Highly Efficient Nanocrystalline Photovoltaic Devices

Abstract

A new molecular photovoltaic system for solar light harvesting and conversion to electricity has been developed. It is based on the spectral sensitisation of a nanocrystalline semiconductor film by transition metal complexes. The film consists of nanometre-sized colloidal titanium dioxide particles sintered together to allow for charge carrier transport. Ruthenium and osmium based sensitisers have so far achieved the best performance both from the efficiency as well as the stability point of view. Carboxylated polypyridyl complexes of these two metals give extraordinary efficiencies for the conversion of incident photons into electric current, exceeding 90 per cent within the wavelength range of their absorption band. The outstanding performance of cis-di(thiocyanato) bis(2,2'-bipyridyl-4-4'-dicarboxylate)ruthenium(II)is unmatched by any other known sensitiser. The present paper discusses the underlying physical principles of these astonishing findings.Exploiting this discovery, we have developed a low-cost photovoltaic cell whose overall light to electric energy conversion yield is 10 per cent under direct (AM1.5) solar irradiation. For the first time a deuice based on a simple molecular light absorber is attaining a conversion efficiency commensurate with that of silicon baed photovoltaic cells, but at a much lower cost.