Receiving my household fuel bills, as I do with monotonous regularity, tends to focus my mind both on energy conservation and the cost of energy generation. Of course, this line of thinking quickly extends into the realms of professional interest as well and increasingly we see one of our particle characterization systems being used in a process that helps manage costs in the manufacture of photovoltaic (PV) cells.
Manufacturing PV solar cells
Photovoltaic (PV) solar cells, which convert light energy directly into electricity, are fabricated from a polysilicon ingot. This is turned into wafers using a silicon carbide (SiC) wire-saw cutting process. PV cells are then created by coating these wafers with conductive inks and silicon nitride passivation layers.
Two of the most significant areas for savings in the manufacture of PV solar cells have been identified within the wire-saw wafer cutting process. This process involves a stainless steel wire passing through a slurry of SiC particles – the steel wire is the ‘blade’, the SiC the ‘teeth’ and glycol is present as the ‘coolant/glue’.
Over time the slurry becomes less effective as it is contaminated with waste silicon particles – ‘kerf’–and as the SiC particles wear down.
Reducing kerf loss
Kerf is the particulate silicon gouged from the ingot when cutting each wafer from it. The narrower the cut, the smaller the kerf loss and the more wafers that can be cut from each ingot.
Cut width and kerf loss can be managed by reducing the particle size and particle size distribution (PSD) of the SiC particulate ‘teeth’ in the slurry. Finer particles make finer cuts with the same size steel wire.
Recycling the SiC slurry
The cost of the SiC slurry is the second most important cost in wafer manufacture after the polysilicon itself. One estimate places the operating cost of running 10 wire saws as high as $16M per year.
Recycling of the slurry can recover up to 80-90% of the original SiC content, so the potential for cost savings are huge – some estimates are as high as 40-50%. The ability to monitor the slurry for build up of fines to is therefore key.
Applying FPIA 3000 imaging
Combining the ruggedness and speed of measurement with enough image definition to detect the presence of fines at the sub micron level, the FPIA 3000 delivers particle size, PSD and shape data needed to optimize both these resource saving measures.
The application note: Analysis of wiresaw abrasive slurries to facilitate recycling using the FPIA-3000 provides specific details on this topic and, is well worth the read for those PV solar cell manufacturers out there who – like me – find themselves preoccupied with the idea of saving money!