Effects of hole injection layer thickness on the luminescent properties of white organic light-emitting diodes

This work investigates how the thickness of the hole injection layer (HIL) influences the luminescent characteristics of white organic light-emitting diodes (WOLED). Experimental results indicate that inserting a thin HIL (<200 Å) into a WOLED without an HIL reduces the brightness and clearly changes the chromaticity because the surface of the 4,4′,4″-tris{N,-(3-methylphenyl)-N-phenylamino}-triphenylamine) (m-MTDATA) film is extremely rough. In contrast, a dense film structure and the fine surface morphology of m-MTDATA of moderate thickness (350-650 Å) provides a uniform conducting path on which holes cross the indium tin oxide (ITO)/HIL interface, improving luminescent performance, associated with the relatively stable purity of the color of the emission, with Commission Internationale 1′Eclairage (CIE) coordinates of (x = 0.40, y = 0.40). However, inserting a thick HIL (>650 Å) reduces the luminescent performance and causes red-shift, because the holes and electrons in the effective emissive confinement region become less optimally balanced. Moreover, optimizing the device structure enables a bright WOLED with CIE coordinates of (x = 0.34, y = 0.33) to reach a luminance of 7685 cd/m² at a current density of 100 mA/cm², with a maximum luminous efficiency of 1.72 lm/W at 5.5 V.