Energy. There's never enough of it, at least in terms of our ability to harness it to produce useful work. And right at the center of our solar system, taunting us by being so near, and yet so far, is the Sun -- a big, powerful nuclear generator that produces enough nuclear reactions in a single day to pretty much meet any energy needs we could imagine well into the future. It's just so darned difficult to turn that raw solar energy into a kind of energy we can use.

That's why photovoltaics (solar cells, solar panels and similar technologies) is such an active area of research, attracting sharp minds like Paul Alivisatos, a professor of physics at the University of California, Berkeley, and one of the featured speakers at the 2006 Industrial Physics Forum. Such devices perform quite well in a laboratory setting, but the cost per unit and overall efficiency just don't scale up to sufficient performance levels to make them useful for anything more than select niche energy applications. After all, the US is a major energy hog: we need between 1 and 3 Terawatts of energy just to go about our daily lives. There is no solar technology currently in existence that can generate energy at that enormous scale while still being affordable and efficient.

What to do? If you're Alivisatos, you find a useful analogy in a jewelry store. Economically speaking, diamonds obey a fundamental scaling law that places constraints on what the buyer can afford: the bigger the diamond, the more it will cost. (We'll leave aside questions of quality, etc. for simplicity's sake.) So Alivisatos thought, instead of trying to make large crystals for solar cell applications, why not try to make them as small as possible instead? In fact, why not look to nanotechnology, specifically, colloidal nanocrystals?

Ten years ago, this wouldn't have been a viable solution, but Alivisatos pointed out that there have been many significant advances in our ablity not only to grow colloidal inorganic nanocrystals, but also to control their size, shape, and even branching (called interconnection), as well as their topology (they need to be hollow and nested to achieve the desired properties). Like most nanoscale materials, many of their unique properties are size dependent, but colloidal nanocrystals are also quite stable and can be processed in solution, just like polymers. Alivisatos thinks this makes them an attractive candidate for solar cell components.

He's working on a project (dubbed HELIOS) that seeks to turn solar energy into a usable transport fuel. It's brand new -- they submitted the proposal to DOE earlier this week -- but the concept is to convert photon energy into the energy found in the chemical bonds in gasoline. The goal is to go from the current dismal 1% efficiency of most solar cells to around 6% efficiency. There's some significant challenges, of course: it's really, really hard to control a material's properties and behavior at the nanoscale to a suitable level of precision. We've heard about solar's potential (and the pitalls) before, and it's true at the nanoscale as well: if those challenges can be met -- admittedly a substantial "if" -- nanoscale photovoltaic technology would combine very high efficiency with good scalability, making it a highly desirable energy source.

Alivisatos is interesting in building a type of hybrid solar cell out of the usual polymers paired with randomly oriented, self-assembling nanorods. Over the past few years, his research group has succeeded in controlling the growth of these nanosctructures to achieve very precise multiple branching, resulting in something that resembles a model of a carbon atom -- "tetrapods" of CdTe.

They've also managed to incorporate these nanostructures into hybrid solar cels to achieve a slight improvement in efficiency: currently around 3%. It's still far short of the target 6% efficiency, and Alivisatos admits his team is struggling with controlling material behavior at the crucial interfaces. But he believes they're on the right track, and that nanocrystal based solar cells will be a perfectly viable commercial technology in the future.