Organic magnetoresistance near saturation: mesoscopic effects in small devices

TL;DR

This paper investigates mesoscopic effects in organic magnetoresistance near saturation, revealing how finite site numbers and hyperfine field averaging lead to current fluctuations driven by slow spin state beatings.

Contribution

It introduces a statistical framework for mesoscopic current fluctuations in small organic devices, highlighting the role of hyperfine fields and spin state dynamics.

Findings

Mesoscopic fluctuations develop at high magnetic fields near saturation.

Current fluctuations are linked to slow beatings between spin states.

Fluctuations are prominent in sparse electron-hole pairs with aligned hyperfine projections.

Abstract

In organic light emitting diodes with small area the current may be dominated by a finite number, N of sites in which the electron-hole recombination occurs. As a result, averaging over the hyperfine magnetic fields, b_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^(-1/2). To quantify the statistical properties of {\Delta}I(B) we calculate the correlator K(B, {\Delta}B)= <{\delta}I(B - {\Delta}B/2){\delta}I(B + {\Delta}B/2)> for parallel and perpendicular orientations of {\Delta}B. We demonstrate that mesoscopic fluctuations develop at fields B>>b_h, where the average magnetoresistance is near saturation. These fluctuations originate from the slow beating between S and T_0 states of the recombining e-h spin pair-partners. We identify the most relevant processes responsible for the current fluctuations as due to anomalously slow beatings that develop in sparse e-h polaron pairs at sites for which the b_h projections on the external field direction almost coincide.