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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 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
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 as “shallow acceptors” when 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 like “band gap” and “fermi levels” kept 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.
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