Study On Inkjet Printed Phosphorescent OLEDs With Ternary Blend Hosts

Organic light-emitting diode（OLED）,with its unique advantages and a broad application prospect in the display and lighting,has attracted widely attention in research and industry.Thanks to the heavy atom effect,phosphorescent OLED has achieved 100%internal quantum efficiency,leading a huge leap in OLED efficiency.However,the energy level and carrier transport capacity of the host can’t meet the requirements of high-performance phosphorescent OLED devices.Phosphorescent OLEDs based on blended hosts have a variable device structure design,and their device performance can be improved by adjusting the charge injection and transport by varying the composition of blend hosts.Inkjet printing technology can further promote the low-cost and high throughput manufacturing of OLED.Though inkjet printing technology has been successfully applied in the manufacturing OLED devices,there are still technical hinders need to overcome such as the stable droplet injection and uniform film formation.In this dessertation,with the results obtained in the studies on spin coated phosphorescent OLED devices,where the effects of blended hosts on the device performance were investigated,a triplet host is selected for inkjet printing phosphorescent OLED on both rigid and flexbile substrates.With optimization on the the inkjet printing conditions,such as the driving waveform,film drying and printing spacing,inkjet printed phosphorescent OLED devices are fabricated on rigid and flexible substrates.The main work in this dissertation includs the following two parts:1.Study on spin coated phosphorescent OLED devices with blended hosts.The effect of blend-hosts on the performance of phosphorescent OLED device has been studied.In this work,m CP,PVK and TCTA are selected as the fluorescence hosts,Ir（mppy）₃ as phosphorescent dopant.Phosphorescent OLED devices with single-host,double-hosts and triple-hosts are designed and fabricated.Among the single-host devices,TCTA based single-host phosphorescent OLED devices have the lowest efficiency roll-off,but TCTA has the characteristics of easy crystallization,which is difficult to be applied in inkjet printing process.Blending TCTA with other subjects is an effective way to solve this problem.The efficiency roll-off of phosphorescent OLED with TCTA and PVK blends decreases with the increase of hole transport capacity.By comparing the photoluminescence spectra with the electroluminescence spectra,it can be concluded that the exciton recombination zone for the double-hosts phosphorescent OLED device based on TCTA and PVK is located at the interface between the emissive layer（EML）and the electron transport layer（ETL）,and will move towards to the ETL with the increased hole transport capacity.The efficiency roll-off of the device is reduced,but other photoelectric characteristics remain unsatisfactory.In order to further improve the device performance,a common host of Ir（mppy）₃,m CP,is introduced and blend with PVK.It is found that when the mass ratio of m CP and PVK is 6:3,the double-host device has shown the best maximum external quantum efficiency（EQE）of 16.2%and it inhibits the quenching of the phosphorescent triplet exciton,thus improving the EQE of the device.However,when the brightness increases from 1000cd/m² to 2000 cd/m²,the device shows a serious efficiency roll-off of 14.7%.TCTA is added into m CP and PVK as the third host to reduce the efficiency roll-off of the device.When the mass ratio of m CP,PVK and TCTA is 6:3:1.5,the triple-hosts device has the best device performance,with V_on of 2.9V,a brightness of 10214 cd/m² at 7V,and a maximum EQE of 16.9%.When the brightness rises from 1000 cd/m² to 2000 cd/m²,the efficiency roll-off drops to 7.5%.2.Study on inkjet printed rigid and flexible OLEDs.With the optimization of inkjet printing conditions including driving waveform,print spacing and film drying,the inkjet printed phosphorescent OLED devices based on rigid and flexible substrates are fabricated.A binary solvet of butylbenzoate and chlorobenzene is used for formulating the EML ink with selected blend of m CP、PVK and TCTA（4:2:1）as the fluorescence host for phosphorescent Ir（mppy）₃emitter.When the volume ratio of butyl benzoate to chlorobenzene is 7:3,the stable droplet jetting of the emissive layer ink is achieved with the optimized driving waveform,and a relatively uniform emissive film is obtained with adjusted printing spacing.Combining all the optimized printing processes,a printed phosphorescent OLED device is obtained on rigid glass.The device has a V_on of 2.9V,a maximum brightness of 8359 cd/m²,and a maximum EQE of 2.1%.By inserting a PVK HTL,the maximum brightness and maximum EQE of printed device have been increased to 18520cd/m²,and 4.8%,respectively,but the V_on increased to 4.8V.On flexible transparent PI film,a flexible OLED with a V_on of 4.9V,a maximum brightness of 1073 cd/m²,and a maximum EQE of 3.0%is successfully fabricated by inkjet printing using same process.