Room-temperature coupling between electrical current and nuclear spins in OLEDs

Malissa, H.; Kavand, Marzieh; Waters, D. P.; Schooten, Kipp J. van; Burn, Paul L.; Vardeny, Z. Valy; Saam, B.; Lupton, John M.; Boehme, Christoph

doi:10.1126/science.1255624

Cited by 86 publications

(102 citation statements)

References 24 publications

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“…At room temperature, charge-carrier spins in the OLED material poly[2-methoxy-5-(2 -ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) are characterized by long spin coherence and relaxation times, T 2 ≈ 350 ns and T 1 ≈ 40 µs, respectively 1 . These parameters ensure that even tiny static magnetic fields (weaker than nuclear hyperfine fields) modify spin precession of localized carriers and alter spin-permutation symmetry of the pair, which controls the yields of electron-hole recombination and dissociation 1,[10][11][12][13] . OLEDs therefore exhibit lowfield magnetoresistance [14][15][16] owing to spatial variations in the local magnetic field experienced by the pairs precessing around hydrogen nuclear magnetic moments.…”

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confidence: 99%

“…Pairs of charge-carrier spins in organic semiconductors constitute four-level systems that can be driven electromagnetically 1 . Given appropriate conditions for ultrastrong coupling 2 -weak local hyperfine fields B hyp , large magnetic resonant driving fields B 1 and low static fields B 0 that define Zeeman splittingthe spin-Dicke e ect, a collective transition of spin states, has been predicted 3 .…”

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The spin-Dicke effect in OLED magnetoresistance

et al. 2015

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Pairs of charge-carrier spins in organic semiconductors constitute four-level systems that can be driven electromagnetically 1 . Given appropriate conditions for ultrastrong coupling 2 -weak local hyperfine fields B hyp , large magnetic resonant driving fields B 1 and low static fields B 0 that define Zeeman splittingthe spin-Dicke e ect, a collective transition of spin states, has been predicted 3 . This parameter range is challenging to probe by electron paramagnetic resonance spectroscopy because thermal magnetic polarization is negligible. It is accessed through spin-dependent conductivity that is controlled by electron-hole pairs of singlet and triplet spin-permutation symmetry without the need of thermal spin polarization 4 . Signatures of collective behaviour of carrier spins are revealed in the steady-state magnetoresistance of organic light-emitting diodes (OLEDs), rather than through radiative transitions. For intermediate B 1 , the a.c.-Zeeman e ect appears. For large B 1 , a collective spin-ensemble state arises, inverting the current change under resonance and removing power broadening, thereby o ering a unique window to ambient macroscopic quantum coherence.Macroscopic phase coherence is a hallmark of many exotic states of matter such as superconductivity, ferromagnetism or BoseEinstein condensation. Such coherence may also emerge between two-level systems, where it is mediated by electromagnetic fields, as described by the Dicke effect in collisional narrowing 5 and superradiance 6 . Collective behaviour may already arise within a pair of interacting two-level systems 7 , an observation that can potentially be extended to the prototypical two-level system of an electron spin. For pairs of charge-carrier spins in organic semiconductors, with driving fields B 1 exceeding the hydrogen-induced random local hyperfine field 1 B hyp and approaching the magnitude of the static magnetic field B 0 , a collective macroscopic spin phase has been predicted to emerge 3 . Under these conditions, when the spin-Rabi splitting becomes comparable to the Zeeman splitting, the electromagnetic field links individually resonant spin pairs together, forming a spin-Dicke state analogous to that in the dipolar Dicke effect [5][6][7] . These macroscopic effects are observable through measurements of electronic recombination rates, which depend on spin-permutation symmetry of the pair 8 .We monitor the electron-hole recombination current in an OLED, where positive and negative charges are injected into a thin film of an organic semiconductor from opposite electrodes. As the charges drift through the material, they can capture each other on intermolecular length scales owing to weak dielectric screening. These weakly coupled intermolecular electron-hole pairs 9 can ultimately recombine on individual molecules to form a molecular excited state, or exciton, which gives rise to electroluminescence. The subsequent discussion focuses on carrier pairs and not on excitons, which have spin S = 0 or 1. Because the carriers possess spi...

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The spin-Dicke effect in OLED magnetoresistance

et al. 2015

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“…A magnetic resonance technique widely used to investigate the hyperfine interaction of paramagnetic centers is electron spin echo envelope modulation (ES-EEM) spectroscopy. 20,21 The pEDMR implementation of this technique, applied to organic polymer poly[2-methoxy-5-(2 ′ -ethyl-hexyloxy)-1,4-phenylene vinylene] (MEH-PPV), was recently reported by Malissa et al 15 Employing a version of ESEEM, the authors of Ref. 15 have been able to resolve the proton spectral line in MEH-PPV and that of the deuteron and proton in partially deuterated MEH-PPV.…”

Section: Introductionmentioning

confidence: 99%

“…Importantly, pEDMR (and pODMR) offer the implementation of various spin-echo based spectroscopic techniques in the study of organic semiconductors. 14,15 This motivates the present theoretical study of a spectroscopic method based on the two-pulse (Hahn) echo and three-pulse echo sequences. 17 In many organic semiconductors the spin-orbital coupling is very week, and the polaron spin dynamics is governed mainly by the hyperfine interaction of polaron spin with the surrounding proton spins.…”

Section: Introductionmentioning

confidence: 99%

“…[7][8][9] Substantial progress in this direction was made by the pulsed EDMR (pEDMR) experiments. [10][11][12][13][14][15][16] Unlike the continuous wave measurements, these experiments are capable of probing the coherent spin dynamics, and thus provide a closer view on the spin-dependent processes. Importantly, pEDMR (and pODMR) offer the implementation of various spin-echo based spectroscopic techniques in the study of organic semiconductors.…”

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Polaron spin echo envelope modulations in an organic semiconducting polymer

Mkhitaryan

Dobrovitski

2017

Phys. Rev. B

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Theoretical treatment of the electron spin echo envelope modulation (ESEEM) spectra from polarons in a semiconducting π-conjugated polymer is presented. The contact hyperfine coupling and the dipolar interaction between the polaron and proton spins are found to have distinct contributions in the ESEEM spectra. However, since the two contributions are spaced very closely, and the dipolar contribution is dominant, the detection of the contact hyperfine interaction is difficult. To resolve this problem, a recipe of probing the contact hyperfine and dipolar interactions selectively is proposed, and a method for detecting the polaron contact hyperfine interaction is formulated. The ESEEM decay due to the polaron random hopping is analyzed, and the robustness of the method against this decay is verified. Moreover, this decay is linked to the transport properties of polarons, providing an auxiliary probe for the polaron transport.

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Electrically Detected Magnetic Resonance Spectroscopy

Boehme

Malissa

2017

eMagRes

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Room-temperature coupling between electrical current and nuclear spins in OLEDs

Cited by 86 publications

References 24 publications

The spin-Dicke effect in OLED magnetoresistance

The spin-Dicke effect in OLED magnetoresistance

Polaron spin echo envelope modulations in an organic semiconducting polymer

Electrically Detected Magnetic Resonance Spectroscopy

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