Synthetic Metals (v.160, #3-4)

Editorial by Markus Wohlgenannt (203).

We describe device models for spin injection, transport, and magneto-resistance in structures consisting of an organic semiconductor layer sandwiched between two ferromagnetic contacts. Carrier transport in the organic semiconductor is modeled with spin-dependent transport equations in drift-diffusion approximation. The effectiveness of spin-selective tunnel contacts on spin-polarized injection and magneto-resistance is examined on the basis of a simple analytical model. In agreement with earlier results, we find that spin injection from ferromagnetic metallic contacts into organic semiconductors can be greatly enhanced if (spin-selective) tunneling is the limiting process for carrier injection. We then explore the effects of the injected space charge and of spin relaxation in the semiconductor by comparing the results of a numerical calculation with the analytical model. For relatively thick organic semiconductor layers the injected space charge has strong effects on charge injection and, hence, on spin injection at high bias. Lastly, we consider a simple model for the bias dependence of the tunnel contacts and find that this effect may limit spin injection to relatively low currents.
Keywords: Organic semiconductors; Spin injection; Magneto-resistance; Spin valves;

Recent advances in organic spin-valve devices by Fujian Wang; Z. Valy Vardeny (210-215).
Organic Spintronics has been considered to be the physics and applications of spin polarized electron injection, transport, manipulation and detection in organic diodes by the application of an external magnetic field. The prototype device is the organic spin-valve (OSV), which is based on an organic semiconductor spacer placed in between two ferromagnetic electrodes having different coercive fields, of which magnetoresistance changes with the applied field. Immense progress has been achieved in the past few years in fabricating, studying and understanding the underlying physics of these devices. We highlight the most significant advance in OSV research at the University of Utah, including the magnetoresistance response temperature and bias voltage dependencies; and show significant room temperature operation using LSMO/C60/Co structure. We also report positive OSV-related magnetoresistance at low temperature, which was achieved using LSMO/polymer/Co OSV structure, where the polymer is a poly[phenylene-vinylene] derivative.
Keywords: Organic Spintronics; Organic spin-valve; Spin polarized carrier injection; Magnetoresistance;

Tunneling vs. giant magnetoresistance in organic spin valve by Jung-Woo Yoo; H.W. Jang; V.N. Prigodin; C. Kao; C.B. Eom; A.J. Epstein (216-222).
We studied magnetoresistance (MR) in La2/3Sr1/3MnO3 (LSMO)/organic semiconductor (OSC)/Fe heterojunction devices using rubrene (C42H28) as an organic semiconductor. Efficient spin polarized tunneling using a hybrid barrier (oxide (1.2 nm)/rubrene (5 nm)) was observed. Devices with a thin layer of rubrene as the barrier may have magnetic clusters and/or pinholes in the barrier, which could explain significant variations of MR among devices. As the thickness of the rubrene layer is increased, device current becomes strongly limited by carrier injection resulting in strong temperature and bias dependent device resistance. The carrier injection in these devices can be described with thermionic field emission at the metal/OSC interface and is analyzed with both empirical and theoretical models. The effect of carrier transport through the spacer on the magnetoresistance for organic-based spin valve is discussed. The observed giant magnetoresistance (GMR) in 20 nm rubrene device demonstrates the spin polarized carrier injection and transport through the rubrene OSC layer.
Keywords: Organic spin valve; Tunneling magnetoresistance; Giant magnetoresistance; Spin injection; Thermionic field emission;

Spin relaxation and magnetoresistance in disordered organic semiconductors by P.A. Bobbert; T.D. Nguyen; W. Wagemans; F.W.A. van Oost; B. Koopmans; M. Wohlgenannt (223-229).
Because of the light elements involved, the spin–orbit coupling in organic materials is small. Therefore, the spin of charged polarons in these materials is expected to be a well conserved quantity. The conviction in the community grows that the main source of spin relaxation is in fact the coupling of the polaron spin to the random hyperfine fields of the hydrogen nuclei. By considering reactions between polarons forming bipolarons or excitons in the presence of these hyperfine fields we explain line shapes of the intrinsic magnetoresistance observed in disordered organic semiconductors. We also show how these hyperfine fields determine the spin-diffusion length in these semiconductors and how this affects the magnetoresistance line shapes of organic spin valves.
Keywords: Organic semiconductor; Magnetoresistance; Spin valve; Spin relaxation; Disorder;

Organic light-emitting devices (OLEDs) with a phosphorescent molecule Ir(ppy)3 as the emitter and with an Fe cathode as the spin injector were fabricated for the observation of the large degree of circular polarization. The OLED structure was a glass-substrate/ITO/α-NPD/CBP doped with Ir(ppy)3/BCP/Al-oxide/Fe/Al. The mixing ratio of CBP and Ir(ppy)3 in the emissive layer was optimized for high luminescence efficiency. The OLEDs showed circular polarization, and the maximum degree of circular polarization of the OLEDs was 0.4% at the applied magnetic field of 1.6 T at room temperature. On the other hand, no circular polarization was observed from the OLEDs with an Al cathode.
Keywords: Molecular spintronics; Spin injection; Spin transport; Organic light-emitting device; Circular polarization;

Spin-valve behavior in porous alumina-embedded carbon nanotube array with cobalt nanoparticle spin injectors by M.B. Murphey; J.D. Bergeson; S.J. Etzkorn; L. Qu; L. Li; L. Dai; A.J. Epstein (235-237).
We report the operation of spin-valve structures formed from arrays of aligned carbon nanotubes grown and embedded in porous anodized aluminum oxide. The devices incorporate cobalt nanoparticles acting as both a ferromagnetic layer and carbon nanotube array-growing catalysts, requiring only one deposited ferromagnetic layer as the other magnetic electrode. A peak in the resistance occurs clearly as a result of the reversal of the magnetization of the electrodes. Device magnetoresistance ratios reach 1.5% at 40 K, yielding an estimate of the maximum spin scattering length of 2 μm at temperatures up to 40 K. This device architecture presents not only an opportunity for massively parallel patterned circuits with devices the size of an individual nanotube diameter, but also the potential for multi-state switching devices with the selective incorporation of various ferromagnetic catalyst nanoparticles of different coercive fields within the same porous anodized aluminum oxide array.
Keywords: Aligned carbon nanotubes; Co nanoparticles; Spin-valve; Porous alumina;

A photoemission study of interfaces between organic semiconductors and Co as well as Al2O3/Co contacts by M. Grobosch; C. Schmidt; W.J.M. Naber; W.G. van der Wiel; M. Knupfer (238-243).
We have studied the energy-level alignment of ex situ, acetone cleaned Co and Al2O3/Co contacts to the organic semiconductors pentacene and rubrene by combined X-ray and ultraviolet photoemission spectroscopy. Our results demonstrate that the work function under these conditions is smaller than in the in situ cleaned, atomically clean case. Moreover, the studied interfaces are characterized by very small, short range interfaces dipoles and substantial injection barriers for holes. This represents essential information in view of their use in organic spintronic devices. Our core-level photoemission spectroscopy measurements rule out chemical reactions.
Keywords: Spintronics; Photoemission spectroscopy; Energy-level alignment; Pentacene; Rubrene;

Spin dynamics control of recombination current in organic semiconductors by Vladimir N. Prigodin; Arthur J. Epstein (244-250).
The recombination rate of coulombically bound electron–hole pairs depends on their spin configuration. Because of low spin–orbit coupling the spin dynamics of these well separated coulombic pairs is determined by weak hyperfine and exchange interactions. In this case a weak magnetic field produces strong effect on the spin dynamics and hence on the recombination rate of e–h pairs. We have shown that the recombination current in organic semiconductors may have a maximum as a function of recombination constant. For high recombination constants the current is space charge limited and decreases with increasing the e–h recombination constant. This decay of current is due to decrease of the region where the recombination takes place. At a low recombination constant the recombination takes place in the whole volume and the current increases with increasing the recombination constant. The characteristic recombination constant separating those two regimes depends on the thickness of sample, applied voltage, and charge carrier mobilities. The model predictions are consistent with experimental data for magnetoresistance of organic semiconductors.
Keywords: Organic semiconductor; Magnetoresistance; Recombination; Spin conversion; Space-charge limited transport;

Tuning of organic magnetoresistance by reversible modification of the active material by U. Niedermeier; S.A. Bagnich; C. Melzer; W. Sarfert; H. von Seggern (251-255).
We investigate the mechanism of the recently discovered phenomenon that the organic magnetoresistance effect can be enhanced by electrical device conditioning. In organic light emitting devices based on poly(paraphenylene vinylene) appropriate conditioning can increase the magnetoconductance values from 1% to 25% at 40 mT. While the conditioning procedure increases the magnetoconductance it leads to a reduction of the electro-optical device performance. The favorable effect of conditioning is non-permanent, leading to a slow reduction of the magnetoconductance values once the conditioning procedure has been turned off. We demonstrate that this relaxation process can be accelerated by thermal annealing of the devices. Higher annealing temperatures cause a stronger reduction of the magnetoconductance values as well as a more pronounced improvement of the previously reduced electro-optical performance. We attribute our results to a modification of the charge carrier transport properties in the bulk polymer material during conditioning and annealing. Changes in polymer morphology or in molecular conformation during conditioning are assumed to result in the formation of energetic trap states which are beneficial for the organic magnetoresistance effect. Thermal annealing reverses the material modifications, reduces the number of trap states in the material and thus decreases the magnetoconductance.
Keywords: Organic magnetoresistance; OMR; OLED; Magnetic field effect; PPV; Charge carrier traps;

Magnetophotocurrent effect in organic photovoltaic cells at low temperatures by H. Tajima; M. Miyakawa; H. Isozaki; M. Yasui; N. Suzuki; M. Matsuda (256-261).
The photocurrent responses were investigated for the bulk-hetero-junction of poly(3-hexylthiophene) (P3HT) and 1-(3-methoxycarbonyl)-propyl-1-phenyl-1-phenyl-(6,6)C61 (PCBM) (1), and the single-active-layer junction of (5,10,15,20-tetraphenyl porphyrinato)zinc (II) (Zn(TPP)) (2), in the temperature range between 300 K and 1.5 K under the magnetic field up to 8 T. Magnetic field effect (MFE) of the photocurrent was observed in both devices. In the device (1), MFE at low temperatures was small for the continuous light illumination, but remarkable for the chopped-light illumination. An experiment of photoinduced charge extraction by linearly increasing voltage (photo-CELIV) revealed a drastic increase in the lifetime for trapped carriers under applied magnetic field. In the device (2), the photocurrent decrease under magnetic field was observed both for the continuous-light and the chopped-light illumination.
Keywords: Photovoltaic effect; MIM junctions; Magnetophotocurrent; Magnetic field; Low temperature;

Enhanced performance of P3HT/PCBM bulk heterojunction photovoltaic devices by adding spin ½ radicals by Ye Zhang; Golda Hukic-Markosian; Debra Mascaro; Zeev Valy Vardeny (262-265).
We have studied bulk heterojunction organic photovoltaic devices (solar cells) based on polymer/fullerene blends using both electrical and magneto-optical methods. We show that adding spin ½galvinoxyl radicals to the device active layer, consisting of a regio-regular polythiophene/fullerene derivative [P3HT/PCBM] blend, significantly improves the device performance. Compared to pristine photovoltaic devices, the radical-rich devices show improved short-circuit current density, fill factor, and power conversion efficiency. The enhanced device performance is attributed to a reduced geminate recombination rate and improved carrier transport, both of which result from spin–spin interactions between the radical impurities and the photogenerated carriers. Optically detected magnetic resonance, a technique that is sensitive to spin-lattice relaxation rates, is used to verify the proposed mechanism.
Keywords: Organic photovoltaic devices; Bulk heterojunctions; Spin–spin interaction; Solar cells; Power conversion; Optically detected magnetic resonance;

Separating photocurrent and injected current contributions to the organic magnetoresistance by W. Wagemans; W.J. Engelen; F.L. Bloom; B. Koopmans (266-270).
The magnetoresistance in the photocurrent and injected current of an organic device has been studied. When such a device is illuminated under bias, both photo-generated charges and electrically injected carriers will contribute to the current. We used a magnetic field modulation to successfully remove the time-dependent change in the current. This was combined with a modulation of the intensity of the illumination to separately measure the effects of the photocurrent. At low bias, without illumination no magnetoconductance (MC) is observed in leakage current, while with illumination an MC is observed due to the photo-generated charges. With increasing voltage, as electrical charge injection starts, the width of the MC curves as a function of magnetic field of the photocurrent remains constant and is larger than the curve observed for the electrically injected current, even when both currents are present. This result shows that the contributions from the photocurrent and the injected current cannot be combined, but should be treated separately.
Keywords: Organic magnetoresistance (OMAR); Organic semiconductors; Photocurrent;

We report large magnetoresistance (up to 55% at T = 2.5  K and H = 8  T) for polyaniline nanofiber networks. Magneto-transport behavior has been studied at low and high magnetic and electric fields for temperatures 2–250 K. A crossover from positive magnetoresistance (MR) to negative MR is observed at ∼ 87.5  K. Two competing mechanisms which account for the MR behavior are shrinkage of localized electron wavefunctions and destruction of the quantum interference of wavefunctions of electrons propagating along various paths during charge hopping processes in the applied magnetic field. We also observed that the magnitude of positive magnetoresistance decreases with increasing electric field which we propose is due to decrease in the activation energies in hopping process.
Keywords: Polyaniline; Nanofibers; Magnetoresistance; Magnetic and electric field; Temperature dependence;

Magnetic field effect on the photocarriers in self-assembled hexabenzocoronene nanotubes by Yusuke Wakikawa; Tadaaki Ikoma; Yohei Yamamoto; Takanori Fukushima; Takuzo Aida (275-279).
By means of the integral mode time-of-flight measurement of the drift motion of the photoinjected charge at room temperature, we observed magnetic field effect (MFE) on the photocharge in self-assembled nanotube of hexabenzocoronene (HBC) at the configurations where the magnetic field is parallel and perpendicular to the electric field. The detected MFEs are interpreted in terms of two mechanisms that are caused by the fast charge transportation (Hall effect) and the spin selective recombination (electron–hole (e–h) pair mechanism). A high mobility in the nanotubes of π-stacked HBC was implied from the Hall data. The time dependence of the low field MFE due to the e–h pair mechanism clarified the recombination from both the singlet and triplet e–h pairs with the initial rate of ∼108  s−1.
Keywords: Self-assembled nanotube; Electron–hole pair mechanism; Hall effect; Mobility; Recombination;

Spin–orbit coupling and spin relaxation rate in singly charged π -conjugated polymer chains by J. Rybicki; T.D. Nguyen; Y. Sheng; M. Wohlgenannt (280-284).
In inorganic semiconductor spintronics the spin-diffusion length is usually limited by spin–orbit coupling. Here we examine the effect of spin–orbit coupling in organic spintronics. We consider singly charged π -conjugated polymer chains. We show that the diagonal matrix elements for spin–orbit coupling are zero. Even the off-diagonal matrix elements are zero or negligibly small unless a twisted, non-planar polymer chain is considered. We calculate these matrix elements as a function of twist angle using tight-binding wavefunctions. We show that time reversal symmetry prevents spin–orbit induced spin-precession and propose a phonon-assisted spin-flip process.
Keywords: Spin–orbit coupling; Spintronics; Organic semiconductors;

Giant magnetoresistance due to electron-hole pair mechanism in poly(N-vinylcarbazole) by Tadaaki Ikoma; Toshinari Ogiwara; Yutaka Takahashi; Kimio Akiyama; Shozo Tero-Kubota; Yuka Takahashi; Tomohiro Suzuki; Yusuke Wakikawa (285-290).
An amorphous molecular semiconductor, poly(N-vinylcarbazole) (PVCz), exhibits negative giant magnetoresistance (MR) under ambient conditions. The application of a weak magnetic field of 10 mT to PVCz films doped with lumichrome, in which light excitation immediately creates triplet electron-hole (e-h) pairs that are precursors of photocarriers, causes a steep decrease in resistivity by more than 20% at ambient temperature. Further, the resistivity of the doped film gradually reduces by approximately one-half under a field of 1 T, equivalent to an MR ratio of −55%. In addition, anomalous spikes are also detected at 0.07, 0.30, and 9.0 T, indicative of an exponential dependence with a decay distance of 0.1 nm in the exchange interaction of the e-h pair. A quantum mechanical calculation based on the density operator formalism clarifies that the observed MR effect can be comprehensively understood by the spin-selective charge dynamics and the coherent and incoherent spin dynamics of the e-h pair in a quasi-one-dimensional lattice for photocarrier generation. Model calculations also indicate the importance of the spin-lattice relaxation for the giant MR effect in organic molecular semiconductors.
Keywords: Magnetoresistance; Electron-hole pair; Spin-selective recombination; Spin relaxation; Photoconduction;

Organic semiconductor (OSC) devices have been shown to have a large magnetoresistance (MR) response at room temperature for relatively small-applied magnetic fields of 0.1–100 milli-Tesla (mT). This large MR is not limited to one class of organics, but is seen in small molecules, oligomers, conjugated polymers, and non-conjugated polymers. In this paper, data is presented on the MR effect for the poly(phenylene vinylene) (PPV) derivative “Super Yellow,” for poly(vinylenecarbazole) (PVK), for alpha-sexithiophene (α-6T), and for tris(8-hydroxyquinoline) aluminum (Alq3). The data is analyzed in the context of the Magnetoresistance by the Interconversion of Singlets and Triplets (MIST) model. The MR data of Alq3 for magnetic fields of less than 1 mT are fitted to a polynomial expansion, and an estimate for the hyperfine interaction constant, which is consistent with values for small molecules, is extracted from the fitting parameters. Curve fits at fields in the 100 mT range are also presented and they show that there exist two kinds of magnetic field behavior, inverse square root, and inverse even orders. Furthermore, the scaling factor at this range is one order of magnitude larger than that found in the 3 mT range.
Keywords: Organic semiconductor; Magnetoresistance; Recombination; OLED;

Spin dependent reactions of polaron pairs in PPV-based organic diodes by F. Wang; C.G. Yang; E. Ehrenfreund; Z.V. Vardeny (297-302).
We characterize spin dependent kinetics of polaron pairs (PP) reactions in polymer organic light emitting diodes (OLED) based on a derivative of poly(phenylene-vinylene) [PPV], namely MEH-PPV as the active layer, using the dynamic response of the magnetic resonance effect on electroluminescence, injected current, and photocurrent. We found that at 10 K and under forward bias, the in-phase component of both electroluminescence detected magnetic resonance (ELDMR) and current detected magnetic resonance (CDMR) responses are positive at low microwave modulation frequency, f; but both reverse sign at a frequency f 0 that depends on the microwave power, current density, and device architecture. The similarity between ELDMR and CDMR response dynamics show that the two phenomena share a common origin. We identify the underlying ELDMR mechanism as due to current density increase under resonance conditions. From a model fit to the data that involves both spin singlet and triplet PP dynamics, we obtained their effective recombination and spin-lattice relaxation rates. We found that the spin-lattice relaxation rate in the active layer increases with the current density in the device, showing the importance of spin–spin interaction in spin dependent reactions in OLED. Unlike ELDMR and CDMR the reverse bias photocurrent detected magnetic resonance (PCDMR) is negative at low f. The negative sign is the result of the dynamic equilibrium between the resonantly reduced PP density and the photogenerated mobile charge carrier density.
Keywords: Polaron pairs; Electroluminescence; Conductivity and photo-conductivity detected magnetic resonance;

Ferromagnetism in indium tin-oxide (ITO) electrodes at room temperature by Himadri S. Majumdar; Sayani Majumdar; Daniel Tobjörk; Ronald Österbacka (303-306).
In this article we review the experimental observation of ferromagnetism in metal-oxide systems. This is relevant for the application of the most commonly used metal-oxide, indium tin-oxide (ITO), in organic electronic devices where organic magnetoresistance phenomenon has recently been observed. We provide experimental evidence of ferromagnetism in commercial ITO substrates and investigate the role of impurities in giving rise to such effects. Magnetoresistance have also been observed in organic diodes without any ITO contacts. However, it is important to take into account the intrinsic ferromagnetism in the ITO when interpreting the experimental data for devices with ITO. The possible limitations of using ITO are discussed in the article.
Keywords: Organic magnetoresistance; Metal-oxide; Ferromagnetism;

Positive magnetoresistance (MR) has been observed to increase linearly up to 32 T in the magnetically ordered state of organic-based ferrimagnetic semiconductor V(TCNE) x films (x ∼ 2; TCNE = tetracyanoethylene) with T c above room temperature ( > 350  K). In this material conductivity takes place via electrons activated from 3d level of V2+ to upper π * subband of [TCNE] • − . We show an unusual MR behavior without any sign of saturation up to a magnetic field of 32 T. For temperatures less than T c MR exhibits a linear behavior in the entire field range and above T c it has a quadratic dependence at low fields. Temperature and field dependent behaviors of MR in this material are explained on the basis of spin polarizations in V2+ 3d level and the upper π * subband of [TCNE] • − formed by Coulomb repulsion.
Keywords: Organic magnets; Spintronics; Magnetoresistance; Spin polarization;

We report the effect of spin ½ radical impurity, Galvinoxyl on the ultrafast photoexcitation dynamics in annealed films of regio-regular poly(3-hexylthiophene) (RR-P3HT)/[6,6]-phenyl C61 butyric acid methyl ester (PCBM) blend. The addition of Galvinoxyl radical impurities to the blend reduces the geminate recombination rate of photogenerated polaron pairs. Consequently organic photovoltaic solar cells made from the radical/blend mixture as the active layer show increased short-circuit current (J sc ), fill-factor, and power conversion efficiency. We speculate that the spin–spin interaction of the radical impurity with the electron and hole forming the polaron pairs is the main mechanism responsible for the obtained reduced recombination rate.
Keywords: Organic photovoltaic solar cells; Spin ½ radicals; Spin–spin interaction; Geminate recombination; Photogenerated polaron pair;

In the quest for better power conversion efficiency of organic photovoltaic cells (OPVC) various architectures have been considered. Bulk heterojunction type OPVC proved to be the most successful, where the polymer as electron donor is blended with fullerene molecule as electron acceptor. With the goal of improving power conversion efficiency of OPVC we studied 1-(3-methoxycarbonyl)-propyl-1-phenyl-(6,6)C61 (PCBM) as a film and dispersed in polystyrene matrix. We used a variety of continuous wave (cw) optical probes including photoinduced absorption (PA), photoluminescence (PL), and optically detected magnetic resonance (ODMR). The steady state photophysics of PCBM is dominated by triplet exciton. For both PCBM dispersed in polystyrene and PCBM film, the triplet exciton specie is characterized by a PA band at about 1.7 eV and spin 1 PA detected magnetic resonance (PADMR) powder pattern resonance around g  ∼ 2. However, in PCBM film the occurrence of a polaron PA band at 1.2 eV and spin 1/2 PADMR are also noticed.
Keywords: Photoinduced absorption (PA); Optically detected magnetic resonance (ODMR); PADMR; Photoluminescence (PL); Triplet exciton; Spin 1; Polaron; Spin 1/2;

Magnetic field effects on the conductivity of organic bipolar and unipolar devices at room temperature by J.A. Gómez; F. Nüesch; L. Zuppiroli; C.F.O. Graeff (317-319).
Magnetic field effects on the conductivity of different types of organic devices: undoped and dye doped aluminium (III) 8-hydroxyquinoline (Alq3)-based organic light emitting diodes (OLEDs), electron-only Alq3-based diodes, and a hole-only N,N′-diphenyl-N,N′-bis(1-naphthyl)-1,1′-biphenyl-4,4′-diamine (α-NPD)-based diode were studied at room temperature. Only negative magnetoresistance (MR) was observed for the Alq3-based devices. The addition of a rubrene dye in Alq3-based OLEDs quenches the MR by a factor of 5. The α-NPD hole-only device showed only positive MR. Our results are discussed with respect to the actual models for MR in organic semiconductors. Our results are in good agreement with the bipolaron model.
Keywords: Organic semiconductors; OLEDs; Magnetoresistance; Bipolaron model;

Device-spectroscopy of magnetic field effects in several different polymer organic light-emitting diodes by T.D. Nguyen; Y. Sheng; J. Rybicki; G. Veeraraghavan; M. Wohlgenannt (320-324).
We perform charge-induced absorption and electroluminescence spectroscopy in a set of organic magnetoresistive devices made from different π -conjugated polymers. These experiments measure the singlet exciton, triplet exciton and polaron densities in live devices under an applied magnetic field. In most devices we find that the singlet exciton, triplet exciton and polaron densities and conductivity all increase with increasing magnetic field. However, in regio-regular polythiophene devices the polaron density decreases with increasing field. Our experiments test the predictions of three different models that were proposed to explain organic magnetoresistance. These models are based on different spin-dependent interactions, namely exciton formation, triplet-exciton polaron quenching and bipolaron formation.
Keywords: Organic magnetoresistance; OMAR; Organic magnetoconductance; Charge-induced absorption; Sexithiophene; Methyl-substituted ladder-type poly(p-phenylene) polymer; Regio-regular polythiophene; Polyfluorence;