Silicon wafers are at the heart of solar cells, and global demand has risen sharply in recent years. However, a barrier for further uptake is cost. Manufacturing and material production are not cheap, with over 50% of the silicon machined into unusable dust. Wastage has become a serious cost burden that is causing increasing concern.
Researcher Dr Stefan Janz said: “With our method, we can avoid almost all of the losses that occur during the conventional production process.”
For the new process, researchers first manufacture chlorosilane and heat it to over 1000°C and mix it with hydrogen. Critically, however, the silicon isn't just grown randomly, instead it is coaxed into a desired crystalline form.
The process uses chemical vapour deposition. Here, the gaseous silicon flows past the substrate – itself a silicon wafer – and coats its surface. In this way, the wafer grows atomic layer, by atomic layer.
To make the wafers separable from the substrate, researchers introduce a mechanical breakpoint beforehand in the form of porous silicon. These substrates can be reused several dozen times. Not only do the substrates serve as a ‘backing plate’, they also provide the necessary crystal.
For solar cells, a silicon crystal is needed for the atoms to be arranged in a regular array, much as they are in a diamond. The substrate ‘informs’ the atoms, so to speak, about how they should arrange themselves from the gaseous silicon.
Janz added: “In this way we get a very good mono-crystal, which is the best type of crystal, and the wafers are of the same quality as those produced using conventional methods.”
By growing the wafer exactly as needed, it does away with the need for cumbersome machining. Another advantage of the new process is that it allows the wafers to be made thinner, further reducing material and processing costs.
A spin-off company, NexWafe, will produce wafers for the general market from 2017.