General Information on Solar PV
Pioneers of this technology were scientists from Bell Laboratories in the United States of America, who in the early 1950’s discovered the PV Effect upon observing that an electric charge was generated when sunlight irradiated silicon. In essence, the technology has not changed significantly, but it has improved in versatility, quality, price and efficiency, making it one of more popular renewable energy applications implemented around the world.
A single PV unit is referred to as a solar cell. In order to form an electric field, solar cells are usually manufactured from semi-conductive materials (predominantly silicon or a variation thereof) and other conductive materials. A particular advantage of PV is that it can be designed to any scale depending on the consumption pattern or requirement of the user. A solar cell is capable of producing 1-2 watts of power each. For a larger output, several cells can be connected to form modules or panels. In turn, these modules or panels can be connected to form arrays, which are the building blocks of a PV system.
The most common classification of solar cell technologies is according to first, second or third generation, referring to the way they are produced and the materials utilised. The main difference between each generation class is the material used for the semiconductor component and the structure of its crystal.
First generation solar cells include both monocrystalline and polycrystalline cells. These are generally flat plate cells used for rooftop PV applications. Monocrystalline (‘one crystal’) cells are those produced from silicon wafers cut from a silicon crystal. These solar cell types are usually the most expensive, but also have the highest efficiency rating i.e. between 15 – 24%, due to the purity of the silicon used.
Polycrystalline cells are those produced from molten silicon or thin silicon wafers cut from multiple crystals that were formed together. As a result, this type of cell has a lower production cost but has a lower efficiency rating of 13 – 18%.
Production of crystalline silicon cells
(DGS Scholar School / ©GIZ)
Cells derived from an assortment of non-silicon materials, for example copper-indium-diselenid, cadmium telluride or amorphous silicon, are referred to as second generation cells. These cells are also known as thin film cells as the thickness of the silicon element is only a few micro-meters in width.
Various types of second generation cells
(DGS Scholar School / ©GIZ)
Second generation products have a lower cost and efficiency, but are suitable for applications that require the solar cells to be more flexible.
There are also some manufacturers who use a more innovative approach and produce cells from organic materials. Examples include those cells with a polymer or nanocrystal base; concentrated or dye sensitised solar cells. These are referred to as third generation cells and can be considered emerging technologies under research, as there are not commercially proven yet.
Each solar cell constitutes 2 sheets of silicon i.e. an ‘N’ type (has an excess of electrons) and a ‘P’ type (has fewer electrons and more capacity to accept electrons). As a result, an electric field is formed internally due to the oppositely charged layers. Silicon atoms in solar cells are strongly bonded to one another. In this natural state, there is no flow of current; however when sunlight irradiates a cell with sufficient energy, electrons in silicon atoms become energised and move out of their natural state. Through metal contact points on the surface of the cell, the displaced electrons are collected and conveyed to the wires. If a circuit is connected to the panel, then electricity can be generated.
For a comprehensive visual on how solar panels work, watch this short Ted-Ed video by Richard Komp (2016).