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 angstrom) 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 angstrom) 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 angstrom) 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/m(2) at a current density of 100 mA/cm(2), with a maximum luminous efficiency of 1.72 Im/W at 5.5 V. (c) 2006 Elsevier B.V. All rights reserved.