The term photovoltaic (often shortened to "PV") is used to describe materials that are capable of converting electromagnetic energy - or light - into electrical energy.
The photovoltaic effect is the creation of a voltage - or of a corresponding electric current - in a material upon exposure to light. Electromagnetic radiation creates an electrical potential differences (or voltage) in the material, which then induces a continuous flow of electrons in a single direction if the circuit connecting the two terminals of the photovoltaic material is closed. The amount of current that flows is directly proportional to the amount of light that strikes the photovoltaic surface.
The most fundamental component of a photovoltaic module is its individual photovoltaic cells. They are made up of a semiconductor material that exhibits the photovoltaic effect. The material most commonly used in PV technology is silicon because of its abundance and affordability, but other semiconductors are sometimes used for particular applications. By doping the silicon with other elements, two oppositely charged layers are created in the cell, a positive (p) layer and a negative (n) layer, which meet at a p-n junction. The p-n junction is essentially a diode, meaning that current can only flow in one direction through the junction. Therefore, a voltage exists between the two layers of the PV cell.
When photons strike the cell, this will give electrons the energy they need to move from the valence band to the conduction band, making them available to move through the circuit. The voltage across the cell will then cause these electrons to flow if there is a closed circuit connecting the negative layer at the top cell to the positive layer at the bottom. This is the electric current, which corresponds directly to the amount of solar energy available to elevate electrons into the conduction band. Once these electrons have traveled through the circuit to the bottom p-layer, they will then travel back through the p-n junction and return to the n-layer. This cycle will continue as long as the circuit is closed and the cell is exposed to light.