Solar cells that imitate plants


Every year, approximately 2.5 million exajoules of solar energy reach the Earth. That's about 5,000 times the amount of energy consumed by people each year. The trick is collecting it and converting it into electricity cheaply and efficiently. Plants do a good job of that. Turns out scientists have been working on ways to imitate nature's photosynthesis since 1912. And they're still at it. This week, the scientific journal Chemical & Engineering News posted two deep articles on the subject. The first is about the molecular mysteries of photosynthesis, including whether it's as efficient as one would expect from a process that has more than a couple billion years of evolution behind it. From the article, "Harnessing Light":

Water-splitting is key to the renewable production of hydrogen gas and other energy fuels, and doing so with inexpensive catalysts, as plants do a billion times per day, would be a huge step forward for solar power research. But the photosynthetic process has some other secrets, too, that scientists are only just figuring out, such as how photosynthetic organisms can tame light without suffering too much radiation damage, the plant equivalent of a sunburn…

(One) contentious area is the question of how efficient photosynthesis actually is..

One reason that plants don't store fuel efficiently is that "plants' goals are different than our goals," says (Devens Gust, a photochemist at Arizona State University). "The plant's goal is to live and reproduce, not to store energy for humans."

Even so, many researchers turn to photosynthesis for inspiration on how to achieve humanity's energy needs. They hope to mimic the early, energy-efficient light-harvesting steps of photosynthesis, but then direct the harnessed light energy entirely toward producing fuel instead of growing a plant.

"When we think about ways to harvest sunlight," Gust says, "it seems natural to look to photosynthesis for ideas. It has been around for the longest time, and it works on the largest scale."

That's where biomimicry comes in. The companion article in C&EN describes research by Gust and others to build small molecular systems that imitate what plants do using a biological "machine" consisting of 20 proteins. From the second article:

The overall efficiency of photosynthesis for making sugar fuels is low–only about 2-3%–because plants' primary goal is to live and reproduce, not store fuel. But the first steps performed by (the plant's photosynthesis system) are much more energy efficient, about 30%. So researchers want to "take the basic chemistry and the basic physics of the photosynthetic reaction center" and build a molecular version in the lab, says Devens Gust, a photochemist at Arizona State University. In effect, they want to produce a molecular photovoltaic that, like (the plant's version), produces a current of electrons that could be used to split water and thus drive the production of a fuel such as hydrogen gas.

Harnessing Light
Attempts To Mimic A Plant's Light-Harvesting And Water-Splitting Megamachinery