Omer Mermer

Large magnetoresistance in nonmagnetic π-conjugated semiconductor thin film devices

Following the recent observation of large magnetoresistance at room temperature in polyfluorene sandwich devices, we have performed a comprehensive magnetoresistance study on a set of organic semiconductor sandwich devices made from different pi-conjugated polymers and small molecules. The study includes a range of materials that show greatly different chemical structure, mobility, and spin-orbit coupling strength. We study both hole and electron transporters at temperatures ranging from 10 K to 300 K. We observe large negative or positive magnetoresistance (up to 10% at 300 K and 10 mT) depending on material and device operating conditions. We discuss our results in the framework of known magnetoresistance mechanisms and find that none of the existing models can explain our results.

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.

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

We describe a large magnetoresistance (MR) effect in organic light-emitting diodes that reaches up to 10% at fields of 10 mT at room temperature. This MR effect occurs both in /spl pi/-conjugated polymers and small molecules. Our devices do not contain any magnetic materials. We found that the MR effect is only weakly temperature dependent and does not depend on sign and direction of the applied magnetic field. This is a novel type of MR-yet to be explained theoretically-that does not have, to the best of our knowledge, an analog in inorganic semiconductor devices.

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

The maximum efficiency in organic light-emitting diodes (OLEDs) depends on the ratio,

Large magnetoresistance at room temperature in organic semiconductor devices

r=k_S/k_T

Single-step multiphonon emission model of spin-dependent exciton formation in organic semiconductors

, where

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

k_S

Large magnetoresistance at room temperature in semiconducting polymer sandwich devices

(