Photovoltaic energy (PV) refers to a process that converts the light into clean electricity using semiconductor materials and the photovoltaic effect.
The photovoltaic effect is common today in the solar energy sector where solar photovoltaic cells (solar PV cells) are used to generate clean electricity using the sunlight.
What Are Photovoltaics?
Photovoltaics (PV) is a term used today to describe the process of converting the light directly into clean electricity using photovoltaic solar cells.
The photovoltaic effect is a physical and chemical phenomenon closely related to the photoelectric effect and refers to the creation of electrical energy in a semiconductor material due to the exposure to light.
The photovoltaic (PV) effect was discovered in 1839 by Alexandre-Edmond Becquerel (a French physicist).
Becquerel explained his discovery in the French scientific journal called “Les Comptes Rendus de l’Académie des Sciences”, where he showed that an electric current can also be produced by using two plates of gold or platinum immersed in an acid, neutral, or alkaline solution and exposing them to sunlight.
However, the first photovoltaic solar cell (PV solar cell) consisting of a layer of selenium covered with a thin film of gold, was first created by Charles Fritts (an American inventor credited for the creation of the first working Selenium cell) in 1884, but his PV cell featured a very low efficiency.
In the twentieth century, space science was the main promoter of the photovoltaic technology because many space ships, satellites, space probes and even the International Space Station (ISS) have used and are still using solar panels to generate their own electricity in the space.
On the planet, solar power increases its generation capacity every year, mostly due to the fact that the photovoltaic (PV) technology has become more affordable, and due to the incentives provided by the governments to promote the development of the renewable energy.
What is a Photovoltaic System?
A photovoltaic system (solar PV system) represents a power system that relies on the photovoltaic effect to produce clean electricity using a source of light (sunlight).
Photovoltaic systems are used today all over the planet to produce clean electricity using only the sunlight.
To convert the sunlight into electricity, we are using photovoltaic solar cells that capture the photons present in the sunlight and transforms them into green electricity (no emissions are released in the process).
The semiconductor materials used by solar cells today consist of silicon (Si) or a thin layer of Cadmium Telluride (CdTe), amorphous Silicon (a-Si), Copper Iridium Gallium Selenide (CIS/CIGS), or organic PV cells.
How Do Solar Photovoltaic Cells Work?
Solar panels consist today of smaller units called solar cells, which can be made from silicon or other semiconductor material.
Solar cells made of silicon (Si), consist of a crystalline layer (mono or poly) of silicon that is sandwiched between two conductive layers.
Each silicon atom in the silicon layer is connected to its neighbors using four strong bonds that keep the electrons in place so no electric current can flow.
The silicon solar cell uses two layers of silicon (an N-type and a P-type).
The N-layer of silicon uses extra electrons, while the P-type layer of silicon has extra spaces for electrons (holes).
When the two layers of silicon meet, electrons can wander across the P/N junction, leaving a positive charge on one side and creating a negative charge on the other side.
The Photovoltaic Effect
When the photons in the sunlight are hitting the silicon cell, they remove some of the electrons from their place by breaking their bonds.
The negatively charged electron and the positively charged hole are now free to move around, but due to the electric field at the P/N junction they will only move one way.
The electrons will be drawn to the N-side, while the holes will be drawn to the P-side of the solar cell.
The mobile electrons will be collected by thin metal fingers located at the top of the solar cell.
From there the electrons will flow through an external circuit doing electrical work like powering a light bulb or an appliance before returning through the conductive aluminum sheet on the back of the solar cell.
One single silicon solar cell cannot generate more than a few watts of power, and about half a volt, but by putting them together in a solar panel we can generate more power.
A solar panel that contains a small number of solar cells is just enough to power a smartphone, but to power the entire house with solar energy, we need a pretty large number of solar panels.
Types of Photovoltaic Solar Panels
On the market today, we can find a pretty wide variety of solar panels.
Some of them are made to generate the most amount of power per square meter (the most expensive ones), while others generate less power per square meter, but they are way more affordable and also flexible, so more practical.
Solar for Home provides monocrystalline and polycrystalline solar PV systems designed for residential use both on and off-grid.
1. Monocrystalline Solar PV Panels
These solar panels rely on monocrystalline solar cells that are made from a single ingot of silicon with high purity.
These solar cells are one the most efficient solar cells available on the market today, and they are also more expensive because they generate the most amount of solar power per square meter, however, they have a few downsides.
2. Polycrystalline Solar PV Panels
Being made from melted pieces of silicon, polycrystalline solar PV cells are more affordable (compared to the monocrystalline solar cells), but they generate less power per square meter (less efficient).
Besides of being more affordable, they also have a few downsides.
3. Thin Film Solar Panels
This type of solar cells is even more affordable, and they are also flexible and greener, but their efficiency is even smaller than the efficiency provided by polycrystalline solar cells.
Thin-film solar panels rely on a combination of materials such as Cadmium Telluride (CdTe), Copper Iridium Gallium Selenide (CIS/CIGS), amorphous Silicon (a-Si), and even organic PV (photovoltaic) cells.
However, these PV solar panels are more flexible than the other types of solar panels, and they can also generate power even in the evening.
Future Developments in Photovoltaics
Today, we can find on the market only solar PV panels that use crystalline solar cells (mono and poly), and flexible thin-film solar panels that are more affordable, and their production has a smaller impact on the environment.
However, to make solar energy a main source of power on the planet, we need to develop the solar technology even further.
1. Perovskite PV cells
Perovskite solar PV cells are considered today an emerging alternative and a promising solar technology in the field because it relies on an organic/inorganic hybrid solar cell that is made of methylammonium lead halide perovskites.
The Perovskite PV cell structure includes a metal back contact (can be made of aluminum, gold or silver), a hole transfer layer (can consist of Spiro-MeOTAD, PTAA, P3HT, CuSCN, NiO or CuI), an absorber layer (CH3NH3PbIxBr3-x, CH3NH3PbI3 or CH3NH3PbIxCl3-x), a layer for transporting electrons (TiO, Al2O3, ZnO or SnO2) and a top contact layer (consists of fluorine doped tin oxide or tin doped indium oxide).
For the moment, the highest efficiency obtained by a single-junction perovskite solar PV cell was 22.1%, and was obtained in March 2016 by researchers from the Korea University of Science and Technology (KRICT) and Ulsan National Institute of Science and Technology in South Korea (UNIST).
2. Copper zinc tin sulfide (CZTS) solar cells
Copper zinc tin sulfide (CZTS or Cu2ZnSnS4) solar cells provide favorable optical and electronic properties that are similar to the properties of CIGS (thin-film copper indium gallium selenide solar cells), but the materials used in the manufacturing process are more abundant on the planet and less toxic for the environment.
Actually, CZTS solar cells have been developed due to the concerns raised by the price and availability of indium in CIGS and tellurium in CdTe, as well as the toxicity of cadmium.
The highest efficiency achieved by a CZTS solar cell in the lab by now was 12.6%, but they are considered more affordable and more friendly with the environment compared to the other PV solar cells available on the market today.
3. Zinc phosphide (Zn3P2) solar cells
Zinc phosphide (Zn3P2) is an inorganic chemical compound used today as a rodenticide (can be found in rat poison).
Being also a semiconductor material, zinc phosphide can be used to manufacture photovoltaic cells.
Zinc phosphide is considered today an ideal candidate for the production of cheaper thin film photovoltaic cells, due to its strong optical absorption and an almost ideal band gap (1.5EV).
Zinc and phosphorus are considered abundant materials on the planet and they are also non toxic (compared to the other materials used to produce thin-film solar cells such as Cadmium Telluride (CdTe).
4. Organic and polymer photovoltaic (OPV) solar cells
Organic PV solar cells or plastic PV solar cells represent a new type of photovoltaic solar cells that rely on organic electronics such as conductive organic polymers or small organic molecules that are used to absorb light and ensure the charge transport for electricity generation using the photovoltaic effect.
Most organic solar PV cells are polymer PV solar cells (OPV).
The molecules used by the organic solar cells are cheap, which results in low manufacturing costs for the mass production of organic solar PV cells.
Even being considered so affordable, organic solar PV cells can provide an efficiency between 10 and 15%.
Conclusion
Solar photovoltaic systems represent the future in terms of solar power generation on planet Earth and as time passes, the lowering prices will make solar power one of the main sources of clean energy for mankind.
