“Porphyrin-Fullerene Electropolymers for Solar Energy Conversion”.

D. GUST, T. A. MOORE, A. L. MOORE, P. A. LIDDELL, M. GERVALDO, G. KODIS, B. BRENNAN AND J. BRIDGEWATER

Annapolis, MD, USA.

Conferencia; 31st DOE Solar Photochemistry Research Meeting; 2009

Organic photovoltaics, which are based on conducting polymers that absorb visible light, arepotentially very inexpensive and easy to manufacture, and they can have other advantages suchas flexibility and partial transparency. However, the efficiency of organic photovoltaics iscurrently only ~5%. This is due in part to the fact that the conducting polymers used were ingeneral developed for other purposes, and have not been optimized for solar energy applications.We are using the basic chemical and physical principles underlying natural photosyntheticenergy conversion to design new conducting polymers that overcome some of the problemsinherent in the present generation of materials, and may ultimately be useful as components of efficient, inexpensive organic solar cells. An example is the polymer shown in Fig. 1. The polymer is readily formed as a film on a transparent electrode by electropolymerization of a porphyrinfullerene dyad. Film thicknesses >100 nm can be easily achieved. In cyclic voltammetry, the film shows reversible waves at 0.95 and -0.63 V vs. SCE, corresponding to oxidation of the porphyrin and reduction of the fullerene moities, respectively. The absorption  spectrum of the film, (Fig. 2) shows that the porphyrin Soret and Q-bands are still present, but broadened and red-shifted, compared to the monomer. Absorbance is seen throughout the visible spectral region. Fig. 1 The film is virtually non-fluorescent; the porphyrin excited singlet state lifetime is quenched to a few ps due to photoinduced electron transfer to the fullerene.Similar behavior is observed in a model monomeric porphyrin-fullerene dyad. Thus, the polymer acts at a “molecular heterojunction” wherein light absorption is followed by immediate charge separation, without the need for exciton migration. This is in contrast to the “bulk heterojunctions” used in conventional organic photovoltaic systems, in which excitation must migrate through a material until it encounters a phase junction where photoinduced electron transfer can occur, leading to energy conversion losses due to decay of excited states. In preliminary experiments, the new polymer has been found to produce photocurrents upon illumination.