 Welcome back to the lecture series on bioelectricity. So, today we will be starting our 35th lecture. So, as of now, so while dealing with the plant bioelectricity, we have dealt with the process of photosynthesis, the photosystem 1, photosystem 2 and the electron transfer followed by the breaking of the water molecule in the manganese cluster and the overall implication of this whole process. So, one common theme which kind of emerged out of the plant bioelectricity or from the photosynthesis is that the harvesting of the solar energy is the key to all the evolving processes of life across the earth or across this ecosystem. So, whether it is chlorophyll or in one of the lectures after this we will be talking about some of the very ancient inorganic molecules or if we talk about the synthetic world of energy harvesting talk about silicon. So, it is all these different light absorbing molecules which has the ability to absorb light and eject out an electron and in that process leads to a current generation these kind of molecules remain a key in the whole evolving process. So, while we finish the basic photosynthesis, we highlighted the different areas where all you know the technological intervention could take place. So, you could have a technological intervention. So, if we just enumerate again where all the technological interventions could take place and what are the inspiration we are drawing from photosynthesis. The inspiration drawn from photosynthesis one of the molecule the most inspiring molecule is of course, chlorophyll itself which has the ability to absorb light and eject an electron and then of course, leading to the whole electron transport and energy synthesis or synthesizing energy rich molecules. Then the next inspiration is within the photosystem water splitting cluster or manganese cluster where your water molecule is split up into oxygen and hydrogen. This is a second inspiration where people are working we have already discussed this part previous lecture. Now, there is another inspiration which is if we could use there are several other dyes several other molecules which has the ability to eject out electron upon absorbing light. So, similar molecules which could be utilized say for example, the inspiration can be drawn from you know the flower dyes like anthocyanin and several other dyes which are involved in it or you could you know synthesize synthetic dyes inorganic dyes and all of them have one common feature. They could utilize the solar energy in order to generate electron having said this if we see the whole photosynthesis process out here and if we recall what is happening in photosynthesis where there are two photosystem standing there if you just for your recall let me do it out here. So, photosystem one photosystem two sitting and both the photosystem are ejecting an electron the one of the electron which is ejected by one of the photosystem comes back and brings back the chlorophyll molecule. So, if you just recollect what is happening in the actual thylakoid membrane when the light falls electron is ejected out from one of the photosystem. So, as soon as the electron is ejected out from. So, what is essentially happening is the chlorophyll molecule is losing an electron as soon as the it is losing an electron this electron jumps out, but then this particular molecule the chlorophyll molecule almost behave like a free radical after losing that electron. So, this chlorophyll molecule has to be brought back to its ground state while this electron start to take part into the cascade of reaction in order to bring this photosystem or this chlorophyll molecule back to its ground state another electron is needed that electron is supplied by the second photosystem. You guys remember that how the electron is kind of coming. So, this second electron which is ejected by another chlorophyll molecule sitting in another photosystem is bringing this chlorophyll molecule devoid of electron back to its ground state, but if when it does. So, it the second chlorophyll molecule loses an electron. So, it has to be brought back to its ground state and it is being brought back to its ground state by this process where the electrons are ejected this electron brings back the other one into its ground state. So, now having said this just with this brief recap one has to realize for these molecules which has the ability to eject an electron. So, if a molecule eject an electron it has to come back to its ground state if it fails to come back to its ground state and this all these different molecules will behave like a free radical and there is something called a whole electron in a pairing and coupling and all those things. So, this there is a very complex process which goes on. So, in order to derive energy from these kind of material it is imperative that we need some source of free electron nature has already devised in its armory of workstation called water from where it is deriving by splitting the water it is deriving sufficient amount of electron to you know take care of the chlorophyll molecule which is devoid of one electron. But here we have to do that synthetically and today we come now. So, start formally what will be talking about is a third generation of solar cells which is called dye sensitized solar cells and you can draw the inspiration from the natural flower dyes. And there is a small kind of cartoon shown here, but we will be talking about this cartoon later. But you have to understand the basic principle of dye sensitized solar cell before I go into that just have a clearly clear notion that what is the current energy status on the floor of earth about 1.8 billion of people are without electricity. So, if we really dreaming of a sustainable society we have to ensure that all the people across on the planet earth receives sufficient amount of electrical energy in order to you know sustain their livelihood. So, if you look at all the traditional energy sources. So, you will realize all our traditional energy sources except wind and hydrothermal are product of photosynthesis. If you see coal it is basically the trees and the plants which over a period of time in the deep core inside the earth get transform oil the same way gas the same way the biomass the same way. It is a photosynthesis produces 8 times the current energy needs of the world. So, we have always depend on photosynthesis in order to you know sustain our day to day livelihood. And if you look at the overall reaction of the photosynthesis which just a kind of a recap for you people the carbon dioxide plus water leading to glucose and oxygen and symbolic we can show it. But in that whole process it is basically you can termed it as an electrochemical process a redox reaction of excited chlorophyll molecules chlorophyll and organic dye. So, exactly that is what I was trying to tell you this is an organic dye which upon absorbing light ejects out the electrons. An organic dye absorbs the light photons to produce excited electrons carbon dioxide acts as a electron acceptor while oxygen is produced as oxidation product. But remember carbon dioxide is an electron acceptor, but that excited chlorophyll has to be brought back to it is ground state and that is what is trying to explain. In order to do that there should be a infinite supply of electrons and those electrons are supplied by the water molecules by the splitting of the water molecule in the manganese cluster cage what we have just discussed in the previous lecture. Now, if you look at the thin membrane this is the thylakoid membrane and this is what I was trying to explain. So, you have this photo system 1 photo system 2. So, photo system 1 is ejecting out electron whereas, just drawing it I should have done it like this sorry this is the 2. So, photo system 1 if you now follow the slide. So, this whole process is taking place in the thylakoid membrane which is shown in the slide now it is a very thin membrane having embedded with photo system 1 and photo system 2. Photo system 1 chlorophylls are receiving the light energy ejecting an electron that electron is being absorbed in order to reduce the carbon dioxide. Whereas, this chlorophyll molecule has to be brought back to its ground state that is done by the electron ejected out from the photo system 2 and photo system 2's electron comes back and brings the chlorophyll back to its ground state. Whereas, photo system 2's chlorophyll is being brought back to its own ground state by the splitting of water and that was what I was showing you out here where you are the water splitting is taking place and this is how this whole process continues. So, now what is the very basis of the dye sensitize solar cells now if you replace chlorophyll out here. So, for example, instead of chlorophyll you want to use any of these as a organic dye or inorganic dye or whatsoever ok. So, you have to ensure. So, you have a dye layer here say for example, just like the chlorophyll you have this dye out here and the light is falling on it H nu. So, when the light is falling these dye molecules are ejecting electrons and if they are ejecting electrons. So, it means these dyes are getting oxidized they are losing electrons. So, they have to be brought back to its ground state in order to bring them back to their ground state you need continuous supply of electron from some other source which is not this source otherwise they will recombine this is not going to really work. So, the other source in case of photosynthesis is manganese cluster. So, you need something else here or you mimic the manganese cluster ok. So, coming back what is that what is being replaced here. So, if you see why the duplication of photosynthesis has not been really successful as of now a very thin layer is required to minimize the electron loss as I was trying to tell you when you see the dye layer this layer really really has to be as much thin as you can think of thinner the better there would not be any further loss of electrons. A thin layer absorbs very little light and nature has solved this problem of course, in the form of photosynthesis, but if you look at this picture by stacking the thylakoid membranes if you look at the thylakoid membrane there is stack like this microscopic stacking which has taken place or nanoscopic stacking which has taken place. So, nature has its own way to solve that problem providing successive layers and the anti the molecule and can afford to recycle the whole system on an annual basis. So, in order to have an artificial photosynthesis for energy harvesting what all we need something to absorb the solar energy that is could be an organic dye could be a floral dye could be a synthetic synthesizing synthetic dyes could be an inorganic dye anything which could you know absorb light in the whole spectrum a mechanism to separate two types of charges. So, there are there will be always a electron and a hole which is devoid of electron. So, they should we should be able to you know separate out the charges which very efficiently has been done in photosynthesis by the membrane by the thylakoid membrane a mechanism to pick up one type of charge. So, when these electrons are ejected you need a mechanism by which you can you know siphon out the charge if you cannot siphon out if you cannot put it in a circuit you really cannot put it to the load to global light or you know run a machine and a medium for transport of other type of charges. So, the other kind if there is a hole and the electron pair. So, there should be a way that you should be able to you know transfer those charges. So, these are the four fundamental requirement in order to mimic the photosynthesis what you are seeing here. So, there is a new generation of solar cells which is called disensitized nanocrystalline solar cells which is also called after the name of its innovator professor Gretzel it is also called a Gretzel cells. And I request you people to go through the work of Gretzel in detail over the websites it is a really beautiful pieces of work which has been done by Gretzel. So, which are called the third generation of the solar cells it mimics the natural photosynthesis processes there is a light absorption process is separated from the charge transfer process unlike in the older cells. An organic dye absorbs the sunlight and a nanocrystalline wide band gap semiconductor is used to transport the charges. So, what essentially is being done you are playing with the band gap of different known or existing semiconductor material and you are coupling it what you are doing here this is organic dye is as one of the component it could be any of these or plus semiconductor material there should be significant difference in the band gaps and this is also called the Gretzel cells. Now, coming back how really the Gretzel cells work. So, this is a very kind of a busy slide, but we will go through this slide very carefully. So, what you see in the center out here. So, what you see is let me just you see this molecule out here. So, imagine this is the dye molecule. So, the light is falling on the dye molecule and is ejecting an electron. So, now this dye is in a oxidized state which is devoid of electron. So, now let us follow the slide you will see. So, the dye is absorbing the electron h nu and it is ejecting the electrons. Now, the first thing which has to be done is that these electron which are ejected out has to be separated. So, if you go back to the previous slide what you all you need is that you basically need a mechanism to separate the two charges and mechanism to pick up one type of charges. So, here what you are doing is these ejected electron has to be picked up which is being picked up out here on the top you see the titanium dioxide. So, this is the titanium dioxide which is picking up those electrons. So, now these electrons are being transported along with the electrode across that electrode to the other end from the anode to the cathodes you could see the electrons are moving on there. Now, in that whole process what has happened the iodide molecule has sorry the dye molecule has got oxidized. So, it has to be brought back. So, it is being brought back by the presence of an electrolyte which is present there in the form of iodine. So, if you see this reaction out here iodine to iodide reaction. So, this is one of the common mode by which that electron is being supplied by the iodine iodide transformation and that electron comes back. So, whatever this electron which is generated out here these helps to. So, let us clean this up and let us do it in a right way. So, here you have this cell. So, you have the electrolyte here you have the dyes present light is falling on them electrons are ejected these electrons are picked up by specific electrode and transported out for you know running any kind of device and in the meantime there is iodine to iodide transformation the electron which is ejected out helps it to come back to its ground state. Whereas, if there is a depletion which is taking place here is being taken care by the other electrode which supplies the necessary electron in order to bring it back. So, in this whole process is good enough to you know power several devices of course not very high end devices as of now and, but this is how. So, you can use any of these different kind of dyes you can use an organic dye you can use a floral dye you can use synthetic dye you can use inorganic dyes like and so on and so forth. You can use several kind of dyes by which you can start mimicking the photosynthetic machinery. So, when now this was the first cartoon what I was trying to show you. So, if you look at this. So, you will see there is a glass sheet on the top is basically transparent. So, if both side is transmitted in order to get maximum amount of sunlight you have the dye sensitize titanium oxide which is present there. Then you have iodine iodide electrolyte and you have a platinum catalyst which is present there. So, this is how and there is a load on the other side. So, this is the whole overall compact assembly of this kind of dye of this kind of cells and if you try to figure out what is the reaction which is taking place. So, this is how it is going. So, titanium oxide these molecules of titanium oxides are like this a lot of surface area where you know these dyes are bound like this. So, the picture what you are seeing is T I O 2 bound with the dye. So, and then there is a platinum on top of the I T O sheet if you see on the slide. So, here you have the I T O sheet and the reaction what is taking place now if you look at it carefully. So, first of all the dye loses an electron you see the red arrow going up there is a photo excitation process from the Homo state which goes to the Lomo state and where the dye is not is devoid of one of the electrons. So, it is kind of almost like a free radical there. Now, this dye has to be brought back it has to be reduced. So, you see the green color there number 3 which says the reduction. So, that is where the dye is getting reduced. So, you are bringing the dye back into its ground state whereas, that the other electron which was present there it is injected into the circuit. So, there are at least 4 or 5 processes taking place. So, the first step is the photo excitation what you could see there. So, this first step photo excitation process. Second process if you look at followed by your injection of the current. So, this is where this electron is being injected out this is the second process and there is a third process which is involved where this photo excited molecule is being brought back you see the reduction step which is basically bringing it back. Then there is the fifth and the sixth where basically there is a recombination and there is a dark current which is involved. So, we are not getting in depth on those ones, but at this point what I expect you people to understand the basic circuit and the external circuit the electron comes back and activates the load and moves on. So, this is how dye sensitized solar cell works and if you want to get a much more schematic picture exactly where the electron is you can use in carbon electrode you can use organic dye like cyanates you have conducting glass. So, this is what is happening you have all these dyes which are you know embedded on the titanium dioxide and light is falling the electron is being ejected out and the electrolyte which is present there. So, if you follow the whole sequence on the right hand side organic dye which is essentially the light harvester or sensitizer. So, then you have the nano crystalline titanium oxide which is helpful in transporting the electrons because these electrons have to be transported. Electrons reach the carbon electrode via external connection and a current flows. So, this is what you are seeing there is an external connection and if you follow this picture there is an electron flow you could see on the left hand side of the slide and of course, the current flow is in the reverse direction as per the assumption. So, electron reach the carbon electrode via external connection and the current flows what are the reactions at the carbon electrode which is on your right hand side on the slide there is I 3 minus plus an electron from I minus. So, it is ejecting an electron whereas, at the photo electrode that same I minus gets an proton and it forms I 3 minus which comes back to its original state plus an electron so you see there are two reversible reactions which are taking place, but one is at the cathode one is at the anode. So, the carbon and the carbon electrode there is one set of reaction taking place where this I 3 minus is you know picking up an electron and getting reduced whereas, in the second situation the it is you know ejecting out an electron and getting oxidized and maximum photo voltage is equal to the difference between the redox potential of the electrolyte and the Fermi level of the semiconductor those of your interested there are several other very good concise mechanisms which are involved in order to you know kind of understand this whole thing which is this is beyond the scope of this course. So, I am not really getting into the what are the different Fermi level electrons and how they are kind of you know formulas and how they are kind of involved in this whole process, but for your reference you can go through the work of Gratzel, Rathory and I will follow you some of these work and you can go through them and you will see that nicely people have you know model this whole system. So, coming to the next slide let us look at the efficiency of these kind of processes it is around 10.5 percent percent efficient there are commercial solar cells efficiency is 15 to 17 percent, but that we know very you know 17 percent efficiency is in a very very I should say ideal condition. So, it is slightly less it is almost 5 percent less than solar cell efficiency, but this is a technology which is newer as compared to the solar cell and it could really go further up tropical forest ecosystem efficiency is around 1 percent theoretical limit for natural for synthesis is 13 percent. So, if you now compare these numbers this is very interesting number to compare 13 percent is the theoretical limit of natural for synthesis. Solar cell efficiency is 15 percent 15 to 17 percent with the best great devices using crystalline silicon. Your efficiency achieved for the dye sensitized solar cell is around 10 and the tropical forest ecosystem efficiency is around 1 percent that is of course the tropical forest ecosystem has a has a lesser efficiency for different reason. So, if you look at where all these have been used various colors of dyes several corporations which are involved in it and they are coming in a big way you know to you know promote this. So, now let us take a look at some of the special dyes which have been prepared. So, if you look at it you will see. So, you can please I mean on the print these pictures are not really coming neat. So, you can really refer to the work of grad cells and others which I will be putting on the on the reference you can go through them and you can exactly get the picture of these dyes how they look like, but what is important for you to understand is what are the properties which are essential they must contain a chemical group that can attach to titanium oxide surface. So, this is where thing is that. So, this molecule whatsoever this molecule is this dye molecule this dye molecule should have a hinge by which chemical hinge by which it can bind to T I O 2. So, that is what it meant by we should be able to bind it to T I O 2 because that will minimize the loss of electron transfer must have a energy level at the proper position. So, it has to be oriented in such a way that you get the best out of it. The dyes which are currently in use if you look at there are ruthenium dyes, n 3 dye, there are black dyes, co-marined dyes these are the organic. So, several different kind of dyes which are currently available in the market. Now, yet beyond that there are people who are working this is from Athol Rathuri's work in Fiji island. You will see there are experimentations which are being done using natural light these some of these lights what I have collected from his own presentation. So, if you see. So, there are dyes which have been utilizes the anthocyanin dye and there are and I will give you some of the things where you can really develop these kind of dye sensitize solar cell in your lab you know these are very I mean highly doable these are not something like you know out of the world they are very straight forward thing and I will send you I will put as some of the reading materials or you know practical material. So, experimenting with the natural dyes Rathuri's work if you see they have used anthocyanin dye they have used several floral dyes available or the flowers available in Fiji island. So, the way it works is the nano crystalline porous film of titanium oxide was coated on a conducting glass light which is an I2O in the empty oxide sensitizer dye was derived from the berries or you know anthocyanin or whatsoever flower you want to use titanium oxide was coated with the dye the cell was constructed by putting the second conducting glass on top of the coated one with few drops of electrolyte between them cell characterization was done the VOC the open circuit voltage and efficiency and everything is being measured. So, if you look at the nature what is most important is now coming to what are the different commercialization market if you look at it currently this is again I have derived it from Rathuri's professor Rathuri's one of his slides I was kind going through. So, this is very neatly given. So, this is the kind of numbers what we are looking forward to in the years to come like this technology is will definitely grow in a big way where you know several different things which could be developed out of it and this is the classic example what is believed as you know is the case of biomimicry dye sensorized solar cell have the great potential providing low cost photovoltaic power to billions of people around the world living in remote areas without any access to electricity these cells can be produced easily without expensive setup need for conventional solid state solar cells. So, now coming back what I wanted to highlight is if you look at nature carefully nature has given us abundant resources of different kind of energy trapping molecules energy transforming molecules it is us we have to look or we have to kind of thing very sustainable manner in order to you know explore harvest and make the best out of these different available opportunities which are which are scattered all over the nature. So, if you look at dye sensorized solar cells what we discussed today you can draw inspiration from anything I mean like in the remote places like Fiji remote places of the world like where you do not have you cannot have a do not have extraordinary facilities to develop a current crystalline silicon systems you can develop these kind of wonderful energy harvesting devices of course, at this stage their efficiency is only 10 percent, but you know with intense research who know someday they may outsmart the existing solar cell technology is just a matter of time how much investment we do. So, overall what we see bioelectricity we started with the bioelectrical phenomena and different bioelectrical things in plants and everything what is very essential to appreciate that we have to think very globally very holistically that you can derive inspiration from multiple things across nature and you can you know bio mimic them and you can you know develop some extraordinary thing for the futuristic sustainable societies. So, with this lecture I am pretty much closing on on the photosynthesis and other inspirational devices which could be or the technological intervention which could help you know to appreciate photosynthesis as well as mimic it. So, post this we will be talking about some of the plant movements, but let us close in here and I will leave it to you people to you know dream and think in a very sustainable way how these developments can be taking place and again with the request that please go through the work of I will be anyway I will be providing all the relevant literatures of greatsills as well as rathuris and several other people please go through them you will be able to appreciate it lot better. Thank you.