Organized Langmuir-Blodgett monolayers and multilayers based on semiamphiphilic binuclear phthalocyanine: Structural and photovoltaic characteristics

A kind of water-soluble binuclear cobalt phthalocyaninesulfonate (bi-CoPc) was synthesized and used to prepare Langmuir-Blodgett (LB) films by the sem iamphiphilic technique. On the bi-CoPc-containing subphase, monolayers were built up from the mixtures of octadecylamine (C18NH2) and methyl octadecan oate (OME). The surface pressure-area (pi-A) isotherms indicated that C18NH 2-OME molecules could form well-defined condensed-type monolayers. The mono layers of C18NH2:OME = 1:1 and 1:4 were deposited onto solid substrates (Ca F2 and silicon substrates). Small-angle X-ray diffraction (XRD) patterns of C18NH2-OME/bi-CoPc LB multilayers indicated the formation of highly ordere d multilayers in the vertical direction. The long spacing; of LB multilayer s were 5.94 and 5.11 nm for C18NH2:OME = 1:4 and 1:1, respectively. This su ggested that in the LB multilayer of C18NH2:OME = 1:4 the hydrocarbon chain s stood more vertically than that of 1:1. FTIR spectra confirmed the format ion of the supermolecular assembly of C18NH2 and bi-CoPc. W-vis spectra rev ealed that bi-CoPc existed in aqueous solution as both monomers and aggrega tes to give a broadened Q-band spectra. The LB films of C18NH2-OME/bi-CoPc produced significantly redshifted Q-band spectra compared with the monomer spectra in solution. These results indicated that bi-CoPc formed dimers in LB multilayers, and the dimers formed between two adjacent layers. From the surface photovoltage spectra (SPS), it could be followed that the photovol tage response of the n-type silicon increased by a factor of about 4 times when modified by the C18NH2-OME/bi-CoPc monolayer. On the contrary, the pho tovoltage response value of p-Si was reduced by about 30 times compared wit h that of blank p-Si. These results showed that the C18NH2-OME/bi-CoPc LB m onolayer effects different photosensitization processes on n-Si and p-Si.