Yugang Sheng

Hyperfine interaction and magnetoresistance in organic semiconductors

We explore the possibility that hyperfine interaction causes the recently discovered organic magnetoresistance (OMAR) effect. We deduce a simple fitting formula from the hyperfine Hamiltonian that relates the saturation field of the OMAR traces to the hyperfine coupling constant. We compare the fitting results to literature values for this parameter. Furthermore, we apply an excitonic pair mechanism model based on hyperfine interaction, previously suggested by others to explain various magnetic-field effects in organics, to the OMAR data. Whereas this model can explain a few key aspects of the experimental data, we uncover several fundamental contradictions as well. By varying the injection efficiency for minority carriers in the devices, we show experimentally that OMAR is only weakly dependent on the ratio between excitons formed and carriers injected, likely excluding any excitonic effect as the origin of OMAR.

Magnetoresistance in π-conjugated organic sandwich devices with varying hyperfine and spin–orbit coupling strengths, and varying dopant concentrations

We study a recently discovered organic magnetoresistive (OMAR) effect whose underlying mechanism is currently not known with certainty. We examine the hypothesis that OMAR is caused by spin-dynamics by varying the strength of the hyperfine and spin–orbit coupling in the organic semiconductor material. We show that unlike most other materials, C60 devices do not exhibit the OMAR effect. Therefore hydrogen atoms and the resulting strong nuclear hyperfine coupling is a necessary prerequisite for the observation of OMAR. We investigate the dependence of OMAR on the strength of the spin-coupling introduced by heavy atoms in two widely used organic semiconductors, and find the characteristic magnetic field scale shifts to much larger fields. We also study OMAR sandwich devices made from the conducting polymer PEDOT. We show that in PEDOT the observed effect is caused by interface resistance, distinct from the case of intrinsic devices where OMAR is related to the bulk resistance of the (undoped) organic semiconductor.

An 8

We present the design principles for a pen-input organic light-emitting diode (OLED) display based on the recently discovered organic magnetoresistance effect (OMAR). In the prototypical OLED material Alq3, OMAR is as large as 10% for small magnetic fields, B=10 mT at room temperature. We construct a pen-input screen consisting of an 8 times 8 pixel OMAR array made from Alq3, together with a magnetic pen that emits an ac magnetic field. We describe a multiplexed detection scheme that uses a single filter/amplifier circuit to sequentially scan the individual pixels for the presence of the magnetic pen. For this scheme to work efficiently, it requires using frequencies on the order of 100 kHz. We demonstrate that our OMAR devices can indeed follow such high frequencies under certain operating conditions

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We report on the experimental characterization of a recently discovered large magnetoresistive effect in polyfluorene and in Alq3 organic light-emitting diodes. We also observe similar magnetic field effects (MFEs) of comparable magnitude in electroluminescence and photocurrent measurements. We provide a comprehensive overview of all these three types of MFE. To the best of our knowledge, the mechanism causing these MFEs is not currently known with certainty. Moreover, we show that experiments in bipolar, electroluminescent devices do not allow determination of whether the MFE acts on the carrier density or carrier mobility, making any attempt at explaining it ambiguous. As a remedy, we perform magnetoresistance measurements in hole-only polyfluorene devices and show that the MFE acts on the carrier mobility rather than carrier recombination.

8 Pixel Array Pen-Input OLED Screen Based on Organic Magnetoresistance

Abstract We present magnetoconductivity and magnetoluminescence measurements in sandwich devices made from films of a π-conjugated molecule and demonstrate effects of more than 30 and 50% magnitude, respectively, in fields of 100 mT at room-temperature. It has previously been recognized that the effect is caused by hyperfine coupling, and that it is phenomenologically similar to other magnetic field effects that act on electron–hole pairs, which are well-known in spin-chemistry. However, we show that the very large magnitude of the effect contradicts present knowledge of the electron–hole pair recombination processes in electroluminescent π-conjugated molecules, and that the effect persists even in almost hole-only devices. Therefore, this effect is likely caused by the interaction of radical pairs of equal charge.

Magnetic field effects on current, electroluminescence and photocurrent in organic light-emitting diodes

We perform charge-induced absorption and electroluminescence spectroscopy in a polyfluorene organic magnetoresistive device. Our experiments allow us to measure the singlet exciton, triplet exciton, and polaron densities in a live device under an applied magnetic field and to 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. We show that the singlet exciton, triplet exciton, and polaron densities and conductivity all increase with increasing magnetic field. Our data appear to be inconsistent with the exciton formation and triplet-exciton polaron quenching models.

Magnetoconductivity and magnetoluminescence studies in bipolar and almost hole-only sandwich devices made from films of a π-conjugated molecule

We report on the experimental observation of large magnetoresistance in polyfluorene organic light-emitting diodes (OLEDs). Very similar magnetic field effects (MFEs) of comparable magnitude were also observed in electroluminescence and photocurrent measurements. We provide a comprehensive overview of these three types of MFE. To the best of our knowledge, the mechanism causing these MFE is currently not known. Moreover, we show that these experiments do not allow determination whether the MFE acts on the carrier density or carrier mobility making any attempt of explaining it ambiguous. As a remedy, we performed magnetoresistance measurements in holeonly OLEDs and show that the MFE acts on the carrier mobility rather than carrier density.

Device spectroscopy of magnetic field effects in a polyfluorene organic light-emitting diode

We report on the extensive experimental characterization of a recently discovered (Francis et al., 2004) large and intriguing magnetoresistive effect in OLEDs 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 existence of this effect is highly surprising, since it has generally been believed that large room-temperature magnetoresistive effects can exist only in ferromagnetic devices, whereas our devices are constructed entirely from non-magnetic materials.

Magnetic field effects on current, electroluminescence and photocurrent in polyfluorene organic light emitting diodes

We report on the experimental observation of large magnet field effect (MFE) on current and electroluminescence (EL) in polyfluorene organic light emitting diodes (OLEDs). We provide a comprehensive overview of the observed MFE. To the best of our knowledge, the mechanism causing these MFE is currently not known. Moreover, we show that these experiments do not allow determination whether the MFE acts on the carrier density or carrier mobility making any attempt of explaining it ambiguous. As a remedy, we performed magnetoresistance measurements in hole-only OLEDs and show that the MFE acts on the carrier mobility rather than carrier density.

Characterization and Application of Large Magnetoresistance in Organic Semiconductors

We describe magnetic field sensors based on a recently discovered magnetoresistance (MR) effect in nonmagnetic organic semiconductor sandwich devices. The MR effect reaches up to 10% in a magnetic field of 10 mT at room temperature. We perform an extensive experimental characterization of this effect. We found that the MR effect is only weakly temperature dependent and does not depend on sign and direction of the applied magnetic field. We also measured the device response to alternating magnetic fields up to 100 kHz. To the best of our knowledge, the discovered MR effect is not adequately described by any of the MR mechanisms known to date.