Olivier Stéphan

Two-photon absorption and optical power limiting of bifluorene molecule

We have studied the nonlinear absorption spectra and optical limiting properties of 2,2′-(9,9-dihexyl) bifluorene. Measurements were done in chloroform solution, in the visible range (450–650 nm), and for nanosecond time-duration pulses. The two-photon absorption (TPA) spectrum, obtained by the up-conversion fluorescence method, shows a resonance at λ=534 nm with a cross-section σTPA=60 10−50 cm4⋅s/photon-molecule. Semiempirical quantum chemistry calculation on fluorene and bifluorene suggests an enhancement of the bifluorene TPA due to coupling effects between monomers. In nonlinear transmission measurements, two-photon absorption is reinforced by the excited-state absorption that occurs during the pulse duration. At resonance, the three-photon absorption coefficient is α3=14 000 cm3/GW2 for a bifluorene concentration of 600 g/L. This strong nonlinear absorption leads to an efficient optical power limiting in the green and blue parts of the spectrum. The maximum transmitted energy is lower than 10 μJ for an input energy of up to 200 μJ in a F/5 optical geometry.

Electrochemical behaviour of 3, 4-ethylenedioxythiophene functionalized by a sulphonate group. Application to the preparation of poly(3, 4-ethylenedioxythiophene) having permanent cation-exchange properties

We report the synthesis of a new 3,4-ethylenedioxythiophene monomer functionalized by a sulphonate group. Its electrochemical polymerization in water allows the generation of water-soluble oligomers. Furthermore the electrochemical polymerization of an equimolar aqueous solution of this new monomer and the unsubstituted one produces a polymer film having permanent cation-exchange properties. This phenomenon has been investigated with hexamine ruthenium(III) and uranyl cationic species using electrochemical and radiochemical methods.

Two-photon absorption spectrum of poly(fluorene)

Abstract We report here on the two-photon absorption (TPA) properties of poly(fluorene). The two-photon absorption spectrum has been characterized in the range 550–680 nm. A resonance at λ =625 nm is observed with a cross-section of 2×10 −46 cm 4 s/photon-molecule. This large value is interpreted as the result of dipolar coupling between 12 monomer units, i.e. the polymer coherence length. In the framework of a three-level model, the TPA transition is related to the second excited state 2A g lying at 6200 cm −1 above the lowest excited state 1B u .

Light-emitting electrochemical cells using a molten delocalized salt

The device performances of light-emitting electrochemical cells are improved by adding a room-temperature molten salt (tetrahexylammonium-bis-trifluoro-methyl-sulfonyl imide) directly into the light-emitting layer. For poly(9,9-dihexyl-fluorene-2,7-diyl) with an indium-tin-oxide anode and an aluminum cathode, the power efficiency can be increased by more than one order of magnitude. An even more pronounced effect is observed for poly [2-methoxy-5-(2′-ethyl-hexyloxy)-1,4-phenylenevinylene]. Equally important for both luminescent polymers, the operating voltage is drastically reduced.

Blue light electroluminescent devices based on a copolymer derived from fluorene and carbazole

Abstract Thin layers of poly(dihexylfluorene-co-N-hexylcarbazole) (poly(DHF-co-NHK) sandwiched between indium tin oxide (ITO) and aluminium electrodes show stable photoluminescence (PL) and electroluminescence (EL) at room temperature and ambient atmosphere. I(V) and EL vs. voltage curves present a much lower threshold than device made up of poly(DHF) homopolymer. The behaviour of both materials has been quantitatively investigated through a Fowler–Nordheim analysis allowing the determination of the energy offsets between the Fermi level of the ITO electrode and the HOMO of polymers. When using the copolymer, the barrier height for hole injection is reduced to a value close to 0.17 eV compared to 0.83 eV for the homopolymer. This difference in offsets has been confirmed by electrochemical investigations.

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

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...

Organic Light-Emitting Electrochemical Cells Based on Polyfluorene. Investigation of the Failure Modes

The discovery of organic light-emitting electrochemical cells (OLECs) has opened new directions for organic light-emitting devices based on luminescent conjugated polymers However, such systems face two major problems: slow turn-on speed and short lifetime. If the p-n time formation can be efficiently reduced by increasing the ionic conductivity of the active layer by simply adding salt-laden poly(ethylene oxide), especially when the salt is fully dissociated, short lifetimes remains a key issue. A study of the decay of the electroluminescence intensity vs s. current ratio as a function of time is representative of the quantum efficiency and can be compared with the evolution of the cyclic voltammograms, representative of the system electroactivity. We report that the main degradation process involved in the decrease of the light emission intensity during device operation is based on a current-driven degradation of the semiconducting polymer, mainly a loss of capacity on the n side. Possibly this is associated with an irreversible reduction of the anions from the added salt.

Blue-green light-emitting diodes and electrochemical cells based on a copolymer derived from fluorene

Abstract A copolymer derived from fluorene has been synthesised using a fluorene monomer functionalised with poly(ethylene oxide) (PEO)-like segments as comonomer in a poly(dihexylfluorene) main chain. Efficient blue-green polymer light-emitting diodes (LEDs) based on this material and working under ambient atmosphere are reported. When using ITO as anode and aluminium as cathode, operating voltage at around 25 V are required to obtain emission intensity of 1 μW cm−2. By decreasing the film thickness from 250 to 110 nm and using calcium instead of aluminium, the corresponding operating voltage can be reduced by approximately 50% without significant loss of luminescence efficiency. Light-emitting electrochemical cells (LECs) are also demonstrated, leading to threshold operating voltages close to the electrochemical gap of poly(fluorene).

Double-layer formation in organic light-emitting electrochemical cells

We present a systematic analysis of the current transients accompanying the formation of the electrode–electrolyte double layers in organic light-emitting electrochemical cells. By using various room-temperature molten salts, conducting polymers, and electrodes, we show that the current I always decreases as a power law of time, I∝t−n. The current transients are formed of various time domains, each one being characterized by a power-law exponent n<1. Impedance measurements conducted from 5 Hz to 5 MHz demonstrate that these transients represent the time response of a simple combination of constant phase angle (CPA) impedances, Zn∝(jω)n, and of the electrolyte ionic conductivity. The physical origin of the CPA impedance is attributed to the roughness of the interface between the electrodes and the electrolyte, and to the phase separation within the salt–polymer blend.

Two-photon induced fabrication of gold microstructures in polystyrene sulfonate thin films using a ruthenium(II) dye as photoinitiator

Gold microstructures are produced with a femtosecond laser in thin films of a polystyrene sulfonate matrix containing gold ions. Two-photon induced metal reduction is obtained by addition of 0.1wt% of ruthenium(II)tris(bipyridine) in the formulation. Laser power is reduced to 5mW, thereby limiting thermal effects. Lines of typically 150nm heights and 1μm widths are fabricated as well as freestanding bidimensional structures. An additional electroless plating step produces gold structures with conductivities only ten times smaller than the bulk metal.