CHARACTERIZATION OF A HIGHLY PHOTOACTIVE MOLECULAR SEMICONDUCTOR - OXOTITANIUM PHTHALOCYANINE

Oxotitanium phthalocyanine (OTiPc) thin films have been sublimed at va rious thicknesses on SnO2 and glass substrates. The morphology, crysta llinity, and photoelectrochemical activity of the films have been stud ied. It was found that the physical and photoelectrochemical propertie s of the films are greatly influenced by the temperature reached by th e substrate during the sublimation. Below 80-degrees-C, amorphous film s are obtained while the films are partially crystalline when the subs trate is allowed to reach about 140-degrees-C. Amorphous films are mad e of tightly packed aggregates of circular section while partially cry stalline films consists of platelets. All films are porous and permeab le to the I3-/I- redox system. The dominant polymorph in partially cry stalline films is not the same for all film thicknesses. It is phase I V OTiPc (as deduced by electron diffraction) for films thinner than ab out 2000 angstrom. On the other hand, for films thicker than about 800 0 angstrom, phase I OTiPc becomes the dominant polymorph (as deduced b y X-ray diffraction). It is replaced by phase II for 20 000-angstrom-t hick films of OTiPc. Partially crystalline films are the only ones to absorb in the near-infrared (NIR) region. This typical absorption is g oing along with an improvement of the photoactivity of the films. In p artially crystalline films, energy is transferred from the amorphous t o the crystalline regions where most of the charges are generated. Qua ntum yields for electron collection per incident photon may reach over 25% in short circuit conditions, at 850 nm, the Q-band absorption max imum for 8000-angstrom-thick films. Those films are phase I OTiPc. Und er 35 MW CM-2 polychromatic illumination, the same films are character ized by short circuit photocurrents of 1.5 mA cm-2. A NIR absorbance i s an important factor required to obtain a high photoactivity, but it is not the only one. Interaction of OTiPc with oxygen at the purificat ion level of the crude material is very important as well.