Harnessing the spin degree of freedom in semiconductors is generally a challenging, yet rewarding task. In recent years, the large effect of a small magnetic field on the current in organic semiconductors has puzzled the young field of organic spintronics. Although the microscopic interaction mechanisms between spin-carrying particles in organic materials are well understood nowadays, there is no consensus as to which pairs of spin-carrying particles are actually influencing the current in such a drastic manner. Here we demonstrate that the spin-based particle reactions can be tuned in a blend of organic materials, and microscopic mechanisms are identified using magnetoresistance lineshapes and voltage dependencies as fingerprints. We find that different mechanisms can dominate, depending on the exact materials choice, morphology and operating conditions. Our improved understanding will contribute to the future control of magnetic field effects in organic semiconductors.

The large effect of a small magnetic field on the current, magnetoconductance (MC), in organic semiconductors-so-called organic magnetoresistance-has puzzled the field of organic spintronics during the last decade. Although the microscopic mechanisms regarding spin mixing are well understood by now, it is still unknown which pairs of spin carrying particles are influencing the current in such a drastic manner. Here, a model for the MC is presented based on the spin selective formation of metastable trions from triplet exciton-polaron pairs. Additionally, the magnetic-field and voltage dependence of the MC are experimentally investigated in materials showing large effects. Using a combination of analytical and numerical calculations, it is shown that the MC is perfectly described by a process in which trions are created at polaron trap sites.

Triplet exciton (TE) formation pathways have systematically been investigated in prototype bulk heterojunction (BHJ) SY-PPV:PCBM, SY-PPV:PC70BM, and SY-PPV:ICBA solar cell devices of varying compositions using complementary optoelectrical and electrically detected magnetic resonance (EDMR) spectroscopies. In this investigation it is shown that EDMR spectroscopy allows the unambiguous demonstration of fullerene triplet production in BHJ polymer:fullerene solar cells. EDMR triplet detection under selective photoexcitation of each blend component and of the interfacial charge transfer (CT) state unravels that low lying fullerene TEs are produced by direct intersystem crossing from singlet excitons (SEs). The direct CT-TE recombination pathway, although energetically feasible, is kinetically suppressed in these devices. However, high energy CT states in the CT manifold can contribute to the population of the fullerene triplet state via a direct CT-SE conversion. This undesirable energetic alignment could be one of the causes for the severe reduction in photocurrent observed when the open circuit voltage of polymer:fullerene solar cells is pushed to or beyond 1.0 V.

Large negative magnetoconductance (MC) of ∼12% is observed in electrochemically doped polymer light-emitting diodes at sub-band-gap bias voltages (V bias ). Simultaneously, a positive magnetoefficiency (Mη) of 9% is observed at V bias = 2 V. At higher bias voltages, both the MC and Mη diminish while a negative magnetoelectroluminescence (MEL) appears. The negative MEL effect is rationalized by triplet-triplet annihilation that leads to delayed fluorescence, whereas the positive Mη effect is related to competition between spin mixing and exciton formation leading to an enhanced singlet:triplet ratio at nonzero magnetic field. The resultant reduction in triplet exciton density is argued to reduce detrapping of polarons in the recombination zone at low-bias voltages, explaining the observed negative MC. Regarding organic magnetoresistance, this study provides experimental data to verify existing models describing magnetic field effects in organic semiconductors, which contribute to better understanding hereof. Furthermore, we present indications of strong magnetic field effects related to interactions between trapped carriers and excitons, which specifically can be studied in electrochemically doped organic light-emitting diodes (OLEDs). Regarding light-emitting electrochemical cells (LECs), this work shows that delayed fluorescence from triplet-triplet annihilation substantially contributes to the electroluminescence and the device efficiency.

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The dependence of the magnetic field sensitive current on the orientation of the magnetic field has been investigated in organic semiconductor devices where the active layer consists of the poly(p-phenylene vinylene) derivative "Super Yellow." Previous work on Alq 3 suggested that the anisotropy was caused either by anisotropic spin-spin interactions or by anisotropic hyperfine fields, but no discrimination could be made. In the present work, the anisotropy at the hyperfine field scale is best explained by dipolar coupling between the spin of polarons. In addition, a high field anisotropy is found with an opposite sign, different angle, and voltage dependence. Spin density matrix calculations were performed of polaron pair interactions for the low field effect, and a g-mechanism, triplet-polaron, or triplet-triplet interaction for the high field effect. The simulations confirm that the low field anisotropy can indeed be explained by dipolar coupling. However, the proposed models can not entirely account for the high field anisotropy. These results show that, although contemporary models can account for (anisotropic) magnetic field effects in organic semiconductors at low field scales, more experimental and theoretical research of high field effects is highly desirable.

Fringe fields emanating from magnetic domain structures can give rise to magnetoresistance in organic semiconductors. In this article, we explain these magnetic-field effects in terms of a B mechanism. This mechanism describes how variations in magnetic-field strength between two polaron hopping sites can induce a difference in precessional motion of the polaron spins, leading to mixing of their spin states. In order to experimentally explore the fringe-field effects, polymer thin-film devices on top of a rough in-plane magnetized cobalt layer are investigated. The cobalt layer can be described by a distribution of out-of-plane magnetic anisotropies, most likely induced by thickness variations in the cobalt. With a magnetic field perpendicular to the cobalt layer, fringe fields are created because some domains are magnetized out of plane whereas the magnetization of other domains remains approximately in plane. By varying the distance between the polymer layer and the cobalt layer, we find that the magnetoresistance arising from these fringe fields reduces with the gradient in the fringe fields, in agreement with the B mechanism.