Study On The Mechanism Of Excitonic Ferromagnetism In Chiral Organic Semiconductors

Compared with the traditional inorganic semiconductor,organic semiconductor has many advantages such as light weight,good flexibility,translucent,low price,easy to prepare and so on.As a new generation of electro-optical materials,organic semiconductor has a very broad application prospect.In the past forty years,people have carried out systematic research on the internal physical mechanism of organic semiconductor,developed organic light-emitting diode,organic photovoltaic cells,organic thin film transistor,organic laser and other optoelectronic devices.In addition,it is noted that organic devices are sensitive to magnetic field,and it is hoped to develop organic magnetic storage,organic spin valve and other magnetic devices.Now,people have a systematic and comprehensive understanding of the charge properties of the carrier in organic semiconductor,but the study of organic magnetic devices is not mature,and there are still many problems to be solved in organic spintronics.In order to control the spin properties,a series of studies have been carried out on the phenomena of electron spin injection,transport and polarization in organic semiconductors.With the development of organic spintronics,the unique spin polarization phenomenon in chiral molecules or chiral nanostructured materials has attracted extensive attention.In the early studies,most people focused on the optical properties of chiral materials,such as optical rotation,circular dichroism,polarized photoluminescence,etc.Recently,many experiments have shown that the spin polarization phenomenon exists in the carriers of materials with chiral characteristics,which is called the chiral-induced spin selectivity(CISS).On the one hand,the chiral organic semiconductor has strong electroacoustic coupling interaction,which makes the carrier type different from the electron and hole,but the polaron coupled with the lattice.On the other hand,when polarized light excites to form excitons,there is a coupling interaction between photon angular momentum and electron angular momentum,which causes the spin-polarized electrons to be selectively excited.In this paper,the physical image of excited ferromagnetism in chiral organic semiconductors is simulated by theoretical calculation based on SSH model,considering the chiral spin-orbit coupling caused by the helical configuration of organic molecules.1.Magnetic moment of spontaneous charge transfer in ground stateBy introducing the chiral spin-orbit coupling caused by helical structure,we use the SSH model under tight binding approximation to construct the chiral organic molecular D/A heterojunction system,which theoretically explains the phenomenon of magnetic moment enhancement after organic semiconductor has chiral characteristics.We found that the helical configuration of organic molecules can generate a built-in electric field,and thus produce chiral spin-orbit coupling.The single group donor and acceptor molecules are both closed shell structures.When electrons are transferred from donor to acceptor after the formation of charge transfer complex,the spin polarization of donor and acceptor molecules is different,and the net magnetic moment appears in the system.When the polymer chain is affected by chiral spin-orbit coupling,the spin polarization will change and the net magnetic moment of the system will be changed.2.Polarized light excites charge transfer magnetic momentIn quantum mechanics,left-handed and right-handed circularly polarized light can be treated as photons carrying different spin angular momentum.Because of the conservation of angular momentum when the polarization photons are coupled with electrons in organic semiconductors,the electron spins excited by photons with different polarization states are different.In aciral organic semiconductor,the spin polarization of donor and acceptor is always opposite due to the spin-spin interaction,and the electron spin polarization orientation excited and transferred does not affect the net magnetic moment of the system.When the organic semiconductor has chiral characteristics,the symmetry of time inversion is broken,and the system appears chiral induced spin selective polarization.Since the spin polarization of donor molecules mainly depends on the chiral structure,and the electron excited by different polarized light will affect the spin polarization of acceptor molecules,the net magnetic moment of the system will also change with the polarization state of light.