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

Abstract

under the application of relatively weak magnetic fields point toward their application as quantum sensors at room temperature. [17,18] Their fabrication flexibility and industrial use in commercial displays [19] underlie recent advances toward the miniaturization and integration of quantum technologies based on these materials. [20][21][22] For effective integration or miniaturization, reproducibility is crucial, and this extends to those properties of materials which impact spin-dependent processes. In this work, we aim to measure the variation of a critical spin property common to organic materials, the Overhauser field, which arises from the hyperfine coupling of charge-carrier spins to the bath of randomly oriented nuclear spins in the molecular environment. [23] The Overhauser field underlies a range of phenomena observed in organic devices, including magnetoresistive and magnetic resonant effects in organic light-emitting diodes (OLEDs), [24][25][26][27][28] both of which have been proposed as a mechanism to enable magnetic field sensing. [17,26] To date, the Overhauser field has been treated as a bulk property of the device, characterized by a single value that describes its impact. [26,29] If this is not a good assumption, it will present a significant challenge as we move toward high spatial resolution of magnetic fields [30,31] in organic devices. [32,33] We will demonstrate in the following sections that the Overhauser field indeed shows substantial intra-device variation by spatially resolving the magnetoluminescence effect exhibited by both a copolymer Super Yellow poly(phenylene-vinylene) (SY-PPV) and a small-molecule tris-(8-hydroxyquinoline) aluminum (Alq 3 ) OLED. As the Overhauser field is central to a wide range of spin-enabled functionality in organic devices, this variation presents a fundamental challenge for efforts to miniaturize and integrate such devices. Concept Magneto-Electroluminescence in OLEDsWe employ a stable and widely investigated active material-SY-PPV-to characterize the spatial variation of the Overhauser field in a simple vertical structure OLED (Figure 1a) and extract 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 (...

Inter‐ and Intra‐Device Variation and Correlation of Hyperfine Interactions in Micron‐Scale Organic Light‐Emitting Diodes

“…Our results for the 40 μm device is consistent with measurements on bulk SY-PPV devices showing statistically significant spatial auto-correlation up to 9 μm lag distances for both A and B hf , tending toward zero at long distances. [19] However, for the smaller devices the characteristic correlation length is altered; We find similar behavior for the 20 and 10 μm devices, with correlation lengths of 3.5 μm before trending toward zero. For the 5 μm device the B hf parameter auto-correlation decays more rapidly than in the A case, with a positive correlation for points at 2.5 μm radius and 3.5 μm for B hf and A. Interestingly at large lag distances the correlation in both parameters inverts, leading to a significant negative correlation.…”

“…Consistent with a previous result on bulk devices, we measure an average variability in B hf exceeding 20% in the 40 µm device with reduced variability in the corresponding MEL amplitude ( A ). [ 19 ] We see persistent spatial variation in the two smaller devices, with the 5µm device showing 16 % varation in B hf and 5% in A .…”

“…[15] Traditionally, hyperfine interactions in organic devices have been studied at the macroscopic level by measuring magnetic field effects [16,17] or via optically/electrically detected magnetic resonance. [7,12,18] Recently, spatially resolved magneto-electroluminescence (MEL) [19] has been developed as a technique to probe the microscopic variation and underlying structure of hyperfine interactions.…”