Apparent thickness dependence of mobility in organic thin films analyzed by Gaussian disorder model

The authors report the investigation of thickness dependence of mobility in N, N'-bis(naphthalen-1-yl)-N, N(')-bis(phenyl) benzidine (NPB), and tris(8-hydroxyquinoline) aluminum (Alq(3)) thin films based on the Gaussian disorder model. The energetic disorder of 118 meV at thickness of 50 nm is larger than that of 88 meV at 300 nm in NPB. From the mobility prefactor and intersite distance, the carrier hopping distance between NPB molecules of thinner film is larger than that of thicker film. It suggests that the lower mobility at thinner thickness of NPB is affected by not only the interfacial trap states but also the molecular arrangement on the surface. Contrary to the results of NPB, the energetic disorder of Alq(3) at various thicknesses was obtained to be 150 +/- 7 meV that is almost independent of the thickness of film. The small changes in hopping distance of Alq(3) at different thicknesses have been observed, and this trend is clearly different from the case of NPB. (c) 2008 American Institute of Physics.