nanoHUB U Organic Electronic Devices/Lecture 1.5: Structural and Optical Characterization
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[Slide 1] Hello and welcome back to nanoHUB-U's course on Organic Electronic Devices. Today's lecture will conclude the first week worth of lectures and will be called "Structural and Optical Characterization" of our polymer semi-conducting materials. Throughout the course of these previous lectures, we talked about how to synthesize the materials using a variety of polymerization techniques, we’ve spoken about how to characterize the molecular and thermal properties, and we want to conclude by looking at how to characterize the structural and optical properties. Once we have these tools in our kit of available characterization techniques, we'll be able to start talking about how electronic materials are actually used and how we can actually think about the fundamentals of charge transport in these materials. 

[Slide 2] So what we'd like to go over today is how to characterize the crystallinity and the crystal structure in organic semiconductors. Because if we know how crystalline the material is, and we know what kind of crystal structure it has, we'll be able to have a very good feel of what it’s electronic properties would be like, and we'll be able to understand how to improve the crystal structure in order to improve the electronic properties. We'll also talk about the difference between bulk and thin film measurements. So many times, we'll want to characterize the bulk properties in the materials to understand the general properties of new polymeric or organic semiconductors. However, oftentimes our devices will be thin film in nature and we'll need to be able to correspond the difference between our bulk properties and our thin film characteristics. Finally, we'll want to talk about the difference between our solution and solid state optical properties of our materials, because this will make a difference depending on the application we want to use the polymers in. So our learning objectives for today are be able to, by the end of the lecture, you should be able to utilize Bragg's Law and the definition of a wave vector to calculate domain spacings in organic semiconductors. Beyond that, hopefully we'll be able to explain how the grazing incidence wide-angle X-ray scattering, or GI-WAXS works, and describe how atomic force microscopy can be used to actually image crystalline domains in polymer semiconductors. And lastly, we'd like to be able to employ Beer's Law to calculate the concentration of polymer semiconductor in solution or to calculate the absorption coefficient of a polymer thin film. 

[Slide 3] So when we think about crystallinity in polymers, the easiest way to think about it is by starting with a polymer that has been annealed above its melting transition, so that it's in a fluid-like state. So it's liquid-like in nature. Here I've drawn the polymer chains as some kind of free-floating polymer segments that are all red here in some kind of isotropic melt, meaning that there's no direction and that they can move with relative ease. And when we're in this liquid-like stage, generally we call it "the melt”. As we start to cool, what we'll see is that these polymer chains will start to order, And it forms small grains that have some degree of order within them. And we call this the semicrystalline state, because in between those grains, obviously these polymers are connected but there's a lot of grain boundaries in between them where we won't have a lot of order. So within the grain boundaries, we don't have a lot of order. Within the grains, there's a relatively high degree of order. Finally, we can crystallize in such a manner that we end up with semicrystalline grains and hopefully, if we process the material well, there'll also be some of what will align. So we'll have nice columns of ordered materials. And we'll talk about how we can characterize how much the materials crystalline versus amorphous, and also we'll talk about how we can characterize how well aligned these different domains are. 

[Slide 4] Here, I want to show you with one of our typical examples we've been working with throughout the course. And this is poly(3-hexylthiophene) again. So in the next slide we're going to talk about X-ray diffraction patterns. So we're going to have to name certain crystal phases. So I want to go through common nomenclature of that. So here we see our repeat units, so here's one, two, three, four units of our polymer chain, and there are three directions that we care about. The first is what we call the alkyl stacking direction, and that will be the h, where h can be any integer value, 0 is 0 reflection. And that really characterizes the length of this alkyl chain. So that will tell us how long that the stacking direction is, before we have another repeating unit up here of another alkyl chain. One that's very important is this pi-pi stacking distance. So the distance between this pi-ring and this pi-ring, going back and forth like this. And this would be the 0k0 direction, of the 0k0 plane, where k again is an integer value. And we'll want to know the distance between those pi to pi stacks because that will change the electron transfer properties of materials very greatly. And the third direction we'll talk about is the 00l direction, where l again is some kind of an integer, and that's the length, the long the polymer chain that these crystallites are. So we have the h, k, and l planes or h, k and l directions. And this is a typical crystal structure for a lot of polymer semiconducting materials with poly(3-hexylthiophene), again being one of our prototypical examples. 

[Slide 5] So when we think about this, we can characterize these materials using Bragg's Law of Diffraction. In Bragg's Law, we're going to have some kind of ordered planes, and here we've symbolized them as blue dots. When we characterize these materials, we're going to shine a light on them. This light's going to come in and it's going to be relative to the crystal plane. That's going to be coming in at some angle theta. If this light hits some kind of ordered structure here, here it's shown as a blue dot but it can easily be a crystallite of a polymer. It's going to diffract and it's going to diffract back at angle 2 theta. You know that there's going to be multiple different ways of light to come in, but they're all going to have a single wavelength. If you recall back to your physics days, you’ll remember that if they leave in-phase, they'll be coherent scattered in other words. These two waves will leave in such a manner that they add in sum together constructively. They'll have constructive interference. They leave exactly out of phase, there'll be destructive interference and we actually, we'll have canceling of the waves. But if they leave together, just by a geometric argument, if they leave together to leave in a constructive manner, which when we detect it will appear as a point on our detector, they'll leave it a difference of 2d sine theta, and that will be lead to constructive interference of that's what the distance is. So that tells us, Bragg's Law tells us that if we want constructive interference, we know that lambda is equal to 2d sine theta, where lambda is the wavelength of the incoming light, d is the characteristic domain spacing between ordered structures, theta is the angle between the incoming light in this ordered structure plane, and here we'll introduce a new parameter that you may not have seen before. A lot of times in the polymer semiconducting literature, we'll talk about q, and q is nothing more than the 2 pi divided by the domain spacing d. So this is called the wave vector, and it's a measure of inverse space. So if we have this, we can look and say, that means that if my domain spacing is fixed, which it should be for a crystal, and my wavelength is fixed, which it should be for a monochromatic source of light, and in general for X-ray generating machines, we'll use 1.54 Angstrom wavelength of light. If we do that, then we should see peaks of certain thetas. We should see peaks of certain thetas that correspond to different ordered planes and we should see no peaks at other thetas that correspond to regions where we don't have this order. This is exactly what we do see. So here is our intensity of our detected X-rays coming off from our material. We'll take P3HT as our example here, and we'll see that we see the (100), the (200), and the (300) peaks. So those will correspond to different repeat units or different levels of degrees of order along that alkyl chain direction. We also see the peak associated with the pi-pi stacking direction, so that's (010) peak. And we can see that these correspond to different domains and space at different q values. So if we have a higher q value, that means that d must be smaller, so this is a much smaller distance in these alkyl chain stacking directions, which makes sense from a geometric point of view. One thing you'll know is if this was a perfect crystal, all this piece would just be straight-step functions, very sharp with almost no width, whatsoever. But you'll see at certain points here, especially right here, we have a lot of broad peaks, and as to the random scattering associated with amorphous domains. So one way that we think about, these materials, is the relative degree of crystallinity can be measured looking at how sharp these peaks are relative to how much amorphous domain we have. So if you see a spectrum with a lot of sharp peaks, that will generally correspond to a more crystalline kind of structure. 

[Slide 6] What do these structures look like at the nanometer scale. Well, if we have a polymer chain, it's some big one chain, but eventually, it can start to order in pack into these little crystallites on something on the order of about 10 nanometers. It's going to be able to-- what we call chain-fold a number of times to generate our ordered structure. Eventually, part of that chain is going to come out and not be part of the crystal. When that happens, it will have this amorphous regime right here. So we're going to have a crystal, crystal, crystal, and then a bunch of amorphous regime. So any polymer semiconductor is probably going to have a great deal of amorphous regime just because there's going to be this difference between trying to orient the chain into a nice ordered structure, and the relative degree of disorder associated with trying to force a big long chain into some confined crystalline structure. It's relatively hard to do, so as we saw on the last slide, trying to correlate the difference between crystalline and amorphous domains, helps us give a feel about the ultimate charge transport, because the more crystalline a material is, generally, that leads to better charge transport in the solid state. 

[Slide 7] Now like I said, we'll be able to look at this both in the book which is what we saw for the last wide-angle X-ray scattering patterns, but we can also look at these in the thin film state. So in the thin film state, we'll take a substrate, for instance, SiO2 is a very common one, and we'll shoot in our X-ray source, and we'll be able to look off of a very thin film anywhere from 10 nanometers in thickness up to roughly 100 nanometers in thickness, and we'll be able to see some of the diffracted light that comes off of the material. This is called grazing-incidence wide angle X-ray scattering, or sometimes grazing-incidence X-ray diffraction. Wide angle X-ray scattering and X-ray diffraction are somewhat used interchangeably in a polymer semiconductor literature. Not only will we be able to see different crystalline domains, different peaks like we saw before, we'll also be able to tell the relative orientation of the chains in the polymer semiconductor film. 

[Slide 8] So let's look at an example. So before we saw a 1D representation. Here this is a 2D detector. So in our first example that I showed earlier, it would have been an area of the intensity along one of these lines, we'll say this line for instance. And then we get a 1D plot if we just integrate that area. Here we're looking at a 2D detector. We have a 2D area that allows us to go ahead and think about the crystal structure. So let's call this line right here, let's call this chi equal 0 and we'll say chi runs from 0 all the way out to chi equals 90 on the horizontal axis here. If that's the case, we see that some peak intensities are very sharp right around the chi equal 0. And some are sharper around chi is close to the 45 degrees. That tells us about the relative crystal orientation. So here, I've drawn 3 different ones for our P3AT molecule, or poly(3-alkylthiophene). These materials here, this long, like this, corresponds to the pi direction. So this is the thiophene ring. At 0 degrees, it's standing straight up. At 90 degrees, it would be lying flat down relative to the substrate. So if we look at this and we integrate along this guy right here, we can see that almost all the intensities right around chi is equal to 0. So that tells us that almost all of our alkyl chains are pointing right at the substrate and that our pi stacking is almost parallel to the substrate. So not only can we get a feel for how crystalline it is, but we also can get a feel for what we call a crystal texture, or how these are aligned relative to something like the substrate or the air interface. Not only that, so that if we pick our materials correctly and our experimental sampling materials correctly, we can actually watch materials crystallize in thin films. So here we have a material that crystallizes slowly at room temperature. You can see that right now there's almost 0 peaks associated with material. This has actually has been sitting at room temperature for 10 minutes, OK? If you go up to 20 minutes and 30 minutes, you start to see very small peaks start to appear, and then at 40 minutes, we start to see a whole lot of peaks appear. If you wait 50 minutes, now things are really starting to come in, 60 minutes, they're very, very plain. And after that they'll just finally grow in. So you can see that this material goes from having a lot of peaks at the very end to almost no peaks at the very beginning of its crystallization process. So you can see that this increase in peaks corresponds to an increase in crystallinity. You can actually quantify this degree of crystallinity using some techniques associated with the manuscripts in the further reading section if you're interested. 

[Slide 9] We can image these crystalline domains and look at them directly using atomic force microscopy. So here's a P3HT thin film and here's another poly(3-alkylthiophene) thin film. But if you look at these materials, you can actually see these long wires in the materials. Here's one where a junction of four wires come together right here at this point. So those bright domains, those wire-like domains, those are actually the crystalline regimes of the P3HT. You can see that they're on the order of roughly 10 nanometers in width, and almost a micron or 500 nanometers in length. So we have very long, but very thin crystalline domains. And that tells us the charge transport, say for instance along this long wire, is probably a lot easier than trying to go between multiple wires this way. So looking at this morphology in the alignment of the polymer chains allows us to think about how we'd like to texture our material, how to control the structure of a material, such that it performs better when we put electrodes on either side. And the regions around those nanowires, where you don't see the nanowires, so like this red region right here, that would be an amorphous regime where charge transport are likely to be lower than in the crystalline regime. 

[Slide 10] So now we've talked about how to look at the crystal structure of the material, let's wrap up by talking about the optical properties. The optical properties of these polymer semiconductors are usually characterized by UV-Vis spectroscopy, or ultraviolet-visible light spectroscopy. In this setup it's relatively simple. You have a light source or a lamp, or two lamps, depending if you want to change the wavelength of light. You'll also have pre-sample optics and a lot of that will have to do with making sure that you only shine one wavelength of light at a time. You'll then have your sample chamber, and this can either be a solution, it's a liquid or a thin film solid. You'll have some post-sample optics to make sure that you detect it properly and then you'll have your detector which will tell you how much of the light you absorb at a given wavelength. So it's very simple, it's just basically shining light in a controlled manner at a very specific wavelength and seeing how much is absorbed or, by the sample, or how much is transmitted. So it's either absorbed by the sample or transmitted and detected. If you say, whatever is not transmitted must have been absorbed, then you can get the absorption profile, the polymer semiconductor out relatively quickly. When we do solution, we talk about the absorbance, and that absorbance must be equal to, according to Beer's Law if it's a dilute solution, the more absorptivity of the materials, so this is a material's property, times b, the path length, so that's how long this cubet is, times C, the concentration of the polymer or the organized semiconductor and solution. So if you know the absorption and you know the material's molar absorptivity, and you know your experimental setup, you can then calculate the concentration of the solution. Alternatively, if you make solutions of varying concentration and measure the absorbance, you can backout the molar absorptivity of the material, so this works either way. That's for a liquid-like solution. What if we have a solid, thin film sample? Well then we talk about Beer's Law on the solid state where we have I over, or I, which is the intensity that's detected by the detector, I naught, which is the intensity of the light coming out of the light source. And that I equals I naught times e to the negative alpha d where alpha is the thin film absorption coefficient, and d is the thickness of the film, and you can measure the thickness of the film, and really, I over I naught is just the absorption of the film. So if you know those, if you measure the absorption, you measure the film thickness, then you can back up the absorption coefficient in the thin film. Why would you want this? Well the higher the absorption coefficient, the more photons you can absorb over a shorter distance, which is better for organic photovoltaic devices. 

[Slide 11] So let's look at some examples. So here's a normalized absorbance of P3AT. So these are all poly 3, some alkyl chain, thiophenes, so P3AT’s. You can see that, in solution of chloroform at roughly 1 micromolar, that they all absorb somewhere between 350 and 550 nanometers in wavelength. That's a typical solution absorption spectrum for polymer semiconductors. If we move to a solid-state, we'll see a couple of things. I've left the solution here in gray just so you can see this. The first thing you'll notice is that there's a clear shift to longer wavelengths. We call this the red shift, and that's because red is longer wavelengths. We see that the red shift occurs in all of the samples, znd this red shift is associated with more inter-chain interactions. So in solution, it's more or less one polymer chain, all by itself. And this is a very dilute solution, 1 micromolar. So the chance that one polymer chain is going to be seen in other polymer chain is relatively low, but in the solid state, there's going to be many, many polymer chains all right next to each other. When this happens, especially when it crystallizes. We see this red shift and we see that when we order things or when we crystallize them, they red shift. This red shift is higher wavelength, but that means lower energy. So this ordering causes a lowering of the energy, and from here, we can see that our polymers will absorb all the way out to 650. When we see that, this red shift is very common for polymer semiconductors that crystallize in thin films. So we can actually see order happening between the solution state and the solid state just for simply UV-Vis spectroscopy. 

[Slide 12] So what we've talked about today is we've talked about the techniques we would use to look at the nanostructure of polymer semiconductors. That includes X-ray diffraction, or wide-angle X-ray scattering, both in the bulk and in the grazing incidence geometry, GI geometry. We talked about how atomic force microscopy is used to visualize both the crystalline domains and the texture of the crystalline domains. We've talked about how UV-Vis spectroscopy can help us look at where these polymers absorb light and how we can use this technique to see any kind of increase in order especially when I'm going from the solution to the thin film state. With that, I'll thank you for your time and remind you that coming up next is a discussion about atomic and molecular orbital theory so we can start talking about the electron transport properties of these polymer semiconductors that we know, now know how to synthesize, characterize, and utilize in various different kinds of experimental characterization procedures. With that, thank you and I look forward to seeing you next time on organic electronic devices.