Organic magnetoresistance near saturation: Mesoscopic effects in small devices

In organic light-emitting diodes with small area the current may be dominated by a finite number, N, of sites in which electron-hole (e-h) recombination occurs. As a result, averaging over the hyperfine magnetic fields, b sub(h), that are generated in these sites by the environment nuclei is incomplete. This creates a random (mesoscopic) current component, delta I(B), at field B having relative magnitude ~ N super(-1/2). To quantify the statistical properties of delta I(B) we calculate the correlator (ProQuest: Formulae and/or non-USASCII text omitted) for parallel, Delta B B, and perpendicular, Delta B [perpendicular] B orientations of Delta B. We demonstrate that mesoscopic fluctuations develop at fields B >> b sub(h) where the average magnetoresistance is near saturation. These fluctuations originate from the slow beating between the singlet (S) and triplet (T sub(0)) states of the recombining e-h spin-pair partners. We identify the most relevant processes responsible for the current fluctuations as being due to anomalously slow beatings that develop in sparse e-h polaron pairs at sites for which the b sub(h) projections on the external field direction almost coincide.