Photovoltaic Cells

The different types of photovoltaic cells

A photovoltaic cell or solar cell is made of a semiconductor material that absorbs light energy and transforms it directly into electrical current.

Production of photovoltaic cells requires energy, and it is estimated that a photovoltaic cell must run for about 2 to 3 years depending on its technology to produce the energy that was needed for its manufacture.

Principle of operation

An individual cell, the basic unit of a photovoltaic system, produces only a very low electrical power, typically 1 to 3 W with a voltage of less than one volt.

To produce more power, the cells are assembled to form a module (or panel).

The series connections of several cells increase the voltage for the same current, while the parallel connection increases the current by maintaining the voltage. The output current, and thus the power, will be proportional to the surface of the module.

Advantages:

  • High reliability, no moving parts
  • Reduced maintenance, little or no running costs

Disadvantages:

  • High manufacturing cost
  • Intermittent operation, dependent on sunshine
  • Low yield

The main types of photovoltaic cells

Multi-junction cell

multi-junction-photovoltaic-cells

Multijunction photovoltaic cell
Credit: Spectrolab

The multi-junction cells are composed of different layers which allow to convert different parts of the solar spectrum and thus to obtain the best conversion efficiency.

Advantages:

  • Unmatched performance

Disadvantages:

  • No commercial application

Additional Data:

  • Record laboratory yield: about 40% (under a concentration of 240 suns)
  • Developed for space applications, this type of cell is not yet commercially available

Monocrystalline photovoltaic cell

monocrystalline-silicon-cell

Monocrystalline photovoltaic cell

During cooling, the molten silicon solidifies by forming only one large crystal. The crystal is then cut into thin slices which will give the cells. These cells are generally of uniform blue color.

 Advantages:

  • Very good efficiency (about 150 Wc / m²)
  • Long life (+/- 30 years)

 

Disadvantages:

  • High cost
  • Low output at low illumination

Additional Data:

  • Commercial module efficiency: 12 to 20%, Record laboratory yield: about 25%
  • High cost

Polycrystalline photovoltaic cell

polycrystalline-silicon-cell

Polycrystalline photovoltaic cell

During the cooling of the silicon, several crystals are formed. This kind of cell is also blue, but not uniform, we can distinguish patterns created by different crystals.

Advantages :

  • Good efficiency (about 100 Wc / m²)
  • Long life (+/- 30 years)
  • Better market than monocrystalline

Disadvantages:

  • Low output at low illumination.

Additional information:

  • Commercial module efficiency: 11 to 15%, Record laboratory yield: about 20%

This type of cells have for the moment the best value for money

CIS thin fil silicon cell

copper-indium-selenium-cell

Photovoltaic cell
Copper – indium – selenium (CIS)
Credit: 
Solar World

CIS cells represent the new generation of solar cells in the form of copper-indium-selenium (CIS) thin films. The raw materials needed to manufacture CIS cells are easier to obtain than the silicon used in conventional photovoltaic cells. Moreover, their energy conversion efficiency is the highest to date for thin-film photovoltaic cells.

Advantages :

  • Provides the best performance compared to other thin-film photovoltaic cells
  • Allows to get rid of the silicon
  • Materials used do not cause toxicity
  • The cell may be constructed on a flexible substrate

Disadvantages:

  • Thin layer cells require a larger area to achieve the same yields as thick cells

Additional Data:

  • Commercial module efficiency: 9 to 11%, Record laboratory yield: approximately 19.3%

Amorphous photovoltaic cell

amorphous-photovoltaic-cell

Amorphous photovoltaic cell

The silicon during its transformation produces a gas, which is projected onto a sheet of glass. The cell is very dark gray or brown. It is the cell of so-called “solar” calculators and watches.

Advantages :

  • Operate with low illumination
  • Cheap compared to other cell types
  • Less sensitive to high temperatures

Disadvantages:

  • Low yield in full sun (about 60 Wc / m²), the cells in thin layer requires a larger surface to reach the same yields as the thick cells
  • Short life (+/- 10 years), performances that decrease significantly with time

Additional information:

  • Commercial module efficiency: 5 to 9%, Record laboratory yield: about 13.4%

CZTS Cell(Copper Zinc Etain Sulfur)

czts-cells

Still in the development phase (as of 2016) and thus not yet commercialized, CZTS cells, made from non-toxic minerals – unlike silicon – have the advantage of being fine, and can therefore be applied to flexible supports.

CZTS cells belong to the category of “thin film” solar cells, which constitute the new generation of solar technology. These solar cells as thin as a film film are manufactured by affixing a thin layer of solar absorbing material to a support such as glass or plastic which has the advantage of being flexible.

In April 2016, the team of Dr. Xiaojing Hao of the Australian Center for Advanced Photovoltaics was able to achieve a record 7.6% for cells of one cm2. These results are constantly improving. The team was achieving a 5.5% return in 2013 and a 6.6% return in 2015. The target is to reach 20%, which would allow this technology to be put on the market.

CZTS have many advantages. They are thin, and measure barely 1 to 5 μm thick, whereas the silicon cells make 200 to 350 μm. Currently, nearly 90% of installed panels are composed of silicon cells, with an average efficiency of 21%. CZTS thin cells can be used on all types of substrates, unlike silicon cells, which makes it possible to design curved, transparent, or superimposed surfaces of other materials.

Advantages :

  • Use of common and non-toxic raw materials
  • Applicable on flexible supports

Disadvantages:

  • Reliability unknown
  • Average yield

Summary

The challenges of photovoltaic cells remain: to continue to lower the cost of solar energy, to find ways to give more solar cells more durability, to use abundant and non-toxic materials; which will give solar power its full potential.

 

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