Black silicon could exploit potential of multicrystalline technology

Would you say that black silicon has a future?

I believe that it solves a major problem today in the PV industry concerning multicrystalline silicon, because it allows the use of diamond wire saws on wafers, which provides a substantial cost-benefit over slurry saws. In addition, black silicon provides a pathway to higher efficiency on those wafers through both the obvious optical gain of lower reflectivity, which results in higher current, but also in approved fill factor in voltage.

In recent years we have seen a shift from multi to mono wafers. One reason for this was the introduction of diamond wire sawing for monocrystalline wafers and the difficulty of processing diamond wire sawed multi wafers with the standard etching processes for multi. How can you solve this problem with black silicon?

Black silicon provides a nano-texture which is essentially independent of the surface of the wafer, so it works very well with diamond wire sawed cell and wafers. At least our technology does. I think that prior to the diamond wire saw and its rapid adoption, there wasn’t a strong argument to convert to black silicon simply for the efficiency gain, but when you combine this gain with the cost-reduction available from using the diamond wire sawing, then you get a solid argument to make this conversion.

So you developed a process? What is the background of your company?

Yes, we did. We do consulting and licensing in this area, and we also do small amounts of custom fabrication in applications outside of photovoltaics.

What is different in black silicon compared to standard silicon?

The key difference is the surface texture. We are using a nano-texture where the features are smaller than the wavelength of light. This allows you to get reflection down to single digit percentages. This happens on the surface of the wafer itself, so one gets more light into the cell, which consequently increases the current of the cell. If you take if further, in optimizing the cell process, you also get gains in the filling factor of the cell and also in the open circuit voltage increases.

What is the process that allows this to happen?

In our process we use a wet chemistry process called Metal Assisted Chemical Etching. This is a proprietary process that we own.  What we do is we take a very similar front-end process to what is currently used in acid etching of multicrystalline wafers, but we change it somewhat to adapt to the Metal Assisted Chemical Etching, where essentially we are using silver as a catalyst to do the nano etching on the surface of the wafer. One of the advantages of this process is that we can do it on existing equipment with minor modifications, so you don’t need substantial new capital expenditure to incorporate it.

Why don’t you get these advantages with mono wafers?

The standard process for texturing already achieves a relatively high absorptive surface, so there is much less gain in applying the black silicon technology to monocrystalline silicon. However we do believe that once the black silicon technology is more widespread, there is a possibility that it could be adapted also in the mono sector.

We already see the emergence of products with black silicon coming to the market — could you estimate current existing production?

This is hard to estimate because it is on a really rapid adoption curve, and it only really started being used in production in the last year.