Staebler–Wronski effect

Transcription

Hey, guys. So as you all know, we expect the solar cells to produce electricity. You know, convert sunlight into electricity for a long period of time. So for example, usually you assume that these panels will work for 25 years or even more than that. But I want to discuss this, this special issue which is of light induced degradation and it's, it's a special concern for this amorphous silicon kind of solar cell. So, thin-film solar cells which are made out of amorphous silicon. So when you make these solar cells out of of amorphous silicon, what is often observed is that if you expose them to sunlight, you know, if you just make these cells and expose them to sunlight, you get a certain efficiency. In this case, efficiency of 8.5%. But as you keep on, keep it exposed to light, the efficiency, it starts degrading, and you can see that you know, just in a period of five hours, the efficiency has dropped from 8.5 to, you know, close to 7 point you know, close To, 6.8%. And if you, can, keep on exposing it to light even more the efficiency it starts to degrade. Thankfully it, the rate of degredation slows down, but nevertheless it's still degrading. And, you know, it reduces substantially as compared to what you started at, at t equal to zero. Now, even more interestingly what people have observed, is now if you take this solar cell and anneal it or bake it for a couple of hours. So you know, you put it in a chamber which has a temperature let's say equal 250 degrees centigrade. And then you take it out, and again expose it to sunlight, you know, expose it to sun. And measure its efficiency. So, miraculously what occurs is that the efficiency recovers back. You know, you get part of the efficiency you lost back, and you again have higher efficiency. But unfortunately, again, when you expose it to light again, you know, it starts to degrade again and the efficiency starts to fall. If again, you expose it to, or you anneal it again this efficiency rises up. And it starts to fall again if you keep on exposing it to light. So you get the idea that you know, essentially the efficiency degrades if you expose it to light. It improves if you anneal it or, you know, if you subject this solar cell to this high temperature. And or you are usually efficiency, it can degrade very easily to around 30%. Or, efficiency can degrade by 30%, you know your efficiency can degrade by 30% off where you started off from. And it starts to stabilize after awhile, and, you know, the rate of the decrease, or the rate of this, degradation slows down and, it reaches a stable state, stable state after sometime. But nonetheless you degrade, you know you are operating at an efficiency which is 30 percent lower than what you started at when the cell just came out from the factory. In fact, many of these these [INAUDIBLE] lines, they measure, you know, they measure these cells when after they're manufactured for the purpose of bending them into the different categories. And they observed that as soon as, you know, they measured the cell, they, it starts to degrade. So again, that does not bode very well with what I said earlier, that these things you know, need to work for more than 25 years. But it's not that, you know, when people you know, started when this amorphous silicon became really hot and people were selling these. A lot of these solar panels back in back in 2008 or 2009 kind of time frame. It was not that they, you know, they just discovered about this after they started manufacturing these cells. In fact, this phenomena is very well known for the case of amorphous silicon. It was discovered all the way back in 1977 by these two bright at that time, young scientists Staebler and Wronski, who used to work at RC Labs at that time. And what they observed was they measured the conductivity, so they measured the conductivity of this amorphous silicon. And they measured it you know right after it was made. And they exposed it to light for some time. And what they saw was that the conductivity it fell to a much lower level after you, after you expose it to light. So this, this phenomena of degradation of this amorphous silicon has been known for for you know quite some time. And people have proposed different theories for it. So you know as is with this degration or these reliability kind of phenomena. People propose different models, and there are ways you, you can be assured that there will be more than one competing models to explain the observed experimental behavior. So the first of these models which is used to explain this degradation is what is called is a hydrogen bond. Switching model to hydrogen bound, switching model. And this was proposed it was one of the earliest models to explain this degradation, so it's more widely accepted as well. And the way this model tries to explain this degradation is that it says that suppose the others amorphous materials, amorphous silicon, in this case. So you have these you know you have these silicon items and they are bonded to each other. Now what happens when you shine light on this material? So when you shine shine light on this material, you generate these electron in whole pairs which are generated throughout throughout the bulk of this absorbing material. And now these electron and hole pairs, they can essentially either get connected, they can recombine and they can release out a photon, out from the system. So, they can relatively recombine. More commonly, they essentially, they can recombine, and they can give away this energy to you, to the lattice itself. So they can recombine and they can [INAUDIBLE] give away this energy to the lattice. Now if such an electron and hole pair, it recombines very close to this bond which is there between these two silicon atoms, it can essentially give out that energy to this bond. And that may result in the breaking of this bond. So that's how essentially you, you break this bond, because you get [UNKNOWN] energy from this recombination of this electron and whole. And that essentially creates, that essentially leads to creation of the dangling bond on each of these silicon atoms. Now, what this model says is that now, this this bond is essentially switched by hydrogen, so essentially, what what happened is that this hydrogen, which is in [INAUDIBLE], which is, you know, always, almost always present in this amorphous silicon material, it opportunistically comes in, and it binds with one of these One of these silicon atoms. But this other bond on the other silicon atom is essentially still still unsatisfied, so it results in creation of this dangling bond on one of the silicon atoms. But overall this bond between the silicon these two silicon atoms is essentially switched by this hydrogen hydrogen molecule. So this is one way to explain, y'know, how this, how this how this degradation occurs, it occurs because as you shine light, your density of these are dangling bond. It increases because, because of the recombination of these electron and hole pairs, which are generated because you shine light. They result, in breaking of these bonds. And it results in creation of these, dangling bind states. So another model which can, explain the creation of these, dangling bind, states as well, is, this hydrogen, collision model. And this is a model which has been recently become which has recently become more popular to explain this phenomena. And the way it says that these dangling bonds are created. It says that you have this amorphous material, so you have a lot of these silicon items, which are, you know, which are bonded to hydrogen anyway, because, you know, not all the, all the bonds in amorphous material are not satisfied with other silicon items. You have a lot of these silicon items which are bonded, which are bonded with the hydrogen. So now what happens is that when you shine light it alerts, it alerts in the breaking of these binds, so what the picture that alerts is that it creates these dangling binds on each of these silicon atoms. And this hydrogen, which is subsequently set free, or these hydrogen atoms, which are subsequently set free, because they are now released, they essentially they essentially, you know, go and form a metastable state somewhere within the lattice array. So these two models are, you know, often frequently used to explain this phenomena.