Study On The Exciton Quenching And Exciton Fission In Organic Fluorescence Molecules

In the past two decades,organic semiconductor materials have developed rapidly.At present,great progress has been made in the practical use of devices.Although large-scale industrialization has begun,there are still some basic physical problems in the microscopic mechanism of materials that are worth exploring.In organic light-emitting materials,when the moving electrons and holes are close to each other,the most basic excited state,i.e.,"excitons" are formed.The generated excitons will last for a certain lifetime before they decay to the ground state by radiation.During this time,excitons may interact with carriers(including electrons and holes)or other molecules in the material.And these interactions constitute other decay pathways for excitons,which deserve attention.In this thesis,we mainly carry out experimental research on two important physical processes in organic fluorescent molecules,i.e.,the processes of exciton quenching and exciton fission.The main contents of this thesis are as follows:(1)The first chapter mainly describes the microscopic processes of exciton quenching and exciton fission,which are common in organic luminescent molecules.The exciton quenching phenomenon mainly occurs between electrons/holes and excitons in light-emitting devices.As a result,the excitons undergo non-radiative decay to the ground state,which is an important factor that restricts the increase of quantum efficiency of organic light-emitting devices.The exciton fission phenomenon mainly exists in some special organic molecules.In these materials,the energy of one singlet exciton is approximately equal to the energy of two triplet excitons,thus enabling the transition of exciton from one singlet to two triplet states.The effects of these two physical processes on organic optoelectronic devices are described in the first chapter.(2)The second chapter is mainly about the experimental design and measurement process for studying exciton quenching and exciton fission.The description of pre-experimental preparation,fabrication experimental samples,and measurementtechniques are present.The main equipment used in the research was explained one by one.(3)The third chapter mainly gives a detailed explanation of the experimental research process of exciton quenching.Since both electrons and holes are present in the light-emitting device,they all show quenching effects on the excitons,and there may be a difference in the intensity of the interactions.Therefore,a clear distinction must be made experimentally to study the interaction of excitons with electrons and the interaction of excitons with holes.The quenching effect can be expressed as:S1+e/h?S0+e/h,where S1 is a singlet exciton,So is a molecule at ground state,e represents an electron,and h represents a hole.In our experiments we designed electronic-only devices and hole-only devices in order to study the quenching effect of two carriers on excitons.In an electronic-only device,the singlet excitons in the luminescent layer are excited by photoluminescence,and then a voltage is applied to generate a pure electron current in the device,so that the quenching effect of singlet excitons by the electrons flowing in the light-emitting device can be simulated.Similarly,in a hole-only device,photoluminescence is still used to excite singlet excitons in the luminescent layer,and then an applied voltage to generate pure hole current in the device.In addition,there may be a large number of polarized charges in the interior or interface of the luminescent layer.These fixed charges may be electrons or holes,and they can also quench the excitons.In order to study the quenching effect of static charge on excitons,a certain thickness of LiF insulating layer was inserted into the device to form a capacitor structure.When a positive(negative)bias is applied,a positive(negative)polarization charge can be generated on one side of the luminescent layer.The singlet excitons are still produced by photoluminescence,so that the quenching effect of excitons by static charges can be simulated.(4)The fourth chapter mainly gives a detailed explanation of the experimental research process of exciton fission.Traditionally,exciton fission process can be represented by a two-step,three-state reaction model:S1+S0(?)1(TT)i(?)T1+T1,where T1 represents a triplet exciton,and 1(TT)i state represents a triplet pair with a singlet character.Usually,the exciton fission process is reversible,the process from left to right is called singlet exciton fission,and the process from right to left is also called triplet exciton fusion.Usually both of them can coexist in fission molecules.Experimentally,the microscopic mechanism of exciton fission can be studied by analyzing the transient fluorescence decay process.In this work,the transient fluorescence decay of rubrene is measured at different temperatures.Firstly,the experimentally measured decay curves were fitted by using the differential equations based on the three-state reaction model.The theoretical curves showed good agreement with the experimental results,but there are still evident deviations.In fact,these deviations between fitting curves and experimental results stem from the fact that the above three-state model is too simple to fully describe the actual state-evolution in fission process.To this end,the fluoresce decay curves were fitted again by using differential equations based on a three-step,four-state reaction model.The four-state reaction model can be expressed as:S1+S0(?)(TT)i(?)1(T...T)(?)T1+T1,where1(T...T)state represents two triplet exciton pairs that are spatially separated but still have spin interactions.The four-state reaction model shows that the spin interaction can still exists although two molecules are spatially separated,which actually provides an idea for studying the coherent distance of the spin interaction.The fitting curves obtained by using the four-state reaction model are in good agreement with the experimental results,indicating that the four-state model is better to reflect the actual evolution process in singlet fission.On the other hand,using Arrhenius's law,the rate parameters obtained in fitting process are exponentially fitted,and the experimental result of energy difference between two triplet excitons and one singlet exciton are obtained.It shows that the results show that the energy difference ?E obtained from four-state model is closer to the theoretical value than the ?E value obtained from three-state model.This again shows that the four-state reaction model is superior to the three-state reaction model.(5)The fifth chapter summarizes the research work described in this thesis and lists the main research results and conclusions of this work.