| In recent years,organic light-emitting diodes(OLEDs)have developed rapidly with the advantages of low power consumption,wide viewing angle,ultra-thin,fast response speed,flexible,and so on.At present,OLEDs have been commercialized and widely used in display and solid-state lighting.However,the external quantum efficiency(EQE)is limited by the lower output coupling efficiency.The orientation of molecular dipole moment of the emitting layer(EML)can be adjusted and controlled to enhance the output coupling efficiency and EQE.The solution method can be used to prepare the OLEDs,which has great potentials with the characteristic of simple and low cost.In this thesis,the EML was fabricated by solution method and the orientation of molecular dipole moment adjusted by the annealing temperature,the solvent polarity,and the induced addition of PFO were studied as well as the effect on the performance of OLEDs.The physical mechanism was also analyzed systematically.The main works in this paper are as follows:1.The EML of 2DPAc-MPM: DPEPO composite films were fabricated by solution method,which 2DPAc-MPM is a kind of thermally activated delayed fluorescence(TADF)material.The effect of annealing temperature on the dipole moment orientation in the EMLs prepared by solution method and the performance of the corresponding OLEDs were investigated.When the annealing temperature is 115°C,the highest ratio of horizontal molecular dipole moment orientation,the maximum luminescence intensity and photoluminescence quantum yield(PLQY)were obtained in the EML,and the optimal carrier injection and transmission performance was found in the corresponding single-carrier devices.At the same time,the turn-on voltage reduced from 13.5 V to 8.5 V and the maximum brightness enhanced nearly 6 times compared with that of the reference device at 55°C.The analysis suggests that the combination of molecular dipole moment orientation,PLQY of the EML and the carrier mobility enables the OLEDs to reach the optimal performance at an annealing temperature of 115°C.2.The molecules of DMAC-MPM and DMAC-PPM,two TADF materials with different acceptors,were prepared by solution method using chloroform,chlorobenzene,and their mixed solutions of different ratios as solvents of different polarities,respectively.The effect of solvent polarity on the luminescence properties of TADF materials with different acceptors were investigated,and the luminescence properties of the corresponding OLEDs were also analyzed.The analysis of the angle-dependent fluorescence spectra shows that lower polarity is beneficial to the formation of horizontal molecular dipole moment orientation,which can effectively improve the carrier injection and transport in single-carrier devices.The corresponding OLEDs have a significantly lower turn-on voltage compared to that of the OLEDs prepares by using a highly polar solvent spin-coating EMLs.At the same time,the horizontal molecular dipole moment orientation tends to formulate in DMAc-PPM with the benzene ring substituent group,which has a higher proportion of horizontal orientation in the same solvent.3.The effect of inducer PFO doping on the optical properties and molecular dipole moment orientation of DMAc-PPM: DPEPO EMLs was investigated.With the increase of PFO doping concentration,the orientation factor Θ gradually decreases,indicating that PFO can induce the horizontal molecular dipole moment orientation in the EMLs.At the same time,the doping of PFO will improve the carrier injection and transport performance to a certain extent.The OLEDs prepared based on PFO-doped EMLs have little change in the maximum luminescence intensity,but the EQE is increased by a factor of 2.6.It is shown that PFO as an inducer can effectively increase the horizontal dipole moment orientation of molecules in the EML and improve the carrier injection and transport performance,and enhance the output coupling efficiency and external quantum efficiency of OLEDs. |
