ArtsAutosBooksBusinessEducationEntertainmentFamilyFashionFoodGamesGenderHealthHolidaysHomeHubPagesPersonal FinancePetsPoliticsReligionSportsTechnologyTravel

How Solar Cells Work

Updated on June 18, 2014
Photovoltaic (solar cell) array.
Photovoltaic (solar cell) array. | Source

Alternative energy has become an increasingly popular topic since the 1973 Oil Crisis when members of the Organization of Arab Petroleum Exporting Countries (OAPEC) proclaimed an oil embargo on the United States forcing oil prices to skyrocket. Since then technology has attempted to exploit natural, renewable resources as sources of energy, such as wind, geothermal, hydroelectric, biomass, etc. It wasn't until recently, however, that green technology started gaining ground and became a national trend. Maybe filling the tank in your car at a cringeworhty $3-$4 per gallon (almost triple the price of gas when I first got my license just 9 years ago) had something to do with it. Or maybe it is an increasing knowledge of global environmental change and how humans are irrevocably changing this planet for the worse. Whatever the reason may be, green is the new thing and with supplies of oil and natural gas dwindling every day, renewable technology is the key to our future.

Green technology has primarily been successful in the generation of electricity. There are seven fundamental methods of generating electricity: static, electromagnetic, chemical, photoelectric, nuclear, piezoelectric and thermoelectric. Currently,However there are currently only two ways to produce electricity commercially: electro electricity is produced commercially only three of those ways: nuclear, electromagnetic (wind, hydro, and coal energy), and via the photoelectric process (solar energy).

Solar energy is the most abundant renewable form of energy on Earth and the current amount of solar energy incident on our planet is enough to not only meet, but exceed, current global energy demands. I talked a little bit in my last post about why solar energy isn’t more widely used, the two main reasons being that it is expensive and inefficient, and in places like WNY, where I live, I literally have not seen the sun peak out behind layers and layers of clouds in probably weeks and I am not exaggerating.

While making the snow go away and the sun to shine all year in WNY would not be only extremely detrimental to Earth and the survival of all its species, it’s also impossible. I will have to suffer another dreary and miserable season of sunset at 4pm and back breaking shoveling. However, new research may be the solution to the problem of expensive and inefficient solar cells.

But first, what is a solar cell?

Examples of Incident Solar Radiation

Location
Solar Radiation in July
Solar Radiation in Jan.
Northern Europe
4.5-5 kWh/m2
0.5 kWh/m2
Southern Europe
6-7.5 kWh/m2
1.5-2 KWh/m2
Examples of solar radiation incident on Earth in kilowatt-hours per meter squared per day. 5 kWh is enough energy to heat the water for a rather hot bath. Source: Renewable Energy: Power for a Sustainable future by Godfrey Boyle.

Wave-Particle Duality

Quantization

History of Photovoltaics

Before we continue, I am going to assume two things: (1) that you know what an electron is, and (2) that you know that the flow of electrons is, by definition, an electric current i.e. electricity. With that being said, the next thing you must know is that a solar cell is a photovoltaic. Photovoltaics (PVs) describes the generation of electricity from light. In other words, for our purposes, photovoltaics is simply a fancy word for solar cell, so don't be fooled by the terminology.

The discovery of primitive PVs is credited to a French Physicist name Edmond Becquerel who in 1839 discovered during his experiments that the voltage in his ‘wet cell’ battery increased when its silver plates were exposed to sunlight. That's great, but what did that mean? What was next? Absolutely nothing. The discovery was just another notation in the scientific community until a very important discovery more than six decades later.

That very important discovery that paved the way for PVs was made in 1905 by one of the world’s most famous scientists. The discovery was the photoelectric effect theororized by the one and only Albert Einstein. Many people think that it was his theory of relativity that won him the Nobel Prize, but it was actually his work on the photoelectric effect that won Einstein the coveted award.

One way to produce commercially used electricity is through the photoelectric effect. But what is the photoelectric effect? The photoelectric effect occurs when electrons are emitted from matter as a result of absorption of electromagnetic radiation, creating a current. That's just a mouthful, isn't it? So let me try to break that down.

The sun emits energy, that travels through space and time until it hits Earth. You may call this light, but scientists call this electromagnetic radiation. Visible light, however, is just a small part of the electromagnetic radiation spectrum that also includes radio and mircowave, as well as ultraviolet (UV), x-ray and gamma ray. This energy travels through space as a wave until millions of years after being emitted from the sun, it hits an object on Earth. Once the light hits an object, it can cause the electrons to free themselves from the object and be emitted. This, in a nutshell, is a photolectric effect.

But how does this happen? How does electromagnetic radiation cause an electron to be emitted from an object? In 1887, Heinrich Hertz was the first person to observe the photoelectric effect, but he declined to attempt to describe a theory that would explain his experiment. In 1905, Albert Einstein proposed a simple theory to explain the phenomena. Light, he realized, was quantized. Up until this point, people thought light was just a wave. However, Einstein postulated that light wasn't just waves, but it was also particles, a groundbreaking discovery that is what we now call the wave-particle duality. These particles are called photons. When photons hit an object, they transferred all of their energy to an electron in the object during the collision causing the electron to be emitted from the matter. I like to think of it as a joust. You have two knights on their horses, one is a photon, the other is an electron. When the photon knight hits the electron knight with his lance all of the energy from the photon is transferred to the electron knight causing his to be 'emitted' from his horse.

Robert Millikan, an American physicist, thought Einstein was full of crap, so for the next ten years he diligently worked to disprove Einstein’s theory only to be disappointed when he provided the experimental data needed to prove the theory in 1916. (He can't be too mad, though, as is work on the photoelectric effect earned him a Nobel Prize as well). The photoelectric effect is the driving force behind photovoltaics (PV).

I highly suggest watching the videos I posted on both wave-particle duality and quantification from Quantum Made Simple at http://www.toutestquantique.fr/. The animations are a good way to visualize the abstract concepts. In the first video, what they call a quantum object is a representation of wave-particle duality, an object that has the properties of both.

Photoelectric Effect

Pictorial representation of the photoelectric effect; hv stands for light.
Pictorial representation of the photoelectric effect; hv stands for light. | Source

How an Inorganic Semiconductor Photovoltaic Works

It wasn’t until the late 1940s to early 1950s that the breakthrough occurred that set in motion the development of modern day, high-efficiency solar cells. It was the discovery of the doping of semiconductors that changed the course of history.

Semiconductors are non-metallic materials, such as germanium and silicon, whose electrical characteristics lie between those of conductors, which offer little resistance to the flow of electric current, and insulators, which block the flow of current almost completely. The characteristic physical property of a semiconductor is its ability to increase electrical conductivity with increasing temperature. Doping is a process where tiny quantities of carefully selected impurities are deliberately diffused into an extremely pure crystalline semiconductor (usually silicon).

Now that we got that out of the way, you need to know there are a lot of things that go on inside an inorganic semiconductor photovoltaic, and a lot of basic chemistry that I've already cut out and won't go into detail about (like how the electronic bands of metals are different from insulators and semiconductors) but I will do my best to stick to the basics because this is not a hub for scientists, this is a hub for everyone. The basic and most common PV consists of a junction between a p-type and n-type semiconductors and are usually made of Silicon (Si). An n-type, or negative, semiconductor is usually made from crystalline silicon that has been doped with small impurities, such as phosphorus, that cause a surplus of free electrons. A p-type, or positive, semiconductor, on the other hand is usually made from crystalline Si that has been doped with small impurities, usually boron, so that there is a deficit of free electrons, or ‘holes.’ (If you want to know more about this, comment and I will write another hub explaining holes in greater detail.) The joining of these two types of semiconductors creates a p-n junction that sets up an electrical field.


Cross-section of thin film polycrystalline solar cell.
Cross-section of thin film polycrystalline solar cell. | Source

When light falls on the p-n junction, the photons transfer their energy to some of the electrons and promote them to a higher energy level (*cough*the photoelectric effect *cough*). Think of it as a parking garage with two floors. Before the light energy hits, all of the cars are parked on the lowest floor in the basement. There are no free parking spots in the basement and none of the cars are allowed to leave. After the light hits, however, several of the cars are promoted to the floor above. The cars in the basement are now free to move around inside the basement, while the cars on the first floor are also free to move around, however they are now also free to leave the building. In this case, the promoted electrons, or excited electrons become ‘free’ to conduct an electric current by moving through the material. As they move through the material, they are also leaving a positively charged hole behind, which can also move. Holes, in the p-type are attracted to electrons in the n-type and can combine to form an electron-hole pair.

The net effect is the formation of a reverse electric field. The p-type semiconductor becomes negative and the n-type semiconductor becomes positive. Under the influence of the reverse electric field at the junction, the electrons will tend to move back into the n-region where they originated and the holes back to the p-region. This flow of electrons to the n-region is an electric current that generates a potential difference or electromotive force. This force drives the electrons out of the semiconductor into a metallic contact on the top of the solar cell and through a load in an external circuit to do electrical work. This electrical work is electricity. In order to produce power, however, the solar cell must generate voltage as well as current provided by the flow of electrons. However, the p-n junction naturally produces a small voltage right where the two layers meet, also called the depletion zone. These process combined are considered the photovoltaic effect, where two dissimilar materials in close contact produce an electrical voltage upon exposure to light.

There is a lot more that goes into a PV cell. There is a lot of math, and a lot of drawings of energy levels and band gaps and directions in which electrons and holes move to make all of this happen. I took an entire class on just those things; on how the electrons and holes move, the voltage created by junctions variant on the doping of each semiconductor, so on and so forth. It is interesting, but not within the realm of this blog.

Semiconductor PVs were first used in 1958 to power a small radio transmitter in the second US Space satellite, Vanguard I, and have not only seen rapid progress, but have also seen rapid growth for both industrial and commercial purposes.

However, there is a theoretical limitation of inorganic semiconductor photovoltaics. In 1961, William Shockley and Hans Queisser first calculated the Shockley-Queisser approximation. Shockley and Queisser assumed that an electron-hole pair must annihilate at some point and that during this annihilation light would be emitted. However, that is the ideal case and does not always happen. Generally the extra energy of the excited proton is wasted as heat. They also postulated that only electrons of a certain energy would be able to produce power. As such, a vast amount of energy is wasted. The Shockley-Queisser limit is then calculated by examining the amount of electrical energy that is extracted per photon of incoming sunlight. Shockley and Queisser calculated a limit of only 31%. What this means, that even under the best conditions possible, an inorganic semiconductor PV can only hope to achieve a 31% efficiency and it can never be any better than that. This limit is fundamental to solar energy production and has fueled the research of various alternatives to semiconductors, such as Quantum Dots and Organic PVs, which I will talk about in greater detail in another hub to follow.

Now that you know how a solar cell works, use your new knowledge to impress your friends!

Comments

    0 of 8192 characters used
    Post Comment

    • profile image

      Luana 

      3 years ago

      All power sources have dckabarws. The two you mention, like nuclear, are relatively clean, which is a plus.Hydro is very cheap for the amount of energy we get, but our rivers are basically all dammed up already there isn't a lot more hydropower we can get.Solar is one of the most expensive ways to generate electricity on a utility scale, but could be the cheapest way to generate electricity on a homeowner scale, at least for many urban homes. Unless one has a stream running through their property to run a water wheel, or strong steady wind, the other choices for getting power are using a gas- or oil-powered generator, or buying retail electricity from the grid. The generator will cost more per kWh over the life of the system, and the grid electricity could be more or less expensive, depending on where one lives. At the moment, the grid is usually cheaper, but not in all parts of the country.

    working

    This website uses cookies

    As a user in the EEA, your approval is needed on a few things. To provide a better website experience, hubpages.com uses cookies (and other similar technologies) and may collect, process, and share personal data. Please choose which areas of our service you consent to our doing so.

    For more information on managing or withdrawing consents and how we handle data, visit our Privacy Policy at: https://hubpages.com/privacy-policy#gdpr

    Show Details
    Necessary
    HubPages Device IDThis is used to identify particular browsers or devices when the access the service, and is used for security reasons.
    LoginThis is necessary to sign in to the HubPages Service.
    Google RecaptchaThis is used to prevent bots and spam. (Privacy Policy)
    AkismetThis is used to detect comment spam. (Privacy Policy)
    HubPages Google AnalyticsThis is used to provide data on traffic to our website, all personally identifyable data is anonymized. (Privacy Policy)
    HubPages Traffic PixelThis is used to collect data on traffic to articles and other pages on our site. Unless you are signed in to a HubPages account, all personally identifiable information is anonymized.
    Amazon Web ServicesThis is a cloud services platform that we used to host our service. (Privacy Policy)
    CloudflareThis is a cloud CDN service that we use to efficiently deliver files required for our service to operate such as javascript, cascading style sheets, images, and videos. (Privacy Policy)
    Google Hosted LibrariesJavascript software libraries such as jQuery are loaded at endpoints on the googleapis.com or gstatic.com domains, for performance and efficiency reasons. (Privacy Policy)
    Features
    Google Custom SearchThis is feature allows you to search the site. (Privacy Policy)
    Google MapsSome articles have Google Maps embedded in them. (Privacy Policy)
    Google ChartsThis is used to display charts and graphs on articles and the author center. (Privacy Policy)
    Google AdSense Host APIThis service allows you to sign up for or associate a Google AdSense account with HubPages, so that you can earn money from ads on your articles. No data is shared unless you engage with this feature. (Privacy Policy)
    Google YouTubeSome articles have YouTube videos embedded in them. (Privacy Policy)
    VimeoSome articles have Vimeo videos embedded in them. (Privacy Policy)
    PaypalThis is used for a registered author who enrolls in the HubPages Earnings program and requests to be paid via PayPal. No data is shared with Paypal unless you engage with this feature. (Privacy Policy)
    Facebook LoginYou can use this to streamline signing up for, or signing in to your Hubpages account. No data is shared with Facebook unless you engage with this feature. (Privacy Policy)
    MavenThis supports the Maven widget and search functionality. (Privacy Policy)
    Marketing
    Google AdSenseThis is an ad network. (Privacy Policy)
    Google DoubleClickGoogle provides ad serving technology and runs an ad network. (Privacy Policy)
    Index ExchangeThis is an ad network. (Privacy Policy)
    SovrnThis is an ad network. (Privacy Policy)
    Facebook AdsThis is an ad network. (Privacy Policy)
    Amazon Unified Ad MarketplaceThis is an ad network. (Privacy Policy)
    AppNexusThis is an ad network. (Privacy Policy)
    OpenxThis is an ad network. (Privacy Policy)
    Rubicon ProjectThis is an ad network. (Privacy Policy)
    TripleLiftThis is an ad network. (Privacy Policy)
    Say MediaWe partner with Say Media to deliver ad campaigns on our sites. (Privacy Policy)
    Remarketing PixelsWe may use remarketing pixels from advertising networks such as Google AdWords, Bing Ads, and Facebook in order to advertise the HubPages Service to people that have visited our sites.
    Conversion Tracking PixelsWe may use conversion tracking pixels from advertising networks such as Google AdWords, Bing Ads, and Facebook in order to identify when an advertisement has successfully resulted in the desired action, such as signing up for the HubPages Service or publishing an article on the HubPages Service.
    Statistics
    Author Google AnalyticsThis is used to provide traffic data and reports to the authors of articles on the HubPages Service. (Privacy Policy)
    ComscoreComScore is a media measurement and analytics company providing marketing data and analytics to enterprises, media and advertising agencies, and publishers. Non-consent will result in ComScore only processing obfuscated personal data. (Privacy Policy)
    Amazon Tracking PixelSome articles display amazon products as part of the Amazon Affiliate program, this pixel provides traffic statistics for those products (Privacy Policy)