We thank the Office of Research and Sponsored Programs for supporting this research, and Learning & Technology Services for printing this poster. INTRINSIC MAGNETORESISTANCE IN ORGANIC LIGHT-EMITTING DIODES Austin Riedl with Faculty Mentor Dr. James Rybicki | Department of Physics and Astronomy A N INVESTIGATION OF O RGANIC MAGNETORESISTANCE IN OLED S MADE WITH P3HT W HY O RGANIC E LECTRONICS ? O BJECTIVE AND P ROCEDURE B IPOLARON M ODEL Light is emitted when an electron (e-) and hole (h+) recombine. The organic layer is an insulator, so it does not have intrinsic charge carriers. As a result, an anode and cathode are needed to facilitate the injection of charge. H OW OLED S W ORK Spin Blocking reduces the current at high B-fields Pauli Exclusion Principle prevents charges with the same spin from occupying the same state Applied magnetic field aligns the spins of the charge carriers Most likely to increase the resistance, and decrease the conductance W ORKS C ITED A CKNOWLEDGMENTS O RGANIC M AGNETORESISTANCE (OMAR) Lightweight, thin, and efficient Improved picture quality Lower production costs Flexible devices due to the amorphous molecular structure of the organic layer Determine if current theoretical models of Organic Magneto-resistance (OMAR) can explain observed magnetoresistance in organic light-emitting diodes (OLEDs) made with the organic polymer P3HT. Fabricate OLEDs without exposing the devices to damaging x-rays (thermal evaporation) and observe the unaltered, intrinsic OMAR effects. Attempted to fit our data with two existing OMAR models O RGANIC L IGHT -E MITTING D IODES (OLED S ) Organic Layer (light-emitting) Electron Injection Layer Hole Injection Layer Aluminum Calcium P3HT ITO (-) Cathode Anode OLED structure (+) Anode (ITO) Organic Layer (P3HT) Electron Energy Conduction Band Valence Band e- e- e- e- Cathode (Ca) h+ h+ h+ h+ e- h+ Position Electrical resistance of OLEDs changes in a magnetic field. We measure magnetoresistance by looking at magneto-conductance. Magneto-conductance is expressed as a percentage and is given by: I(B) is the current at a particular magnetic field (B) and I(0) is the current through the device when the applied magnetic field is zero Tesla. The potential (voltage) applied across the device remains constant. T HEORETICAL M ODELS There are two leading models that attempt to explain OMAR, the Bipolaron Model and the Electron-Hole Model. E LECTRON -H OLE M ODEL Excitons are bound electron-hole pairs that can be in either a singlet state or a triplet state. Recombination is spin dependent; high recombination decreases magneto-conductance. Singlet state: net spin = 0, recombination more likely Triplet state: net spin = 1, recombination less likely At high B-fields, the spins of the holes and electrons are likely to be aligned, forming triplet, rather than singlet, excitons. Less recombination allows more charge carriers to stay in the device, increasing the magneto-conductance. Dr. Markus Wohlgenannt - University of Iowa University of Wisconsin - Eau Claire Department of Physics and Astronomy E XPERIMENTAL R ESULTS P. A. Bobbert, T. D. Nguyen, F. W. A. van Oost, B. Koopmans, and M. Wohlgenannt, Phys. Rev. Letters 99, (2007). Prigodin, V. N., Bergesona, J. D., Lincoln, D. M. & Epstein, A. J. Synth. Met. 156, 757–761 (2006). O. Mermer, G. Veeraraghavan, T. Francis, Y. Sheng, D. T. Nguyen, M. Wohlgenannt, A. Kohler, M. Al-Suti, and M. Khan, Phys. Rev. B 72, (2005). which is referred to as a “Fully Saturated” curve, or which is referred to as a “Weakly Saturated” curve, where x is the magnetic field and A, B, C, and D are constants. OMAR is believed to be able to be described by either of the following empirical line shapes. In an attempt to get an even better fit, we fit the data with the sum of a weakly saturated and a fully saturated fitting function. C OMBINED C URVE F IT : P OSITIVE M AGNETO - CONDUCTANCE Neither the fully saturated curve nor the weakly saturated curve match the data very well. S EPARATE C URVE F ITS It appears that the combination of the existing models and fitting functions may be able to explain the small OMAR effects observed. Although we can fit of our data with the combination of fitting functions, our fits do not have consistent values for the constant terms. More data collection and analysis will be required to determine if the combination of functions is necessary and sufficient to describe the observed OMAR effects. Two magnetic fields, the applied field and the hyperfine field Spin of the electron processes around the net magnetic field C OMBINED C URVE F IT : N EGATIVE M AGNETO - CONDUCTANCE Electron Spin Adj-R Squared = Adj-R Squared = Adj-R Squared = Adj-R Squared = D ATA FITTING FUNCTIONS D ISCUSSION Magneto-conductance