Incorporation of silicon nano sized particles in a solar cell device structure for increased efficiency

Increases in the efficiency of terrestrial photovoltaic devices are important for their widespread acceptance as a form of power generation. One of the main factors limiting the efficiency of Si based solar cells is the imperfect match between bandwidth o ver which it converts photons into current and the incident solar spectrum. The solar spectrum is very broad and contains significant power density both below and above the band gap of silicon. Therefore the conversion efficiency limit for a single-juncti on crystalline Si solar cell under a standard AM 1.5 solar spectrum is about 30 %. The 70% of incident solar power which is not converted to electricity includes about 50% converted to heat through phonon emission. Therefore effective generation of multip le excitons per absorbed photon of energy greater than twice the band gap represents one possible route toward enhanced photon conversion efficiency. The multiple exciton generation (MEG) is a process whereby multiple electron-hole pairs, or excitons, are produced upon absorption of a single photon in semiconductor nanocrystals. It has been demonstrated recently that for Si nano-crystals with diameter about 9.5 nm the exciton production yield is about 2.5 excitons per absorbed photon at 3.4Eg. To implemen t this finding for the actual Si based solar cell processing it is necessary to realize the dissociation of excitons into free separated electrons and holes and collect them afterwards. Another concept, which can be considered as a method to overcome the restrictions of the Si solar cell base is termed "down conversion". The down-converter ideally transforms incident photons with energies exceeding n-times the band-gap energy of the solar cell material into n lower energy photons, which can then be used for the generation of n electron-hole pairs inside the solar cell. Thus down conversion represents a method for multiple electron-hole generation per incident high-energy photon and can be