So photo voltaic effect has a very long history, and
what I want to do in this couple of videos is
to present you a brief timeline, or you know, summarize
some of the ways key events which happened in this field.
And this is by no way a comprehensive summary, in fact you know, it's a
very long history lesson and there have been
many, many many, many developments in his field.
So I just want to present
a few, few, you know, few key events which happened.
So the very first photovoltaic effect, the very first
observation of photovoltaic effect, the discovery of the photovoltaic effect, was
done in 1839, and it's credited to this gentleman
Edmond Bacquerel, who was a, who was a French, who was a French physicist.
And, he was a child prodigy, so as well, and at the age of 19
while working with his father, he discovered this photovoltaic effect.
So the experiment that he performed, I'm showing it over here.
So he, in fact, observed this, for, for electrolytes and he had this,
equipment where, you know, he was
essentially measuring this, voltage across these
two electrodes, and he was shining light on one of these electrode.
And he, he measured a potential difference when
you shine the light on one of these electrodes.
So, this was the first discovery of photovoltaic effect.
But this was not a solid state device and in fact this with a electrolytic solution.
So the, first demonstration of what you can call
a solid state photovoltaic effect in a solid state device, and a first
solar cell made of a semiconductor could be, could be credited
to this gentlemen, Charles Fritts, and he was an American inventor.
And he was experimenting with selenium.
A lot of the earlier
demonstrations of solar cells were done in this in this material selenium.
Because this was one of the semiconductor material
whcih you could get your hands around easily.
in those days, remember this is 1883 and the processing technology for silicon.
hadn't been developed, yet.
So the way Charles Fritts, designed his experiments was that he
sandwiched, thin layer of selenium.
He sandwiched this, thin layer of selenium.
between, two different, Between, metals of two different, work functions.
So he chose.
He, he, he took a metal plate, which was made, for example, of brass.
And then he placed this, sheet of.
Selenium which was in thickness, between 25 to,
you know, 100 to 125 micron.
And then on the top he basically based this very thin sheet of gold on the top.
And when you, when he then subsequently shown
light on top of this device You could measure photovoltaic effect.
And you could in fact extract a reasonable amount of electrical energy out
of this solar cell.
So, you know just to illustrate how this solar cell might work.
So this is a short key contact solar cell.
So you have this semiconductor which is selenium, and I'm drawing this band
diagram of selenium so it has a conduction band and let's say valance band.
And now you're essentially, you're
essentially you know, you're, you're you're
placing a semiconductor material.
gold, which has a higher work function on the top.
And then you're placing another semiconductor material.
Which have your, different work functions at the bottom.
So the built in voltage, in this, in this, are, in this junction is
essentially coming from the difference, difference in
the work function between, between, these two metals.
So it's not a junction based, sort of sell as we will.
See later but it's a, it's short key contact base solar cell and the, the built
in voltage is essentially resulting from this difference
in the work function of these two metals.
But Nonetheless the efficiency of these solar
cells which were made out of selinium was
very poor, in fact most of these solar cells in those days had efficiency of less
than 1%. So the next big breakthrough in terms of
effiency came in Came around in 1954
and it's credited to this trial of these three
people who used to work at Bell labs and shown here is
Pearson Chapin and Fuller.
And in fact there's interesting you know, background story behind this.
So, Chapin was the original solar energy enthusiast and he
was pursuing this research of solar cells based on selenium.
And Pearson was a good friend of good friend of Chapin, and he persuaded him
to, you know, look at this other material
called silicon, which was being developed for especially
for transistors around the same time in 19 54, at the same place in at.
Bell Labs.
And he introduced him to Fuller who was essentially another
scientist who was working on silicone, especially doping of silicone.
So, he you know they together and with, you know, using
the help of Fuller who was an expert in doping these layers
of silicone very precisely.
Using, using you know, phosphorus or boron.
So they came up with this design where, where, Fuller essentially took an N type
wafer and Then he uniformly doped a, p type.
You know in a furnace kind of doping process where
it was doped on all sides, all sides using this,
using this boron doping.
And then as they lead from this PN junction,
PN junction, at the very near to the surface.
And then essentially they etched out, they etched out
the, the P region for a small portion, and
then they made a contact to the N region, they also made a contact to the P region.
And then when you shine, when you shine light From
the top.
Of course, this device will generate will generate solar energy.
An interesting point to note is that this device is all back-contact solar cell.
You know, both the contacts are located at
the back of the device, and this is interesting.
This, we'll, you know, we'll come back to
this later, and we'll discuss that, you know, the.
some of the most efficient cells made
in silicon also need to realize this back contact to avoid effects of shadowing etc.
But
so this device when you know, when this was measured
and optimized this could achieve efficiency of close to 6%,
which was a big jump from the previous selenium based
solar cell which had a efficiency of less that 1%.
So this was again, you know, Bell labs.
Made a big announcement about about this and mention that
this is you know, one of the photographs that I'm borrowing
from their archives.
And, it's interesting to note that, you know, people used to wear
scientists used to wear coats and a tie, at that time, to work.
Which is something I haven't seen you know, recently.
The culture of dressing and in in labs at least it has changed.
And then they made you know, they made you know a, they tried
to generated some press, publicity, around this around this invention.
So this is one of the images that was published in
one of the popular magazines, again, you know, by popularized by
Bell Labs in It's showing this family having this solar panel
and, you know, they, it all seems very blissful, sunlight coming in.
And there was a big article in the
New York Times announcing this intervention in 19 54.
And I'm borrowing a expert excerpt from that article.
And it proclaimed that this is the beginning of a new era, and
leading eventually to the realization of mankind's one of the most
cherished dreams of mankind, that is to harness the almost limitless energy
of the sun for the use of civilization.
Of course, here, you know, it says it says this article claimed
that that might be the eventual future, but we are not there
yet, and I'll continue in the next video why it has, it took so long
for adoption of this solar cell into mainstream technology.