So, let us start again ok. So, introduction to solar PV semiconductors for PV cells we want to use a semiconductor material and why semiconductor I will come back to that. This we already discussed PVs electricity can be generated in decent flies way it is modular. So, you can add the capacity whenever you want. So, today if you have 1 kilowatt requirement you install 1 kilowatt tomorrow if you have 2 kilowatt you can add one more kilowatt to make it 2 kilowatt there is no other power plant which actually provide you that kind of facility. It can generate a very small power in milli watt, micro watt, 2 megawatt that I told you yesterday and it is feasible to fulfill all our energy requirement using solar and you can actually have the big power plant and stand alone system also I just want to give a glimpse of what is happening in the world in terms of the photovoltaic production and if you look at this almost exponential graph and which is very pleasing to see that in 2010 the worldwide production was more than 20 gigawatt ok, 20 gigawatt, 20,000 megawatt the whole world production that is the red line and you can see what is happening in other part of the world as well. So, which is a very good sign the worldwide PV production is increasing tremendously there is a very good government policies exist and as the secretary was telling yesterday that there is a need for more and more manpower in this area. So, generations of photovoltaic just a brief overview basically people talk about 3 generations of solar photovoltaic generation 1 is basically a crystalline silicon technology mono crystalline and multi crystalline I will come back to the what it is 2nd generation is referred as thin film technologies this 1st generation has a based on silicon crystalline silicon. So, it is abundant non-toxic good efficiencies, but also have relatively high cost based on silicon. The 2nd generation of thin film technology are basically amorphous silicon cadmium telluride CIGS that is copper indium gallium selenide and this are actually low material cost, but also have the lower efficiency as compared to crystalline silicon we will discuss more about that in detail. So, what people are looking is 3rd generations of solar cell which is extremely low production cost extremely high efficiency. So, that your module cost eventually the module cost is more important. So, that your module cost becomes low. So, that is the benefit of the generation 1 that is the good quality material high efficiency and the benefit of generation 2 that is the low cost generation 3 should have the both of them. But let me tell you that in the in the real picture when you see the real commercial picture there is not much difference in terms of the cost that you can get yes thin film technologies can be relatively cheaper and they are relatively cheaper, but the cost margin that we expect is not that much that one can always decide to go for a thin film. So, is still a big chunk of whatever is produced in the world and whatever is consumed in the world is coming from crystalline silicon thin film. So, crystalline silicon is about 80 percent of the market thin film is about 20 percent. Now, the final cost that we pay as a customer which is in terms of rupees per watt peak the solar PV module power is given as a watt peak peak power. Why it is given as a watt peak? So, let me explain you that briefly why it is given as a. So, PV rating is given into watt that is the power, but peak power. Why this p is added p is actually referring to the peak and why p is added to just emphasize that whenever manufacturer gives you module it is giving you for air mass 1.5 spectrum and air mass 1.5 spectrum is corresponding to 1000 watt per meter square 1000 watt per meter square. Now, we know that 1000 watt per meter square solar intensity is not there always in the morning you will have very low intensity 100 watt per meter square then towards after it will increase from 200, 500 it may go to 800, 900 or 1000 if it is the best sunny day and then it will come down right. So, therefore, manufacturer gives the rating at this power and so this is the peak power that the power rating is actually maximum power that you can get from your module and that is the peak power and that is why we should always refer to not watt, but it is watt peak remember that. So, now the money that we pay as a rupees per watt peak or dollar per watt peak depends on two main parameters and very important to understand. It depends on the price of the module that is rupees per watt peak rupees per watt peak can be written as a rupees per meter square and watt peak per meter square right rupees per meter square is basically how much money you spend to manufacture 1 meter square of module. So, it refers to the production cost rupees per meter square is actually refers to the production cost how much money that you spend in manufacturing 1 meter square of module and watt peak per meter square is basically how much watt peak or how much wattage you can get from your module per meter square. Remember, this is under the air mass 1.5 ok. By the way, this air mass 1.5 is part of what is called the SPC standard test condition. The two standard test condition one parameter is referred to the prediction that is 1000 watt per meter square other parameter is a temperature ok. So, other parameter is a temperature and that comes to 25 degree centigrade. So, standard test condition is referring to the 1000 watt per meter square of solar radiation, air mass 1.5 spectrum and 25 degree centigrade. This is referred as STC. So, your watt peak rating is given under standard test condition ok. This is given under standard test condition. So, this is keep that in mind again all the power is rated at a standard test condition of 1000 watt per meter square and 25 degree centigrade ok. So, coming back to this. So, your price of your module in rupees per watt peak depends on the production cost and the efficiency. What do you want? We want low production cost and we want high efficiency. So, that is what is all the scientists keep on doing all over the world that you try to minimize the production cost, try to increase the efficiency I want to mind you again that only increase in efficiency is not the target right. When it comes to the actual commercial module only increase in the efficiency not is not the target which is clearly described here ok. So, suppose if you have a technology with a 10 percent efficiency and by doubling the production cost you are doubling the efficiency in making 20 percent cell. Will it result in the decrease in the price of the module? Let me repeat if you are having a 10 percent technology today by some advancement you are actually increasing the efficiency to 20 percent that is double, but you are also increasing your production cost to double. Will it reduce the price of your module? Answer is no right because both the numerator and denominator is doubling and therefore, price of module is not decreasing and one most important parameter for the user is how much money you pay per watt peak of module. So, therefore, very important to note that it is the production cost as well as efficiency both are important when it comes to the PV module. So, let us look at what are the various technology and then we will come back to the materials ok. The various commercially available technologies are. So, there is two distinction one is called the crystalline silicon wafer based technology wafer is basically a thin sheet of silicon or 10 disc of silicon or you can say thin plate of silicon which is used for making a solar cells in module. When you say thin film actually use a glass substrate or within the glass substrate you deposit your material and that deposited material it become your solar cell ok. So, main difference is in the one case the solar cell itself is a the material or a plate in which the wafer itself is a material plate in which you make solar cell in other case you deposit the material and you make your solar cell. So, the crystalline silicon cells are actually thick cells 180 micron thick current standard and therefore, the cells are made in the wafer or we can say crystalline silicon wafer. So, there are three different crystalline silicon wafer technology one is mono crystalline multi crystalline ribbon silicon and I will come back to that what is mono what is multi crystalline ribbon and this are the various companies ok. So, one category of technologies are crystalline silicon wafer based technology then the other technologies are thin film technology and remember what I told thin film technology thin film technology is the one where you take some substrate which is not actually doing a job of solar cell it is actually only there to help the thin film that you are going to deposit. You take your substrate you deposit your thin film and the deposited thin film is your solar cell the substrate is not substrate is only helping it. So, those kind of technology are referred as a thin film technology and as the name suggests thin basically means that your material which is acting as a solar cell is very thin less than a micron for crystalline silicon the wafer is 180 micron 200 micron thick, but crystalline for a thin film the material is less than a micron or sometime 2 or 3 micron, but much lower than the crystalline silicon. So, what are the thin film technology? They are silicon based I hope you can see this silicon based you have amorphous silicon and other thin film which is nano crystalline silicon that is silicon based and then non silicon based. So, material like cadmium telluride CDT or CIGS copper indium gallium selenide or CIS copper indium selenide or disensitized solar cell. So, these are the non silicon based solar cell. Now, the thin film technologies whether it is amorphous silicon or or CIGS can be flexible it can be deposited at a low temperature and therefore, at low temperature you can use plastics to deposit that or you can use metal foil like very thin sheet of stainless steel can or actually use. So, the modules can be flexible or it can be rigid also, but thin film does not always mean flexible module thin film modules can be flexible cadmium telluride modules are not flexible at all they are always rigid. So, these are so depending on this it can be flexible or rigid and the various thin film technologies are amorphous silicon cadmium telluride and CIGS that are commercially available some disensitized solar cells are also commercially available right. So, now let us look at the challenges that PV technology have for example, the high cost per unit watt is a is a is a big challenge that scientists are trying to reduce it, but let me tell you that when solar cell production started in 70s or when solar cell were used for the space application in 1960s the cost of solar cell were 1000 dollars per watt and what is the cost of solar modules today? Cost of modules you can get as low as dollar 1 dollar or 1.5 dollar per watt. So, significant reduction has already occurred it is very exciting time and more reduction is expected in future. Efficiencies are moderate I will I will give the numbers efficiency of crystalline silicon is about 15 percent module for thin film it is about 8, 9, 10 percent, 11 percent. So, we can further improve the efficiency material because when we are talking about production of solar modules for a very large quantity gigawatt, terawatt ok 1 terawatt is 12. So, when we are talking about terawatt production we require huge amount of material and therefore, we should use the material which is available in large quantity it should also be stable in the long term. So, if you are making a module it is expected that module should work for 25 years that is a guaranteed lifetime and therefore, we should use the material which is having long stability. The energy payback period should be low. So, whatever energy has gone into making a solar cell should itself be low and for crystalline silicon the energy payback period is 2 to 3 years and for thin film technology the energy payback period is about 1 year or less than 1 year and whatever money you should spend should be recovered back. So, these are the various challenges that any solar cell technology must overcome whether it is a crystalline silicon whether it is a thin film whether it is a second generation or third generation whatever generation you are talking these are the challenges that must be overcome in order to be a successful solar PV technology ok. So, now the question which material can be used for solar cells can I use metals for solar cells can I use semiconductors for solar cells can I use insulators for solar cells ok. So, these are the three questions we must answer and before answering that we should understand what is the role of a solar cell. What solar cell does? I am sure everybody knows that what solar cell does is you put a light on solar cell it generates electricity ok. Then the next question what is electricity? The electricity flows whenever you have the potential difference ok whenever there is a potential difference you connect a wire to it electricity will flow. Then the next question is what is potential difference? The potential difference occurs or the voltage is generated whenever a positive charge is separated by a negative charge right. So, there is a line of thought we want electricity and therefore, we want voltage we want voltage to be generated in a solar cell. So, therefore, we want positive charge to be separated by a negative charge. So, that is the job solar cell must do it must separate a positive charge from negative charge. So, that they can measure a voltage across the terminal p side and n side or positive side negative side many of you must have done it yesterday measure the voltage when light falls on a solar cell. So, that is the job a solar cell must perform generation of voltage right. Now, can a metal do this job can a metal do this job the answer is no. I will get come back to this, but in a brief basically a metal semiconductor or insulator can be described in terms of the energy level that they represent within the material energy level that represent and we will come back to that again this point. But if you look at the metal and metal. So, basically conduction band and valence band this energy level are all mixed right and we want to we want to create that positive and negative charge, but here all this energy levels are continuous and therefore, the separation is not possible. And therefore, metals cannot be used metals are very good in converting the light energy into heat energy. So, that metals can be used for a solar water heater it can be used for the solar concentrator thermal powered, but it cannot be used for the solar photovoltaic power But look at this diagram this is a diagram of this is a diagram for semiconductor semiconductor have a conduction band and valence band and there is a in between there is no gap there is a gap of energy which means there is no energy there is no house which a carrier can occupy there are no levels there and I will come back to that, but let me quickly. So, when you do that when you have this kind of material then basically you can actually separate a positive charge and a negative charge across this energy gap or band gap and when you can do this you can actually generate voltage. Then now somebody can tell me that insulators also have similar band gap. So, why not to use insulators any answer to that any answer to that why not to use insulator. So, the so my semiconductor will have this kind of band gap. So, this is my valence band this is my conduction band and I will come back to that again and this is a gap this is the energy gap. My insulator will have a normally many times this diagram can be shown only by this line that is the age of the conduction band and this line which is the age of the valence band. So, in a simplified I can show only this diagram that this is the conduction band age this is the valence band age and this is the energy gap. Now, what are the energies of the photon that I receive in my spectrum in our solar spectrum in our galaxy whatever sun is there with us what are the photon energy that we have done the calculation yesterday right. We receive ultraviolet photon we receive violet photon we receive green photon yellow photon red photon and infrared photons right. You have done the calculation of energy what is the range of energy we receive normally the photon energy range in solar spectrum what is your answer we receive about 3.5 electron volt at the higher side and lower side we receive about 3 electron ok. Remember these numbers are very important again photon energy at the higher side is about 3.5 electron volt in the spectrum at the lower side infrared side it is about 0.3 electron. Now, so this is a semiconductor SC now insulator will have very high band gap ok. So, semiconductor band gap you know can vary from let us say 0.3 electron volt to let us say 1 it can go actually 2.5 electron volt something like this insulators have this is the conduction band for insulator valence band for insulator and this band gap for insulator this is insulator. This band gap for insulator is very large it can be more than 5 electron volt. So, because my photons do not have enough energy to excite electron in this gap because the gap is so high it cannot give the energy and jump the electron from lower energy level to higher energy level. By the way this all this band diagrams the this is the this is the energy excess remember this excess is the energy excess and what is this excess? This excess is a distance or a space ok. So, because in insulator my band gap is so high that no photon in my solar spectrum can excite the electron, but on the other end semiconductor have the band gap about 0.3 to 2.5 electron ok. For example, silicon the most commonly used material for solar cell have the band gap of 1.12 electron volt ok. The silicon band gap the band gap of silicon is 1.12 electron volt. Look at my photons there are enough photons in the spectrum which will have enough energy to excite which will have enough energy to excite electron in silicon and therefore, insulator cannot be used metal cannot be used what I can use is only semiconductors is that clear to everybody ok. So, what are the semiconductors we can use many. So, if you look at the periodic table column 4 group 4 5 6 2 3 silicon is sitting here which is the most commonly material used in solar cell, but we can use germanium we can scar burn boron aluminium gallium indium zinc cadmium phosphorus arsenic antimony sulphur selenium tellurium all these materials either in one way or other way are being used for solar cell application all these materials ok. So, silicon is used in elemental form. So, we can use elemental form like silicon and germanium we can actually make the compound. So, we can make two materials together like gallium and arsenic together indium and phosphorus together make indium phosphate or we can put three of them together aluminium gallium arsenide mercury cadmium telluride or we can put four of them together five of them together whatever it is ok. So, the question is why do we put many semiconductor material together why do we put many semiconductor material together to make a solar cell and the answer is that we want best properties for our solar cells and what are those base properties we will come back to that ok, but basically the idea here is that yes you can combine more than one you can combine more than one material and make your solar cells. So, look at what is commercially available today silicon solar cells are commercially available cadmium telluride solar cells are commercially available gallium arsenide solar cells are commercially available, but a very expensive use for the space application and you can also actually put other materials and make a solar cell ok. So, these are about the semiconductor that can be used now what are the properties that we are interested in ok. So, we are interested in the structure of the material. So, how the atoms in the materials are arranged we are interested in the band gap of the material as I said band gap is very important if it is too high not many photons will be able to excite electrons if it is too low then there is a problem in the voltage I will come to that and we want a semiconductor which has a good absorption right. For example, glass glass is a transparent to the solar spectrum right we get the light through the glass because glass does not absorb anything, but we want our solar cell to absorb the light and convert it into the electricity. So, absorption is also very important. So, these are the important parameters we look for, but from the commercial perspective we must also look for ease of fabrication. So, you may have a very wonderful technology, but if it is difficult to fabricate then it is of no use. Temperature of fabrication is also important. So, normally we want low temperature. So, that your budget or your electricity spent in making the solar cells low and we want high throughput we should be able to make 2000 solar cells to 1 hour 3000 solar cell 1 hour. So, these are the various properties we try to look into. So, let us look at the structural properties of the material. The structure of the material are basically we are talking about how the atoms are arranged in a given material. Atoms can be arranged in a very systematic manner like this what I shown here. Look at in any direction all the atoms are arranged by a rule. There is some order and this order is extended in all directions. So, this order if your material is 1 centimeter long this order is extended 1 centimeter long. If your material is 10 centimeter long your order is extended 10 centimeter or if your material is infinitely long the order is extended infinite. And therefore, this kind of material is called mono crystalline. This kind of material is called mono crystalline fine. Mono crystalline materials are of very high quality they are very high quality because very very well order is maintained, but it requires lot of energy. We will come back to that. Look at the multi crystalline. So, multi crystalline are similar to mono there is a order, but this order is available in some page of the material and different order is available in a different page different order is available in different page. And the in between this two order there is a disorder in between the two orders there is a disorder at the boundary there is a disorder at the boundary. And these orders are not good these are the defects in semiconductors and they are not good for the solar cell operation. Nevertheless the cost of this kind of material is lower and multi crystalline silicon this grains can be very large this grain can be 1 millimeter or it can be 1 centimeter also. So, this is referred as a multi crystalline many different crystal therefore, multi crystalline. This is also what is called the micro crystalline silicon micro crystalline silicon you have small order. So, this is a there is only small area where there is some order and this small area may be you know 100 nanometer 500 nanometer only that range there is atomic arrangement in order format in a well order, but other there is no atomic arrangement there is all random. So, then this is referred as a micro crystalline or some people call it also nano crystalline because the range of the order is in range of hundreds of nanometers. Then there is this material which is amorphous absolutely no order all items are randomly arranged absolutely it is like a crowd when you go to the when you go to the mela the crowd or the people will be randomly distributed there is no order then it is referred as a amorphous material. And because there is no order there is a lot of defects and because there is a lot of defects it is not good for solar cell I will come back to why it is not good it is not good for solar cell, but still people have found a way to minimize the defect density. So, in this case the defect density is in the range of you know tens for 18 tens for 19 defects per centimeter square people have per centimeter cube sorry people have reduced this by hydrogen passivation. So, all the amorphous silicon commercial technology that you see today in the market has actually amorphous in nature. So, all the items are randomly arranged, but it is the defect density is reduced by putting hydrogen inside it. So, it is called as the passivated amorphous. These are the 5 categories of the structures material structure mono crystalline is best multicrystalline is little lower grade micro crystalline even lower grade passivated amorphous silicon is even lower grade and amorphous silicon is really worth the material. But as you go from amorphous to mono crystalline your cost of making the material increases. Also as you go from amorphous silicon towards the mono crystalline your efficiency is also increasing. So, then as I told you earlier that the cost of the module both things are important production cost is important efficiency is also important it is not the only efficiency that is the matter and that is why you see in the market that is the both amorphous silicon technology is also there and mono crystalline silicon technology is also there. So, this is a fine balance of the cost and the efficiency. So, material structure whether it is amorphous or mono crystalline or micro crystalline affects the optical properties. Optical properties are band gap and how much it absorbs it also affects the electrical properties which is the conductivity and the mobility. So, these are the parameter we should discuss now how it affects optical properties conduct and what are the optical properties what is the value of the band gap absorption how do we define absorption how do you define mobility etcetera. Let us look at the band gap of the material and one question if you can answer what is the ideal band gap that should that can be used for the solar cell uplift. Is there any ideal band gap or any band gap is ok we know that insulators have very high band gap not good metals have very low band gap or 0 band gap they are not good for solar cell. So, definitely somewhere in between there is a number which is the most ideally suited for a solar cell and we should try to answer that question what is the ideal band gap of the material that can be used for solar cell.  So, I think most of you know probably that when you look at the single atom within the single atom the electrons are orbiting there is a nucleus in the atom and then the electrons are orbiting around the nucleus and the energy of this electron can be different and depending on that the orbit of the electron will also be different. So, for example, if you have the nucleus if you have the nucleus here the electron can orbit at one level or it can orbit at other level or it can orbit other level and also there is a this quantum theory we says that in the first orbit you know there are quantum number there can be only two electrons and the next there can be eight electrons and there can be eighteen electrons and so on and this continues within this orbit there are various energy levels. So, basic idea is that in one orbit there is one fixed energy level in between energy is not possible in between energy is not possible. So, this is what is the this is what happens and this is example of a hydrogen atom for example, that the energy levels are either here or here or here or here or here or here. This are given as a negative energy because this is measured with respect to the nucleus, but the basic concept that here is that yes within an atom if you look at there are various energy levels that electron can occupy it cannot take any energy level and therefore, it is referred as a quantization of energy. When I talk about the speed of a motorcycle it can be 10 kilometer per hour 10.1 kilometer per hour it can be 10.11 kilometer per hour or it can be 12.1 kilometer per hour, but when I talk about energy of a electron it can be either one. So, either it can be this level or it can be that level it cannot be in between. So, that is referred as a quantization. So, I have try to produce it in a simple way. So, if you look at one atom there is one energy level there is another energy level there is another energy level. Then what happens if you put two atoms together when two atoms together. So, one atom has its own energy level the other atom will have its own energy level and this energy level as per the there is a principle called the exclusion principle that both the energy level at cannot be at the same level. So, there has to be some speed in energy level that occurs as per the Pauli's exclusion principle. So, when two atoms comes this happens when many atoms comes this happens. So, energy level cannot be exactly the same because no two electron as per the Pauli's exclusion principle no not more than two electron can occupy the same energy level and that law must always fulfill and therefore, the energy level split little bit. So, what you see if you go from single atom to many atoms you create many an energy level at one level that many energy level at one level and many energy level at one level. So, you are getting a hint what is happening this you are actually resulting in a formation of not energy level, but energy bands resulting in a formation of energy band. So, when you put trillions of atoms together billions and trillions of atoms together what happens is you find an energy band. So, this is for example, when you isolated energy levels or isolated atoms you have isolated energy level when you start pulling many atoms together there is a split in an energy level and you form energy bands. So, there are many energy levels at low energy or the energy level which are close to the nucleus do not take part in all this interaction with the energy level with the outer side takes part. So, typically when we represent the band diagram we are actually only showing you this. So, what you can see there is a one band here there is a energy gap there and there is another band here and this is when you show the energy band diagram this is the picture we are showing this band energy gap and this band and that is how we show here. So, we show this band energy gap and this band. If you look at the crystalline silicon or the silicon atom silicon atom has silicon atom has 14 electrons in silicon there are 14 electrons. Now, this electron are distributed. So, this orbit can have 2, 8 or 18 in order to have stable it requires minimum 8 it requires minimum. So, if I want to distribute electrons of silicon 2 electrons will be in this orbit 8 electrons will be in this orbit. So, 2 plus 8 10 I have left with the 4 electrons. So, the 4 electrons can actually go in here. So, there are 8 let us say there are in there is a sub shell there are 8 locations are available and we have only 4 electrons left. So, out of the 8 4 electrons will occupy the 4 locations and other 4 locations will be available for electron to occupy or we can say empty space those are the locations which are available. So, 4 locations are available. So, when the formation of bands occur that is what says the this band will have all the states occupy this band will have all the states empty and that is why it refers. So, if you look at the simplified energy band diagram this refers the lower band refers to the valence band and it is completely filled all the energy levels available in this bands are completely filled this is referred as a conduction band and it is completely empty. Empty means there are all energy levels are available electron can actually go and live there. So, these are the houses which are available, but this houses do not have any electron all the houses which are available here are filled by the electron. So, this is the typical scenario of silicon at 0 Kelvin this is scenario of and as I showed simplified way we can show it like this the energy band diagram can be shown like this this is the edge of the conduction band this is the edge of the valence band and the energy gap in between. What is the axis keep always note this point never forget this axis is the energy axis and this axis is a distance this is very important to understand the solar cell operation is that clear. So, there is a band gap all the states above the conduction band are empty electron actually can go and occupy the space or occupy the energy level all energy levels below this a valence band age are filled. One more property of semiconductor that this is the diagram of a semiconductor in the space of energy and the distance we can also draw the diagram in the space of energy and the momentum. So, there are two types of semiconductors that are available one is called the direct semiconductor other is called indirect or basically direct band gap semiconductor and indirect band gap semiconductor. What is the meaning of direct band gap? So, when we want to excite electron this is the highest energy in this valence band by the way this is the valence band energy level this are the conduction band energy level. So, when we want to excite electron it will actually go from this minimum energy level is this right minimum energy. So, it can gallery go here from the maximum to minimum because this is the minimum energy gap and this axis is momentum. So, what it means that in a direct band gap semiconductor no change in the momentum is required only a change in the energy is required. If you want to put a photon on a semiconductor the photon will the electron it will give its energy to electron electron will go from lower energy level to higher energy level and this can happen without change in momentum. So, this is a direct band gap semiconductor there is other kind of semiconductor which is called indirect band gap semiconductor look at here what is here the semiconductor the conduction band is actually shifted with respect to the valence band in momentum and because of that if you want to excite electron it will have to go from here to here. So, change its energy and change its momentum. So, two steps are required if you want to excite electron in this kind of semiconductor arrangement the electron will have to increase its energy and it will have to increase its momentum. So, two steps are required change in the energy change in the photon and whenever more than one people are involved in a job what happens it becomes slower or the possibility or the probability of that happening becomes smaller and therefore, absorption of the material absorption of the light or absorption of the photon in indirect band gap semiconductor become weak as compared to the absorption in direct semiconductor ok. So, high absorption probability in a direct semiconductor low absorption probability in a indirect sense semiconductor because of the high absorption probability the direct band gap semiconductor should be thinner only ok only thin amount of material is good enough and therefore, all thin films cadmium telluride, CIGS, gallium arsenide, amorphous silicon all the materials are actually direct band gap semiconductors are behaves like a direct band gap semiconductor and have very thin material requirements ok. So, cadmium telluride for example, can be only 2 to 3 micron thick amorphous silicon can be only couple of hundreds of nanometers unfortunately silicon is a indirect band gap semiconductor and because of that the absorption probabilities are low and therefore, you need to have thicker material to absorb the more light and because of the thicker material you need to use thick material which is about 180 to 200 micron. So, that is another property of semiconductor that is of importance. So, direct band gap semiconductor and indirect band gap semiconductor I told you and the examples let me write for you here. So, the direct band gap which means thin material right direct band gap is amorphous silicon, you have cadmium telluride, you have CIGS and you have gallium arsenide and these are all commercially available technology and they are all thin film technology ok and there is a indirect band gap semiconductor ok. Indirect band gap is basically a thick material you have to use an example is silicon ok. So, silicon is indirect and therefore, it has to be thicker. These are the various band gaps of the solar materials used for solar cell applications. So, look at crystalline silicon 1.12 electron volt indirect CIGS 1.1 electron volt, but it can vary depending on the composition of the material because of therefore, elements sitting there copper, indium, gallium, selenium. So, because the composition will change the band gap you can have the variation in the band gap. Cadmium telluride typically 1.42 electron volt, but band gap can be sorry different will normally will be about 1.4 not this range. Gallium arsenide you can change the band gap in a huge range it can be by mixing the compound 3, compound 4, compound can be as low as 0.5 electron volt, it can be as high as 2.5 electron volt. Amorphous silicon again you can change the band gap typically it is about 1.7, 1.8 electron volt, but you can change the band gap depending on the composition. Remember these numbers because they are useful to understand the operation of solar cells. What are the typical band gaps 1.12 silicon CIGS 1.1, CDT 1.42, gallium arsenide 1.45, amorphous silicon 1.7. This is what we have seen that if you know the energy of the photon, if you know the wavelength of the photon we can find out the energy or if you know the energy of the photon we can find out the wavelength. Why I am asking that that one question that you should do in your tutorial I have given this as a problem. Find out what is the minimum wavelength or the energy of the photon that will get absorbed. For example, gallium arsenide is having a band gap of 1.45 electron volt. What does it mean that what does it mean that if the band gap of the material is this EG this is the conduction band H, this is the valence band H and if the photon falling on the solar cell is having energy H nu normally it is given H is the constant use of frequency. So, the requirement is that H nu should be either greater than or equal to EG. So, energy of the photon should be equal to or greater than the band gap energy and that must happen. So, for example, if you are talking about gallium arsenide solar cell the photon of energy having less than 1.45 electron volt will not get absorbed will not get absorbed and will just transmit. We will discuss that more in detail when we discuss the solar cell. So, we are almost at the end just want to focus again that finally, there are many material parameters which are important the band gap is important the structural arrangement is important the carrier lifetime is important, but the most important is the price of the module in terms of the rupees per watt which depends on the production cost and the efficiency not only the efficiency. There are some other parameters which are important. For example, how much money that you are paying per kilowatt hour of energy generated. Another parameter is how much kilowatt hour you will generate if you installed 1 kilowatt of module or 1 kilowatt peak of module. What is the price of module? What is the production cost of the module? What is the efficiency? So, these are the various parameter we will discuss over this course. There is sufficient time I will discuss more of this, but the idea of this lecture was to actually discuss what are the material that can be used you know what is the material that can be used for solar cell you know which are the direct band gap which are the indirect band gap you know what are the thin film solar cell and what is the thick or the solar cell you know what are the crystalline silicon technology and what are the thin film technology. You know about the structure of the material that it can be amorphous, it can be micro crystalline, it can be polycrystalline, it can be monocrystalline and you know the various band gaps of the material that is available. In the later lecture we will actually look at the more of these properties. So, what are the optical properties of the material? What are the electrical properties of the material? How they affect the performance? We look at the manufacturing and the main and the performance of the solar cell. So, these are the various parameters we will continue to discuss when we discuss the theory of the solar cell. So, with this thank you for your attention. I hope I have summarized it and I hope this is a good beginning to discuss solar photovoltaic solar cells and the operation of it manufacturing. Spend time on this lecture whenever you get and try to absorb the basic themes about the materials about the band gap structural properties direct versus indirect thin thick film versus thin film etcetera. So, now it is time to take the questions. Government call is Salim. On what basis we have to select the PV model whether we have to monocrystalline or multi crystalline? So, this is a very good question million dollar question everybody is trying to find answer. I will not answer this question now, but we wait for some more time some more lectures and over a period of this course you will get the answer, but in general there is no specific answer. It is a very important question for industry what to do. If anybody knows the answer he can get a million dollars. C O E T Pune. I would like to ask about mechanism design. Can you include in your presentation brief idea of mechanism design for continuous sun trekking? About the continuous sun trekking. Yes actually the slides that I have shown and I have not gone through I have not discussed. Basically the basic idea of the continuous trekking is to make that theta that we have discussed equal to 0 right. So, when so the whole idea of continuous trekking is to make theta equal to 0 this can be done in two way. One is by using the equations that I have given to you in the presentation. Other way is to use the sensors. So, if we use the sensor in a such a way that it can monitor the position of the sun. So, you can actually direct your motors to do that. So, both ways are possible and people actually use both of them together in order to increase their accuracy. So, sensors will actually sense a position of the sun. For example, let me show you that for example, if you have a if you have a if for example, sun is having this position you are having a tube here which is having a sensor here right. So, now if sun ray is like this then the this sensor will actually sense the radiation, but if sun's position becomes like this then the no light is falling. So, this sensor will immediately find out that sun has actually moved and based on this you can actually design electronic circuit so that your panel is also moving. So, either you can sense the position of the sun and do the tracking or you can actually use the equations that I have given and not discussed to find out. Okay sir, thank you. Okay, Jabalpur College. Sir, my question is does the efficiency of PV module includes the life of PV module also? The efficiency of the PV module does not include the life. So, efficiency is an instantaneous value and the efficiency of a module keeps decreasing over a period of time. So, when we say the life of the module is 25 years which means that after 25 years as per the international standard the efficiency decreases by 20 percent. So, after 20 years if today let us say efficiency is 10 percent and after 25 years the efficiency will decrease by 20 percent that is how the life of a module is defined. Okay sir, you have given one question that whether that company will survive or not then for that we could able to determine only price of module. So, how we could comment on that key whether that company will survive or not? Okay, so, good that you pointed that the price of the module we should compare what are the other prices of different companies. Okay, so, the prices of the different company today is as I told you is available at you know 60 rupees per watt, 50 rupees per watt, 70 rupees per watt and if any company is producing module which is higher than that cost will not survive. So, basically you have to compare the price of the standard commercial module with respect to this company. But you have given the given only one company data. So, how we could compare that? Other you should know basically, other you should know what are the current what is currently happening in the market that you should know. Engineering college Pune, go ahead. Sir, I just wanted to ask you said that indirect band gap is has an example of silicon. Okay. Right. So, why are we using silicon even if it is having low bandwidth means low absorption capacity. Okay, good question. So, because silicon is having low absorption capacity, why do we use silicon? And the answer is that silicon has been used being used for the micro electronics industry for a very long time. Okay, now six decades, almost 60 years you are using silicon and there is a lot of knowledge available of silicon, there is a lot of manufacturing capacities available of silicon. Silicon is by the way the single largest material produced in the highest purity on the earth and silicon is also available in the large quantity. Silicon also gives stable efficiency solar cell. So, despite it is being a indirect band gap semiconductor, there are so many other advantages that almost 80 percent of the solar modules in the world are produced using silicon. Can we reduce this means deficiency by doping it with some other thing? No doping does not affect the band gap, it does not affect the indirect nature of the silicon. By the way the silicon when it goes to the amorphous silicon form when the then it actually starts behaving like a direct band gap. So, by doping we cannot do that, but by changing the structural form we can do that, but then it actually reduces the quality of the material etc. But the knowledge about the crystalline silicon is so good and people are actually trying to make it thin and thin. So, earlier crystalline silicon cells were available at 400 micron thick, 500 micron thick and now today commercially it is available at 180 micron thick and people are going to try to as thin as 150 micron. So, there is a lot of work going on to reduce the thickness of silicon. Thank you all. Let me stop here. Thank you.


Hey guys, how are you doing? This is Rich from RichTVLive here at Walmart. We're at Walmart in Mexico. You actually have to wear a mask here. We're going to wear a mask on and we're going to go to Walmart in Mexico. It's not easy! Me a poco and no. Thank you!


Hi, I'm Ricky Anderson and welcome to the Fort report today. We're going to talk about hoops So I brought out my old hula hoop. I used to be pretty good at this not to brag But in fact, I was a member of the 2016 Olympic hula hoop team in Rio I remember this one story from Rio where I tripped on a curb it ended up chasing a rolling hoop for a couple of blocks What? Loop what what's a loop? Great, then I'm completely off topic I guess let's just watch the report and find out what this loop is Loop is a cross between a music box a zoetrope and a railway hand car People sit inside the giant wheel and pump the lever together to activate the beautiful images inspired by 12 different childhood fairy tales This free one-of-a-kind interactive art exhibit will be at the Burnett Park from September 28th through October 28th Here's a look at other events happening in and around Fort Worth in the next few weeks For a listing of more events in city meetings, you can always check the calendar at the city's website


All right, welcome everyone to today's Webmaster Central Office Hours Hangouts. My name is Joe Mueller. I'm a Webmaster Trends Analyst here at Google in Switzerland. And part of what we do are these Office Hours Hangouts, where webmasters and publishers can join in and ask about their websites in search. Since it's a bit of a holiday, I guess maybe there won't be that many people, but we'll see how it fills up.  But before we get started with them, maybe one of you has something on their mind that they really want to kind of talk about in the beginning. If not, that's OK too. Nick, you're also welcome, of course, to jump in in between. Hello, John. Oh, hi. Hi. Do you take question about Google My Business? I can't really help much with Google My Business. OK. Is it specific to Google My Business, or does it relate to Shown and so on? It was about the maps. For example, we got an issue with people creating fake, positioning fake, listing around main establishment. And they are doing it multiple times. We are trying to remove them by doing the reports. And every time, after a little bit of time, we see something appearing with false images, false address. Do you know if there is a solution for this too? I don't know. So I would recommend, on the one hand, using the tools that you have available there. And on the other hand, maybe checking in with the help forum for Google My Business to see if the folks there have some more information. OK, thank you. All right. Any other questions around web search that maybe I can help with? Otherwise, I'll just get started with what was submitted. And like I mentioned, feel free to jump in if there's something that you want more information on, or if you have related questions, any of that. All right, fairly long question. Let me see if I can simplify it a little bit. In Search Console, it's written that the unexpected 404 errors might be generated by Googlebot trying to follow links in JavaScript, flash files, or other embedded content. For example, some Google Analytics link. In Search Console Help, it's also written that this error has no effect on crawling and ranking of your site. Is this always true? I'm asking because we have on our site many cases like this, specifically related to image files, checking our logs related to Googlebot image. We see that last October, Googlebot image requested more than 3,000 resources. And we know for sure that these requests are related to 404 URLs generated by Googlebot trying to follow links in embedded content. Could this be a problem? Could it be affecting Googlebot image's ability to crawl our actual images? So I guess the one thing that you don't have mentioned in the question here is, are your images otherwise being indexed normally?  What can sometimes happen is that sites embed their own images with a wrong URL. And then when users look at those pages, those images don't work. And then when we try to crawl and index those images, we can't get to those images either. So that would be the kind of situation where I'd say that's something that you'd need to fix. However, if it's just the case that in addition to all of the images that we have crawled and indexed on your website, we try out a number of URLs that return 404, then that's absolutely not a problem. For the most part, especially when we're crawling images, a 404 error is something that we can process really quickly. It doesn't distract us from processing the images that are really on your website. So in that regard, I would not worry about this. But I would really double check if you're already looking at your server logs, double check that we're actually crawling the URLs that you do care about. And if we're not crawling the URLs that you do care about, then double check why Googlebot might be missing out on those. We're looking to incrementally modernize our WordPress website with some modern web frameworks like React. Since the site is PHP WordPress, we currently don't have the infrastructure to server-side render JavaScript and React code. So we're thinking about possibly foregoing server-side rendering in favor of just server rendering of structured data like JSON-LD for the short term. Our long-term goal is to move away from WordPress. So this would mean that a site's HTML would only be client-side rendered, but there would still be some server-side rendered structured data. Is this feasible? Would this cause significant SEO hit? How does Google consider structured data in search results and in rankings? The alternative would be dynamic rendering. However, we're wondering if we can get away with not managing puppeteer clusters. So there's a lot packed into this question. And I feel if I just say yes or no, it's probably not going to be a great answer for you. In practice, these kind of changes can have a significant effect on your SEO. In particular, what you're essentially doing is changing the layout of your website and moving from something that is pure HTML to something that has a different layout and that uses a different technology, namely client-side rendering JavaScript, which means along that path, there are a lot of small changes that add up, and they can have a significant effect on how Google indexes your pages. On the one hand, layout changes can have an effect, especially if you're making changes that would affect the internal navigation of a website or the URL structure of the website. That's something that can have a significant effect. That's even without looking at the trickier problem of moving to a JavaScript framework. So that's something that can definitely have an effect. The JavaScript framework part is something where I would be kind of cautious taking that on in the sense that it can definitely work. It's not something that by design would not work, but it's also something where you need to make sure that you're implementing it in a way that does actually work for search. So what I would recommend doing there is setting up a bunch of test pages, either on your website or on kind of a testing website, and using the various tools and the documentations that we have available, really double check to make sure that everything on those pages is actually indexable. And then that's something where you could step by step try to take that and see how does this react within my normal website. So in particular, what I usually recommend doing is taking some pages where you see enough traffic from search that there are things that you can measure there, and then converting those to your new setup, maybe kind of splitting that up depending on how you have your server setup, and really double checking to see what is kind of the mid-turn effect from changing those pages over to the new infrastructure that you have, the new frameworks that you're using, the new layouts that you're using, and really double checking that those changes are OK for you. So I wouldn't just see is it indexable or not, but I would kind of leave it running for maybe a month or so to see where does it actually settle down. Because especially when you make bigger changes, you will see some fluctuations for kind of the short term. And the question here I think is more in the long term, will these fluctuations kind of settle down at a state that are similar to what we have before? And that's something that you can really only test. So from that point of view, I wouldn't take this as something like, oh, I will just swap out the whole back end and add some JSON-LD structured data to these pages. And we'll see what happens. But I'd really try to take this on in a controlled way so that you know along the way where things go wrong, where things are actually working out well. And as always, with any kind of site redesign, it's not always the case that the changes that you see will be negative. It's very possible that you do a revamp of your website. And that results in Google understanding your content a lot better in Google being able to crawl and index your pages a lot better. And then kind of the ultimate effect, being more visible in search. So changes alone are not something where I would say that would be a problem. It's really a matter of like, where do these changes go? OK. And then I have my website is getting thousands of links from a porn site through some domain where it's kind of redirecting through an error page, apparently. What is happening? And what's the best solution for this problem? Is my website being penalized due to this? So these kind of links from spammy sites are really common. I wouldn't necessarily worry about that. One thing I did notice looking at your website is that it's built on an expired domain. So there used to be completely different content there. And that, I think, is probably the bigger effect that you'd be seeing. Not so much kind of the links from adult content sites. But really, it's like, well, your website is not what it used to be. There's like a really big change there with regards to the way that the content used to be and what you have there now. So on the one hand, I wouldn't expect that your current website ranks the way that the old domain used to rank. I would expect that things are slightly different. And on the other hand, if you're building on an expired domain, then obviously all of the things that this domain has collected over the years, they're still around there, too. So it's not that you can just say, well, there are some random adult content links linking to my website, and I don't know what the effect is. But rather, you're building on a very shaky foundation already, and whether or not those particular links are the problem or like the whole shaky foundation, that's kind of the question. For new websites that have been internally scored well enough to be given a chance for head keywords, I've seen Google continuously test the pages by giving them small bursts of traffic a few days or weeks apart. Is this part of the testing signals? It's collected over time on those pages, sort of like Google saying, you're new, and you're good, and I want to trust you, but I want to take my time. I'm not aware of any particular system on our side that would be saying, well, we need to kind of double check how these pages work, and we will send them some sample traffic from time to time. But in general, with new websites, it is something where we don't have a lot of signals. So if we don't have a lot of signals, we have to guess sometimes. And our algorithms might end up trying things out. Our algorithms might end up kind of guessing to see what the stable state could be. And then over time, over a couple of months, you'll see that things settle down into something more stable. And this is something that can go both ways in the sense that maybe our algorithms will guess in the beginning that this is actually something that is really, really important, and we need to show it a lot more visibly. And later on, we realize maybe it's not that critical. And similarly, it can happen that we look at it and say, well, we're not really sure how to show this in search. And over time, we learn that this is actually something that is really good and worth showing a little bit more visibly. So it can go up. It can go down. But especially in the beginning, it's something where it's a little bit of an unclear situation for us. Now that Google can index JavaScript, we're thinking of using more JavaScript to increase our website's performance. At the moment, all of our content is in the initial HTML. IT says, loading the content using JavaScript would decrease page loading time. If we made this and also implemented dynamic rendering so that our content would be indexed faster, what could happen? Is it a terrible idea? Or should it be fine? Is it better to leave it as is? This is kind of, I guess, similar to the previous question where someone wanted to swap out WordPress against some React setup. In general, this is something that you can do in a way that works well. It's also possible to do it in a way that causes trouble. So like with the previous question, I would be cautious in kind of going down this direction. But I wouldn't say that it's impossible to do that. There are lots of speed improvements that you can do with static HTML. Sometimes there are really cool improvements that you can do with JavaScript as well. So I would try these things out, test them out, test them out with your actual pages. Like if you can do something like an A-B test where you have some set of pages where you know you're getting good traffic from search enough that you can measure it, swapping those out, and seeing what the reaction is over maybe a course of a month or so. Some questions regarding AMP. My website is in WordPress. Oh my gosh, so many WordPress sites. And I've enabled AMP for it. Please give me a hint about native AMP and how Google sees it as there will be no canonical URL. Also, I want to ask how Google ranks AMP pages. Is there any specific criteria for AMP pages or is it the same as a normal web page? So I don't know which AMP plugin you used. I know Google has been working on one, which it seems to be working out really well. If you're using native AMP, I assume you don't have the legacy HTML pages anymore, but rather you switch completely to AMP. In general, that's kind of the approach that I recommend because then you just have one URL and you don't have this problem of having an AMP URL and a normal kind of HTML URL. If you can switch completely to AMP, then I think that makes a lot of sense. Obviously, not all sites are really suitable to switching completely to AMP. So that's something you have to figure out. If it's mostly a content site and it's hosted on WordPress and it's more a matter of using a plugin to just turn it on and off, then that seems like a pretty easy approach. We rank AMP pages the same as we rank any other pages. So there is no inherent advantage to using AMP. There is no ranking boost from AMP. Some search features do require AMP pages so that we can show them properly because of the pre-caching and all of that. But otherwise, there's no ranking advantage from using AMP. Obviously, AMP pages can be a lot faster than the legacy HTML pages. You can make really fast HTML pages, too. So it's not that you need to use AMP to have a fast page, but they can be quite fast. And speed for us is a ranking factor, especially on mobile. So all of these things kind of add up. But ultimately, I'd say if you think that it works well for your website to have AMP pages and you can go to a pure AMP setup, then I don't see any problems with that. I just wouldn't expect any magical boost to the top of the search results from just switching to AMP. Hey, John, you want to take a question on APIs? Sure. So in Discover, sometimes our website shows AMP pages, and sometimes mobile pages show up. The template is the same, the speed is the same, the story are different. But sometimes mobile pages showing up, and sometimes AMP pages are showing up.  So the reason for the images part is because it has a direct impact on the CTRs. So do you see any logic there, like why AMP site not AMP, while the speed is pretty much same in both the templates? I don't know. My hunch would be that it's more a matter of us having discovered and recognized the AMP version of those pages. So if we do know there is an AMP version of the page and it's properly connected with the canonical back and forth, then that's something where we would try to show the AMP version as much as possible on mobile devices where that would be compatible. So it's not something, at least as far as I know, it's not that there is any kind of heuristic that would say, oh, in special cases, only then show the AMP version, otherwise show the kind of traditional mobile version. So it feels to me it's more a matter of timing and maybe proper understanding of those two pages that we can actually understand. This is the HTML version, this is the AMP version. Or do you consider the first crawled page type as one of the source to rank in APS instantiate?  Because when we looked at this page type, there's no error. Everything is perfect. And fortunately, what happens is the first page AMP site and the second link is, again, AMP page. So there is no correlation that we are not at all coming there as an AMP, but the same category, same folder, one in AMP site, one in AMP site. So you have basically desktop pages, AMP site version, and the AMP version? Or do you just have kind of responsive and AMP? No, we have three different versions. OK. Yeah. So what I would make sure there is that from the mobile version, you also have a link to the AMP HTML version so that if we get a copy. Yeah? OK. I guess the thing that I'm not completely sure about there as well is from the AMP version, would you have a link back to the M.version or to the desktop version? We have both. Canonical. We have desktop. We have both. I can only tell you some questions. Yeah. I could see maybe that might be something that's confusing us. But I'd need to double check with the team on that. Yeah. I can push to that URL. And second question is like, for the videos, the image always goes in the smaller icon than the bigger icon. Yeah. And that has a direct impact in graphics. That's something. So for AMP pages, we can show the bigger preview image in Discover. And what you can also do is use the meta tag for what is it, max image size, I think, where you can specify large to let us know that for this particular page, we can also use a large preview image. So that's something that would also need to be on, I guess, on the mobile and on the desktop page. So for videos, what we have noticed is all the websites are showing only mobile pages. Earlier, it used to be AMP pages. But now, not only us, but competitors also, all websites, even YouTube, any video that shows up in Discover are mobile only. Never it is AMP since last couple of months. OK. That's weird. If you can send me some examples, I'm happy to take a look at that with the team. Sure. Sure, we will do that. Also, one last question on APIs is that, I mean, they're multiple. But what we're generally seeing in India is that the cache rate or the refresh rate of APIs is pretty bad, right? So in the morning, we see yesterday articles, which I thought which is OK. But in the afternoon, I'll see like, Sanna Sego article, which is not based on the real time. So do you see any cache issue that is happening? Or is that something we have to do with that? I'm not aware of anything specific. But I haven't heard from other people either about this. But I'm happy to take a look at that, yeah. OK. Hey, John, I have a question. Sure. Yeah, hi. So being a news publisher, we can see most matters the most, right? So being a news publisher, we publish all the news in real time. But in Google News Carousel or AMP Carousel, generally I see three weeks ago article, two weeks ago article, 14 hours ago, 20 hours ago. And there are some news publishers, as you know, there are domain crowding where only two or three publishers are coming. But everybody is contributing to Google. So I don't understand what is the reason that the old articles are there and only two, three publishers are ranking for every query. It's something I can't really say in general, because it really depends on the query and the timing there. But it's definitely not the case that we would say we try to show older content in kind of the past story. I completely agree with the timing on the point. But I just want to understand why we are showing like in the morning, there was some big news in the parliament, in the Indian parliament. And if I search for Google, I can see articles two weeks ago, one week ago. And it's like four hours ago, when the news breaks. I don't know, that seems suboptimal. Yeah. I think John, to add to that, in India, basically, this is the biggest problem we always see is that the timestamps don't generally get updated, especially when you have an event, right? It goes really for a toss. This is something, it's a common feedback. I think we already shared once. But I think this is pretty much what we are living today in the search. OK, so the timestamps in particular are not, don't match the article or? Yeah, I mean, while the article updates, Google still shows that six hours ago is six hours ago. But interestingly, sometimes what happens is that the article title that is still showing in Google is like the latest one. But the timestamps still shows six hours ago. OK. Interesting. OK. If you all can send me some examples, I'm happy to take a look at that. Sure, we can share some examples with you. Cool. All right, so a completely different question. My competitor is sending fake traffic to my website using a bot in huge amount. How does Google see it? Will I get penalized for that? What do I do? With regards to SEO, this has absolutely no effect at all. So you can ignore it. You can block it, whatever you want. If I use the same website template to create multiple websites, will Google consider it plagiarism and does it affect SEO? So using the same template itself is not so much of a problem. But using the same content on multiple websites essentially means that you kind of dilute the value of your content. So it makes it a lot harder for us to recognize this is actually something really good and important if you're saying, well, it's across all of these different websites, it's all the same content. Then when it comes to trying to rank things, we might say, well, something else looks really important. And this is kind of like each of these individual pieces has a little bit of value, but not enough to actually be shown like competitively in the search results. So that's something where just using the same template is no problem. Using the same content across multiple sites, that's generally just kind of a bad practice. Does AMP increase direct traffic? So direct traffic is when people come to your site directly by entering a URL or kind of like using the bookmarks. And that's not something that AMP really directly affects. I mean, indirectly, maybe, in the sense that if you have a really great and fast website, that could be something where people might say, oh, I like this. I'll continue coming back. But directly, there's definitely no effect on direct traffic for AMP pages. I have a site that supports multiple languages. Sometimes we have content in four or five languages, but you can still see a page in untranslated content. We're being flagged as having an issue with reciprocal hreflang tags. So I think the general issue is you have multiple language versions of your website, and you don't have all of the content in all of those language versions. So in general, that's kind of OK in the sense that you can have hreflang for those pages anyway, because probably the template of the website is translated. So for example, if you don't have a piece of content in Korean in this case, but the template of the website is in Korean, then that's still a useful page to point Korean users to. So the hreflang there would still be OK. At the same time, you could also say, well, I don't have this page in Korean. I don't even want to send Korean users to the English version of the article on a Korean templated website, then that might be something where you say, well, I just won't use the hreflang on that Korean page. So from my point of view, you can do it either way. Either include that page in the hreflang set or don't include that page in the hreflang set. The other thing to keep in mind here is that hreflang is handled on a per page basis. So if we can recognize that, like say, three or four of the five hreflang links that you have on a page are OK, then we will just use those. And if we can recognize that that one is not OK, then we will just skip that one. It's not that there is a negative effect on the hreflang set for that piece of content. It's also not that there is a negative effect on the website itself if one of the hreflang links does not work. So from that point of view, it's not going to be critical if you know that this page is kind of iffy with regards to translation and the hreflang set up for that particular page is not OK. My main service page is not ranking, even in the first 100 pages of Google, where all other pages are performing well. I doubt this is due to duplicate URLs as I couldn't find any old URL in the different file format and different protocols. This might have happened when the website was converted to WordPress and the page is redesigned long ago. I've redirected, 301, redirected all these URLs to the present service page. How will I inform Google to recheck the URL for duplication or if the issue is solved? So I don't completely follow what has happened here and what is happening with your pages at the moment. But you can use the Inspect URL tool to let us kind of reindex a specific page. So if you're worried that maybe something shifted along the way and maybe the content isn't really what or the content has been updated and you want to make sure that Google has kind of taken a new version into account, then using the Inspect URL tool and submitting that to indexing is definitely something that you can do. I wouldn't be shy about using something like that. That's kind of what it's meant for. With regards to changing website structures, like I mentioned with the other cases, you can make website structure changes. And sometimes these just take a while to be reprocessed. So depending on what you mean with these pages were redirected long ago, that might be something where maybe it's still being processed. If it was, I don't know, a couple of weeks ago or maybe they're already processed and already kind of in that stable new state. If you made this change, say several months or maybe even years ago. What's the best course of action? If you have a website, pages drop completely out of search index and you've received no manual action notifications. Hard to say. So I think, first of all, it's worth saying that we don't index all pages. That can be completely normal. Sometimes with the new features in Search Console where you have the coverage report, it can be tempting to focus on some of the issues there and say, I need to fix this problem. I need to get all of these URLs indexed. When in practice, it's just, well, we've indexed a lot of pages from your website. And at the moment, we don't feel that it's worthwhile to index even more. And that's not something where there's a technical reason involved. It's more that, well, we're not sure if it's really worthwhile to spend so much time on these things. So that could be, for example, if you have a ton of different URLs on your website and they're all kind of similar. If you have, I don't know, a directory of all businesses in your country and their phone numbers and we've crawled through one third of those and we're like, well, these are all things that we've seen before. Do we actually need to index all of these or not? So that's kind of one aspect there. The other aspect could also be that maybe there is a technical issue involved, that it's actually worthwhile to kind of dig into those individual pages to see what exactly has been happening there. So that's kind of the general split that I would try to focus on is first, try to get kind of some rough understanding of is this perhaps just the case that Google is indexing as much as it feels is necessary for my website and maybe some of these other things would be worthwhile to kind of rework in a way to really prove to Google and to users that this is really important content. That's something where maybe it's worthwhile to have a second pair of eyes look at these issues with you and really give you some more honest feedback if possible. And on the other hand, if you feel that these are actually really good and important pages for your website and for some reason they're just dropping out, then that's something where I would try to dig in for technical reasons and try to figure out why these particular pages are not being indexed is maybe another version of that content being indexed is maybe there's something weird happening with my server that I could look into all of those more technical questions. Are attachment pages bad for SEO? Will Google penalize a website if its attachment pages are indexable? So assuming these attachment pages are for attachments that you have maybe on a blog and these are reasonable things on a reasonable website otherwise, then there's nothing against having the index. So there's definitely not a penalty on our side where we would say attachment pages are indexable. Therefore, we will demote the website. That's definitely not the case. The general question I'd say is more are these attachment pages valuable for your website? Is there something of value that you're providing on these pages that when users go there, they say, oh, this is really fantastic website.  I will recommend this to my friends. Or are these attachment pages essentially just kind of placeholders where when people go there, they're like, how did I end up here? And what do I need to do next? And why did Google send me here? Then that's the kind of pages where I'd say probably not worth getting those indexed. But that's not an issue of attachment pages or not, but rather what content do you want to have indexed. And that's something where you can decide this is a good use of my time or a good use of my visibility in search or maybe this is something where I'd rather have people actually go to different pages within my website. John, if you have one minute to explain about why YouTube brands much higher is the general question you would have faced many times, but do you think you can spend some time to explain that what factors are, why the search is so much part with YouTube for an intent of a query which is about a video, right? And it has a mix. I'm not saying it's only YouTube, but historically the way we are seeing is that YouTube is actually taking up the entire listing because I'm sure it's a Google product, but do you want to throw any light around that to give us some indication that on videos, why is YouTube so important than a original publisher who actually has the video? I don't actually know how much we're showing YouTube, so that's kind of tricky. But in general, we spend a lot of time to make sure that we don't give any of our own services any preferential treatment in search in that regard. So YouTube pages or video pages like any others, they obviously work to try to make these pages work well. I think in particular, YouTube almost has a little bit more challenging than other video pages in the sense that most users on YouTube probably go to the app. So that's where they put most of the content. And kind of the mobile version of their pages is not really that well suited for search. And with mobile-first indexing, we try to index the mobile version of those pages. So that's something where they struggle with SEO just as much as any other website. So from that point of view, I don't think there is any inherent bias to say, we should show more YouTube content in search than anything else. But there is a lot of content there. And they have a lot of good content. So from that point of view, I think it makes sense to show some things from there. But obviously, there are lots of other video sites too. And we need to make sure that we can show their content as well. Over the years, I've been in touch with a number of these video sites. And sometimes they do things really well. And sometimes they also struggle with SEO, like anyone else. And especially for really large websites where you have a lot of content that is in video form, where it's hard for us to recognize what is actually in the videos if we don't have any text around that, then that can be sometimes really tricky to crawl and index really large sites like that. But that's something where everyone struggles with SEO. And especially bigger sites, they also have the inherent organizational challenges that come together with any bigger business in that you want to make some changes on these pages because there's something technically wrong, then it just takes a really long time until you have all of those permissions. So I don't know. From that point of view, I don't particularly believe that we have any inherent YouTube bias, that we show more content from YouTube in search. But they do a lot of things well. There's a lot of good content there. Other sites do well too. And we really need to make sure that we're showing kind of a balance mix of sites. I think we'll share some samples where the publisher, first publishers, and then goes to YouTube and to take the advantage of YouTube as a YouTube.com maybe bigger in terms of ranking the domain. Maybe it is taking up the first slot and then the publisher comes. There would be reasons, but we'll share the samples with you to dig in separately on that. OK, I'm happy to take a look. I think the other aspect is also tricky if you have a video that you host on YouTube and you also put it on your own pages, then that's something where our algorithms try to figure out which is the best landing page to show there. That's always kind of a tricky situation. But happy to take a look at some examples. And second question, follow up to the video, says that when you search for a query, generally the tradition of search results starts with like news and then videos. But these days we are doing a lot of field wherein even if you search for, let's say, Indian news as a query, then the videos are coming first and then the AMP news stories coming at the somewhere at the bottom of the page. Do you see any foresee changes that are going to come in search? Is it something like a test, or do you really see that it's going to be the case now? So kind of forward looking, I would always expect to see changes in search. I don't think that really helps you at the moment, but I know the team is always working on trying to find the right balance there. And that's something where generally what happens is we have a number of algorithms that try to determine which of these kind of search results blocks to show above the others. And essentially they're trying to determine what is the most relevant for users for those specific queries. And sometimes it can happen that videos are shown on top, sometimes images, sometimes a news block. That's something where I think we generally get the mix fairly well, but sometimes maybe we don't. And in particular, I could imagine for non-English content, it might be even trickier for us to understand what specifically is being searched for with this query. Are they looking for more kind of up-to-date news? Or are they looking for background information? Are they looking for images or videos on this topic? Sometimes there are also videos that are indexed that are very kind of news friendly in the sense that they're really fresh. And it's also worth showing kind of together with news content. But this kind of mix is something that I know the teams work really hard on. And it's really tricky to get right, because I don't think there is one answer that always works. Yeah, I agree with that. But I think are you guys seriously taking the publisher's feedback as well? Like what is the impact on that at the end? That is the content that is going to surface on the search results, right? So do you see that it should be open forum to the publisher also, because the reason this question comes because we're lost in the traffic, right? So do you see that as a feedback? There's a channel to that. It's good to do an experiment. We're happy to see the experiments, but there should be a give and take. Yeah, I mean, we do take that kind of feedback seriously. But it's also important from our side that these search results work well for users. So kind of having feedback from publishers saying, I want my content to be shown on top rather than someone else's content. That's something where, OK, we can kind of have a reasonable discussion about that. But ultimately, what's important for us is that users get the search results that are relevant to them at the time when it is relevant for them. And sometimes publishers see these kind of issues a lot clearer than maybe users would be able to formulate them. So that's always useful to have. But sometimes it's also kind of just that conflict between publishers would like to have more visibility in search. And we think that maybe there are other things that are also important to be shown in search. Yeah, I think it's not something like we have one publisher wants to rank better. But because of this click, YouTube is something which is getting benefited, right? So it's always like the YouTube videos ranks first, and then the YouTube always takes the slot completely. So that is the kind of question that's why I asked you to spend some time to talk about the YouTube. Because I mean, Google is benefiting, but not the publisher, but the YouTube is taking the benefit out of it. So that's where I think the feedback is a question, not to rank between the publishers. But the news versus videos and videos takes advantage. If that is the case, then YouTube is taking 90% share. Yeah, I don't know. I could also imagine situations where maybe some fairly fresh videos are really kind of comparable to news content. So it's, I don't know the specific queries that you're looking at. Sure. Yeah. All right. And a question relating to kind of one of the previous office hours. So some websites are copying their content, and they're using a piece of JavaScript that kind of adds a link back to the website with a hash after a fragment identifier. And the general question is, is this a problem, or is this not a problem? From our point of view, that's perfectly fine. If you have JavaScript that includes kind of a link when people copy and paste content from your site, then that's something that kind of makes sense. The fragment identifier at the end of the link is something that we would drop, because we try to use the normal kind of URL instead. So with the path and the file name and the fragment identifier, we would drop. So that's something where we would see that as a normal link to that normal URL. Would Google consider these to be natural links? I think there are two aspects here. On the one hand, if these are scraper sites that are copying and pasting content from your website, then probably those links are not something that we would value that much anyway. So it's not something that would be high on our radar. On the other hand, if people are putting content on their website and linking back to you as a source, then that seems like a normal natural link to me. So I wouldn't necessarily worry about this. But kind of how I mentioned there, on the one hand, these are probably not going to be that useful links for your website anyway. So on the one hand, I wouldn't worry about this as being a kind of natural or unnatural link, because they're not valued that much. But also, I wouldn't expect to see any positive effect from these kind of links, because probably those sites are not going to be things that we, say, are kind of useful references for a website. Sure, hi. Yes, hi. So we are a new publisher, and we have a business news website, which happens to follow all the Google guidelines, the news guidelines, and everything. The sitemaps are working perfectly fine. The feeds are working perfectly fine. We are getting indexed. We are getting indexed in Google News as well. But somehow, we are not appearing in the top box. So we have raised this query multiple times on Google support and to the Google team. But we have never got a satisfactory response. We have never received any message in our search console about what is happening and what is the issue. So can you please enlighten me something about it, what is happening here, and what is the exact issue? So the, what is it called, the kind of the box on top there, that's something that is, from our point of view, is an organic search feature. And it's not something where news publishers automatically show up. It's not something that's limited to news publishers. Yeah, that is a thing. But the thing is that we used to appear in the top box. But it's been a one and a half year that somehow we got disappeared from there. Without getting any notice in the search console or something, we never, never, ever appear for a single query in the top search. We get appeared in the search results, but never in the top box. So we don't have anything, as far as I know, we don't have any kind of manual action that would block a site from appearing in the box. I forgot what the official name is. But from that point of view, it's not that there is kind of a setting for your site or anything like that where we would say, this site should never show up. But rather, this is some top stories. Yeah, top stories. That's what I meant. So from that point of view, it's not that there is any setting or anything like that, which will be blocking it. But rather, we see this as an organic search feature. And over time, our algorithms evolve. And it can happen that a site is visible in Google News and does not appear in the top stories section. So from that point of view, it's not that I'd say this is particularly kind of a bug on Google's side or something that you're doing incorrectly. But rather, we try to show the appropriate results in the top stories section. And sometimes that includes some sites. Sometimes that doesn't include other sites. But it's not the case that we kind of like block sites from appearing there. So that's from that point of view, it's kind of tricky to say. I understand your point. But we have other websites as well, other English websites in the website and regional websites. They do appear for search queries in the top box. But for a business website, there's not even a single query. Not even a single query that appears in the top box. So I can understand that it happens sometimes. One website shows and the other one doesn't show. But it cannot happen in every day's business that not even a single news is getting picked in the top story. Right? It can happen. But you know what? I don't know your website. So it's really hard to say if that's something that I would expect for your website. But it can certainly happen that a website is not a one month or one week old thing. It's been quite two years. And despite of writing multiple queries on this webmaster search forum and the Google people and asking them in these seminars, I have not got any response that could actually relate to the issue or what I can do on my part to solve this problem. Because I have seen everything that everything is getting followed, all the guidelines, all the schemas, speeds, everything is working fine. Still, it does not appear. That can happen. I mean, on the one hand, the technical things are things that are always useful to follow. But it's not that there is a technical requirement where we say, you need to do this. And then we will show you there. But rather, there are lots of also quality issues that are involved there with regards to trying to figure out what type of website to show there. And it can certainly happen that a website exists for a longer period of time. And it is even indexed in Google News. But we still think, well, we're not convinced that it makes sense to show this website in the top search, top story section. So that's something where I wouldn't say, offhand, it's impossible that any new website is able to show up in there. Because new websites show there all the time. But it's also not the case that it's a sign that there's something specifically wrong with a website when we don't show it in the top story section. Because it's an organic search feature. And the criteria to be shown there from an algorithmic point of view, we have a lot of things that play into that. And it's not guaranteed that we would always show a site there or that we would ever show a site there. Does BERT affect the ranking of websites which have content in other languages than English? So I struggle a lot with all of these BERT questions when they come up. Because from our point of view, BERT is a way of understanding text better. And we use that, on the one hand, to understand queries better. So to better understand when someone writes something complicated into the search box. And on the other hand, to understand pages a little bit better. So from that point of view, it's something, even in English, when we use things like BERT to understand the pages and the queries a little bit better, it's not that you'd have to do anything particularly different with regards to your pages. But rather, we're just trying to understand your pages better. We're trying to understand which queries better fit to your pages. Especially things where the queries, for example, might be kind of complicated, where people are asking things that we can't map one to one on a keyword basis, where we have to understand what it is that they actually mean. So from that point of view, it's something where, when we do use BERT, it's not that you need to change anything on your website. When we don't use BERT, it's also not something that you need to do anything different on your website. So when we kind of try to expand our understanding of queries in English, often that's kind of one of the places where we start, because we have a lot of people that are active in English. And also, there are lots of research that's being done in English. So maybe we'd start there. But we do try to apply this technology to other languages. And for other languages, for other language content, it essentially means that we have to do a lot of research to try to make sure that we're understanding things correctly. We have to do a lot of testing to make sure that we're actually improving things overall and that we're not causing more confusion in the search results. That's something that, in English, we did spend a lot of time to really test these algorithms to try to fine tune them. So that's something where I could imagine it just takes a lot longer for us to get that done in a way that is easily transferable to other languages. And then even there, we'd still have to test that significantly. So my guess is, over time, we will learn to understand a lot more content, a lot better, understand queries a lot better. But there is nothing specific that you need to do on your pages to make them compatible with BERT or to make them work better with BERT. But rather, if you're creating good content, if you're creating content in a way that is easily understandable to users, then that's what we're looking for. We're not looking for new meta tags to try to find things a little bit better, but rather, we're trying to understand your content a lot better. Let's see. We're kind of out of time. There's still a few questions left, but it feels like these are kind of tricky ones that need a lot of time to kind of go through. Maybe I'll just switch over to more questions from you all if there's anything specifically on your mind that you'd like to talk about. I've got a question, John, if you have the time. So last time we met, I mentioned about a website where certain commercial pages were doing really bad, average position 70 to 80. So worse than it would be just competition there. Looks like there might be something else going on. You mentioned let's just try switching the URL, see what happens, see if it kind of wiggles it around. So we did that, but the issue is the old URL is now 404, and we did a new URL, which is the exact same content for that page. It's just a different URL. But when I go to Search Console and do a URL inspection on the old URL, it doesn't show it as a 404. It shows it as Google selected canonical to the new URL. So I think that kind of defeats the whole purpose of switching it and 404ing the old one, because Google doesn't consider it 404, it just migrates all the ranking signals and possible issues to the new URL as well. So it doesn't really validate the experiment I guess. I totally don't remember what the context was. So it's hard for me to say. So you're not seeing any positive effects in short? No, but one of the reasons that I think I'm not is because Google doesn't see the old URL as a 404, and the new URL is a brand URL. It just sees the old URL as a canonical to the new URL. And I don't know how to make it kind of see that one is a 404. Using the test live URL, it does show a 404. But doing the normal URL inspection and just show that Google selected canonical is now the new URL instead of a 404. No. So I don't know how to push it to kind of re-crawl it and take the 404 into account. I could imagine our systems are doing that on purpose to help people who aren't doing redirects properly. So maybe that's a case where we're being more helpful for you than you want us to be. Yeah, quite. Because if we can tell that the content is the same and it disappeared one URL and it reappeared somewhere else, then I could imagine our system saying, well, you're kind of shifting the canonical from one to the other. So that's something. I guess the approach there could be to make sure that the content is significantly different. But then your experiment is getting more and more complicated. Yeah, so it's kind of a product page. So can't really change much of the content. There is just a description of some of the features of the product. Yeah. I don't know. Yeah, tricky one, I guess. Tricky, yeah. And the new URL seems to have inherited the keyword positions from the old URL, which is, again, 70 to 80 average position for any keyword, which is very dubious. OK, I mean, maybe that's just the normal ranking that we would see there. That might also be. I mean, nobody wants to hear it. Sure, yeah. Have you started? Sorry. OK, let's take a break here. I'll pause the recording. I still have a bit of time. So if any of you want to stick around and just chat, that's fine as well. All right, so thank you all for joining in. Thanks for watching the recording, I guess. Thanks for submitting so many questions. I'll set up the next batch of Office Hour Hangouts, probably also in two weeks, where we also have the Webmaster Conference in two weeks. So not quite sure how the timing will work out, but we'll try to figure something out. We also have Webmaster Conferences in Japan and Korea happening the next couple of weeks. Next week there's one in Tel Aviv, and then two weeks in Zurich. So lots of things happening here. So yeah, hopefully we can catch up there. All right, so with that, let me pause here. Thanks again, everyone, and see you in one of the future Hangouts.


We have to show what she is capable of doing to the rest of the world. I'll help my sister. Every single action of the Indian government is designed to strengthen five or six big business people and weaken India's real strength, the laborers, the workers and the weavers. These are the foundation and the walls of any building that we are going to build.