Spin injection and transport in a solution-processed organic semiconductor at room temperature

Abstract

Spin injection and transport in solution-processed 6,13-bis(triisopropylsilylethynyl)-pentacene (TIPS-pentacene) are investigated using vertical CoPt/TIPS-pentacene/AlOx/Co spin valve architectures. The morphology and the molecular orientation of TIPS-pentacene spin-coated from a chloroform solution as grown in the spin valve devices are characterized by means of atomic force microscopy and x-ray diffraction, where the $\ensuremath{\pi}$-$\ensuremath{\pi}$ stacking is found to be in planes oriented parallel to the substrate plane on CoPt (not $L{1}_{0}$-ordered). The magnetization hysteresis loops recorded with a superconducting quantum interference device show an individual switching of the magnetization of the two ferromagnetic layers. The antiparallel state of the relative orientation of CoPt and Co is achieved due to their different coercive fields. A spin valve effect is observed at room temperature. The scaling of the magnetoresistance (MR) with the bulk mobility of TIPS-pentacene as a function of temperature together with the dependence of the MR on the interlayer thickness clearly indicates spin injection and transport in TIPS-pentacene. From organic semiconductor-spacer thickness-dependent MR measurements, we estimate a spin relaxation length of TIPS-pentacene of ${\ensuremath{\lambda}}_{s}=(24\ifmmode\pm\else\textpm\fi{}6)$ nm and a spin relaxation time of approximately ${\ensuremath{\tau}}_{s}=3.5$ $\ensuremath{\mu}$s at $T=300$ K, taking the measured bulk mobility of holes into account.