CdS/chloroaluminum phthalocyanine heterojunctions: effect of CdS preparation method on the efficiency of photovoltaic cells

Abstract

Heterojunctions of the type CdS/chloroaluminum phthalocyanine have been obtained by using the following methods to prepare the CdS layer: electrochemical deposition in dimethylsulfoxide (I); electrochemical aqueous deposition at constant reduction potential (II), or cyclic potential (III), or constant oxidation current (IV); and chemical aqueous deposition (V). The CdS films were characterized by SEM, EDAX, optical microscopy with Nomarski interference, X-ray diffraction, and photoelectrochemistry in polysulfide solution. The best cells show a conversion efficiency of about 0.2% under white light illumination of 90 mW cm−2. They were obtained with a 1.0 μm thick layer of the organic semiconductor and a 0.4 μm thick CdS film prepared by method I. This method yields a polycrystalline electrode characterized by large plates (1–3 μm) of tightly packed single oriented (111) cubic CdS crystallites showing low sub-bandgap absorption. The better performances of the CdS/ClAlPc heterojunctions obtained by method I can be understood in terms of absorbance of the CdS layer of optimum thickness, hole diffusion length in the CdS and electron–hole recombination efficiency at the CdS/ClAlPc interface. Keywords: CdS, phthalocyanine, heterojunction, organic semiconductor, photovoltaic cell.