We will write our own version of this demonstration eventually, but now here is a link to a ready-made kit.
Dye-sensitized solar cells convert sunlight into electrical energy using dye molecules that absorb the light and then transfer charge to an electrode. The resulting electric potential flows through an external circuit and is collected at the other side of the solar cell where it re-reduces the dye molecule back to its neutral state. There are highly specialized molecules designed to work in these devices, but you can also use naturally occurring dyes such as anthocyanin.