May 03 2019

Organic Solar Cell Breakthrough

It’s interesting to try to envision the energy infrastructure 50 and 100 years in the future. Which technologies will prove the most cost and carbon effective? Of course, this can be a bit of a self-fulfilling prophesy – the technologies we think will work will be the ones we invest in and develop. This is why I think we should hedge our bets by developing every viable option. Can nuclear fission be made cost-effective? Will fusion ever be practical? How efficient can solar get, and what grid storage options will work best?

Having said that, I do think it’s pretty clear that solar energy will play an increasing role in our energy infrastructure. As of 2018 solar represents only 1.6% of energy production in the US, but is growing rapidly. Most solar panels in use today are silicon based. They are getting cheaper and more efficient, and have already crossed the line to cost-effectiveness in most areas, and within a decade should be cost effective everywhere.

Solar, of course, is an intermittent energy source, not on-demand. Right now, with such little penetration, you can basically use the grid as your battery – put energy into the grid when you produce more than you use, and take energy from the grid when you need more than you produce. This is why net metering laws are so important – making sure that customers with solar get full credit for the excess energy they produce. This strategy will work until we get to about 20% penetration. Then storage will be necessary to get to higher levels of solar in the mix.

Storage can be individual, like having a power wall in your home. Or it can be massive grid storage run by the power companies themselves. Likely we will have both in the future. Exactly what we will do for storage, in my opinion, is one of the open technological questions.

Meanwhile increasing the use of solar energy will be improved by continued advances in solar photovoltaic cell technology. Here there are three main contenders. The first is silicon-based rigid (wafer-based) solar cells. This is the kind already in use (about 95% of the solar market), and is experiencing steady incremental improvement every year. This technology will also benefit from increased mass production.

The second is thin-film photovoltaics, which use a variety of materials but at present represent only about 5% of the market. However, there is a potential emerging technology based on perovskites, a class of abundant materials with excellent properties for solar cells and possibly even batteries. Perovskite thin-film solar panels can be cheaper, thinner, and more efficient than silicon wafers. However, they have a fatal flaw – they are not stable. They break down too quickly over time. The focus of perovskite solar research is figuring out ways to make them stable, without sacrificing too much of their benefits. I suspect this will eventually work (the problem does not seem insolvable) but we don’t know. They may not figure this out before other technologies make it obsolete.

The third category, and the focus of the news item, is organic solar cell technology. These are made from organic polymers, and so also use abundant materials without the need for any rare or precious material. Organic solar cells have two limitations, one currently fatal and one not. The not-so-fatal limitation is efficiency. They are about half as efficient current silicon solar cells on the market.

Commercial silicon solar cells are in the 17-20% efficiency range, with prototypes already hitting 26% – which is getting close to the theoretical limit of 29% for silicon (called the Shockley–Queisser limit). So pretty soon we will likely have commercial silicon solar cells with close to 29% efficiency. This is more than enough so that an average home can generate more than 100% of its electricity use through rooftop solar. The theoretical limit for perovskite is 31%, a little higher. Using expensive exotic materials, we have developed solar cells with 44% efficiency, but this will never be for commercial use (but good for satellites and similar applications).

There are ways around this limit, by the way, such as using multiple junctions and quantum dots and other emerging technologies. The real question is always cost effectiveness and scalability.

Current organic solar cells are at about 11% efficiency. Just 15 years ago, however, that is where silicon solar cells were. This is low, but not fatally so. The advantages of organic solar cells are more than enough to counterbalance this disadvantage. Organic solar cells are thin, flexible, cheap, easy to mass produce, and easy to install. There are many situations in which installing solar that is half as efficient but at a quarter the price makes sense. The flexibility, and also the ability to make it translucent, also increases possible applications. So you could have more of your house covered with organic solar, to make up for the lower efficiency, with a lower overall price.

This is all good, and as the efficiency of organic solar climbs it will become relatively more and more cost effective as well. For these reasons organic solar may be the ultimate solar of the future.

However – there is still one fatal flaw that needs to be overcome, organic solar is also not stable. The surface tends to react with oxygen in the air and in moisture and breaks down, losing its efficiency. You can cover it in a clear sealant, but this increases the cost, makes the cell thicker, and may reduce flexibility. So you sacrifice some of the benefits or organic by fixing its major problem, and then you basically have something closer to silicon but half as efficient.

This brings us finally to the news item – researchers have developed a method for making organic solar cells more stable without sacrificing any of its advantages. The main problem with organic solar cells is that they have conjugated fullerene derivative Phenyl-C61-butyric acid methyl ester (PCBM) on their surface. These are electron-accepting molecules that are responsible for its reactivity with oxygen. They developed a method that essentially uses adhesive tape, along with pressure and heating, to pull the PCBMs off the surface. This removes 94% of the PCBMs and significantly improves the durability of the organic cells, even in wet conditions.

The researchers also argue that this is a scalable cost-effective process. Will this result in an organic solar-cell technology boom? It’s too early to tell, and the researchers are calling this a “pathway” to commercialization, so it sounds like we are not there yet. But this is a significant improvement that does make organic solar technology seem more viable.

If you follow solar technology news, then news items like this are a familiar, even weekly, event. It’s always hard to tell which of these “breakthrough” will pan out, and which will never reach commercialization. They do tend to collectively contribute to the steady incremental improvement in technology. Many of the techniques in the lab that we read about are probably 10 years or more away from commercial products, if they do work out. So we are always getting a blurry vision of the future.

But my take-away from all this collective solar science news is that there are various options and the technology is making steady progress. We are seeing the fruits of this progress (if a decade delayed) in commercial products. It is highly likely that by 2030 we will have significantly cheaper and more efficient solar cells, and by then we may be seeing commercial pervoskite or even flexible organic solar cells. The solar cells of 2050 may be using a technology I have not even read about yet.

The fact is the Earth is bathed in free solar energy, more than enough to run our current civilization. It’s hard to imagine that harvesting that energy will not be a major part of our energy infrastructure in the future. It may eventually get to the point that much of our stuff is covered in cheap, thin, flexible solar harvesting material. Anything that uses energy will take the free energy from the sun at least to supplement its needs. Sure, this will take decades to achieve, but we are steadily getting there.

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