Mesoporous oxide junctions and nanostructured solar cells

Mesoporous films of wide-band gap semiconductor oxides are an important new class of electronic materials. They are constituted by a network of nanocrystalline particles of oxides, such as titania, niobia or zinc oxide, sintered together to allow for charge carrier transport to take place. The pores between the nanoparticles are filled with an electrolyte or a solid state organic hole conductor forming an interpenetrating heterojunction of very large contact area. These junctions exhibit extraordinary opto-electronic properties due to their large surface area to volume ratio leading to applications in different domains, such as photovoltaics, intercalation batteries, electrochromic and electroluminescent displays, photocatalysis and chemical sensors. Of particular interest are dye-sensitized heterojunctions, where photo-induced charge separation occurs at the interface between the mesoporous oxide and the hole conductor or the electrolyte. Photovoltaic cells based on this concept form a viable alternative to conventional silicon cells. Solar to electric power conversion efficiencies exceeding 10% have been reached with mesoporous titania films derivatized with molecular charge transfer sensitizers and used in conjunction with organic iodide/triiodide-based redox electrolytes. Long-term accelerated light-soaking tests have shown the system to be intrinsically stable. This article summarized recent developments in this field including a discussion of solid state dye-sensitized heterojunctions employing spirobifluorene-connected arylamines as hole transport materials.