Star-shaped molecules based on a triphenylamine core derivatized with various combinations of thienylenevinylene conjugated branches and electron-withdrawing indanedione or dicyanovinyl groups have been synthesized. UV-vis absorption and fluorescence emission data show that the introduction of the electron-acceptor groups induces an intramolecular charge transfer that results in a shift of the absorption onset toward longer wavelengths and a quenching of photoluminescence. Cyclic voltammetry shows that all compounds present a reversible first oxidation process whose potential increases with the number of electron-withdrawing groups in the structure. Prototype bulk and bilayer heterojunction solar cells have been realized using fullerene C60 derivatives as acceptor material. The results obtained with both kinds of devices show that the introduction of electron-acceptor groups in the donor structure induces an extension of the photoresponse in the visible spectral region, an increase of the maximum external quantum efficiency, and an increase of the open-circuit voltage under white light illumination. These synergistic effects allow reaching power conversion efficiencies of approximately 1.20% under simulated AM 1.5 solar irradiation at 100 mW cm(-2).

An overview of some recent developments of the chemistry of molecular donor materials for organic photovoltaics (OPV) is presented. Although molecular materials have been used for the fabrication of OPV cells from the very beginning of the field, the design of molecular donors specifically designed for OPV is a relatively recent research area. In the past few years, molecular donors have been used in both vacuum-deposited and solution-processed OPV cells and both fields have witnessed impressive progress with power conversion efficiencies crossing the symbolic limit of 10 %. However, this progress has been achieved at the price of an increasing complexity of the chemistry of active materials and of the technology of device fabrication. This evolution probably inherent to the progress of research is difficult to reconcile with the necessity for OPV to demonstrate a decisive economic advantage over existing silicon technology. In this short review various classes of molecular donors are discussed with the aim of defining possible basic molecular structures that can combine structural simplicity, low molecular weight, synthetic accessibility, scalability and that can represent possible starting points for the development of simple and cost-effective OPV materials.