E. Ehrenfreund

Isotope effect in spin response of [pi]-conjugated polymer films and devices

Recent advances in organic spin response include long polaron spin-coherence times measured by optically detected magnetic resonance (ODMR), substantive room-temperature magnetoelectroluminescence and magnetoconductance obtained in organic light-emitting diodes (OLEDs) and spin-polarized carrier injection from ferromagnetic electrodes in organic spin valves (OSVs). Although the hyperfine interaction (HFI) has been foreseen to have an important role in organic spin response, no clear experimental evidence has been reported so far. Using the chemical versatility advantage of the organics, we studied and compared spin responses in films, OLED and OSV devices based on pi-conjugated polymers made of protonated, H-, and deuterated, D-hydrogen having a weaker HFI strength. We demonstrate that the HFI does indeed have a crucial role in all three spin responses. OLED films based on the D-polymers show substantially narrower magneto-electroluminescence and ODMR responses, and as a result of the longer spin diffusion obtained, OSV devices based on D-polymers show a substantially larger magnetoresistance.

Spin-Polarized Light-Emitting Diode Based on an Organic Bipolar Spin Valve

Spin-Dependent Light Emission Spintronic devices exploit electronic currents that are spin polarized, which have an excess of one spin current over the other. One way to detect this polarization would be to create a light-emitting diode that is sensitive to spin polarization. Along these lines, Nguyen et al. (p. 204) constructed a bipolar device in which an organic semiconductor was sandwiched between two ferromagnetic contacts whose relative polarization could be controlled by an applied magnetic field. Magneto-electroluminescence of the order of ∼1% was observed at a bias voltage of ∼3.5 V. The use of a deuterated organic polymer interlayer improved spin transport relative to polymers with hydrogen side groups, and a thin LiF buffer layer on the ferromagnetic cathode improved electron injection efficiency. The light emission from an organic light-emitting diode depends on the spin polarization of the injected current. The spin-polarized organic light-emitting diode (spin-OLED) has been a long-sought device within the field of organic spintronics. We designed, fabricated, and studied a spin-OLED with ferromagnetic electrodes that acts as a bipolar organic spin valve (OSV), based on a deuterated derivative of poly(phenylene-vinylene) with small hyperfine interaction. In the double-injection limit, the device shows ~1% spin valve magneto-electroluminescence (MEL) response, which follows the ferromagnetic electrode coercive fields and originates from the bipolar spin-polarized space charge–limited current. In stark contrast to the response properties of homopolar OSV devices, the MEL response in the double-injection device is practically independent of bias voltage, and its temperature dependence follows that of the ferromagnetic electrode magnetization. Our findings provide a pathway for organic displays controlled by external magnetic fields.

Photoinduced absorption and resonant Raman scattering of polythiophene

Abstract Resonant Raman scattering (RRS) and photoinduced absorption (PA) measurement in polythiophene are reported. The RRS is analyzed by the amplitude mode model modified to include an additive extrinsic component Δ e . For polythiophene we obtain a ld gap 2 Δ = 2.2 eV and 2 Δ e = 0.22 eV. The dominant photo-carriers generated with an above the gap excitation are charged bipolarons with two correlated electronic PA bands at 0.45 eV and 1.25 eV and several IR active vibrations. The PA spectrum yields for the bipolaron mass M ≅ m e in good agreement with theoretical predictions.

Negative capacitance in organic semiconductor devices: Bipolar injection and charge recombination mechanism

The authors report negative capacitance at low frequencies in organic semiconductor based diodes and show that it appears only under bipolar injection conditions. They account quantitatively for this phenomenon by the recombination current due to electron-hole annihilation. Simple addition of the recombination current to the well established model of space charge limited current in the presence of traps yields excellent fits to the experimentally measured admittance data. The dependence of the extracted characteristic recombination time on the bias voltage is indicative of a recombination process which is mediated by localized traps.

The first decade of organic spintronics research

The first decade of organic spintronics research has benefitted from the analogy and previous experience of the inorganic spintronics field, coupled with the unlimited versatility of organic materials synthesis. At the same time, the field of organic spintronics has developed into an attractive and promising field of its own, with rich physics and promising unique potential applications. We review here a set of significant milestones achieved in organic spintronic devices such as organic spin valves, bipolar spin-valves, and hybrid organic/inorganic light emitting diodes in comparison with representative inorganic spintronic devices. We also point out acute problems that need to be resolved before the young field of organic spintronics can mature.

Raman scattering and infrared phonons in polyacetylene

Abstract The Raman active normal modes of polyacetylene are described in terms of amplitude modes associated with the Peierls gap. A narrow distribution in the effective electron-phonon coupling constant accounts for the spectrum and its dependence on laser frequency. The resonant Raman condition determines a functional dependence of the gap on the coupling constant which is consistent with the Peierls relation. All infrared phonons induced by either doping or photogeneration are determined by the Raman data and a pinning parameter.

Short-lived charge-transfer excitons in organic photovoltaic cells studied by high-field magneto-photocurrent

The main route of charge photogeneration in efficient organic photovoltaic cells based on bulk hetero-junction donor-acceptor blends involves short-lived charge-transfer excitons at the donor-acceptor interfaces. The cell efficiency is critically affected by the charge-transfer exciton recombination and dissociation processes. By measuring the magneto-photocurrent under ambient conditions at room temperature, we show here that magnetic field-induced spin-mixing among the charge-transfer exciton spin sublevels occurs in fields up to at least 8.5 Tesla. The resulting magneto-photocurrent increases at high fields showing non-saturating behaviour up to the highest applied field. We attribute the observed high-field spin-mixing mechanism to the difference in the donor-acceptor g-factors. The non-saturating magneto-photocurrent response at high field indicates that there exist charge-transfer excitons with lifetime in the sub-nanosecond time domain. The non-Lorentzian high-field magneto-photocurrent response indicates a dispersive decay mechanism that originates due to a broad distribution of charge-transfer exciton lifetimes.

Thermal and magnetic studies of the nearly one-dimensional antiferromagnetic system CsNiBr3

Abstract Magnetic susceptibility, specific heat and 133Cs magnetic resonance measurements in a single crystal of CsNiBr3 are reported. The data reveal two magnetic transitions separating the paramagnetic phase from the antiferromagnetic ground state. At the higher transition temperature TN2 = (14.25 ± 0.05)K a net magnetic moment is observed only along the hexagonal c-axis, while only below the lower transition temperature TN1 = (11.75 ± 0.05)K a perpendicular component of the magnetic moment appears also. Above TN2 CsNiBr3 can be described as a one-dimensional antiferromagnet with intrachain exchange interaction J k B = −(17.0 ± 0.2) K and single-ion anisotropy constant D k B ≊ −1.5 K . Below TN1, the data are consistent with the non-colinear triangular structure of the Ni2+ moments proposed previously for the isomorphic crystal CsNiCl3. A reduced value of the zero-temperature susceptibility over the classical value is found and atrributed to the zero point deviations.

Photoluminescence and optically detected magnetic resonance in polythiophene

Abstract Photoexcitations in polythiophene were studied by photoluminescence, its excitation spectrum and its optical detected magnetic resonance. The main radiative centres are identified as weakly bound interchain excitons, while the non-radiative centres are intrachain spin 1 2 defects with g=2.003 which also dominate the ESR measurements.

Radiative lifetimes of single excitons in semiconductor quantum dots — manifestation of the spatial coherence effect

Using time correlated single photon counting combined with temperature dependent diffraction limited confocal photoluminescence spectroscopy we accurately determine, for the first time, the intrinsic radiative lifetime of single excitons confined within semiconductor quantum dots. Their lifetime is one (two) orders of magnitude longer than the intrinsic radiative lifetime of single excitons confined in semiconductor quantum wires (wells) of comparable confining dimensions. We quantitatively explain this long radiative time in terms of the reduced spatial coherence between the confined exciton dipole moment and the radiation electromagnetic field.