The Evolution And Energy Transfer Process Of The Excited State In Thermally Activated Delayed Fluorescence Devices

Organic light-emitting diode?OLED?is gradually entering people's daily life because of its advantages of simple preparation,low price,energy saving and environmental protection,ultra-thin and flexible folding.Only 25%of the common fluorescence OLED can be combined into a single state exciton excitation emission,and the remaining 75%of the triplet exciton can dissipate the energy in the form of non-radiation excitation,and the internal quantum efficiency is limited to 25%.Phosphorescent OLED devices,relying on the strong spin orbit coupling of metal complexes such as iridium and platinum,can make use of the single state exciton and the triplet exciton,although the internal quantum efficiency can be as high as 100%in theory,but it is restricted by the high cost of precious metals,the pollution of the environment and the efficiency of rolling-off in the large current.The recently discovered thermal activated delayed fluorescence?TADF?has not dependent on the precious metal its quantum efficiency as high as 100%,and its luminous efficiency is several times that of the common fluorescent device.The design of TADF material is mainly composed of two parts,one is electron donor and the other is an electron acceptor.The two parts are spliced into one molecule,independent in the molecule and can form the intramolecular charge transfer state?CT?.The highest occupied molecular orbital?HOMO?energy level and the lowest unoccupied molecular orbital?LUMO?energy level electron cloud of the TADF material will be separated.The energy level difference(?E_ST)of the single excited state?¹CT?and the triplet excited state?³CT?is very small.³CT will have the opportunity to convert to ¹CT by the absorption of the environmental heat through the reverse intersystem crossing?RISC?.It is used to improve the luminous efficiency of the device.At present,the research on TADF materials and devices is still at the initial stage,and TADF theory is still a big framework.There are still many problems to be solved.In this paper,we have measured the magneto-electroluminescence?MEL?and magneto-conductance?MC?characteristic curves of the TADF devices.Through the analysis of MEL and MC of TADF devices,the process of spin interaction in TADF devices,the transfer of energy between the host material and the guest material,and the influence of the charge trap in the organic materials on the carrier and excited state of the TADF device are obtained.The main chapters were listed below:In the first chapter,we first introduce the development of OLED,its application in lives and its basic principles.Next,we introduce the basic principle of TADF and some problems to be solved.Subsequently,the principle and application of MEL and MC of organic light-emitting devices are introduced.In the end,the main work and significance of this paper are described.The process of spin interaction in TADF devices and the transfer process of excited state energy between host and guest are studied by MEL and MC.The second chapter first introduces the preparation methods and matters needing attention of OLED devices and TADF devices.We mainly use spin coating and evaporation deposition.Subsequently,the measurement of the electroluminescence and magnetic properties of TADF devices is introduced.The test is carried out under different current voltages and different ambient temperatures.In the third chapter,4CzTPN-Ph,2CzPN and PIC-TRZ2 are selected as the adulteration system to prepare TADF devices.The MEL study shows that the MEL of the TADF device shows a similar lines with the Alq3 fluorescent device at room temperature.At low field?<40 mT?,the MEL is rapidly rising,and then gradually saturated at the high field?>40 mT?.In the TADF device,there is a competition between the intersystem crossing?ISC?in polaron and the RISC in charge transfer state?CT?.The number of the polaron of the TADF devices is more than the number of CTs,so it shows the characteristics of the ISC process between the polaron pair.Secondly,the MEL of TADF devices will become stronger with the increase of injection current,which is opposite to the result of Alq3 fluorescent devices.The rate of the conversion of the triplet polaron to the triplet exciton?K_t?and the single polaron to the single exciton?K_s?is an important parameter affecting the process.In addition,at low temperature,the MEL of Alq3 device shows obvious characteristics of TTA process,while the energy of single and triplet excited states of TADF devices is close,it does not have the conditions of TTA process,and the RISC process is suppressed at low temperature,so the TADF device shows a stronger ISC process.In the fourth chapter,the energy transfer process between host and guest is studied by using MEL at different doping concentration of TADF devices.When the TADF material concentration is low,the FRET between the host and guest is the main way,which will cause the utilization of the triplet exciton in the host material.Meanwhile,the RISC process in the guest material decreases with the increase of the dissociation rate of CTs at high current density.When the concentration is high,the energy transfer between host and guest is mainly in DET mode.Although the RISC process can increase the utilization of triplet and improve the luminous efficiency of the devices,the high doping concentration will lead to the quenching of the single excited state and reduce the luminous efficiency of the TADF devices.Therefore,for the TADF guest doped TADF system,there is mutual inhibition of doping concentration and RISC process.The fifth chapter mainly studies the influence of charge traps in organic materials on carriers and excited states of TADF devices by using MC.It is found that when the TADF device contains the polaron and exciton bound by the charge trap,MC satisfies the Lorenzain equation at a small current density at room temperature,showing the ISC characteristics.With the increase of the current density,MC attenuates in the range of10⁴0 mT,and is fitted by Lorenzain function to show the triplet trapped-polaron interaction?TP_t I?.When the temperature drops to 200 K,the previous attenuation is gradually transformed into a platform,which is the result of the interaction between trapped-triplet polaron interaction?T_tPI?and TPtI.Moreover,MC shows a positive and negative transition in the hyperfine range at low temperature,which indicates that TADF devices exhibit a transition between ISC and RISC at low temperatures.These results reveal the ISC,RISC,TPtI and TtPI processes in TADF devices.