Photovoltaic effect

Light consists of photons carrying energy. When a semiconductor absorbs a photon with sufficient energy, it can promote an electron into a mobile state and leave behind a positively charged vacancy called a hole. The resulting electron–hole pair is a charge carrier pair. The fraction of light absorbed and the energy required to create carriers depend on the semiconductor and the photon wavelength.

Generating mobile charges alone does not guarantee a useful current: in an undirected material, electrons and holes can recombine. A photovoltaic device incorporates a junction or another charge-selective structure that establishes an internal electric field and separates carriers. The separated charges create a voltage; when the cell is connected to a load, their movement through the external circuit supplies current. The process can continue while illumination supplies carriers and the device remains operational.

A typical silicon photovoltaic cell contains semiconductor regions with different electrical properties that form a junction, along with front and rear electrical contacts. Photons absorbed in or near the active region generate electron–hole pairs. The junction's internal field and selective contacts direct the two carrier types toward opposite sides, producing a potential difference. Conductive contacts collect the charges, and an external circuit provides a path for current.

Individual cells produce limited voltage and power, so manufacturers connect many cells into a module; modules can in turn be combined into arrays. Photovoltaic cells produce direct current. In grid-connected systems, an inverter converts that output to alternating current, while batteries or other power electronics can support storage and system control.

Crystalline silicon is the most widely used photovoltaic semiconductor, but it is not the only option. Thin-film technologies include cadmium telluride and copper indium gallium selenide, while perovskites, organic semiconductors and other materials are also studied or used in particular applications. Each material brings trade-offs in absorption, charge transport, manufacturing, durability, cost and environmental management.

Some devices stack semiconductors with different band gaps. A top layer can absorb some parts of the solar spectrum while transmitting other photons to lower layers, potentially using a broader range of incident light than a single-junction cell. Such tandem and multijunction designs can achieve high conversion efficiencies, but fabrication complexity and cost shape their practical uses.

Photovoltaic conversion efficiency is the fraction of incident radiant power converted into electrical power under specified measurement conditions. It depends on the spectrum and intensity of the light, the semiconductor band gap, reflection and absorption losses, carrier recombination, electrical resistance, temperature and device design. Efficiency values therefore need context: a laboratory cell result is not automatically the performance of a commercial module in outdoor conditions.

Photovoltaic generation also varies with available light. Systems can feed electricity to a grid or pair generation with storage and other sources to manage periods of low sunlight. The effect itself describes light-to-electricity conversion in a device; it does not mean every illuminated material will produce useful power.