The main driving force for developing plastic electronics – the use of semiconducting polymers to make logic devices, light emitting diodes and displays, and solar cells – is the hope that these materials can be processed very cheaply. Because these materials are soluble, devices can be made by processes like ink-jet printing or screen printing. Compared to standard silicon-based electronics, the performance of these devices is often not very good, but the fact that you won’t need the massive capital expenditure of a conventional silicon fab tilts the economics towards the plastic materials, at least for applications where cost is more important than performance. But Nature this week reports an interesting twist – a group from the Seiko-Epson labs in Japan report a new way of printing silicon directly from solution (see also the Epson press release here).
The method is based on using polysilane as a precursor material. Polysilane is essentially the silicon based analogue of the well-known polymer polyethylene, consisting of a long chain of silicon atoms, each of which has two hydrogen atoms attached. But unlike polyethylene, polysilane is both unstable and very difficult to work with, being insoluble in most common solvents. The Japanese group got over this problem by starting with a five-membered ring version of the polysilane molecule – cyclo-pentasilane (this is the silicon analogue of cyclopentane). They found that polysilane was soluble in solutions of this precursor, and these solutions could be ink-jet printed and converted into pure silicon layers by a simple heating step.
The silicon layers formed this way are amorphous, not crystalline, and their electronic properties are not very good compared to silicon films prepared by more conventional routes (though they are better than most polymer semiconductors). Plastic electronics still has some advantages over this new process, which requires temperatures too high for the use of plastic substrates. The printing step is also complicated by the need to exclude water and oxygen. Nonetheless, it’s an important step forward towards the development of low-cost electronics for applications like large area displays and cheap solar cells.