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The Mystery Flaw of Solar Panels - YouTube
Channel: Real Engineering
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this episode of real engineering is
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brought to you by brilliant
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the problem-solving website that teaches
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you to think like an engineer
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installed global capacity of solar cells
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has increased
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year on year for the past decade fueled
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by the plummeting prices
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and rising efficiency of solar cells
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forcing fossil fuel producers out of the
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market
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through technological advance at the end
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of 2019
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the total installed capacity of
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photovoltaic cells
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exceeded 630 000 megawatts
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an astounding figure that is going to
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continue to rise in the coming decades
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however in the 40 years we've been using
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solar cells
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there has been a mystery flaw that has
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been sapping away
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potential electricity from the
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photovoltaic cells
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upon testing in the laboratory newly
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manufactured solar cells
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display an efficiency of about twenty
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percent
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meaning they can convert twenty percent
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of the incoming energy
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from sunlight into electric current
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however
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within hours of operation that
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efficiency would drop to eighteen
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a 10 drop in total electric generation
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losing 10 of 630 000 megawatts of power
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is no small problem that's equivalent to
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about 30
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nuclear power plants worth of power
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capacity if the solar panels could
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operate
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all day which they can't but you get the
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point
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there's a lot of potential electricity
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being lost
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it's no wonder that scientists and
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engineers have been hunting down the
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cause of this problem
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termed light-induced degradation for 40
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years
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and last year we may have finally
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cracked the problem and found the cause
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behind
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this mysterious loss in power to
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understand it
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we first have to understand how
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photovoltaic cells work
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photovoltaic cells use the photovoltaic
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effect
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to generate a current an effect where
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photons of a particular threshold
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frequency
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striking a material can cause electrons
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to gain enough energy
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to free them from their atomic orbits
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and move freely
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in the material this is best achieved
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with semiconductors
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whose unique properties lying between
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conductors and insulators
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allows them to most easily elevate
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electrons from atomic orbit
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to moving freely among their atoms some
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of the first solar cells were created
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with selenium
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like this one created by charles fritz
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sitting atop a new york roof
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in 1884 a revolutionary device that
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produced a consistent current of
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electricity
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but it was achieving an efficiency of
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just one percent
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converting one percent of the energy
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striking it in the form of light
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into electricity this in combination
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with the high cost of selenium
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made it an unviable source of
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electricity
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to succeed these devices needed to
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compete with fossil fuel power sources
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before the photovoltaic effect could
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power the world
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scientists and engineers would need to
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figure out
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how to increase that efficiency
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percentage and to do it
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with cheaper materials enter silicon
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a common semiconductor material that has
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formed the bedrock of the electronic age
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this is going to be our starting
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material for our solar cell
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let's build the solar cell from scratch
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and see how
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efficiencies were gradually increased
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over time
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let's first look at what happens when
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light interacts with the pure silicon
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crystal
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like this incoming light can do one of
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three things
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it can be reflected absorbed or simply
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pass right through it
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if light is reflected or passes through
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it cannot produce the photovoltaic
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effect
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step one to improving our efficiency is
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to minimize the amount of light
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that gets reflected off the material
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this is wasted energy that affects our
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efficiency level
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in fact 30 percent of light that strikes
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untreated silicon is reflected so
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before we even start our maximum
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efficiency drops to 70
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for this reason silicon is often treated
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with a layer of silicon monoxide which
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can reduce the light reflected
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to just 10 percent while a second layer
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with a secondary material
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like titanium dioxide can reduce it as
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low
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as 3 percent texturing the surface of
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the material can further increase the
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probability of light being absorbed
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if it is textured like this light that
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is initially reflected
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has another chance to strike the
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material and be absorbed
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only light that is absorbed can
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potentially cause the photovoltaic
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effect
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but not all light will we need photons
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above a threshold energy to
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increase an electron's energy enough to
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allow it to move freely in the material
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a photons energy is defined by
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multiplying planck's constant
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by its frequency silicon requires
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photons with 1.1
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electron volts to produce the
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photovoltaic effect
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which corresponds to a wavelength of
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1110 nanometers
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this lies around here in the light
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spectrum and any lower energy from here
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down
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cannot produce the photovoltaic effect
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this light will simply cause the atom to
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vibrate and create heat
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this graph shows the total solar energy
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being emitted by the sun
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however a good deal of this does not
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reach the earth's surface
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as it is absorbed by the atmosphere this
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is a more realistic graph
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about four percent of the energy
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reaching earth's surface
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is in ultraviolet as the sun emits
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relatively little
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ultraviolet photons 44 percent is in the
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visible spectrum
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and 52 is in the infrared spectrum
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this may sound surprising as infrared is
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lower energy
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but it covers a wider range of the
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spectrum and thus accounts
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for more energy because silicon cannot
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make use of light with a wavelength
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greater than 1110 nanometers everything
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from here up
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is energy we cannot convert into
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electricity
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this represents about 19 of the total
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energy
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reaching earth another thing to note is
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that light with higher energy
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does not release more electrons it
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simply produces
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higher energy electrons blue light has
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roughly twice the energy of red light
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but the electrons that blue light
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releases simply lose their extra energy
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in the form of heat
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producing no extra electricity this
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energy loss results in about 33 percent
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of sunlight's energy being lost
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so these spectrum losses alone cause a
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52
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loss in efficiency this is a lot of
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energy to lose
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but silicon sits near the ideal
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threshold frequency that balances
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these two energy losses capturing enough
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of the lower energy wavelengths
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while not losing too much efficiency as
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the result of the material heating up
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this is such a large loss in power that
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in some climates
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active cooling which takes some of the
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electricity the panels create
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to cool the panels actually results in
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more electricity being generated
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the reason solar panels lose efficiency
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as they get hotter
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is quite complicated and outside the
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scope of this video
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but for now all you need to know is that
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silicon balances
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these factors best for terrestrial
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purposes
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on to the next problem knocking an
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electron free
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by itself does not create an electrical
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current in our circuit
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it just frees an electron to move freely
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about the material
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to create a useful current we have to
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force this electron around an external
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circuit
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where it can do work freeing electron
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also creates a positively charged hole
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in its place that is also free to move
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about the material
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if an electron meets a hole it simply
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fills it and our energy is wasted
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the next trick to maximize efficiency is
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to limit the chances electrons have
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to fill these holes and to force them
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into our circuit
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as quickly as possible to do this we
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used the unique properties of silicon
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silicon has four electrons in its outer
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shell and thus
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readily forms a crystal structure with
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four neighboring atoms
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using covalent bonds a bond where
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neighboring atoms share an electron pair
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we can manipulate this behavior and
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tailor the crystal's material properties
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by adding impurities called dopants
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say we add boron atoms to the silicon
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crystal wafer
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these boron atoms have three electrons
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available for bonding
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with the silicon crystal but silicon
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wants four
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so this creates a hole in the crystal
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that wants to be filled
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with an electron we call this a p-type
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as it has positive charge carriers now
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let's say we create another wafer of
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silicon
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but this time we add atoms with five
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electrons available for bonding
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like phosphorus again the phosphorus
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bonds with the silicon
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but this time we have an extra electron
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that can float freely about the material
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we call this an n-type because it has
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negative charge carriers
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now let's sandwich these two materials
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together and see what happens
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the positive holes and negative
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electrons migrate towards each other
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the electrons will jump into the p-type
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and the holes jump into the n-type
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this causes an imbalance of charge
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because now the p-type has more negative
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charges and the n-type
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has more positive charges we have just
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formed an electromagnetic valve
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that allows electrons to pass in one
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direction let's see how this works
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suppose a photon with sufficient energy
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enters the p-type side
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of the solar cell and knocks an electron
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free
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the electron starts bouncing around the
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material and one of two things can
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happen
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it can recombine with a hole resulting
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in no current
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or it can come into the electromagnetic
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field
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at the junction of the materials here
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the electromagnetic field
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actually accelerates the electron across
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the junction
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into the n-type side where there are
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very few holes for it to fill
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and to boot the junction's
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electromagnetic field
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actually prevents the electron from
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passing back to the other side
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a similar thing happens on the n-type
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side where holes are selectively
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transported
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across the junction before they can
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recombine
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this means one side of the junction
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becomes negatively charged
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while the other side becomes positively
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charged we have created a potential
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difference
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or in other words a voltage if we add
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some metal contacts and an external load
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circuit
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these electrons will pass along the
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circuit to recombine with the holes
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on the other side we've just created a
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solar cell
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you may notice a problem here though by
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adding metal contacts to the upper
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surface of the solar cell we've just
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blocked light from entering the cell
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and thus reduced its efficiency this is
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yet another problem
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engineers have had to think carefully
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about in their quest
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to optimize solar cell efficiency over
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the years
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engineers have optimized both the shape
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and manufacturing techniques
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to minimize the area covered by the
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metal electrodes
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while also minimizing the resistance the
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electrons will face in
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entering the external circuit one
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research paper used topology
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optimization
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to design these electric contacts
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topology optimization
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uses algorithms to optimize the design
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of objects
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using constraints the engineer inputs
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using this method for the electric
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contacts produce something remarkably
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like the vasculature of a leaf
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that shouldn't really surprise you
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vasculature on a leaf
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does not perform photosynthesis it
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instead brings the water that is
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essential for
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photosynthesis to the leaf and extracts
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the useful products
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serving a similar purpose as our
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electric currents
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so of course plants have developed the
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perfect shape
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to optimize the energy they can absorb
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from the sun
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plants have had millions of years to
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evolve this shape
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however most solar cells use a simple
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grid shape
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as it is cheaper to manufacture this
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typically results in an efficiency loss
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of about
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eight percent all told these effects
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result
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in a typical modern solar cell having a
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laboratory tested efficiency of 20
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so what was happening to cause that drop
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to 18
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after a couple of hours of operation
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this problem was the focus of hundreds
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of scientific papers
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and many found clues to the problem
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manny noted that the efficiency drop was
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correlated
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to the concentration of boron and oxygen
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in the silicon
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and noted that the drop did not occur
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when boron was substituted
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with gallium thus it was known a boron
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oxygen defect was causing the issue
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others found that the defects could be
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reversed by heating the silicon
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in the dark at 200 degrees for 30
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minutes
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but it would return once again upon
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exposure to the light
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efforts in reducing the problem have
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primarily focused on reducing the
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concentration of oxygen impurities
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in the silicon wafers which occur as a
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result of the silicon manufacturing
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technique
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that is the source of 95 of silicon
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solar cells
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these manufacturing techniques are still
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a point of research
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and the engineers and scientists were
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working blindly
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little was known about the actual defect
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creation process
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and how exactly it was causing such a
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large drop in efficiencies
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leaving engineers with less information
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to solve the problem with
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this paper used a special imaging
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technique and observed
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these boron oxygen molecules converting
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into something the paper refers to
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as shallow acceptors when exposed to
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light
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in essence they observe the defects
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transforming into little electron
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traps that acted as recombination sites
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and thus reduced the time and
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probability of electrons
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entering the circuit to do work with
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this knowledge
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engineers can now develop better
[854]
techniques for preventing this
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phenomenon
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and hopefully increase our renewable
[858]
energy capacity
[860]
in the coming years it's easy to think
[862]
that technology has reached a point of
[864]
being so advanced
[866]
that knowing where things can be
[867]
improved is practically impossible
[870]
for the average person but that simply
[872]
isn't true
[873]
a little bit of research into any area
[875]
will reveal
[876]
countless problems humans are still
[878]
grappling with fixing
[880]
when i started researching this video i
[882]
knew little about solar panels
[884]
beyond the basics in order to make this
[886]
video
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i took a week to deep dive into some
[889]
college textbooks using my knowledge
[891]
of material science and electronics to
[893]
guide my research
[895]
but i had some gaps in understanding
[897]
that the college textbooks just assumed
[899]
i had pre-existing knowledge of
[901]
terms like band gap and fermi levels
[903]
kept appearing and without understanding
[905]
these terms
[906]
i couldn't make complete sense of the
[909]
explanations
[910]
these were like canyons in my journey
[912]
for knowledge i couldn't advance until i
[914]
filled them in
[915]
so halfway through the research in
[917]
writing process
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i decided to stop what i was doing i
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changed my tactics
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and i decided to take the brilliant
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course on solar energy
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because i knew brilliant would guide me
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through the very basics of the subject
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right through to the more complicated
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concepts it worked a treat
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all the little gaps in my understanding
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were filled in and i could now read
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scientific papers
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and college textbooks without feeling
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like i was reading a foreign language
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this is the magic of brilliant
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every day as always thanks for watching
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