Preparation of substrates, absorber and transporting layers

Beyond the fabrication of a patterned substrate there remains the need to prepare conformal and void-fdling layers on these substrates. Recently, several solution and gas-phase techniques have been developed which allow conformal or void-filling deposition in very deep and narrow structures, and we will next outline how these techniques can be adapted for solar cell fabrication. 

Porous materials with large internal surface areas have attracted considerable attention in surface chemistry, catalysis and chromatography. In principle, a vast choice of porous materials is available. While some inspiration can indeed be drawn from chemical applications, the choices for useful photovoltaic substrate materials are considerably narrowed by the requirement to have thin-film constituency, transparency and electronic conductivity. Indeed, these requirements are so stringent that only a few substrate types and materials have so far been found suitable. Sintered nanocrystalline oxide films are one major class of these. Much development work on these materials has been reported in the context of dye-sensitised cells, and this is 

Chalcogenide compounds (Olea and Sebastian, 1998) and Cu compounds such as Cul (Rost, 1999), CuSCN (Engelhard and Konenkamp, 2001) and CUAIO2 (Kawazoe et al, 1997 Tsuboi et al, 2003) have been suggested and explored, the Cu compounds as p-type substrates. 

From the experimental results it appears that electron transport in nanoporous films is strongly affected by grain boundaries. If the porous structure has a depth of 5 //m and the typical grain size is 10 nm, a minimum of 500 grain boundaries have to be crossed in the transport vertical to the film plane. It may therefore be worthwhile to consider alternative surface-enhanced substrates, which feature fewer grain boundaries, or those where these are completely avoided. There are essentially two possibilities for such films porous structures etched in multicrystalline films, or grown columnar films with single-crystalline columns. 

Si (Canham, 1990), SiC (Shor et al, 1993), GaP (Belogorokhov et al, 1994) and ZnTe (Zenia et ai, 1999) may be taken as examples of etched single-crystalline porous structures. Solar cells have indeed been fabricated with some of these materials, but it appears that the final product is as costly as the original multicrystalline precursors, and the cost advantages may therefore be limited. 

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