Tuning the performance of organic spintronic devices using x-ray generated traps

Trap-Filling Magnetoconductance as an Initialization and Readout Mechanism of Triplet Exciton Spins

Photoexcited triplet states are promising candidates for hybrid qubit systems, as they can be used as a controlling gate for nuclear spins. But microwave readout schemes do not generally offer the sensitivity needed to approach the single-molecule limit or the scope to integrate such systems into devices. Here, we demonstrate the possibility of electrical readout of triplet spins at room temperature through a specific mechanism of magnetoconductance (MC) in polycrystalline pentacene. We show that hole-only pentacene devices exhibit a positive photoinduced MC response that is consistent with a trap-filling mechanism. Spin and magnetic-field-dependent quenching of photogenerated triplets by holes quantitatively explains the MC response we observe. These results are distinct in both sign and proposed mechanism compared to previous reports on polyacene materials and provide clear design rules for future spintronic devices based on this spin-sensing mechanism.

Simulation and Theory of Classical Spin Hopping on a Lattice

The behavior of spin for incoherently hopping carriers is critical to understand in a variety of systems such as organic semiconductors, amorphous semiconductors, and muon-implanted materials. This work specifically examined the spin relaxation of hopping spin/charge carriers through a cubic lattice in the presence of varying degrees of energy disorder when the carrier spin is treated classically and random spin rotations are suffered during the hopping process (to mimic spin–orbit coupling effects) instead of during the wait time period (which would be more appropriate for hyperfine coupling). The problem was studied under a variety of different assumptions regarding the hopping rates and the random local fields. In some cases, analytic solutions for the spin relaxation rate were obtained. In all the models, we found that exponentially distributed energy disorder led to a drastic reduction in spin polarization losses that fell nonexponentially.

Organic Single-Crystal Spintronics: Magnetoresistance Devices with High Magnetic-Field Sensitivity

Organic spintronics is a new emerging field that deals with the spin-related phenomena of organic materials under the influence of a magnetic field. However, there remain some challenges in organic spintronics including (i) low conductivity and massive disorders of organic thin films blocking the way to controllable spin transport, (ii) relatively low magnetic-field sensitivity of organic magnetoresistance (OMAR) devices with tangled working mechanisms and short of methods for sensitivity improvement. Here, we report the realization of OMAR devices based on organic single crystals. The lesser amount of impurities and defects in crystals guarantees a reduction in spin and charge scatterings, so that the OMAR devices exhibit both a small Lorentz function fitting parameter B₀ of 2.3 mT and a non-Lorentz function fitting parameter B₁ of 0.86 mT in the strictly limited bipolaron model. Moreover, we demonstrate the effect of aggregation and intrinsic trap states, pointing out a way for the improvement of the sensitivity.

Electrical Detection of Singlet Fission in Single Crystal Tetracene Transistors

We present the electrical detection of singlet fission in tetracene by using a field-effect transistor (FET). Singlet fission is a photoinduced spin-dependent process, yielding two triplet excitons from the absorption of a single photon. In this study, we engineered a more deterministic platform composed of an organic single crystal FET rather than amorphous or polycrystalline FETs to elucidate spin-dependent processes under magnetic fields. Despite the unipolar operation and relatively high mobility of single crystal tetracene FETs, we were able to manipulate spin dependent processes to detect magnetoconductance (MC) at room temperature by illuminating the FETs and tuning the bias voltage to adjust majority charge carrier density and trap occupancy. In considering the crystalline direction and magnetic field interactions in tetracene, we show the MC response observed in tetracene FETs to be the result of the singlet fission process.

Magnetoresistance Effect and the Applications for Organic Spin Valves Using Molecular Spacers

Organic spin devices utilizing the properties of both spin and charge inherent in electrons have attracted extensive research interest in the field of future electronic device development. In the last decade, magnetoresistance effects, including giant magetoresistance and tunneling magnetoresistance, have been observed in organic spintronics. Significant progress has been made in understanding spin-dependent transport phenomena, such as spin injection or tunneling, manipulation, and detection in organic spintronics. However, to date, materials that are effective for preparing organic spin devices for commercial applications are still lacking. In this report, we introduce basic knowledge of the fabrication and evaluation of organic spin devices, and review some remarkable applications for organic spin valves using molecular spacers. The current bottlenecks that hinder further enhancement for the performance of organic spin devices is also discussed. This report presents some research ideas for designing organic spin devices operated at room temperature.

Unraveling the Characteristic Shape for Magnetic Field Effects in Polymer-Fullerene Solar Cells

Spin-dependent effects in organic solar cells (OSCs) are responsible for tuning the electric current when an external magnetic field is applied. Here, we report the magnetic field effect (MFE) on wide-bandgap (WBG) solar cells based on the polymers PBDT(O)-T1 and PBDT(Se)-T1 blended with PC₇₀BM. Furthermore, we propose an experimental method based on the electrical transport (i-V) measurements to unveil the negative magneto conductance (MC) at small bias. The observed curves in a double-logarithmic scale display a particular S-like shape, independent of the OSC power conversion efficiency (PCE) or MC amplitudes. Additionally, from the slope of the S-like shape curve, it is possible to identify the fullerene concentrations that would result in the minimum MC and the maximum PCE. Our work opens up a door to find more patterns to describe MFE and PCE in polymer-fullerene solar cells, without the application of external magnetic or luminous sources.

Spin-chemistry concepts for spintronics scientists

Spin chemistry and spintronics developed independently and with different terminology. Until now, the interaction between the two fields has been very limited. In this review, we compile the two "languages" in an effort to enhance communication. We expect that knowledge of spin chemistry will accelerate progress in spintronics.

Magneto-conductance characteristics of trapped triplet-polaron and triplet-trapped polaron interactions in anthracene-based organic light emitting diodes

The effects of a magnetic field on the dissociation of triplet excitons by free charges (TCI) are well understood. However, the magneto-conductance (MC) characteristics of trapped triplet-polaron interactions (T_tPI) and triplet-trapped polaron interactions (TP_tI) within organic light emitting diodes (OLEDs) are not well understood. We have studied these interactions in an anthracene-based OLED. The electroluminescence spectra, current-voltage characteristics and magneto-electroluminescence indicated that the anthracene layer contained many defects that could trap either triplet excitons or polarons, which led to TP_tI and T_tPI. The MC curves at low temperature exhibited a complex line shape, which indicated that intersystem crossing, TP_tI, T_tPI, and TCI occurred simultaneously in the device. The individual MC characteristics of TP_tI and T_tPI were extracted from temperature dependant MC curves by fitting them to three empirical Lorentzian functions and one non-Lorentzian function. The MC of TP_tI exhibited a negative sign, while that of T_tPI exhibited a positive one, with characteristic magnetic fields (B₀) of ∼10.5 and ∼15 mT, respectively. Both processes were prominent below 150 K and weakened with increasing temperature. TP_tI was neglected above 200 K, while T_tPI was observed even at ambient temperature. These results add significant insight into the magnetic field effects on triplet-polaron interactions.

Exchange-Dominated Pure Spin Current Transport in Alq3 Molecules

We address the controversy over the spin transport mechanism in Alq3 utilizing spin pumping in the Y3Fe5O12/Alq3/Pd system. An unusual angular dependence of the inverse spin Hall effect is found. It, however, disappears when the microwave magnetic field is fully in the sample plane, excluding the presence of the Hanle effect. Together with the quantitative temperature-dependent measurements, these results provide compelling evidence that the pure spin current transport in Alq3 is dominated by the exchange-mediated mechanism.

Spin Polarization Inversion at Benzene-Absorbed Fe4N Surface

We report a first-principle study on electronic structure and simulation of the spin-polarized scanning tunneling microscopy graphic of a benzene/Fe(4)N interface. Fe(4)N is a compound ferromagnet suitable for many spintronic applications. We found that, depending on the particular termination schemes and interface configurations, the spin polarization on the benzene surface shows a rich variety of properties ranging from cosine-type oscillation to polarization inversion. Spin-polarization inversion above benzene is resulting from the hybridizations between C p(z) and the out-of-plane d orbitals of Fe atom.

The first decade of organic spintronics research

The significant milestones in organic spintronics achieved during the first decade of research are reviewed.

Charge and spin transport in PEDOT:PSS nanoscale lateral devices

The electrical transport of the highly conductive poly-(3,4-ethylenedioxythiophene):poly(styrenesulfonic acid) (PEDOT:PSS) is investigated with Ohmic and spin-polarized tunnel contacts at nanoscale lateral dimensions. Temperature-dependent charge transport measurements reveal that electrical conductivity scales non-linearly as a function of electrode spacing, which is attributed to the localization of carriers induced by the disorder introduced by the PSS polyelectrolyte. In addition, we demonstrate the integration of this conducting polymer in nanoscale lateral spin-valve devices by increasing the pH of the PEDOT:PSS solution. We present charge and magnetotransport measurement results of NiFe/AlOx/PEDOT:PSS/AlOx/NiFe lateral structures for various thicknesses of the alumina tunnel barriers. We discuss the absence of magnetoresistance of our spin valves within the framework of Valet-Fert theory, and estimate an upper limit for the spin lifetime of carriers in PEDOT:PSS to τsf ≤ 50 ns.