Metal vs. Polymer Electrodes in Organic Devices: Energy Level Alignment, Hole Injection, and Structure

We have investigated the electronic, structural, and charge injection properties of interfaces formed between three electroactive conjugated organic materials, i.e., N,N′-bis-(1-naphthyl)-N,N′-diphenyll-1,1-biphenyll-4, 4′-diamme (a-NPD), pentacene, p-sexiphenyl, and two high work function electrode materials, i.e., gold and the conducting polymer poly (3,4-ethylenedioxythiophene)/poly(styrenesulfonate) (PEDOT:PSS). Ultraviolet photoelectron spectroscopy shows that the hole injection barrier between the three organic materials and PEDOT:PSS is lower by 0.6-1.0 eV as compared to Au, despite a similar work function of the pristine electrode material surfaces (ca. 5 eV). This very large difference is due to an effective change of the metal work function due to the deposition of organic molecules, i.e., a decrease of the Au surface dipole due to adsorption. Accordingly, model device structures built from α-NPD and pentacene on the two different electrode materials show much higher current densities for hole injection from PEDOT:PSS than from Au. Hole injection from Au for α-NPD devices is independent of deposition sequence and substrate. Pentacene devices exhibit significant asymmetries in that respect, due to a strong dependence of the morphology and preferred molecular orientation of the crystalline material on the substrate, as shown by atomic force microscopy and X-ray diffraction. Consequently, great care must be taken when modeling current-voltage characteristics of devices comprised of crystalline organic solids, especially when the influence of film thickness or different substrate materials is to be studied.