0 1 00:00:05,990 --> 00:00:09,010 Welcome back in this section. 1 2 00:00:09,010 --> 00:00:22,670 We are going to discuss Solar cell and its properties most solar cell are based on PN junction. 2 3 00:00:23,290 --> 00:00:30,820 So the question is what is the PN junction we take as an example. 3 4 00:00:30,820 --> 00:00:43,310 The case for silicon If we have semiconductor (silicon) in which one part is doped p-type 4 5 00:00:43,320 --> 00:00:47,280 and another part is doped n-type 5 6 00:00:47,750 --> 00:00:54,870 we have created a so-called p-n junction. 6 7 00:00:54,870 --> 00:01:03,060 In p-type the holes are the majority charge carriers and in n-type the electrons are the majority charge carriers. 7 8 00:01:07,440 --> 00:01:18,970 There are two different mechanisms control the transport of charge carriers in semiconductors: first one is diffusion 8 9 00:01:19,910 --> 00:01:27,500 which is controlled by a density gradient. 9 10 00:01:27,660 --> 00:01:38,700 The second one is drift is controlled by electric fields, which you can build in the p-n junction or apply externally. 10 11 00:01:39,150 --> 00:01:43,220 or apply externally. 11 12 00:01:43,450 --> 00:01:45,540 Let's take an example. 12 13 00:01:45,700 --> 00:01:54,940 light absorption only affects the density of the minority charge carriers in doped semiconductor materials 13 14 00:01:54,940 --> 00:01:55,530 light absorption only affects the density of the minority charge carriers in doped semiconductor materials 14 15 00:02:01,400 --> 00:02:10,220 let's discuss the operation of that solar cell. solar cell in the dark the drift of the minority carrier 15 16 00:02:10,700 --> 00:02:23,150 and diffusion of majority charge carriers are in balance. so there where no current if we apply reverse 16 17 00:02:23,150 --> 00:02:31,320 bias on such p-n junction in the dark, the depletion zone gets wider, 17 18 00:02:32,810 --> 00:02:36,890 the diffusion of majority charge carriers is suppressed and only an extreme small current 18 19 00:02:37,830 --> 00:02:43,830 and only an extreme small current related to drift of minority charge 19 20 00:02:43,830 --> 00:02:52,440 carriers is generated. 20 21 00:02:55,440 --> 00:02:57,120 or in other words, 21 22 00:02:58,660 --> 00:03:01,690 The density gradient is becoming smaller. 22 23 00:03:03,080 --> 00:03:14,950 The current density related to diffusion of both electrons and holes is reduced. 23 24 00:03:14,950 --> 00:03:20,170 The drift is enhanced Since the electric field E is larger 24 25 00:03:20,170 --> 00:03:30,140 the drift of the electrons and holes is slightly enhanced. In this case the drift current density 25 26 00:03:31,290 --> 00:03:32,510 is dominant. 26 27 00:03:32,520 --> 00:03:42,960 The diffusion current density Since the drift current density is ruled by the density of the minority charge carrier. 27 28 00:03:42,960 --> 00:03:55,890 in the p and n region, the total net current is extremely small. or in other words in the reverse 28 29 00:03:56,010 --> 00:04:06,410 bias condition an extreme small current will move from the contact at the p-region to the n-region. 29 30 00:04:06,410 --> 00:04:08,000 the outer circuit. 30 31 00:04:08,270 --> 00:04:17,640 This means that on average electrons are walking from the contact of the n-region to the p-region 31 32 00:04:17,640 --> 00:04:21,910 shown in the figure. 32 33 00:04:21,980 --> 00:04:32,620 If we apply a forward bias on such p-n junction in the dark, the width of the depletion 33 34 00:04:32,620 --> 00:04:35,440 zone is getting smaller 34 35 00:04:35,440 --> 00:04:44,800 the diffusion of the majority charge carriers is significantly enhanced and overrules the drift of minority 35 36 00:04:44,800 --> 00:04:54,820 charge carriers. The p-n junction becomes conductive and is able to generate a current. 36 37 00:04:55,350 --> 00:04:58,810 or in other words, the density gradients become much larger. 37 38 00:04:58,900 --> 00:05:07,660 As a consequence this the current density related to diffusion becomes larger. 38 39 00:05:07,870 --> 00:05:17,730 On the other hand, the electric field E is reduced , which means that the current density 39 40 00:05:17,800 --> 00:05:20,750 related to drift is getting slightly smaller. 40 41 00:05:22,790 --> 00:05:33,800 But Note that the effect of the increased flux due to diffusion is much larger than the small change 41 42 00:05:33,800 --> 00:05:35,480 in drift. 42 43 00:05:35,480 --> 00:05:42,740 This means that we generate a net current over the depletion zone. 43 44 00:05:42,800 --> 00:05:54,060 More electrons are diffusing from the n-region to the p-region, than are drifting from the p-region to the n-region. 44 45 00:05:54,110 --> 00:06:05,000 Means More holes are diffusing from the p-region to the n-region, than are drifting for the n-region to the p-region. . 45 46 00:06:05,000 --> 00:06:05,990 region. 46 47 00:06:05,990 --> 00:06:15,860 This means by applying a forward bias over a p-n junction in the dark we produce a net current through the electrical 47 48 00:06:15,860 --> 00:06:17,040 circuit. 48 49 00:06:17,120 --> 00:06:18,570 as shown in the figure 49 50 00:06:21,770 --> 00:06:27,890 we can summarize that in is a dark under forward bias 50 51 00:06:28,220 --> 00:06:32,040 the diffusion of charge carriers over the depletion zone 51 52 00:06:32,040 --> 00:06:33,680 is dominant, 52 53 00:06:36,410 --> 00:06:38,100 but under reversed bias 53 54 00:06:38,210 --> 00:06:47,810 the drift of charge carriers over the depletion zone is dominant. Under forward bias in the dark, 54 55 00:06:47,810 --> 00:06:57,470 the p-n junction can produce relatively large currents, whereas in reverse bias, it generates very small 55 56 00:06:57,470 --> 00:06:58,660 currents. 56 57 00:06:59,150 --> 00:07:02,740 Such a device is called a diode 57 58 00:07:02,750 --> 00:07:13,160 It has a high conductance in forward bias, but has a low conductance in reverse bias. 58 59 00:07:13,910 --> 00:07:27,070 solar cell and dark like diamond. 59 60 00:07:27,100 --> 00:07:32,050 Now we are going to shine the light on is a device. 60 61 00:07:32,050 --> 00:07:36,310 This means that we are looking as a solar cell. 61 62 00:07:38,390 --> 00:07:47,410 Light is incident from the left on the p-region. For the moment 62 63 00:07:47,410 --> 00:07:59,290 we assume the light is being absorbed in the p-region and the n-region. The absorption of the photon will generate 63 64 00:07:59,290 --> 00:08:03,150 electron and hole pairs. 64 65 00:08:03,280 --> 00:08:12,850 Important to realize and as discussed earlier, light absorption only affects the density of the minority charge carriers 65 66 00:08:12,860 --> 00:08:17,700 in doped semiconductor materials. 66 67 00:08:17,710 --> 00:08:28,870 This means that the light excited charge carriers significantly increase the density of the electrons in the 67 68 00:08:28,870 --> 00:08:38,440 p-region and the density of the holes in the n-region. so we increase the drift. 68 69 00:08:38,450 --> 00:08:50,150 This can be easily recognized by looking at the equations for the current densities 69 70 00:08:50,210 --> 00:09:03,090 (Je = nqµeE). We see that we are significantly 70 71 00:09:03,810 --> 00:09:11,100 increasing the drift over the depletion zone, which is indicated by the larger arrows. 71 72 00:09:17,040 --> 00:09:25,710 Many electrons drift from the p-region to the n-region and many holes drift from the nregion to 72 73 00:09:25,710 --> 00:09:36,360 the p-region. The current density related to drift can be easily increased by many orders of magnitude under 73 74 00:09:36,360 --> 00:09:46,650 illumination in reference to the p-n junction in the dark. By illuminating a p-n junction we can generate a current. 74 75 00:09:46,740 --> 00:09:47,280 current 75 76 00:09:52,590 --> 00:10:02,750 Finally, we're looking at the working principle of a solar cell. Using an electrical circuit we connect 76 77 00:10:02,750 --> 00:10:13,900 the contact at the p-doped silicon with that of the n-doped silicon, or in other words we short-circuit the p-n 77 78 00:10:13,990 --> 00:10:14,650 junction. 78 79 00:10:17,600 --> 00:10:27,840 Short circuit In this condition, the illuminated p-n junction will produce only an electrical current. We call 79 80 00:10:27,870 --> 00:10:38,590 this current the short-circuit current of a solar cell. We can make a very simplified animation of 80 81 00:10:38,600 --> 00:10:49,670 the journey of the generated charge carriers. On average a minority electron will drift to the ntype material 81 82 00:10:50,210 --> 00:11:01,370 and diffuses to the metal contact in which the electron is injected. The electron moves to the contact 82 83 00:11:01,670 --> 00:11:17,410 at the p-side and is injected into the p-type silicon. and recombines with a hole. The minority holes in 83 84 00:11:17,410 --> 00:11:27,580 the p-type drift across the depletion zone and diffuse to the back contact to recombine with the electrons. 84 85 00:11:27,580 --> 00:11:28,270 electrons 85 86 00:11:31,400 --> 00:11:33,640 the second to thing is open circuit. 86 87 00:11:34,140 --> 00:11:38,770 However, no current is created when we open 87 88 00:11:38,820 --> 00:11:49,480 the electrical circuit. In that case, the dominant drift current of light excited charge carriers 88 89 00:11:49,490 --> 00:11:56,860 will positively charge the p-region with holes and negatively 89 90 00:11:56,910 --> 00:11:58,160 charge 90 91 00:11:58,190 --> 00:12:10,430 the n-region with electrons. This charging creates an electric field opposite to the built-in electric 91 92 00:12:10,430 --> 00:12:15,150 field and reduces the net drift current 92 93 00:12:15,170 --> 00:12:24,650 again. This charging of free holes in the p-region and free electrons 93 94 00:12:25,000 --> 00:12:32,970 in the n-region will build up until both the drift currents are in equilibrium. 94 95 00:12:37,100 --> 00:12:48,110 This means that the device does not generate a current, but builds up an electric field, or voltage. 95 96 00:12:48,110 --> 00:12:49,250 voltage. 96 97 00:12:49,250 --> 00:12:57,770 The voltage created by an illuminated solar cell under open-circuit conditions is called the open-circuit voltage. 97 98 00:12:57,770 --> 00:12:59,030 voltage. 98 99 00:12:59,030 --> 00:13:06,260 If you couldn't understand the open circuit condition and short circuit condition we are going to discuss 99 100 00:13:06,470 --> 00:13:18,830 by equations in the next section so we recommend you to see the next section open circuit voltage and 100 101 00:13:18,830 --> 00:13:23,240 short circuit current. 101 102 00:13:23,490 --> 00:13:33,560 The last thing we are going to talk about today is the equivalent circuit of PVsolar so to understand 102 103 00:13:33,690 --> 00:13:45,420 To understand the electronic behavior of a solar cell in light, it is useful to create a model which is electrically equivalent. 103 104 00:13:48,140 --> 00:13:57,290 An ideal solar cell may be modelled by a current source in parallel with a diode; 104 105 00:13:57,290 --> 00:14:09,260 in practice no solar cell is ideal, so a shunt resistance and a series resistance component 105 106 00:14:09,740 --> 00:14:16,250 are added to the model as shown in the figure blow. 106 107 00:14:16,790 --> 00:14:25,790 To understand the main parameters of solar cell we are going to simplify the above circuit by eliminating 107 108 00:14:26,720 --> 00:14:33,770 the shunt resistance and the series resistance as shown below. 108 109 00:14:37,700 --> 00:14:50,120 so the solar cells or ideal solar cell can be simplified by a current source in parallel with diode 109 110 00:14:51,950 --> 00:15:04,350 in the next section we are going to define the open circuit and short circuit from this equivalent circuit 110 111 00:15:05,790 --> 00:15:09,240 so see you in the next block.