What Is Photoelectricity?

Photoelectricity is the phenomenon by which electromagnetic radiation, particularly light, interacts with matter to produce electrical effects. The term covers two related but distinct processes: the photoelectric effect, in which photons eject electrons from a material's surface, and photovoltaic effects, in which absorbed photons generate electron-hole pairs that produce a sustained current or voltage. Both processes are governed by quantum mechanics, and understanding them requires treating light as a stream of discrete energy packets rather than as a continuous wave.

Albert Einstein's 1905 explanation of the photoelectric effect, for which he received the Nobel Prize in Physics in 1921, established the quantum nature of light. His analysis showed that the energy of an ejected electron depends on the frequency of the incident light, not its intensity, a result that classical wave theory cannot explain. This insight launched a century of photodetector and photovoltaic technology development.

The Photoelectric Effect and Work Function

When a photon strikes a metal or semiconductor surface, it can transfer all of its energy to a bound electron. If that energy exceeds the work function of the material, the electron escapes the surface as a photoelectron. The work function is the minimum energy required to remove an electron from the surface to a point just outside it in vacuum. Metals have work functions ranging from about 2 eV for cesium to 5 eV for platinum, which sets the threshold photon frequency for emission. NIST's online database of electron work functions provides tabulated values for common materials used in photodetector and emitter design.

Photovoltaic Effects and Solar Cells

The photovoltaic effect occurs in semiconductor p-n junctions when absorbed photons with energy above the bandgap generate electron-hole pairs. The built-in electric field at the junction separates carriers before they can recombine, producing a net current. This is the operating principle of solar cells, photodiodes, and image sensors. Silicon dominates commercial solar cell production because of its 1.1 eV bandgap, good carrier mobility, and abundant availability. Tandem cells stack multiple junctions with different bandgaps to absorb a broader solar spectrum, achieving efficiencies well above the Shockley-Queisser limit for single-junction devices. Research published through Nature Energy on photovoltaic efficiency records tracks the progress of various cell technologies.

Quantum Efficiency

Quantum efficiency (QE) quantifies how effectively a photodetector or solar cell converts incident photons into collected carriers. External quantum efficiency is the ratio of collected electrons to incident photons at a given wavelength; internal quantum efficiency excludes reflection and transmission losses, measuring only photons that actually enter the device. High QE requires minimizing surface recombination, bulk defects, and contact resistance. Photomultiplier tubes can achieve quantum efficiencies above 40 percent in the ultraviolet; silicon photodiodes typically achieve 80 to 95 percent in the visible range. IEEE Journal of Photovoltaics is the primary archival venue for quantitative photoconversion research.

Photoconductive and Secondary Emission Effects

Beyond the standard photoelectric and photovoltaic effects, light can also increase the electrical conductivity of a semiconductor by creating free carriers without a junction, a process called photoconductivity. Image sensors based on photoconductive materials such as amorphous selenium are used in flat-panel X-ray detectors. Secondary photoemission, where an initial photoelectron triggers the emission of additional electrons through impact, amplifies weak signals in photomultiplier tubes and microchannel plate detectors used in particle physics and night-vision systems.

Applications

• Photovoltaic solar panels for residential, commercial, and utility-scale electricity generation
• CMOS and CCD image sensors in cameras, smartphones, and medical imaging equipment
• Photodiode-based optical fiber receivers in telecommunications networks
• Photomultiplier tubes and silicon photomultipliers in PET scanners and high-energy physics detectors
• Photoemissive cathodes in electron guns for electron microscopy and free-electron lasers