The size of the challenge for OPV

In a recent paper titled Green chemistry for organic solar cells, published (open access) in Energy and Environmental Science, the authors Burke and Lipomi ask an interesting question: how much organic semiconductor do we need in order to make a sizeable dent in global energy consumption with organic photovoltaics (OPV)?

[I]f 10 TW of the 30 TW of power demanded in the year 2050 is to be generated by photovoltaics, and if organics account for 500 GW to 5 TW, then 10–100 kilotonnes of organic semiconductors will be required… given an average solar flux of 200 W m-2, a module efficiency of 5 %, a typical thickness of the active layer of 200 nm, and a density 1000 kg m-3. This extremely rough estimate assumes 100 % yield of working modules, no waste in the coating processes, and infinite lifetime of devices.

To put 10,000–100,000 tonnes in perspective, polyethylene is the most common plastic with an annual production of 80 million tonnes (according to Wikipedia, but it sounds reasonable). Taking the higher end of their rough estimate, we’re talking 0.1 million tonnes over nearly 40 years—basically nothing compared to commodity polymers. But organic semiconductors have more complicated structures than commodity polymers and require more complicated chemistry. Burke and Lipomi also compare their estimate with pharmaceuticals, with their figure “2–3 orders of magnitude greater than those of top-selling small-molecule drugs of similar structural complexity”.

Making this amount of high quality polymer—with a specific molecular weight and acceptable PDI, high purity and low batch-to-batch variation—is a challenge. Material currently on the market is terrible, meeting none of these requirements.

Both conjugated polymers and structurally complex drugs require synthetic sequences of 5–10 steps to produce. The multi-tonne synthesis of conjugated polymers will be a challenge in process chemistry with few precedents, and will in consequence the materials that could be seriously considered for installations that cover many square kilometers.

The challenge is made even greater by the need for it to be cheap because if OPV isn’t cheap, it’s commercially inviable. If OPV is to stand a chance researchers must remember that they’re working on a technology that has to cost <$10 m-2 to compete with fossil fuels.[^price] Burke and Lipomi point out is roughly the same price as carpet.

No matter how efficient your polymer, if its synthesis can’t be scaled up or is too expensive (e.g. PCDTBT), it will fail. It worries me a little that this is lost in the quest for high device efficiency (because that’s what makes for a HIGH IMPACT[^IMPACT] paper). Taking a “not my problem” approach is unacceptable. We must stop passing the responsibility off to process chemists and engineers and instead remember the scale of the problem that OPV is trying to solve, otherwise it’ll never make the slightest dent in our global energy consumption.

*[PCDTBT]: poly[N-9′-heptadecanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole)]

[^IMPACT]: Just to be clear, I’m being flippant. [^price]: Burke and Lipomi get this price from Lewis and Nocera.

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