Abstract
We predict that singly occupied carrier traps, produced by electrical stress or irradiation within organic semiconductors, can cause spin blockades and the large room-temperature magnetoresistance known as organic magnetoresistance. The blockade occurs because many singly occupied traps can only become as doubly occupied in a spin-singlet configuration. Magnetic-field effects on spin mixing during transport dramatically modify the effects of this blockade and produce magnetoresistance. We calculate the quantitative effects of these traps on organic magnetoresistance from percolation theory and find a dramatic nonlinear dependence of the saturated magnetoresistance on trap density, leading to values ∼20%, within the theory's range of validity.
Highlights
We consider traps in a recent theory20–23 of OMAR based on percolation theory24 and surmise that occupied traps are commonly the cause of OMAR instead of the much more dilute bipolarons or excitons
We predict that singly occupied carrier traps, produced by electrical stress or irradiation within organic semiconductors, can cause spin blockades and the large room-temperature magnetoresistance known as organic magnetoresistance
We calculate the quantitative effects of these traps on organic magnetoresistance from percolation theory and find a dramatic nonlinear dependence of the saturated magnetoresistance on trap density, leading to values $20%, within the theory’s range of validity
Summary
We consider traps in a recent theory of OMAR based on percolation theory and surmise that occupied traps are commonly the cause of OMAR instead of the much more dilute bipolarons or excitons. When one of the spins hops off the trapping center at a later time, the other spin solely occupies the trap and returns to the lower energy state. Several spin-evolution mechanisms permit spin-triplet states to evolve to spin-singlet states, including the different hyperfine fields at the two sites and spin-orbit interactions These mechanisms are influenced by an applied magnetic field, which affect the charge transport and produce magnetoresistance. Deep traps are highly localizing centers, so the doubly occupied triplet state has an exchange energy assumed to be large enough to neglect the triplet state as a possible intermediate configuration
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