Organic molecular films on gold versus conducting polymer: Influence of injection barrier height and morphology on current-voltage characteristics

The current-voltage characteristics I(V) of model organic devices are studied under ultra-high-vacuum conditions. Active materials are N,N'-bis-(1-naphthyl)-N,N'-diphen-yl1-1,1-biphenyl1-4,4'-diamine (alpha-NPD) and pentacene, electrode materials are polycrystalline Au and the conductive polymer poly(3,4-ethylenedioxythiophene)/poly(styrenesulfonate) (PEDOT/PSS). Despite a similar work function of electrode material surfaces (similar to5 eV), hole injection from PEDOT/PSS is significantly more efficient than from Au, due to a smaller hole injection barrier. Hole injection characteristics from Au electrodes for devices made from alpha-NPD are independent of deposition sequence and substrate used. Pentacene devices exhibit serious asymmetries in that respect. These are caused by a strong dependence of morphology and preferred molecular orientation on the substrate for the crystalline material. (C) 2003 American Institute of Physics.