The next few slides will look at the details of some harvesting technologies, starting with solar cells. Solar cells are composed of an array of PN junctions and operate on the photovoltaic effect. The cell can be simply modeled as a current source in parallel with a diode. The shunting resistance models the leakage, and the series resistance models contact resistance. The blue line on the plot shows the current versus voltage characteristic. Increasing illumination increases the short circuit current and has a slight effect on the open circuit voltage of the cell.  From this information, one can also plot the power as a function of voltage. This plot makes intuitive sense since the power is zero at both ends, where either the voltage or the current is zero.  It can also seen that the power is maximum at a given voltage and current. This is the maximum power point of the cell, and is primarily a function of the temperature of the cell and incident power. This means that having electronics which are able to operate the cell at its maximum power point is very critical.