Jean-Charles Ribierre

Non-volatile organic memory with sub-millimetre bending radius

High-performance non-volatile memory that can operate under various mechanical deformations such as bending and folding is in great demand for the future smart wearable and foldable electronics. Here we demonstrate non-volatile solution-processed ferroelectric organic field-effect transistor memories operating in p- and n-type dual mode, with excellent mechanical flexibility. Our devices contain a ferroelectric poly(vinylidene fluoride-co-trifluoroethylene) thin insulator layer and use a quinoidal oligothiophene derivative (QQT(CN)4) as organic semiconductor. Our dual-mode field-effect devices are highly reliable with data retention and endurance of >6,000 s and 100 cycles, respectively, even after 1,000 bending cycles at both extreme bending radii as low as 500 μm and with sharp folding involving inelastic deformation of the device. Nano-indentation and nano scratch studies are performed to characterize the mechanical properties of organic layers and understand the crucial role played by QQT(CN)4 on the mechanical flexibility of our devices.

Amplified spontaneous emission and lasing properties of bisfluorene-cored dendrimers

A study of the amplified spontaneous emission (ASE) properties of three bisfluorene-cored dendrimers in the solid state is reported. The results show that the dendron type has a strong impact on the photoluminescence quantum yield and affects the ASE threshold, the optical gain, and loss coefficients. Optically pumped distributed feedback lasers operating in the blue spectral region were fabricated by spin coating the dendrimer films on top of a two-dimensional corrugated fused silica substrate. A best lasing threshold of 4.5 mu J/cm(2) and a slope efficiency of 8.3% were obtained, which demonstrate the high potential of these materials for laser applications. (c) 2007 American Institute of Physics.

Reversible Conversion of the Majority Carrier Type in Solution‐Processed Ambipolar Quinoidal Oligothiophene Thin Films

There has been a growing interest in recent years in ambipolar organic semiconductors for their applications in light-emitting transistors, [ 1 , 2 ] sensors [ 3 ] and complementary metal oxide semiconductor (CMOS)-like integrated circuits. [ 4–8 ] A number of ambipolar organic fi eld-effect transistors (OFETs) have been previously reported using blends [ 4 , 5 , 9 ] or heterostructures [ 7 , 10 , 11 ] of pand n-type materials. However, the most attractive approach for the realization of ambipolar FETs is based on the use of single-component low bandgap organic semiconducting materials that enables the injection of both holes and electrons from a same metal electrode and notably simplify the device fabrication procedures. [ 4 , 6 , 12–15 ] Soluble organic ambipolar materials, which can be deposited into thin fi lms by spin-coating or inkjet printing techniques, have shown hole and electron mobilities higher than 0.01 cm 2 V − 1 s − 1 and are particularly attractive for low-cost, large-area fl exible electronic devices. [ 15–17 ] The possibility to selectively and spatially modify or even suppress the conduction of one type of charge carrier in ambipolar organic materials is extremely attractive for the development of high performance and low-power consuming CMOS organic logic circuits. A few approaches have been reported so far to control the ambipolar charge transport properties of organic thin fi lms. This has been achieved in pentacene OFETs by modifi cation of the dielectric/semiconductor interface or by changing the source/drain metal electrodes. [ 18 , 19 ] Conversion from n-type to p-type has been also observed in titanyl-phthalocyanine after exposition of the organic thin fi lm to oxygen. [ 20 ] Another method to selectively control the ambipolar transport has been the introduction of buffer layers in OFETs based on polythiophene/fullerene blends. [ 21 ] On the other hand, unipolarization from ambipolar p-type dominant to n-type behavior has been achieved by thermal annealing

Direct laser writing of complementary logic gates and lateral p-n diodes in a solution-processible monolithic organic semiconductor

2010 WILEY-VCH Verlag Gmb Fabrication of p–n structures is a key issue in a number of electronic devices, including rectifying diodes, solar cells, and bipolar transistors. Complementary metal oxide semiconductor (CMOS) technology based on the integration of discrete pand n-channel field-effect transistors (FETs) on a same substrate has been widely used in a variety of electronic applications and enables the fabrication of integrated circuits with low-power dissipation and high operational stability. In the growing field of organic electronics, efficient large-scale organic CMOS integrated circuits as well as lateral p–n diodes have already been realized by thermally evaporating pand n-type semiconducting organic molecules separately through shadow masks. Alternatively, fabrication of organic inverters by inkjet printing, self-assembly, or spin-coating has also been reported and has demonstrated the enormous potential of solution processing for low-cost, large-area, flexible electronics. However, in spite of remarkable recent advances in high-mobility organic FET materials and nanostructuration techniques, there has still been no reliable approach for effectively patterning p–n bipolar and CMOS microstructures made from solutionprocessible organic semiconductors. We have addressed this issue by exploiting the unique charge-transport properties of the solution-processible dicyanomethylene-substituted quinoidal quaterthiophene [QQT(CN)4], which can be converted from an ambipolar p-type-dominant to n-type semiconductor by either thermal annealing or direct laser writing. The chemical structure of the QQT(CN)4 oligomer is shown in the inset of Figure 1a. Quinoidal oligothiophene derivatives generally show a low highest occupied molecular orbital (HOMO)–lowest unoccupied molecular orbital (LUMO) energy bandgap and are very promising candidates for n-type or single-component ambipolar organic FETs. We have fabricated top-contact bottom-gate organic FETs under ambient conditions by spin-coating a QQT(CN)4 thin film on top of a polyimide gate-dielectric layer and using chromium drain/source electrodes. The transfer and output characteristics displayed in Figure 1 show that the device based on as-prepared QQT(CN)4 thin film exhibits an ambipolar behavior with a predominant hole-transport character. The charge-carrier field-effect mobilities were extracted from the transfer characteristics in the saturated regions. Hole and electron mobilities were measured to be 2 1 10 3 and 4 1 10 4 cm V 1 s , respectively. Compared to the QQT(CN)4–based device performance obtained using octadecyltrichlorosilane-treated SiO2 as gate-dielectric layer and gold top-contact electrodes, these mobility values are substantially lower. However, the use of a polyimide gate dielectric presents the significant advantage of strongly reducing

Majority carrier type conversion in solution-processed organic transistors and flexible complementary logic circuits

We report on the realization of high performance solution-processed ambipolar organic transistors based on a quinoidal oligothiophene derivative. The devices show hole and electron field-effect mobilities in air as high as 0.1 and 0.006 cm2 V−1 s−1, respectively, and can be converted from ambipolar p-type dominant to n-type transistors by thermal annealing. The conversion of the majority carrier type is assigned to strong variations in molecular packing. The demonstration of complementary flexible inverters suggests an effective strategy for patterning lateral pn-bipolar structures in solution-processed thin films made from a monolithic ambipolar organic semiconductor.

Direct comparison of mechanical and electro-optic responses of a low Tg photorefractive doped polymer

The temperature dependence of the electro-optic responses in a low glass transition temperature (Tg) photorefractive polymer was investigated using an ellipsometric technique. The sample was composed of a carbazole functionalized polysiloxane doped with a push–pull chalcone derivative. The results provide information on the orientational dynamics of the chromophores doping the polymer host. For this purpose, the electro-optic response is directly compared, for different temperatures above Tg, to dynamic shear compliance measurements characterizing the mechanical macroscopic behavior of the material. We demonstrate here that these orientational processes are entirely ruled by the mechanical properties of the material.

Influence of the dendron chemical structure on the photophysical properties of bisfluorene-cored dendrimers

A detailed study of the photophysics of a family of bisfluorene-cored dendrimers is reported. Polarized time-resolved fluorescence, singlet-singlet exciton annihilation and fluorescence quantum yield measurements were performed and used to understand how the dendron structure affects the light-emitting properties of the materials. The exciton diffusion rate is similar in all films studied. An increase in the nonradiative deactivation rate by nearly one order of magnitude is observed in films of dendrimers with stilbenyl and carbazolyl based dendrons as compared to solutions, whereas the dendrimers with biphenyl and diphenylethylenyl dendrons showed highly efficient emission (photoluminescence quantum yields of 90%) in both solution and the solid state. The results of the materials that show fluorescence quenching can be explained by the presence of quenching sites at a concentration of just a fraction of a percent of all macromolecules. A possible explanation of this quenching is hole transfer from the emissive chromophore to the dendron in a face-to-face geometry. These results are important for the design of efficient blue emitters for optoelectronic applications.

Influence of the liquid carbazole concentration on charge trapping in C60 sensitized photorefractive polymers

We study by two-beam coupling experiments the photorefractive properties of a poly(Nvinylcarbazole) (PVK) matrix plasticized with different concentrations of 9-(2-ethylhexyl)carbazole (EHCz), so-called liquid carbazole, and doped with the photosensitizer C60 and the electro-optic push-pull chromophore 4-piperidinobenzylidene malonitrile. The steady state and dynamic photorefractive performances of these materials are optimized by tuning the concentration ratio between PVK and EHCz. In parallel, the trap density values are determined by a spectroscopic method based on the strong absorption of the C60− anion in the near infrared region and are compared with those obtained from the photorefractive measurements. The results obtained from both techniques are in good agreement and show the important role played by the liquid carbazole in the charge trapping processes and the photorefractive properties of C60 sensitized photorefractive polymers.

Tuning the Direction of Intramolecular Charge Transfer and the Nature of the Fluorescent State in a T‑Shaped Molecular Dyad

Controlling photoinduced intramolecular charge transfer at the molecular scale is key to the development of molecular devices for nanooptoelectronics. Here, we describe the design, synthesis, electronic characterization, and photophysical properties of two electron donor-acceptor molecular systems that consist of tolane and BF2-containing curcuminoid chromophoric subunits connected in a T-shaped arrangement. The two π-conjugated segments intersect at the electron acceptor dioxaborine core. From steady-state electronic absorption and fluorescence emission, we find that the photophysics of the dialkylamino-substituted analogue is governed by the occurrence of two closely lying excited states. From DFT calculations, we show that excitation in either of these two states results in a distinct shift of the electron density, whether it occurs along the curcuminoid or tolane moiety. Femtosecond transient absorption spectroscopy confirmed these findings. As a consequence, the nature of the emitting state and the photophysical properties are strongly dependent on solvent polarity. Moreover, these characteristics can also be switched by protonation or complexation at the nitrogen atom of the amino group. These features set new approaches toward the construction of a three-terminal molecular system in which the lateral branch would transduce a change of electronic state and ultimately control charge transport in a molecular-scale device.

Solvent-free fluidic organic dye lasers

We report on the demonstration of liquid organic dye lasers based on 9-(2-ethylhexyl)carbazole (EHCz), so-called liquid carbazole, doped with green- and red-emitting laser dyes. Both waveguide and Fabry-Perot type microcavity fluidic organic dye lasers were prepared by capillary action under solvent-free conditions. Cascade Förster-type energy transfer processes from liquid carbazole to laser dyes were employed to achieve color-variable amplified spontaneous emission and lasing. Overall, this study provides the first step towards the development of solvent-free fluidic organic semiconducting lasers and demonstrates a new kind of optoelectronic applications for liquid organic semiconductors.