What Are Photovoltaic Cells?

Photovoltaic cells are semiconductor devices that convert light directly into electrical energy through the photovoltaic effect. When photons are absorbed at or near a semiconductor junction, they excite electrons into higher energy states, and the internal electric field of the junction separates these excited carriers before they can recombine, producing a voltage and driving a current through an external circuit. This phenomenon, which can be understood as a specific instance of photoelectricity occurring in a solid-state device rather than through electron emission into vacuum, was first demonstrated in selenium by Charles Fritts in 1883 and became commercially viable with the development of silicon cells at Bell Laboratories in 1954.

A single cell produces a voltage of roughly 0.5 to 0.7 volts at its maximum power point, depending on the semiconductor material and the intensity of illumination. Cells are interconnected in series and parallel arrays to form modules and panels, which are then combined into larger photovoltaic systems for residential, commercial, and utility-scale electricity generation. The efficiency of a cell, defined as the ratio of electrical power output to incident optical power, is the central metric by which cell technologies are compared.

Cell Structure and the p-n Junction

A photovoltaic cell consists of a thin semiconductor absorber layer sandwiched between electrical contacts, with the p-n junction formed either within the absorber or at an interface between two different materials. In a conventional silicon cell, a lightly boron-doped (p-type) base is paired with a thin, heavily phosphorus-doped (n-type) emitter at the illuminated surface. Photons absorbed within a diffusion length of the junction generate electron-hole pairs; the junction's built-in electric field sweeps electrons toward the n-side and holes toward the p-side, creating a separation of charge that appears as an open-circuit voltage across the cell's terminals. Metal fingers on the front surface and a reflective back contact collect current while minimizing shading and recombination losses.

Anti-reflection coatings, typically silicon nitride or magnesium fluoride, reduce reflection losses at the cell surface; textured surfaces with pyramidal or inverted-pyramid features scatter light at oblique angles, increasing the optical path length inside the absorber and improving absorption near the bandgap. Modern passivated emitter and rear cell (PERC) designs add a dielectric rear passivation layer that reduces surface recombination and reflects unabsorbed long-wavelength photons back into the silicon, raising commercial cell efficiencies into the 22 to 24% range.

Generations of Photovoltaic Cell Technology

The evolution of photovoltaic cells is often organized into three or four generations distinguished by materials and fabrication approach. A thorough description of these generations and their efficiency milestones appears in a PMC review of photovoltaic cell generations.

First-generation cells use bulk crystalline silicon and account for roughly 90% of installed global capacity. Monocrystalline silicon cells reach 22 to 24% efficiency in production; gallium arsenide single-junction cells exceed 28% but are reserved for space and concentrator applications because of cost.

Second-generation thin-film cells deposit absorber layers only a few micrometers thick onto glass, metal, or polymer substrates. Cadmium telluride (CdTe) and copper indium gallium diselenide (CIGS) are the leading thin-film technologies, each with certified efficiencies above 22%. The U.S. Department of Energy Solar Energy Technologies Office supports research across these material platforms, and the National Renewable Energy Laboratory (NREL) maintains the authoritative chart of best research-cell efficiencies across all technologies.

Third-generation approaches include perovskite solar cells, which advanced from about 10% efficiency in 2012 to above 25% in a decade, and perovskite-silicon tandem cells, which have exceeded 33% in laboratory settings. Multi-junction III-V cells achieve above 47% under concentrated sunlight by stacking subcells tuned to successive spectral bands.

Applications

Photovoltaic cells have applications in a wide range of disciplines, including:

• Utility-scale solar power plants supplying electricity to the grid
• Rooftop and building-integrated photovoltaic installations for distributed generation
• Satellite and spacecraft power systems requiring lightweight, radiation-tolerant cells
• Consumer electronics ranging from solar-charged wearables to remote monitoring equipment
• Rural electrification in off-grid communities in developing regions