that is a good electron-accepting material, as it displays a reversible two-electron reduction. The reduction onset potential of −0.875 V against Ag/Ag+ corresponds to a lowest unoccup...

Ladder-type pentaphenylenes and their polymers: efficient blue-light emitters and electron-accepting materials via a common intermediate.

We have synthesized and characterized a new fluorene copolymer exhibiting bright yellow luminescence. In order to ensure a complete π-stacking of the active layer, a 9-fluorenone monomeric unit (FOne) has been used as comonomer in conjunction with the more classical 9,9-di-n-nonylfluorene unit. As expected with fluorene-based materials, when excited at 370 nm, the corresponding dilute copolymer solution photoluminescence spectra exhibit a main peak centered at 450 nm in the blue part of the visible spectrum. However, in the solid state, immediate structural reorganization of the layer occurs, leading to a red-shifted emission (bright yellow emission) centered at 550 nm. The origin of the emitted light has been attributed to excimers and/or aggregates based on short FOne segments and involves mainly exciton transfer between nonaggregated fluorene segments and aggregated ones. It is noteworthy that organic light-emitting devices based on these new materials exhibit no spectral evolution upon device operatio...

Fluorene–fluorenone copolymer: Stable and efficient yellow-emitting material for electroluminescent devices

Carrier dynamics properties of poly(alkylthiophene) are investigated via their radiative and nonradiative recombination rates, depending on the length of the alkyl side chain. The recombination rates are determined by luminescence quantum efficiency and photoluminescence decay time measurements in the picosecond range. A large decrease in radiative rate, going along with an increase in nonradiative rate, was observed in the solid state when compared to dilute solution, and was associated with the well known π stacking of the thiophene rings. As already reported from field effect mobility measurements, the study of poly(hexylthiophene) and poly(octylthiophene) in the solid state showed an increase of the carrier transport for polythiophene with the shortest alkyl chain length. We then discuss the differences between poly(dinonylfluorene) and poly(alkylthiophene), both prone to aggregation. In the solid state, the origin of the emission band in the yellow part of the emission spectrum, which appears upon oxidative aging or annealing, is still controversial. Nevertheless, we explain here that this redshift results from fluorenone excimer/aggregate emission. However, the amount of aggregated species is too small to allow good carrier transport.

S. Panozzo

Papers

Fluorene–fluorenone copolymer: Stable and efficient yellow-emitting material for electroluminescent devices

Interplay between carrier dynamics and structure of poly(alkylthiophene) and poly(dialkylfluorene) films