If you're making a solar-powered circuit, it is important to include a diode in series with the solar cell. Without one, your battery or capacitor storing the gathered energy will be discharged through the cell. The solar cell behaves a lot like a series of diodes. Current will flow back through it if enough voltage is applied. Adding a diode in series with the solar cell blocks the reverse current flow. It's not a perfect solution -- a diode will waste a bit of energy and reduce the maximum vol