What Is Photovoltaic?

Photovoltaic (PV) refers to the direct conversion of light into electrical energy through semiconductor materials. The term derives from the Greek word for light and from Alessandro Volta, whose work on electrochemistry underlies modern cell physics. In a photovoltaic device, photons with energy at or above the semiconductor's bandgap excite electrons from the valence band into the conduction band, creating electron-hole pairs. A built-in electric field at a p-n junction then separates these carriers before they can recombine, driving a net current through an external circuit. The open-circuit voltage and short-circuit current density of a cell determine how much power it can deliver under a given spectrum.

Photovoltaic technology is the fastest-growing segment of the global energy supply. The U.S. Department of Energy Solar Energy Technologies Office supports research across crystalline silicon, cadmium telluride, copper indium gallium diselenide (CIGS), perovskites, multijunction III-V devices, and organic photovoltaics, targeting utility-scale system costs of $0.02 per kilowatt-hour by 2030. Unlike concentrating solar thermal plants, photovoltaic systems have no moving parts in the conversion stage, which contributes to their low maintenance requirements and long operational lifetimes.

Photovoltaic Conversion Principles

The photovoltaic conversion process begins when a photon is absorbed within a few diffusion lengths of the p-n junction. The probability of absorption depends on the semiconductor's absorption coefficient and thickness; direct-bandgap materials such as gallium arsenide (GaAs) and cadmium telluride (CdTe) absorb sunlight within a few micrometers, while indirect-bandgap silicon requires material thicknesses of hundreds of micrometers for comparable absorption. The theoretical efficiency limit for a single-junction cell under the AM1.5 solar spectrum, established by Shockley and Queisser in 1961, is approximately 33% for an optimal bandgap near 1.1 eV, close to that of silicon (1.12 eV).

Energy losses arise from three mechanisms: photons with energy below the bandgap pass through the material without absorption; photons with energy well above the bandgap lose the excess as heat through rapid carrier thermalization; and recombination of electron-hole pairs at defects or surfaces reduces the collected current. Passivated emitter and rear cell (PERC) silicon designs address surface recombination, while multijunction cells using III-V semiconductor stacks capture a broader portion of the solar spectrum by stacking materials of progressively smaller bandgaps, achieving laboratory efficiencies above 47% under concentrated illumination.

Cell Technologies and Materials

Silicon dominates the global PV market, accounting for roughly 90% of installed capacity. Monocrystalline silicon cells, grown by the Czochralski process, achieve efficiencies of 22 to 24% in commercial production and hold laboratory records above 26%. Polycrystalline silicon cells are manufactured at lower cost by casting and slicing silicon ingots, with typical commercial efficiencies of 17 to 20%. Cadmium telluride thin-film modules, produced by companies such as First Solar, require only a few micrometers of absorber material and have reached certified efficiencies above 22%, with manufacturing costs among the lowest in the industry.

Perovskite solar cells have advanced from roughly 10% efficiency in 2012 to certified values above 25% for single-junction devices, as documented in a PMC review of photovoltaic cell generations. Tandem cells combining a perovskite top cell with a silicon bottom cell have exceeded 33% in the laboratory. NREL maintains the chart of best research-cell efficiencies, the primary public record for tracking progress across all cell types.

Applications

Photovoltaic technology has applications in a wide range of disciplines, including:

• Utility-scale solar farms connected to the electrical grid for bulk power generation
• Distributed rooftop and building-integrated systems for residential and commercial energy harvesting
• Off-grid electrification in remote areas and developing regions
• Space power systems on satellites and planetary exploration vehicles
• Consumer electronics, from calculators to wearable energy-harvesting devices