A Study On Manipulating The Charge Carriers And Exciton Recombination Zones Of Solution-Processed Electroluminescent Devices

With the excellent properties of self-emission,fast response speed,high contrast and flexibility,organic light-emitting diode?OLEDs?and quantum dot light-emitting diodes?QD-LEDs?based on electroluminescence are shining brightly in the field of display and lighting.To form multiple-layer films in such devices,solution processing can simplify the fabrication process,reduce the production cost and help to achieve large-sized panels.However,for solution-processed OLEDs and QD-LEDs,many issues,such as low luminous efficiency,high driving voltage and poor stability still generally exist and some working mechanisms of QD-LEDs are not clear.In view of these problems in OLEDs and QD-LEDs,from the aspects of design of device structures,selection of functional materials and optimization of device fabrication processes,we investigated the charge carrier injection and transport properties,tuned the charge carrier balance and manipulated the exciton recombination zones.As a result,the emission properties such as luminous efficiency,efficiency roll-off and color purity of the devices have been improved effectively.The main work and achievements of the thesis are as follows:1)For solution-processed phosphorescent OLEDs with emitting layers?EMLs?composed of host-guest systems,we employed small molecules with high purity,triplet energy and chemical stability as hosts.Compared to the hosts of polymers,hole-transporting small molecules possess higher hole mobility and more effective energy transfer to the guest emitter.However,the driving voltage of the device with small molecular host is high due to the large hole injection barrier to the EML.To enhance hole injection,another hole-transporting small molecule with higher highest occupied molecular orbital?HOMO?level was introduced into EML to form mixed hosts.By adjusting the composition ratio of the hole-transporting mixed hosts,the charge carrier balance is promoted and the exciton recombination zone is broadened.As a result,the optimized device realizes low driving voltage and high luminous efficiency.At the same time,its efficiency roll-off is improved.2)In QD-LEDs,it is generally much more difficult for holes to inject into the deep valance band of QDs than electrons into the conduction band of QDs for the larger hole injection barrier,which limits the luminous efficiency.As a result,on account of the energy levels of hole-transporting polymer PVK and the energy transfer from PVK to the green QDs,we employed a doped EML of PVK:QDs in the device to realize QD emission.To suppress the weak emission from electron transporting layer?ETL?,a buffer layer of TmPyPB which possesses deep HOMO level and high electron mobility was introduced between the doped EML and ETL.With the TmPyPB buffer layer,electron injection into the EML is enhanced and holes are blocked from transporting across the EML.As a result,more excitons are formed and confined within the EML,contributing to the improvement of the color purity of the device.And the maximum external quantum efficiency of the device obtains a 1.0-fold improvement compared with that of the reference without the buffer layer.3)To improve the charge carrier balance in the QD EML,we firstly combined the good solution-processed film quality of hole-transporting polymer TFB,and the deep HOMO level and moderate hole mobility of small molecule CBP.By mixing TFB with CBP in a certain propotion to form a hybrid hole transport layer?HTL?and introducing a buffer layer between QD EML and ETL to weaken electron injection properly,hole transport to the QD EML is facilitated and the charge carrier balance is enhanced.Accordingly,the luminous efficiency and color purity of the device is further improved.Next,all-solution-processed QD-LEDs were realized with ZnO ETL,and we found that due to the higher HOMO level of the hole-transporting polymer of TFB than that of PVK,holes accumulate at TFB/QDs interface and the hole injection into QD EML is assisted by the non-radiative Auger recombination.Hence,the devices obtain larger current densities at low voltages and lower turn-on voltages?defined as the voltage at 1 cd/m²?,but their luminous efficiencies are much lower than that of the device employing individual PVK as HTL instead.Moreover,we employed inorganic phosphomolybdic acid?PMA?which has the properties of low price,simple solution processing,good stability and high transmittance as hole injection layer?HIL?in the device.The effects of different preparation conditions on its hole injection property were investigated.And the optimized PMA possesses stronger hole injection property than the traditional PEDOT:PSS,and the turn-on voltage of the corresponding device is dramatically decreased from 4.4 V to 2.7 V,along with a 65.4%improvement on the maximum power efficiency.4)In the inverted QD-LEDs,we investigated the effects of small molecular HTLs with different energy levels and charge carrier mobilities on the device performances.It shows that the higher HOMO level of TAPC HTL causes Auger-assisted hole injection,and its strong electron blocking ability,resulting in sharply lowered turn-on voltage and higher luminious efficiency.On this basis,by introducing a thin HATCN layer,which possesses deep LUMO level and strong electron withdrawing ability between TAPC HTL and MoO₃ HIL,the electrons can easily transport from the HOMO of TAPC to the lowest unoccupied molecular orbital?LUMO?level of HATCN,and generates more free holes.As a result,the hole injection and transport is further enhanced,promoting the charge carrier balance in the QD EML and decreasing the driving voltage.The optimized device obtains a maximim power efficiency of 26.2 lm/W,meaning a 50%improvement compared to the device without the HATCN interlayer.