Fabrication of titanium oxide/phthalocyanine hybrid multilayers and their application to interface control for an organic diode

Abstract

Hybrid multilayers of phthalocyanines (Pc) and titanium oxide (TiOx) have been effectively fabricated on the surfaces of quartz and indium-tin-oxide (ITO) according to the following layer-by-layer sequence; (i) the formation of a titanium butoxide layer (Ti(OBu)_m) grafted onto the substrate surface by immersion of a substrate into a solution of Ti(OBu)₄, (ii) hydrolysis of Ti(OBu)_m to a TiOx layer with a Ti–OH surface, (iii) dipping of the treated substrate into a solution of Pc1–Pc3 bearing four or eight (triethoxysilyl)alkoxy substituents to form a Pc layer on the TiOx surface, presumably through Ti–O–Si bonding, (iv) hydrolysis of excess Si(OEt)₃ groups in the Pc layer to form a silanol surface and (v) cyclic repetition of steps (i)–(iv). Atomic force microscopy (AFM) has shown that the multilayer surfaces are smooth, with surface roughnesses similar to that of a pristine substrate. The ratios of Ti atoms/Pc molecule for three different multilayers determined by X-ray photoelectron spectroscopy (XPS) were found to be roughly constant (3.8–4.3). For hole-only diodes of ITO/N,N′-bis(3-methylphenyl)-N,N′-diphenylbenzidine/Au, it has been found that the surface modification of ITO with the hybrid multilayers of Pc1 causes remarkable current enhancements at low applied biases depending on the cycle number of the layer-by-layer deposition.