Research On Single Layer Organic Light Emitting Devices Based On Thermally Activated Delayed Fluorescence

In the past decades,organic light emitting diodes(OLEDs)have shown great commercial value and application prospects in the field of lighting and display because of their advantages of high contrast and low power consumption.In order to improve the efficiency of OLED devices,OLED devices usually adopt complex multilayer device structure,but it also increases the cost of devices.Therefore,single-layer organic light-emitting devices with low-cost advantages have become a research hotspot in this field.However,due to the lack of transport layer and barrier layer,the charge injection efficiency is low and the carrier transmission is unbalanced,and the performance of single-layer devices is significantly lower than that of multi-layer devices.In order to improve the performance of single-layer devices,multi doped systems are often used in single-layer OLEDs,which also increases the cost and complexity of device preparation.In recent years,TADF materials with nearly 100%quantum yield and bipolar transmission characteristics have been favored by researchers.Introducing TADF material into single-layer devices and making use of the unique characteristics of TADF to prepare undoped single-layer organic light-emitting devices can further simplify the preparation process while improving the device performance,which is of great significance to promote the practicability of single-layer organic light-emitting devices.Firstly,we explore the analytical models of light extraction efficiency and external quantum efficiency of monolayer organic light emitting devices with wide exciton recombination region.Through the derivation of classical electromagnetic theory of optical extraction efficiency and external quantum efficiency of single-layer OLED devices,the factors affecting the optical extraction efficiency and external quantum efficiency of single-layer OLED devices are analyzed.Then,we use the high-performance thermally activated delayed fluorescent material Cz DBA as the light-emitting layer of the device to study the hole injection ability of different anode modified layer(AML)devices.The device with AML structure of C₆₀(2 nm)/MoO₃(3 nm)/C₆₀(2 nm)has better injection effect than other devices,The reason is that the transition metal oxide MoO₃with high work function will form ohmic contact with the high ionization energy semiconductor C₆₀,which greatly eliminates the hole trap and reduces the non-radiation recombination loss.In addition,C₆₀forms an interface dipole on the ITO anode surface,which helps to reduce the hole injection barrier from ITO to MoO₃.Through the optimization of anode modified layer and light-emitting layer,a high-performance single-layer bottom emitting TADF device is finally prepared,and the external quantum efficiency is 13.24%.After that,we prepared the probe device and determined the exciton spatial distribution characteristics of the device.Combined with the quantum yield of TADF material(close to 100%in theory)and the theoretical external quantum efficiency formula of single-layer device,the theoretical maximum external quantum efficiency of the device is26.65%.The reason why the actual value of the maximum external quantum efficiency is lower than the theoretical value may be that the electrical efficiency is reduced due to the imbalance of charge injection and transmission,resulting in the reduction of the internal quantum efficiency of the device,and then the external quantum efficiency of the device.Finally,we fabricated a single-layer top emitting undoped TADF device.The maximum power efficiency of the device is 43.21 lm/W,the maximum current efficiency is 34.24 cd/A,and the maximum external quantum efficiency is 10.95%.Similarly,using the probe devices,we determined the exciton spatial distribution characteristics of the device.Combined with the theoretical external quantum efficiency formula of the single-layer device,we obtained that the theoretical maximum external quantum efficiency of the top emitting device is 12.06%.The probe device proved that most excitons are formed at 37.5 nm away from the interface between AML and the light emitting layer,and the exciton distribution is closer to the middle position,Therefore,the charge transfer is relatively balanced.