1 00:00:05,950 --> 00:00:12,049 So, how can we get electricity directly from sunlight? 2 00:00:12,049 --> 00:00:17,689 The mechanism in which solar light is directly converted into voltage or current is called 3 00:00:17,689 --> 00:00:20,369 the photovoltaic (PV) effect. 4 00:00:20,369 --> 00:00:26,090 In week 2, we will discuss the photovoltaic effect in greater detail. 5 00:00:26,090 --> 00:00:33,090 But to give you a first idea, I will show its principle using this simple animation. 6 00:00:33,290 --> 00:00:39,750 Here we see a simplified representation of a silicon based solar cell. 7 00:00:39,750 --> 00:00:48,440 It consists of the c-Si absorber layer, a pn-junction to separate the light excited 8 00:00:48,440 --> 00:00:54,650 charge carriers and a metal front and back contact. 9 00:00:54,650 --> 00:00:59,470 Here we see the same structure, but then in a cross-sectional view. 10 00:00:59,470 --> 00:01:03,100 The light enters the solar cell from the front side, 11 00:01:03,100 --> 00:01:07,740 in this illustration that is the top side. 12 00:01:07,740 --> 00:01:14,420 The light is transmitted into the absorber layer where its energy is absorbed. 13 00:01:14,420 --> 00:01:19,319 The energy is used to excite charge carriers in the semiconductor material, 14 00:01:19,319 --> 00:01:24,859 which are a negatively charged electron, indicated by the red dot, 15 00:01:24,859 --> 00:01:31,249 and a positively charged hole, indicated by the blue dot. 16 00:01:31,249 --> 00:01:35,100 These charge carriers diffuse around and need to be separated, 17 00:01:35,100 --> 00:01:39,810 which occurs at the depletion region between the n- and p-type doped silicon 18 00:01:39,810 --> 00:01:45,450 and the depletion region at the back of the solar cell. 19 00:01:45,450 --> 00:01:51,080 You don't have to understand yet why this happens, I will explain this in detail next week. 20 00:01:51,929 --> 00:01:57,880 Then the charge carriers have to be collected at the contacts. 21 00:01:57,880 --> 00:02:04,369 In this example the contacts are connected with a load, in this case a lamp. 22 00:02:04,369 --> 00:02:08,880 The electron will move through the load back to the solar cell. 23 00:02:08,880 --> 00:02:14,170 Both charge carriers recombine at the metal/p-layer interface. 24 00:02:14,170 --> 00:02:20,920 It shows that the photovoltaic process is based on three important principles: 25 00:02:20,920 --> 00:02:27,920 the first is excitation of free mobile charge carriers due to light absorption, 26 00:02:28,159 --> 00:02:32,510 the second is separation of the charge carriers 27 00:02:32,510 --> 00:02:38,959 and the third one is collection of the charge carriers at the contacts. 28 00:02:38,959 --> 00:02:42,900 A variety of PV technologies exist today. 29 00:02:42,900 --> 00:02:46,930 We can categorize them in various ways. 30 00:02:46,930 --> 00:02:53,930 The first logical way is to categorize them based on the type of absorber material; 31 00:02:54,049 --> 00:02:56,260 we will do that in a minute. 32 00:02:56,260 --> 00:03:01,430 Another categorization approach is based on the generations, 33 00:03:01,430 --> 00:03:05,890 which is often used in books about PV technology. 34 00:03:05,890 --> 00:03:11,500 I have some personal objections against the generation-based categorization, 35 00:03:11,500 --> 00:03:16,890 but as it's widely used, I will introduce it here. 36 00:03:16,890 --> 00:03:23,739 For that we use the graph in which we plot on the horizontal axis the cost-price of the 37 00:03:23,739 --> 00:03:29,370 solar cell per area, expressed in dollar per square meter. 38 00:03:29,370 --> 00:03:34,589 So solar cells made of expensive materials or using expensive processing methods 39 00:03:34,589 --> 00:03:38,569 will be further to the right on this axis. 40 00:03:38,969 --> 00:03:43,250 The conversion efficiency is the fraction of the energy in the solar light, 41 00:03:43,250 --> 00:03:49,829 which is converted into electricity, which is represented by the vertical axis. 42 00:03:49,829 --> 00:03:54,859 The efficiency scales with the energy yield of the solar cell. 43 00:03:54,859 --> 00:04:01,230 The larger the efficiency, the larger the generated power per area will be. 44 00:04:01,230 --> 00:04:04,469 This power is expressed in watts per square meters. 45 00:04:04,469 --> 00:04:13,369 For example, standard test conditions for solar cells means an irradiance of 1000 watts per square meter. 46 00:04:13,689 --> 00:04:20,670 This means a solar cell with conversion efficiency of 10%, produces under standard test conditions 47 00:04:20,670 --> 00:04:25,760 a power output of 100 watts per square meter. 48 00:04:25,760 --> 00:04:30,730 It means that the slope of the dashed line is equal to the watt per dollar. 49 00:04:30,730 --> 00:04:36,380 Or in other words it is reciprocally linear with the cost price per watt-peak. 50 00:04:36,380 --> 00:04:41,230 The cost price per watt-peak corresponds to the cost price of the energy generated 51 00:04:41,230 --> 00:04:42,510 by the solar cell. 52 00:04:42,810 --> 00:04:48,910 In this example the dashed line represents 0.5 dollars per watt. 53 00:04:48,910 --> 00:04:55,820 If the slope of these dashed lines is very steep, the cost price per watt is low, 54 00:04:55,820 --> 00:05:01,850 whereas when the slope of the dashed lines becomes less steep the cost price per watt-peak 55 00:05:01,850 --> 00:05:04,780 is getting significantly higher. 56 00:05:04,780 --> 00:05:10,510 To compete with other energy sources, you would like that your PV technology overlaps 57 00:05:10,510 --> 00:05:15,860 with the steepest lines in this graph. 58 00:05:15,860 --> 00:05:22,510 The first generation PV technology is based on using very pure bulky semiconductor materials, 59 00:05:22,510 --> 00:05:24,860 like crystalline silicon (c-Si). 60 00:05:24,860 --> 00:05:31,750 Pure materials means less defects and in general solar cells with a relative high efficiency 61 00:05:31,750 --> 00:05:33,740 can be manufactured. 62 00:05:33,740 --> 00:05:39,940 However, high quality materials requires more expensive production processes, 63 00:05:39,940 --> 00:05:45,310 which in general makes the cost price per area solar cell larger as well. 64 00:05:45,310 --> 00:05:51,050 The light grey circle roughly shows the area in which you would find the first generation 65 00:05:51,050 --> 00:05:53,610 PV technology. 66 00:05:53,610 --> 00:05:57,400 What is the second generation PV technology? 67 00:05:57,400 --> 00:06:02,650 These are PV devices, which are firstly based on thin-film solar cells. 68 00:06:02,650 --> 00:06:08,980 Thin film implies that less material is used which makes the solar cells cheaper. 69 00:06:08,980 --> 00:06:15,980 Secondly, these solar cells are manufactured using cheaper processing technology. 70 00:06:16,190 --> 00:06:23,190 As a consequence, the materials have more defects resulting in lower performances. 71 00:06:23,400 --> 00:06:29,120 Although the solar cell efficiency is lower, due to the lower cost price per area, 72 00:06:29,120 --> 00:06:36,120 the cost price per watt of the second generation PV technology is significant lower. 73 00:06:36,830 --> 00:06:43,830 The blue area represents the so-called Shockley-Queisser limit, which we will introduce in week 3. 74 00:06:44,990 --> 00:06:49,740 Given the shape of our solar spectrum and the band gap of the materials used, 75 00:06:49,740 --> 00:06:56,890 the Shockley-Queisser limit tells us the theoretically maximum conversion efficiency of the solar cell. 76 00:06:58,050 --> 00:07:04,720 Third generation PV technologies are based on solar cell concepts, which try to tackle 77 00:07:04,720 --> 00:07:07,610 the Shockley-Queisser limit. 78 00:07:07,610 --> 00:07:14,030 So, third generation PV technology would be solar cells with higher conversion efficiencies 79 00:07:14,030 --> 00:07:18,410 in reference to the first and second generations. 80 00:07:18,410 --> 00:07:23,470 The current solar cell concepts being studied to beat the Shockley-Queisser limit 81 00:07:23,470 --> 00:07:26,210 will be discussed in week 6. 82 00:07:26,310 --> 00:07:32,810 Furthermore, the cost price of the materials and processing techniques used to process 83 00:07:32,810 --> 00:07:39,090 the third generation solar cells are expected to be cheap as well. 84 00:07:39,090 --> 00:07:45,340 The result is that the third generation solar cell technology overlaps with the steepest 85 00:07:45,340 --> 00:07:50,960 dashed lines in this graph, meaning that third generation technology would have the lowest 86 00:07:50,960 --> 00:07:53,570 cost price per wattpeak (Wp). 87 00:07:53,570 --> 00:08:00,020 However, beating the Shockley-Queisser limit for most concepts is a challenge itself, 88 00:08:00,020 --> 00:08:06,360 making these complex concepts cheaply is a completely different ball game. 89 00:08:06,360 --> 00:08:12,430 Next to categorizing the PV technologies in the three generations, we indicate the various 90 00:08:12,430 --> 00:08:19,900 PV technologies based on the semiconductor material used as absorber layer in the solar cell. 91 00:08:20,840 --> 00:08:26,090 The most dominant PV technology is based on c-Si wafers. 92 00:08:26,090 --> 00:08:32,810 This technology represents around 90% of the current PV market and belongs to the first 93 00:08:32,810 --> 00:08:35,550 generation PV technology. 94 00:08:35,550 --> 00:08:40,380 We will discuss this technology in great detail in week 4. 95 00:08:40,380 --> 00:08:46,670 Another PV technology based on silicon is thin-film silicon. 96 00:08:46,670 --> 00:08:54,650 In this case no c-Si wafers are used but very thin layers of silicon, which are deposited 97 00:08:54,650 --> 00:08:58,970 on glass or a flexible substrate. 98 00:08:58,970 --> 00:09:05,580 The silicon does not have the same lattice structure and can be amorphous or nanocrystalline. 99 00:09:05,580 --> 00:09:11,570 This technique belongs to the second generation PV technologies and will be discussed in week 5. 100 00:09:11,810 --> 00:09:19,010 An alternative thin-film PV technology is based on II-VI semiconductor, 101 00:09:19,750 --> 00:09:21,310 the cadmium telluride (CdTe). 102 00:09:21,310 --> 00:09:26,900 CdTe PV technology belongs to the so-called second generation technologies as well. 103 00:09:27,200 --> 00:09:33,780 The CdTe has currently the largest market among the thin-film PV technologies. 104 00:09:33,780 --> 00:09:40,080 In week 5, we will discuss this technology in great detail. 105 00:09:41,630 --> 00:09:47,810 Another thin-film PV technology, based on a chalcogenide alloy is CIGS, 106 00:09:47,810 --> 00:09:52,370 which stands for copper indium gallium selenide. 107 00:09:52,370 --> 00:09:57,360 Among the thin-film PV technologies, it has the highest demonstrated conversion efficiency 108 00:09:57,360 --> 00:09:59,670 on lab scale, just above 20%. 109 00:09:59,670 --> 00:10:04,920 It belongs to the second generation PV technologies as well. 110 00:10:04,920 --> 00:10:11,070 Another thin-film PV technology is based on organics, also referred to as the plastic solar cell. 111 00:10:11,840 --> 00:10:17,700 The absorption and charge transport in the solar cell occurs in conductive organic polymers 112 00:10:17,700 --> 00:10:19,960 or molecules. 113 00:10:19,960 --> 00:10:26,960 The dye-sensitized solar cell is a kind of photoelectrochemical system, in which a semiconductor 114 00:10:27,370 --> 00:10:34,770 material based on molecular sensitizers is placed between a photo-anode and an electrolyte. 115 00:10:34,900 --> 00:10:41,900 We will discuss both the organic and dye-sensitized PV technology in week 5. 116 00:10:42,070 --> 00:10:47,750 The final PV technology we will discuss is based on III-V semiconductor materials 117 00:10:47,750 --> 00:10:49,600 such as gallium arsenide (GaAs). 118 00:10:49,600 --> 00:10:56,600 III-V materials are being used in multi-junction devices, often processed on germanium wafers 119 00:10:56,870 --> 00:10:59,340 as substrate. 120 00:10:59,340 --> 00:11:05,520 The multi-junction based on III-V semiconductors are the most efficient solar cells today. 121 00:11:05,520 --> 00:11:13,570 The record conversion efficiency of 44% was obtained with a metamorphic triple junction in 2012. 122 00:11:13,890 --> 00:11:18,940 The III-V semiconductor solar cells are being used in concentrator PV technology 123 00:11:18,940 --> 00:11:21,390 and in space applications. 124 00:11:21,490 --> 00:11:25,479 This technology will be discussed in week 5. 125 00:11:25,479 --> 00:11:32,600 Third generation PV technologies are based on various concepts, trying to beat the Shockley-Queisser limit. 126 00:11:32,600 --> 00:11:38,779 We will discuss third generation concepts in great detail in week 6. 127 00:11:38,779 --> 00:11:45,290 Third generation PV technology covers a wide range of novel and innovative ideas, 128 00:11:45,290 --> 00:11:49,660 the most successful being multi-junctions. 129 00:11:49,860 --> 00:11:54,040 You have to realize that most of these ideas still need to be proven. 130 00:11:54,040 --> 00:12:02,740 Some of these ideas are quantum dots solar cells, absorber layers exhibiting multiple-exciton-generation, 131 00:12:02,870 --> 00:12:07,770 intermediate bandgap solar cells, hot carrier solar cells, 132 00:12:07,820 --> 00:12:12,950 spectral-conversion using down-converters or up-converters. 133 00:12:13,560 --> 00:12:19,060 Finally, I show you here the famous chart of NREL. 134 00:12:19,060 --> 00:12:25,740 It summarizes the worldwide research effort of the last 40 years and it shows the current 135 00:12:25,740 --> 00:12:29,970 record efficiencies of solar cells at research scale. 136 00:12:29,970 --> 00:12:36,970 These solar cells are fabricated in a lab environment and have very small sizes, 137 00:12:36,970 --> 00:12:41,610 often not larger than 1 square centimeter. 138 00:12:42,300 --> 00:12:48,120 The purple colored markers represent the III-V technology based on single, double and 139 00:12:48,120 --> 00:12:57,120 triple junctions and have efficiencies ranging from 26% up to 44% under concentrated light conditions. 140 00:12:58,590 --> 00:13:05,120 The blue lines and dots represent the crystalline silicon technology based on monocrystalline 141 00:13:05,120 --> 00:13:08,240 and multicrystalline silicon. 142 00:13:08,240 --> 00:13:15,240 The record efficiency ranges from 20% up to 25% under standard 1 sun illumination conditions 143 00:13:15,500 --> 00:13:21,050 and 27% can be achieved under 92 suns illumination. 144 00:13:21,850 --> 00:13:29,410 The inorganic thin-film technologies, like thin-film silicon, CdTe and CIGS are indicated 145 00:13:29,410 --> 00:13:35,760 by the green markers and their record efficiencies range from 13.4 % up to 20%. 146 00:13:35,760 --> 00:13:41,760 The red colored lines and markers indicate the emerging PV technologies, 147 00:13:41,760 --> 00:13:44,529 like organic solar cells. 148 00:13:44,529 --> 00:13:51,330 You have to be aware that these are lab results of very small area solar cells and this chart 149 00:13:51,330 --> 00:13:57,680 does not tell you anything of the long term stability of some PV technologies, 150 00:13:57,680 --> 00:14:01,250 certainly not the ones indicated in red. 151 00:14:01,250 --> 00:14:07,430 They indicate the potential efficiency of many PV technologies. 152 00:14:07,430 --> 00:14:13,410 Nevertheless, you have to realize that most PV technologies still have a large gap between 153 00:14:13,410 --> 00:14:22,910 the record conversion efficiency of the lab cells and the conversion efficiency of large commercial modules. 154 00:14:23,000 --> 00:14:28,200 It will be my pleasure to discuss these challenges in weeks 4 and 5. 155 00:14:28,200 --> 00:14:33,490 What is the history of solar energy technology and what is the current status of PV technologies 156 00:14:33,490 --> 00:14:35,210 in the real world? 157 00:14:35,210 --> 00:14:39,380 I mean the world of large area modules. 158 00:14:39,380 --> 00:14:45,010 I will discuss that in the next two blocks. I will begin with the history.