Tuning the Current of Organic Semiconductors by Magnetic Fields Arthur J. Epstein , Ohio State University, DMR 08052

1. Tuning the Current of Organic Semiconductors by Magnetic Fields Arthur J. Epstein, Ohio State University, DMR 0805220 • The reports that a weak magnetic field ~10 mT can increase the room temperature electrical conductivity (up to 10%) of organic semiconductors revealed a mystery. We proposed that the Magnetoresistance (MR) is due to Interconversion of Singlets and Triplets (MIST mechanism) of correlated electron and hole. • In this model, hyperfine coupling interconverts triplets into singlets and vice versa, while an applied magnetic field lifts triplet degeneracy by Zeeman splitting, leading to carrier recombination and electrical transport to be magnetic field dependent. We showed that the MIST mechanism accounts for the observed positive and negative MR in alpha-6T, and other organic semiconductors. • New results at high magnetic fields (a few T) show that MR can increase or decrease. Within our model we suggest that this behavior is related to S-T level crossing and reflects the distribution of e-h pairs spatial separations. • Total citation of MIST model is over 100. 300K100 nm7V Alq3 O Spin Levels 2gmBH S 2J(r) Sr r T1 T0 e-h distribution Upper panel: MR for Alq3 devices at high magnetic fields. Lower panel: Schematic dependence of S and T levels on e-h separation. At high magnetic fields the S-T interconversion is for e-h pairs separated with distance at which S-T level crossing happens. r

2. Tuning the Current of Organic Semiconductors by Magnetic Fields Arthur J. Epstein, Ohio State University, DMR 0805220 • Much of the research involves assembling small teams of students from different departments to achieve successful experiments. The students maintain a 1,000 sq. ft Class 1000 clean room with extensive experimental facilities. • About a dozen students from Physics and Chemistry Departments took a course during the Winter Quarter 2010 from the P.I. on magnetic phenomena in organic-based materials • Students also contribute to the writing of reports to funding agencies so that they are aware of the process for applying for research funding and how to report one‘s results. 1st Year Graduate Student Chris Wolf (right) carries out the measurements of his spintronic device fabricated under guidance of Dr. J-W Yoo as part of laboratory training.