How do Solar Cells Work - Definitive Guide

Sun is a big power station that provides clean, non-stop energy for free. Sun is a ball of nuclear energy that has enough fuel onboard to run our solar system for another five billion years and solar panels can convert this energy into the infinite supply of electricity.

Photovoltaic solar panels covering the roof of a house

Solar power may seem an advance technology, but it's already in use. Solar-powered quartz watch, calculator, car, aircraft, etc are already used in daily life. Also, NASA has developed a solar-powered plane whose upper wing surface is covered by solar panels that power the plane's engine.

Topics covered in this article:

  • How much energy Sun can provide us?
  • What are Solar Cells?
  • How solar cells are manufactured and its working? 
  • The efficiency of Solar Cells 
  • Different types of photovoltaic solar cells
  • How much power a solar cell can create?
  • Why Solar cells are not popular in use?

How much energy Sun can provide us?

The amount of energy received from the sun is minimum at sunrise or sunset and maximum at noon when the sun is right above the head.

Dark red sunrise
 
On average, every square meter of Earth's surface gets 170 watts of solar energy. In other words, if we cover just one percent of the Sahara desert with solar panels, we could light up the whole world. This makes Solar power an amazing source of energy.
 
But it's not as easy as it looks, Energy from the sun arrives on Earth as a combination of Light and Heat. Both of these are essential for our survival on earth but they can't be directly used to run our television, fan, etc, instead, we have to find an alternative way to convert it into a useable form of energy. That's what solar cells do. 

What are Solar Cells?

A solar cell is a device that captures U.V radiations of the sun and turns it directly into electricity. Its size is equal to a palm, octagonal in shape and black colored. Many solar cells are combined to make larger units called solar modules, which further bundled into bigger units called solar panels (black colored slabs on the rooftop of homes).

Tracking solar panels

Solar cells work like a battery. A battery converts chemical energy into electricity whereas a solar cell converts sunlight into electricity. Solar cells sometimes called photovoltaic (PV) cells because they convert sunlight into electricity ( "Photo" derived from a Greek word which means "light" and "voltaic" refers to an Italian electricity prodigy Alessandro Volta, 1754-1827).

Sunlight is made up of tiny particles called "photons" which is a source of energy. So a beam of sunlight contains trillions of photons that are captured by solar cells and converted into electricity. Each cell produces few volts of electric current, so a solar panel's work is to combine these volts into a large amount of current for commercial use.



Solar cells are made up of silicon particles (a most common element found in sand). Solar cells are made up of other particles too which we will study later in this article. When sunlight falls on the solar cell, electrons start ejecting from silicon atom which initiates the flow of electric current in the circuit. Simple!

How solar cells are manufactured and its working? 

Solar cells are made up of silicon which is a semiconductor. Elements (commonly metals) allow electrons to flow through them; they are called conductors. Other elements (commonly non-metals) don't allow electrons to flow through them; they are called insulators. Semiconductors like silicon are neither conductors nor insulators, but under certain circumstances, they can behave as conductors as well as insulators.
Closeup of a single solar panel

A solar cell is a sandwich of two differently doped layers of silicon. The lower layer is doped in such a way that it contains very few electrons, it's called p-type or Positive type silicon. The upper layer is doped in such a way that it contains too many electrons, it's called n-type or negative type silicon.

When a layer of n-type silicon is placed over a p-type silicon layer, an electron barrier is created at the junction (a borderline where two layers of silicon meet) of two layers. No electrons can cross this barrier under normal conditions even if you connect a bulb to the solar cell, it will not light up as there is no electric current flowing through it. 

Now, when photons enter the sandwich they give their energy p-type silicon layer and knockouts the electrons from the valence shell of silicon. The electrons jump the barrier and accepted by n-type silicon and an electric current generated in the circuit. The more photons fall on the solar cell, the more electric current will be generated. That's why we refer it as a photovoltaic cell!

The efficiency of Solar Cells

The very first solar cell has only 6% efficiency; the most efficient solar cell tested in laboratory conditions has 46% efficiency. Most of the cells are first-generation types that can maintain approximately 8% efficiency.
 
As physics says "Energy can't be created nor be destroyed but can be converted into one form to another". That means a solar cell can't produce more energy than it receives from a photon. A solar cell has efficiency only about 10-20% which goes up to 30% (only theoretically), known as Shockley-Queisser limit.

Chart comparing the efficiency of first, second, third generation and other solar cells.

Why solar cells are not much efficient?

That is because sunlight contains a wide mixture of photons with different wavelengths and energies and a solar cell is only capable to capture specific wavelength photons, wasting the rest. Some photons falling on solar cells don't have enough energy to knock out electrons from silicon whereas some of them have too much energy, both are wasted effectively. 

The most efficient solar cells maintain only 46% efficiency in absolutely perfect conditions using different junctions to catch photons.
Daily used home solar panels might achieve about 15% efficiency because all kinds of pesky real-world factors will vanish the nominal efficiency, including the construction of panels, how they are placed and angled on the roof, whether they are cleaned frequently, climatic conditions, outside temperature, etc.

Different types of photovoltaic solar cells

Most of the solar cells that we use daily are just silicon sandwiches, doped to make them good electrical conductors. Scientists called them "first generation" solar cells whereas modern technologies cells designated as "second and third generation" solar cells. Let's discuss them in detail!
  

First-generation solar cells

The majority of the world's solar cells are made from wafers of crystalline silicon, sliced from ingots, which are made in modern laboratories that can take months to complete. These ingots either molded in single crystals (monocrystalline) or multiple crystals (polycrystalline). First-generation solar cells use a single junction between n-type and p-type silicon layers, which are sliced from different ingots. 


First generation solar cell

So, n-type ingots are made by heating silicon with tiny amounts of phosphorus, antimony or arsenic as dopant, whereas a p-type ingot would use boron as a dopant. Slices of n-type and p-type silicon material are then combined to make a junction. Also, few extra accessories are added like anti-reflective coating, protective glass front, plastic backing, and metal connections. That's how all photovoltaic cells have worked since 1954.  

Second-generation solar cells

The classic solar cells are thin wafers, usually a millimeter deep (about 200 micrometers). They are absolute slabs compared to second-generation cells, also known as thin-film solar cells (TPSC) or thin-film photovoltaics (TFPV), which are 100 times thinner.
Second generation solar cell

Most of the second-generation solar cells are made up of silicon (known as amorphous silicon, in which atoms are placed randomly instead of precisely ordered). Some are made up of other materials like cadmium-telluride (Cd-Te) and copper indium gallium diselenide (CIGS) because they are very light, thin and dynamic to use. Second-generation solar cells can be laminated on house windows, skylights, roof tiles and all types of other materials like metal, glass, and plastics. 

Second-generation solar cells are not much efficient as first-generation solar cells. First-generation solar cells can give efficiency up to 20%, amorphous silicon solar cells are 7% efficient, thin-film Cd-Te cells are 11% efficient and CIGS cells are efficient up to 12%. That's why second-generation cells do not have a great impact on the solar industry.

Third-generation solar cells

Third generation solar cell

 Third-generation solar cells have the best features than first and second-generation solar cells. They also give efficiency up to 30% which are way much higher than first and second-generation solar cells. Third-generation solar cells are made up of other than "normal" silicon, like organic polymers, perovskite crystals and feature multiple junctions. These all properties make them cheaper, efficient and practical to use.

How much power a solar cell can create?

Theoretically, Solar cells can produce a gigantic amount of energy but practically their efficiency is low. A single solar cell can create 3-4.5 watts of energy and a module made up of 40 solar cells could create 100-300 watts of energy. The majority of solar panels are made up of 3-4 modules, which means they can provide enough energy to power a house. 


Many other factors like maintenance, geographic location, climate, weather and outside temperature affect the power-generation ability of a solar cell. For example- If you live in an area, where the sky is clear, sunlight is falling perpendicular to the solar panel and has the optimum climate, then more power will be produced by your solar panel whereas if you have just opposite conditions then power generation would be low.

Why Solar cells are not popular in use?

The primary reasons are economic, political and technological factors. In most of the countries, the power generated by burning fossil fuel is still cheaper than solar energy. That's why people prefer electricity coming from the national grid instead of solar power. In many countries especially in the middle east, oil and gas companies are still more influential than solar energy. 

Percentage of US electricity generated by solar cell

Their whole economy is depended on the sale of oil and natural gas. This makes it another reason why the government doesn't promote solar energy over traditional energy. Solar cells indeed need maintenance, good climatic condition, etc but if the government provide incentives like subsidies, cheap maintenance service, etc then use of solar cell could be boosted. 

Technologically, solar cells are not much efficient compared to fossil fuels. They give only up to 15% of efficiency which is far low than energy produced by fossil fuel. We need to do some advancements in solar technology so that their efficiency could be increased and people will motivate to buy them.

Solar farms can promote the use of solar energy but the huge cost in building them does not allow it. So, considering the above facts it may take several more decades to increase the use of solar energy.

Cheers
Sciencedoze

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