Welcome back in this section. 1
We are going to discuss Solar cell and its properties most solar cell are based on PN junction. 2
So the question is what is the PN junction we take as an example. 3
The case for silicon If we have semiconductor (silicon) in which one part is doped p-type 4
and another part is doped n-type 5
we have created a so-called p-n junction. 6
In p-type the holes are the majority charge carriers and in n-type the electrons are the majority charge carriers. 7
There are two different mechanisms control the transport of charge carriers in semiconductors: first one is diffusion 8
which is controlled by a density gradient. 9
The second one is drift is controlled by electric fields, which you can build in the p-n junction or apply externally. 10
or apply externally. 11
Let's take an example. 12
light absorption only affects the density of the minority charge carriers in doped semiconductor materials 13
light absorption only affects the density of the minority charge carriers in doped semiconductor materials 14
let's discuss the operation of that solar cell. solar cell in the dark the drift of the minority carrier 15
and diffusion of majority charge carriers are in balance. so there where no current if we apply reverse 16
bias on such p-n junction in the dark, the depletion zone gets wider, 17
the diffusion of majority charge carriers is suppressed and only an extreme small current 18
and only an extreme small current related to drift of minority charge 19
carriers is generated. 20
or in other words, 21
The density gradient is becoming smaller. 22
The current density related to diffusion of both electrons and holes is reduced. 23
The drift is enhanced Since the electric field E is larger 24
the drift of the electrons and holes is slightly enhanced. In this case the drift current density 25
is dominant. 26
The diffusion current density Since the drift current density is ruled by the density of the minority charge carrier. 27
in the p and n region, the total net current is extremely small. or in other words in the reverse 28
bias condition an extreme small current will move from the contact at the p-region to the n-region. 29
the outer circuit. 30
This means that on average electrons are walking from the contact of the n-region to the p-region 31
shown in the figure. 32
If we apply a forward bias on such p-n junction in the dark, the width of the depletion 33
zone is getting smaller 34
the diffusion of the majority charge carriers is significantly enhanced and overrules the drift of minority 35
charge carriers. The p-n junction becomes conductive and is able to generate a current. 36
or in other words, the density gradients become much larger. 37
As a consequence this the current density related to diffusion becomes larger. 38
On the other hand, the electric field E is reduced , which means that the current density 39
related to drift is getting slightly smaller. 40
But Note that the effect of the increased flux due to diffusion is much larger than the small change 41
in drift. 42
This means that we generate a net current over the depletion zone. 43
More electrons are diffusing from the n-region to the p-region, than are drifting from the p-region to the n-region. 44
Means More holes are diffusing from the p-region to the n-region, than are drifting for the n-region to the p-region. . 45
region. 46
This means by applying a forward bias over a p-n junction in the dark we produce a net current through the electrical 47
circuit. 48
as shown in the figure 49
we can summarize that in is a dark under forward bias 50
the diffusion of charge carriers over the depletion zone 51
is dominant, 52
but under reversed bias 53
the drift of charge carriers over the depletion zone is dominant. Under forward bias in the dark, 54
the p-n junction can produce relatively large currents, whereas in reverse bias, it generates very small 55
currents. 56
Such a device is called a diode 57
It has a high conductance in forward bias, but has a low conductance in reverse bias. 58
solar cell and dark like diamond. 59
Now we are going to shine the light on is a device. 60
This means that we are looking as a solar cell. 61
Light is incident from the left on the p-region. For the moment 62
we assume the light is being absorbed in the p-region and the n-region. The absorption of the photon will generate 63
electron and hole pairs. 64
Important to realize and as discussed earlier, light absorption only affects the density of the minority charge carriers 65
in doped semiconductor materials. 66
This means that the light excited charge carriers significantly increase the density of the electrons in the 67
p-region and the density of the holes in the n-region. so we increase the drift. 68
This can be easily recognized by looking at the equations for the current densities 69
(Je = nqµeE). We see that we are significantly 70
increasing the drift over the depletion zone, which is indicated by the larger arrows. 71
Many electrons drift from the p-region to the n-region and many holes drift from the nregion to 72
the p-region. The current density related to drift can be easily increased by many orders of magnitude under 73
illumination in reference to the p-n junction in the dark. By illuminating a p-n junction we can generate a current. 74
current 75
Finally, we're looking at the working principle of a solar cell. Using an electrical circuit we connect 76
the contact at the p-doped silicon with that of the n-doped silicon, or in other words we short-circuit the p-n 77
junction. 78
Short circuit In this condition, the illuminated p-n junction will produce only an electrical current. We call 79
this current the short-circuit current of a solar cell. We can make a very simplified animation of 80
the journey of the generated charge carriers. On average a minority electron will drift to the ntype material 81
and diffuses to the metal contact in which the electron is injected. The electron moves to the contact 82
at the p-side and is injected into the p-type silicon. and recombines with a hole. The minority holes in 83
the p-type drift across the depletion zone and diffuse to the back contact to recombine with the electrons. 84
electrons 85
the second to thing is open circuit. 86
However, no current is created when we open 87
the electrical circuit. In that case, the dominant drift current of light excited charge carriers 88
will positively charge the p-region with holes and negatively 89
charge 90
the n-region with electrons. This charging creates an electric field opposite to the built-in electric 91
field and reduces the net drift current 92
again. This charging of free holes in the p-region and free electrons 93
in the n-region will build up until both the drift currents are in equilibrium. 94
This means that the device does not generate a current, but builds up an electric field, or voltage. 95
voltage. 96
The voltage created by an illuminated solar cell under open-circuit conditions is called the open-circuit voltage. 97
voltage. 98
If you couldn't understand the open circuit condition and short circuit condition we are going to discuss 99
by equations in the next section so we recommend you to see the next section open circuit voltage and 100
short circuit current. 101
The last thing we are going to talk about today is the equivalent circuit of PVsolar so to understand 102
To understand the electronic behavior of a solar cell in light, it is useful to create a model which is electrically equivalent. 103
An ideal solar cell may be modelled by a current source in parallel with a diode; 104
in practice no solar cell is ideal, so a shunt resistance and a series resistance component 105
are added to the model as shown in the figure blow. 106
To understand the main parameters of solar cell we are going to simplify the above circuit by eliminating 107
the shunt resistance and the series resistance as shown below. 108
so the solar cells or ideal solar cell can be simplified by a current source in parallel with diode 109
in the next section we are going to define the open circuit and short circuit from this equivalent circuit 110
so see you in the next block.