Study On The Chirality And Thermal Effect In Organic Optoelectronic Devices

Organic semiconductors include small molecules and conjugated polymers.Benefiting from the advantages of abundant raw materials,low prices,simple preparation,flexibility,and environmental protection,optoelectronic devices based on organic semiconductors have been developed and applied in lighting,solar cells and other fields.Although organic functional devices exhibit many advantages compared to inorganic devices,the efficiency and stability of the organic devices still need to be improved.It is urgent to further study the the intrinsic mechanisms and factors in organic optoelectronic processes to realize efficient and stable organic functional devices.An important property of organic semiconductors is the electron-phonon(e-ph)interaction,so that the charge carriers are no longer electrons and holes in conventional inorganic semiconductors,but self-trapped states coupled to the lattice structure,such as solitons,polarons,and bipolarons;In addition,the main excitation of organic semiconductors is no longer Wannier-Mott excitons,but tightly bound Frenkel excitons,double excitons,etc.These localized states exhibit many interesting phenomena in organic optoelectronic devices.For example,the transport mechanism is no longer a simple band transport,but hopping mechanism or tunneling mechanism;excitons will not directly dissociate into free carriers,but charge transfer states(CT states)due to the larger binding energy.Carrier transport and exciton dissociation are critical processes in organic optoelectronic devices.It’s of great research significance to consider the thermal effect on these processes for further improving the optoelectronic efficiency.Furthermore,the chirality-induced spin selectivity(CISS)effect has injected vitality into the organic optoelectronic field.Paltiel et al.proposed a chiral molecule-assisted charge separation mechanism.Ouchi and Brown et al.successively used chiral molecules as the active layer and carrier-transporting layer of solar cells,and found the advantages of chiral structure in photovoltaic devices.Nuzzo et al.controled the polarization of light by chiral organic light-emitting devices and the polarization rate was as high as 74%.Kim et al fabricated spin light-emitting diodes using chiral molecules as hole-transporting layers.Combining the organic and chiral properties of molecules,these issues need to be studied urgently,that is,how chiral configuration affects carriers transport and excited states when chiral organic materials are introduced into organic optoelectronic devices as the carrier transport layer and light-emitting layer,and how to use chiral configuration to design organic optoelectronic devices,etc.Combining the extended Su-Schrieffer-Heeger(SSH)model and the method of non-adiabatic molecular dynamics,the thesis focuses on the physical processes in organic optoelectronic devices.The effects of temperature and chiral configuration on exciton diffusion,dissociation,and yields are investigated.Some important conclusions have been obtained,which can reveal experimental phenomena and provide theoretical guidance for the preparation of organic optoelectronic devices.The research contents and results of this thesis are as follows:1.Thermally induced exciton diffusion and dissociation in organic semiconductorsIn organic semiconductors,the binding energy of exciton not only originates from electron-electron interactions,but also from lattice distortion.The exciton binding energy is much larger than the room temperature thermal energy,so it seems difficult to dissociate excitons by thermal energy.However,experiments have found that thermal effect is favorable for exciton dissociation and exciton diffusion exhibits different behaviors at high and low temperature regions.In Chapter 3 of this thesis,in the framework of the tight-binding model,by introducing the thermal effect on π-electrons with Fermi-Dirac distribution and atomic nuclei with Boltzmann distribution,the effects of non-uniform temperature on the exciton diffusion and dissociation in an organic polymer are studied.It is found that thermal effect can separate the electron-hole in an exciton,and the separation becomes apparent with increasing temperature.Especially,a linear-gradient temperature distribution along the polymer will drive the exciton to migrate along the chain with the direction towards to the high-temperature region.Based on this,we propose a physical mechanism for built-in thermal field to drive exciton diffusion and dissociation,which provides a new tunneling to enhance the OSCs efficiency.2.Thermally assisted charge transfer and dissociation in organic heterojunction photovoltaicsIn order to further improve the dissociation efficiency of excitons,the bulk heterojunction(BHJ)structures based on donor(D)and acceptor structures(A)have become the mainstream of OSCs.In a BHJ,excitons are generated by light absorption,and subsequently migrate or diffuse to the D/A interface where they dissociate into CT states.In Chapter 4 of this thesis,we theoretically investigated the thermal effect on the charge transfer process,revealing the difference between phonons in the D/A system caused by elastic energy acts as the thermally assisted driving force for charge transfer.It is found that the system exhibits a quite different CT process in the high and low temperature regions.Remarkably,combined with the entropy driving mechanics,the thermally assisted charge separation was as high as 70%at room temperature.Our results provide theoretical guidance for improving the efficiency of OSCs.3.Polaron transport-induced spin selectivity effect in organic chiral moleculesWith the in-depth study of organic spintronics,organic chiral materials with unique structures have begun to attract attention.Because of its unique spin-orbit coupling(SOC),it predicts the abundant spin-charge related phenomena exsit in organic chiral materials,such as CISS effect.In recent years,organic chiral molecules have been widely applied as carrier transport layers or active layers in organic optoelectronic devices.It found experimentally that chiral molecules act as carrier transport layers to generate spin polarization,which can break the 25%singlet excitons ratio.In order to study the source of spin polarization caused by carrier transport in chiral organic materials,in Chapter 5 of this thesis,the behavior of polaron transport in helical spin filtering devices is studied.It found that the transport behavior of the polarons with the spin state initialized as up and down is asymmetry.In particular,by calculating the I-V relationship for ferromagnet/helix/nonferromagnetic-metal,we revealed the complex dependence of spin polarization on the applied voltage and attributed it to the competition between carrier velocity and density,which provides a general microscopic view to reconsider the CISS phenomenon and theoretical guidance for the design of organic chiral devices.4.Helical packing-induced singlet-triplet exciton conversion in organic chiral electroluminescent devicesWhen chiral organic molecules with a helical configuration act as a light-emitting layer,the potential field gradient of the helix causes SOC when electrons move along the helical chain,and the SOC will lift spin degeneracy.Generally speaking,SOC can reduce the energy difference between singlet and triplet states,which is beneficial to the conversion between singlet and triplet.Therefore,the arrangement of the helical configuration of the atoms not only affects the space state of the electrons,but also affects the spin state the electrons.In Chapter 5 of this thesis,we studied the effect of the helical configuration on the exciton yield from the two aspects:the strength of chiral SOC and the helical structure parameters.It found that helical packing can induce the conversion between singlet and triplet states,breaking the traditional spin statistics.By changing the helical structure,the regulation of exciton yield can be achieved.Our results well explained the experimentally observed phenomenon of helical stacking-induced single-triple conversion,and provided theoretical support for the fabrication of chiral OLED.