Jul 20 2017

Bismuth and Solar Cells

bismuth ocyodideThere is something appealing about the fact that while there was so much controversy and public debate about global warming and energy production, solar cell technology incrementally improved in the background, largely unnoticed, until it became an actual cost-effective option for energy production. There were no breakthroughs or big announcements, just a slow increase in efficiency and decrease in cost.

Slow incremental changes add up, like a conservative but reliable investment. In 1941 the first silicon-based solar cells were developed with <1% energy conversion efficiency. In 2009 we broke 25% efficiency. The current record efficiency for mass-produced solar cells is 26.6%.

While more efficiency is always welcome, this is now more than enough for most practical uses of solar cells. The solar panels on the roof of my house (which doesn’t even cover my entire available roof space) produces 100% of the energy (averaged over the year) that my house consumes. Obviously, your mileage may vary, depending on roof orientation, shading, and geographic location. I live in CT, which is hardly a sunny state, so anywhere in the US should be viable for solar energy. If you life somewhere like Arizona, it’s a no-brainer.

With many technologies there is one feature that is considered a limiting factor, or at least the most important factor, at least by the public, and tends to dominate discussion. For computers it used to be processor speed, for cameras it was megapixels. Once, however, we get to the point where these parameters are generous enough to no longer be the dominant limiting factor, we start to pay attention to other factors. Make no mistake, these other factors were important all along, they were just often neglected by the public who focused on one number.

I have been reading about solar panels for decades, and definitely popular discussions were dominated by reference to the energy conversion efficiency. Now that we are north of 20%, however, it is clear that other factors and now more critical. The parameter that I think now dominates discussion is the overall cost of mass produced solar panels. While the price has also been dropping every year, the cost is still high – tens of thousands of dollars for a single home, depending on how much of your roof needs to be covered.

The primary factor that is keeping costs relatively high, apparently, is the fact that silicon-based solar cells have a very low tolerance for defects. This means the silicon purity needs to be very high, which in turn drives high manufacturing costs.

Solar researchers have therefore been looking for alternatives to silicon that have a much higher tolerance for defects. A great deal of recent attention has been focused on perovskite. If you have not heard about perovskite yet, you probably will (or, as we will see, perhaps not). A recent report noted:

In the past decade, research into perovskite solar cells has boomed. At least in the lab, the efficiency of perovskite devices is now more than a match for those based on silicon. In 2006, the first perovskite photovoltaic converted 2.2% of photons into electrons1; by 2016, that figure was 22.1%. Silicon rooftop panels have an efficiency of 16–20%; perovskite cells could in theory could reach 31%. And even higher efficiencies might be achieved by combining silicon and perovskite devices.

So again we are well into the acceptable efficiency range, but the huge advantage of perovskite is that they have a higher defect tolerance than silicon and are therefore much cheaper to mass produce. This technology could break out of the small incremental advances and put us on a different path to cheap and abundant solar panels.

But there is one issue – current perovskite solar cells are alloyed with lead. It is not clear that this is a problem, but it is a concern. Lead is a highly toxic heavy metal, and its use in mass producing anything is a concern. It would also be a concern for disposal. I don’t think this is a deal-killer, but it is not optimal.

As with other similar technologies (like batteries) the optimal solar cell has multiple simultaneous properties: high energy conversion efficiency, long lifespan, resistant to damage, cheap to produce, easy to install, and the manufacture, use and disposal (or recycling) has low environmental impact (meaning not a lot of toxic substances).

Perovskite is efficient and cheap, but contains a toxic heavy metal. Researchers, therefore, are continuing to search for still other solar panel materials that would be even better than perovskite.

A recent study explores the potential of bismuth oxyiodide. Bismuth is similar to lead (in that it is a dense metal), but is extremely non-toxic. The researchers found that bismuth oxyiodide is tolerant to defects, therefore inexpensive to manufacture, has potential efficiencies rivaling perovskite, is stable, and non-toxic. Further, it can be mass produced with existing techniques.

It is too early to tell what the next generation solar panels will be. There is also a class of solar panels that are thin, flexible and cheap. They have less efficiency than current silicon, however, and make up a minority of the market.

What is encouraging is that the basic science of solar voltaics has advanced to the point that applications are now coming fairly quickly. We can’t predict what will dominate the market in a decade, but there are now several viable options all of which promise to be significant improvements over current silicon-based solar panels (which are already cost effective in many contexts).

This can only improve our energy infrastructure, with the potential of greatly expanding our green energy production. However, the grid will need to be significantly upgraded to handle distributed production of energy. Right now the grid can only handle about a quarter of houses having solar panels. As more homes try to add solar, they (or the electric company) will have to pay to update the grid in their area to handle the production.

Battery technology (or some mass storage technology) also needs to improve. If every home could also have a power wall type storage that was cheaper with a greater capacity and life cycles, that could have a huge impact.

Further, while solar energy will help reduce our greenhouse gas production, we cannot assume it will be enough fast enough. But I am excited to see this technology really come into its own.

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