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  • This episode of Real Engineering is brought to you by Brilliant, the problem solving website

  • that teaches you to think like an engineer.

  • Installed global capacity of solar cells has increased year on year for the past decade,

  • fueled by the plummeting prices and rising efficiency of solar cells. [1] Forcing fossil

  • fuel producers out of the market through technological advance.

  • At the end of 2019, the total installed capacity of photovoltaic cells exceeded 630 thousand

  • Megawatts, an astounding figure that is going to continue to rise in the coming decades.

  • However, in the 40 years we have been using solar cells, there has been a mysterious flaw

  • that has been sapping away potential electric current from the photovoltaic cells.

  • Upon testing in the laboratory, newly manufactured solar cells display an efficiency of about

  • 20%. Meaning they could convert 20% of the incoming energy from sunlight into electric

  • current. However, within hours of operation, that efficiency would drop to 18%. [2] A 10%

  • drop in total electric generation. Losing 10% of 630,000 Megawatts of power is no small

  • problem. That's equivalent to about 30 nuclear power plants worth of power capacity, if the

  • solar panels could operate all day, which they can't, but you get the point. There's

  • a lot of potential electricity being lost.

  • It's no wonder that scientists and engineers have been hunting down the cause of this problem,

  • termed light induced degradation, for 40 years. And last year, we may have finally cracked

  • the problem and found the cause behind this mysterious loss in power. To understand it

  • we first have to understand how photovoltaic cells work.

  • Photovoltaic cells use the photovoltaic effect to generate a current. An effect where photons

  • of a particular threshold frequency striking a material can cause electrons to gain enough

  • energy to free them from their atomic orbits and move freely in the material. [3]

  • This is best achieved with semiconductors, whose unique properties lying between conductors

  • and insulators allows them to most easily elevate electrons from atomic orbit to moving

  • freely among their atoms.

  • Some of the first solar cells were created using selenium, like this one, created by

  • Charles Fritts, sitting atop a New York in 1884 [4] A revolutionary device that produced

  • a consistent current of electricity, but it was achieving an efficiency of just 1%. Converting

  • 1% of the energy striking it in the form of light, into electricity. This, in combination

  • with the high cost of selenium, made it a unviable source of electricity.

  • To succeed these devices needed to compete with fossil fuel power sources.

  • Before the photovoltaic effect could power the world, scientists and engineers would

  • need to figure out how to increase that efficiency percentage and do it with cheaper materials.

  • Enter Silicon. A common semiconductor material that has formed the bedrock of the electronic

  • age. This is going to be our starting material for our solar cell. Let's build a solar

  • cell from scratch and see how efficiencies were gradually increased over time.

  • Let's first look at what happens when light interacts with a pure silicon crystal like

  • this.

  • Incoming light can do one of three things. It can be reflected, absorbed, or simply pass

  • right through it. If the light is reflected or passes through, it cannot produce the photovoltaic

  • effect.

  • Step one to improving our efficiency is to minimize the amount of light that gets reflected

  • off the material. This is wasted energy that affects our efficiency level. In fact, 30%

  • of light that strikes untreated silicon is reflected. So before we even start, our maximum

  • efficiency drops to 70%. [5]

  • For this reason, Silicon is often treated with a layer of silicon monoxide which can

  • reduce the light reflected to just 10%, while a second layer with a secondary material,

  • like Titanium Dioxide can reduce it as low as 3% [6]

  • Texturing the surface of the material can further increase the probability of the light

  • being absorbed. If it is textured like this, light that is initially reflected has another

  • chance to strike the material and be absorbed.

  • Only light that is absorbed can potentially cause the photovoltaic effect, but not all

  • light will. We need photons above a threshold energy to increase an electron's energy

  • enough to allow it to move freely in the material.

  • A photon's energy is defined by multiplying planck's constant by its frequency. Silicon

  • requires photons with 1.1 electron volts to produce the photovoltaic effect, which corresponds

  • to a wavelength of 1,110 nanometres [7]

  • This lies around here in the light spectrum and any lower energy light from here down

  • cannot cause the photovoltaic effect. This light will simply cause the atom to vibrate

  • and create heat.

  • This graph shows the total solar energy being emitted by the sun, however a good deal of

  • this does not reach the Earth's surface as it is absorbed in the atmosphere. This

  • is a more realistic graph. About 4% of the energy reach earth's surface is in ultraviolet,

  • as the sun emits relatively little ultraviolet photons. 44% is in the visible spectrum, and

  • 52% is in the infrared spectrum. This may sound surprising, as infrared light is lower

  • energy, but it covers a wider range of the spectrum and thus accounts for more energy.

  • Because silicon cannot make use of light with a wavelength greater than 1,110 nanometres,

  • everything from here up is energy we cannot convert to electricity. This represents about

  • 19% of the total energy reaching earth.

  • Another thing to note is that light with higher energy does not release more electrons, it

  • simply produces higher energy electrons. For example, blue light has roughly twice the

  • energy of red light, but the electrons that blue light release simply lose their extra

  • energy in the form of heat. Producing no extra electricity. This energy loss results in about

  • 33% of sunlights energy being lost.[8]

  • So these spectrum losses alone cause a 52% loss in efficiency. This is a lot of energy

  • to lose, but silicon sits near the ideal threshold frequency that balances these two energy losses.

  • Capturing enough of the lower energy wavelengths, while not losing too much efficiency as a

  • result of the material heating up. [9]

  • The reason's solar panels lose efficiency as they get hotter is quite complicated and

  • outside the scope of this video, but for now all you need to know is that silicon balances

  • these factors best for terrestrial purposes.

  • This is such a large loss in power that in some climates active cooling, which takes

  • some of the electricity the panels create to cool the panels, actually results in more

  • electricity being generated.

  • Onto the next problem.

  • Knocking an electron free by itself does not create an electrical current in our circuit.

  • It just frees an electron to float freely about the material. To create a useful current

  • we have to force this electron around an external circuit where it can do work. Freeing an electron

  • also creates a positively chargedholein its place, that is also free to move about

  • the material. If an electron meets a hole, it simply fills it and our energy is wasted.

  • The next trick to maximise efficiency is to limit the chances electrons have to fill these

  • holes and to force them into our circuit as quickly as possible. To do this we use the

  • unique properties of silicon.

  • Silicon has 4 electrons in its outer shell, and thus readily forms a crystal structure

  • with 4 neighbouring atoms using covalent bonds, a bond where neighbouring atoms share an electron

  • pair.

  • We can manipulate this behaviour and tailor the crystals material properties by adding

  • impurities, called dopants.

  • Say we add boron atoms to the silicon crystal wafer. These boron atoms have 3 electrons

  • available for bonding with the silicon crystal, but silicon wants 4. So this creates a “hole

  • in the crystal that wants to be filled with an electron. We call this a p-type as it has

  • positive charge carriers

  • Now, let's say we create another wafer of silicon, but this time we add atoms with 5

  • electrons available for bonding, like phosphorus. Again the phosphorus bonds with the silicon,

  • but this time we have an excess electron that can float freely about the material. We call

  • this an n-type, because it has negative charge carriers.

  • Now, let's sandwich these two materials together and see what happens.

  • The positive holes and negative electrons migrate towards each other. The electrons

  • will jump into the p-type and the holes jump into the n-type. This causes an imbalance

  • of charge, because now the p-type side has more negative charges, and the n-type has

  • more positive charges.

  • We have just formed an electromagnetic valve that allows electrons to pass in one direction.

  • Let's see how this works.

  • Suppose a photon with sufficient energy enters the p-type side of the solar cell and knocks

  • an electron free. The electron starts bouncing around the material and one of two things

  • can happen. It can recombine with a hole, resulting in no current, or it can come into

  • the electromagnetic field at the junction of the material. Here the electromagnetic

  • field actually accelerates the electron across the junction into the n-type side [10], where

  • there are very few holes for it to fill, and to boot, the junction's electromagnetic

  • field actually prevents the electron from passing back to the other side. A similar

  • thing happens on the n-type side, where holes are selectively transported across the junction

  • before they can recombine. This means one side of the junction becomes negatively charged,

  • while the other side becomes positively charged, we have created a potential difference or

  • in other words a voltage. If we add some metal contacts and an external load circuit, these

  • electrons will pass along the circuit to recombine with the holes on the other side. We have

  • just created a solar cell.

  • You may notice a problem here though, by adding metal contacts to the upper surface of the

  • solar cell, we have just blocked light from entering the cell, and thus reduced it's

  • efficiency.

  • This is yet another problem engineers have had to think carefully about in their quest

  • to optimize solar cell efficiency.

  • Over the years engineers have optimized both the shape and manufacturing techniques to

  • minimize the area covered by the metal electrodes, while also minimizing the resistance the electrons

  • will face in entering the external circuit.

  • One research paper used topology optimization to design these electric contacts. [11] Topology

  • optimization uses algorithms to optimize the design of objects using constraints the engineer

  • inputs. Using this method for the electric contacts produced something remarkably like

  • the vasculature of a leaf, and that shouldn't really surprise you.

  • Footage: Vasculature tissue on a leaf does not perform

  • photosynthesis. It instead brings the water that is essential for photosynthesis to the

  • leaf and extracts the useful products,, serving a similar purpose as our electric contacts,

  • so of course plants have developed the perfect shape to optimize the energy they can absorb

  • from the sun. Plants have had millions of years to evolve this shape[10] However, most

  • solar cells use a simple grid shape, as it is cheap to manufacture. This typically results

  • in an efficiency loss of about 8%.

  • All told, these effects result in typical modern silicon solar cells having a laboratory

  • tested efficiency of 20%.

  • So, what was happening to cause that drop to 18% after a couple of hours of operation?

  • This problem was the focus of hundreds of scientific papers and many found clues to

  • the problem. [12] Many noted that the efficiency drop was correlated to the concentration of

  • boron and oxygen in the silicon and noted that the drop did not occur when boron was

  • substituted for gallium. Thus, it was known a boron oxygen defect was causing the issue.

  • Others found that the defects could be reversed by heating the silicon in the dark at 200

  • degrees for 30 minutes, but it would return once again upon exposure to the light. Efforts

  • in reducing the problem have primarily focused on reducing the concentration of oxygen impurities

  • in the silicon wafers, which occur as a result of the czochralski silicon wafer manufacturing

  • technique that is the source of the 95% of silicon solar cells. These manufacturing techniques

  • are still a point of research [14] and the engineers and scientists were working blindly.

  • Little was known about the actual defect creation process and how exactly it was causing such

  • a large drop in efficiencies, leaving engineers with less information to solve the problem

  • with.

  • This paper used a special imaging technique and observed these boron oxygen molecules

  • converting into something the paper refers to asshallow acceptorswhen exposed

  • to light. [13] In essence, they observed the defects transforming into little electron

  • traps that acted as recombination sites, and thus reduced the time and probability of electrons

  • entering the circuit to do work.

  • With this knowledge, engineers can now develop better techniques for preventing this phenomenon

  • and hopefully help increase our renewable energy capacity in the coming years.

  • It's easy to think that technology has reached a point of being so advanced that knowing

  • where things can be improved is practically impossible for the average person, but that

  • simply isn't true. A little bit of research into any area will reveal countless problems

  • humans are still grappling with fixing.

  • When I started researching this video, I knew little about solar panels beyond the basics.

  • In order to make this video, I took a week to deep dive into some college textbooks using

  • my knowledge of material science and electronics to guide my research, but I had some gaps

  • in understanding that the college textbooks just assumed I had preexisting knowledge of.

  • Terms likeband gapandfermi levelskept appearing, and without understanding

  • these terms, I couldn't make complete sense of the explanations.

  • These were like canyons in my journey for knowledge. I couldn't advance until I filled

  • them in. So, half way through the research and writing process, I decided to stop what

  • I was doing. I changed my tactics. I decided to take the Brilliant course on solar energy,

  • because I knew Brilliant would guide me through the very basics of the subject right through

  • to the more complicated concepts. It worked a treat. All the little gaps in my understanding

  • were filled in and I could now read scientific papers and college textbooks without feeling

  • like I was reading a foreign language.

  • This is the magic of Brilliant. Brilliant's courses are curriated incredibly well and

  • allow you to go from knowing nothing to being an expert.

  • This is just one of many courses on Brilliant. Brilliant's thought-provoking math, science

  • and computer science courses help guide you to mastery by taking complex concepts and

  • breaking them up into bite-sized understandable chunks. You'll start by having fun with

  • their interactive explorations and over time you'll be amazed at what you can accomplish.

  • If you are naturally curious, want to build your problem-solving skills, want to develop

  • confidence in your analytical abilities, or like me, find yourself struggling to parse

  • the language of advanced college textbooks because you are missing some fundamental knowledge,

  • then get Brilliant Premium to learn something new every day.

  • As always, thanks for watching and thank you to all my patreon supporters. If you would

  • like to see more from me, the links to my twitter, instagram, discord server and subreddit

  • are below.

This episode of Real Engineering is brought to you by Brilliant, the problem solving website

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B1 US silicon solar electron energy efficiency material

The Mystery Flaw of Solar Panels

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    joey joey posted on 2021/04/23
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