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The kitchen chemistry of photovoltaics

Ted Sargent is introduced as “the boy wonder of nanotechnology”, based on the strength of his book, “The Dance of Molecules”.

Sargent introduces us to the beautiful silicon lattices that make information processing possible. We can etch so many transitors in silicon and pass electrons through them because they’re so pure. A sheet of silicon has one atom of a trillion out of place.

This purity comes at a price – high temperatures, the inability to interact with other materials. Sargent wonders whether we could make discoveries based on paradigms other than purity and perfection. When we look at nature – the conversion of 6% of the sun’s energy via photosynthesis in a leaf – we’re looking at much messier, goopier stuff. But there’s perfection on a molecular layer – the “absolutely beautiful, seductive, curvy, gorgeous” shape of DNA. The perfection on the molecular level allows the creation of the goopy, spatially unique structures for photosynthesis.

Goopiness can camoflague amazing abilities – an image of the retina looks messy, but is capable of detecting as few as 5 photons. (The stage lights, he points out, are each putting out a billion billion photons.)

The dream of nanotech is to build things outside the abilities of microelectronics, using these “perfect molecules” for social and human benefit. We don’t want to move the molecules, Sargent tells us, though we can – the production of buckyballs is evidence that we can work on that level. But what we really want to do is let chemistry do the heavy lifting for us. The question he’d like to answer is, “Can we specify function, like photosynthesis or detection of cancer, and then let nature do the work for us?”

Sargent shows us a colleague doing “kitchen chemistry”, mixing compounds to create molecule-sized semiconductor particles. We see eight different beakers, each containing particles of different sizes – they’re a different color due to the quantum size effect. “We change the size of the tympani over which the electron wave passes and we change the color of light.” He tells us, “Nature did the work, the scientist choreographed it.”

In Sargent’s lab, he’s “painting” sheets of glass with particles suspended in a solvent. The particles are identically sized quantum dots. Coat a piece of glass with them, put electrical contacts on the other side and you’ll collect charge from solar light.

A more efficient process involves spraypainting nanodots onto glass. Sargent’s lab has demonstrated that these quantum dots can capture the infared as well as visible light. A recent piece of research (not Sargent’s) demonstrates that photosensors built via this process can be 100 times more efficient than traditionally etched ones – this is a clue that we may be able to use nanodots to make more efficient photovoltaics.

This technique doesn’t rely on being perfect, but on letting molecules be perfect enough. “Perfection scales down beautifully, but molecular design scales up.” The underlying molecular architecture could allow us to “open our arms wide and capture huge amounts of solar energy.”