Organic magnetoelectroluminescence for room temperature transduction between magnetic and optical information

Degradation Analysis of Organic Light-Emitting Diodes through Dispersive Magneto-Electroluminescence Response

Understanding the stability and degradation of organic light-emitting diodes (OLEDs) under working conditions is a significant area of research for developing more effective OLEDs and further improving their performance. However, studies of degradation processes by in situ noninvasive methods have not been adequately developed. In this work, tris-(8-hydroxyquinolino) aluminum (Alq3)-based OLED degradation processes have been analyzed through the investigation of the device dispersive magneto-electroluminescence (MEL(B)) response measured at room temperature. By studying the change in the MEL(B) response during the device degradation under different external stimuli, such as exposing the device to the atmosphere and prolonged illumination by a strong visible light source, we have gained insight into the microscopic spin-dependent phenomena that control the recombination of e-h polaron pairs in the device. We found that the device degradation leads to a shorter e-h polaron lifetime, smaller dispersive parameter, and broader lifetime distribution function that shows increased disorder in the active layer. This study could offer a potential tool that may be beneficial for assessing the degradation of OLED devices based on various active layers.

Resolving the Spatial Variation and Correlation of Hyperfine Spin Properties in Organic Light-Emitting Diodes

Devices that exploit the quantum properties of materials are widespread, with quantum information processors and quantum sensors showing significant progress. Organic materials offer interesting opportunities for quantum technologies owing to their engineerable spin properties, with spintronic operation and spin resonance magnetic-field sensing demonstrated in research grade devices, as well as proven compatibility with large-scale fabrication techniques. Yet several important challenges remain as moving toward scaling these proof-of-principle quantum devices to larger integrated logic systems or spatially smaller sensing elements, particularly those associated with the variation of quantum properties both within and between devices. Here, spatially resolved magnetoluminescence is used to provide the first 2D map of a hyperfine spin property-the Overhauser field-in traditional organic light-emitting diodes (OLEDs). Intra-device variabilities are found to exceed ≈30% while spatially correlated behavior is exhibited on lengths beyond 7 µm, similar in size to pixels in state-of-the-art active-matrix OLED arrays, which has implications for the reproducibility and integration of organic quantum devices.

Magneto-Electroluminescence Study of Fringe Field in "Magnetic" Organic Light-Emitting Diodes

Magneto-electroluminescence (MEL) represents the electroluminescence intensity change upon application of an external magnetic field. We show that the MEL field response in "magnetic" organic light-emitting diodes, where one electrode is ferromagnetic (FM), is a powerful technique for measuring the induced fringe field, B⃗_F, from the FM electrode in the organic layer. We found that the in-plane fringe field, B⃗_F∥, from 3 nm Co and Ni₈₀Fe₂₀ FM electrodes is proportional to the applied field, B⃗_∥. The fringe field of the 3 nm Ni₈₀Fe₂₀ film was also investigated for an applied out-of-plane magnetic field, B⃗_⊥. We found that the out-of-plane fringe field has two components: a component that is parallel or antiparallel to B⃗_⊥ and remains unchanged with the distance, d, from the FM electrode and the other component that is highly inhomogeneous, parallel to the surface, and steeply decreases with d. We show that the obtained B⃗_F is independent of the underlying mechanism for the MEL(B) response and thus may be considered universal.

Enhancing Light Emission in Interface Engineered Spin-OLEDs through Spin-Polarized Injection at High Voltages

The quest for a spin-polarized organic light-emitting diode (spin-OLED) is a common goal in the emerging fields of molecular electronics and spintronics. In this device, two ferromagnetic (FM) electrodes are used to enhance the electroluminescence intensity of the OLED through a magnetic control of the spin polarization of the injected carriers. The major difficulty is that the driving voltage of an OLED device exceeds a few volts, while spin injection in organic materials is only efficient at low voltages. The fabrication of a spin-OLED that uses a conjugated polymer as bipolar spin collector layer and ferromagnetic electrodes is reported here. Through a careful engineering of the organic/inorganic interfaces, it is succeeded in obtaining a light-emitting device showing spin-valve effects at high voltages (up to 14 V). This allows the detection of a magneto-electroluminescence (MEL) enhancement on the order of a 2.4% at 9 V for the antiparallel (AP) configuration of the magnetic electrodes. This observation provides evidence for the long-standing fundamental issue of injecting spins from magnetic electrodes into the frontier levels of a molecular semiconductor. The finding opens the way for the design of multifunctional devices coupling the light and the spin degrees of freedom.

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.

In situ investigation of energy transfer in hybrid organic/colloidal quantum dot light-emitting diodes via magneto-electroluminescence

Energy transfer (ET) and charge injection (CI) in the hybrid organic/colloidal quantum dot light-emitting diodes (QD-LEDs) have been investigated by using magneto-electroluminescence (MEL) as an in situ tool. The feasibility and availability of MEL as an in situ tool were systematically demonstrated in the typical QD-LEDs based on CdSe-ZnS core-shell QDs. Our results suggest that the ET and CI processes can be well discerned by MEL measurements since these two processes exhibit distinct responses to the applied magnetic field. Through measurement of the MEL and current efficiency, we indicated that ET would be the main mechanism for light emission in the present hybrid QD-LEDs. This study strongly suggests that MEL could be a highly sensitive fingerprint for ET, which provides a facile and efficient method for the in situ investigation of fundamental processes in hybrid organic/colloidal QD-LEDs and other organic/inorganic composites.

A Model of Charge-Transfer Excitons: Diffusion, Spin Dynamics, and Magnetic Field Effects

In this Letter, we explore how the microscopic dynamics of charge-transfer (CT) excitons are influenced by the presence of an external magnetic field in disordered molecular semiconductors. This influence is driven by the dynamic interplay between the spin and spatial degrees of freedom of the electron-hole pair. To account for this interplay, we have developed a numerical framework that combines a traditional model of quantum spin dynamics with a stochastic coarse-grained model of charge transport. This combination provides a general and efficient methodology for simulating the effects of magnetic field on CT state dynamics, therefore providing a basis for revealing the microscopic origin of experimentally observed magnetic field effects. We demonstrate that simulations carried out on our model are capable of reproducing experimental results as well as generating theoretical predictions related to the efficiency of organic electronic materials.

Hyperfine interaction vs. spin–orbit coupling in organic semiconductors

We report experimental and theoretical studies on hyperfine interaction vs. spin–orbit coupling in a thin film of organic semiconductor poly[9,9-di-n-hexylfluorenyl-2,7-diyl] and the dramatic influence of doping the PFO with bis[2-(2′-benzothienyl)pyridinato-N,C3′]Ir(acac).

Singlet-to-triplet interconversion using hyperfine as well as ferromagnetic fringe fields

Until recently the important role that spin-physics ('spintronics') plays in organic light-emitting devices and photovoltaic cells was not sufficiently recognized. This attitude has begun to change. We review our recent work that shows that spatially rapidly varying local magnetic fields that may be present in the organic layer dramatically affect electronic transport properties and electroluminescence efficiency. Competition between spin-dynamics due to these spatially varying fields and an applied, spatially homogeneous magnetic field leads to large magnetoresistance, even at room temperature where the thermodynamic influences of the resulting nuclear and electronic Zeeman splittings are negligible. Spatially rapidly varying local magnetic fields are naturally present in many organic materials in the form of nuclear hyperfine fields, but we will also review a second method of controlling the electrical conductivity/electroluminescence, using the spatially varying magnetic fringe fields of a magnetically unsaturated ferromagnet. Fringe-field magnetoresistance has a magnitude of several per cent and is hysteretic and anisotropic. This new method of control is sensitive to even remanent magnetic states, leading to different conductivity/electroluminescence values in the absence of an applied field. We briefly review a model based on fringe-field-induced polaron-pair spin-dynamics that successfully describes several key features of the experimental fringe-field magnetoresistance and magnetoelectroluminescence.