Photovoltaic modules — the principle of photovoltaic cell operation

Nowadays, it is a common situation for household illumination systems or even electric cars to be supplied with solar energy provided by photovoltaic cells.

However, it does not mean that everybody, including PV system users, knows the principle of operation of such systems and recognises the processes transforming solar energy into electric power. In this article, most of these issues are precisely explained.

What is photovoltaics?

Photovoltaics is the technology of converting solar radiation into electric power through the use of photovoltaic panels and DC-to-AC converters to supply energy to consumers connected to a power network or to store energy in batteries. This is the simplest and most narrow definition, so it is worth adopting a slightly broader perspective and going back to the beginnings of this technology.

By definition, “photovoltaics” is a science interested in converting solar energy into electric power that has evolved into an important field of technical engineering that is now present in the majority of highly developed countries. In parallel, numerous technologies related to obtaining energy from renewable energy sources have been developed. However, obtaining energy from the sun’s radiation is the most important and remains the leading method of providing ‘free-of-charge’ and infinite energy. Moreover, note that solar energy could completely replace fossil fuel-based energy sources, which would significantly reduce air pollution levels. However, there is one major drawback to it, namely the demand for a large surface area resulting from a PV panel size.

Photovoltaic panels — principle of operation

The process of converting solar energy into electric power taking place in the internal cell layers starts under the reflective layer, where two silicon plates are located, ie, the upper one acts as a negative conductor made of silicon with a phosphorus admixture. The lower plate acts as a positive conductor and is made of silicon with a boron admixture. When a photon strikes a silicon atom, it knocks the electron out of its place and forces it to move. Since there are a lot of photons and such knocks, the electrons attempt to pair up with the vacant places left by the previously knocked out neighbouring electrons, and, as a result, they begin to circulate between the plates and in this way generate electric charge movement. However, the electric field present at the contact point of the positive and negative layers separates the electron-vacant space pairs, which results in arranging the electron movement and generating voltage.

Image caption: Cell with polycrystalline silicon offered by Cellevia Power. Image credit: Transfer Multisort Elektronik

This process is easy to connect with a local power grid; however, the system must be provided with a frequency converter to transform DC into AC. As mentioned above, silicon is used as a basic material of PV cells, as the photon energy is equivalent to the energy required to move one electron in a silicon atom. Despite the fact that pure silicon is not a perfect conductor, the best solution is obtained when it is mixed with phosphorus and boron. Manufacturers also offer panels made from other materials, eg, cadmium combined with tellurium, copper, indium, gallium and selenium. However, regardless of the elements used, photovoltaic panels always consist of photovoltaic cells, which are their smallest components and can be connected in series or in parallel.

Main PV cell types

Currently, there are two basic photovoltaic cell types available on the market. These are:

• silicon-based cells (panels) (monocrystalline, polycrystalline and amorphous) — market share is definitely dominant
• cells (panels) made of other elements (CdTe and CIGS).
   

Image caption: Cellevia Power panel with monocrystalline silicon. Image credit: Transfer Multisort Elektronik

Silicon-based panels are most common due to their simple design and high efficiency, ie, 17–23% for monocrystalline panels and approx. 14–18% for polycrystalline panels. Monocrystalline panels are characterised by the highest efficiency, but they are also most expensive. They are made of circular silicon plates that are trimmed to a square shape with chamfered corners. Polycrystalline panels (efficiency: 14–18%) are made from multicrystalline silicon, in the form of a compressed block with a non-uniform surface, cut into thin square bluish plates. Matt amorphous panels are made of very cheap non-crystallised silicon, which results in their lower efficiency (8–10%), but also the lowest price.

The other group of PV cells includes products made from elements other than silicon. This group includes CdTe and CIGS cells with a negligible photovoltaic layer thickness and lower efficiency as compared to silicon-based panels.

CdTe panels comprise a single cell, with a thickness of a few microns, made of cadmium telluride, and their efficiency is slightly higher (10–12%) in comparison with CdTe panels (12–14%), whose design is based on four elements, ie, copper, indium, gallium and selenium.

Factors influencing PV panel efficiency

Although PV cell manufacturers usually do not disclose any proprietary manufacturing process details for reasons of commercial confidentiality, it can be assumed that the quality of these cells is determined by the cutting conditions of the crystals and quartz blocks, as well as the monocrystalline crystal culture conditions. Other factors affecting quality include conditions in which the panels are polished, as well as cleanliness, ventilation and temperature of the rooms where the key processes are run. However, it is easiest to compare the efficiency and performance of identical panels supplied by different manufacturers, as they largely depend on the manufacturing process quality and materials used in production. To obtain objective results, tests must be conducted under identical real-world conditions which are constantly influenced by diverse factors common to all the panels (cells) affected by a test.

However, regardless of whether a comparative test is conducted or real-life PV system operation is analysed, there is always the same group of factors influencing the end results:

• roof surface area
• roof inclination in relation to sunlight
• season of the year, ie, sun radiation intensity
• sunlight absorption time in hours
• operating temperature of the cells.
   

The latter is quite problematic, as the efficiency of cells decreases when they reach their temperature limit, which happens during very hot and sunny days. This is therefore a paradox situation, as these are the days with the most solar radiation intensity. The other factors mentioned above, including the lack of shade in panel operating locations, also directly affect the amount of electric power obtained. Seasons of the year are also important, as in spring and summer the number of sunny days is greater, and the intensity of the sunlight is at its highest, which, together with the right angle of cell inclination, gives much better results.

Image caption: Miniature cell manufactured by Panasonic. Image credit: Transfer Multisort Elektronik

PV cells in the TME product catalogue

The range of PV cells offered by TME includes products provided by three manufacturers: Panasonic, Green Power and Cellevia Power. The products offered by the first one are mini-amorphous cells for internal use (clocks, minor electronic equipment) and the range provided by the other two manufacturers includes larger mono- and polycrystalline cells for outdoor applications.

Top image credit: iStock.com/xijian