Solar cells are becoming more viable sources of energy -- and as they become more efficient, they're only getting smaller and cheaper to produce. Liquid nanocrystal cells are traditionally inefficient at converting sunlight into electricity, but by adding a synthetic ligand to help transmit currents, researchers at USC have improved their effectiveness. The advantage of these liquid solar cells? They're cheaper than single-crystal silicon wafer solutions, and they're also a shockingly minuscule four nanometers in size, meaning more than 250 billion could fit on the head of a pin. Moreover, they can be printed onto surfaces -- even plastic -- without melting. Ultimately, the goal of this research is to pave the way for ultra-flexible solar panels. However, the scientists are still experimenting with materials for constructing the nanocrystals, since the semiconductor cadmium selenide they've used thus far is too toxic for commercial use.