Magnetic field effects in organic semiconductors and their applications in displays

Organic semiconductors have been widely expected to replace inorganic semiconductors in the field of display electronics since the advent of organic light emitting diodes (OLEDs). Organic pi-conjugated materials have also been used to manufacture photovoltaic cells and field-effect transistors. Recent years have seen a surge in interest in the magnetotransport properties of these materials because they show great potential for such applications as magnetic field sensors and spintronics devices.; In this thesis, I experimentally characterize a large magnetoresistance effect, which was recently discovered at the University of Iowa and was dubbed organic magnetoresistance (OMAR). We focus on two materials that show particularly great promise; one a macromolecular polymer called polyfluorene, the other a prototypical small molecule called Alq3.; Building on the work of a prior student; in this thesis I report on the magnetic field dependence of current, photocurrent and electroluminescence in OLEDs made from Alq3 and polyfluorene. We provide a comprehensive overview of all these three types of magnetic field effects. In particular, we show that they are all caused by the same mechanism, in contrast to some claims by others. To the best of our knowledge, the mechanism causing OMAR and the other magnetic field effects is currently not known with certainty. Moreover, we show that experiments in bipolar, electroluminescent devices do not allow determination whether the mechanism acts on the carrier density or carrier mobility. This is a crucial gap in knowledge since it makes any attempt of explaining it ambiguous. As a remedy, we performed magnetoresistance measurements in hole-only polyfluorene devices and show that the mechanism acts on the carrier mobility rather than carrier recombination. We present an outline of a possible explanation which will be studied in more detail in another thesis.; In my thesis I focus on possible applications of OMAR. I present the design principles for a pen-input OLED display based on OMAR effect. The pen-input screen consists of an 8x8 pixel Alq3 OMAR diode array together with a magnetic pen that emits an high frequency magnetic field. The design principles of a multiplexed detection scheme that uses a single filter/amplifier circuit to sequentially scan the individual pixels for the presence of the magnetic pen are discussed. For this scheme to work efficiently, it requires using frequencies of order 100kHz. We demonstrate that our OMAR devices can indeed follow such high frequencies. We believe the described prototype demonstrator constitutes an elegant and inexpensive solution to make interactive OLED screen applications a reality.