IMPROVEMENT OF PHOTOELECTROCHEMICAL PROPERTIES OF CHLOROALUMINUM PHTHALOCYANINE THIN-FILMS BY CONTROLLED CRYSTALLIZATION AND MOLECULAR-ORIENTATION

An orientation-controlled thin film of AlPcCl was prepared by vacuum deposition on a (001) surface of a KCl substrate. In the epitaxially grown film on KCl AlPcCl molecules stack cofacially, staggering around the Al-Cl-Al bonds and holding the molecular planes parallel to the KCl surface. At a low substrate temperature the deposit was a uniform and closely oriented thin film. As the substrate temperature increased, the film became rugged and cracked due to growth of a large size of crystallites. The photovoltaic properties of the oriented AlPcCl film were measured in a photoelectrochemical cell of ITO/AlPcCl/I3-,I-/Pt. The oriented AlPcCl films showed good rectifying I-V curves of p-type conductance and cathodic photocurrents under illumination of white light. As compared to a morphous or polycrystalline AlPcCl films, photovoltaic efficiency was improved especially in increase of I(sc) by the epitaxial orientation. The photocurrent quantum yield of the uniform thin film with epitaxial orientation was 25 times as high as that of the rugged and cracked polycrystalline film with large crystallites. The epitaxial AlPcCl film showed an intense peak at 830 nm in the photocurrent action spectrum, which could be attributed to carrier generation from charge-transfer excitons of the cofacially stacked molecules. It was concluded that the reason for the improvement of the photovoltaic properties was that exciton diffusion and charge carrier mobility were larger through the molecular column in the epitaxial film, while exciton trapping and recombination of carriers were probably caused in grain boundaries between crystallites in the polycrystalline film.