OK I've been posting really crummy stuff on my blog recently, so here, in the nick of time, comes redemption for my poor blog :)
It was Saturday evening and I was all decked up with my fancy new high heels. (High heels *always* make me feel grown up and all..like i have all the cares of the world on my shoulder..starting with balance for instance :P)
This was because my dad, for the second time in his life was going to be singing at a big concert and I was selected to represent the family and see him through his ordeal!:D
You see, my dad's a great guy and all. He plays the guitar really well and has given several concerts, but he's only sung once before in public- (well basically he thought no one was home and was singing some serious Italian opera in his bedroom, while my friends and I were rolling on the floor in a convulsive heap as each new note brought more and more hysteria :D)
Well, this time my dad was not going to be singing Italian opera (Thank Heavens for that!) but this really cute song called L-O-V-E, first sung by Nat King Cole. It's a real peach of a love sing-or so I thought, before hearing my dad practice singing it for like the nth time!!
He had practiced it so many times that I thought that if anyone mentioned that 4 letter word- love to me I would put another 4 letter word-Fear into his goddam heart so that he would never use it again!
So you see, none of us were particularly keen on hearing dad's song for the n+1 th time, and I volunteered rather reluctantly to be the lamb in this case and sacrifice myself on the altar of family duty and honour :)
My dad was really nervous and as we drove to the auditorium, kept saying
1) his throat was hurting, so maybe he should not sing
2) he suddenly remembered he had to check up on a patient, may be his duty lay there, maybe he should leave
I know he came up with a lot more, probably picked up from listening to my sister and I trying to dodge dentist appointments, but I had learnt something from my parents stony silence as we pelted them with heart breaking excuses. So, I merely said nothing and kept on assuring him that he would do great.
finally, i had got my dad back stage.
Now, while my dad was studying for his doctor thing. He had taken up karate. He had become so good at it that he had even been a karate instructor for some length of time.
So suddenly, as we counted down the performers, he broke into some karate exercises to calm himself down!
An astonished compere watched as my dad went "haiyya" etc. :D
(it was embarrassing max!!)
Several of the other performers were back stage as well,
"Oh, don't worry," I told them. "He's always like this when he's hungry," I said.
Anyways, finally dad went on stage and sang. It was really good! He got the entire crowd singing along with him. I was really happy! Life was Bee-Yew-Ti-Fulllll :):)
Monday, March 22, 2010
Constructal Theory
"For a finite-size (flow) system to persist in time (to live), its configuration must evolve such that it provides easier access to the imposed currents that flow through it."
The above is the statement of Constructal Theory, developed in 1996 by Professor Adrian Bejan.
Why is it important?
Because it describes the evolution of flow!
Flow of what?
The flow of ANYTHING! This ranges from the flow of energy, mass to the flow of heat, goods, people etc.
Let me explain the Constructal Theory to you know, in the context of thermodynamics.
The second law tell us that a system will evolve to a state of maximum entropy.
The minimum work principle tells us that to optimize a process, minimum entropy must be generated during the process.
Constructal Theory tells us that every natural flowing system will want to globally optimize itself. It accepts the imperfections in the process and tells us how the system will evolve such that these imperfections will be minimum.
It says that the maximization of the flow access= minimization of the various global resistances at the scales at which they act.
For instance if we look at a tree and see how it evolves as time, we see that it evolves such that the flow of air in its interstitial spaces (which occurs primarily due to conduction) is maximized by the minimization of the resistance by increasing the interstitial surface area. (This is why when drawing a tree the white is as important as the black)
For the flow of gases/water in the 'black' of the tree, the resistance is less as compared to the high resistance of the flow of air due to conduction, therefore the flow channel area is more. Thus we see how a tree, evolves such that the two flows that occur are maximized, by considering each of the resistances that act on the system globally, and by optimizing each resistance at the scale at which it acts.
Let me give you one more striking example, that is the flow of mass. We shall see how Constructal Theory gives us various allometric relations, already observed in Nature by approximating running, swimming and flying as the flow of mass.
There are two resisitve forces the body has to compete against. One is drag and the other is the gravitational force.
Constructal Theory says that to optimize this flow of mass, the various resistances per scale has to be minimized. That is
W1 of the object against drag/L + W2 of the object against gravitation/L has to be minimized. Or we have to minimize the work being wasted across the scales at which these 2 resistive forces act. Therefore we must minimize the work done per unit length traveled by the object.
Basically, constructal theory accepts, that when an animal ingests food, there is nothing that can be done in the input stage, or in the conversion of food to energy. however, at the output stage we can globally minimize the effects of the two resistances.
With the model given by constructal Theory, we can derive allometric relationships between the mass of the body and the velocity with which it travels according to this model. Thus Constructal Theory seems to generate various scale relationships which had been observed in various fields in various things.
Constructal theory gives us a basis to understand these relations and to actually derive such relations across various fields.
In this post I will not elaborate on further examples. I will only tell you some of the remarkable effects Constructal Theory has predicted.
It has predicted the accurate relationship between the length and width of a block in a city. It predicts the changing dimensions of streets with the faster velocities that are now attainable. It even predicts globalization as a better method for the flow of goods and services.
Lots of papers on this theory can be found on the net on this wonderful theory, and I hope that you, like me will find this theory intriguing! :)
The above is the statement of Constructal Theory, developed in 1996 by Professor Adrian Bejan.
Why is it important?
Because it describes the evolution of flow!
Flow of what?
The flow of ANYTHING! This ranges from the flow of energy, mass to the flow of heat, goods, people etc.
Let me explain the Constructal Theory to you know, in the context of thermodynamics.
The second law tell us that a system will evolve to a state of maximum entropy.
The minimum work principle tells us that to optimize a process, minimum entropy must be generated during the process.
Constructal Theory tells us that every natural flowing system will want to globally optimize itself. It accepts the imperfections in the process and tells us how the system will evolve such that these imperfections will be minimum.
It says that the maximization of the flow access= minimization of the various global resistances at the scales at which they act.
For instance if we look at a tree and see how it evolves as time, we see that it evolves such that the flow of air in its interstitial spaces (which occurs primarily due to conduction) is maximized by the minimization of the resistance by increasing the interstitial surface area. (This is why when drawing a tree the white is as important as the black)
For the flow of gases/water in the 'black' of the tree, the resistance is less as compared to the high resistance of the flow of air due to conduction, therefore the flow channel area is more. Thus we see how a tree, evolves such that the two flows that occur are maximized, by considering each of the resistances that act on the system globally, and by optimizing each resistance at the scale at which it acts.
Let me give you one more striking example, that is the flow of mass. We shall see how Constructal Theory gives us various allometric relations, already observed in Nature by approximating running, swimming and flying as the flow of mass.
There are two resisitve forces the body has to compete against. One is drag and the other is the gravitational force.
Constructal Theory says that to optimize this flow of mass, the various resistances per scale has to be minimized. That is
W1 of the object against drag/L + W2 of the object against gravitation/L has to be minimized. Or we have to minimize the work being wasted across the scales at which these 2 resistive forces act. Therefore we must minimize the work done per unit length traveled by the object.
Basically, constructal theory accepts, that when an animal ingests food, there is nothing that can be done in the input stage, or in the conversion of food to energy. however, at the output stage we can globally minimize the effects of the two resistances.
With the model given by constructal Theory, we can derive allometric relationships between the mass of the body and the velocity with which it travels according to this model. Thus Constructal Theory seems to generate various scale relationships which had been observed in various fields in various things.
Constructal theory gives us a basis to understand these relations and to actually derive such relations across various fields.
In this post I will not elaborate on further examples. I will only tell you some of the remarkable effects Constructal Theory has predicted.
It has predicted the accurate relationship between the length and width of a block in a city. It predicts the changing dimensions of streets with the faster velocities that are now attainable. It even predicts globalization as a better method for the flow of goods and services.
Lots of papers on this theory can be found on the net on this wonderful theory, and I hope that you, like me will find this theory intriguing! :)
Friday, March 5, 2010
Solar Cells Part 2
Solar cells from Blueberries:
This is essentially a dye sensitized solar cell.
To explain how this works in more detail we need to go back to Nature.
The problem with silicon solar cells is that achieving the optimum band gap is difficult. This was already explained in the last post. But wait, who am I kidding?! :P No one would have gone through that anyways. So let me explain again.
If the band gap is high, high energy photons can be captured. However, if the band gap is low, then more number of photons can be captured. There therefore must be an optimum band gap. Tandem solar cells, quantum dots are two of the ways used to capture all the wavelengths of light.
Also, in solid state devices. Silicon is used as the electron generator and an electric field is produced in it which causes charge transport. A simplistic explanation of this process is:- A photon is incident on n type Si. If it's energy is more than the band gap, it excites an electron which moves towards the bottom of the cell: p junction and then through an electrical wire to the load.
Now the problem is to prevent the recombination of the electron with a hole as it travels from the top negative junction to the bottom positive junction.
Dye sensitized cells solve the fist problem by mimicking photosynthesis. They consist of an electrolyte with two electrodes. Silicon basically acts as a charge transporter and does not generate the electron.
What happens is, a photon falls on the organic dye which excites the dye molecule to an excited state releasing an electron. This electron moves into the TiO2 and exits through the conductive layer into the electrical wires. This is how current flow occurs.
Then the electron returns to the cell via the bottom conductive layer. With the help of a catalyst (usually platinum), the electron enters the electrolyte and interacts with the tri iodide molecule. It reduces it to form an iodide ion. The iodide ion moves towards the activated dye molecule and transfers an electron to the dye. Thus the dye molecule reverts to its previous ground state and the iodide ion reverts back to tri iodide.
What happens in photosynthesis is, an electron is absorbed by a pigment. The energy of the electron is used to synthesize dihydro nicotinamide di hydrogen phosphate (NADPH), a molecule that will eventually produce carbohydrates. To get the pigment back to it's initial state, an electron is donated in the oxidation of water to produce oxygen.
The difference between the two processes is that the storage of energy in the form of starch occurs in leaves, while the storage of energy in a dye sensitized solar cell is still not possible.
The similarities of the two processes is evident. Both are regenerative cycles which involve redox reactions. They use a multi layer structure similar to a thylakoid membrane. (They are the site of the light dependent reactions in leaves)
In DYSC the organic dye replaces the pigment in Nature. The wide band gap semi conductor used replaces the oxidized NADPH and carbon dioxide as the electron acceptor. The electrolyte replaces the water and oxygen as the electron donor.
Now I will talk a little about the materials involved.
Typically a ruthenium metalorganic dye (Ru-centered) is used as a monolayer of light-absorbing material. The dye-sensitized solar cell depends on a mesoporous layer of nanoparticulate titanium dioxide to greatly amplify the surface area (200-300 m2/g TiO2, as compared to approximately 10 m2/g of flat single crystal). The photogenerated electrons from the light absorbing dye are passed on to the n-type TiO2, and the holes are passed to an electrolyte on the other side of the dye. The circuit is completed by a redox couple in the electrolyte, which can be liquid or solid. This type of cell allows a more flexible use of materials, and is typically manufactured by screen printing and/or use of Ultrasonic Nozzles, with the potential for lower processing costs than those used for bulk solar cells. However, the dyes in these cells also suffer from degradation under heat and UV light, and the cell casing is difficult to seal due to the solvents used in assembly.
The absorption of incident light depends upon the number of dye molecules attached per unit volume of the semi conductor. if the dye is on a flat surface less than 1% of light will be absorbed.
Light absorption is maximized by the use of sintered nano meter size anatase (tetragonal crystal structure) titanium dioxide. The surface are is increased by 200-300%. This surface has pores in the range of 20-500Angstrom. It is advantageous to use TiO2 because it is cheap, easily available, bio compatible and non toxic. The anatase phase is used because it has a wide band gap that is transparent to visible light and thus ensures that the light is absorbed only by the dye molecules.
The thickness of the TiO2 layer is a compromise between maximum surface area and minimum recombination losses.
They dyes used by commercial dye sensitized solar cells generally use ruthenium bi pyridyl based dyes. The wavelengths of light absorbed are 510-570nm and adequate conversion between 450-650nm. This absorption spectrum overlaps well with the diffuse sunlight spectrum. However such dyes are hard to synthesize.
A little about these dyes
Ruthenium bipyridyl complexes have been used as efficient sensitizers for photoelectrochemical cells based on nano-porous films of . It is found that cis-dithiocyanate-bis(2,2-bipyridyl-4,4-dicarboxylate) ruthenium (II) can be used as the sensitizer in a solid-state photovoltaic cell in which a monolayer of the sensitizer is sandwiched between nano-porous n- and p-CuI.
A significant photo current can be obtained using natural anthocyanin dyes which can be extracted from blueberries as natural water based substitutes. The absorption of cyanin on the surface of TiO2 is a rapid reaction and the OH counter ion is displaced from the ti(4) work site that combines with a proton donated by anthocyanin. This strong chemical affinity is the reason blue berries can be used :D.
An Experiment that was conducted: (i found this paper on the net)
A commercially bought titanium dioxide coated glass slide was stained with a berry dye (e.g. raspberries, blueberries, beetroot); the dye was made by crushing the fruit (or leaves) and purifying them in a solution of methanol, acetic acid and water . This slide was washed with water, and dried with propanol, and a transparent conducting glass slide (tin dioxide coated) was secured over it using metal clips.
The electrolyte solution (0.5M potassium iodide and 0.05M iodine in ethylene glycol) was drawn up into the porous titanium dioxide structure . Raspberry, blueberry, beetroot and orange leaf cells were constructed.
All cells produced a photocurrent when a voltage was applied , but its magnitude varied between the dyes; the region of negative current and positive voltage represents photocurrent activity. It was found that blueberries were most efficient –they produced a photocurrent of 0.2mA (~0.02% efficiency) - followed by raspberries, then beetroot. The orange leaf dyed cells produced virtually no current.
Initially the blueberries, raspberries and beetroot showed improvement, whereas there was none in the orange leaf dye. There was significant improvement over the first 9 hours, before degradation began. There was improved photocurrent as the concentration of electrolyte solution was increased for raspberries, blueberries and beetroot, but not for the orange leaf cell.
It is hard to take into account factors like electrolytic degradation and the oxidation of fruit etc.However people feel that the improvement is independent of the degradation process and when this degradation process becomes the dominant process then a decrease in efficiency is noted.
The improvement process can be explained by looking at the energy levels associated with the titanium dioxide, the dye and the electrolyte solution. In the blueberry cell: the ease with which an electron moves through the system is determined by the energy barriers, or band gaps that must be overcome.
The electron must have a significant energy to transfer from the electrolyte to the dye. As the cell is exposed to the light the cell heats up, causing evaporation of the electrolyte solution. This increases the concentration of KI and I2, and the energy level of the valence band is raised, making the transfer of electrons to the dye a far more favourable process. This explains why the photocurrent improved when placed underneath the light.
The conduction band of the blueberry dye lies above that of the titanium dioxide conduction band. Hence it is energetically favourable for the electrons excited from the valence band into the conduction band of the dye to be transferred to the conduction band of the titanium dioxide.
This is not the case in the orange leaves. The conduction band of the orange leaves lies below that of tiO2.
Thus in choosing materials it is important to look at the energy considerations.
A significant short coming of the DYSC model is the leaking of the electrolyte which reduces the life time of the cell. Although we can use sold electrolytes where the dye would eject an electron into the TiO2 and a hole into the solid electrolyte the efficiency is really low due to the poor penetration of the sold into the pores of TiO2.
Currently there is research going on, on the use of polymer gel to quasi solidify the electrolyte. The addition of Poly(viny1idene fluoride co-hexafluoropropylene) to the KI/I2 electrolyte has improved the fill factors and the energy conversion efficiency of the DYSC by about 17 %.
Thus dye sensitized solar cells seem to be a viable option for the future
This is essentially a dye sensitized solar cell.
To explain how this works in more detail we need to go back to Nature.
The problem with silicon solar cells is that achieving the optimum band gap is difficult. This was already explained in the last post. But wait, who am I kidding?! :P No one would have gone through that anyways. So let me explain again.
If the band gap is high, high energy photons can be captured. However, if the band gap is low, then more number of photons can be captured. There therefore must be an optimum band gap. Tandem solar cells, quantum dots are two of the ways used to capture all the wavelengths of light.
Also, in solid state devices. Silicon is used as the electron generator and an electric field is produced in it which causes charge transport. A simplistic explanation of this process is:- A photon is incident on n type Si. If it's energy is more than the band gap, it excites an electron which moves towards the bottom of the cell: p junction and then through an electrical wire to the load.
Now the problem is to prevent the recombination of the electron with a hole as it travels from the top negative junction to the bottom positive junction.
Dye sensitized cells solve the fist problem by mimicking photosynthesis. They consist of an electrolyte with two electrodes. Silicon basically acts as a charge transporter and does not generate the electron.
What happens is, a photon falls on the organic dye which excites the dye molecule to an excited state releasing an electron. This electron moves into the TiO2 and exits through the conductive layer into the electrical wires. This is how current flow occurs.
Then the electron returns to the cell via the bottom conductive layer. With the help of a catalyst (usually platinum), the electron enters the electrolyte and interacts with the tri iodide molecule. It reduces it to form an iodide ion. The iodide ion moves towards the activated dye molecule and transfers an electron to the dye. Thus the dye molecule reverts to its previous ground state and the iodide ion reverts back to tri iodide.
What happens in photosynthesis is, an electron is absorbed by a pigment. The energy of the electron is used to synthesize dihydro nicotinamide di hydrogen phosphate (NADPH), a molecule that will eventually produce carbohydrates. To get the pigment back to it's initial state, an electron is donated in the oxidation of water to produce oxygen.
The difference between the two processes is that the storage of energy in the form of starch occurs in leaves, while the storage of energy in a dye sensitized solar cell is still not possible.
The similarities of the two processes is evident. Both are regenerative cycles which involve redox reactions. They use a multi layer structure similar to a thylakoid membrane. (They are the site of the light dependent reactions in leaves)
In DYSC the organic dye replaces the pigment in Nature. The wide band gap semi conductor used replaces the oxidized NADPH and carbon dioxide as the electron acceptor. The electrolyte replaces the water and oxygen as the electron donor.
Now I will talk a little about the materials involved.
Typically a ruthenium metalorganic dye (Ru-centered) is used as a monolayer of light-absorbing material. The dye-sensitized solar cell depends on a mesoporous layer of nanoparticulate titanium dioxide to greatly amplify the surface area (200-300 m2/g TiO2, as compared to approximately 10 m2/g of flat single crystal). The photogenerated electrons from the light absorbing dye are passed on to the n-type TiO2, and the holes are passed to an electrolyte on the other side of the dye. The circuit is completed by a redox couple in the electrolyte, which can be liquid or solid. This type of cell allows a more flexible use of materials, and is typically manufactured by screen printing and/or use of Ultrasonic Nozzles, with the potential for lower processing costs than those used for bulk solar cells. However, the dyes in these cells also suffer from degradation under heat and UV light, and the cell casing is difficult to seal due to the solvents used in assembly.
The absorption of incident light depends upon the number of dye molecules attached per unit volume of the semi conductor. if the dye is on a flat surface less than 1% of light will be absorbed.
Light absorption is maximized by the use of sintered nano meter size anatase (tetragonal crystal structure) titanium dioxide. The surface are is increased by 200-300%. This surface has pores in the range of 20-500Angstrom. It is advantageous to use TiO2 because it is cheap, easily available, bio compatible and non toxic. The anatase phase is used because it has a wide band gap that is transparent to visible light and thus ensures that the light is absorbed only by the dye molecules.
The thickness of the TiO2 layer is a compromise between maximum surface area and minimum recombination losses.
They dyes used by commercial dye sensitized solar cells generally use ruthenium bi pyridyl based dyes. The wavelengths of light absorbed are 510-570nm and adequate conversion between 450-650nm. This absorption spectrum overlaps well with the diffuse sunlight spectrum. However such dyes are hard to synthesize.
A little about these dyes
Ruthenium bipyridyl complexes have been used as efficient sensitizers for photoelectrochemical cells based on nano-porous films of . It is found that cis-dithiocyanate-bis(2,2-bipyridyl-4,4-dicarboxylate) ruthenium (II) can be used as the sensitizer in a solid-state photovoltaic cell in which a monolayer of the sensitizer is sandwiched between nano-porous n- and p-CuI.
A significant photo current can be obtained using natural anthocyanin dyes which can be extracted from blueberries as natural water based substitutes. The absorption of cyanin on the surface of TiO2 is a rapid reaction and the OH counter ion is displaced from the ti(4) work site that combines with a proton donated by anthocyanin. This strong chemical affinity is the reason blue berries can be used :D.
An Experiment that was conducted: (i found this paper on the net)
A commercially bought titanium dioxide coated glass slide was stained with a berry dye (e.g. raspberries, blueberries, beetroot); the dye was made by crushing the fruit (or leaves) and purifying them in a solution of methanol, acetic acid and water . This slide was washed with water, and dried with propanol, and a transparent conducting glass slide (tin dioxide coated) was secured over it using metal clips.
The electrolyte solution (0.5M potassium iodide and 0.05M iodine in ethylene glycol) was drawn up into the porous titanium dioxide structure . Raspberry, blueberry, beetroot and orange leaf cells were constructed.
All cells produced a photocurrent when a voltage was applied , but its magnitude varied between the dyes; the region of negative current and positive voltage represents photocurrent activity. It was found that blueberries were most efficient –they produced a photocurrent of 0.2mA (~0.02% efficiency) - followed by raspberries, then beetroot. The orange leaf dyed cells produced virtually no current.
Initially the blueberries, raspberries and beetroot showed improvement, whereas there was none in the orange leaf dye. There was significant improvement over the first 9 hours, before degradation began. There was improved photocurrent as the concentration of electrolyte solution was increased for raspberries, blueberries and beetroot, but not for the orange leaf cell.
It is hard to take into account factors like electrolytic degradation and the oxidation of fruit etc.However people feel that the improvement is independent of the degradation process and when this degradation process becomes the dominant process then a decrease in efficiency is noted.
The improvement process can be explained by looking at the energy levels associated with the titanium dioxide, the dye and the electrolyte solution. In the blueberry cell: the ease with which an electron moves through the system is determined by the energy barriers, or band gaps that must be overcome.
The electron must have a significant energy to transfer from the electrolyte to the dye. As the cell is exposed to the light the cell heats up, causing evaporation of the electrolyte solution. This increases the concentration of KI and I2, and the energy level of the valence band is raised, making the transfer of electrons to the dye a far more favourable process. This explains why the photocurrent improved when placed underneath the light.
The conduction band of the blueberry dye lies above that of the titanium dioxide conduction band. Hence it is energetically favourable for the electrons excited from the valence band into the conduction band of the dye to be transferred to the conduction band of the titanium dioxide.
This is not the case in the orange leaves. The conduction band of the orange leaves lies below that of tiO2.
Thus in choosing materials it is important to look at the energy considerations.
A significant short coming of the DYSC model is the leaking of the electrolyte which reduces the life time of the cell. Although we can use sold electrolytes where the dye would eject an electron into the TiO2 and a hole into the solid electrolyte the efficiency is really low due to the poor penetration of the sold into the pores of TiO2.
Currently there is research going on, on the use of polymer gel to quasi solidify the electrolyte. The addition of Poly(viny1idene fluoride co-hexafluoropropylene) to the KI/I2 electrolyte has improved the fill factors and the energy conversion efficiency of the DYSC by about 17 %.
Thus dye sensitized solar cells seem to be a viable option for the future
Solar Cells Part 1
OK, so here's the thing, I'm taking part in Padarth, which is the material science fest of IITB in the exposition on materials. My topic is 'Whacky Solar Cells'. The exposition happens to be tomorrow and I'm really nervous. So I'm going to lay down the structure of my talk here.
This post will just have the history of solar cells and the different types.
I have taken most of this post from an article I wrote along with 2 seniors on Solar cells. This article has already appeared in the Meta department's magazine/newsletter.
History of Solar Cells:
Photovolataic devices have become a buzzword now in the times of energy crisis but the history of photovoltaics dates more than 160 years back. The phenomenon of photoconductivity was first discovered in 1839 by a nineteen years old French experimental physicist, Alexandre Edmond Becquerel, who was experimenting with selenium electrodes to be used as high resistance rods for use in submarine applications. He observed that the conductivity of the rods increased with illumination.
In 1873, Willoughby Smith discovered photovoltaic effect in selenium. Later, in 1876, R.E. Day and William G. Adam found that illuminating a junction between selenium and platinum also has a photovoltaic effect. These discoveries led to the construction of the first solar cell based on selenium and gold junction in 1877, which was described in detail by Charles Fritts, an American scientist, in 1883. In 1888, Edward Weston received the first US patent for "solar cell" and later, in 1901, Nikola Tesla received a US patent for "method of utilizing, and apparatus for the utilization of, radiant energy".
From 1900-1949, scientists tried to find the theoretical explanation of Photovoltaic effect and first functional solar cells were made. In 1904, Albert Einstein published a paper explaining the effect, for which he was awarded the Nobel Prize in 1921. Einstein's theoretical explanation was experimentally proved by Robert Millikan in 1916. In 1914, the existence of a barrier layer in PV devices was reported.
The start of silicon based solar cells and devices can be attributed to the discovery of a method of preparing monocrystalline silicon by Jan Czochralski in 1918. The first monocrystalline silicon solar cell was made in 1941 by Russel Ohl.
Solar cell research got a big boast during 1950-1970 due to extensive interest in space research. In early 1950s, extensive laboratory research on solar cells took place. In 1951, the first germanium solar cells were made. In 1954, Rappaport, Loferski and Jenny of the RCA Laboratories published a report on photovoltaic effect in CdS.
In the same year, Bell Labs researchers Pearson, Chapin, and Fuller reported an efficiency of 4.5% in silicon solar cells; this was raised to 6% only a few months later. By the year 1955, the energy cost for PV devices was US$ 1,785 per W and the maximum efficiency achieved out of a device was 2%. By 1960, the efficiency of commercially available solar cells had touched 14%.
The energy crisis and oil embargoes of 1970s increased the interest in renewable sources of energy and development of photovoltaics got a big boast. Large photovoltaic corporations were set up around the world during this time, like the Solar Power Corporation in 1972, the Solarex Corporation in 1973 Solec International and Solar Technology International in 1975. In 1974, Japan formulated “Project Sunshine” to fuel PV research and development. By this time, a silicon solar cell of US$ 30 per W had been produced. This opened up the possibility of using solar cells in small day-to-day applications like pocket calculators, watches, radios, lanterns and other small battery-charging applications. The Solar Energy Research Institute (SERI), later to become the National Renewable Energy Laboratory (NREL), opened in Golden, Colorado in 1977. Total PV manufacturing production exceeded 500 kW by that year.
In 1984, the fist amorphous silicon solar cell was made and in 1986, ARCO Solar introduced the first commercial thin film photovoltaic module. By then University of New South Wales, Australia had made a solar cell with efficiency of 20%. In 1989, a concentrator solar cell was developed in which sunlight is concentrated onto the cell surface by means of lenses. This system achieved an efficiency of 37% due to the increased intensity of the collected energy. New thin film deposition techniques were perfected in 21st century, leading to the development of more efficient and cost-effective novel designs solar cells like amorphous silicon solar cells, organic solar cells and quantum dot solar cells. Recently, a research group at Fraunhofer Institute for Solar Energy Systems ISE, Germany, reported a whopping efficiency of 41% with tandem solar cells. Hopefully, that day is not far when solar cells will become the ultimate energy source with cheap solar panel installations on each and every household in the world!
Novel Types of Solar Cells:-
1. Quantum Dot Solar Cells
Quantum Dots are semiconductor nano crystals composed of periodic groups of II-VI, III-V, or IV-VI materials that exhibit size and compositionally tunable band gaps. This is because the size of such a semiconductor crystal is so small that it approaches the size of the material's Exciton Bohr Radius (An exciton bohr radius is the distance in an electron-hole pair) and the electron energy levels can no longer be treated as continuousi.e that there is a small and finite separation between energy levels. The absorptive and emissive behavior of a quantum dot depends strongly on its size.
The greater the bandgap of a solar cell semiconductor, the more energetic the photons absorbed, and the greater the output voltage. On the other hand, a lower bandgap results in the capture of more photons including those in the red end of the solar spectrum, resulting in a higher output of current but at a lower output voltage.
Thus, there is an optimum bandgap that corresponds to the highest possible solar-electric energy conversion, and this can also be achieved by using a mixture of quantum dots of different sizes for harvesting the maximum proportion of the incident light. Another advantage of quantum dots is they can easily be molded in convenient shapes by combining with organic polymers, dyes, or made into porous films on substrates of various materials like glass, plastics and metal sheets.
2. Organic solar cells
Organic solar cells emerged in the late 1970s, based on conjugated polymers – polymers with alternating double and single carbon-carbon bonds – when it was discovered that doping these materials – i.e. slightly contaminating with appropriate chemical elements - increased conductivity several orders of magnitude . Since then these materials have found applications in the making of solar cells.
Organic photovoltaic solar cells bear an important potential of development in the search for low-cost modules for the production of domestic electricity. One of the main differences between inorganic and organic solar cells is that photo-excitation in these materials does not automatically lead to the generation of free charge carriers, but to bind electron–hole pairs (exciton) with a binding energy of about 0.4 eV.
One of the biggest obstacles to organic solar cells is that it is difficult to control what happens after light is absorbed: whether the desired property is transmitting energy, storing information or emitting light. Experiments, however suggests it is possible to achieve control using quantum effects, even under relatively normal conditions. Organic solar cells — made out of plastic-like polymers — are much cheaper when compared by the conventional inorganic, silicon and metal-based solar cells typically favored by the solar industry. They are also flexible and are very light. Their one main disadvantage, of course, is their relatively low efficiency of just 5.4% as compared to the 20 - 40% efficiencies regularly attained by inorganic crystalline solar cells.
3. Dye sensitized solar cells
A dye sensitized solar cell is a relatively new class of thin film, low-cost solar cells. It is based on a semiconductor formed between a photo-sensitized anode and an electrolyte which functions like a photochemical system. They are also known as Grätzel cells after their inventor.
Dye-sensitized solar cells separate the two functions provided by silicon in a traditional cell design. Normally the silicon acts as both the source of photoelectrons, as well as providing the electric field to separate the charges and create a current. In the dye-sensitized solar cell, the bulk of the semiconductor is used solely for charge transport, the photoelectrons are provided from a separate photosensitive dye.
Charge separation occurs at the surfaces between the dye, semiconductor and electrolyte. In quantum efficiency (the chance that one photon (of a particular energy) will create one electron) terms, these cells are extremely efficient. Due to their "depth" in the nanostructure there is a very high chance that a photon will be absorbed, and the dyes are very effective at converting them to electrons. DSSc's are currently the most efficient third-generation solar technology available.
4. Tandem solar cells
Multijunction photovoltaic cells are a sub-class of solar cells developed for higher efficiency. These multijunction cells consist of multiple thin films. Each type of semiconductor will have a characteristic band gap energy which, loosely speaking, causes it to absorb light most efficiently at a certain color, or more precisely, to absorb electromagnetic radiation over a portion of the spectrum. The semiconductors are carefully chosen to absorb nearly all of the solar spectrum, thus generating electricity from as much of the solar energy as possible.
Tandem solar cells based on monolithic, series connected, gallium indium phosphide (GaInP), gallium arsenide GaAs, and germanium Ge pn junctions, are seeing demand rapidly rise as we can see reflected in the irse in cost of gallium and germanium. These solar cells are currently being utilized in the Mars rover missions. Triple-junction GaAs solar cells were also being used as the power source of the Dutch four-time World Solar challenge winners Nuna in 2005 and 2007, and also by the Dutch solar cars Solutra (2005) and Twente One (2007).A Dutch university set the record for thin film solar cell effiency using a single junction GaAs to 25.8% in August 2008 using only 4 µm thick GaAs layer which can be transferred from a wafer base to glass or plastic film.
My next post will cover my topic in more detail which is -Solar Cells from Blueberries
This post will just have the history of solar cells and the different types.
I have taken most of this post from an article I wrote along with 2 seniors on Solar cells. This article has already appeared in the Meta department's magazine/newsletter.
History of Solar Cells:
Photovolataic devices have become a buzzword now in the times of energy crisis but the history of photovoltaics dates more than 160 years back. The phenomenon of photoconductivity was first discovered in 1839 by a nineteen years old French experimental physicist, Alexandre Edmond Becquerel, who was experimenting with selenium electrodes to be used as high resistance rods for use in submarine applications. He observed that the conductivity of the rods increased with illumination.
In 1873, Willoughby Smith discovered photovoltaic effect in selenium. Later, in 1876, R.E. Day and William G. Adam found that illuminating a junction between selenium and platinum also has a photovoltaic effect. These discoveries led to the construction of the first solar cell based on selenium and gold junction in 1877, which was described in detail by Charles Fritts, an American scientist, in 1883. In 1888, Edward Weston received the first US patent for "solar cell" and later, in 1901, Nikola Tesla received a US patent for "method of utilizing, and apparatus for the utilization of, radiant energy".
From 1900-1949, scientists tried to find the theoretical explanation of Photovoltaic effect and first functional solar cells were made. In 1904, Albert Einstein published a paper explaining the effect, for which he was awarded the Nobel Prize in 1921. Einstein's theoretical explanation was experimentally proved by Robert Millikan in 1916. In 1914, the existence of a barrier layer in PV devices was reported.
The start of silicon based solar cells and devices can be attributed to the discovery of a method of preparing monocrystalline silicon by Jan Czochralski in 1918. The first monocrystalline silicon solar cell was made in 1941 by Russel Ohl.
Solar cell research got a big boast during 1950-1970 due to extensive interest in space research. In early 1950s, extensive laboratory research on solar cells took place. In 1951, the first germanium solar cells were made. In 1954, Rappaport, Loferski and Jenny of the RCA Laboratories published a report on photovoltaic effect in CdS.
In the same year, Bell Labs researchers Pearson, Chapin, and Fuller reported an efficiency of 4.5% in silicon solar cells; this was raised to 6% only a few months later. By the year 1955, the energy cost for PV devices was US$ 1,785 per W and the maximum efficiency achieved out of a device was 2%. By 1960, the efficiency of commercially available solar cells had touched 14%.
The energy crisis and oil embargoes of 1970s increased the interest in renewable sources of energy and development of photovoltaics got a big boast. Large photovoltaic corporations were set up around the world during this time, like the Solar Power Corporation in 1972, the Solarex Corporation in 1973 Solec International and Solar Technology International in 1975. In 1974, Japan formulated “Project Sunshine” to fuel PV research and development. By this time, a silicon solar cell of US$ 30 per W had been produced. This opened up the possibility of using solar cells in small day-to-day applications like pocket calculators, watches, radios, lanterns and other small battery-charging applications. The Solar Energy Research Institute (SERI), later to become the National Renewable Energy Laboratory (NREL), opened in Golden, Colorado in 1977. Total PV manufacturing production exceeded 500 kW by that year.
In 1984, the fist amorphous silicon solar cell was made and in 1986, ARCO Solar introduced the first commercial thin film photovoltaic module. By then University of New South Wales, Australia had made a solar cell with efficiency of 20%. In 1989, a concentrator solar cell was developed in which sunlight is concentrated onto the cell surface by means of lenses. This system achieved an efficiency of 37% due to the increased intensity of the collected energy. New thin film deposition techniques were perfected in 21st century, leading to the development of more efficient and cost-effective novel designs solar cells like amorphous silicon solar cells, organic solar cells and quantum dot solar cells. Recently, a research group at Fraunhofer Institute for Solar Energy Systems ISE, Germany, reported a whopping efficiency of 41% with tandem solar cells. Hopefully, that day is not far when solar cells will become the ultimate energy source with cheap solar panel installations on each and every household in the world!
Novel Types of Solar Cells:-
1. Quantum Dot Solar Cells
Quantum Dots are semiconductor nano crystals composed of periodic groups of II-VI, III-V, or IV-VI materials that exhibit size and compositionally tunable band gaps. This is because the size of such a semiconductor crystal is so small that it approaches the size of the material's Exciton Bohr Radius (An exciton bohr radius is the distance in an electron-hole pair) and the electron energy levels can no longer be treated as continuousi.e that there is a small and finite separation between energy levels. The absorptive and emissive behavior of a quantum dot depends strongly on its size.
The greater the bandgap of a solar cell semiconductor, the more energetic the photons absorbed, and the greater the output voltage. On the other hand, a lower bandgap results in the capture of more photons including those in the red end of the solar spectrum, resulting in a higher output of current but at a lower output voltage.
Thus, there is an optimum bandgap that corresponds to the highest possible solar-electric energy conversion, and this can also be achieved by using a mixture of quantum dots of different sizes for harvesting the maximum proportion of the incident light. Another advantage of quantum dots is they can easily be molded in convenient shapes by combining with organic polymers, dyes, or made into porous films on substrates of various materials like glass, plastics and metal sheets.
2. Organic solar cells
Organic solar cells emerged in the late 1970s, based on conjugated polymers – polymers with alternating double and single carbon-carbon bonds – when it was discovered that doping these materials – i.e. slightly contaminating with appropriate chemical elements - increased conductivity several orders of magnitude . Since then these materials have found applications in the making of solar cells.
Organic photovoltaic solar cells bear an important potential of development in the search for low-cost modules for the production of domestic electricity. One of the main differences between inorganic and organic solar cells is that photo-excitation in these materials does not automatically lead to the generation of free charge carriers, but to bind electron–hole pairs (exciton) with a binding energy of about 0.4 eV.
One of the biggest obstacles to organic solar cells is that it is difficult to control what happens after light is absorbed: whether the desired property is transmitting energy, storing information or emitting light. Experiments, however suggests it is possible to achieve control using quantum effects, even under relatively normal conditions. Organic solar cells — made out of plastic-like polymers — are much cheaper when compared by the conventional inorganic, silicon and metal-based solar cells typically favored by the solar industry. They are also flexible and are very light. Their one main disadvantage, of course, is their relatively low efficiency of just 5.4% as compared to the 20 - 40% efficiencies regularly attained by inorganic crystalline solar cells.
3. Dye sensitized solar cells
A dye sensitized solar cell is a relatively new class of thin film, low-cost solar cells. It is based on a semiconductor formed between a photo-sensitized anode and an electrolyte which functions like a photochemical system. They are also known as Grätzel cells after their inventor.
Dye-sensitized solar cells separate the two functions provided by silicon in a traditional cell design. Normally the silicon acts as both the source of photoelectrons, as well as providing the electric field to separate the charges and create a current. In the dye-sensitized solar cell, the bulk of the semiconductor is used solely for charge transport, the photoelectrons are provided from a separate photosensitive dye.
Charge separation occurs at the surfaces between the dye, semiconductor and electrolyte. In quantum efficiency (the chance that one photon (of a particular energy) will create one electron) terms, these cells are extremely efficient. Due to their "depth" in the nanostructure there is a very high chance that a photon will be absorbed, and the dyes are very effective at converting them to electrons. DSSc's are currently the most efficient third-generation solar technology available.
4. Tandem solar cells
Multijunction photovoltaic cells are a sub-class of solar cells developed for higher efficiency. These multijunction cells consist of multiple thin films. Each type of semiconductor will have a characteristic band gap energy which, loosely speaking, causes it to absorb light most efficiently at a certain color, or more precisely, to absorb electromagnetic radiation over a portion of the spectrum. The semiconductors are carefully chosen to absorb nearly all of the solar spectrum, thus generating electricity from as much of the solar energy as possible.
Tandem solar cells based on monolithic, series connected, gallium indium phosphide (GaInP), gallium arsenide GaAs, and germanium Ge pn junctions, are seeing demand rapidly rise as we can see reflected in the irse in cost of gallium and germanium. These solar cells are currently being utilized in the Mars rover missions. Triple-junction GaAs solar cells were also being used as the power source of the Dutch four-time World Solar challenge winners Nuna in 2005 and 2007, and also by the Dutch solar cars Solutra (2005) and Twente One (2007).A Dutch university set the record for thin film solar cell effiency using a single junction GaAs to 25.8% in August 2008 using only 4 µm thick GaAs layer which can be transferred from a wafer base to glass or plastic film.
My next post will cover my topic in more detail which is -Solar Cells from Blueberries
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