Following cool solar experiments at jeelabs.org, I also got idea to attach something to a solar cell. But first, figure out what types of them exists and available. Regarding the first, you can read in Wikipedia. But that's example of dried-out wikipedia article, giving a lot of formal facts, but little practical knowledge. What I'd like to know is what types are typically available in small consumer products.
Here's what I researched. First type is brownish, with regular vertical or horizontal lines. It's the type of solar cell which have been see for at least 20 years in calculators. So, that's amorphous silicon solar cell, less expensive (explains why we had it for 20 years around) and less efficient, they say. But they also say that there're special subtypes, designed to work in low-light conditions (specifically, provide enough voltage, even though current may be miniscule). Here's how it typically looks:
Last years, another type became widely available in cheap stuff - crystalline silicon solar cells. In cheap stuff, they look like black (or dark) colored plate, covered with few millimeter of something like epoxy, in which you can clearly see metal wires - oftentimes in irregularly spaced groups. Looking closely, it can be seen that epoxy actually holds individual bars of crystalline silicon, and irregular spacing of builtin wires are in particular due to irregular cuts of these bars in the original wafer.
Here's close up:
They also say that under dimmer lighting, crystalline cells provides reduced voltage.