In this video, we will discuss Solar Cell. Solar Cell is another example of an Optoelectronic device based on p-n junction, and the operating mechanism of a solar cell is essentially the same as that of Photodiode in that, a p-n junction is illuminated by light and the photogenerated carriers are separated by the built-in electric field across the p-n junction within the depletion region, and they are collected by the outer electrode to register a photo-current. Now, in Photodiode you measure the photo-current to know the intensity of the incoming light. In solar cell, you take that photo-current to power something else. So, here is a schematic of a Solar Cell p-n junction under light illumination, and a light of various color will come in, and the absorption quotient or the penetration depth will be different for different color light. Therefore, on average these different colors of light will be absorbed at different location. Now, if the absorption takes place within the depletion region, then the photogenerated carriers are immediately separated by the built-in electric field, electrons get pushed to the n side and the holes get pushed to the p side. Then, when they reach the neutral region, then they diffuse and reach the ohmic contact on the backside of the p-n junction and then they register current. Now, if the absorption takes place outside the depletion region here, for example in the neutral p-type region, then you have to wait until the minority carrier electron diffuse into the depletion region. Then the electron can get drifted across the depletion region reach the n-type region and then gets collected. Similarly, if the absorption takes place in the p-type region, then you need to wait for hold to diffuse into the depletion region, and then gets drifted, and then reach the neutral p-type region, and then continue to diffuse to reach the backside electrode. So, in order to understand the mechanism of Solar Cell operation. Consider the simplest circuit where you have a p-n diode here on the light illumination, and it is connected to a load resistor here. Now, imagine for a moment that your load is the 0, your load the resistance is 0, and this is a situation called a short circuit condition. Because you have a short circuit, the voltage across the diode is 0. When the voltage across the diode is 0, then the diode current should be 0, so whatever current that you have in the short-circuit condition is entirely due to light absorption. So, the current flowing here in the short circuit condition is equal to the photo-current, but if you look at the direction of the photo-current, it is in the direction corresponding to the reverse biased current. You can easily see why this is the case by simply looking at the direction of the electric field, built-in electric field within the depletion region. So, the short circuit current is simply equal to the negative of photo-current. Now, you come back to the situation where your load resistance is not 0, so this is your load that you're powering. In that case, the photo-current is going to produce a finite voltage drop across your load resistor, and therefore there is a finite voltage between the p and n side of the p-n junction, and that finite voltage across the p-n junction is going to induce, drive a current in the p-n diode. So, the total current in this situation is the diode current, which is driven by the voltage across the diode, p-n diode minus the photo current, if the current produced by the generation of carriers by light illumination. So, you can do the simple circuit analysis and you have the diode equation, diode I-V equation here for this diode, which is basically the ideal diode equation minus the photo-current, the photo-current which is proportional to the light intensity. Then, of course you have the load line equation, which is basically the Ohm's law across this load resistance. So, you can plot the I-V equations for the Solar Cell, the diode, which is again the diode equation here minus the photo-current. That's shown here in the left figure, so the purple curve is the regular diode equation, so that's the situation under dark when there is no light illumination. When you illuminate with light, then you push, you shift the diode equation lower to below, and the shift amount is equal to the photo-current and that of course is proportional to the light intensity. You then add the load line equation which is basically a straight line because the Ohm's law says that the I and V should be proportional to each other, and the proportionality constant is given by the resistance. So, this red line here on this right curve, right figure is the load line equation, and the green curve is the portion of the I-V characteristics of a diode under illumination. So, the intersection of these two curves gives you the operating point, that's the voltage and the current for the diode circuit, and that is the produced power. So, the area of this rectangle here, or V times I is the total power generated and that's the power that you use to drive other devices. Now, notice the difference in operating condition between the photo diode and the Solar Cell. In Solar Cell, you operate under forward bias, your diode is under forward bias. The forward bias diode that drive the forward current, and the forward current plus the photogenerated current is used to power something else. If you recall photodiodes case, in the case of photo-detector photodiode, the photodiode is used in reverse bias condition. So, in that case, you produce current due to light illumination, but you actually consume power, you have to power your detector by applying a reverse bias in order to generate photo-current. So, the photo-current is used just to detect the incoming light intensity, but not to power anything else and if you are actually consuming power in that situation.
