| 11 January 2012
Picosun Oy, a global Atomic Layer Deposition (ALD) equipment manufacturer based in Finland, reported the final results of a multinational three-year research project aimed at pioneering nanomaterials that would make thin-film solar cells much more efficient and cheaper to manufacture.
Funded through the European Union Seventh Framework Programme (FP7), this project— dubbed ROD-SOL—led to energy conversion efficiencies of greater than 9%, with good long-term cell stability. The new silicon nanorod-based concept requires significantly less active photovoltaic material by growing light-trapping nanorod "forests" (thickness from < 1?m to a few ?m at most) on cheaper substrates, such as glass or flexible foils. Because these densely packed tiny silicon columns are three-dimensional, their active surface area is much larger. In addition, the location of the p-n junction is much closer to the surface than in conventional solar cells, which improves the minority carrier charge transport and, therefore, the amount of electricity that can be extracted from the cell.
To prevent recombination losses in the active photovoltaic layer, the nanorods were coated with a passivation barrier, in this case an ultrathin ALD-deposited Al2O3 film. “The gas-phase, surface-controlled and self-limiting nature of the ALD process ensures that even the deepest and narrowest between-the-rods nooks and crannies will be reliably covered with 100% uniform, conformal and pinhole- and defect-free passivation film,” says Picosun's Managing Director, Juhana Kostamo. The current collector on the top of the cell is a transparent conductive oxide (TCO) layer, which Picosun can now produce with a High Volume Manufacturing (HVM) batch ALD system developed specifically during the ROD-SOL project.
Picosun concludes that ROD-SOL's novel silicon nanorod cell concept shows promising potential for increasing the efficiencies of thin film solar cells and reducing manufacturing costs. As might be expected, the Finnish company is “especially satisfied” that their ALD played a central role in making this happen. “The next step for us will definitely be [to continue] developing ALD technology and equipment for solar energy applications,” says Minna Toivola, who coordinated the ROD-SOL project on Picosun’s end. “ALD for Si solar cell passivation—which can increase the cell efficiency [by] 1-2% and is, in many ways, [a] better method than what has been "traditionally" used [for] this purpose—, for example, has evoked a lot of interest recently in the global PV community.” The technology used in ROD-SOL to create the more efficient thin-film solar cells is still very new and might require some time and further development to go commercial. “But, the promising over 9% efficiencies and good long-term stability achieved already in this three-year project really make this technology worth of further investigation and hopefully we can see the first Si nanorod cells available to consumers in already a few years,” Toivola says.
Written by Sandra Henderson, Research Editor, Solar Novus Today
