Theoretical Simulations Of Carrier Mobility In Organic Light Emitting Diodes

With the rapid development of OLED,its efficiency and performance have been greatly improved,but it still suffers problems such as low carrier mobility and imbalanced electron and hole mobilities.In addition,the electronic structure,molecular stacking,and energy level arrangement at the OLED interfaces can seriously affect the charge injection and transport.Therefore,improving the carrier mobility of the single-layer thin film and the interface layer can not only reduce power consumption,but also helpful for improving charge balance,which can directly improve the performance and efficiency of OLED devices.Based on the above two issues,in this paper,we use molecular dynamics to simulate single-layer thin film structures and multi-layer thin film interface structures under different vapor deposition conditions.Combining with the charge transport theory,the charge transport properties of different deposited structures are studied.First,based on molecular dynamics simulations,linear bipolar host CBP molecules were deposited at different substrate temperatures Tsub and initial velocities vd.Within the range of parameters studied,both lower Tsub and higher vd are favorable for obtaining a more ordered structure.The kinetic Monte Carlo simulations show that the deposited films with higher order have higher charge mobility,which is due to the decreased site energy disorder and the enhanced site energy spatial auto-correlation.In addition,the molecular order of the film has a more significant effect on electron transport than on hole transport.The larger increase of the electron mobility compared to the hole mobility can promote the balance between the electron mobility and the hole mobility in the CBP film,thereby improving its bipolar transport characteristics.Secondly,linear hole transport molecules(?-NPD)and CBP molecules were successively deposited on substrates with different temperatures Tsub and different initial velocities vd.Our research shows that Within the range of parameters studied,higher Tsub and lower vd are easier to obtain rough two-phase mixed organic interfaces.For organic interfaces with higher degrees of mixing,the distribution of sites energy at the interfaces can change more significantly.This is because interfaces with higher degrees of mixing are more affected by the electrostatic and polarization effects of two different molecules.In addition,when the interface barrier is large,the charge injection rate is mainly dominated by the interface barrier,and the charge rates can be affected by transfer integral only when the charge transfer integral is large.Our work not only links molecular morphology to charge transport properties,but also provides strategies for improving charge mobility by optimizing molecular deposition conditions.