Study On The Mechanism Of Pure Spin Current Transport In Organic Semiconductors

The special sp2 orbital hybridization of organic semiconductor materials provides an effective channel for electrons transfer.The carriers are localized solitons,polarons and excitons due to the strong electron-phonon coupling in organic semiconductors,which makes them exhibit rich electrical,magnetic,and optical properties.In addition,organic materials have the advantages of low cost,roll-to-roll processing,flexibility,translucency,and pollution-free.Therefore,organic semiconductors are focus for development and preparation of a new generation of cheap,excellent performance,and environmentally friendly multifunctional electronic devices.So far,a variety of functional devices based on organic semiconductors,such as organic solar cells,organic light-emitting diodes,and organic field effect tubes have been implemented in laboratories.The organic light-emitting diodes have achieved commercial applications in display screens.However,the internal physical mechanism is not completely clear,In addition to charge properties,electrons also have spin properties.Both charge and spin can be used as carriers for information storage and transport,such as integrated circuit devices that use charge as a carrier and magnetic disks that use spin as a carier.Limited by Moore’s Law,the development of integrated circuit chips has been severely hindered.Compared with charge,spin as carriers opens up new channels for information storage and transport.In 1988,Albert Fert and Peter Grunberg independently discovered the phenomenon of giant magnetoresistance,which achieved the improvement of the storage density of computer hard disks.With the discovery of giant magnetoresistance effect and tunnel magnetoresistance effect in organic semiconductor,the subject of organic spintronics came into being.Organic spintronics is the cross-combination of spintronics and organic electronics.Compared with inorganic semiconductors,organic semiconductors are mainly composed of elements with a smaller atomic number.Therefore,organic semiconductors have weaker spin-orbit coupling and hyperfine interaction,resulting in longer spin relaxation time,which is conducive to long-distance spin transport.The basic research content of organic spintronics are spin injection,transport and detection.In recent years,organic spintronics experiments have developed rapidly,and the frontier topics involved the injection and transport of pure spin currents and unique electrical,magnetic,and optical phenomena caused by spin-selective effects in organic semiconductors with chiral structures.In 2013,using the spin pumping technology and inverse spin Hall effect,Ando K.et al.,observed the pure spin current injection and transport in the organic conjugated polymer for the first time.At present,it have been realized in a variety of polymers and small molecules,and an ultra-long spin diffusion length of about 1 μm has been obtained.In 2020,Yan Y.et al.,have detected up to 80%spin filtration in the self-assembled super-coiled conductive polymer fiber molecular device.Compared with the development of experiments,many basic problems and physical mechanisms in organic spintronics have not been solved or are still lacking.For example,the source of organic magnetic effects,the theoretical mechanism of pure spin current transport,and the source of spin selectivity in organic chiral molecules et.Considering that organic semiconductors have strong electron-phonon coupling,the paper is based on the tightly bound Su-Schrieffer-Heeger(SSH)model,and also includes electron-electron interactions,spin-spin interactions,spin orbital coupling and hyperfine interaction,the effect of spin correlation on the yield of singlet excitons in organic light-emitting diodes was studied.In addition,in order to study the transport mechanism of pure spin current in organic semiconductors,we have further established the dynamic evolution equations.By solving the equations,the spin current and spin diffusion length are obtained.The research content and results of this paper are as follows:1.Strongly enhanced luminous efficiency of organic light-emitting diodes in molecular heterojunctionsThe internal quantum luminous efficiency of organic light-emitting diodes mainly depends on the proportion of singlet.According to the statistics of excitons,the fraction of singlet is 25%indicating the internal quantum efficiency is limit of 25%.However,the prerequisite for this conclusion is that the formation rate of singlet and triplet in organic light-emitting diodes is same and there is no mutual conversion between them.When spin related effects,such as spin-orbit coupling or hyperfine interaction,is included,the yield of the singlet can be changed.On the one hand,it is possible to break the same formation probability of singlet and triplet affected by spin related interactions.On the other hand,the presence of spin related effects makes the singlet or triplet become spin mixed state,which breaks the spin forbidden between singlet and triplet,which causes the mutual conversion between singlet and triplet.In chapter 3,we have studied spin-related effects in organic systems,including spin-orbit coupling and hyperfine interaction on the singlet faction.Under the effect of spin correlation,we project the excitons of spin-mixed to pure singlet and triplet,and count the singlet fraction.We found that both the spin-orbit coupling and hyperfine interaction can be increase singlet fraction,and the fraction of singlet in inter-chain excitons is higher than that in intra-chain excitons.We attribute it to the difference in the localization of the excited states.In addition,it shows that singlet fraction depends on the two organic semiconductor materials forming the heterojunction and the heterojunction structure.2.Spin transport based on exchange coupling in highly doped organic polymersIn recent years,the successful realization of the injection and transport of pure spin currents excited by spin-pumping in organic semiconductors have stimulated in-depth study on the spin transport mechanism.Compared with inorganic semiconductors,organic semiconductors usually have a large electron-phonon coupling strength.Thus,carriers are localized polarons,which cannot be transported by band transport.So far,mechanisms of spin transport in organic semiconduetors include exchange coupling,polarons hopping and impurity band.However,which mechanism plays a dominant role is still full of controversy.In chapter 4,we quantitatively study pure spin transport based on exchange coupling.We found that the exchange coupling-mediated spin transport can be achieved only when the carrier density is higher than 8×1017cm-3.Through fitting experiments,we found that the theoretical results are in good agreement with the experimental data,which further indicates that the exchange coupling between polarons is the main channel for spin transport in organic semiconductors with high impurity concentrations.In addition,we also found that the relationship between the pure spin current and the carrier density is non-monotonous.In the spin transport dominated by exchange coupling,although a larger carrier density can produce a stronger exchange coupling,however,spin relaxation is also present.When the carrier density is large,the frequency of spin relaxation is large during the transport process,which leads to a rapid decay of the spin current.Therefore,we predict a maximum pure spin current at a certain carrier concentration.3.Hopping-dominated spin transport in unintentionally doped organic semiconductorsIn previous work,we found that polarons exchange coupling can only occur in organic materials with a high carrier concentration.However,The injection and transport of pure spin current in organic semiconductors have also been confirmed in the low carrier density.For example,in 2020,Groesbeck and Wittmann et al.observed spin diffusion length of about 37 nm and 40 nm in SY-PPV organic conjugated polymer with a carrier concentration of about 4.4±2×1015cm-3 and dinaphtho[2,3-b:2,3-f]thieno[3,2-b]thiophene(DNTT)molecules with a concentration of about 1014-1015 cm-3,respectively.Obviously,the exchange coupling mechanism is no longer applicable.Therefore,the polaron hopping may be dominant when the low carrier concentration is low.It is demonstrated that polarons hopping dominated pure spin current transport does occur in organic semiconductors with low carrier density.The spin difusion length in the DNTT molecule is consistent with the experimental data.Interestingly,spin diffusion length can be prolonged by restraining molecular geometry structure disorder and reducing the reorganization energy.In comparison with anisotropic organic materials,the spin diffusion length in isotropic ones increases up to 60%.Our results open up a new avenue to design organic spintronics devices with long SDL and low carrier density.