 Alright guys, so we have I think 10 more minutes so I can briefly just introduce the next topic write down optoelectronic Junction devices Okay, now why it is called optoelectronic because Light has a major role to play in these devices Okay, so Light can be you know created If it is getting created because of the junction it is also an optoelectronic junction device or if Light is used for for its functioning then also we call it to be an optoelectronic device Okay, so we are going to talk about these devices which Utilize light or create light Can you think of few devices optoelectronic junction devices? No solar cells solar cells correct and LEDs Okay, LED is also optoelectronic device LED is light emitting diode. Okay, then we have photo diodes also So I'll just quickly discuss them one by one so photo diode Let's finish this particular topic. It will take I think 20 minutes more after 430, but then it is fine Okay, at least this will complete one chunk of the chapter, which is about diodes Fine so photo diodes is Used to detect em waves Okay second LED converts electrical energy into light energy and third photovoltaic cells or Simply we can say solar cells this converts light energy into electrical energy Okay, let us see how photo diode actually works photo diode first of all works in reverse bias Okay This is the circuit of the application of photo diode Ramu Kondi, you're there, right? Yes, Ramu is not there So I'm here only this is the reverse bias Micro emitter like this Okay, and there will be a junction or a barrier Okay, this is the barrier zone and if a light Hits this barrier Then current gets created. Let's see first what we are talking about here is First of all PN junction is connected to reverse bias So right now negligible current or you can say no current is flowing Why no current is flowing because in the barrier There are no charge carriers Okay So when the light Whose photons energy is h mu hits the barrier Okay, and it hits the barrier. There's a chance that Electrons from the valence band gain sufficient energy and The you know the bonds get broken away and free electrons get generated fine So when this light hits the junction some charge carriers can get generated and because of that current may be visible Fine, so you can detect the radiation using a photo diode. So if you know that What particular frequency or beyond what frequency the radiation should be so that Electron breaks away from a bond Using that knowledge you can roughly guess what is the frequency of the light you're detecting okay Not only just frequency you can actually Detect the intensity of the light also because there is a characteristic curve between intensity voltage and current Let's see how it is While I'm drawing this can you answer me why it is working under the reverse bias? Why not for our backs? So because very little current will be generated So So reverse bias will be like Micramp here, right? So That's how you agree of reverse bias No, that's not the reason see in reverse bias almost no current is flowing So even if slightest of the current flows Then you'll be able to detect it's like you know The moon is there when sun is there, right? But it is very difficult to detect moon during daytime Because a lot of light is already coming from the sun but during nighttime when sun is not There you can easily detect the moons and stars It's like that only So the amount of current that this radiation will generate is so less That you'll be able to detect only when There was no current earlier and that is why we are having reverse bias and also in a reverse bias The junction barrier will be wider So you have wider space on which radiation can fall and current could get generated Okay So your junction should be transparent to the radiation so that radiation falls onto it And then photodiode will work Under the reverse bias mode Fine any doubt? No, sir Okay, now let's talk about functioning of leds Light emitting diodes So is this chapter therefore advanced also? It was there I guess last year it was there For mains definitely a couple of questions come And For mains whatever question comes there will be you know, it is like It's like inorganic chemistry if you do it properly you will Get it right with almost Good amount of certainty Okay, so do this chapter properly because couple of questions will definitely come from this particular topic Fine, let's talk about leds now. You might have seen leds and I think you might be using led at your home also So this is widely accepted as a device which can efficiently convert Electrical energy into light energy It has very very high efficiency. Okay, so led works in So led works under the forward bias mode So in a forward bias what happens is Let's say this is The p and junction Okay, you have This as barrier Okay, so we have discussed that uh, there is a dynamic equilibrium Isn't it so The electron and holes They recombine and they separate again and again. So there is a dynamic equilibrium that is happening fine Now every time electron combined with a hole at the p side There's a difference in energy, right whenever electron combined What will happen? It goes to lower energy Okay And from the external circuit it gets energy to again come out of the hole So there is an equilibrium Now when electron goes from higher energy state to lower energy state As in when the hole captures it then the difference in energy difference in energy is given away as light h mu Okay, and if this light that is coming out if the frequency of the light is in the visible spectrum We can say that this is led Okay, so you'll have red light then you have blue light white light Depending on what frequency of the light is coming out different different colors of leds will be there Yeah Now leds have certain advantages over the conventional lighting So let's write it down one by one First one. What do you think the first one will be? quickly Any advantage you can think of? Pondi Uh, so like for a small amount of current it can produce a lot of light Oh, you mean to say higher efficiency, right? Yeah Correct that that is given like that that that's like That comes with it any other advantages you can think You might have seen that In led lamps How many voltage or how many watts typically is written in uh in a box of led light? Have you ever noticed? No, sir never And you have never ever seen the box of led Do you have an idea typically what volts the typically what? Power led light consumes No idea Anyways, so typically, uh, you see that For an incandescent light which Which is around uh, yes. I'm correct. So let's say 100 watt bulb Which is uh, I'm talking about the The the conventional lamp. Okay, so 100 watt bulb is like similar to let's say 18 watt Uh CFL and this is similar to Around three to five watt of led Okay, so you can see that Such small amount of power generate light, which is equal to 100 watt of the conventional light So it is very efficient Okay, so low operational Voltage and less power Right down Low operational voltage And less power Okay Then you have fast action Fast action as in it quickly switched on Okay No warm-up is required The the frequency which comes out from the led is near monochromatic Okay Which is around 100 am strong To 500 am strong typically there are like multiple wavelength that comes out from the light But with led since the the the cause of light Coming out is a combination of electron in the hole. So that remains almost constant So that is a near monochromatic light This is an advantage if you want A near monochromatic light, okay Then you have uh long life and ruggedness Okay, which is a A biggest plus point of the led light it will have a long life So even if you purchase it for a higher amount over a period of time since your electricity bill will be very less Your running cost is very less compared to the convention lamp and within like a few years you will recover your cost And then it it becomes all the more meaningful to use led Then the last advantage is fast on and off switching capability Now you might be knowing already that there are uh, you know Some research going on To use led as a li-fi device So li-fi is similar to wi-fi So rather than using radio waves for internet Uh wireless connectivity. We are going to use light Okay, so all one I have to do is to transmit zero and one signal right So if you switch on a light so that switching on could mean one switching off could mean zero So if you have fast on and off switching capability Then using sensor you can detect How many times it switch on and off so very quickly you can transmit the signal or very quickly you can transmit the entire data And the kind of uh speed we are talking about is gigabytes per second So leds Can uh be really useful Uh, you know in future you can expect li-fi to be there And once it comes, uh, this technology will just Capture entire space very quickly that that's how things are happening nowadays like for example, nobody knew that There will be ola and uber couple of years back and suddenly it has captured entire thing So the acceptability of the market when it comes to anything which is better or uh, efficient is very quick nowadays All right. So this is leds. Do you have any doubts till now? Any doubt? No, sir. No, sir Okay, let's talk about solar cells now. So this is the last topic we are going to discuss today Okay solar cell actually, uh Converts light energy into elliptic energy. What do you think solar cells will be forward bias or reverse bias quickly? So reverse bias Ramcharan How can it be biased it itself creates a voltage, right? There is no point connecting battery if it itself is a battery Okay, so PN junction itself Is a battery you're not connecting anything outside Fine. So what happens the light falls At the junction barrier and again what we discussed If the energy of the photon is more than the energy required to break the bonds Then what will happen? The free electron gets generated and the current will start to flow Okay, why current is flowing because There is an electric field here. You can see that Once Electron get generated here Electron will move this side because positive charge will attract and holes will go that side Fine. So the current because of the electron movement is this side and because the whole movement also that side Fine hence We have because of h mu current flowing in the circuit And current times resistance is the voltage Okay, if you draw the characteristic graph of the solar cell Okay It will be on the fourth quadrant Because you can see that The battery potential is plus over here, but then current flow is Opposite like usually what happens if battery is like that The current flows like this, isn't it but right now positive potential is on the right hand side But current is flowing towards the left hand side Okay So I can say that I can assume voltage to be positive Then current I have to assume to be negative because current is flowing in opposite direction of the usual way That is why it is coming in the fourth quadrant And the kind of graph you get is like that This is open circuit voltage Current is not there and this is short circuit I short circuit As in the resistance once it becomes zero voltage is also zero close to that and current will be very large Okay, so like this you can understand how solar cell works Fine So that's it from my side. You have any doubts the entire chapter till now? Anything no, sir Sir, how do we ensure that light emitted is in the visible spectrum? Amog it is uh, you know experimental and We may have documented stuff that okay, if you have a p-ingension of this material Then uh, you know light from this frequency will get emitted Okay, and if you know exactly what is the energy difference between a hole and electron So when once they recombine The difference in energy gets emitted as a photon. So because of that you will estimate what frequency will get emitted And you know the frequency range of the visible spectrum Okay Fine, so that's it from my side and guys, uh, I know your school may have a lot of work It might be pressurizing But then trust me if you focus at the right thing You will come out as much more happier person, okay, so many a times, uh You have to take tough calls Fine, so that is your individualistic call. You have to take a call. What is more important? What can affect you more? All right, so according to me if you're planning to be in india There is uh, I mean there is hardly any thought you should give Other than just what is coming in january first week Okay, so that's it from my side. We'll meet next week. Thank you. Okay, sir. Thank you, sir. Thank you