Quantum-Sized PbS, CdS, Ag2S, Sb2S3, and Bi2S3 Particles as Sensitizers for Various Nanoporous Wide-Bandgap Semiconductors

In semiconductor particles of nanometer size, a gradual transition from solid state to molecular structure occurs as the particle size decreases. In this very size regime, the physical and chemical properties of the particlesstrongly depend on their size.14 One of the first utilizations of the fascinating properties of these quantum-sized particles (Q-particles) for typical semiconductor applications was to embed the particles into porous Ti02 films and to use those modified layers as light-converting electrodesael Visible light was absorbed by theQ-particles which, consequently, transferred electrons into the porous Ti02 substrate. The photocurrent quantum yield reached values of more than 70% for Q-CdS and Q-PbS sensitized electrodes. Similar results were also obtained in a later work by Hotchandani and KamatI2 with Q-CdS on ZnO electrodes, who measured quantum yields of up to 15%. The basic principle of theseelectrodes, namely, the sensitization of a wide-bandgap semiconductor, has been investigated intensively during the past 3 decades whereby organic dyes were used as sen~itizers.~~15 Low coverage of the semiconductor surface by the dye molecules, typically much less than a monolayer, was found as a requirement for an effective charge carrier separation. Under these conditions, however, the amount of light absorption is negligibly small. A few years ago, a remarkable advance was made by Grltzel and co-workers, who used a highly porous Ti02 substrate electrode with a tris(bipyridy1)ruthenium or a coumarine dye as sensitizer.1618 The internal surface area of these electrodes was so large that less than a monolayer of adsorbed dye molecules was already sufficient for total light absorption. Electrochemical cells based on these electrodes are currently being discussed as a possible, cheap alternative to amorphous silicon solar cells. The crucial part in the cells is the dye itself only a very limited number of dyes give high photocurrent quantum yields and are reasonably stable against photodegradation. As already pointed out: the use of Q-particles as sensitizers principally implies several advantages as compared to organic dyes: the bandgap and thereby the absorption range are easily adjustable by the size of the particles, the band edge type of absorption behavior is most favorable for effective light harvesting, and the surface properties of the particles can be modified in order to increase the photostability of the electrodes. In contrast to organic dyes, however, the photophysics and photochemistry of Q-particles are still only poorly understood, and most of the knowledge is empirically based. Experiments on Q-particle sensitization are, therefore, still considered more at a level of basic research than at a level of practical application.