在过去二十几年 太阳能对于 in the last two decades the contribution
这整个世界的能源供应 of solar energy to the world's total
energy supply has grown significantly
this video will show how solar cell or
photovoltaic cell produce electricity
energy from the Sun is the most abundant
and absolutely freely available energy
on planet earth in order to utilize this
energy we need help from the second most
abundant element on earth sand the sand
has to be converted to 99.999% pure
silicon crystals to use in solar cells
to achieve this the sand has to go
through a complex purification process
as shown
the raw silicon gets converted into a
gaseous silicon compound form this is
then mixed with hydrogen to get highly
purified polycrystalline silicon
these silicon ingots are reshaped and
converted into very thin slices called
silicon wafers the silicon wafer is the
heart of a photovoltaic cell when we
analyze the structure of the silicon
atoms you can see they are bonded
together when you are bonded with
someone you lose your freedom
similarly the electrons in the silicon
structure also have no freedom of
movement
to make the study easier let's consider
a 2d structure of the silicon crystals
assume that phosphorus atoms with five
valence electrons are injected into it
here one electron is free to move in
this structure when the electrons get
sufficient energy they will move freely
let's try to make a highly simplified
solar cell only using this type of
material when light strikes them the
electrons will gain photon energy and
will be free to move however this
movement of the electrons is random it
does not result in any current through
the load
to make the electron flow unidirectional
a driving force is needed an easy and
practical way to produce the driving
force is a PN Junction let's see how a
PN Junction produces the driving force
similar to n-type doping if you inject
boron with three valence electrons into
pure silicon there will be one hole for
each atom this is called p-type doping
if these two kinds of doped materials
join together some electrons from the N
side will migrate to the P region and
fill the holes available there
this way a depletion region is formed
where there are no free electrons and
holes due to the electron migration the
n-side boundary becomes slightly
positively charged and the P side
becomes negatively charged
an electric field will definitely be
formed between these charges this
electric field produces the necessary
driving force let's see it in detail
when the light strikes the PN Junction
something very interesting happens
light strikes the end region of the PV
cell and it penetrates and reaches up to
the depletion region this photon energy
is sufficient to generate electron hole
pairs in the depletion region
electric field in the depletion region
drives the electrons and holes out of
the depletion region
here we observe that the concentration
of electrons in the end region and holes
in the P region become so high that a
potential difference will develop
between them as soon as we connect any
load between these regions electrons
will start flowing through the load the
electrons will recombine with the holes
in the P region after completing their
path in this way a solar cell
continuously gives direct current
in a practical solar cell you can see
that the top and the layer is very thin
and heavily doped whereas the P layer is
thick and lightly doped this is to
increase the performance of the cell
just observe the depletion region
formation here you should note that the
thickness of the depletion region is
much higher here compared to the
previous case this means that due to the
light striking the electron hole pairs
are generated in a wider area compared
to the previous case this results in
more current generation by the PV cell
the other advantage is that due to the
thin top layer more light energy can
reach the depletion region
now let's analyze the structure of a
solar panel you can see the solar panel
has different layers one of them is a
layer of cells you will be amazed to see
how these PV cells are interconnected
after passing through the fingers the
electrons get collected in busbars the
top negative side of this cell is
connected to the back side of the next
cell
through copper strips here it forms a
series connection when you connect these
series connected cells parallel to
another cell series you get the solar
panel a single PV cell produces only
around point five voltage the
combination of series and parallel
connection of the cells increases the
current and voltage values to a usable
range the layer of evie a sheeting on
both sides of the cells is to protect
them from shocks vibrations humidity and
dirt
why are there two different kinds of
appearances for the solar panels this is
because of the difference in the
internal crystalline lattice structure
in polycrystalline solar panels Multi
crystals are randomly oriented if the
chemical process of silicon crystals is
taken one step further the
polycrystalline cells will become mono
crystalline cells even though the
principles of operation of both are the
same
mono crystalline cells offer higher
electrical conductivity
however mono crystalline cells are
costlier and thus not widely used
even though running costs of PV cells
are negligible the total global energy
contribution of solar voltaic is only
1.3 percent this is mainly because of
the capital costs and the efficiency
constraints of solar voltaic panels
which do not match conventional energy
options
solar panels on the roofs of homes have
the option to store electricity with the
help of batteries and solar charge
controllers however in the case of a
solar power plant the massive amount of
storage required is not possible so
generally they are connected to the
electrical grid system in the same way
that other conventional power plant
outputs are connected with the help of
power inverters DC is converted to AC
and fed to the grid
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