Investigation of charge injection and transport behavior in multilayer structure consisted of ferromagnetic metal and organic polymer under external fields

Significance 

A derivative of the tetraiodopyrrole has been reported to have a high conductivity of about 1 Ωcm-1 for several years now. Later it was discovered that polyacetylene could be oxidized with halogens to generate conducting materials from semiconducting and insulator materials. Therefore, organic electronics has gained much interest recently, and has become a research area in high demand. Organic materials are π-conjugated polymers, conducting and semiconducting polymers, dyes, and small molecules.

In organic electronic devices, electrodes are made of metals. Therefore, these gadgets are normally made of a sandwich structure: metal/organic layer/metal. Thus, researchers have focused on the electrical characteristics of the organic electronic devices with an objective of acquiring excellent current-voltage features for possible organic diode device. They have as well concentrated on studying optoelectronic features for perfect light-emitting performance.

Professor Hua Zhao and his graduate student Wei-Feng Meng from ChongQing University in China, studied in depth a five-layer organic electronic device with alternately placed ferromagnetic metals and organic  polymers. The device configuration was ferromagnetic metal/organic layer/ferromagnetic metal/organic layer/ferromagnetic metal, and was injected a spin-polarized electron from the outer sides. The system implemented 1-dimensional tight binding model Hamiltonian. They also look at the transport features of an injected electron in the multi-layer system exposed to varying external electric fields theoretically and also analyzed the entire transport system of the injected electron in the multilayer system physically. Their research work is published in journal, Superlattices and Microstructures.

The authors found that the first electron-polaron in the right-hand organic layer moved towards the left end of the layer at the positive electric field and then entered the middle ferromagnetic layer at a stronger positive electric field and drifted in the ferromagnetic layer. The electron-polaron finally jumped into the left-hand organic layer at a stronger electric field. It stayed at the left-hand organic layer because the Fermi energy level of the left-hand ferromagnetic layer was much higher as compared to the electron-polaron energy level of the left-hand organic layer.

They also observed that in the positive external electric field, the negative charges piled up at the left ends of each layers, and this led to induced local interface dipoles at the interfaces between the ferromagnetic layer and the organic layer. The dipoles were directed to the left-hand direction and their effects were like barriers that affected transport of the injected electron. The dipoles were not intrinsic and they disappeared when the external electric field was withdrawn.

In the case of the negative external electric field, the authors observed two electron-polarons whose energy levels had the same spin polarization that appeared in the right hand organic layers at the external electric field. When the external electric field was increased to a much stronger value, these two electron polarons stayed in the right-hand organic layer.

The findings of Hua Zhao and Wei-Feng Meng study indicated that a pin-polarized electron injected into the organic layer could stride over the middle ferromagnetic layer to another organic layer with increasing external electric fields as opposed to using a constant electric field. The results provided the necessary information for future non-equilibrium transport study of an injected electron in a similar multilayer system and a superlattice system, which would be helpful in experimental investigation of the organic electronic gadgets, for instance, organic transistor device and organic light emitting devices.

About the author

Prof. Hua Zhao has been studying theoretical physics at Chongqing University, Chongqing, China and has been Head of Institute of Condensed Matter Physics since his entering Chongqing University as a professor in 2005. He received his PhD of theoretical condensed matter physics in 1998 from Department of Physics in Fudan University (1995-1998) at Shanghai of China where his main study was electronic structure, electron-electron interaction, and electronic correlation of organic pi conjugated polymers.

In 1989, he used low temperature vibrating magnetometer to study magnetic properties of high temperature superconductors and did theoretical study at MASPEC, Parma, Italy. In 1998, he studied electron-electron interactions in pi electronic systems at Department of Chemistry, Hong Kong University, Hong Kong, China. In 1998-1999, he studied physical properties of molecular systems as a Postdoctoral at Department of Chemistry and Center of Super-function Material,Pohang University of Science and Technology, Pohang, Korea. In 2000-2003, he studied electronic structure of many-electron systems, electron transport through superconductor quantum junction as a Postdoctoral at Department of Astronomy and Physics, McMaster University, Hamilton, Canada In 2003, he studied semiconducting quantum well as an Associate Researcher at Department of Computer and Electrical Engineering, McMaster University, Hamilton, Canada.

Professor Hua Zhao studies electron-electron (diagonal and off-diagonal) interactions, long-range electronic correlation, electronic structure, elementary excitations such as polaron, exciton, etc, in organic conjugated polymers for a long time. He has extensive interesting in physics such as superconductivity, topological insulators, quantum transition, quantum magnetism, quantum Hall effects, spontaneous symmetry broken, Spin polarization, and so on. Specially, He proposed an analytic and concise theoretical formula and correlation Hamiltonian that can be used to calculate long-range electronic correlation energies for ground state and excited states for organic pi electronic systems, which remedies a shortcoming of the density functional theory (DFT).

He has published research papers on the organic conjugated pi electronic systems in Phys. Rev. B, Eur. Phys. J. B, J. Appl. Phys., J. Phys. Condens. Matter, Physica B, Synth. Met., Acta. Phys. Sinica, Superlattice and Microstructure,Computing and Theoretical Chemistry, etc.

Reference

Hua Zhao and Wei-Feng Meng. Investigation of charge injection and transport behavior in multilayer structure consisted of ferromagnetic metal and organic polymer under external fields. Superlattices and Microstructures, volume 110 (2017), pages 31-48.

 

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