Bulletin of the American Physical Society
2007 APS March Meeting
Volume 52, Number 1
Monday–Friday, March 5–9, 2007; Denver, Colorado
Session W3: Magnetotransport in Organic Conductors and Semiconductors |
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Sponsoring Units: DCMP Chair: Zeev Vardeny, University of Utah Room: Colorado Convention Center Korbel 2A-3A |
Thursday, March 8, 2007 2:30PM - 3:06PM |
W3.00001: Spins and Organic Materials: The Spin-dependent OLED Invited Speaker: |
Thursday, March 8, 2007 3:06PM - 3:42PM |
W3.00002: Spin correlations in organic semiconductors Invited Speaker: Organic semiconductors differ from their inorganic counterparts by large exchange interactions and weak spin orbit coupling. As a result, parallel and anti-parallel spin configurations are highly non-degenerate and spectroscopically well-defined. Whereas singlet excitons are highly emissive, triplet excitons generally decay non-radiatively. Addition of heavy metal centres to the polymer backbone induces localized spin-orbit coupling, which can activate radiative triplet decay through phosphorescence [1]. By tuning the concentration of these triplet acceptors to match the diffusion length of the triplets (which exceeds that of the singlets), triplets can be harvested radiatively without significantly affecting the actual triplet formation pathway through intersystem crossing of the singlet [2]. Using this technique we can study the interconversion between spin states of exciton precursors (charge carrier pairs) as a function of time, temperature, and electric and magnetic fields [3]. We find that the probability of a spin change occurring in the exciton precursor state is extremely small, which suggests that the primary recombination pathway in organic light-emitting diodes is governed by spin statistics [3]. Phosphorescence spectroscopy of organic semiconductors has a number of immediate applications. Stimulated emission competes with intersystem crossing required for triplet generation so that a phosphorescent polymer laser acts as a highly non-degenerate all-optical excitonic switch [4]. Singlet-triplet mixing in metallorganics with strong spin-orbit coupling also provides a versatile method for ultrafast luminescence based molecular thermometry [5,6]. \newline \newline [1] Lupton et al., \textit{Phys. Rev. Lett.} \textbf{89}, 167401 (2002). \newline [2] Reufer et al., \textit{Phys. Rev. B Rapid} (in press 2006). \newline [3] Reufer et al., \textit{Nature Mat.} \textbf{4}, 340 (2005). \newline [4] Reufer et al., \textit{Appl. Phys. Lett.} \textbf{89}, 141111 (2006). \newline [5] Stehr et al., \textit{Adv. Mater.} \textbf{16}, 2170 (2004). \newline [6] Balouchev et al., \textit{US Patent} \textbf{7097354} (2006). [Preview Abstract] |
Thursday, March 8, 2007 3:42PM - 4:18PM |
W3.00003: Characterization and Application of Large Magnetoresistance in Organic Semiconductors Invited Speaker: Recent years have seen a surge in interest in magnetoresistive and spintronic properties of organic semiconductors, whereas this field was previously almost exclusively concerned with their electrooptical properties. We report on the extensive experimental characterization of a recently discovered large and intriguing magnetoresistive effect in organic light- emitting diodes that reaches up to 10\% at room temperature for magnetic fields, B = 10mT. This magnetoresistive effect is therefore amongst the largest of any bulk material. The study includes a range of materials that show greatly different chemical structure, mobility, hyperfine and spin-orbit coupling strength. We show that the applied magnetic field affects the carrier transport inside the bulk semiconductor. By demonstrating that the effect is critically altered by the presence of strong spin- orbit coupling and that it does not occur in fullerene devices, we prove that the transport in organics sensitively depends on spin-dynamics induced by hyperfine interaction with the hydrogen protons. We discuss a possible relation between organic magnetoresistance and other magnetic field effects in organics that were known long before its discovery. As a possible mechanism we describe how Pauli's principle restricts carrier hopping between singly occupied sites near the Fermi level. However, spin-mixing by the hyperfine interaction may partially lift this restriction. Since the devices we describe can be manufactured cheaply they hold promise for applications where large numbers of magnetoresistive devices are needed, such as magnetic random- access-memory (MRAM); and applications related to organic light- emitting diode displays such as touch screens where the position of a magnetic stylus is detected (patent pending). We will show a video of a simple demonstrator device. [Preview Abstract] |
Thursday, March 8, 2007 4:18PM - 4:54PM |
W3.00004: Spin-Orbital Coupling and Magnetoresistance Tuning in Organic Semiconductors Invited Speaker: Magnetoresistance can be readily obtained from non magnetic organic semiconductors in light-emitting diodes. Tuning this novel magnetoresistance is an important issue in using an external magnetic field to control optoelectronic response in organic semiconductors. The experimental results indicate that weak-spin-orbital coupling materials exhibit much more significant magnetoresistance as compared to strong-spin-orbital coupling molecules. We find that uniformly mixing strong-spin-orbital-coupling fac-tris (2-phenylpyridinato) iridium [Ir(ppy)$_{3}$] molecules and weak-spin-orbital-coupling poly(N-vinyl carbazole) (PVK) leads to a concentration-dependent magnetoresistance. There are three possible processes, namely intermolecular spin-orbital interaction, energy transfer, and charge transport, that can contribute to the concentration-dependent magnetoresistance. The magnetic field-dependent electroluminescence shows that an intermolecular spin-orbital interaction is formed in the PVK+Ir(ppy)$_{3}$ mixture. This intermolecular spin-orbital interaction modifies the singlet/triplet exciton ratio, changing further charge injection when the space charge carriers from exciton dissociation are considered. Based on this intermolecular spin-orbital interaction effects, metal electrode-dependent spin-orbital coupling and magnetoresistance have been demonstrated. This presentation will discuss the effects of spin-orbital coupling on magnetoresistance tuning through exciton dissociation and exciton-charge reaction in organic light-emitting diodes through controlling energy transfer and bipolar injection. [Preview Abstract] |
Thursday, March 8, 2007 4:54PM - 5:30PM |
W3.00005: Low Field, Large Magnetoresistance in Nonmagnetic Organic Semiconductors Invited Speaker: Transport in various thin-film organic semiconductors has been shown to have an anomalously high sensitivity to low magnetic fields at room temperature (RT). Early experiments on polydiacetylene single crystals and poly(phenylenevinylene)s revealed increases in photoconductivity of a few percent at RT.\footnote{E.L. Frankevich, et al., Mol.\ Cryst.\ Liq.\ Cryst.\textbf{175}, 41 (1989); E.L. Frankevich, et al., Phys.\ Rev.\ B \textbf{46}, 9320 (1992).} Further magnetotransport studies showed larger effects in $\pi$-conjugated backbone polymers and small molecules.\footnote{\"{O}. Mermer, et al., Phys.\ Rev.\ B \textbf{72}, 205202 (2005).} We report magnetoresistance (MR) for semiconducting oligomer and nonconjugated polymer materials in addition to small molecule and conjugated backbone polymer materials. For example, films of the light emitters poly(N-vinylcarbazole) and Alq$_{3}$ each have an MR response greater than 5\% at an unusually low magnetic field of 100 Oe $(\mu_{B}H \sim \mbox{0.0006 meV})$ at an unusually high temperature of 300 K $(k_{B}T \sim \mbox{26 meV})$. Increasing the spin-orbit coupling in Alq$_{3}$ films by doping with the phosphorescent sensitizers Ir(ppy)$_{3}$ or PtOEP strongly suppresses the MR signal. MR in thin films of the oligomer $\alpha $-sexithiophene can be negative, similar to the behavior of other organic semiconductors, or positive depending on the temperature, layer thickness, or applied voltage. We have developed a model, termed Magnetoresistance by the Interconversion of Singlets and Triplets (MIST), accounting for this anomalous MR.\footnote{V.N. Prigodin, et al., Synth.\ Met.\textbf{156}, 757 (2006).} At zero field, the singlet and triplet e-h pair states are degenerate and the states can readily interconvert due to hyperfine interaction. Finite magnetic fields lift triplet degeneracy which affects the hyperfine interconversion of e-h pairs between singlet and triplet states. By changing the carrier recombination the MIST mechanism gives rise to a space-charge-limited current that depends on magnetic field, producing MR. [Preview Abstract] |
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