Abstract
Spin-dependent space charge limited carrier conduction in a Schottky barrier diode using polycrystalline \emph{p}-type $\pi$-conjugated molecular pentacene is explored using multiple-frequency electrically detected magnetic resonance (EDMR) spectroscopy with a variable-angle configuration. The measured EDMR spectra are decomposed into two components derived respectively from mobile and trapped positive polarons. The linewidth of the EDMR signal for the trapped polarons increases with increasing resonance magnetic field for an in-plane configuration where the normal vector of the device substrate is perpendicular to the resonance magnetic field, while it is independent of the field for an out-of-plane configuration. This difference is consistent with the pentacene arrangement on the device substrate, where pentacene molecules exhibit a uniaxial orientation on the out-of-substrate plane. By contrast, the mobile polarons do not show anisotropic behavior with respect to the resonance magnetic field, indicating that the anisotropic effect is averaged out owing to carrier motion. These results suggest that the orientational arrangements of polycrystalline pentacene molecules in a nano thin film play a crucial role in spin-dependent electrical conduction.
Highlights
Organic semiconductors exist in various polycrystalline forms, the morphologies of which affect the characteristics of electronic devices incorporating such semiconductors (Thorsmølle et al, 2009)
In another previous paper published by us, we focused on the relationship between electrically detected magnetic resonance (EDMR) line and J-V characteristics (Fukuda and Asakawa, 2017), but we did not perform mf-EDMR measurements on the pentacene device
The forward current density lies in the so-called trap-free spacecharge-limited conduction (SCLC) regime (Sze, 1981)
Summary
Organic semiconductors exist in various polycrystalline forms, the morphologies of which affect the characteristics of electronic devices incorporating such semiconductors (Thorsmølle et al, 2009). For π-conjugated molecules, the device structure determines the optimal orientation of π-stacking planes, since carrier mobility varies depending on whether the direction of carrier transport is aligned with the direction of π-stacking (Lee et al, 2006). For devices in which electrical current flows in-plane, such as typical field-effect transistors, an edgeon orientation of plate-like π-conjugated molecules on the substrate surface, with the direction of π-stacking parallel to the surface, is appropriate (Sakanoue and Sirringhaus, 2010). For structures such as typical solar cells and light-emitting devices, a face-on orientation of plate-like molecules, with the direction of π-stacking perpendicular to the substrate surface, is appropriate. Organic semiconductors have attracted interest in the field of spintronics, because they have longer spin coherence times than inorganic semiconductors (Sanvito and Dediu, 2012), owing to
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