| Organic light-emitting devices show unique advantages in solid-state lighting and display,owing to light,thin,bendable,simple structure,low energy consumption,high color rendering,uniform and soft illumination,healthy eye protection,green environmental protection,etc.Phosphorescent materials can achieve 100%internal quantum efficiency due to their ability to make singlet and triplet excitons illuminate together.The internal quantum efficiency of the device can reach 100%theoretically.It has a high external quantum efficiency and has certain application prospects.The efficiency and efficiency roll-off of phosphorescent devices has been a problem that researchers in various countries need to overcome.In this paper,Cs is used as the N dopant to improve the efficiency and efficiency roll-off of green phosphorescent devices and yellow phosphorescent devices.Based on this,two new charge generation units?CGUs?are proposed and prepared tamden OLED devices.Finally,we prepared blue phosphorescent devices and studied the efficiency roll-off problem and microcavity effect of the devices.The main contents are listed as follows:1?Using Cs as N-dopant in B3PyPPM and Bphen respectively,the experimental result showed that the green phosphorescent device prepared by B3PyPPM:Cs can increase the conductivity of the device to 1.59×10-6S cm-1,current efficiency increasing to 68.5 cd/A,improving efficiency roll-off,making it conform to the efficiency roll-off model of triplet-triplet annihilation?TTA?;The yellow phosphorescent device prepared by Bphen:Cs increases the critical current value of the device to 189 mA/cm2,higher than the critical current of 141 mA/cm2 without N-doped devices.N doping increases the injection of electrons and makes more holes combined with electrons to form excitons and improves the efficiency roll-off.2?Tandem devices were prepared using B3PyPPM:Cs/Al/HAT-CN and B3PyMPM:Cs/Al/HAT-CN as CGUs,respectively.Experiments show that B3PyMPPM:Cs and B3PyMPM:Cs can reduce the injection barrier of electrons.The addition of thin layer of Al can further reduce the electron injection barrier and enhance the stability of CGU.The voltage and brightness of the Tandem device fabricated by B3PyPPM:Cs/Al/HAT-CN are 8.12V and 1450cd/m2 at a current density of 5mA/cm2,which is close to the sum of voltage and brightness of two single-section devices.The peaks of the spectrum are from two light-emitting units.We have fabricated an efficient tandem OLED based on Ir?ppy?2acac emitter adopting B3PyMPM:Cs/Al/HAT-CN as CGU.At a current density of 5 mA/cm2,the driving voltage and brightness of the tandem device are 2.04 times and 2.84 times that of a single device,and the maximum current efficiency is 172.2 cd/A,which is 2.18 times that of a single device.At the brightness of 1000 cd/m2,the current efficiency is still up to 165.4 cd/A,and the efficiency roll-off is only 3.9%.The peak value of the spectrum of the device does not change with the change of the observation angles,meaning that the high efficiency of the tandem device is a result of collectively illuminating of the light-emitting layers of the two light-emitting units,not because of the microcavity effect.3?Two kinds of Firpic-based blue phosphorescent devices were prepared by using B3PyPPM as the electron transport layer,Cs as the N dopant,and TCTA and mCP as the precursors.When the brightness of the device with mCP is raised from500 cd/m2 to 3000 cd/m2,the external quantum efficiency roll-off is only 10.1%.The turn-on voltages of the two devices are 2.3V and 2.8V,respectively,which is lower than other electron transport materials.The Firpic device demonstrates that B3PyPPM has excellent electron injection characteristics,which is not found in other transport layer materials.The intensity of the vibration peak of the device spectrum changes with the thickness of the electron transport layer B3PyPPM,which has a significant microcavity effect. |
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