Organic Magnetic Field Effects Research Articles

Perspective on the Development and Integration of Hydrogen Sensors for Fuel Cell Control

The measurement of hydrogen concentration in fuel cell systems is an important prerequisite for the development of a control strategy to enhance system performance, reduce purge losses and minimize fuel cell aging effects. In this perspective paper, the working principles of hydrogen sensors are analyzed and their requirements for hydrogen control in fuel cell systems are critically discussed. The wide measurement range, absence of oxygen, high humidity and limited space turn out to be most limiting. A perspective on the development of hydrogen sensors based on palladium as a gas-sensitive metal and based on the organic magnetic field effect in organic light-emitting devices is presented. The design of a test chamber, where the sensor response can easily be analyzed under fuel cell-like conditions is proposed. This allows the generation of practical knowledge for further sensor development. The presented sensors could be integrated into the end plate to measure the hydrogen concentration at the anode in- and outlet. Further miniaturization is necessary to integrate them into the flow field of the fuel cell to avoid fuel starvation in each single cell. Compressed sensing methods are used for more efficient data analysis. By using a dynamical sensor model, control algorithms are applied with high frequency to control the hydrogen concentration, the purge process, and the recirculation pump.

Spin Injection and Transport in Organic Materials.

This review introduces some important spin phenomena of organic molecules and solids and their devices: Organic spin injection and transport, organic spin valves, organic magnetic field effects, organic excited ferromagnetism, organic spin currents, etc. We summarize the experimental and theoretical progress of organic spintronics in recent years and give prospects.

Understanding the mechanisms of light emission from DCJTB charge-transfer states in OLEDs using organic magnetic field effects

Although energy transfer (ET) and direct charge trapping (DCT) are two competing mechanisms that are known to occur in dye-doped organic light-emitting diodes (OLEDs), which of these processes governs the electroluminescence is not clear. Additionally, each process influences the formation of excitons, and the interactions between excitons, differently. If ET is the dominate process, the mechanism that governs an OLED involves both the host and guest materials. In contrast, if DCT is the dominant process, the mechanism that governs an OLED will only involve the guest material. Magneto-electroluminescence (MEL) is an effective technique for exploring and understanding the mechanisms that occur within OLEDs. It exhibits sensitive fingerprint responses to intersystem crossing (ISC), reverse intersystem crossing (RISC), triplet-triplet annihilation (TTA) and triplet-charge annihilation (TQA). In this work, we fabricated three different OLEDs and measured their MEL curves. Tris-8-hydroxyquinoline aluminum (Alq3), 4,4′- N , N ′-dicarbazolebiphenyl (CBP) and 2,4,6-tris[3-(diphenylphosphinyl)phenyl]-1,3,5-triazine (PO-T2T) were used as the host materials, each of which had a different triplet exciton energy. 4-(dicyanomethylene)-2-tert-butyl-6-(1,1,7,7-tetramethyljulolidin-4- yl -vinyl)- 4 H -pyran (DCJTB) was used as a charge-transfer (CT) dopant that had a small energy difference between the singlet and triplet excited states. By analyzing the energy level structure of the different devices, the absorption spectrum of the dopant, and the emission spectra of the host materials, we determined that ET was the dominant process when DCJTB was used as a dopant with Alq3. However, DCT dominated when DCJTB was used as a dopant with both CBP and PO-T2T. The current- and temperature-dependent MEL curves from all devices were composed of low field components (| B | ≤| B | B because both TTA and TQA were competing processes, but their low-field changes were completely different. The MEL curves from the DCJTB/Alq3 device exhibited a positive MEL because polaron-pair (PP) states can occur on the host material via ISC (PPS→PPT). Conversely, the DCJTB/CBP and DCJTB/PO-T2T devices exhibited negative MEL responses because RISC (1CT←3CT) occurred in the CT states of the dopant. However, the triplet energy levels of the host material simultaneously impacted RISC. Hence, these devices exhibited a diverse range of MEL curves. Low temperatures enhanced RISC and TTA of the triple CT state within DCJTB, which was attributed to the increased concentration and lifetime of triplet excitons at low temperature. This work serves as a reference for improving the internal quantum efficiency of fluorescent OLEDs by enhancing the use of triplet states, and importantly gives a deeper understanding of the mechanisms that govern light-emission from devices that incorporate DCJTB CT materials.

Abnormal magnetic field effects in doped organic light-emitting devices

Organic magnetic field effects (OMFEs), which primarily includes magneto-electroluminescence (MEL) and magneto-conductance (MC), is a non-contact, non-destructive and sensitive tool for probing the mechanisms that occur within in organic light-emitting diodes (OLEDs). Not only OMFEs can be used to detect charge transport, exciton evolution and luminescence mechanisms in OLEDs, and can also be used in magnetic devices, including magnetic switches, magnetic storage and magnetic sensors, etc. Stable magnetic devices are usually required for practical applications. There is a wealth of research that shows that doped devices exhibit improved stability when compared with non-doped devices, however, they always show weaker OMFEs responses. Therefore, it is important to achieve strong OMFEs responses from stable, doped devices. In this work, we have fabricated OLEDs using the thermally activated delayed fluorescence (TADF) material 2,3,5,6-tetrakis(3,6-diphenyl-carbazol-9-yl)-1,4-dicyanobenzene (4CzTPN-Ph) of different doping concentration as the dopant, and either tris-8-hydroxyquinoline aluminum (Alq3) and 4,4′- N , N ′-dicarbazole-biphenyl (CBP) as the host materials. The light-emitting layer in these OLEDs was Alq3: x % 4CzTPN-Ph (studied devices) or CBP:5% 4CzTPN-Ph (reference device). The MEL and MC curves from these devices were measured with different injection currents and doping concentrations at room temperature, and with varying temperatures at a constant injection current and doping concentration. Compared with the reduced OMFEs observed from the reference device, the studied devices exhibited increased amplitudes in their MEL and MC responses. For example, at an injection current of 150 μA at room temperature, the amplitude of the MEL from the studied device reached ~10% when exposed to a magnetic field of 300 mT, which was approximately 13 times larger than that observed from the reference device (~0.75%). The corresponding amplitude from the MC from the studied device was ~6% at 300 mT, which was approximately 50 times larger than that observed from the reference device (~0.12%). In addition, the MEL and MC responses from the studied devices were determined by the doping concentration. At a doping concentration of approximately 15%, the MEL and MC values from the studied devices at 300 mT reached maximum values of 13.4% and 9.3%, respectively. Both MEL and MC values from the studied device reduced with decreasing temperatures. Analysis of the energy levels and optical spectra from these studied devices indicated that the special energy level arrangement between the guest and host molecules resulted in a weak energy level trap. The external magnetic field suppressed the scattering of charges by triplet excitons, a channel for triplet-charge annihilation (TQA), which generated significant MEL and MC responses. Therefore, OMFEs that were stronger than those from the reference device were obtained. In addition, since TQA is affected by the concentration and mobility of triplet excitons and carriers, the doping concentration and working temperature can also mediate TQA by modifying the concentration and mobility of triplet excitons and carriers, which eventually affects the OMFEs. This work gives insight into the mechanisms that occur within 4CzTPN-Ph-based light-emitting diodes, and also importantly promotes practical applications of magnetic organic light-emitting diodes.

Tuning the polarity of organic magnetic field effects in polymer light-emitting diodes by incorporating a colloidal quantum dots thin layer

Organic magnetic field effects (MFEs) have attracted a lot of interests in foundation research and practical applications. In this work, a thin layer of colloidal quantum dots QDs was incorporated into the planar heterojunction-based polymer light-emitting diodes (PLEDs) as a sensitizer to tune the organic MFEs. As a result, the magneto-electroluminescence (MEL) and magneto-conductance (MC) in the poly [2-(4-(3′,7′-dimethyloctyloxyphenyl)-1,4-phenylene vinylene)] (P-PPV)-based PLEDs can be tuned synchronously from positive to negative. The current and temperature dependences of the negative MEL and MC were studied elaborately, showing normal and anomalous dependences compared to those of typical positive MEL and MC, respectively. The underlying mechanism of negative MEL and MC was examined. It is revealed that efficient Förster energy transfer between P-PPV and QDs is responsible for the effective tuning of MEL and MC via promoting the reverse intersystem crossing between electron-hole pairs.

Investigation of excitons fission and annihilation processes in Rubrene based devices by utilizing magneto-electroluminescence curves

That the energy of triplet exciton in Rubrene is about half of its singlet leads to energy resonance. This resonance not only allows two triplets to annihilate into a singlet, but also makes a singlet probably fission into two triplets in different molecules. On the other hand, the π-π conjugation of two Rubrene molecules could be formed during molecules stacking, and this spatial relationship will affect the charge transport property enormously. In this article, we use organic magnetic-field effect as a convenient approach to explore the influence of the energy resonant excited states in the Rubrene molecules and the π-π conjugation between the different molecules on the luminescence property of Rubrene. Firstly, we fabricate organic light emitting diodes based on pure Rubrene and modulate the thickness of Rubrene. Experimental measurements of these devices at room temperature exhibit that the thickness can affect the devices' magneto-electroluminescence (MEL) curves substantially. Values of high-field MEL increase with the thickness of Rubrene and gradually saturate after reaching 30 nm. This can be attributed to the fact that the ratio of π-π conjugation in Rubrene molecules to the stacking will grow with increasing thickness, and then saturate at a proper thickness. Subsequently, we modulate the concentration of Rubrene by doping Buthocuproine (BCP) in the active layer. Experimental results at room temperature show that the values of high-field MEL decrease as the concentration of Rubrene decreases. These results verify that the influence of π-π conjugation is not only on the MEL curves, but also on the singlet fission. Furthermore, all the MEL curves exhibit a high-field decay at low temperatures since the endothermic fission process in the Rubrene molecules becomes weaker as the temperature decreases, and the longer triplet lifetime at lower temperatures also enhances the process of triplet annihilation. Besides, the extensively existent intersystem crossing between singlet and triplet polaron pairs may affect these devices as well. Finally, the MEL curves of 20% Rubrene device at room temperature changing with various currents are successfully fitted through the combination of two exponential functions and a Lorentzian function. By means of the fitting, we confirm that the singlet exciton fission, the triplet-triplet exciton annihilation, and the intersystem crossing between singlet and triplet polarons coexist in the devices. Therefore, the varieties of these MEL curves can be attributed to the competition of these processes. The fittings reveal that the triplet-triplet exciton annihilation rate increases more obviously than the singlet exciton fission rate with increasing current. Compared with the rates of the two bimolecular interactions given before, the change of the intersystem crossing rate could be neglected because of its small magnitude. This work is helpful to expand the understanding of the internal mechanism of organic optoelectronic devices.

Theoretical investigation on magnetic field effect in organic devices with asymmetrical molecules

A mechanism of organic magnetic field effect (OMFE) based on Lorentz effect in organic light-emitting devices with asymmetrical molecules is suggested and the magnetoconductance (MC) value is calculated. By considering the collision of a positive and a negative charged polaron and exciton formation in the organic layer through a non-adiabatic dynamic process, it is found that the exciton yield can be changed by applying a magnetic field due to the Lorentz effect on the moving polarons’ phase factors. By calculating the current through the device and the MC, we obtain that the calculated MC is well consistent to some experimental observations. It is also found that the MC value is sensitive to the asymmetrical structure and the electron–phonon (e–ph) coupling of the organic material, which explains why magnetic effect in an organic semiconductor is much more apparent than its inorganic counterpart.

Magnetic field effect spectroscopy of C60-based films and devices

We performed spectroscopy of the magnetic field effect (MFE) including magneto-photoinduced absorption (MPA) and magneto-photoluminescence (MPL) at steady state conditions in annealed and pristine fullerene C60 thin films, as well as magneto-conductance (MC) in organic diodes based on C60 interlayer. The hyperfine interaction has been shown to be the primary spin mixing mechanism for the MFE in the organics. In this respect, C60 is a unique material because 98.9% of the carbon atoms are 12C isotope, having spinless nucleus and thus lack hyperfine interaction. In spite of this, we obtained substantial MPA (up to ∼15%) and significant MC and MPL in C60 films and devices, and thus mechanisms other than the hyperfine interaction are responsible for the MFE in this material. Specifically, we found that the MFE(B) response is composed of narrow (∼10 mT) and broad (>100 mT) components. The narrow MFE(B) component is due to spin-dependent triplet exciton recombination in C60, which dominates the MPA(B) response at low pump intensities in films, or the MC response at small current densities in devices. In contrast, the broad MFE(B) component dominates the MPA(B) response at high pump intensities (or large current densities for MC(B)) and is attributed to spin mixing in the polaron pairs spin manifold due to g-factor mismatch between the electron- and hole-polarons in C60. Our results show that the organic MFE has a much broader scope than believed before.

The influence of the excition recombination zone on the organic magnetic-field effect

In this work we explore the influence of the exciton recombination zone (RZ) on magnetic-field effect in tris-(8-hydroxyquinolinato) aluminum (Alq3) based organic light-emitting diodes by changing the thickness of Alq3. The magneto-electroluminescence and magneto-conductance (MC) in these devices are investigated at various temperatures and bias voltages. It is found that the sign of MC changes from positive to negative, and then back to positive with the reduction of the thickness of Alq3 at 50 K. The phenomenon observed is ascribed to the change of the exciton density in the exciton RZ. Based on the mechanisms including the hyperfine mixing, the triplet-charge interaction and interfacial dissociation or quenching of excitons, the observed results are explained qualitatively.

Direct evidence for the electron–hole pair mechanism by studying the organic magneto-electroluminescence based on charge-transfer states

Organic magneto-electroluminescence (MEL) based on the charge-transfer (CT) states was investigated to clarify the electron–hole (e–h) pair mechanism for the organic magnetic field effects. The CT state is an ideal object because its emission is a direct intermolecular recombination process without forming intramolecular exciton. We found that the MEL of the CT states is not only greater than that of the exciton, but also exhibits almost no high-field decrease at low temperatures. Our results directly prove the e–h pair mechanism. Meanwhile, the transient electroluminescence measurements with and without magnetic fields confirm that magnetic field has no effect on the charge mobility but on the charge recombination process, implying the charge mobility-related mechanisms may be less dominant above the turn-on voltage.

Effects of Magnetic Field on Conductance and Electroluminescence in Organic Devices

AbstractIn this contribution we review recent advances of the organic magnetic field effects, namely magneto‐conductance and magneto‐electroluminescence, in both experiment and theory. We describe a number of processes, including the polaron‐pair and triplet‐triplet pair mechanisms that are responsible for the magnetic field effects in organic semiconductor systems. For the polaron pair mechanism, we review in detail the effect of the hyperfine interaction, both isotropic and anisotropic; different g‐factors of the two polarons composing the pair; and the spin orbit interaction. The effect of the triplet‐triplet annihilation process is also analyzed. Experimental examples for each case treated theoretically are given.

Progress in the magnetic field effects in organic semiconductor devices

The magnetic field effects(MFEs) in organic semiconductor devices are used to describe the changes of the current and electroluminescence when none magnetic organic semiconductor devices are submitted to an external magnetic field.The MFEs in organic semiconductor devices have attracted a broad interest in the past few years because of their scientific importance and wide application.Up to date,large progresses in this field have been made.In this article,we intend to summarize the history,advances and the major results made in the field of organic MFEs,specifically the underlying mechanism for the intriguing MFEs.In the end,an outlook of the organic MFEs is made.