Study On The Mechanism Of Excitonic Ferromagnetism In Organic Semiconductors

Over the last four decades,as important electromagnetic optical functional materials,the field of organic semiconductors has made great progress.Compared with inorganic counterparts,organic semiconductor devices are easy preparation,flexibility and have lower price,which have broad prospects in application.Many organic electronic devices,such as organic light-emitting diodes,organic solar cells,organic spin valves and organic thermoelectric devices have been developed.Organic light emitting diode(OLED)technology is probably the most mature areas of organic semiconductor devices,and a lot of flexible,transparent,ultrathin displays have been launched.Through many methods,such as organic electrophosphorescent devices,thermally activated delayed fluorescence and using open-shell molecules,the internal quantum efficiency of an OLED can up to 100%in theory.Organic solar cells are photovoltaic energy conversion devices,and the photoelectric conversion efficiency have been setting new records by means of selecting the appropriate materials and structures.The discovery of giant magnetoresistive effect caused a hard disk revolution of "large capacity and miniaturization".Compared with inorganic counterparts,the spin-orbit coupling in the organic semiconductors is weaker,which means a longer spin relaxation time.Therefore,organic semiconductors are more suitable for spin transport materials.An important challenge for organic spin valves is to improve the magnetoresistance at room temperature.Organic thermoelectric devices can realize direct conversion between thermal and electrical energy.A variety of ways can improve the figure of merit of organic thermoelectric device,such as low-dimension,nanocrystallization and organic-inorganic compounds.Thermal effect is an important factor which affecting the performance of these organic devices.It may cause burning down when an organic device is operated under a not high bias or not long sustained duration as the Joule heat could not be released immediately.Temperature dependent photoluminescence(PL)is a primary consideration in organic devices.A number of temperature-dependent PL studies have provided valuable information about the absorption and emission characteristics of conjugated polymers.These studies can provide deeper understanding of physical processes in conjugated polymers.In 2003,Guha et al.have presented PL studies as a function of temperature for a series of conjugated polymers.They have found that the PL transition energies blue shift with increasing temperatures.Similarly,in 2006,Kong et al.have found that the intensities of the PL spectra of poly-[2-methoxy-5-(2-ethyl-hexyloxy)-1,4-phenylene-vinylene])(MEH-PPV)decrease and the peak positions blue shift with increasing temperature.Recently,spin and charge dependent thermoelectric transport has gained great attention for thermal energy conversion.Organic polymers indubitably make a main opportunity for developing spintronic and thermoelectric nanodevices because of their dominant properties such as adjustable energy gap,strong electron-phonon coupling and weak spin-orbit interaction.In 2010,Jin et al.presented an experimental length-scale method that measured electrical and thermal conductivity.They obtained a 500%and 200%increase in the thermal and electrical conductivity respectively,as the film thickness was increased from 20 nm to 1000 nm.In 2012,for porous polyaniline coated with multi-walled carbon nanotubes,Zhang et al.obtained the thermoelectric properties of figure of merit ZT=0.01.Recently,Golsanamlou et al.studied the spin thermoelectric properties of polythiophene molecular junction.They showed that the increase of molecular length results in the increase of the spin figure of merit.Although the great technological interest toward organic materials for electronics and spintronics,the charge-transport mechanism in polymers as well as molecular semiconductors around room temperature is not well understood.For example,it is usually accepted that electronic charges are localized polarons or bipolarons in organic semiconductors.However,in molecular crystals majority of the experimental observations in the 100-300 K range indicate a decrease of the mobility with temperature,which suggests that the transport is described as bandlike or delocalized charge carriers.The spectroscopic studies also seem to indicate that charge localization,due to the formation of a small polaron with the charge localized on a single molecule is unlikely.These contrasting results show that organic semiconductors are much more sensitive to temperature than their inorganic counterparts.From the analyses above,thermal effect have a great influence on photoluminescence,electrical conductivity,thermal conductivity,figure merit,and transport properties of organic semiconductors.While many theoretical and experimental results were not uniform,which maybe the fascination of physics.Therefore,the thermal effect in organic semiconductors,especially the properties under room temperature is worthwhile of further study.The excitonic room-temperature ferromagnetism in organic photovoltaic cells has provided deeper understanding of organic semiconductors.Compared with inorganic counterparts,organic photovoltaic cells are solution-processable,low-cost,degradable and light,which have aroused great interest.To improve the efficiency of the organic photovoltaic cells,many efforts were made.In 1994,Yu et al.firstly composited donor-acceptor organic photovoltaic cell which consist of MEH-PPV and fullerenes,and the energy conversion efficiency were approximately 5.5%.Photoinduced charge transfer across a donor-acceptor interface provides an effective way to overcome early time-carrier recombination in organic systems and thus to enhance their optoelectronic response.Considering the requirements of industrialization,one of the effective strategies to further improve the photoelectric conversion efficiency is to use active materials with different spectral absorption range to fabricate tandem organic photovoltaic cells.In 2007,Kim et al.fabricated tandem organic photovoltaic cells with poly(3-hexylthiophene)(P3HT)and[6,6]-phenyl-C71 butyric acid methyl ester(PC70BM),in which two solar cells with different absorption characteristics are linked to use a wider range of the solar spectrum.A transparent titanium oxide TiO2 layer separates and connects the front cell and the back cell.Power-conversion efficiencies of 6.5%were achieved.In 2013,You et al.reported the development of a high-performance low bandgap and high mobility polymer.The polymer enable a solution processed tandem solar cell with certified 10.6%power conversion efficiency,which was the first certified polymer solar cell efficiency over 10%.In 2016,Chen et al.In Nankai University reported a high-performance solution-processed,tandem solar cell based on the small molecules,which offer efficient,complementary absorption when used as electron donor materials in the front and rear subcells,respectively.Optimized devices achieve a power conversion efficiency of 12.5%.This is a major breakthrough of our country in the research of the organic solar cells.In 2012,Ren et al.investigated the magnetic properties of organic charge-transfer heterojunction.They found the excitonic room-temperature ferromagnetism in semiconducting single crystal poly-3(hexyl-thiophene)nanowires(nw-P3HT)doped with buckminsterfullerene(C60),which are wildly used in OPV cells.The pristine nw-P3HT and C60 single component do not exhibit any ferromagnetic behavior.The appearance of ferromagnetism in nw-P3HT/C60 should comes from the charge transfer between nw-P3HT and C60.Especially it is revealed that the saturated magnetization has an apparent enhancement once under light illumination.For example,the magnetization in darkness is about 12 emu/cm3.Illumination of this sample with a 615 nm laser makes the moment increase to about 30 emu/cm3.Later in 2014,the excitonic ferromagnetism in a nano-carbon bulk heterojunction device consisting of C60 and semiconducting single-walled carbon nanotubes was also demonstrated.These reports tell us that some new magnetic phenomena may appear in organic charger transfer composites in the excited state that may be absent in the ground state.Further,it was found that the magnetization changes under manipulation of electric field and mechanical stress,so it seems that the organic charge transfer composites have the characteristics of multiferroics.At the same time,the similar phenomena were also found in nw-P3HT/PCBM(PCBM=1-(3-methoxycarbonyl)propyl-l-phenyl[6,6]C61)charge transfer composite.All these experimental results imply that organic charge transfer composites have more abundant properties than their single components.Organic ferromagnets combine organic materials and ferromagnetism together,which can be realized by using spin radicals which are containing an unpaired electron.There are 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,and the magnetism can be obtained in this material.However,the molecular structures of the organic ferromagnets and the organic charge-transfer composites are different.The organic ferromagnets are a kind of open-shell magnetic molecules,in which there is only one electron in the highest occupied molecular orbital(HOMO).Organic ferromagnets show ferromagnetic ground state because of the unpaired electron in the side radical.While the materials in organic charge-transfer composites are closed-shell.There are two electrons in the HOMO,one is spin up and the other one is spin down.The whole system has no net spin moment.So the ferromagnetic mechanism in the organic ferromagnets and the organic charge-transfer composites are different.Up to now,the mechanism of ferromagnetism in organic charge-transfer composites is still not clear.In summary,the thermal effect in organic semiconductors needs further study,and the ferromagnetic mechanism in organic photovoltaic cells with closed-shell structures is still not clear.In this dissertation,we use one-dimensional tight binding Su-Schrieffer-Heeger(SSH)model study the thermal effect and excitonic ferromagnetism in organic semiconductors.The following are the brief introductions and conclusions.1.Thermal effect on the organic semiconductors.The thermal effect is introduced by considering electrons with Fermi-Dirac distribution and atomic fluctuation with square-random distribution.It is found that in a neutral polymer chain,the band gap is decreasing as the temperature increases.For a doped polymer chain,we defined the width and formation energy to describe the localization and the stability of the electronic state.With the temperature increasing,more electrons will occupy higher energy levels.The localization and stability of the electronic self-trapped state will decrease with temperature.We also show that at temperature high enough,a hot electron will get rid of the trap of the lattice and become extended over the whole polymer molecule.In addition,the effects of the quantity of charge carriers and the electron-phonon coupling at room temperature are also investigated.With the increase of the quantity of charge carriers and the electron-phonon coupling strength,the localization and stability of the electronic self-trapped state will increase.2.Excitonic ferromagnetism in organic semiconductors.We reveal the spin polarization or ferromagnetism observed in organic composite nw-P3HT/C60 with closed-shell structures.We find that there may exist spontaneous charge transfer because of the e-ph coupling in organic materials,which will not realize in inorganic system.Different from the organic ferromagnets with an open-shell structures,the ferromagnetism of nw-P3HT/C60 comes from the charge transfers from the polymer to the small molecules.The transferred electrons become spin polarized and they are coupled together through the holes in the polymer.Finally a ferromagnetic order appears in the pure organic composite.The magnetic moment of the system is mainly provided by the spin polarized small molecules.The magnetization is dependent upon the density of the transferred charges,which is consistent to our experimental observation under illumination.3.Thermal effect on spin polarization of organic semiconductors.The temperature not only affects the charge character but also the spin polarization of organic semiconductors.Besides,the charge transfer composites show ferromagnetism at room temperature.So we also investigate the magnetic moment of the polymer under different temperatures,and analysis the thermal effect on the excitonic ferromagnetism qualitatively.Besides,the electron-phonon coupling,electron-electron interaction and spin-orbit coupling will also affect the magnetic moment of the polymer at room temperature.