Spin Transport Properties Of Organic Semiconductors Studies By Inversed Spin Hall Effect Measurements

Spintronics studies the injection,transport,detection and manipulate of spin polarized electrons to achieve high-speed and low-energy electronic devices.Compared with the traditional inorganic materials,the spin-orbit coupling and hyperfine interaction of organic semiconductors are weaker.Semiconductive polymers also have the advantages of flexibility and low-cost device fabrication,therefore they are expected to be ideal candidates for spin polarized transport.In the past decade,numerous important progresses have been made in organic spintronics,but there are also many limitations and challenges.For example,the performance of organic spin valve devices is still very low at room temperature,and the injection and transport mechanism of spin polarization is also controversial.One way to solve these problems is to find suitable organic materials as the transport medium of spin polarization,and to develop new detection methods to study spin transport and relaxation behavior.In recent years,the use of spin pumping to excite spin current,combined with the measurement of inverse spin-Hall effect(ISHE),can overcome the mismatch of interface conductance in spin valve devices.It becomes a powerful tool to research the intrinsic spin transport behavior and mechanism.It has been successfully applied to the study of spin transport in a variety of small molecules and conjugated polymers.In this paper,the spin transport properties of some new semiconductive polymers are investigated by using the spin pumping method of FMR and ISHE voltage measurement.The purpose of this work is to find new materials with excellent spin transport properties,and to explore how the molecular structure and film structure factors(such as molecular orientation and stacking order,morphology and crystallinity)of organic semiconductors affect the spin transport behavior.The main results are as follows:1.A measurement system of ISHE based on electron spin resonance spectrometer(ESR)is built,and a sample holder(installed in the ESR resonator)is designed for the measurement of ISHE voltage with low noise.The system is simple,stable and has high signal-to-noise ratio(SNR).It can suppress voltage noise to 10-20 nV.With the support of this measurement system,we systematically studied the ISHE of several ferromagnetic/paramagnetic bilayers and measured the ISHE voltage of the bilayers composed of ferromagnetic permalloy(Py)and "sinker"materials such as Pt,Pd and Au.By optimizing the measurement conditions and the geometry of the device,the ISHE voltage of up to 100 ?V is detected in Py/Pt bilayer structure.The spin mixed conductance at the interfaces of Py/Pt,Py/Pd and Py/Au,and the spin Hall angle of various "sinker" materials are calculated.The results are in good agreement with those reported in the latest literature.It is also found that the interface effect(such as inserting a thin buffer layer)has a great influence on the spin injection ability of the bilayer structure.Our experimental results have proved the reliability and accuracy of our measurement system.We also have a deeper understanding of the spin injection and spin orbit coupling characteristics of ferromagnetic/paramagnetic bilayer system.2.The donor-acceptor polymer semiconductor PBDTTT-C-T with excellent charge transfer performance was selected as the spin transport medium,and the spin transport characteristics of PBDTTT-C-T are investigated by using FMR-induced spin pumping effect.Therefore,we have prepared high quality Py/organic layer/Pt sandwich devices.The polymer spacer has a flat and compact surface morphology and a clear and sharp interface with the ferromagnetic electrode Py.By measuring the ISHE voltages of these devices,the spin current in PBDTTT-C-T film is observed.The spin diffusion length(37 nm)of PBDTTT-C-T at room temperature is calculated by fitting the change of ISHE voltage with the thickness of the spacer.It is found that PBDTTT-C-T has a long spin relaxation time.In addition,the change of ISHE voltage with temperature in sandwich device is measured.It is found that PBDTTT-C-T shows weak temperature dependence.The results support the spin relaxation mechanism based on spin orbit coupling(SOC).A small amount of PBDTTT-C-T is blended with polystyrene(PS)to control the film structure of the semiconductive polymer.The spin transport characteristics of the blend film are investigated for the first time by ISHE measurement.Compared with "pure" PBDTTT-C-T,the spin diffusion length(up to 56 nm)and carrier mobility of the blend films are significantly improved.The microstructure of the blend films is studied by AFM,EDS and UV-Vis absorption spectroscopy.It is found that the nano bundles formed by PBDTTT-C-T skeleton chains can form a film-through,interconnected nano grid in the insulated PS matrix.It provides a fast intra-chain charge conduction path,which can explain the enhancement of spin/charge transport in the blend film.Our work not only proves that the morphology and structure of organic semiconductor films have an extremely important influence on spin transport behavior and properties,but also provides an effective way to develop low-cost and high-performance organic spintronics materials.3.We also studied the spin transport behavior of organic-inorganic hybrid halide perovskite MaPbBr3 films.The ISHE of Py/MaPbBr3/Pt sandwich devices are measured by FMR-induced spin pumping technique.The results show that although Pb has a strong spin orbit coupling and Pb2+ion has a great absorption of spin current,the spin current can still be transported in MaPbBr3 film.The spin diffusion length is 94 nm and the spin relaxation time is 0.81 ?s measured at room temperature.The spin transport mechanism of MaPbBr3 film is also analyzed.Our results show that MaPbBr3 is an excellent spin polarized transport medium.