Recently, researchers from the Center for High Magnetic Field Science, Hefei Institute of Material Science, China Academy of Sciences have made new progress in the research on the spin flow detection of polymer semiconductors and the relationship between their thin film structure and spin transport properties. The related research results are published in ACS Applied Materials &: Interfaces) published online.
organic semiconductor materials have weak spin-orbit coupling and hyperfine interaction, and can be used as promising spin-polarized transmission media, so it is of great significance to find new organic spin-electron materials and explore their spin-polarized transmission process and mechanism. Previously, most of the research in this field measured the electron transport with spin polarization by preparing organic spin valve devices, but there were some problems such as conductance mismatch at the ferromagnetic/semiconductor interface, which seriously restricted the quantitative and in-depth study on the spin transport characteristics of organic semiconductors. In recent years, spin pump excitation and detection of pure spin current (without net charge current) have gradually become a powerful means to explore the intrinsic spin transport properties of semiconductor materials because they can overcome the interface conductance mismatch problem.
in cooperation with researcher Tong Wei, Zhang Fapei's research group of High Magnetic Field Center studied the spin-polarized transmission characteristics of a new polymer semiconductor PBDTTT-C-T by using ferromagnetic * * * vibration (FMR) spin pumping technology combined with reverse spin Hall effect (ISHE) measurement. By designing a sample frame suitable for low-noise voltage measurement, they detected a clear ISHE signal in NiFe/ polymer /Pt sandwich structure. By measuring the change of ISHE voltage with the thickness of PBDTTT layer, they observed the pure spin flow transmission and long spin relaxation time in PBDTTT layer.
Surprisingly, researchers used the semiconductor/insulator polymer * * * mixed film as the spin polarization transmission medium for the first time. In the * * * mixed film formed by low-content PBDTTT and insulating polystyrene (PS), strong ISHE voltage signal can still be measured, and it is found that the spin diffusion length and carrier mobility of the * * * mixed film are significantly improved compared with the "pure" PBDTTT film. Through the comprehensive measurement of the microstructure of the film, they found that the PBDTTT skeleton chain bundle formed an interconnected network of nanofilaments in the insulating PS matrix, which formed a fast charge conduction path through the film, which could explain the higher charge and spin transmission ability of the * * * mixed film. In addition, it is found that the spin diffusion length of PBDTTT has weak temperature dependence, which is consistent with the spin relaxation mechanism based on spin-orbit coupling.
these results clearly show that the structural characteristics of organic semiconductors, such as molecular orientation, stacking mode and film morphology, have a key influence on their spin transport properties. This work is of great significance to understand the microscopic process and mechanism of spin polarization transmission in organic semiconductors, and provides a new way to find low-cost and high-performance organic spintronics materials.
this research is supported by the national natural science foundation project and the national key research and development project.
article link: https://pubs.acs.org/doi/1.121/acsami.9b1662
figure (a) the generation of ISHE effect on Py/PBDTTT-C-T/Pt sandwich structure device, (b) the total voltage spectrum measured by the device (varying with magnetic field) and its deconvolution. Where VLorentz corresponds to ISHE voltage, and (c) ISHE voltage varies with the thickness of PBDTTT-C-T dielectric layer and PBDTTT/PS*** mixed film dielectric layer respectively. From this, different spin diffusion lengths ls of polymer films are calculated.