What Are Photovoltaic Effects?

Photovoltaic effects are the generation of a voltage or electric current in a material when it is exposed to light. The term encompasses several related but mechanistically distinct phenomena in which photon absorption drives the separation of positive and negative charge carriers, producing a measurable electromotive force without any moving parts or external heat cycle. The most technologically important instance is the p-n junction photovoltaic effect that underlies solar cells and photodetectors, but anomalous photovoltaic effects in ferroelectric materials and bulk effects in non-symmetric crystals represent distinct physical phenomena with their own device implications.

The photovoltaic effect is distinct from the photoelectric effect: in the photoelectric effect, photons cause electrons to be emitted from a material's surface into vacuum; in the photovoltaic effect, excited carriers remain within the solid, and charge separation is accomplished by an internal electric field rather than by emission. The first observation of a photovoltaic-type response was made by Edmond Becquerel in 1839, who noticed that an electrochemical cell with a silver chloride-coated platinum electrode produced additional current when illuminated. The solid-state form of the effect, in selenium and then silicon, was developed over the following century and became commercially viable when Bell Laboratories produced the first practical silicon solar cell in 1954.

The Photovoltaic Effect in Semiconductor Junctions

In a semiconductor p-n junction, the built-in electric field arises from the diffusion of majority carriers across the junction boundary, leaving behind a charged depletion region. When photons with energy greater than the semiconductor's bandgap are absorbed within a diffusion length of this depletion region, electron-hole pairs are generated. The built-in field sweeps electrons toward the n-type side and holes toward the p-type side, preventing their recombination and establishing a charge separation that manifests as an open-circuit voltage across the device terminals. Connecting a load allows this potential difference to drive a current, delivering electrical power.

The power produced depends on both the photogenerated current density and the open-circuit voltage. The open-circuit voltage is bounded by the quasi-Fermi level splitting of the absorber under illumination, which is always less than the bandgap energy. As analyzed in the ScienceDirect overview of photovoltaic effects, the Shockley-Queisser limit for a single-junction device under the standard terrestrial solar spectrum is approximately 33% for a bandgap near 1.1 eV. In practice, silicon p-n junction cells achieve 25 to 26% in the laboratory and 22 to 24% in commercial production. The development of photovoltaic cells across multiple generations is reviewed in a PMC study on photovoltaic cell generations.

Anomalous and Bulk Photovoltaic Effects

In non-centrosymmetric crystals such as ferroelectric materials, photovoltaic effects can occur without any junction or interface. This bulk photovoltaic effect (BPE) arises because the lack of inversion symmetry in the crystal structure causes photo-excited carriers to be preferentially scattered or shifted in a specific direction. In ferroelectric perovskites such as barium titanate and lithium niobate, the spontaneous electric polarization assists carrier separation, and the resulting open-circuit photovoltage can far exceed the material's bandgap: values of 10 to 100 volts per centimeter, and occasionally above 1,000 volts per centimeter, have been reported in thin-film ferroelectrics.

The anomalous photovoltaic effect in obliquely deposited polycrystalline thin films of II-VI and III-V semiconductors has also been documented, where grain boundary effects and the Dember diffusion mechanism cooperate to generate high voltages. These phenomena are studied in the context of photodetectors and emerging non-volatile optical memory elements. Research on these materials is supported by the U.S. Department of Energy Solar Energy Technologies Office.

Applications

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

• Solar energy conversion in photovoltaic cells, modules, and large-scale power plants
• Solid-state photodetectors for imaging, sensing, and optical communications
• Position-sensitive detectors for optical alignment systems
• Radiation detectors in nuclear instrumentation using compound semiconductors
• Ferroelectric-based photoelectric memory and neuromorphic computing elements