A study on the concentration and distribution of the mixed host material in the blue phosphorescent OLED with the dual emitting layers

We have investigated a relationship between the device performance and the mixed host material in the blue phosphorescent organic light-emitting diodes (PHOLEDs) with the dual emitting layers (EMLs). Firstly, a series of devices with EML1 (host material: mCP) close to the hole transport layer and EML2 (host material: mCPx: TPBi1-x) close to the electron transport layer are fabricated. When the thicknesses of dual EMLs are same, the best performance appears as x is 1/2. Then, another series of devices with the same total quantity of TPBi in EML are fabricated by means of changing the concentration and distribution of EML2, the maximum values of the luminance, current and power efficiency can reach to 13390 cd/m2 (at 8.75 V), 37.89 cd/A and 36.63 lm/W when the thickness ratio is 1:2 between EML1 and EML2 and the concentration of TPBi is 3/8 in EML2, which are improved by about 14%, 12% and 16% compared with that in the mixed host single EML device. These results show that it is beneficial for electro-optic performance in PHOLEDs by changing the concentration and distribution of EML2 in the dual EMLs, which can not only broaden the recombination zone but also balance the charge carriers.