Mechanism Study On The Effect Of Molecular Stack Structure On Ultrafast Phenomenon Of Organic Photovoltaic System

Organic semiconductor materials have the characteristics of flexibility,easy processing,no pollution,and low cost.In recent years,they have also been found to have rich magnetic,optical and electrical properties,which are expected to bring innovation and improvement in the fields of healthcare,communication and energy.At the same time,organic semiconductor is a field full of challenges and opportunities,and its research involves materials science,physics,chemistry,and many other disciplines.Research in organic semiconductors could promote the development of advance science and technology.Ultrafast phenomenon in organic photovoltaic system is an advanced topic in the field of organic semiconductor and an important opportunity to improve the performance of organic photovoltaic system.With the relatively mature framework of exciton theory,every step of the physical process is slower,on the order of picosecond and nanosecond.However,it is difficult to explain the phenomenon of ultrafast charge generation in organic systems.At present,the theoretical understanding of ultrafast phenomena in organic semiconductors can be roughly divided into two aspects.On the one hand,it is believed that the microstructure of heterojunction system greatly reduces the transport time of exciton,which is based on the theory of exciton.Another theory holds that there is coherent resonance in initial photoexcitation products,interface charge transfer states,and free carriers.There is no consensus on the ultrafast phenomenon.In this thesis,we mainly study the influence of morphology on the ultrafast charge generation process in organic semiconductor,trying to find out the mechanism of ultrafast charge generation and related modulation parameters in single-phase organic uniform structure,and predict the device state with higher internal quantum efficiency.This thesis includes the following points:1.Based on the understanding of the basic properties of organic semiconductor materials,and the tracking of the research hotspot in the field of organic semiconductor,the possible microscopic mechanism of organic ultrafast phenomenon is summarized.In order to highlight the important physical properties of the strong electro-lattice coupling of organic conjugated polymers,the tightly bound SSH model combined with a non-adiabatic kinetic method is used to study organic semiconductors.With this method,a multi-molecule stacked structure is established to explore the mechanism of ultrafast charge generation in the molecular aggregation structure.2.Through the theoretical simulation process,we found that in the uniform structure of a single polymer,the excited state generated by the initial photoexcitation can be directly separated into free charges by means of the weak electric field prevalent in the bulk material.The time scale is in the order of 30 femtoseconds,and the charge separation efficiency can be up to 97%.3.The polarization charge dipole moment and the amount of separated charge in ultrafast dynamic process are defined,and the standard of charge separation in ultrafast process is established,which lays a foundation for marking the length of ultrafast process.4.By changing the molecular spacing in the domain of uniform polymer,we obtained several systems with different intermolecular coupling strengths.By calculating the ground state energy,the initial excited state energy,and the free charge energy under each system,we achieved the separation energy of the initial excited state and the relaxation exciton.It can be concluded that the initial excited state has the possibility to evolve towards a free charge.As inferred by the Frank-Condon principle,it tends to evolve towards a charge separation state under the current parameters.As a result,ultrafast charge generation is reasonable in terms of energy.5.From the spatial distribution of excited states,we give the range and degree of the excited state lattice distortion under different molecular coupling intensities and electro-lattice coupling intensities.It is confirmed that the two interactions have opposite effects on the delocalization degree of excited states,that is,electro-lattice coupling makes small local range and large distortion degree,while intermolecular coupling makes large local range and small distortion degree.This conclusion gives the corresponding relation between delocalization degree and ultrafast phenomenon.This thesis provides a reasonable mechanism for the ultrafast charge generation in single-phase organic materials,and also has a certain guiding role for the exploration of anisotropy of physical properties in organic semiconductors.