a, Optical image showing the as-fabricated control device. The structure of this control device is similar to that of the DEG, but without an aluminium electrode. b, Diagram showing its detailed working mechanism. i, Before the droplet contacts the PTFE, the amount of (positive) charges on the ITO is the same as the (negative) charges on the PTFE, owing to electrostatic induction. Thus, there is no current flow from ITO to the ground. ii, When a water droplet contacts the PTFE surface, the droplet becomes positively charged while the PTFE becomes more negatively charged as a result of contact electrification. iii, When the positively charged droplet leaves, it causes the ITO electrode with positive charges to be unable to screen the more negatively charged PTFE. iv, Accordingly, a flow of current (I) between the ground and ITO electrode is induced by electrostatic induction. c, Variation in voltage output from the control device as a result of continuous droplet impinging. The inset shows the time-dependent variation in voltage from a single droplet. The frequency of impinging droplets is set at 1.0 Hz, with a total of nine droplets. d, In each test of the control device, the amount of transferred charge (in green) is identical to the charge carried by the departing droplets (in blue), showing that electricity generation from the control device indeed originates from contact electrification. The frequency of impinging droplets is 1.0 Hz, with a total of nine droplets.