Nature's Blueprint for Going Green

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A model of a self-assembling group of molecules that mimic photosynthesis by transferring electrons. Molecular structures like this one may someday be used in organic solar cells.

The sun—our lives literally revolve around it. Now, researchers are working to harness its power as the ultimate renewable energy source, making it an unlikely ally in the fight against climate change.

You might be thinking, “Aren’t we already using solar energy for power?” We are. From small solar-powered calculators to the larger solar-powered highway traffic signs, limited applications of solar energy can be found in our everyday lives. However, the current technology used to capture and convert the sun’s energy is very expensive to produce, preventing solar power from becoming a widespread solution to our world’s dependence on carbon-emitting fossil fuels.

One strategy is to develop solar cells that are easier and cheaper to manufacture. Current solar cells are made of silicon, which is derived from sand and formed into crystals through an energy-intensive, expensive process. “Organic photovoltaics,” a promising alternative, are solar cells made primarily of organic molecules—molecules that contain carbon. The atoms that compose these molecules are linked in ways that make them very flexible. By contrast, silicon atoms are arranged in tightly packed units, making silicon products very rigid.

Because of their inherent flexibility, organic photovoltaics may be manufactured in large rolls, or even as inks or paints, which could easily be spread over large surfaces. While organic photovoltaics will not be nearly as efficient as silicon-based solar cells, they may eventually be produced at just a fraction of the cost. Learn more about the cost and production of solar cells.

Applications of solar energy encompass more than just advancements in organic photovoltaics. For example, at night, solar cells can no longer receive energy from the sun. Therefore, it is important to be able to store the energy acquired during the daylight hours and use it at night. Photochemical fuel cells do this. They utilize similar strategies found in organic solar cells to collect light, but couple it to methods that produce usable fuels, such as hydrogen. These fuels can then later be used to power your home or workplace, or the electricity generated from the solar fuels can even be shipped to a new location where sunlight is not as abundant.

To design these solar-powered devices, we look to nature’s model for converting sunlight into usable energy—photosynthesis. For eons, photosynthesis has been sustaining life on our planet, and the evidence is not difficult to find. From the food we eat to the air we breathe, it is an evolved, complex, and finely tuned process that supplies the inspiration for a variety of applications.

Of particular interest is photosynthesis’ initial steps, in which sunlight is collected by a plant’s light-absorbing molecules. That energy is transferred to a place where it strips an electron from a special pair of molecules. This free, high-energy electron is then transferred from molecule to molecule within the plant cell until it is far enough away from its source to be stable. This process then provides the energy to convert carbon dioxide into usable products such as carbohydrates (sugars).

We are now working to apply these fundamental principles to develop new solar energy conversion technologies, known as “artificial photosynthesis.” Our goal is to capture and transfer energy as nature does, but to also generate fuel we can use. The diverse talents of many scientists from many different fields— chemistry, biology, physics, and engineering, to name a few—will be required to recreate this entire process. But first, to fully understand photosynthesis as a whole, each step must be carefully studied and replicated.

Our lab focuses on the basics of energy and electron transfer processes. How is light harvested and transferred? How quickly are electrons moved from one molecule to another? And how do we finally take those electrons and couple them to systems that can split water into oxygen and hydrogen to use as fuels?

First, we can monitor an electron’s travels by observing how the molecules through which the electron is moving absorb light. Visible light, or white light, is actually comprised of seven different colors—red, orange, yellow, green, blue, indigo, and violet. Objects absorb certain colors of light and reflect the others—for example, if your shirt is green, it is reflecting the green light and absorbing all the others.

When a molecule loses an electron, its structure changes slightly, causing it to absorb different colors of light than it would if the electron was still present. Likewise, when the electron returns, the molecule’s light absorption pattern changes back to its original form. Observing these patterns allow us to infer where the electron is traveling and how quickly. With advanced instrumentation, we can detect changes that occur within a picosecond, or a millionth of a millionth of a second (10-12second)!

Our goal is to create a series of molecules, or building blocks, through which electrons will travel, like a circuit, to eventually power a light bulb or generate fuel. We want these molecules to give up electrons quickly, take them back slowly and travel long distances efficiently, allowing the electrons to effectively reach their final destination where they can be used appropriately.

These building blocks must be easy to produce and be able to self-assemble. “Self-assembly” refers to the process of disorganized molecules in solution arranging themselves into a specific pattern spontaneously, forming a larger structure. This property is crucial to producing solar cells from liquids, like a paint or ink, rather than solids.

We have recently made exciting progress in developing these self-assembling structures with photosynthetic capabilities. Molecules we’ve designed arrange themselves in solution into distinct horizontal units that stack vertically. Shining light on them results in electron transfer both within each building block, or horizontal units, and also between their self-assembled neighbors, or vertical units. This means that the electrons can potentially travel far enough away from their source to be utilized in a circuit.

Although this is an important step forward, many more will be necessary before a complete, artificial photosynthetic system can be created. For example, while the building blocks that we’ve created do self-assemble, they stack together in a helical formation (each level is rotated slightly relative to the previous), rather than directly on top of one another. This makes it difficult for the electrons to travel between all subunits. We are now working on a different arrangement of these building blocks so that electrons can travel more freely and efficiently.

Even when this is achieved, the transition from basic research findings to prototypes to final mass production will not occur overnight. The challenge facing scientists today is converting the principles of basic research discoveries into new technologies. Our hope is that someday, by understanding the most basic principles of photosynthesis, we can develop efficient, economical methods for solar energy conversion, providing one answer to the multi-faceted energy problem facing society today.

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