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,N8-bis-(1-naphthyl)-N,N8-diphenyl1-1,1-biphenyl1-4,48-diamine ~a-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 (;5 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 a-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. © 2003 American Institute of Physics. @DOI: 10.1063/1.1565506#