IELTS Reading: Diagram / Plan Completion

Photovoltaic (PV) solar panels convert sunlight directly into electricity through the photovoltaic effect, a process discovered by French physicist Edmond Becquerel in 1839. The basic unit of a solar panel is the solar cell, made primarily from silicon, a semiconductor. Silicon atoms are arranged in a crystal lattice structure; in their natural state, they do not readily conduct electricity. To make a solar cell functional, silicon wafers are treated through a process called doping: a thin layer on one side is infused with phosphorus atoms, which have five outer electrons rather than silicon's four, creating an electron-rich (negative, or n-type) layer. The layer below is doped with boron, which has only three outer electrons, creating an electron-deficient (positive, or p-type) layer. At the boundary where these two layers meet — the p-n junction — an internal electric field is established. When photons from sunlight strike the n-type surface of the cell, they knock electrons loose from silicon atoms. These free electrons are drawn through the internal electric field across the p-n junction. Metal contacts printed on the front surface of the cell collect these electrons, and the electrons flow through an external circuit — producing direct current (DC) electricity — before returning to the p-type layer at the back. The flow of electrons through the external circuit is the electrical current that can be used to power devices. Because most household appliances and the electrical grid use alternating current (AC) rather than DC, the DC output from solar panels must be converted by a device called an inverter. In systems connected to the electricity grid, net metering allows surplus electricity generated during peak daylight hours to be fed back into the grid, with the owner receiving credit against their consumption at other times. Off-grid systems require battery storage to provide power when sunlight is unavailable.