Mechanism Study On Interface Modification For Highly Efficient Polymer Light-emitting Diodes

Organic light-emitting devices(OLEDs)has considered to be the next generation of flat panel display due to its active luminescence,light weight,wide vision angle and flexibility.Polymer lightemitting devices(PLEDs)based on polymer emissive layer have more advantages such as simple preparation process,low cost and easy to realize large area display.PLEDs have developed rapidly in recent years.However,the commonly used cathode materials of PLEDs,such as low-work function metals,alkali metals or alkaline earth metals fluoride,are sensitive to water and oxygen,leading to the degradation of device.Therefore,PLEDs still faces the problems of low efficiency and short lifetime.Kinds of methods were selected to improve the injection of electron,for example,the insertion of interface modifying materials.But,most of them needs complex synthetic processes,which are also expensive.Therefore,simple and effective method is urge to be developed to improve the injection of electron of PLEDs,which will be addressed in this thesis.The main contents are summarized as follows:(1)Polar solvent was used to modify the surface of emissive layer to realize high efficiency PLEDs device with high work-function-metal as cathode.Due to the large energy level difference between the fermi level of cathode metal Al and the LUMO level of luminescent material SY-PPV,the electron injection barrier is high and the current efficiency is low.By modifying the surface of SY-PPV by acetonitrile:methanol,the performance of the device was greatly improved,which also depends on the ratio of acetonitrile and methanol.With the volume ratio of acetonitrile:methanol=1:9,the device had the highest current efficiency,which was improved from 2.79 cd/A of the control device to 6.89 cd/A.The modification of SY-PPV by polar solvent reduced the electron injection barrier,leading to the improvement of electron injection,and therefore the efficiency of PLEDs.(2)Improving the luminous efficiency of PLEDs device by methanol vapor post-treatment.After methanol vapor treatment,methanol can penetrate into the device,the methanol located at the cathode interface can reduce the electron injection potential barrier,thus increase the electron current.While,the methanol located at the anode interface can reduce the injection of hole.Therefore,the balance of electron and hole current of PLEDs was improved.To optimize the conditions of methanol vapor post-treatment,the amount of the used methanol is changed with the treatment time unchanged.PLEDs with the highest current efficiency was achieved by treating with 60 ?L methanol for 24 hours.Compared with untreated devices,the current efficiency increased from 11.39 cd/A to 24.46 cd/A,increased by 114.7%.(3)Highly efficient PLEDs based on double layer alkali halide as electron injection layer.When NaCl and LiF were used as cathode instead of LiF,the device efficiency increased by 286%.In order to study the working mechanism of double-layer alkali halide electron injection layer to improve device performance,we also prepared PLEDs devices with KBr/LiF and CsF/LiF as electron injection layers respectively.The experimental results show that the use of the double-layer electron injection layer can improve the efficiency of electron injection at cathode,reduce the turn-on voltage of the device and greatly increase the current of the electron.The migration of the alkali metal cation toward the anode under the action of the built-in electric field of the device is mainly responsible for the improvement of device performance,leading to the doping of the emissive layer,which improved the injection of electron.