A nano-indentation study of the reduced elastic modulus of Alq3 and NPB thin-film used in OLED devices

Two of the commonly-used Organic Light Emitting Diode (OLED) materials tris-(8-hydroxyquinoline) aluminum (Alq3) and N,N′-bis(naphthalen-1-yl)-N,N′-bis(phenyl)benzidine (NPB) are thermally evaporated as thin-films on two kinds of substrates with different hardness. By using nano-indentation techniques, the reduced elastic modulus of each of the coatings is measured. The data are carefully analysed through the standard Oliver and Pharr method, and the recently developed critical indentation depth method which takes the effect of the substrate more into account.