So now we're going to have a look at electronic devices.

And how they work?



So in the early days of conjugated polymers, one of the first things that people wondered, having discovered that these materials.

Essentially behaved as an organic semiconductor.

I wondered whether they could actually be used as a Silicon substitute.

A semiconductor in an electronic device so too.

Get some sort of idea about how these.

How these materials can be used in such devices? We first of all need to consider how electronic devices work.

And there's quite a useful introduction to this at this website based at Saint Andrews University. Unfortunately, the guy who initiated this website who was in that Electronic Engineering Department.

Is now retired, so this website is.

A little bit elderly. It's a bit clunky and its presentation is a bit old fashioned.

But there's some really quite useful material there.

If you visit the home page, there's a box on there called components, and if you click on that you see a whole list of very different devices, among which are diodes and transistors.

Which are some of the things we're going to consider now? You may wonder why we need to do this, and I guess you could justify this by saying.

But if you're an organic synthetic chemist and you're working in drug discovery.

You may be a specialist in synthetic chemistry, but you're still gonna no need to know.

Some pharmacology, some biochemistry and maybe some toxicology.

So that you can have some idea of what it's actually going to be useful to make and what it might not be a good idea to make, and in the same sort of way as a chemist who's working in organic electronics.

It's going to need to know at least a bit of the polymer science, some basic device physics.

And even a bit of electronic engineering.

So that's why we need an understanding of how these various components work. I suppose the simplest electronic component is a resistor.

And a resistor is just something anything obeys ohm's Law.

And that can be categorized as a resistor.

And to make a current flow through a resistor, clearly you need a voltage to drive it.

And the resistance.

The parameter called resistance, which is measured in ohms.

And resistors are used deliberately in circuits to lower voltages, and they do this by essentially wasting some electrical energy as heat.

And so a material this used to make a resistor has to be something that conducts electricity but doesn't conduct it very well.

Another

component that's important to consider.

Is the is the capacitor?

And this is because, well, not only are they used in circuits, but a thin film transistor inherently behaves in a capacitative way, and so it's important to have some sort of appreciation of capacitance.

A capacitor in a circuit is used to store electrical charge temporarily.

And the most common type is a dielectric capacitor, which is just simply 2 metal plates separated by an insulator, which could just be air.

And if you apply a voltage across the two plates of a capacitor.

Then obviously one becomes negatively charged with respect to the other one.

And you're doing work becausw. You are separating charges.

And.

Capacitance is the charge divided by the voltage which is applied to to generate the charge.

And then the capacitance.

And the units of capacitance are farrants.

Now, in order to make most of the electronic devices, we're going to be interested in, we need to understand how semiconductors work. So here's a little bit of revision, which should be fairly familiar already from Chem. 313 last year.

But nonetheless, it's worth just reminding ourselves about this.

So a chemist generally thinks of a metal as something that's vaguely got a partially filled band.

And the metal has electrons which are essentially free and are free to move under an electric field.

But a physicist is a bit more precise about how they define a metal.

A metal is defined as something whose conductivity decreases as a function of increasing temperature.

And that's because if you increase the temperature, there's more scattering.

Of the charge carriers buy by the lattice atoms, and that's the important thing in controlling the conductivity of a metal.

Right at the other extreme, we have an insulator and an insulator is something which has got.

Only filled and empty buns with quite a large energy separation between the highest lying filled bound and the lowest lying empty band.

So typically greater than about four electron volts.

No semiconductor, something which is intermediate between a metal and an insulator. It still has only filled an empty bands, but now at the energy difference between the highest lying filled band and the lowest lying empty bound is smaller, typically less than three electron volts.

So that's again, that's the kind of chemist picture of a semiconductor. But physicists again will be more precise, and would say that a semiconductor is defined as something.

Weather conductivity increases as a function of temperature because in a semiconductor.

In a pristine semiconductor.

There are not very many charge carriers.

What charge carriers there are? Are there as a consequence of the promotion of Electrons from the filter, the empty bound, and that's difficult if the energy gap is.

Significant and so there are very few charge carriers so that the thing that controls the conductivity of a semiconductor.

As a function of temperature, is the generation of more charge carriers, and if you increase the temperature, you're supplying more energy and therefore you're going to generate more charge carriers, and that's much more important in the semiconductor than any scattering of charge carriers by the lattice atoms.

So.

You should be familiar with the idea that for an inorganic semiconductor like Silicon.

We can greatly increase the conductivity of the material.

By this process called doping.

Where we replace a very small fraction of the lattice atoms.

With some impurity atoms that either have fewer or more electrons.

Non Silicon.

So for example, if we.

Replace some of the atoms of Silicon with boron.

Boron does not have as many electrons as Silicon.

And in order to go into a tetrahedral site in the Silicon lattice, a boron Atom needs to steal an electron from the surrounding Silicon lattice to have enough electrons to form the four bonds.

So the boron becomes negatively charged because it's now got an extra electron.

And the missing electron.

Which isn't in the violence bond of the Silicon.

Is called a hole.

And it will be typically delocalized over several thousand Silicon atoms, so it's mobile if you apply voltage, the charge carrier can move.

But of course, the boron is stuck in the lattice, so that's negatively charged, but it can't move because it's covalently bonded into the lattice.

On the other hand, so that that's called P doping.

Because they charge camera that's generated is positively charged.

But we can do the opposite if we add phosphorus or arsenic.

To replace some of the Silicon atoms.

Then phosphorus or arsenic has too many electrons. Has one electron too many.

And so if it goes into a tetrahedral site in the Silicon lattice.

The arsenic will lose its extra electron. That extra electron will then be free to move about.

In the conduction band of the Silicon, but the arsenic or phosphorus.

Has lost an electron, so it's positively charged. But again it can't move because it's anchored into the lattice. It's the extra electron that's mobile in that case, and so we've introduced a negative charge carrier, and therefore that's called N doping.

Now in the.

In an inorganic semiconductor such as Silicon.

One of the main ways to generate electronic devices.

Is to use junctions between N&P doped semiconductor?

And a typical diode in Silicon is usually made using a PN junction.

And.

What happens in this case? You could picture this. I mean, this isn't how diodes are really created, but you could picture this.

As bringing together an end Opton Oppido piece of Silicon.

So the The N doped form will have some extra electrons in the.

In the conduction band.

And it will also have some positively charged dopant atoms anchored into the lattice.

Now when physicists or electronic engineers draw diagrams likeness, they tend to neglect the presence of these. But for our purposes, we're going to remember that there there.

And similarly.

The P doped.

Silicon will have some acceptor atoms.

Which are negatively charged 'cause they've stolen electrons from the Silicon lattice and that leaves behind holes positive charges.

In the Silicon. So if we bring these two pieces of Silicon together, you can picture that some of the electrons from the end end type Silicon side.

Will go over the interface and they will neutralize some of the holes in the P dope side.

But this can only happen to a limited extent because of the presence of these anchored and charged atoms.

Becausw the electrons from the end oakside.

Will only be able to get so far because they will be repelled by the anchored anionic acceptor atoms in the Silicon lattice, and similarly the holes from the P doped side. They can cross the interface and neutralize some of these electrons, but they can only do that to a certain extent because of the anchored positively charged donor atoms in the lattice, and So what you do what you find is that you create this zone where there are very few charge carriers at the interface.

Which is called the depletion region. Because the charge carriers are depleted.

Cannot access a barrier to charge transport from the PN in across the PN junction.

Now by applying a voltage across the PN junction.

You can alter the the height and width of that barrier.

So imagine that if you.

If you apply the voltage such as such that you made the P type Silicon more positive.

On the N type Silicon more negative you'd pump extra holes.

Into this region here you'd pump extra electrons into this region here and so they'd be able to cross the interface.

And the current will be able to flow.

But if you applied the opposite kind of bias, if you made the anti psid positive.

And the P type side negative.

Making the anti psid positive would essentially get rid of some of these electrons here and making this side negative while you get rid of some of these holes, you'd make this region this interface region wider.

And so that will be a more of a barrier to charge flow. So current wouldn't be able to cross the barrier. And that's that's why the PN junction in an inorganic semiconductor acts as a diode. A diode is an electronic device that will allow charge to flow when the bias is applied in One Direction, but it will oppose charge flow if the bias is applied in the opposite direction.

Now this this is a.

A sort of cartoon.

Of the operation of such a device taken from the Saint Andrews website.

And you can see what's happening here.

Well, here a bonus is being applied.

Now.

It's forward interaction. So in other words, making the P type signing positive and the N type side more negative, the barrier is lowered and if we apply a large enough by barrier bias we can essentially make the barrier disappear so that charge can flow freely.

But if we applied the opposite kind of bias.

Now.

Anne.

You can see that the barrier gets steeper and the charge carriers are even more depleted at the interface region, and so it's it's more difficult for charge to flow.

So the flow of charge is impeded.

So that's how a PN junction diode works.

Now, an important characteristic of organic semiconductors, at least up to about three or four years ago.

Was that most organic semiconductors are water called uni polar?

Silicon is A is an ambipolar material, which means that we can both end and P dope. It it doesn't really matter.

It's equally amenable to both kinds of doping.

But organic semiconductors tend to be unipolar, meaning that they can only be doped in one way.

And that's usually P doping. Certainly it is with Polythiophene because it's electron rich, so it's easy to remove electrons from it. It's very difficult to add extra electrons to polythiophene.

Not creates a problem because you can't really make PN junction devices.

With a material like Polythiophene, because you can't Endo pit so PN junction.

Is not likely to work very well in most organic semiconductors.

So we need a different kind of diode.

If we want to make a diode with such a material.

And the type of dialed that we can make with organic semiconductors. One of the types anyway is called a schottky barrier diode, and again you've probably come across this concept in Chem 313, but it's worth revising.

So.

It turns out that if you make.

A contact between a metal and a semiconductor.

One of two things can happen, depending on the Fermi Energy of the metal.

On the semiconductor.

If they're compatible, they have similar Fermi energies, and you make a contact between the metal and a semiconductor.

Then it may just be gevo Mickley. In other words, it may just obey Ohm's Law.

But if there are different Fermi Energies then you can sometimes get what's called a schottky barrier between the metal and the semiconductor.

And this can. This can result in the possibility of making making a diode.

And this is in fact the the architecture that was used to create the 1st.

Organic electronic devices with conjugated polymers. In this case polyacetylene. So this was why this was why people came up with the Durham route to polyacetylene because they wanted to make.

They wanted to make a precursor polymer that they could put down on any substrate surface and then thermally convert it to polyacetylene in order to test it in diodes.

So this is a very basic diode from those days.

And.

In this particular example.

There was a glass substrate.

Onto, which was evaporated in vacuum efilm of gold to act as one of the Contacts.

The polyacetylene was then created on top of the gold and then some aluminium.

Was spotted coated onto the.

Top of the polyacetylene to make the second contact.

Now it's known that the Fermi Energy of gold is very suitable for making ohmic Contacts to polymers.

So it's not the interface between the Golden the polyacetylene that's important here. It's actually the interface between the aluminium and the polyacetylene.

That is what's called a schottky barrier.

And the net result is that the thing that this device behaves like a diode and you can see that in the characteristics shown here.

So there's the voltage being applied on the bottom on the horizontal axis, and the current in a lot on the log scale on the vertical axis here.

Now, if you forward biased the device. In other words, If you make the aluminium more negative.

And the gold positive.

You can see that quite a large current starts to flow at relatively modest voltages. Current goes up by orders of magnitude.

But if you do the opposite. OK, so if you make the gold negative and the aluminium positive because of the presence of this so called Schottky barrier between the two.

Materials very little currently will flow in the other direction.

So this was an early example of a diode made with conjugated polymer semiconductor polyacetylene.

And this work dates from around the 1980s.

Using usually using the Durham roots and one of the groups that was interested in this topic was the Group A group led by physicist called Richard Friend.

At the University of Cambridge, who will meet later on when we talk about organic light emitting diodes? Because it was his work on this.

This eventually gave rise to an accidental discovery that opened the open, the door to organic light emitting diodes.



OK, so all of the components that we've talked about so far.

I want to call passive components.

But a transistor is an example of something called an active component.

Because it has, it has three electrodes.

And what happens when you apply a voltage between two of those electrodes is determined by the voltage that's applied to the third one.

And so.

There are lots of different kinds of transistors.

But again, because Polythiophene and most indeed most inorganic most organic semiconductors are unipolar.

There's only a limited number of architectures or designs that you can actually use.

To make a transistor with an organic semiconductor material and the main one is the so-called metal oxide semiconductor field effect transistor.

So we'll talk about this in the next of these video presentations for week two.