Solar cells are a great at as a constant, low voltage power source but don't function at night. Therefore, power stored during the day is depleted at night.
What is needed is to modify the solar cell so that it can deliver a constant voltage at night that can extend the loss of power that has been stored during the day.
One way of doing this is to modify the cell to convert power from areas of the spectrum that is delivered at night. One such source that flows freely through Earth's EM window is in the lower end of radio waves.
A technology that was already been proven to convert radio waves to power is the rectenna. A rectenna is a rectifying antenna, a special type of antenna that is used to convert EM energy into direct current electricity.
The Design I have been working on is a layered, 3D power cell. The first layer is the actual solar cell itself. The second and third layers are the rectenna's themselves. Each of the two layers will convert power at a different wavelength.
Research has shown that a rectenna working in the 5 MHz to 30GHz range, size: 100nm X 100nm can produce power at: 10mV, 0.1mA.
This constant slow charge of a battery that has been charged by the daylight solar cell can extend its life by as much as 30% - 40%
Example: a typical solar cell is 100mm X 100mm that gives, with circuits, 5 rectenna's that can be placed with one solar cell. The average voltage for a solar cell is 0.8 volts, with 10 rectenna's (two layers) you get 1V per solar cell or 0.5v with one layer.
To increase power, other layers could be added with different EM tuning.