Name: 太阳能电池板的神秘缺陷（The Mystery Flaw of Solar Panels）
Uploaded: 2021-04-23T07:24:14.000Z
Duration: 16 min 54 s
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基礎被動語態

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

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

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

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

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

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

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

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

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

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

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 charged “hole” in 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

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

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

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

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太阳能电池板的神秘缺陷（The Mystery Flaw of Solar Panels）

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