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

  • N. Koch (a1) (a2), B. Nickel (a3), J. Ghijsen (a4), A. Elschner (a5), J. Schwartz (a3), J.-J. Pireaux (a4) and A. Kahn (a1)...

L 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'-diphenyl1-1,1-biphenyl1-4,4'-diamine (á-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 á-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 á-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.

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