Sorry about the delay in this posting. Real life has been busy.
Solar energy is an obvious candidate for a long-term solution to many of our energy problems. The amount of power reaching the surface of the earth is on the order of 350 W/m2. We could meet the world's projected energy needs in 2030 by covering around 250 km by 250 km with 10% efficient solar cells. Unfortunately, the total surface area of all photovoltaics ever manufactured is less than 0.1% of that. (This is why being able to produce photovoltaic cells by printing processes would be great. Hint: estimate the total area printed by the New York Times in a month.) There are a number of challenges involved in solar. Why might "nano" broadly defined be a big help? Let me give three examples from the large wealth of ideas out there.
1) Semiconductor nanocrystals as absorbers. Because of the beauty of quantum confinement, it is possible to make semiconductor nanocrystals out of a single material, and use different sizes to capture different parts of the solar spectrum. Moreover, there is evidence (after some controversy) that nanocrystals may enhance "multiexciton generation" (e.g., here and here). In a traditional solar cell, a photon with energy twice as large as the semiconductor band gap will generate an electron-hole pair (which must be ripped apart somehow), and inelastic processes will lead to the excess (above the band gap) energy being lost as heat. However, at some rate, instead you can generate two band-gap-energy pairs. The idea is that the rate of that process can be enhanced in nanocrystals, since conservation of "crystal momentum" can be relaxed in materials that are so surface-dominated.
2) Nanostructured materials for photoelectrochemical cells. There are a number of proposals for using electrolytes in solar applications, including dye-sensitized solar cells. In this case, one would like to use a high surface area anode, such as nanostructured TiO2 or some similar nanostructured material. Moreover, instead of using organic dyes as the absorbers and sources of photoexcited electrons, one could imagine again using semiconductor nanocrystals.
3) Plasmon-enhanced photovoltaics. One way to try to boost the efficiency of solar cells is to get the light to hang around the absorber material for longer. One compact way to do so is to use plasmonically active metal nanoparticles or nanostructures as optical antennas. The local fields near these structures can enhance scattering and local intensity in ways that tend to boost performance, though resistive losses in the metal may limit their effectiveness. It's worth pointing out that one can also use plasmonic antennas as sources of hot electrons, also interesting from the photovoltaic angle.
There are many more ideas out there - I haven't even mentioned anything about nanotubes or graphene. While the odds of any individual idea being a truly transformative breakthrough are small, there are probably more clever things being proposed in this area now that at any time ever before, thanks to our ability to manipulate matter on very small scales.