>>[Slide 1] Hello, and welcome to nanoHUB-U's course
on organic electronic devices.
My name is Bryan Boudouris, and I'm in the School
of Chemical Engineering here at Purdue University.
What I'd like to tell you about today is a scientific overview
of a very exciting class of materials and devices
that are based not on inorganic systems but on carbon systems.
And what I'd like to cover
in this class is how we can design these carbon-based
materials to have specific chemical functionality,
specific nanostructures,
and specific molecular transport properties,
such that they can be used in organic electronic devices;
such as photovoltaics, thermal electrics,
and light-emitting diodes.
But these are new, and they're carbon-based materials,
and traditionally when we think of carbon-based materials,
one of the most commercially relevant ones is the class known
as polymers or plastics, and we're very familiar with these.
[Slide 2] For instance, you can think every day of a time
where you've used a plastic bag.
So, here we have a nice blue one.
We know that we can change its color based on the dye.
We can change its strength based on how we process it,
and we can change its change its shape based on the mold.
And this material here is an excellent barrier material.
In fact, it's so good that sometimes we use them
as containers; whether it be plastic cups or plastic bottles
for water or soft drinks.
We can change the materials properties of polymers,
such that they cannot just be soft and moldable,
but also strong and serve as building blocks,
or to be very thin, so they can be processed into large rolls,
and you can see there are very thin sheets.
So the benefits of polymers is
that you can change the molecular
and processing conditions
such that you can tune how they behave in a barrier sense.
And traditionally, polymers have been used as barrier materials
in a lot of different applications.
And we're common and we're familiar with those,
and we've used billions of tons of polymer throughout the time
that they've been around.
Over the past 30 years, how this has changed is now we've been
using organic materials and electronic devices.
[Slide 3] And some example of these include flexible field effect
transistor arrays, flexible blue emitting diodes,
white light emitting diodes.
You can see here from GE Research
that we have a white light emitting diode
that has illuminated the older technology
of the Chinese lantern.
You can also have something akin to a flexible television
or a thin film television that's emitting many colors,
or you can use energy,
or you can harvest energy to make electricity.
And what we see here is a very thin film solar shell that's
being processed in a roll to roll manner, and you can see
that in 2004, these devices were roughly three inches in thickness.
Now a days, you can actually find commercial products
where they could be used to harvest power off a laptop bag
or on top of a surface that's curved here as we can see
on this bus stop right here.
And we'll talk about light emitting devices and solar cells
in this class, and we'll also talk
about flexible memory devices made out of polymers
that you see in this last slide here.
And the benefits of all these materials is
that if you can combine a source that has a logic display,
such as memory, with a readout display, such as the LED,
with something that can harvest power, harvest energy,
and convert into power, then you'd have the makings
of something that could have a completely portable display
that's flexible and bendable, so you can think
of a material that's a foldable tablet.
And this all happens because we can tune
and design the molecular properties of new polymers
that go beyond the barrier polymer paradigm.
[Slide 4] So how do we want to think
about these organic electronic materials?
Well, they come in two general flavors.
The first is that of small molecule semiconductors.
And if you recall from general chemistry class,
generally when you have a semi, a carbon-carbon bond,
we'll draw it as a bond between one point of this hexagon
to the other point of the hexagon,
and this is a carbon-carbon single bond.
We'll also have double bonds, which we'll draw
as two parallel lines.
And you'll notice in all of these materials that I show here
that these bonds alternate between a single line
and a double line, so we have an alternating single double bond,
and this is called Pi conjugation.
And you'll notice in all of these materials,
we have a large degree of Pi conjugation.
And this is how we have delocalization of electrons
and thus charge transport.
One thing you'll note, if you're familiar
with traditional inorganic semiconductors is
that our materials can either be p-type or n-type,
and when we say p-type, we mean they transport holes
preferentially; or n-type,
they can transport electrons preferentially.
And how we design these molecules will allow us to know
which one is which, and we'll cover that in this course.
The second class of organic semiconductors that we'll focus
on here is that of polymeric semiconductors.
And in these materials, we can either have polymers that serve
as conducting materials, like this PEDOT:PSS term
or as low band gap absorbers, they're used very frequently
in organic solar cells, and have organic solar cell efficiencies
that approach ten percent.
Now, how we design these materials very greatly effects
how these materials operate in devices,
but also how we process these
and how we control the nanostructure will impact their
device performance.
[Slide 5] So, one quick example that we'd like to talk about is
that of white light emitting diodes.
So, here we see a laboratory experiment.
We have a set of white light emitting diodes,
all producing a nice bright color.
And the reason they can produce that nice bright color is
because of instead of blocking part of the light
or not allowing some of the light to go through,
they actually emit the light.
We tune the molecular properties of that white light
such that we can have different shades of white light.
We can either have the bright white light there, or if you're
at home and would like a more candle-like feel,
we can also make it more of incandescent white,
as you see on the fins of this fan right here.
You'll note that these materials, because they're built
on flexible bendable substrates, can be incorporated
in many different form factors.
Here we see how we have a nice overhang of material,
and we can even use them into very flexible materials,
as this sculpture represents right here.
So, here we've taken electrical energy,
pass it through our conjugated material,
and use it to emit light.
The opposite of this obviously would be to take light and then
to convert it into electricity,
and that's the general operating mechanism
of a photovoltaic device.
And in fact, organic photovoltaic devices are becoming more
and more popular, and are approaching
commercial viability.
[Slide 6] The way that these devices work, is that we have two metals,
an anode and a cathode, and in between them,
we have our polymeric or organic species.
And if you recall, from solid state physics,
a device output curve for an organic cell or for any kind
of solar cell looks very much like there,
where you have some kind of diode behavior in the dark
where you don't pass current, and then as you apply a voltage,
the current increases, and when you shine light on the device,
you have some kind of photocurrent here.
So, we have a photocurrent, and this is the difference
between when the light's on
and when the light's off, with no applied bias.
And using the standard equation for efficiency
of a photodiode, we can back out the power conversion efficiency
and see how efficient our materials are.
All the theory is the same in terms
of power conversion efficiency as inorganic devices.
However, how you generate that charge, how you conduct
that charge, and the molecules you use
in organic devices is very different.
So, what we'd like to do in our organic photovoltaic section
of the course is to describe the transport differences
and the nanostructural characterization differences
between inorganic devices and organic devices.
[Slide 7] The third example in the course that we'll talk
about is thermoelectric materials.
And as we know, a thermoelectric material is one
where we have a hot side of the device and a cold side
of the device, and this temperature gradient between hot
and cold allows for the flow of charge
between the hot side to the cold side.
So this thermogradient allows us
to convert waste heat energy into electrical energy.
And the way we measure how efficiently we do is
through something called the figure of merit, and the figure
of merit has three key properties.
The first one is the electrical conductivity of the materials.
The second is the Seebeck coefficient, or the amount
of voltage that's received per unit change in temperature,
and the thermal conductivity.
Now, as you might imagine,
you want to keep the thermal conductivity low,
so that the hot side stays hot and the cold side stays cold.
Obviously, you'd want to get that charge out as quickly
as possible, so you want a high electrical conductivity,
and the higher Seebeck coefficient you have is also a
benefit as well.
Now, in traditional inorganic materials, there's a tradeoff
between electrical conductivity and Seebeck coefficient,
and in fact, as you dope the material,
you will increase its electronic conductivity,
but you'll decrease its Seebeck coefficient.
Therefore, the numerator in this equation ends up having a peak
of S squared times sigma.
In organic materials, this necessarily is not the case.
The theory that supports this
and the theory that's derived for this may
or may not apply to polymers.
This is very in depth and undergoing research,
but it seems as though the paradigm of this maximum
and S squared sigma does not apply to polymer semiconductors.
So we'll explore why that is, and how we can use
that in devices that gather waste heat and convert it
into useful electrical energy.
Those have been three key examples of applications
that we hope to explain organic semiconductors in.
We'll cover in this course the fundamentals
in the materials design.
We'll cover how transport is performed
in organic electronic materials, and we'll cover how
that transport then can be affected in different devices,
and how we can tune the materials
to have optimized device performance
for different devices.
[Slide 8] So, what are we going to cover exactly in this course?
Well, the first thing we'll cover is the molecular
properties of organic semiconductors
and how these can be tuned
to change the electronic properties.
Second, we'll talk about how
to characterize the molecular properties
and the nanostructural properties of these materials
in a way that allows us to understand how
to relate the nanostructure to the ultimate device performance.
Thirdly, we'll differentiate between charge transport
in organic materials versus inorganic materials,
and how this can benefit researchers
in certain applications.
Finally, we'll talk about how to implement these materials
in functional organic electronic devices.
As you can see, this covers a wide range of disciplines.
It goes all the way from polymer science in chemistry
through materials design, chemical engineering,
electrical engineering, device physics, and device modules.
As such, this type of material is very rarely covered
in the traditional university setting.
Here we've set up a one stop course that allows us
to have a flavor of each one of these types of avenues,
and provide a broad perspective of organic electronic devices,
such that you can be well prepared to find these
out in the real worlds.
So, who should take this course?
If you are an undergraduate student interested in chemistry,
physics, chemical engineering, material science
or electrical engineering, this course will have something
to provide you a flavor for all of these fields,
and it will let you implement your background and knowledge,
such that you can begin to think
about organic electronic devices.
If you are a grad student researching polymers,
organic electronic materials, or organic electronic devices,
this course will allow you to dig deeper into the literature,
and allow you to bring yourself up to the current state
of the art in a variety of fields.
If you're an industrial professional who's been thinking
about working with organic electronic materials or devices
or taking an entrepreneurial spirit in the field,
this will give you the proper background
so you can make executive decisions in a manner
that is consistent with the proper economics
and scientific background.
[Slide 9] So, what I hope I've shown you today is
that organic electronic materials
and devices are interesting, they're fun,
and that they're very relevant
to the emerging markets in today's society.
With that, I'll remind you the registration
for the course is now open, and if you visit this website,
you can sign up very quickly and efficiently.
With that, I thank you for your time,
and I hope to see you online in the fall.