Photovoltaic Response of Carbon Nanotube-Silicon Heterojunctions: Effect of Nanotube Film Thickness and Number of Walls

We report on the multiwall carbon nanotubes application as energy conversion material to fabricate thin film solar cells, with nanotube network acting both as photogeneration sites either as charge separators, collectors and carrier transport layer. The device consists of a semitransparent thin film of nanotubes coating a n-type crystalline silicon substrate. Under illumination electron–hole (e–h) pairs, generated both in nanotubes and in the silicon substrate underneath, are split and charges are transported through the nanotubes (electrons) and the n-Si (holes). We found that a suitable thickness of the nanotube thin film, high density of Schottky junctions between nanotubes and n-Si and lowest number of nanotube walls are all fundamental parameters to improve the device photovoltaic efficiency. Multiwall carbon nanotubes have been synthesized by chemical vapour deposition in an ultra high vacuum chamber by evaporating a given amount of iron at room temperature and then exposing the substrate kept at 800 _C at acetylene gas. The amount of deposited iron is found to directly affect the size distribution (inner and outer diameters), therefore the number of walls of the nanotubes.