How Everything Works
How Everything Works How Everything Works

Question 1179

How does a photocell absorb light and turn it into power? — MR
A photocell is actually a large diode—a one-way device for electric current. Like most diodes, the photocell consists of two different layers of chemically altered or "doped" semiconductors, the anode layer and the cathode layer, and the junction between these two layers has the peculiar property that it normally allows electrons to cross it in only one direction. There is what's called a "depletion region" at the junction, a very thin insulating layer with two electrically charged surfaces—the surface on the cathode side is positively charged and the surface on the anode side is negatively charged.

When an electron, which is negatively charged, approaches the depletion region from the anode side, it first encounters the depletion region's negatively charged surface and is repelled. But when the electron approaches from the cathode side, it first encounters the depletion region's positively charged surface and is attracted. If it has enough energy when it approaches the depletion region from the cathode side, the electron can cross the depletion region to reach the anode layer. Thus electrons can move relatively easily from the photocell's cathode layer to its anode layer but they can't go back.

When a photocell is exposed to light, some of the light particles (photons) are absorbed in the diode's cathode layer. When such an absorption occurs, the photon's energy may be transferred to an electron in the cathode, giving that electron the energy it needs to cross the depletion region and reach the anode. But once the electron has arrived at the anode it can't return to the cathode directly across the depletion region. Instead, it must flow through an external circuit in order to return to the cathode. As that electron flows through the external circuit, it can give up some of its energy, obtained from the light photon, to devices in that circuit. In that manner, light energy has provided energy to an electrically powered device.


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