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Operating Principle

Photovoltaic (PV) cells use semiconductors to produce electricity. The cell absorbs solar radiation, which excites the electrons inside the cell. A semiconductor must have at least two electric fields. When an electron excited by solar energy leaves its electric field, it seeks to return to its original electric field. In order to do so, it must pass through an external circuit, producing electricity. This is referred to as the photovoltaic effect.

PV technology

The following are the primary components of PV technology.

  • Optics: Different optical elements, such as mirrors and Fresnel lenses, are used to concentrate solar radiation onto a point where a PV cell is located.
  • Photovoltaic Cell: The photovoltaic cell is the semiconductor used to produce the photovoltaic effect.
  • Inverter: Since the photovoltaic effect produces direct current (DC), an inverter must be used to change it to alternating current (AC).

Types of Photovoltaic Cells

There are two predominate PV systems on the market. Each has their own pros and cons regarding application, efficiency, and cost.

1 Crystallized Silicon (~200 µm)

A double layer antireflection coating is used to reduce reflection losses on the front surface of crystalline silicon wafers. The wafers are about 400 µm thick to ensure near-complete absorption of all photons having energy greater than the band gap. At the bottom of the wafer, a SiO2 layer is inserted between the wafer and the aluminum backing to achieve reflectance back toward the cell.

  • Single-Crystalline Si
    The semiconductors of most PV cells are made from single-crystalline Si. This requires highly purified silicon to be crystallized into ingots. The ingots are then sliced into thin wafers to make an individual PV cell.
  • Polycrystalline Si
    Polycrystalline Si cells are produced in a way very similar to single-crystalline cells. The primary difference is that silicon of less purity is used for polycrystalline cells. The result is reduced cost and increased ease of production, but a loss of efficiency.
  • Ribbon Si
    Ribbon type PV cells are produced in a similar fashion to single- and polycrystalline silicon cells. The primary difference is that a ribbon is grown from molten silicon instead of an ingot. These cells often have a prismatic rainbow appearance due to their antireflective coating.
Ribbon Si
Thin film (~5 µm):

Thin film semiconductor technology may not be as efficient as traditional semiconductor technology, but its light weight and low cost make it an ideal solution for certain applications.

Amorphous Si
  • Amorphous Si
    Unlike crystalline semiconductors which have a band gap of 1.1 eV, by manipulating the alloy of amorphous silicon semiconductors the band gap energy can be tuned between 1.1 eV and 1.75 eV. Additionally, because they have a much greater absorbance than crystalline silicon, amorphous silicon semiconductors can be much thinner (less than 1 µm). Although amorphous Si cells can be manufactured at low temperatures (200-500 C) and at low costs, a major drawback is their light-induced degradation.
Amorphous Si
3 Copper Indium Gallium Diselenide Solar Cells
  • 3 Copper Indium Gallium Diselenide Solar Cells (CIS Cu In Se2)(CIGS Cu(InGa)Se2)
    Due to its relatively high efficiency and low material cost, this technology has emerged as one of the most promising thin films. By adjusting the ratio of In to Ga in CIGS cells, the band gap can be tuned between 1.02 eV and 1.68 eV. The absorption elements of CIGS cells are incredibly high, allowing more than 99% of incoming radiation to be absorbed within the first µm of material. Although this technology has a relatively low material cost, the complicated and capital-intensive manufacturing methods remain as significant drawbacks.
CIGS Solar Cell
Cadmium Telluride
  • Cadmium Telluride (TeCd)
    Cadmium Telluride is another thin film technology that has been available longer and undergone more research than any other thin film technology.
    Although there are diverse manufacturing techniques that can be used to produce the films, many of which are promising for large scale production, the cost and potential health concerns remain as drawbacks for this technology.
Cadmium Telluride
  • Micro Si
    Micro silicon cells are expected to surpass the efficiency and performance of amorphous silicon cells and become a competitor with other thin film technologies. The high efficiency and negligible degradation of Micro Si cells has been widely reported.
  • Titanium dioxide (TiO2)
    Instead of the semiconducting materials used in most cells, TiD cells use a dye-impregnated layer of titanium dioxide to generate voltage. Because of their relatively low cost, TiO technology has the potential to significantly reduce the cost of solar cells.
Photovoltaic Concentration

Offers the best efficiency but requires high direct concentration, and is therefore only viable in some geographies.

Fresnel point focus
  • Fresnel point focus (High concentration-GaAs) (GC~500)
    Fresnel point lenses concentrate direct solar radiation onto a focal point. Since Fresnel lens can provide concentration ratios of up to 500, the necessary surface area for PV cells is greatly reduced. Since fewer PV cells are needed, it is possible to use high quality, more expensive materials like Gallium Arsenide for the semiconductors.
    Gallium Arsenide (GaAs) multi-junction semiconductors: Multi-junction semiconductors is a relatively new technology that offers significantly higher efficiencies than traditional, single-junction semiconductors. Each electrical field junction within a semiconductor has only one band gap energy. Incoming solar radiation will either have less energy than the band gap (and therefore will not be used), more energy than the band gap (and therefore some energy will be wasted), or the exact energy as the band gap. By having multiple junctions, GaAs semiconductors are able to utilize more energy from the incoming solar radiation.
  • Fresnel line focus (medium concentration-Si) (GC<500)
    Fresnel line lenses are flat cylindrical lenses that condense or diffuse light in a linear direction. This technology has lower concentration ratios than Fresnel point lenses, so high efficiency silicon semiconductors are used instead of expensive GaAs semiconductors.
  • Low concentration (2-4 times)
    Low concentration (2-4 times) Low concentration technology uses mirrors instead of lenses to concentrate solar radiation. Since the solar radiation is much less condensed, conventional silicon semiconductors are often used because of their affordability.