Tho Duc Nguyen

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.

Hyperfine interaction and magnetoresistance in organic semiconductors

We explore the possibility that hyperfine interaction causes the recently discovered organic magnetoresistance (OMAR) effect. We deduce a simple fitting formula from the hyperfine Hamiltonian that relates the saturation field of the OMAR traces to the hyperfine coupling constant. We compare the fitting results to literature values for this parameter. Furthermore, we apply an excitonic pair mechanism model based on hyperfine interaction, previously suggested by others to explain various magnetic-field effects in organics, to the OMAR data. Whereas this model can explain a few key aspects of the experimental data, we uncover several fundamental contradictions as well. By varying the injection efficiency for minority carriers in the devices, we show experimentally that OMAR is only weakly dependent on the ratio between excitons formed and carriers injected, likely excluding any excitonic effect as the origin of OMAR.

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.

Magnetoresistance in π-conjugated organic sandwich devices with varying hyperfine and spin–orbit coupling strengths, and varying dopant concentrations

We study a recently discovered organic magnetoresistive (OMAR) effect whose underlying mechanism is currently not known with certainty. We examine the hypothesis that OMAR is caused by spin-dynamics by varying the strength of the hyperfine and spin–orbit coupling in the organic semiconductor material. We show that unlike most other materials, C60 devices do not exhibit the OMAR effect. Therefore hydrogen atoms and the resulting strong nuclear hyperfine coupling is a necessary prerequisite for the observation of OMAR. We investigate the dependence of OMAR on the strength of the spin-coupling introduced by heavy atoms in two widely used organic semiconductors, and find the characteristic magnetic field scale shifts to much larger fields. We also study OMAR sandwich devices made from the conducting polymer PEDOT. We show that in PEDOT the observed effect is caused by interface resistance, distinct from the case of intrinsic devices where OMAR is related to the bulk resistance of the (undoped) organic semiconductor.

An 8

We present the design principles for a pen-input organic light-emitting diode (OLED) display based on the recently discovered organic magnetoresistance effect (OMAR). In the prototypical OLED material Alq3, OMAR is as large as 10% for small magnetic fields, B=10 mT at room temperature. We construct a pen-input screen consisting of an 8 times 8 pixel OMAR array made from Alq3, together with a magnetic pen that emits an ac magnetic field. We describe a multiplexed detection scheme that uses a single filter/amplifier circuit to sequentially scan the individual pixels for the presence of the magnetic pen. For this scheme to work efficiently, it requires using frequencies on the order of 100 kHz. We demonstrate that our OMAR devices can indeed follow such high frequencies under certain operating conditions

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We report on the experimental characterization of a recently discovered large magnetoresistive effect in polyfluorene and in Alq3 organic light-emitting diodes. We also observe similar magnetic field effects (MFEs) of comparable magnitude in electroluminescence and photocurrent measurements. We provide a comprehensive overview of all these three types of MFE. To the best of our knowledge, the mechanism causing these MFEs is not currently known with certainty. Moreover, we show that experiments in bipolar, electroluminescent devices do not allow determination of whether the MFE acts on the carrier density or carrier mobility, making any attempt at explaining it ambiguous. As a remedy, we perform magnetoresistance measurements in hole-only polyfluorene devices and show that the MFE acts on the carrier mobility rather than carrier recombination.

8 Pixel Array Pen-Input OLED Screen Based on Organic Magnetoresistance

We investigated curvature-enhanced spin-orbit coupling (SOC) and spinterface effect in carbon-based organic spin valves (OSVs) using buckyball C60 and C70 molecules. Since the naturally abundant 12C has spinless nuclear, the materials have negligible hyperfine interaction (HFI) and the same intrinsic SOC, but different curvature SOC due to their distinct curvatures. We fitted the thickness dependence of magnetoresistance (MR) in OSVs at various temperatures using the modified Jullière equation. We found that the spin diffusion length in the C70 film is above 120 nm, clearly longer than that in C60 film at all temperatures. The effective SOC ratio of the C70 film to the C60 film was estimated to be about 0.8. This was confirmed by the magneto-electroluminescence (MEL) measurement in fullerene-based light emitting diodes (LED). Next, the effective spin polarization in C70-based OSVs is smaller than that in C60-based OSVs implying that they have different spinterface effect. First principle calculation study shows that the spin polarization of the dz2 orbital electrons of Co atoms contacted with C60 is larger causing better effective spin polarization at the interface.

Magnetic field effects on current, electroluminescence and photocurrent in organic light-emitting diodes

We demonstrate that conically tapered cylindrical apertures can be used to efficiently concentrate broadband terahertz (THz) radiation. Keeping the aperture diameter on the input plane fixed, we show that as the diameter of the aperture on the exit plane is decreased, we obtain an increase in the magnitude of the transmitted electric field that varies inversely with the output aperture diameter. Correspondingly, the transmitted THz intensity concentration increases inversely with the square of the output aperture diameter. For the smallest aperture that we fabricated, we obtain a ~50 fold increase in the transmitted THz intensity. We expect further increases in the intensity concentration with smaller output apertures. As the output aperture diameter is decreased with a corresponding increase in the concentration factor, we directly measure an increase in the propagation time delay of a narrowband pulse through the structure. Finally, we demonstrate that further increase in the concentration factor can be achieved by engraving circular grooves around the input aperture.

Curvature-enhanced Spin-orbit Coupling and Spinterface Effect in Fullerene-based Spin Valves.

Abstract We present magnetoconductivity and magnetoluminescence measurements in sandwich devices made from films of a π-conjugated molecule and demonstrate effects of more than 30 and 50% magnitude, respectively, in fields of 100 mT at room-temperature. It has previously been recognized that the effect is caused by hyperfine coupling, and that it is phenomenologically similar to other magnetic field effects that act on electron–hole pairs, which are well-known in spin-chemistry. However, we show that the very large magnitude of the effect contradicts present knowledge of the electron–hole pair recombination processes in electroluminescent π-conjugated molecules, and that the effect persists even in almost hole-only devices. Therefore, this effect is likely caused by the interaction of radical pairs of equal charge.

Concentration of terahertz radiation through a conically tapered aperture

We perform charge-induced absorption and electroluminescence spectroscopy in a polyfluorene organic magnetoresistive device. Our experiments allow us to measure the singlet exciton, triplet exciton, and polaron densities in a live device under an applied magnetic field and to test the predictions of three different models that were proposed to explain organic magnetoresistance. These models are based on different spin-dependent interactions, namely, exciton formation, triplet-exciton polaron quenching, and bipolaron formation. We show that the singlet exciton, triplet exciton, and polaron densities and conductivity all increase with increasing magnetic field. Our data appear to be inconsistent with the exciton formation and triplet-exciton polaron quenching models.