One of the major problems with solar power is that you only get energy when the sun is out. When the sun is down (or even just low on the horizon or it's cloudy), then you need another source of power.

Right now, batteries are often used to store that energy for later use, but weight and inefficiencies limit the technology.

This article tells us about an advance that MIT has announced in energy storage. Using ideas similar to plant photosynthesis, they split water into oxygen and hydrogen and later recombine it when the energy is needed.