A photocell is made up of a semiconductor, the working of which is dependent on the intensity of incident light. When the amount of light exceeds a certain frequency, the electrons of the semiconductor are delocalized or "freed," and a photocell starts conducting. When the light is less, no electrons are freed, and the photocell doesn't conduct.
The semiconductor that is used in a photocell is usually of a very high resistance. This allows the photocell to stop the flow of current completely when there is no light. When light falls on the photocell, it transmits energy into the semiconductor part of the cell. The frequency of incident light is directly proportional to the transferred energy, hence the more light, the more transmitted energy. When this energy exceeds a certain level, the electrons of the semiconductor are delocalized, and the photocell starts conducting. This is the reason why a photocell conducts electricity when a high intensity of light is subjected to it.
A common application of the photocell is the light-dependent resistor. LDRs are used commonly in light sensors, street lights and energy-efficient lighting solutions. A photocell also plays a very important part in efficiently using daylight by turning off lights when natural light reaches a normal level.