During last years, there has been a strong research effort in order to cut down photovoltaics costs. Several possible alternative technologies to silicon-based photovoltaics have been proposed and Organic PhotoVoltaics (OPV) could represent the cheapest way to convert solar energy into electricity. The Bulk Heterojunction (BHJ) solar cell represents a photovoltaic system that can be processed from solution leading to large area devices on transparent and flexible substrates, using cheap techniques like spin-coating, doctor blading, ink-jet printing and screen-printing, in conformity with the request of low cost photovoltaics.One major obstacle to be overcome is the low Power Conversion Efficiency (PCE) that has been demonstrated up to now, a maximum of about 5%.Various parameters influence the low efficiency: among the others, PCBM, a soluble fullerene derivative, is the almost exclusive n-type material (acceptor) in these devices, obliging to optimize the donor in a particular device configuration; only a little amount of the solar spectrum is involved in the energy conversion process; charge carrier mobility is lower than the one of inorganic counterparts, only partly balanced by a higher absorption coefficient and a higher charge mean lifetime.The major objectives to be achieved in the present proposal framework are:1) to study the optical and the electronic properties of new low-bandgap materials, both polymers and inorganic nanoparticles, as donor phase to be used in BHJ solar cells;2) to study the optical and electronic properties of inorganic nanoparticles to be used as acceptor phase in BHJ solar cells.3) to optimize the device structure in order to increase the fraction of absorbed photons from the incoming solar photon flux studying the light harvesting by Resonant Surface Plasmon coupling;4) to optimize the light harvesting by Resonant Energy Transfer and/or new conceptual spectral shaping.