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

Abstract

The current–voltage characteristics $\mathrm{I(V)}$ of model organic devices are studied under ultra-high-vacuum conditions. Active materials are ${\mathrm{N,N}}^{\prime }-\mathrm{bis}\text{-(1-}\mathrm{naphthyl}\mathrm{)-}{\mathrm{N,N}}^{\prime }-\mathrm{diphen-yl}\text{1-1,1-}\mathrm{biphenyl}{\text{1-4,4}}^{\prime }-\mathrm{diamine}$ (α-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 $\text{(∼5\hspace{0.17em}}\mathrm{eV}\mathrm{),}$ 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 α-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.