Accurate thickness/density measurements of organic light-emitting diodes

We report on the use of Rutherford backscattering spectroscopy for thickness analysis of organic light-emitting diode structures (OLEDs) with subnanometer resolution and a spatial resolution <1 mm. A careful study of ion beam induced effects revealed some organic film degradation, but not so severe as to inhibit meaningful measurements. The method is independent of the substrate and is still applicable if the organic film is capped with a metal cathode. Common OLED materials have been the subject of this study: poly(2-methoxy,5-(2′-ethylhexoxy)-1,4-phenylene-vinylene (MEH-PPV), N′ ,N′ -diphenyl-N, N′ -bis(3-methylphenyl)-1,1′ biphenyl-4,4′-diamine (TPD), and tris-(8-hydroxyquinoline) aluminum (Alq₃). The densities of thin films of evaporated TPD (ρ =1.22±0.05 g/cm³) and Alq₃ (ρ=1.51±0.03 g/cm³) have been established.