Triplet exciton controlled current in an organic semiconductor

Organic materials like MEH-PPV have relatively low spin-orbital coupling leading to long spin lifetimes and good spin-selection rules. As a result, the rates of recombination of an ensemble of polaron pairs can be changed by resonant manipulation of either the P$^{+}$ or the P$^{-}$ spins, leading to a flopping between the singlet to triplet manifolds - this can be observed as a small change in the device current [1]. In this study we have used pulsed electrically detected magnetic resonance with electron-rich OLED devices to investigate the possibility of other spin-dependent processes like an exciton-polaron interaction. We expect that devices with excess electrons will show signals from free electrons interacting with long lived triplet excitons. Coherent Rabi nutation experiments were carried out from room temperature down to 5K. At room temperature only a single polaron-pair resonance is observed at g$\sim $2.003. However, as the temperature is decreased a signal at g$\sim $4(triplet exciton resonance) is observed along with a second signal at g$\sim $2.003 corresponding to the rotation of a single polaron.\\[4pt] [1] D. R. McCamey, \textit{et. al.} Nature Mater. \textbf{7}, 723, (2008)