Source collapse refers to a drop in voltage of the source (dark solar cell). This happens when excess current is being drawn form the source. For example in a dark solar cell that typically outputs 1V, the source impedance is in orders of 100 kohms. Hence via ohms law, the current that can be drawn from the cell is 10uA. If a higher current is drawn from the cell, the source voltage will drop thereby collapsing the source.