The worldwide solar panel market is growing at about 30% a year - with
immense growth coming in Asia, Europe, and the United States.
90 to 95% of those panels are made from solar-grade silicon.
I find it fascinating that two of our modern world's most exciting technology
sectors - semiconductors and solar - are so dependent on a single simple slice of silicon.
In this video, I want to talk about another
kind of silicon wafer. Those used for the solar industry.
## Power from the Sun
The idea of using the sun as a power source is not new.
The Ancient Greeks designed their homes to capture the sun's heat during the cold winter days.
In the 1700s, the Swiss scientist
Horace de Saussure built a glass insulated solar box that heated water with solar energy.
In 1839, the French physicist Edmond Becquerel - at the tender
age of 19 - discovered the photovoltaic effect.
If you exposed certain materials to light then they generated voltage and current.
Edmund's son Henri would discover evidence of radioactivity and shared the 1903 Nobel
Prize in Physics with the Curies. What a legit family of physicists!
## The Silicon Rod
Nearly a hundred years later, Russell Ohl was
a materials researcher at Bell Labs working on the behavior of crystals.
He was trying to use crystals to detect the presence of certain radio waves.
These detectors - commonly made with silicon - are vital components for radars.
However, the behavior of his crystal detectors was a bit
erratic. Ohl figured that it was due to impurities. He acquired
increasingly pure ingots of silicon - eventually building his own furnace to do it.
One of these ingots, 99.8% pure, was cut into a rod. Ohl ran a conductivity test
with his oscilloscope and noticed that the rod generated a peculiar loop
on the tool - indicating some form of barrier in the silicon.
He figured that the silicon was still not pure enough and set that rod aside.
A year later, Ohl noticed that the rod had a small crack in it and ran that oscilloscope test again.
He thought that the crack might have been the reason
why he saw that conductivity barrier earlier.
When Ohl ran the test, he noticed that the loop's behavior suddenly changed
when it was placed over a bowl of water. And again when it was exposed to a hot soldering iron.
Ohl showed this phenomenon to a number of his colleagues at Bell Labs. All but one had no
idea why on earth this silicon rod generated a strong current whenever it was exposed to light.
## The Rod of Light
As it turns out, this rod was special. It was a weird hybrid.
While making the silicon ingot from which this rod was made,
the two Bell Labs metallurgists cooled it rather slowly to avoid creating cracks.
In doing so, the impurities already inside the silicon separated into two sections of the ingot -
a commercial-grade section with extra impurities, and another section with much purer silicon.
So when they cut the ingot to create the rod,
they inadvertently gave that rod both sections of the silicon - one at each end.
The end of the rod with more impurities had extra electrons within it.
The other end with the pure silicon had an excess of electron deficits or holes.
Where the two ends meet, you have a barrier. This barrier structure
means that electrons can only go in a certain direction from one end to another.
So when you shine light on the barrier area,
the photons knock loose electrons, setting them free. The free electrons wanna go somewhere.
But due to the barrier already being there, they can only herd in a single
certain direction across the barrier. Thus, creating an electric current.
What Ohl had discovered was the P-N barrier - now known as the P-N junction. The P-N junction is a
fundamental building block of devices like transistors, diodes and photovoltaic cells.
In 1946, Ohl filed a US patent
for a "light sensitive device". This would be the ancestor for the modern solar cell.
Nine years later in 1954, another team at Bell Labs would create the silicon solar cell.
The New York Times said what everyone was thinking at the time:
> "The realization of one of mankind's most cherished dreams - the harnessing
of the almost limitless energy of the sun"
## Solar Silicon
Today, almost every solar cell used on Earth is made from silicon. This market consolidation
occurred in the 1970s as the solar market matured after the Oil Crises of the decade.
Note the phrase "used on Earth". Arrays made to go into space are dominated by
gallium arsenide-based solar cells. This is due to higher efficiencies and slower degradation rates.
While the United States government have researched solar since the late 1950s,
that work was in the context of space travel.
From 1950 to 1970, federal R&D in Earth-based solar energy averaged less than $100,000 a year.
In response to the 1973 Arab Oil Crisis, the United States in 1974
invested billions of dollars into new programs to develop oil alternatives.
Amazingly, 24% of all R&D investments made by the US Federal government in
the 47 years from 1961 to 2008 came during this short 7-year period from 1974 to 1980.
This includes $4 billion into a Synfuel program that tried to make fuel from coal
and oil shale, $16 billion into nuclear energy,
and $2 billion into a large scale solar energy commercialization project.
## Commercializing Solar
The US Department of Energy only wanted to back solar technologies that were
"commercially ready". Photovoltaic cells
were just one of three Solar technologies they backed - the other two relating to solar heating.
Solar heating had seemed promising. Advocates assured the government that the technology
was basically ready. Nobody said otherwise, and so they went ahead with the incentives.
But evaluations later found substantial technical failures and disappointments.
One 1979 evaluation found that just 104 of 238 residential solar heating projects
were ever finished, and of those 104, over half were partially or totally shut down again.
The report concluded that there were "gross inadequacies in the system" - and by the time
Reagan took office in 1980 the industry was almost entirely propped up by government incentives.
Reagan probably did everyone a favor here by shutting it down.
US Government involvement in the photovoltaic industry however had happier effects.
The technology base was much stronger, so the majority of the $450 million of allocated R&D
was plowed into reducing the cost of producing and installing solar cell arrays.
Looking back at it now, probably the biggest missed opportunity was on the demand side,
implementing subsidies or the like to stimulate solar energy adoption
in the US market. Some money was allocated, but programs were not
extensively implemented before Reagan started his budget reductions in 1980.
Nevertheless by then, solar cells saw great success. Sales were up,
private investment was growing and the industry was preparing to export to foreign markets.
And perhaps most crucially, the industry settled upon a single,
well-regarded technology base - the silicon solar cell.
## Economics
Semiconductor-grade silicon - sometimes also referred to as
electronic-grade - requires 99.999 or 9 to 11 nines percent purity.
This corresponds to one impure atom for every billion or hundred billion silicon atoms.
In contrast, the silicon needed for making solar cells does not have to be so pure.
Purer is better, but the industry is fine with about 99.999 ... 6 or 7 nines of purity.
The minimum purity required for a working solar cell is 99.9999% or four 9's.
The semiconductor industry can afford the additional hassle and cost of reaching those
11 nines because it is a $530 billion global industry. The wafer's purity
costs make up only a small percentage of the chip's final cost - less than 1%.
It is a different story for solar grade silicon. Roughly speaking,
the potential return on investment for a solar installation depends on several factors:
How much the solar module costs, measured on a dollar per square meter basis;
The solar module's solar conversion efficiency,
which measures the percentage of sunlight energy the module converts into electricity;
The location in which it is installed;
The cost at which the electricity is sold into the market;
And so on. Raw silicon greatly influences this equation through the solar module's cost.
15-20% of the solar module's final cost derives from the cost of its raw silicon material.
So the industry is not necessarily interested in finding the best performing solar cell ever.
Rather, they want the best balance between performance and cost.
## Production
This economic factor was a critical reason why the solar industry consolidated towards using silicon
in the late 1970s. The reasoning at the time being
that solar could sail in the wake of the booming semiconductor industry.
The steps for making and processing silicon wafers for solar panels may be familiar to you
if you saw my other video on semiconductor silicon wafers.
We begin again with metallurgical or metal-grade
silicon - which is used for producing steel or cast iron.
We make it by putting silica or sand together with charcoal
into an electric arc furnace and heating the whole thing up to 1,900 degrees Celsius.
Metal-grade silicon is 98% pure but still contains significant impurities like iron, aluminium,
and so on. This is not pure enough for solar-grade silicon, so we need additional purification.
The leading way to do this is the Siemens method. Here,
you synthesize trichlorosilane from the metal-grade silicon.
Then you distill and purify this highly explosive liquid before reacting it with
hydrogen inside a Chemical Vapor Deposition reactor to create solar grade silicon.
## Alternative Methods
The Siemens process remains the mainstream process for producing
very pure silicon with some 90% market share.
However, it also suffers from low yields - about 30%. The process deals in and creates various
toxic chlorine gases - which is an environmental hazard. And it is also extremely energy-intensive.
Nevertheless it remains cost effective, but mostly due to significant existing capacity
rather than being necessarily more efficient. Being so simple,
there are largely no more big improvements left to be made.
So researchers have looked for alternate ways to
produce solar-grade silicon from metal-grade silicon.
The leading alternative is the Fluidized Bed Reactor or the FBR method.
It starts out largely the same way as the Siemens
method - turning metal-grade silicon into trichlorosilane.
But where the Siemens method next converts that trichlorosilane directly into silicon,
FBR turns it into an intermediate product called silane - the silicon analogue of methane.
Then we put a mixture of silane and hydrogen into a fluidized bed reactor
and heat it up to about 850 degrees Celsius. Small silicon seed particles are added to the mix.
The silane decomposes onto the seed particles,
creating larger pure silicon granules that fall to the reactor's bottom and are collected.
The process is far more energy efficient than the Siemens method - a nice plus.
However, industrial scaling can be difficult to achieve. Furthermore,
managing and monitoring conditions inside the reactor can be exceptionally challenging.
## Crystals
At this point, the polysilicon is still in chunks. It needs to be turned into a crystal
and cut into wafers before you can make solar cells and cell modules.
You can make two types of crystals: Single-crystal
and multi-crystal silicon. I have heard the latter referred to as polysilicon as well.
Here, I will add the "crystal" modifier to differentiate from the chunks and the crystals.
Single crystals are exactly as the term goes, an unbroken single crystal
all throughout the whole thing. All the atoms are bounded to four other atoms.
The semiconductor industry uses single-crystal silicon for their wafers, largely employing either
the Czochralski or Float Zone methods to produce them. The solar industry uses these methods too.
Single crystal solar cells are more efficient for reasons we will discuss
later but suffer higher costs due to lower yields,
expensive equipment, and longer wait times. In such a competitive industry, this is not ideal.
## Multi-Crystal Silicon
Multi-crystals are not as neatly organized as their single-crystal cousins.
They are referred to as "multi" because they are made up of many "grains".
The dislocations in the resulting atomic bonds are referred to as "grain boundaries". This
interesting structure gives them this distinct, flaky look which I think is kind of cool.
Multi-crystal silicon is largely produced using a method called directional solidification.
We place the silicon feedstock into a coated crucible.
Then we heat it up to something about 1,500 degrees Celsius to create silicon melt.
Then, we slowly and carefully start to remove the heat from the bottom of the crucible.
This can be done in a variety of ways.
But an interesting one is where they use a lift to move the cooling melt
through a "hot" and "cold" zone to slowly cool it.
When done right, the silicon melt crystallizes into a square
shaped crystal that can weigh up to a ton.
Crystals made with directional solidification tend to have more defects
than their single-crystal cohorts. Dislocations due to thermal stress
tend to have unpredictable outcomes on the panel's final solar efficiency and longevity.
On the other hand, multi-crystal silicon is far cheaper to make. And over time,
the industry has managed to reduce the defect rate
and thus improve the panels' efficiency in production from just 8% in 1980 to 18% today.
For these reasons, multi-crystal silicon solar cells currently dominate the
industry - with some 60-70% market share depending on what source you look at.
## Wafers
Finally, we need to turn the crystals into ingots,
make wafers out of those ingots, and turn those wafers into solar cells and modules.
The crystals are cut into ingots using diamond-coated saws.
For multi-crystal silicon slabs, the outsides are contaminated and have to be chopped off.
I am somehow reminded of a dry-aged piece of beef.
Then, the slicing. We take a steel wire and an abrasive slurry
made up of a glycol fluid and a commercially standardized silicon carbide grit powder.
You are essentially scratching the powder particles over the crystal using the wire.
Modern machines can produce about 2,000 to 4,000 wafers from an ingot
in a single run. Each wafer is generally about 200 to 300 microns thick.
The goal is to produce as many wafers as possible with a minimum loss of silicon
and powder grit. And they take it super seriously.
They model everything from wire friction to slurry properties in the computer.
Once we have the wafers, they are doped to create the P-N barriers we talked about at
the start of this video before being packaged and put together into panels.
## Growth and Supply
In the industry's early years, they used the slightly flawed silicon byproduct
from the semiconductor wafer producers - essentially their scrap.
And this was fine as long as the solar industry remained relatively small
compared to semiconductors. Which it was.
As late as 2000, the solar industry used about 3,000 metric tons of silicon.
In comparison, the semiconductor industry consumed about 17,000 metric tons.
This changed in the first decade of the 2000s after
Europe passed new policies favoring solar energy.
Very quickly, the solar industry's silicon demand volumes overtook
and vastly outpaced the semiconductor industry's.
This boom caused shortages, forcing the industry to experiment with new ideas
like slicing their wafers thinner and recycling their waste silicon.
But armed with cheap money, the polysilicon industry scaled up their capacity too fast
and busted out - crashing prices.
By 2016, the solar industry consumed 326,000 metric tons
of solar-grade silicon. This is 11 times the annual consumption of the semiconductor industry.
## Conclusion
Despite its recent near-exponential growth,
the solar industry still provides less than 4% of total worldwide energy consumed.
In order for solar to play a more critical role in the global energy portfolio,
solar cell technologies need to get even cheaper and more efficient.
This might be challenging as the industry grapples with chronic underinvestment
while approaching the upper limits of solar conversion efficiency for silicon solar.
Yet silicon makes up first,
second, and third place in the most commonly-used solar cell installations.
Technically better alternatives might exist out there in the theoretical world,
but history and the market have already decided. Silicon is here to stay.