Tuning Of Molecular Orientation And Its Effect On The Performance Of OLED Devices

Organic light-emitting diodes(OLED)have been widely paid attention for their low power consumption,low driving voltage,high display color gamut and fast response speed.The molecular transition dipole moment(TDM)orientation in the emitting layer(EML)and the molecular orientation in the transport layer of OLED devices play a critical role in improving the performance of the devices.The method of improving the performance of OLED devices by tuning the molecular TMD orientation and molecular orientation is not only simple,convenient and low-cost,but also has great potential to improve the device performance.In this paper,the effects of molecular TDM orientation and molecular orientation on the performance of OLED devices were investigated,and the mechanism was systematically analyzed.The main contents of this paper are as follows:1.The effects of substrate temperature(T_g)on the TMD orientation in the EMLs and the molecular orientation in the hole transport layers(HTLs)were studied.The performance of the corresponding OLED devices was analyzed.When the T_g is 60°C,the maximum luminance and external quantum efficiency(EQE)of the OLED device reach 74320 cd/m²and 19.5%,respectively,which are 3.91 times and 1.27 times higher than the 18990 cd/m²and 15.3%EQE of the reference device at 30°C.The synergistic effects of the TDM orientation in the EML,the molecular orientation in the HTL,and the fluorescence quantum yield(PLQY)of the EML,which enable the OLED device to achieve the optimal performance at the T_g is 60°C.2.The effects of annealing temperature(_aT)on the molecular orientation in the HTLs of TAPC and the performance of OLED devices were investigated.The maximum luminance of the OLED device reaches 43390 cd/m² when the _aT is 60°C,which is 2.65 times higher than that of the reference device at 16630 cd/m² when the _aT is 30°C.The proportion of vertically oriented molecules in the thin films increases with the increase of T_a,and the hole mobility of the corresponding hole-only devices(HODs)increases from8.25×10^-4cm²V^-1s^-1 at 30°C to 1.34×10^-3cm²V^-1s^-1 at 80°C.The improvement of the performance of OLED devices is attributed to the effects of _aT on the molecular orientation and hole mobility of HODs,and the device has the best charge carrier balance at 60°C.3.The molecular orientations in the electron transport layers(ETLs)and the HTLs were controlled by the deposition rate,and the effects of molecular orientations on the performance of OLED devices were studied.The molecular orientations in the films at different deposition rates were investigated and the reasons for the formation of molecular orientations in the deposition process were analyzed.The current density,carrier mobility and capacitance of the corresponding single carrier devices are measured and calculated,and the AC impedance spectrum of the electron-only devices(EODs)is measured.In the deposition process of thin films,the fast deposition rate is easier to obtain the high proportion of vertically oriented molecules,and molecules with vertical orientation are more conducive to the improvement of current density,carrier mobility and capacitance in single carrier devices.The OLED devices were fabricated based on HTL with fixed molecular orientation and ETLs with different molecular orientations.The OLED device achieves the best luminance of 46670 cd/m²,when the proportion of vertically oriented molecules in the ETL reaches the maximum,which is 4.12 times higher than the luminance of 11340 cd/m² in the reference device when the proportion of vertically oriented molecules in the ETL is the lowest.