Loading

Module 1: Solar Energy and Photocatalysis

Notes
Study Reminders
Support
Text Version

Silicon Cell and Photocatalysis

Set your study reminders

We will email you at these times to remind you to study.
  • Monday

    -

    7am

    +

    Tuesday

    -

    7am

    +

    Wednesday

    -

    7am

    +

    Thursday

    -

    7am

    +

    Friday

    -

    7am

    +

    Saturday

    -

    7am

    +

    Sunday

    -

    7am

    +

So, in an amorphous silicon cell, you first need a substrate because by definition it is a film, it is a film that forms. The nice thing about it is because it is a film you can use it in you know in almost any surface. So, therefore, that is an advantage. But in any case, if you want to make it as a cell which we want to sell somewhere, you know as a product then you need a substrate. So, for that typically the substrate is some glass. So, this is being built sort of an in an inverted way, the glass is the front-facing surface and then all these things are happening below it. So, below the glass, they put a transparent conducting oxide. So, these days there is a lot of work on this material the transparent conducting oxide typically it is indium tin oxide. So, it is a transparent material which also has a reasonable electronic conductivity so that acts as your collector for electrons, but at the same time it allows sunlight to go through which is necessary for the p-n junction to function. So, the p-n junction comes below that which is what you see here, comes below it the p-n junction and that is where you know charge separation occurs and your electricity generation happens. And in the back, you again have a contact through which you can pick up electricity which is your reflective back contact, but one other, I mean it is a back contact which has conductivity, but you additionally also add the idea that it is reflective. So, why do we do this? The back contact is made specifically reflective. So, that the sunlight that goes through has sometimes will not you know to participate in the electron-hole generation process it may just miss the junction interacting in the junction it just goes through. So, now, by reflecting it you are giving an additional chance for the sunlight to get captured therefore, you are sort of increasing the efficiency with which the sunlight will be captured by simply putting a reflective surface at the bottom. So, the bottom you have a reflective surface, at the top, you have a transparent surface.
So, those two are things that you do and in the process, you create a cell which is still a single junction p, single p-n junction cell, but it is made out of amorphous silicon. So, the structure is slightly different from what we just saw, because the other things that we just saw did not require a substrate, but here we need a substrate for support. So,
that is the primary difference. (Refer Slide Time: 38:28) In the context of these kinds of cells, one important parameter you have to keep in mind is that the top surface should be anti-reflective. So, if it is reflective then the some of the sunlight that comes to the solar cell is lost simply because it is reflected right. So, the top surface needs to be anti-reflective. So, for example, you should have a thin anti-reflective surface sitting on top of this glass. So, on top of the glass, this is an anti-reflective surface or coating anti-reflective surface or coating. So, this is an area of work that is you know looked at in great detail concerning solar cells and so basically the idea is this, you have a refractive index for air and you have a refractive index for whatever surface is there a present and you know with which you are trying to study. So, the greater the refractive difference in refractive index between the air and whatever that material is the greater the capability of that material to reflect light. So, that is how it ends up. So, you won't do the opposite, you want to make the transition from air to the material more gradual rather than abrupt. So, therefore, you put in a layer in the middle which has a refractive index which is in between that of glass and that of air. So, glass typically has a refractive index of about 1.5, the air has the refractive index of 1 actually and the. So, this is not 0, this is 1. So, the film refractive index is the geometric mean, geometric mean of the two refractive indices that are present n0 and ns. So, you will get a, you know the square root of 1.5 in this case and therefore, that is roughly about 1.225. So, if you have a refractive index of a 1.225, it is significantly anti-reflective concerning sun sunlight coming through the air and entering the glass surface. So, these kinds of anti-reflective coatings are worked on and in fact, in this context a there is a lot of work is done even concerning say biomimicry. So, for example, people look at the eyes of moths and those are considered very non-reflective and there it is done concerning structure. So, what is done is, you have a series of, so to mimic that you will have a series of structures that look like this which are very closely spaced and then the closely spaced in the sense that this spacing should be of the order of the wavelength of the radiation that you are looking at. So, there are some specific calculations that you have to look at, but it is in that order, so the wavelength of the radiation. So, what is happening is as you go down you start by seeing more of air, you see less of air, less of air, less of air, less of air, less of air you see start seeing more of this material. So, therefore, the refractive index gradually changes from that of air to that of the material and that is a much more smooth change of refractive index, and so a lot better anti-reflective properties are there. So, these kinds of structures are also being examined, of course, I have drawn it in large scale, but this is as I said you know we are looking at structures which have dimensions in the wavelength of light. So, there are few microns or you know a few you know fractions of microns that is what you are looking at to get you the structure. So, that is also being a looked out it creates these anti-reflective coatings. (Refer Slide Time: 42:23) So, we looked at how the; we looked at a limitation of the traditional solar cell, traditional single-junction solar cell and we also looked at a how the solar cell is constructed, you looked at various parts of it, looked various parts of the graphitic I mean I am sorry the amorphous silicon solar cell and we also looked at the anti-reflective coatings. So, we looked at all of them. Now, given that we are stuck with this 33.4 per cent efficiency is there anything we can do to make it better is the question that we would like to address. So in fact, there is a very nice solution to it using you know just plain incoming sunlight which is to use something create something called a tandem cell, but it is a bit tricky people have shown it, it can be done and therefore, it is a road worth travelling, but it is not an easy thing to do. So, what we are doing is we are putting two solar cells in series. So, you have Eg1 and Eg2 and the idea is that Eg1 is greater than Eg2. So, what happens? You have radiation coming in, you have solar radiation coming in which has a range of frequencies. So, if the frequency is less than that required to be captured by the first material that frequency will go through. So, that frequency the, so the lower energy photons, lower energy photons will go straight through and arrive at the lower material the higher energy photons will get absorbed. So, lower energy photon and this is the higher energy photon. So, the higher energy photon gets absorbed by that top cell. So, all the photons are arriving at the top cell, but only the higher energy photons which are which have energy greater than the bandgap of that top cell get absorbed by the top cell. The rest of the photons which have energy less than the bandgap of the top cell go through the top cell they go to the cell below it and they get captured by the cell below because that has a lower bandgap. So, this is a nice way in which you now have got two band gaps and therefore, the first one can capture some number of photons effectively, the second one will capture some of the remaining photons more effectively. So, overall you have captured even more photons, so, than you would have done if you are used either one of them separately right. So, you can see it is a very simple way in which you can increase, I mean effectiveness with which the same surface area can be used to capture solar radiation. So, that is how they are constructed there, these are solar cells where they have shown you know two layers, three-layer, etcetera, where you have to create these layered structures such that there is also enough transparency that you know the light can go through. So, that is how these cells are created. So, with several tandem cells and using concentrated sunlight, if this does not require concentrated sunlight, but supposing you also add that as a factor theoretically greater than 80 per cent efficiency is also possible, so 33.4 per cent is one end of this you know of the possibility, but if you do tandem cells, you do other things to help you know the improve the capability of the cell to capture sunlight then you can end up with the numbers which are much more impressive, 80 per cent of what is falling on that location can be captured. So, there is a way in which you can go around the limitation that you know single-junction solar cell presents to us and therefore, it is of very good interest to of all. So, in this context you know it is nice to know that you should be, your, you can do this, but how do you get you to know materials with all sorts of a band gaps right. So, of course, you can look at the literature for every material there is some band gap and therefore, we can pick band gaps that you want, but those are some numbers that you do not have control on, you pick a material that has some bandgap and then you can look at a table you will find them it another material which says 0.3 electron volts away from it and that may be the nearest bandgap material available you have to just take that and go. So, the gradation of band gaps is, at first glance appears something that you do not have much control on. (Refer Slide Time: 46:33) But we have some interesting solution to that what is the interesting solution, the solution is the word that is very popular in material science these days which is nanotechnology. It turns out that when you take a material and let us say it has a bulk bandgap some bulk band gap value and you keep making the size of that material smaller and smaller and smaller. So, you take crystals of the material large crystals and then you start making them smaller and smaller and smaller, when you go when the crystal size goes into the nanometer scale of sizes, it turns out that the bandgap begins to change. This has got to do with how the electron-hole pair is confined within that system etcetera. So, we will not go into the physics of it in great detail, it is called the exciton, how it is confined in the system. But we will not go into that part of it in great detail, there is a something called a excite on Bohr radius and that when this size approaches that radius this effect begins to appear. For us it is sufficient to note that when you go to the nanometer size scale then the same material exact, same material begins to demonstrate at different bandgap same chemically its exactly, the same crystal structure is the same, you are not making any difference to those, but the bandgap begins to change simply because the crystal sizes become very small. So, for example, we already saw this that you know visible spectrum has 400 to 700 nanometers range of wavelengths that corresponds to band gap of 3.1 to 1.8 electron volts and silicon is you know 1.1 electron volt bandgap. So, you have lead sulfide as a material which is a direct bandgap semiconductor, we spoke about direct versus indirect. So, this is a direct bandgap semiconductor. The bulk bandgap of lead sulfide is 0.4 electron volts. So, that is well below this. So, it is well below this. So, it is well into the infrared part of the spectrum, significantly into the infrared part of the spectrum right. So, that is where 0.41 will show up. But if you start going to the nano-crystalline size, same lead sulfide, you start creating a nano-crystalline version of it, it turns out that you can start changing this bandgap, you can keep changing it, you can, this is called bandgap engineering or tuning the bandgap. So, you can change the bandgap from 0.41 electron volts up to 4 electron volts. So, 0.41 to 4 and your visible spectrum is bang in the middle 1.8 to 3.1. So, suddenly you have a material, same material available such that it has a bandgap which can pick up infrared, it can it has a bandgap that can pick up red, it can pick up the visible spectrum, it can pick up violet, it can pick up ultraviolet. So, the entire spectrum can be captured by the same material by creating layer after layer after layer after material each one having a different bandgap. For example, I mean that is just an example there may be other issues that we have to keep in mind, but that possibility shows up. So, now you do not have to go and you know leaf through you you know pages of data and find out which material is the next material you should use, what is the other material you should use, you have 1.3, can you get a 1.4, can you get a 1.5. So, you do not have to do all that. By just doing this you know to change to the material you are now got the material with a different bandgap and therefore, potentially you have a continuous range of band gaps available to you which you can use to create tandem cells of any type that you wish. So, of course, these are like I said you know these have all been shown experimentally, I mean ability to change these band gaps and create all these materials, but to make a solar cell that utilizes all this and then shows you that dramatically improved efficiency is always going to be a technical challenge and it is still a technical challenge to do that. So, we are still not quite out there in terms of the kinds of efficiencies that people would like to see in solar cells or solar panels. So, that sort of the state of affairs right now concerning solar energy, but this shows you the possibility it shows you where the research is right. (Refer Slide Time: 50:42) So, in conclusion, we saw that there are various parts to a solar cell and these parts are designed as you may expect to increase the efficiency of the solar cell. So, that includes everything from you know having an anti-reflective coating, having a finger electrodes, having a backing layer and if it is a thin film based amorphous film based solar cell, then you have the backing layer also being a reflective layer to reflect some of the sunlight so that you can improve the efficiency with which the sunlight is captured. We also saw the Shockley Queisser Limit and it shows us that just because of you know blackbody radiation-related losses, radiation-related losses, then recombination related losses where the electron-hole pairs simply recombine and are lost and also due to spectrum losses. Simply because a single junction solar cell will not be in a position to capture all wavelengths effectively it only captures wavelengths very close to that just marginally above the bandgap that is what it captures most effectively and even other wavelengths it captures it effectively is gives you only energy corresponding to that. Given all that taking all that into account the Shockley Queisser Limit shows us that only about you know one-third of solar incoming solar radiation can be meaningfully captured using a single junction solar cell. We also saw that tandem cells, which you know keep one cell of one bandgap on of a higher bandgap over and above a cell of the lower bandgap, actually overcome, that is one way in which you can overcome this Shockley Queisser Limit, such that in the same region you suddenly have more wavelengths being captured effectively when compared to a system where you have only one bandgap material being used. So, there are interesting ways to work around it and therefore, there is a lot of promise in solar energy and a lot of companies that work on, a lot of a governments push hard for it, mainly because of you know in principle it is quite clean and there is a potential that there is enough energy available for us to capture. But there is still a lot of research to be done and therefore, it remains in the area for active research and development and I expect that it will remain so for some time. So, with that, I would like to conclude this class. Thank you. In the last several classes we have looked at various aspects associated with solar energy, and how we can capture the energy and utilize the energy. More specifically we looked at solar thermal and the solar photovoltaic as these are the two major processes that are used for doing this you know the process of capturing solar energy, and most of the technologies that you see commercially that are out there fall on those in those two categories, that’s what you will see you will see solar water heaters, you will also see solar-based power generation systems, and you also have these solar volt photovoltaic based you know street lamps, the household you know electricity from the rooftop and so on. So, largely it is these two; and so for these two, we looked at in considerable detail the science aspects associated with, the technical aspects associated with it and how it is put together, what are some issues related to them, and how they work etcetera. So, all this we did in the last few classes, within today’s class we will look at a specific aspect of you know utilizing solar energy, which is referred to as solar photocatalysis and really with this topic we will sort of wind up our discussion on or you know our focus on solar energy in these last few classes, will sort off you know come together. In this class and fill wind up with that with this topic on the solar-related aspects. So, photocatalysis is the primary topic, it is different from the solar thermal and the solar you know electric that we looked at photovoltaic that we looked at and that’s why it is interesting to look at. (Refer Slide Time: 01:56) Our learning objectives for this class are to describe the principle of photocatalysis and that is something that. We will look at various considerations energy considerations associated with the photocatalysis, how it is put together, what’s the thought process behind it what’s the science behind it and so to speak. So, to speak and also we look at as always it is important to understand, what are the challenges that one needs to address in the context of photocatalysis. So, these are broadly the ideas that we will look at. Now, photocatalysis is used for a variety of different things, the idea is simply that you are using incident solar energy incident light. In fact, and then solar energy is the most you know the accessible form of light that we have; to do to help catalyze a reaction. So, to help catalyze a reaction and in that process you know some other reaction happens, maybe we have interested in the products of the reaction and therefore, we look at it. In the context of our discussion, we will look at primarily the use of solar energy to generate hydrogen. We have looked at through this course the idea that you know if you are using mostly carbon-based fuel, then necessarily you are going to be generating carbon dioxide. So, this is we are stuck in this situation, and if our understanding of the whole process is correct, then it is not a great idea to keep on generating carbon dioxide and putting it into our atmosphere. So, it helps if we can look at fuel systems or processes, that essentially avoid the introduction of carbon into the system into the atmosphere. So, the good solution that is provided quite often is the idea that you can convert water to hydrogen and then you can use hydrogen as a fuel. So, when you burn hydrogen you are essentially doing you know oxidizing hydrogen and you are generating backwater. So, if you can set up a process by which you take water, supply energy in some form, break it up into hydrogen and oxygen and then later when you utilize it in some location you are simply recombining hydrogen and oxygen and getting backwater. If you keep doing this cycle then you are essentially clean I mean there is no problem at all and in fact, the product water is say if you do it properly, the product water is probably can even be made in such a way that it’s really clean and available for drinking. So, it is a nice cycle to tie yourself into it’s very clean and it is a complete cycle you introduce hydrogen, you take out hydrogen back into the water. From the water, you get out hydrogen and then put hydrogen back into the state, where it is in you know it’s oxidized and you are sitting it is sitting as water. So, this cycle is kind of complete you are not you know suddenly increasing the percentage of hydrogen in the atmosphere that’s not what is happening, and there is no CO2 involve this process if you know how to do it properly and cleanly. And incidentally, if you look at say, for example, manned space, space mission and so on. This is a cycle that people would like to use, they would like to split hydrogen and oxygen, I mean split water to get hydrogen and oxygen and then use hydrogen as some kind of fuel to power various aspects associated with the manned space program and then, in the end, you will the process of powering, you will again re-get you know get back the water that we use and that water can be used for various purposes. So, this a nice cycle as I said to tie into. So, therefore, it is very interesting to do this. So, now, let’s begin by first looking at what is required to do this water splitting. (Refer Slide Time: 05:30) So, the simplest example of doing that would be what you see on your screen, which is simply an electrochemical cell. So, an electrochemical cell, you have here a battery and that is powering a process by which you are taking water and converting it to hydrogen and oxygen. So, we have an electrode, which has you know where you are pushing in the electrons, and you have an electrode from which you are pulling out the electrons okay. So, you are pulling out the electrons in one side, you are pushing in the electrons on the other side. Generally, we say that if you are adding electrons to a species, then we describe that as a reduction process. And if you are pulling electrons away from the species you describe that as an oxidation process and in the electrochemical scheme of things the electrode at which the oxidation occurs from where you are pulling the electrons off into the external circuit is called the anode. So, in this case, this would be the anode, and the electrode where you are pushing electrons in and from the external circuit and therefore, enabling some species in that electrode to get reduced is where the reduction is occurring, and the electrode where the reduction is occurring is referred to as the cathode. So, this is our cathode okay. So, we have anode-cathode and in the middle, we have the electrolyte. So, now, this electrolyte and all we are going to do are take water and split that to hydrogen and oxygen. So, it is a two-step process, we can take water and you can generate 4 H plus ions, which is a proton because once you remove the electron it’s a proton and of course, you will correspondingly also release 4 electrons and you create oxygen ok. So, this is the way it happens, these electrons go into the external circuit. So, this goes to the external circuit and this H plus is going across okay. So, that’s what you see here. So, at the anode this reaction that I have shown you down here okay, this reaction is occurring here this whole reaction is occurring here. So, these electrons that you see here are the electrons that are being pushed into the external circuit there. So, this e minus is going that to it, that is what is happening the same e minus is then pushed back into this circuit here in this part of the system, and then it arrives into this electrode which is the cathode and so, that is this 4 e minus here. So, I have just balanced it out. So, that it looks the same and the proton has also got to arrive there, that proton arrives through the electrolyte which is what I described here, this proton that you generated on the anode side, is now being pushed through the electrolyte and it arrives at the cathode. So, at the cathode that same 4 H plus will now react with the 4 e minus and generate hydrogen. So, this is what we have as our overall reaction. So, this reaction is occurring at the cathode, at this electrode. To do this we are using an external source of energy which is the battery that is sitting here and which is appropriately connected. So, that you are pushing electrons in one side pulling electrons off of the other side and therefore, it encourages the reduction reaction on one electrode it encourages the oxidation reaction on the other electrode. And generally what happens is if you want to do this, you give access enough access to for both the electrodes to water and based on your electrolyte properties typically electrolyte may also desire or prefer to be wet. So, you have to have water in the electrolyte. So, generally, we provide water on both sides of the system, and in principle, you are actually as you keep consuming water, you are consuming water on the anode side. So, water is getting consumed because it is getting split into hydrogen and oxygen, you can, if you supply water on the cathode as well, that water will just transport itself across, the way I have pointed out here right. So, you have a movement of water from your cathode to the anode, you have movement of protons from the anode to the cathode. So, all this is happening and of course, you can also have as I said you can keep supplying more and more water to the anode side. So, it is not only dependent on this water that is coming here. So, it does not have to depend only on this water, you can directly supply water. That’s what I say by may mean by saying electrode with access to water, the same is true here also electrode with access to water okay. So, you do this, usually, in a system like this, this electrolyte that I have put in the middle is some kind of a polymer, that is in a position to transport protons and it’s also in a position to transport water. So, it’s a polymer of that nature, and because it is a polymer it is solid, it’s a solid polymer I mean where it is a solid polymer in this case which we select a solid polymer, solid polymer-based electrolyte and therefore, it acts as a physical barrier. So, acts as a physical barrier between the anode, and the cathode between what is the anode and what is the cathode. So, it acts as a physical barrier, which is a nice thing in this case because on your anode side we are generating O2; that’s a gas and the gas is going to come out. Or if you have some outlet in your system in the physical cell that you build there’s a separate outlet in that outlet the oxygen will come on. On the cathode side, you are generating hydrogen. So, hydrogen is going to come out here. So, this hydrogen gas is going to come out. So, you have hydrogen coming out on from one electrode you have oxygen coming out from the other electrode and because the electrolyte is solid and therefore, acts as a physical barrier you can separately collect the oxygen, you can separately collect the hydrogen and they will not mix and so, whatever hydrogen you generate you can safely collect and store away whatever oxygen you generate you can safely collect and store away and there is no wastage whatever electricity you use does all of this processes ok. So, this is how you can envision, how electrolysis of water can happen. Now you can generate this in this in the manner that I have shown you, using this power supply that we are using here some DC power source which could be a battery. You can use a battery there, you can even connect it to the mains and use some circuit which gets you DC source and that source you can use to do this activity, you can also connect a solar cell. So, in this case, the solar cell will simply act as a source of energy, and from where the electricity is generated that electricity will enter into a cell of this nature, and once it enters the cell of this nature it will do electrolysis and generate your hydrogen and oxygen. When you use a solar cell in this process what you are doing is, you are doing a two-step process. You are first doing a process by which you are using solar energy to generate electricity, then you do a second step where independently the electricity comes into a separate system, where you are doing the electrolysis. So, it becomes a two-step process generally speaking in all in any form of engineering just because of the thermodynamics of you know involved with all of these processes, typically in any engineering process the more steps you in include the more your inefficiency is; because by nature none of these energy conversion processes or most of these energy conversion processes is not necessarily 100 per cent efficient. So, you will invariably find some inefficiency in the process due to heat that is generated, due to movement of ions, lot of different things that are happening here that there is going to be some inefficiency if there is inefficiency. So, that those inefficiencies are multiplied. So, if the one you know the process is 90 per cent efficient, and another process is also 90percent efficient and both of these are in series then you have a process which is only 81 per cent efficient right. So, 0.9 into 0.9 will get to 0.81. So, these kinds of processes will these kinds of issues will be there and therefore, it is not necessarily a great idea to have multi-step processes.