Dynamical Simulations Of Polaron Spin Inversion And Spin Filtering In Organic Semiconductors

Since the first discovery of spin injection from ferromagnetic metal electrode to organic materials, organic spintronics has developed rapidly. Organic semiconductors have their special advantages for potential applications for making organic functional devices such as the very cheap component elements, the soft and flexibility of the organic molecules, the simple processing technology and especially the very long spin diffusion length arises from the very weak spin relaxation mechanisms in organic materials. Due to the advantages above, organic semiconductors have been widely employed in the fabrication of organic functional devices, such as organic spin valves, field-effect transistors, light-emitting diodes, photovoltaic cells and organic thermoelectric generators.On one hand, most organic functional devices exhibit magnetic field effect, it has been observed directly in experiment that the charge transport and luminescence properties of organic spin valves and organic light-emitting didoes are influenced obviously by magnetic fields. As a result, most people pay attention to the spin relaxation mechanism in organic semiconductors. The value of the magnetoresistance in an organic spin valve is affected by the spin diffusion length in organic functional layer according to Xiong and Drew’s measurement. No magnetoresistance can be observed through such device when the spin diffusion length in organic functional layer is not large enough. In 2007, Rolfe et al. made organic spin valves with fullerene and its ramification with H-atoms in the molecule separately, and the phenomenon of magnetoresistance can be observed only in the H-atoms based devices, the maximum value of the magnetoresistance is about 0.1%. Later, Rolfe and Vardeny et al. replaced some of the H-atoms in the molecules of Alq3 and DOO-PPV with D-atoms, they found that the value of the magnetoresistance changed obviously before and after the replacement of the H-atoms after measuring the charge and spin transport properties of the Alq3 and DOO-PPV based devices. The experimental data above indicate that the hyperfine interaction in organic semiconductors play a very important role on the spin relaxation process in such materials. In 2013, Nuccio et al. replaced the Al-atom in Alq3 molecule with K, In and Bi atoms separately, they found that the spin polarization rate of the molecule changed after the replacement of the metal atoms by time-resolved photoluminescence measurements. They demonstrated the fact that spin-orbital coupling also played a very important role on the spin relaxation in organic materials. For instance, temperature-dependent probes of the spin diffusion length in two structures of poly (3-hexyl thiophenes) revealed that distinct spin relaxation mechanisms can be observed under different charge transport mechanisms of these materials. On theoretical side, several works have been performed to investigate the spin relaxation process base on the two mechanisms mentioned above. Yu investigated the spin relaxation and spin scatting process in disordered organic small molecule materials based on spin orbital coupling and hyperfine interaction separately, they explained quantificationally the measured temperature dependence of the spin diffusion length in disordered Alq3 materials. Furthermore, the effects of spin-flip scatting on the performance of organic functional devices have been investigated from several aspects.On the other hand, organic ferromagnets are such fantastic materials that they combine organic and ferromagnetic together. Recently, the field of organic ferromagnet chemosynthesis and organic ferromagnetic devices fabrication are very active, and now several kinds of organic ferromagnets have been synthesized. In general, OFMs can be classified into two categories. One is organic magnetic macromolecules consisting of transition-metal ions (i.e. Mn, Fe, Cr and V) and organic molecular clusters such as charge transfer complex (metal complex together with organic electron acceptor), metallo-organic chelate, single (double) metallic ion magnets and organic magnets made up of metallic ions and organic free radicals. These organic magnets can be synthesized by organometallic chemistry method and most of them can dissolve in organic solvents. Usually, organic magnets with metal elements have very high Curie temperature and the Curie temperature of some materials can even reach room-temperature. The others are OFMs with only purely organic elements (i.e., C, H, O, and N) in the molecule. Mataga et al. first predicted the existence of purely organic magnets theoretically by a model of spin multiplicity in 1968, and this type of organic magnets was first synthesized in 2001. The other kind of purely organic magnets can be realized by using spin radicals which are usually heterocycles containing an unpaired electron. There are strong spin correlations between electrons on the main chain and the side radicals. As a result, the spins of the unpaired electrons on side radicals tend to be the same, so magnetism can be obtained in this material. The first purely organic magnet with side radicals (poly-BIPO) was obtained in 1987 by Ovchinnikov et al. In 1988, some magnets with similar structure such as poly-BIPENO and poly-BIPOC were successfully synthesized. On theoretical side, the properties of magnetism, electrons and energy level, charge density and spin density of the magnet molecule have been investigated as well as the nonlinear excitations such as solitons and polarons. Furthermore, organic ferromagnets are widely used in the fabrication and investigation fields of organic magnetic devices. Yoo et al. measured the spin-dependent electronic transport properties of V[TCNE]x through an Au/OFM/Au structure, and a magnetoresistance with maximum value of about 2.5% was obtained. Later, they used this type of organic magnet as electrode and investigated the charge and spin transport properties of the device, the phenomenon of magnetoresistance was obtained. Li et al. also investigated the magnetoresistance properties of such material by bridging it to magnetic electrodes, and a room-temperature magnetoresistance was observed when the Al2O3 barrier was used. Sugawara et al. investigated the spin-dependent transport properties of spin-polarized wire molecules (ESBN)ClO4 connected to gold nanoparticles, they found that the transport mechanisms were different when the temperature was above and below 30K, and the phenomenon of magnetoresistance was also observed. On theoretical side, organic ferromagnets were also used to design functional devices base on SSH model or density functional theory together with non-equilibrium Green’s function method, and some fantastic phenomena such as spin filtering and spin rectification were obtained.Although great progress has been made in the research of spin relaxation and spin-dependent charge transport in organic semiconductors, the understanding of the phenomena and mechanisms above are not comprehensive enough. First of all, most of the previous works investigating the spin relaxation and spin scatting in organic semiconductors are based on the theory of small polaron hopping, and the research objects are disordered small molecule materials. The condition of polaron drift in organic conjugate polymers are not mentioned. It has been demonstrated that the dominating spin relaxation mechanism in organic semiconductors can be affected by the charge transport mechanisms. Secondly, most of the previous works investigating the spin-dependent transport properties of organic ferromagnets are based on the quantum tunneling theory. In a real device, the thickness of the organic functional layer is usually very thick (a few or hundreds of nanometers, even to the micrometer order of magnitudes) and the charge carriers can inject into and transport in organic materials. So it is necessary to investigate the problems above form dynamical side.In this paper, we investigated the dynamical properties of polaron spin inversion and spin-dependent poalron transport in organic semiconductors (both nonmagnetic and magnetic) based on SSH model together with nonadiabatic dynamical method, the details for our calculation are as follows:1. We investigated the effect of different spin relaxation mechanism on the dynamical properties of polaron spin evolution in organic conjugated polymers, and the results are shown in chapter three. It is found that, while both mechanisms can impact the polaron spin by changing the polaron level from a spin eigenstate to a spin superposition state, substantial difference can be observed in the static and dynamical properties of the polaron. Given the values of model parameters relevant to conjugated polymers, the magnitude of the polaron spin inversion caused by the spin-orbit coupling is much smaller than that of the spin-flip term. When the dynamical properties of the polaron are considered, spin oscillations induced by both mechanisms are observed when the polaron moves along the polymer chain driven by an external electric field. Interestingly, the length of the polaron motion during one spin oscillation period remains constant in the case of spin-orbit coupling, while it is enhanced with increasing the driven electric field in the case of spin-flip term, in which larger spin diffusion length and longer spin relaxation time can be expected.2. The dynamical properties of a polaron in an organic ferromagnetic polymer are also considered as supplementary of quantum tunneling investigation. It is found that, in the presence of an external electric field, the polarons with both up and down spins can get trapped near the side radicals of the polymer chain unless the electric field is stronger than a critical field. However, the magnitudes of the critical electric field vary quite differently for the spin-up and spin-down polarons as a function of the number of the side radicals in the polymer, leading to the exponential change of the range of the electric field within which the spin-filtering take place.3. The effects of the intrinsic factors (i.e. the number and position of the side radicals, the strength of the electron-lattice coupling and the strength of the spin correlation between electrons on the main chain and the side radical) of organic ferromagnetic polymers on the spin filtering are also considered. It is found that, the range of the electric field for spin filtering decreases almost exponentially with the width of the potential barrier (potential well), while it increases linearly with the electron-lattice coupling strength. At last, by changing the strength of the spin correlation between electrons on the main chain and the side radical we found that organic ferromagnets with stronger electron-lattice coupling are more conducive to polaron spin filtering.