Jérôme Faure-Vincent

High tunnel magnetoresistance in epitaxial Fe/MgO/Fe tunnel junctions

We report on spin-polarized tunneling in fully epitaxial Fe/MgO/Fe/Co tunnel junctions. By increasing the thickness of the insulating layer (tMgO), we have strongly enhanced the tunnel magnetoresistance. Values up to ∼100% at 80 K (∼67% at room temperature) have been observed with tMgO=2.5 nm. This tunnel magnetoresistance ratio, which is much larger than the one predicted by the Julliere’s model, can be understood in the framework of ab initio calculations.

Epitaxial MgO layer for low-resistance and coupling-free magnetic tunnel junctions

Epitaxially grown magnetic tunnel junctions MgO(100)/Fe/MgO/Fe/Co/Pd have been elaborated by molecular beam epitaxy, with insulating layer thickness down to 0.8 nm. The continuity of this layer was checked at different spatial scales by means of morphological (high resolution transmission electronic microscopy), electric (local impedance), and magnetic (magnetoresistance and hysteresis loop) measurements. These junctions show a low resistance (4 kΩ μm2), tunnel magnetoresistance up to 17%, and a very small interlayer magnetic coupling.

Enhanced Charge Separation in Ternary P3HT/PCBM/CuInS2 Nanocrystals Hybrid Solar Cells

Geminate recombination of bound polaron pairs at the donor/acceptor interface is one of the major loss mechanisms in organic bulk heterojunction solar cells. One way to overcome Coulomb attraction between opposite charge carriers and to achieve their full dissociation is the introduction of high dielectric permittivity materials such as nanoparticles of narrow band gap semiconductors. We selected CuInS2 nanocrystals of 7.4 nm size, which present intermediate energy levels with respect to poly(3-hexylthiophene) (P3HT) and Phenyl-C61-butyric acid methyl ester (PCBM). Efficient charge transfer from P3HT to nanocrystals takes place as evidenced by light-induced electron spin resonance. Charge transfer between nanocrystals and PCBM only occurs after replacing bulky dodecanethiol (DDT) surface ligands with shorter 1,2-ethylhexanethiol (EHT) ligands. Solar cells containing in the active layer a ternary blend of P3HT:PCBM:CuInS2-EHT nanocrystals in 1:1:0.5 mass ratio show strongly improved short circuit current density and a higher fill factor with respect to the P3HT:PCBM reference device. Complementary measurements of the absorption properties, external quantum efficiency and charge carrier mobility indicate that enhanced charge separation in the ternary blend is at the origin of the observed behavior. The same trend is observed for blends using the glassy polymer poly(triarylamine) (PTAA).

Static and dynamic aspects of spin tunnelling in crystalline magnetic tunnel junctions

Single-crystal magnetic tunnel junctions employing bcc (100) Fe electrodes and MgO(100) insulating barrier are elaborated by molecular beam epitaxy. The magneto-transport properties are investigated in two extreme regimes. First, for extremely small MgO thickness, we show that the equilibrium tunnel transport in Fe/MgO/Fe systems leads to antiferromagnetic interactions, mediated by the tunnelling of the minority spin interfacial resonance state. Second, for large MgO barrier thickness, the tunnel transport validates specific spin filtering effects in terms of symmetry of the electronic Bloch function and symmetry-dependent wavefunction attenuation in the single-crystal barrier. Within this framework, we present giant tunnel magnetoresistive effects at room temperature (125–160%). Moreover, we illustrate that the interfacial chemical and electronic structure plays a crucial role in the spin filtering. We point out imperfect filtering effects and a strong implication of the minority surface state of Fe on the low voltage variation of tunnel magnetoresistance. The insertion of carbon impurities at the Fe/MgO interface changes radically the voltage response of the tunnel magnetoresistance and activates a resonant tunnelling mechanism via the interfacial resonance state.

Charge transport in poly(3-hexylthiophene):CdSe nanocrystals hybrid thin films investigated with time-of-flight measurements

Charge carrier mobilities and transport dispersivity are studied in hybrid films composed of poly(3-hexylthiophene) and CdSe nanocrystals by the time-of-flight method, using a field range of 105–106 V/cm. It is found that charge transport parameters pass through an optimum for 75 wt. % (36 vol. %) of nanocrystals in the polymer matrix, yielding balanced hole and electron mobilities around 10−3 cm2/V s. Changing the nanocrystal shape from spherical to branched increases the intersite coupling disorder in the hybrid whereas changing the surface ligands from stearate/oleylamine to pyridine decreases it. Both these modifications have an impact on the electric field dependence of the measured mobilities.

Synthesis, optoelectronic properties and photovoltaic performances of wide band-gap copolymers based on dibenzosilole and quinoxaline units, rivals to P3HT

Three π-conjugated alternating copolymers, based on dibenzosilole as an electron-rich unit and fluorinated or non-fluorinated quinoxaline as an electron-withdrawing unit, connected through thiazole or thiophene moieties, have been synthesized, fully characterized and applied as donors in polymer solar cells (PSCs). The three copolymers, namely PDBS-TQx, PDBS-TQxF and PDBS-TzQx, belong to the wide band-gap semiconductor materials family, and they show an absorption edge in the visible region close to 650 nm. In order to tune the position of the polymer energy levels, and in particular to decrease their HOMO energy level, we compare the use of a thiazole spacer sandwiching the electron-deficient moiety as an alternative way to the popular backbone fluorination. PSCs based on a blend of PDBS-TQx and [6,6]-phenyl-C71-butyric acid methylester (PC71BM) as an active layer have shown the best device performances with a maximum power conversion efficiency (PCE) of 5.14% for the active area of 0.28 cm2 (under standard illumination of AM 1.5G, 1000 W m−2). Interestingly this polymer outperforms P3HT:(PC61BM) solar cells used as a reference material in this work. In addition to the thorough characterization data, including among other spectroscopy techniques, XRD, OFET, AFM and nc-AFM, we discuss in detail the relationship between the chemical structures of the three polymers, their optoelectronic properties, the phase separation in blends with PC71BM and their photovoltaic performances.

Antiferromagnetic coupling by spin polarized tunneling

By performing magnetic studies on Fe/MgO/Fe magnetic tunnel junctions, we provide experimental evidence of room-temperature antiferromagnetic coupling between two ferromagnetic layers across a very thin insulating barrier. Epitaxial growth of the MgO barrier on a very flat Fe layer leads to an extremely low “orange peel” magnetic coupling. Then, antiferromagnetic coupling is observed for MgO thickness, tMgO, below 0.8 nm. The strength of this coupling increases abruptly when reducing tMgO down to 0.5 nm. The shape of the variation of experimental coupling strength J with tMgO, the quantitative value of |J|, and finally, the thickness range of tMgO for which the antiferromagnetic coupling is observed are in good agreement with the equilibrium interlayer exchange theory by the spin polarized quantum tunneling of electrons between the ferromagnetic layers.

Colloidal CuInSe2 nanocrystals thin films of low surface roughness

Thin-film processing of colloidal semiconductor nanocrystals (NCs) is a prerequisite for their use in (opto-)electronic devices. The commonly used spin-coating is highly materials consuming as the overwhelming amount of deposited matter is ejected from the substrate during the spinning process. Also, the well-known dip-coating and drop-casting procedures present disadvantages in terms of the surface roughness and control of the film thickness. We show that the doctor blade technique is an efficient method for preparing nanocrystal films of controlled thickness and low surface roughness. In particular, by optimizing the deposition conditions, smooth and pinhole-free films of 11?nm CuInSe2 NCs have been obtained exhibiting a surface roughness of 13?nm root mean square (rms) for a 350?nm thick film, and less than 4?nm rms for a 75?nm thick film.

Synthesis, optoelectronic and photovoltaic properties of conjugated alternating copolymers incorporating 2,1,3-benzothiadiazole or fluorenone units: a comparative study

A series of p-type alternating copolymers based on oligothiophene and fluorene electron-donating units, and fluorenone or benzothiadiazole electron-withdrawing units was synthesised via Suzuki coupling reaction. Their optical, electrochemical and photovoltaic properties were investigated in detail allowing rational rules to be drawn on the structure–property relationships. We highlight in this work the crucial role of the side solubilizing groups attached to the polymer backbones that have a significant impact on the morphology of the blends and the solar cells power conversion efficiency. Devices with an active area of 0.28 cm2, based on fluorenone-copolymers and [70]PCBM blends showed Voc up to 0.95 V and Power Conversion Efficiencies (PCE) up to 1.82% under AM1.5 simulated sunlight (100 mW cm−2) conditions. These values are improved compared to the ones reported to date for this class of materials.

XPS depth profiling of organic photodetectors with the gas cluster ion beam

In this work, the authors study active layers of organic photodetector devices containing phenyl-C61-butyric acid methyl ester and Poly[(4,8-bis-(2-ethylhexyloxy)-benzo(1,2-b:4,5-b′)dithiophene)-2,6-diyl-alt-(4-(2-ethylhexanoyl)-thieno[3,4-b]thiophene-)-2-6-diyl)] (PBDTTT-C). Thin films are examined by x-ray photoemission spectroscopy coupled with argon gas cluster ion sputtering. The use of massive cluster projectiles instead of monoatomic ions has the advantage of not destroying the chemical structure of organic materials under study. The authors show how simulated aging influences the chemistry of these blends and how these alterations extend from sample surface into the bulk of the film. The authors identify several possible processes resulting from aging, including C=O bond breakage and PBDTTT-C diffusion.