Direct energy conversion (III), PhotovoItaic energy conversion Solar cell

Semiconductors are used in photovoltaic energy converters to produce
electric energy. Solar cell is not a heat engine ,however, it is rather a quantum
mechanical device. A single crystal of semiconductor material is doped in such
a manner that the bottom portion of the crystal is n-type and the top is p-type.
if we illuminate this thin wafer with light of a given frequency a hole-electron
is produced which diffuses towards the p-n junction, i.e. the transition region.
if a resistor (i.e. electrical load) is placed in the circuit to connect this (p-n) junction
an electric current will pass from that. In this device solar energy is converted
directly to electric power.