The effect of fringe fields from patterned magnetic domains on the electroluminescence of organic light-emitting diodes Nicholas J. Harmon, Markus Wohlgenannt, and Michael E. Flatt´ e Department of Physics and Astronomy and Optical Science and Technology Center, University of Iowa, Iowa City, Iowa 52242, USA ABSTRACT Large magnetic field effects, either in conduction or luminescence, have been observed in organic light-emitting diodes (OLEDs) for over a decade now. The physical processes are largely understood when exciton formation and recombination lead to the magnetic field effects. Recently, magnetic field effects in some co-evaporated blends have shown that exciplexes deliver even larger responses. In either case, the magnetic field effects arise from some spin-mixing mechanism and spin-selective processes in either the exciton formation or the exciplex recombination. Precise control of light output is not possible when the spin mixing is either due to hyperfine fields or differences in the Lande g-factor. We theoretically examine the optical output when a patterned magnetic film is deposited near the OLED. The fringe fields from the magnetic layers supply an additionally source of spin mixing that can be easily controlled. In the absence of other spin mixing mechanisms, the luminescence from exciplexes can be modified by 300%. When other spin-mixing mechanisms are present, fringe fields from remanent magnetic states act as a means to either boost or reduce light emission from those mechanisms. Lastly, we examine the influence of spin decoherence on the optical output. Keywords: magnetic field effects, organic semiconductors, organic light emitting diodes, magnetic patterning, thermally assisted delayed fluorescence, organic magnetoresistance 1. INTRODUCTION Understanding large magnetic field effects (MFEs) in organic semiconductors, such as those comprising organic light emitting diodes (OLEDs), 1 has progressed steadily over the last several years. In typical OLEDs, electron and hole polarons encounter one another in the bulk and temporarily form loosely bound states, known as po- laron pairs, with a statistical ratio of 3:1 between triplet to singlets. These pairs may recombine into excitons [Figure 1(a)]; whether the loosely bound pairs are singlet or triplet may affect the rate of the exciton formation. Only singlet excitons lead to appreciable luminescence which makes most organic semiconductors fluorescent with internal electroluminescence quantum efficiency ≤ 25%. 2 The polaron pair stage is where MFEs play their role (since the exchange splitting is small) and interactions that cause singlet-triplet intersystem crossing, or spin mixing, lead to changes in exciton singlet/triplet formation ratios. Just in the last few years, MFEs in co-evaporated donor/acceptor organic blends have garnered enthusiasm. These systems work differently in that emission occurs via an exciplex (or intermolecular) route and not through an exciton (or intramolecular) pathway. Such blends are interesting for OLEDs since they allow a means to convert optically intet triplets to optically emissive singlets by means of the smaller singlet-triplet exchange splitting (< 100 meV) when compared to excitons. Triplet-to-singlet up-conversion and its concomitant output is known as thermally assisted delayed flu- orescence (TADF) since the emission increases with temperature. 3, 4 The origin of magneto-electroluminescence (MEL) is commonly the hyperfine interaction (HF) between polaron carrier spins and hydrogen nuclear spins which are plentiful in most organic systems. 5, 6 Slight environmental discrepancies can also lead to differences in Lande g-factors (δg) between the positive and negative polarons which is another author of spin mixing. 7–10 If manipulation of spin mixing and light emission is desired in either exciton or exciplex systems, neither the HF or δg mechanisms offer any feasible solution since these parameters are inherent for a given combination of materials. Further author information: (Send correspondence to N. J. H.) N. J. H.: E-mail: nicholas-harmon@uiowa.edu Spintronics IX, edited by Henri-Jean Drouhin, Jean-Eric Wegrowe, Manijeh Razeghi, Proc. of SPIE Vol. 9931, 993102 · © 2016 SPIE · CCC code: 0277-786X/16/$18 · doi: 10.1117/12.2242838 Proc. of SPIE Vol. 9931 993102-1 Downloaded From: http://proceedings.spiedigitallibrary.org/ on 10/07/2016 Terms of Use: http://spiedigitallibrary.org/ss/termsofuse.aspx