Doping effect of blue light-emitting electron transport molecule in blue organic light-emitting devices

Abstract

Here we report the doping effect of blue light-emitting electron transport molecules on the performance of blue organic light-emitting devices (OLEDs). 4,4′-N,N′-dicarbazole-biphenyl (CBP) was chosen as a blue emission material (layer) for two main reasons: its wide band gap makes it suitable for blue emission; and its shifted energy band structure allows for easy electron injection. However, the main drawback of CBP is the low electron mobility and the large hole injection barrier. In order to overcome these weak points, the CBP nanolayer (50nm thick) in blue OLEDs has been doped with 2-(2-hydroxyphenylbenzoxazole) lithium (LiPBO), which is known to be a blue light-emitting electron transport material. The result showed that the device performance (charge injection voltage, turn-on voltage, and current efficiency) was improved by doping the LiPBO molecule, an improvement that can be attributed to the enhanced electron mobility and the reduced hole injection barrier of the LiPBO-doped CBP nanolayer. However, the blue color purity of OLEDs marginally deteriorated as a result of the LiPBO doping. The worst Commission Internationale de l'Eclairage (CIE) color coordinate of OLEDs was x=0.16 and y=0.18 at 16V, while the standard coordinates are x=0.14 and y=0.08, which is possibly due to the formation of exciplexes in a nanoscale.