Aug 26 2022

Organic Solar Cells and Other Solar Advances

When I first started reporting about solar cell technology around 2005 the best commercially available silicon solar cells (photovoltaics) had an efficiency rating of about 11% (the amount of the sunlight hitting the panel that ultimately gets converted into electricity). They were expensive, heavy, and didn’t product that much electricity, but still it was enough for early adopters and we were at the beginning of the commercial solar industry. That was just before the inflection point when solar started to take off.

I remember reading many solar power news item, detailing some incremental advance, but still with some limitations and uncertainty. But slowly, inexorably, these potential advances added up. Every year solar panels because a little better and a little cheaper. Now silicon crystal solar panels commercially available have efficiencies of over 20%, they have a minimum lifespan of 20 years but many are rated for 35-40 years (and some report 40-50 years), and their price has plummeted, down about 90% compared to 2010. The ultimate potential efficiency of silicon solar cells is often cited as 29%, but using various techniques higher efficiencies have been reported, such as this one in 2019 reporting a 31% efficiency.

The question is – how long will this trend continue? It’s hard to say, but it’s clear that advances in silicon solar panels are not done yet. Already the cheapest form of new power, it will likely continue to get cheaper and more efficient for the next 10-20 years, producing phenomenally cheap energy by historical standards. Even without any major new technological breakthroughs, just incremental tweaks on existing solar technology, this is likely to happen.

The question is, however, when and if there will be some major solar power technological breakthrough that changes the game? There are many possibilities here. I wrote recently about perovskite solar cells, which have the potential for higher efficiency than silicon, but have stability issues that still need to be fully worked out. There is also research into using techniques to optimize harvesting, using quantum properties or other high-tech approaches that can coral photons and super high efficiency.

Here is an example that in its current form is not useful for photovoltaics (despite the misleading headline). The technique uses light itself to trap photons in a thin layer of material until they are absorbed, resulting in near-100% absorption. This could be extremely useful for detectors and astronomical applications, but not so much for photovoltaics because it only works on one wavelength of light at a time. But – if they could make a multi-wavelength version, that would be something. I bring it up mainly to highlight the possibility that a single such breakthrough can achieve decades of incremental advance. Researcher are doing amazing things with metamaterials. They are also using AI to accelerate research by finding optimal solutions among billions of possibilities without having to test each one, doing years of research in days. So while we can essentially count on at least a couple more decades of incremental advances, we can also hope for a true breakthrough.

The main solar technology I want to discuss in this essay is organic solar cells. These are photovoltaic materials made from organic (carbon containing) polymers – essentially plastic. But these plastic can be made into an ink-like substance which can literally be printed using existing technology onto a variety of substrates, but hard and flexible. The advantage of organic solar cells is that they are cheap (about half the cost of silicon), versatile, flexible, and light-weight. Imaging just rolling out a sheet of organic cells onto your roof, and installation is mostly done. Or you could paint the side of your house with solar cells, or even replace your windows with them.

There has to be a catch, though, right, or else organic solar cells would be dominating the market. Right now commercial organic solar cells are just getting started with efficiencies of 10-12%. That’s the big disadvantage – much lower efficiency than silicon solar panels. They have half the efficiency, but also at half the price, so the cost-effectiveness is about the same. With mass production experts predict existing organic solar cells could be cut in price half again, making them twice as cost effective as silicon panels. But still, this is considered a huge limiting factor. If you want rooftop solar to produce 100% of your energy needs, there is probably not enough space for organic solar panels.

There is another disadvantage to organic solar panels – their lifespan is about 10 years, compared to about 40 for current silicon panels. So silicon panels are more cost effective over their entire lifespan, if you get 40 years out of them. However, introducing yet another variable, organic solar panels are completely recyclable. Silicon panels are more challenging to recycle.

But to put all this in perceptive, organic solar cells are about where silicon solar cells were 15 years ago in terms of efficiency, but also much cheaper, and with a shorter lifespan. The organic solar panels of today could compete favorably with the silicon panels of 2005, and if there were no competition from silicon panels they would likely be seen as a viable option. Further, we are seeing the same kind of steady drumbeat of organic solar cell breakthroughs as we did (and still do) for silicon, so there is every reason to think that organic solar cells will be much better over the next 10-20 years. For example, in the lab researchers have already achieved 18% conversion efficiency for organic solar cells. Scientists are also figuring out the specific limitations to efficiency, which will provide clues for how to crack it further. Other researchers are figuring out the longevity problem, such as this study showing an organic junction that can survive direct sunlight for 27,000 years.

The real question is – in the actual world, when will organic solar cells become commercially viable. The answer appears to be – just about now. Commercial production facilities are ramping up. However, because of competition from silicon cells, they will find different entry points into the market (other than rooftop solar). For example, organic solar panels could be ideal for the roofs of electric vehicles. They are light and flexible, and a 10 year lifespan is not as much of an issue. These cars won’t run entirely on their solar panels, but they can add a bit of free energy when parking in the sun.

Flexible organic solar panels can also power small wearable devices. Organic solar cells have another advantage over silicon – they can operate on low light levels, such as indoor light, and don’t require direct sunlight like silicon. These would be low energy applications, but could keep such devices operating and obviate the need to replace small batteries in inconvenient locations.

For now, therefore, it seems that organic solar panels will be complementary to silicon panels, rather than competing directly with them in the market. That’s a good thing, and could help not only expand the use of solar power but also bootstrap the industry, leading to more advances and production efficiency. Meanwhile, in the background, we will get that inexorable incremental advance until one day in the not-to-distant future organic panels can compete with silicon, depending on the specific application. It’s difficult to say if they will completely replace silicon (or perovskite) as they will be advancing too. There are even designs for hybrid panels, combining organic with silicon or perovskite, to get even higher efficiencies.

The bottom line is that we are in the middle of the solar energy revolution – and just somewhere in the middle, with plenty of room for further significant advances. As we are planning out our energy infrastructure over the next 20-30 years we need to take this into consideration.

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