A few years ago I bought a bunch of solar cells that had been harvested from broken solar panels. I have used them to add light control to synth projects with basically zero environmental impact (using trash to generate a voltage from sunlight). Each cell provides up to 0.5V, and I have used up to 5 cells to generate approximately 0 to 2.5V. Recently I combined two cells in a junk 8.25"x10" picture frame to provide ~0-1V of control voltage in a compact and relatively sturdy package (the solar cells are VERY fragile, so if you can solder them together without breaking them it is a good idea to package them safely).
Connecting the solar cells is fairly straight-foward, though their fragility can make things a little tricky. Each cell gives a maximum of 0.5V, and to generate higher voltages we connect the solar cells in series. If we consider the blue side to be a negative potential and the grey side to be positive, then to connect them in series we connect the negative (blue) of one cell to the positive (grey) of the next. Now when we place the positive lead of a voltmeter to the positive lead of the first cell, and the COM lead to the negative of the second cell, we will see a range of 0V to 1V (of course depending on lighting).
Continuing to add cells in series we create even higher voltages. The max I have done is 5 cells for a performance where I placed the cells inside of a box and controlled the pitch of a homemade synth by opening and closing this box.
What is behind all of this solar cell voltage generation? Well, it can get complicated pretty quickly, but here's the basic idea:
Semiconductors (the stuff that makes up transistors, diodes, integrated circuit chips, LEDs, and solar cells) are composed of special chemicals that are really good at moving electrons around. A very common semiconductor is Silicon, which is "doped" (chemically bonded) to other chemicals, giving it a tendancy to either accept or reject electrons. In a dormant state, the semiconductor creates a barrier between Silicon that is doped to have extra electrons and Silicon that is doped to "want" electrons. In a solar cell, photons from the sun create a bridge across that barrier, allowing electrons to flow from excess area to wanting area, generating a voltage. Somewhat related, the opposite occurs when a voltage is applied across an LED, generating photons.
Link to write up of a performance with solar synth: http://bedfordandbowery.com/2015/07/apocalypse-chow-zodiac-dinners-and-solar-power-synths-in-a-bed-stuy-garden/