How Does a Photovoltaic Cell Work ?Published on 04.08.2019
5 min read
A photovoltaic (PV) cell, also known as a solar cell, is an electronic component that generates when exposed to photons, or particles of light. This conversion is called the , which was discovered in 1839 by French physicist Edmond Becquerel1. It was not until the 1960s that photovoltaic cells found their first practical application in satellite technology. Solar panels, which are made up of PV cell modules, began arriving on rooftops at the end of the 1980s. Photovoltaic capacity has been growing steadily since the start of the 21st century, led by the construction of huge solar farms.
© Toru YAMANAKA / AFP - A Japanese researcher examines a new type of organic photovoltaic cell with photosensitive pigments.
How a Photovoltaic Cell Works
A photovoltaic cell is made of semiconductor materials that absorb the photons emitted by the sun and generate a flow of electrons. Photons are elementary particles that carry solar radiation at a speed of 300,000 kilometers per second. In the 1920s, Albert Einstein referred to them as “grains of light”. When the photons strike a semiconductor material like , they release the electrons from its atoms, leaving behind a vacant space. The stray electrons move around randomly looking for another “hole” to fill.
To produce an electric current, however, the electrons need to flow in the same direction. This is achieved using two types of silicon. The silicon layer that is exposed to the sun is doped with atoms of phosphorus, which has one more electron than silicon, while the other side is doped with atoms of , which has one less electron. The resulting sandwich works much like a battery: the layer that has surplus electrons becomes the negative terminal (n) and the side that has a deficit of electrons becomes the positive terminal (p). An electric field is created at the junction between the two layers.
When the electrons are excited by the photons, they are swept to the n-side by an electric field, while the holes drift to the p-side. The electrons and holes are directed to the electrical contacts applied to both sides before flowing to the external circuit in the form of electrical energy. This produces direct current. An anti-reflective coating is added to the top of the cell to minimize photon loss due to surface reflection. (See diagram.)
Photovoltaic Cell Efficiency
Efficiency is the of electrical produced by the cell to the amount of sunlight it receives. To measure efficiency, the cells are combined into modules, which are in turn assembled into arrays. The resulting panels are then placed in front of a solar simulator that mimics ideal sunlight conditions: 1,000 watts (W) of light per cubic meter at an ambient temperature of 25°C. The electrical power produced by the system, or peak power, is a percentage of the incoming solar energy. If a panel measuring one square meter generates 200 W of electrical power, it has an efficiency of 20%. The maximum theoretical efficiency of a PV cell is around 33%. This is referred to as the Shockley-Queisser limit.
In real life, the amount of electricity produced by a cell, known as its output, is based on its efficiency, the average annual sunshine of the surrounding area and the type of installation. Incident solar radiation varies significantly, measuring 1 megawatt-hour per square meter per year (MWh/sq.m/y) in the Paris area versus roughly 1.7 MWh/sq.m/y in southern France and nearly 3 MWh/sq.m/y in the Sahara Desert. This means that a solar panel with a 15% efficiency rating will generate 150 kWh/sq.m/y in Paris and 450 kWh/sq.m/y in the Sahara.
Different Types of Photovoltaic Cells
There are three main types of PV cells. Their conversion efficiencies are being improved all the time.
Crystalline Silicon Cells
Silicon is extracted from silica. The latter has many forms, including quartz, which is found in large quantities in sand. Silicon cells account for more than 95% of the solar cell market. In commercial applications, their efficiency ranges from 16.5% to 22%, depending on the technology used.
In the cold processing method, the silicon is composed of many crystals and is called polycrystalline. The cells are easy to manufacture and have a laboratory efficiency in excess of 22%. In the pull-from-melt method, the silicon is converted into a large, single crystal structure and is called monocrystalline. It has a laboratory efficiency of up to 26.6%. (See the infographics). The price of silicon cells has dropped in recent years, making PVs very competitive with other sources of electricity.
Instead of cutting silicon wafers of around 200 microns2, it is possible to semiconductor material in thin layers only a few microns thick on a substrate such as glass or plastic. Commonly used substances are cadmium telluride and copper indium gallium selenide (CIGS), whose laboratory efficiencies are close to that of silicon, at 22.1% and 23.3%, respectively. Amorphous (non-crystalline) silicon can also be used for making thin-film cells. This technology has long been applied in small calculators but is less efficient than silicon.
Organic solar cells that utilize organic molecules or polymers rather than semiconducting minerals are starting to be commercially applied. The cells continue to have a low conversion efficiency and a short lifetime but are potentially a low-cost alternative in terms of production. Another technology, dye-sensitized solar cells with photosensitive pigments, inspired by , is beginning to attract attention.
Earlier research on organic photovoltaics (OPV) led to the discovery of a new type of cell called perovskite, which uses hybrid organic-inorganic compounds as the active material. Perovskites have already reached laboratory efficiencies in line with those of other technologies (the record is 23.7%).
Although a lot of research still needs to be done before the cells can be mass produced (instability is a problem), perovskites have many advantages. In addition to being lightweight and flexible, their materials can be mixed with ink and applied to large surfaces. Furthermore, they are extremely cost effective to produce.
Scientists around the world are working on combining different PV technologies to create multi-junction cells. The utilization of different materials allows the cells to achieve a much higher efficiency than the maximum theoretical limit (33.5%), while keeping manufacturing costs in check. Research is mainly focused on thin-film silicon tandem cells, which yield a theoretical efficiency of 43%. The maximum theoretical efficiency of multi-junction cells is greater than 50%.
- His son, Henri Becquerel discovered the principles of radioactivity in 1903, together with Pierre and Marie Curie.
- 1 micron = 1/1000th of a millimeter.