Low temperature plasma synthesis of silicon nanocrystals: a strategy for high deposition rate and efficient polymorphous and microcrystalline solar cells

It is generally accepted that increasing the deposition rate of amorphous (a-Si : H) and microcrystalline (μc-Si : H) thin films grown by plasma enhanced chemical vapour deposition leads to a deterioration in the films' properties. This 'common sense' rule places an upper limit on the deposition rate for a given film quality and thus on photovoltaic device efficiency. While this applies to a-Si : H and μc-Si : H films produced by the dissociation of silane and grown via the 'radical path', we have found that it is possible to increase the deposition rate and still improve or maintain film properties when the deposition process involves not only radicals but also silicon nanocrystals produced in the plasma. This will be illustrated in the case of polymorphous (pm-Si : H) and microcrystalline (μc-Si : H) materials and solar cells. The transfer of these processes from small area reactors to industrial ones is an exciting challenge for the low pressure plasma community, and one whose solution will open a route to lower cost photovoltaics in particular and large area electronics in general.