1 00:00:05,879 --> 00:00:09,780 We are going to measure the efficiency of a solar panel. 2 00:00:09,780 --> 00:00:16,590 We do this by measuring the J-V curve and its external parameters as discussed in the previous block. 3 00:00:16,590 --> 00:00:20,650 How does such experimental setup look like? 4 00:00:20,650 --> 00:00:23,650 It consists of several components. 5 00:00:23,650 --> 00:00:29,349 The first component is a solar simulator, which is a light source that simulates both 6 00:00:29,349 --> 00:00:36,349 the shape of the AM1.5 solar spectrum and an irradiance of 1000 W/m^2. 7 00:00:36,940 --> 00:00:44,650 Secondly, the setup has a voltage source which applies a varying voltage over the solar cell or solar panel. 8 00:00:44,910 --> 00:00:51,780 An ampere meter measures the current generated by the solar device at every voltage. 9 00:00:51,780 --> 00:00:58,100 Finally, a temperature controlled substrate guarantees that the solar cell is at the required 10 00:00:58,100 --> 00:01:03,130 standard temperature of 25 degrees Celsius. 11 00:01:03,130 --> 00:01:07,759 Let's see how such measurement in reality works. 12 00:01:07,759 --> 00:01:11,710 For that we will go to the Delft Solar Lab. 13 00:01:11,710 --> 00:01:16,789 Here you see the voltage supply and the ampere meter. 14 00:01:16,789 --> 00:01:20,969 Typical panel areas are in the range of 1.5 square meters. 15 00:01:20,969 --> 00:01:27,729 This means that you need a light source with a spectrum shape and irradiance which is homogeneously 16 00:01:27,729 --> 00:01:29,659 distributed over large areas. 17 00:01:29,659 --> 00:01:36,170 In the solar lab, we use a large AAA solar simulator from Eternal Sun. 18 00:01:36,170 --> 00:01:41,689 AAA indicates that the spectral match, the uniformity and the stability is of A class quality. 19 00:01:41,850 --> 00:01:48,850 The c-Si solar panel needs to be connected to the voltage supply and the ampere meter. 20 00:01:50,670 --> 00:01:56,279 In addition a thermocouple is connected to the panel to monitor the temperature 21 00:01:56,279 --> 00:01:59,559 of the panel during the J-V measurements. 22 00:01:59,759 --> 00:02:04,279 Without additional temperature control, the solar panel will heat up in time. 23 00:02:04,279 --> 00:02:11,919 Here we demonstrate that the panel is at the required 25 degrees Celsius using a Fluke infrared thermometer, 24 00:02:11,919 --> 00:02:17,120 before the panel is moved under the solar simulator. 25 00:02:17,120 --> 00:02:20,360 A software program controls the J-V measurements. 26 00:02:20,360 --> 00:02:24,930 The voltage over the panel is varied, while the current is measured. 27 00:02:24,930 --> 00:02:33,520 On the screen the resulting J-V curve and power density is plotted from 0 volts up to the open-circuit voltage. 28 00:02:33,520 --> 00:02:38,280 As the setup measures the total current of the panel, the active area of the panel is 29 00:02:38,280 --> 00:02:41,480 an input parameter for the software. 30 00:02:41,480 --> 00:02:46,450 Using the given active area, the software calculates the current density of the panel. 31 00:02:46,450 --> 00:02:53,450 In this example, the maximum power density is 17.4 milliwatts per square centimeters, 32 00:02:53,490 --> 00:03:00,490 which corresponds to a panel conversion efficiency of 17.4%. 33 00:03:01,430 --> 00:03:07,570 The J-V curve measured does not perfectly match the J-V curve of an ideal solar cell. 34 00:03:07,570 --> 00:03:12,730 In reality a solar cell can have additional electrical losses. 35 00:03:12,730 --> 00:03:19,530 These losses can be represented as parasitic resistances in the electrical circuit 36 00:03:19,530 --> 00:03:22,810 in addition to a non-ideal solar cell. 37 00:03:22,810 --> 00:03:29,810 Here we will discuss two important resistances: the series and the shunt resistance. 38 00:03:30,290 --> 00:03:33,970 The first resistance is called series resistance. 39 00:03:33,970 --> 00:03:39,870 Several effects can be the origin of a series resistance in a solar cell. 40 00:03:39,870 --> 00:03:46,500 Let's consider a standard c-Si solar cell which we will discuss in great detail next week. 41 00:03:47,120 --> 00:03:54,610 First, the current moving through the semiconductor materials of the p-n junction can experience a resistance. 42 00:03:54,820 --> 00:04:02,290 Secondly the interface between the semiconductor material and the metal contacts can act as a resistor as well. 43 00:04:02,290 --> 00:04:08,290 Thirdly, the metal contacts will have a resistance as well. 44 00:04:08,510 --> 00:04:14,180 How does the series resistance appear in an electric circuit? 45 00:04:14,180 --> 00:04:18,739 Here we will use a zig-zag line as a symbol for the resistor. 46 00:04:18,739 --> 00:04:25,380 Note, that sometimes a rectangle is used as a symbol for the resistor as well. 47 00:04:25,380 --> 00:04:31,350 As discussed earlier, the ideal solar cell is a parallel connection of a current source 48 00:04:31,350 --> 00:04:34,190 and a p-n diode. 49 00:04:34,190 --> 00:04:41,060 The series resistance is, as the name already reveals, connected in series with these two elements. 50 00:04:41,550 --> 00:04:48,550 If the solar cell generates current, the solar cell will lose voltage over the series resistance. 51 00:04:49,070 --> 00:04:55,870 The second resistance is the so-called parallel resistance, or also referred to as the shunt resistance. 52 00:04:55,870 --> 00:05:05,270 A shunt is a macroscopic defect in the solar cell, which provides an alternative path for the generated photocurrent. 53 00:05:05,290 --> 00:05:11,000 Examples of a shunt are a crack through the semiconductor layers 54 00:05:11,000 --> 00:05:14,220 or a current path at the edge of the solar cell. 55 00:05:16,220 --> 00:05:22,100 In the electrical circuit the shunt resistance appears as a resistor connected in parallel 56 00:05:22,100 --> 00:05:24,190 with the current source and the diode. 57 00:05:24,190 --> 00:05:31,950 A low shunt resistance means that a large fraction of the photocurrent prefers to travel through the shunt. 58 00:05:31,950 --> 00:05:38,950 While a high shunt resistance means that less or no photocurrent is lost through the shunt. 59 00:05:40,730 --> 00:05:46,460 Important to remind is that you would like to have the series resistance as small as possible 60 00:05:46,460 --> 00:05:54,780 and the shunt resistance as large as possible to come close to an ideal illuminated p-n junction. 61 00:05:55,080 --> 00:06:02,280 The series and shunt resistance result in a more complicated expression for the J-V curve. 62 00:06:03,560 --> 00:06:15,060 The voltage at the terminal is the voltage of an ideal solar cell minus the voltage lost over the series resistance. 63 00:06:17,970 --> 00:06:24,970 The total current density is the photo current density minus the dark current density of the diode 64 00:06:25,360 --> 00:06:31,260 and the current density leaking through the shunt resistance. 65 00:06:31,260 --> 00:06:40,260 The shunt current is given by the voltage of an ideal solar cell divided by the shunt resistance. 66 00:06:41,360 --> 00:06:45,520 This results in the complex expression shown here. 67 00:06:45,520 --> 00:06:52,550 The current density J appears on the left hand side of the equation as well as on the right hand side. 68 00:06:52,550 --> 00:06:57,330 Note, that this expression can not be solved analytically. 69 00:06:59,330 --> 00:07:06,330 How do the series and shunt resistance affect the J-V curve and the FF? 70 00:07:06,729 --> 00:07:11,490 Let's start with the J-V curve of an ideal p-n junction as shown in this figure. 71 00:07:11,490 --> 00:07:17,070 Now we are going to increase the series resistance. 72 00:07:17,070 --> 00:07:24,070 As you see the slope around the open-circuit voltage point starts to become less steep. 73 00:07:24,750 --> 00:07:29,240 The larger the series resistance, the less steep the slope will be. 74 00:07:29,240 --> 00:07:35,740 Furthermore, the maximum power point is affected as well by increasing the series resistance. 75 00:07:35,740 --> 00:07:39,990 The larger the series resistance, the smaller the maximum power point will be. 76 00:07:39,990 --> 00:07:46,910 This also implies that the larger the series resistance will be, the smaller the FF. 77 00:07:46,910 --> 00:07:54,850 In conclusion the series resistance can affect the FF and has to be as small as possible for high FF's. 78 00:07:54,850 --> 00:08:00,910 Note, that the series resistance does not affect the position of the open-circuit voltage. 79 00:08:00,910 --> 00:08:06,690 As at the open-circuit voltage the current density is equal to zero, the voltage drop 80 00:08:06,690 --> 00:08:11,110 over the series resistance is zero as well. 81 00:08:11,110 --> 00:08:15,949 Let's start again with the J-V curve of an ideal p-n junction. 82 00:08:15,949 --> 00:08:19,900 This means that the shunt resistance is infinite large. 83 00:08:19,900 --> 00:08:24,680 Now we are going to decrease the shunt resistance. 84 00:08:24,680 --> 00:08:32,660 As you can see the slope at the short-circuit current density point starts to become more positive. 85 00:08:32,690 --> 00:08:37,019 The maximum power point and FF is affected as well. 86 00:08:37,019 --> 00:08:41,350 The smaller the shunt resistance, the smaller the FF will be. 87 00:08:41,450 --> 00:08:47,050 Let's look in more detail to the slopes in a J-V curve. 88 00:08:47,050 --> 00:08:54,050 The slope is current density divided by voltage which is equal to one over the resistance. 89 00:08:54,230 --> 00:08:59,670 In this week's homework we have included an exercise in which you have to prove that 90 00:08:59,670 --> 00:09:06,430 the slope at the open-circuit voltage point, is equal to 1 divided by the series resistance. 91 00:09:06,430 --> 00:09:11,240 The smaller the series resistance, the larger the slope and the FF will be. 92 00:09:11,240 --> 00:09:17,170 The slope at the point corresponding to the short-circuit current density in a J-V curve 93 00:09:17,170 --> 00:09:20,010 is 1 divided by the shunt resistance. 94 00:09:20,010 --> 00:09:26,090 The larger the shunt resistance the closer the slope will be to zero and the larger the FF will be. 95 00:09:26,310 --> 00:09:32,450 In summary the real solar cells and panels have series and shunt resistance 96 00:09:32,450 --> 00:09:37,780 and in solar cell cell design and manufacturing it is important to minimize the series resistance 97 00:09:37,780 --> 00:09:43,010 and to make the shunt resistance as large as possible. 98 00:09:43,510 --> 00:09:49,750 The J-V curve of a solar cell has a power density which is varying with voltage. 99 00:09:49,750 --> 00:09:56,580 What will determine at which J-V point the solar cell will be operating? 100 00:09:56,580 --> 00:10:02,440 This is determined by the load, which is connected to the solar cell. 101 00:10:02,440 --> 00:10:08,130 If the load has a low impedance the current density will be high and the voltage will be low. 102 00:10:08,130 --> 00:10:13,670 It means that the solar cell is not operating in its maximum power point. 103 00:10:13,670 --> 00:10:20,550 Whereas, if the load has a high impedance the voltage will be high and the current will be low. 104 00:10:20,550 --> 00:10:26,250 Again, the solar cell is not operating in its maximum power point. 105 00:10:26,250 --> 00:10:31,850 This means that we need to tune the impedance of the load to have the solar cell working 106 00:10:31,850 --> 00:10:34,490 in its maximum power point. 107 00:10:34,490 --> 00:10:41,490 If we discuss PV systems later in this course, we will talk about maximum power point trackers. 108 00:10:41,490 --> 00:10:48,160 These are electronic devices which varies the load in time such that the solar panel 109 00:10:48,160 --> 00:10:54,110 is always working in its maximum power point, such that the maximum amount of electrical power 110 00:10:54,110 --> 00:10:56,940 can be generated by the solar panel. 111 00:10:56,940 --> 00:11:04,339 So far, we have discussed the external parameters of an ideal and non-ideal solar cell this week. 112 00:11:04,339 --> 00:11:09,610 The remainder of this week, we will discuss how these external parameters are affected 113 00:11:09,610 --> 00:11:11,959 by the design of the solar cell. 114 00:11:11,959 --> 00:11:17,670 I will introduce you general design rules for solar cells. 115 00:11:17,670 --> 00:11:23,240 These design rules will help you to understand the performance of the different PV technologies 116 00:11:23,240 --> 00:11:26,600 which will be discussed in the coming three weeks.