Study On Carriers Balance In Emitting Layer To Improve Performance Of Blue Phosphorescent Organic Light-emitting Devices

After decades of research on electroluminescence of organic materials, organic light-emitting devices(OLEDs) step gradually into the stage of industrialization from the stage of laboratory. In order to realize full color display and white light, blue light is one of the essential light sources. A lot of researches on the blue organic light-emitting devices have been carried out, in which the problem of carriers balance is one of the main research contents. In this paper, the researches are based on carriers balance in the luminous layer, the main research contents are as follows:1. The traditional structure of ITO / NPB(40nm) /m CP:FIrpic(10wt.% and 20 nm) / Bphen(40 nm) / Li F(1 nm) / Al and ITO / NPB(30nm) /Tc Ta(10 nm) /m CP:FIrpic(10wt.% and 20 nm) / Bphen(40 nm) / Li F(1 nm) / Al were compared to estimate hole concentration higher than the electron concentration in the emission layer of the blue phosphorescent organic light-emitting device based on m CP host, this imbalance directly resulted in more and more serious exciton-polaron quenching.2. By doping FIrpic into the different position of m CP emission layer to research the light-emitting situation of device with the structure of ITO / NPB(30nm) /Tc Ta(10nm) /m CP(x nm) /m CP:FIrpic(10wt%, 5 nm) /m CP(y nm) / Bphen(40 nm) / LIF(1 nm) / Al under different voltages, in where x+y+5=20. It is found that the initial light position of the device is close to the m CP/Bphen interface, and the light-emitting position of the device is gradually extended to the anode side with the increase of the voltage. The holes electrostatic binding effect, which effects the electrons injection, at the m CP/Bphen interface was found by comparing the luminescence spectra.3. The research on inserting electronic transport material(Bphen) with low hole mobility and high HOMO into hole transport layers as a hole block layer, and the inserting position(between NPB and Tc Ta or Tc Ta and m CP) of Bphen layer with 1nm thickness were studied. It is found that the position of Bphen between Tc Ta and m CP makes the efficiency of device lower than that of device without Bphen layer. While the efficiency of device with Bphen between NPB and Tc Ta is improved. Compared to the device without Bphen layer, the maximal current efficiency of device with Bphen between NPB and Tc Ta is improved by 24.39% from 16.07cd/A to 19.99cd/A and the maximal power efficiency is improved by 18.97% from 14.02lm/W to 16.68lm/W. The maximal extern efficiency is improved by 24.57% from 7.53% to 9.38%. At the same time, the current efficiency roll-off of device without Bphen layer is 50% at 8099.86cd/m2. While the current efficiency roll-off of device with Bphen layer between NPB and Tc Ta is 38.32%. Compared to device without Bphen layer, the improvement of roll-off is 23.36%.4. By doping FIrpic into Tc Ta to reveal the reason of low efficiency of device with 1nm thick Bphen between Tc Ta and m CP. Devices with 1 nm doping thickness were prepared by structure of ITO/NPB(30nm)/Tc Ta(9 nm)/ Tc Ta:FIrpic(10 wt%, 1 nm)/m CP(20 nm)/Bphen(40 nm)/Li F(1 nm)/Al and ITO/NPB(30nm)/Tc Ta(8 nm)/ Tc Ta:FIrpic(10 wt%, 1 nm)/Bphen(1 nm)/m CP:FIrpic(20 nm)/Bphen(40 nm)/Li F(1 nm)/Al. Through the analysis, it is found that because Bphen layer blocks holes at the Tc Ta/Bphen interface, and then electrons through Bphen layer from the emission layer reach this interface are consumed by non-radiative with holed, so the efficiency of the device decreases.5. The thickness of the Bphen layer between NPB and Tc Ta was studied. It is found that the efficiency of the device increases first and then decreases with the thickness of Bphen layer increasing, but this change is not obvious. However, the efficiency roll-off of the device gradually become serious caused by electronic field dependence of FIrpic higher than hole field dependence, which leads to the electrons with the electric field increasing faster than the hole. The more thick Bphen layer, which leads to stronger hole blocking effect, can make the hole can not compensate with the rapid increasing electron, resulting in the exciton-polaron quenching more serious.