Prin 2007

Materials and techniques for new-generation systems of photovoltaic conversion

The increasing world-wide demand for energy, enforced by swift economic development of emerging Countries such as China, India and Brazil, is currently largely satisfied by fossil combustibles, which call forward the emission of CO2 and other pollutants. Sustainable development does require the usage energy sources with lowest environmental impact, which are available for both urban and rural areas, in developed and underdeveloped countries, as a network of micro-distributed generators to respond to the increasing demand for energy. Energy generation must rely on robust, fully modular units and span the range from few watts to tens of megawatts. These circumstances are met with photovoltaic technology, though the current production cost of a single unit for each specific application (grid-centralized, distributed, stand-alone) exceeds by a factor of 5 the price of an equally powerful conventional generation unit. Moreover, even under favourable economic condition, assuming that the whole production of silicon yearly produced all over the world would be completely involved in the generation of photovoltaic cells, that would be insufficient for production a non negligible portion (at least 1%) of the national need.
Diffusion of photovoltaic energy is thereby prevented by such limiting factors. Thus, only a big effort into research will be able to support a suitable technological development:
- In the short term, it is mandatory to aim at the improvement of production processes and of conversion efficiency, at the decrease of process costs with cheap low-impact environment-compatible processes and ultimately at the reduction of expensive prime materials. In this scheme, the only method that can be currently pursued is the technique of concentrated photovoltaic, namely the focalization of sunlight onto low-size photovoltaic cells. Indeed, such method is currently employed with high-efficiency silicon cells and constitutes an interesting methodology, inasmuch as it allows an increase in conversion efficiency and as well as a significant decrease in the quantity of used photo-converting material. On the other hand, such scheme does not afford significant improvement because of the limited energy range of the photons for which a single-material cell can efficiently operate.
- In the long term, the research should promote the development of novel devices, which are benefited by cross-fertilization with other material sciences and high-technology industrial fields such as display and aerospace industries. This effort will steer toward a new class of materials with large-scale production and high conversion efficiency. In fact, the target of photovoltaic in the mean term is the achievement of conversion efficiency of the order of 30-40% at the rate of Euro/m2, which is also referred to as third-generation photovoltaic or , alternatively with an efficiency figure about 5-10% but at the rate 15-40 Euro/ m2.
The present project aims at the development of new techniques and novel materials for high-efficiency photovoltaic conversion that can be produced over a large scale. The project naturally assumes the concentration scheme and focuses to the split of sunlight into bands. Due to usage of suitable dichroic filters each band is geometrically separated in a number of beams from the others to form a beam and diverted to a specifically designed semiconductor whose bandgap in tuned to the energy of the photons in the beam.
The combination of concentrated photovoltaic with a system of independent cells, which was recently proposed by some of the members in this project, is highly innovative with respect to traditional being studied semiconductor multi-junction devices because:
- it permits to prevent all the problems and technological drawbacks in the fabrication of a multi-junction;
- it allows to consider a wider class of cheaper and easily producible materials, among which are the polymers, to accompany the traditionally used silicon;
- it affords operation with no need for cooling of photo-sensing units.
The present project aims at seeking for new materials, other than those that are currently in the stage of prototype, and novel techniques for concentration that are eligible for photovoltaic conversion through separation of sunlight by dichroic filters.