We present a theory for spin diffusion in disordered organic semiconductors, based on incoherent hopping of a charge carrier and coherent precession of its spin in an effective magnetic field, composed of the random hyperfine field of hydrogen nuclei and an applied magnetic field. From Monte Carlo simulations and an analysis of the waiting-time distribution of the carrier we predict a surprisingly weak temperature dependence, but a considerable magnetic-field dependence of the spin-diffusion length. We show that both predictions are in agreement with experiments on organic spin valves.

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The stochastic Liouville equation is applied to the field of organic magnetoresistance to perform detailed microscopic calculations on the different proposed models. By adapting this equation, the influence of a magnetic field on the current in bipolaron, electron-hole pair, and triplet models is calculated. The simplicity and wide applicability of the stochastic Liouville equation makes it a powerful tool for interpreting experimental results on magnetoresistance measurements in organic semiconductors. New insights are gained on the influence of hopping rates and disorder on the magnetoresistance.

By combining experiments with simple model calculations, we obtain new insight in spin transport through hybrid, CoFeB/Al2O3(1.5 nm)/tris(8-hydroxyquinoline)aluminium (Alq3)/Co spin valves. We have measured the characteristic changes in the I-V behavior as well as the intrinsic loss of magnetoresistance at the onset of multiple-step tunneling. In the regime of multiple-step tunneling, under the condition of low hopping rates, spin precession in the presence of hyperfine coupling is conjectured to be the relevant source of spin relaxation. A quantitative analysis leads to the prediction of a symmetric magnetoresistance around zero magnetic field in addition to the hysteretic magnetoresistance curves, which are indeed observed in our experiments.

A surprisingly large "organic magnetoresistance" (OMAR) has been found in both polymers and small molecule organic semiconductors at relatively small applied magnetic fields and at room temperature. We review recent highlights in OMAR research and discuss some of the models that have been proposed to explain the effect. In all models spin dephasing due to hyperfine fields plays an essential role. In particular we focus on the characteristic magnetic field dependence, which is generally fitted with either a Lorentzian or a so-called non-Lorentzian function. The shape is determined by both the hyperfine fields and an additional broadening due to microscopic mechanisms, as described in the models. Within the present work, a new empirical function is introduced that captures the two effects separately and converges to the earlier introduced lineshapes in specific limits. Recently it has been demonstrated that an additional feature can be observed at ultra-small magnetic fields. This effect can be easily incorporated in our empirical approach by explicitly treating the limit in which hopping of carriers is no longer slow compared to spin precession in the hyperfine fields. Our approach is used to analyze several theoretical and experimental results. It is shown that experimentally observed trends can be well-understood and important parameters can be obtained from experimental data without prior knowledge about which model applies.

Boron-doped Si-SiO2-Al structures are fabricated to study extremely large magnetoresistance (MR) effects. Current-voltage characteristics show a nonlinear behavior, dominated by an autocatalytic process of impact ionization. At low temperatures, the magnetic field postpones the onset of impact ionization to higher electric fields. This results in a symmetric positive MR of over 10,000% at 400 kA/m. Applying a magnetic field leads to an increase of the acceptor level compared to the valence band as deduced by admittance spectroscopy. A macroscopic transport model is introduced to describe how the MR is controlled by voltage, electrode spacing, and oxide thickness.

We investigated the efficiency and formation mechanism of ammonia generation in recombining plasmas generated from mixtures of N 2 and H 2 under various plasma conditions. In contrast to the Haber-Bosch process, in which the molecules are dissociated on a catalytic surface, under these plasma conditions the precursor molecules, N 2 and H 2 , are already dissociated in the gas phase. Surfaces are thus exposed to large fluxes of atomic N and H radicals. The ammonia production turns out to be strongly dependent on the fluxes of atomic N and H radicals to the surface. By optimizing the atomic N and H fluxes to the surface using an atomic nitrogen and hydrogen source ammonia can be formed efficiently, i.e., more than 10% of the total background pressure is measured to be ammonia. The results obtained show a strong similarity with results reported in literature, which were explained by the production of ammonia at the surface by stepwise addition reactions between adsorbed nitrogen and hydrogen containing radicals at the surface and incoming N and H containing radicals. Furthermore, our results indicate that the ammonia production is independent of wall material. The high fluxes of N and H radicals in our experiments result in a passivated surface, and the actual chemistry, leading to the formation of ammonia, takes place in an additional layer on top of this passivated surface.

Organic semiconductors are interesting materials for spintronics applications because of their long spin lifetimes. In addition, organic spintronics offers the possibility to add magnetic functionality to existing organic electronics. Two main topics of organic spintronics are discussed.First, in organic spin valves, spin transport occurs through an organic spacer layer. As a main source of loss of spin polarization the interactions of the spins with random hyperfine fields originating from surrounding hydrogen nuclei has been identified. Recent progress in the development of organic spin valves and related approaches are discussed, addressing conductivity mismatch and the question whether the observed magnetoresistance is from spin injection or direct tunneling.Second, an intrinsic magnetoresistance is observed in many organic semiconductors. This, so‐called, organic magnetoresistance (OMAR) is interesting because it shows large effects at room temperature, making it interesting for applications, and poses fundamental questions about spin transport and spin interactions in these organic materials.An overview of the main characteristics of OMAR is given and the three main models are discussed. These models are the bipolaron model, the electron–hole pair model and the exciton–charge interaction model. Finally, a comparison is made between the proposed models and experimental results, where it is concluded that the exact origin of OMAR is still open for debate.