The Theoretical Studies On The Electrical And Thermal Properties Of Highly Efficient OLED Materials

Organic light-emitting diode(OLED)technology has received extensive attentions in recent years due to its many advantages,including low cost,high stability,and high transparency.The development of third-generation luminescent dyes,i.e.,thermally Active Delayed Fluorescence(TADF),has lead to a new step for OLEDs towording the direction of low costs,high efficiency and high stability.Achieving efficient TADF luminescence requires small singlet and triplet splitting energy and high fluorescence efficiency.However,for red light TADF materials,the non-radiative emission is strong due to its narrow band gap,thus leads to low quantum efficiency.Although reducing the HOMO and LUMO orbital overlap of TADF molecules can effectively reduce the electron exchange energy,it also weakens the fluorescence radiation efficiency.At the same time,with the increase of power density of OLED devices,its thermal stability also is also an important factor for improving the quality of OLED devices.As a result,it is necessary to study the heat transport capability and the corresponding mechanism of the amorphous organic small molecular materials commonly used in the luminescent layer and the transport layer of OLED devices.Targeting on the above two scientific issues,we use the DFT method and molecular dynamics to study the electronic structure and thermal transport properties of OLED materials,respectively.Firstly,we studied the HOMO and LUMO orbital overlap as well as singlet and triplet splitting energy by adjusting the connection sites between the donor and acceptor groups.Based on our analysis,we found that the site No.7 of BAc and the site No.9 of carbazole are suitable positions to connect with DOAn and DNQX donors.The introduction of a methyl or phenyl group near the connection site increases steric hindrance thereby limiting the twist of the molecules,which can reduces the non-radiative transition rate.Secondly,based on the molecular dynamics simulations,we performed systematic studies on the thermal conductivity of Pentacene,TPD,mCP and Coronene.It has been found that in low thermal conductivity systems,the kinetic heat flux is important for thermal conductivity calculation based on the Green-Kubo formula.Due to the soft behavior of organic materials,the thermal transport of all systems can be greatly enhanced under pressure.Mode diffusivity calculations reveal that the thermal conductivities of TPD and Pentacene can be fully described by diffusons while the low frequency phonons can contribute significantly to the thermal transport of mCP and Coronene.With the increase of pressure,the low frequency phonon group velocity can be enhanced and part of the low frequency modes can convert from diffusons to phonons.The cooperation of the two effects is responsible to the strong pressure dependent thermal conductivity in amorphous organic systems.