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6: Photovoltaics

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    This chapter discusses solar cells and optical detectors, both of which are devices that convert optical electromagnetic energy to electricity. The next chapter discusses lamps, LEDs, and lasers which convert energy in the opposite direction. The photovoltaic effect is the idea that if a light shines on a pure piece of semiconductor, electron-hole pairs form. In the presence of an external electric field, these charges are swept apart, and a voltage develops across the terminals of the semiconductor. It was first demonstrated in 1839 by Edmond Becquerel. In a photovoltaic device, also called a solar cell, this effect typically occurs at a semiconductor pn junction. This same effect occurs on a smaller scale in photodiodes used to detect light and in optical sensors in digital cameras. To understand the physics behind these devices, we need to further study crystallography in semiconductors. Energy level diagrams, which illustrate the energy needed to remove an electron from a material, are another topic studied in this chapter.

    Unlike fossil fuel based power plants, photovoltaic cells produce energy without contributing to pollution. The solar power industry is growing at a fast pace. Worldwide as of April 2017, photovoltaic cells were capable of generating over 303 GW of power, and 75 GW of this total was installed within the past year [67]. This generating capacity was sufficient to satisfy 1.8% of the worldwide demand for electricity [67]. In the United States as of April 2017, photovoltaic cells installed were capable of generating 14.7 GW [67].

    • 6.1: The Wave and Particle Natures of Light
      This page explores the physics of electromagnetic radiation based on Maxwell's equations, detailing optical energy's wavelength (400 nm to 650 nm) and frequency ranges. It examines light's dual nature as a wave and a particle (photon), quantifying energy in joules and electronvolts. The page illustrates energy variation with wavelength, providing examples from the electromagnetic spectrum such as radio waves, UV light, and X-rays.
    • 6.2: Semiconductors and Energy Level Diagrams
      This page covers types of semiconductors (intrinsic, extrinsic, compounds) and their charge carriers (valence electrons, holes), focusing on doping and energy levels of atoms like aluminum and phosphorus. It explains the transition from insulators to conductors via temperature changes and energy gaps that allow semiconductors to absorb photon energy, particularly in applications like solar cells.
    • 6.3: Crystallography Revisited
      This page covers the relationship between real space and reciprocal space in crystal structures, exploring lattices, primitive vectors, and the reciprocal lattice. It connects energy levels, wave vector, and crystal momentum, emphasizing the differences between direct and indirect semiconductors regarding light interaction during electron transitions.
    • 6.4: pn Junctions
      This page explains the importance of pn junctions in semiconductor devices such as photovoltaic cells, LEDs, and photodetectors. It covers their construction, behavior, and the charge carrier movement involved, as well as the formation of depletion layers. The page introduces the electric field and contact potential across the junction, and discusses the impacts of forward and reverse bias on charge flow.
    • 6.5: Solar Cells
      This page covers solar cell efficiency, technologies (crystalline, thin film, multijunction, organic, nanotube), and their applications, including powering spacecraft. It also addresses solar power systems with panels designed for optimal sunlight capture and energy storage. Additionally, it discusses power conditioning systems in grid-tied applications, focusing on AC power phase matching, safety features, and the lifespan of electronic components, which is about 10-15 years.
    • 6.6: Photodetectors
      This page discusses photodetectors that convert light into electricity, classifying them by their materials and operational principles. It covers semiconductor types like silicon p-n junctions and gas-filled devices like photomultiplier tubes, highlighting their mechanisms, including photoelectric and photoconductive effects. Key performance metrics include frequency response, signal-to-noise ratio, detectivity, and response times.
    • 6.7: Problems
      This page covers semiconductor materials, focusing on their properties and energy gaps, particularly in relation to LEDs and solar cells. It includes a ranking of materials by energy gap size, energy level diagrams for silicon pn junctions, and an analysis of LED characteristics. The page also discusses solar cell design and how to select appropriate semiconductor materials based on energy gaps and conversion efficiency, merging fundamental semiconductor physics with practical applications.

    Thumbnail: Photovoltaic systems use semiconductor cells that convert sunlight directly into electricity. Direct current from the PV cells, which are arrayed in flat panels, flows to inverters that change it to alternating current. (Public Domain; Tennessee Valley Authority via Wikipedia)

    This page titled 6: Photovoltaics is shared under a CC BY-NC 4.0 license and was authored, remixed, and/or curated by Andrea M. Mitofsky via source content that was edited to the style and standards of the LibreTexts platform.