1 00:00:00,060 --> 00:00:02,500 The following content is provided under a Creative 2 00:00:02,500 --> 00:00:04,010 Commons license. 3 00:00:04,010 --> 00:00:06,350 Your support will help MIT OpenCourseWare 4 00:00:06,350 --> 00:00:10,720 continue to offer high quality educational resources for free. 5 00:00:10,720 --> 00:00:13,340 To make a donation or view additional materials 6 00:00:13,340 --> 00:00:17,209 from hundreds of MIT courses, visit MIT OpenCourseWare 7 00:00:17,209 --> 00:00:17,834 at ocw.mit.edu. 8 00:00:24,967 --> 00:00:26,550 PROFESSOR: But today we're going to be 9 00:00:26,550 --> 00:00:28,500 talking about crystalline silicon solar cells. 10 00:00:28,500 --> 00:00:32,460 Now, for those of you who do not work in crystalline silicon PV, 11 00:00:32,460 --> 00:00:33,970 the reason this topic is important 12 00:00:33,970 --> 00:00:37,010 is because crystalline silicon comprises about 90% 13 00:00:37,010 --> 00:00:39,110 of all solar cells manufactured today. 14 00:00:39,110 --> 00:00:41,195 It's the dominant technology, and the technologies 15 00:00:41,195 --> 00:00:43,070 that you're working on are going to displace, 16 00:00:43,070 --> 00:00:45,110 or are aiming to displace crystalline silicon, 17 00:00:45,110 --> 00:00:46,829 so it's good to know your enemy. 18 00:00:46,829 --> 00:00:48,870 For those who are working on crystalline silicon, 19 00:00:48,870 --> 00:00:51,017 this is meant to be a background of all 20 00:00:51,017 --> 00:00:52,850 of the different aspects-- the entire supply 21 00:00:52,850 --> 00:00:54,600 chain of crystalline silicon-- so that you 22 00:00:54,600 --> 00:00:56,840 gain insight into the areas that you're not currently 23 00:00:56,840 --> 00:00:57,890 focused on. 24 00:00:57,890 --> 00:01:00,880 You're getting a perspective of the bigger picture. 25 00:01:00,880 --> 00:01:04,989 Crystalline silicon PV has been around since 1954. 26 00:01:04,989 --> 00:01:08,160 The original-- well, in its current incarnation. 27 00:01:08,160 --> 00:01:11,070 That was when Bell Laboratories announced 28 00:01:11,070 --> 00:01:14,690 the development of the modern crystalline silicon PV cell, 29 00:01:14,690 --> 00:01:17,810 and that was 6% efficiency in 1954, 30 00:01:17,810 --> 00:01:19,490 published in general applied physics, 31 00:01:19,490 --> 00:01:23,460 and the cell architecture, it's obviously 32 00:01:23,460 --> 00:01:24,940 evolved over the years but it's not 33 00:01:24,940 --> 00:01:27,870 entirely dissimilar from what we have today 34 00:01:27,870 --> 00:01:32,740 as a cell architecture for our modern PV cells. 35 00:01:32,740 --> 00:01:35,160 So, over the course of-- it's almost 36 00:01:35,160 --> 00:01:37,680 been 60 years of development of crystalline silicon 37 00:01:37,680 --> 00:01:39,660 photovoltaic technology. 38 00:01:39,660 --> 00:01:42,060 That means both the cell itself, the materials 39 00:01:42,060 --> 00:01:44,570 that go into it, and also the manufacturing, or the methods 40 00:01:44,570 --> 00:01:46,995 to produce said materials and device, 41 00:01:46,995 --> 00:01:48,370 over the course of those, almost, 42 00:01:48,370 --> 00:01:51,010 60 years much innovation has happened 43 00:01:51,010 --> 00:01:54,780 both in terms of manufacturing and technology. 44 00:01:54,780 --> 00:01:59,100 So today, we'll be going over kind of a status quo 45 00:01:59,100 --> 00:02:02,980 snapshot of where crystalline silicon stands and we brought 46 00:02:02,980 --> 00:02:05,210 in a number of show and tell items 47 00:02:05,210 --> 00:02:09,660 so that you can see as we talk. 48 00:02:12,790 --> 00:02:14,780 So just for the show and tell, we're 49 00:02:14,780 --> 00:02:17,340 going to be moving from the feedstock materials over here 50 00:02:17,340 --> 00:02:20,615 finally into wafers and cells on that side. 51 00:02:20,615 --> 00:02:21,350 All right. 52 00:02:23,890 --> 00:02:27,290 So, these lecture notes are going 53 00:02:27,290 --> 00:02:29,090 to be valid for both 10 and 11. 54 00:02:29,090 --> 00:02:32,140 We're going to split this up over two classes to really 55 00:02:32,140 --> 00:02:34,364 dive into some of the details. 56 00:02:34,364 --> 00:02:35,780 The first question is why silicon? 57 00:02:35,780 --> 00:02:38,989 Why did silicon evolve as what is currently 58 00:02:38,989 --> 00:02:40,780 the dominant technology, which is currently 59 00:02:40,780 --> 00:02:44,340 90 percent of the PV market, and I think it boils down 60 00:02:44,340 --> 00:02:47,770 to a couple of reasons. 61 00:02:47,770 --> 00:02:49,750 One is scalability. 62 00:02:49,750 --> 00:02:53,680 If you look at the elemental abundance, on the vertical axis 63 00:02:53,680 --> 00:02:56,210 it's abundance, atoms of the element per 10 64 00:02:56,210 --> 00:02:58,955 to the 16 atoms of silicon. 65 00:02:58,955 --> 00:03:01,080 The reason that everything is normalized to silicon 66 00:03:01,080 --> 00:03:04,565 is because there is, well, quite a lot of it 67 00:03:04,565 --> 00:03:05,440 in the earth's crust. 68 00:03:05,440 --> 00:03:08,620 As you can see, it's the second most abundant element 69 00:03:08,620 --> 00:03:10,550 on the Earth's crust. 70 00:03:10,550 --> 00:03:14,260 It just so happens that, out of all the stardust that is here 71 00:03:14,260 --> 00:03:17,630 on the planet, we have a high percentage of silicon 72 00:03:17,630 --> 00:03:19,710 like the moon and like many other planets 73 00:03:19,710 --> 00:03:23,250 in our solar system-- at least the hard ones. 74 00:03:23,250 --> 00:03:24,817 You can see oxygen is probably the, 75 00:03:24,817 --> 00:03:27,400 well, oxygen is the only element with higher natural abundance 76 00:03:27,400 --> 00:03:30,660 in the earth's crust, the upper crust, than silicon 77 00:03:30,660 --> 00:03:32,820 and we go down as we go to higher 78 00:03:32,820 --> 00:03:34,330 and higher atomic number. 79 00:03:34,330 --> 00:03:38,280 The probability of formation due to subsequent fusion reactions 80 00:03:38,280 --> 00:03:40,560 in stars decreases and, hence, it 81 00:03:40,560 --> 00:03:43,080 follows this almost a power law distribution 82 00:03:43,080 --> 00:03:44,530 as you can see there. 83 00:03:44,530 --> 00:03:46,747 So it's scalable. 84 00:03:46,747 --> 00:03:48,830 It is present in the Earth in high enough capacity 85 00:03:48,830 --> 00:03:50,210 to reach terawatt scales. 86 00:03:50,210 --> 00:03:53,890 It's nontoxic and, as Don Sadoway likes to say, 87 00:03:53,890 --> 00:03:55,700 if you want batteries dirt-cheap, 88 00:03:55,700 --> 00:03:57,400 you have to make them out of dirt. 89 00:03:57,400 --> 00:03:59,730 A similar expression is used in the crystalline silicon 90 00:03:59,730 --> 00:04:00,650 community. 91 00:04:00,650 --> 00:04:03,630 I believe the quote in 1366 is, "It's not 92 00:04:03,630 --> 00:04:06,770 only good for the planet, it is the planet." 93 00:04:06,770 --> 00:04:09,250 A variety of riffs off of this particular chart 94 00:04:09,250 --> 00:04:11,550 right here, but from a technological point of view, 95 00:04:11,550 --> 00:04:14,530 why did silicon evolve to the point where it is today? 96 00:04:14,530 --> 00:04:17,589 It forms a very tenacious surface oxide. 97 00:04:17,589 --> 00:04:21,399 So, if you were to expose a piece of pure silicon to air, 98 00:04:21,399 --> 00:04:26,190 the surface oxide that forms is very, very strong and very 99 00:04:26,190 --> 00:04:28,410 resistant, and very dense. 100 00:04:28,410 --> 00:04:30,920 So, unlike some materials that corrode 101 00:04:30,920 --> 00:04:33,240 when exposed to atmosphere, silicon 102 00:04:33,240 --> 00:04:37,180 oxidizes maybe the first few 10s of angstroms, 100 of angstroms, 103 00:04:37,180 --> 00:04:42,530 and then it peters out so it's diffusion-limited oxide growth 104 00:04:42,530 --> 00:04:44,470 mechanism that eventually stabilizes 105 00:04:44,470 --> 00:04:46,760 at a very thin but very dense and very protective 106 00:04:46,760 --> 00:04:48,000 oxide layer. 107 00:04:48,000 --> 00:04:51,340 So the risk of having a silicon wafer degrade inside 108 00:04:51,340 --> 00:04:52,901 of a solar module is very low. 109 00:04:52,901 --> 00:04:55,150 Furthermore, that oxide layer from an electrical point 110 00:04:55,150 --> 00:04:56,820 of view it's very passivating. 111 00:04:56,820 --> 00:04:58,710 So as we studied on, as we solved 112 00:04:58,710 --> 00:05:01,800 in the exam, those interface states or those surface states, 113 00:05:01,800 --> 00:05:03,450 the surface of semiconductor, those 114 00:05:03,450 --> 00:05:05,480 can be reduced or minimized by the presence 115 00:05:05,480 --> 00:05:07,600 of certain passivating layers, and it just so 116 00:05:07,600 --> 00:05:11,610 happens that by the benevolence of nature, 117 00:05:11,610 --> 00:05:15,310 the silicon oxide, which is shown in these red triangles 118 00:05:15,310 --> 00:05:19,980 right here, has a very low surface recombination 119 00:05:19,980 --> 00:05:22,560 velocity, passivates a surface very well, 120 00:05:22,560 --> 00:05:24,880 and results in high-performing devices. 121 00:05:24,880 --> 00:05:27,570 In this particular case, they're plotting emitter saturation 122 00:05:27,570 --> 00:05:30,150 current density in femtoamps per centimeter 123 00:05:30,150 --> 00:05:33,290 squared-- this is very, very low-- versus sheet resistance. 124 00:05:33,290 --> 00:05:36,030 This is essentially the dopant concentration in the emitter, 125 00:05:36,030 --> 00:05:38,830 so they're looking at how the passivation quality changes 126 00:05:38,830 --> 00:05:41,560 as a function of dopant density and silicon oxide 127 00:05:41,560 --> 00:05:44,340 works pretty well, and it's an effective diffusion barrier. 128 00:05:44,340 --> 00:05:46,020 And, probably most significantly, those 129 00:05:46,020 --> 00:05:49,220 are maybe one looking forward rationale 130 00:05:49,220 --> 00:05:52,940 one technological or scientific rationale 131 00:05:52,940 --> 00:05:54,890 and as far as the field is concerned, 132 00:05:54,890 --> 00:05:57,320 as far as engineering community is concerned, 133 00:05:57,320 --> 00:05:59,040 silicon has a lot of momentum. 134 00:05:59,040 --> 00:06:02,290 It's the most common semiconductor material, silicon 135 00:06:02,290 --> 00:06:05,280 and germanium were both purified, more or less, 136 00:06:05,280 --> 00:06:08,930 around the same decades but, because silicon has a wider 137 00:06:08,930 --> 00:06:12,210 band gap, you have a lower thermal carrier concentration, 138 00:06:12,210 --> 00:06:13,800 lower intrinsic carrier concentration, 139 00:06:13,800 --> 00:06:15,920 folks were able to make transistors and devices 140 00:06:15,920 --> 00:06:19,130 with lower noise out of silicon as opposed to germanium 141 00:06:19,130 --> 00:06:22,230 and silicon technology really took off 142 00:06:22,230 --> 00:06:25,330 in terms of the PV industry benefited a lot 143 00:06:25,330 --> 00:06:26,551 by that cross-pollination. 144 00:06:26,551 --> 00:06:28,800 Many technologies came in from the integrated circuits 145 00:06:28,800 --> 00:06:32,336 industry to assist or give a boost to the PV industry. 146 00:06:32,336 --> 00:06:33,710 This number is a little outdated, 147 00:06:33,710 --> 00:06:34,876 it's now about $100 billion. 148 00:06:34,876 --> 00:06:39,840 Hard to keep up with things growing at 68% a year. 149 00:06:39,840 --> 00:06:44,320 Technology acceptance results in lower interest rates. 150 00:06:44,320 --> 00:06:47,080 So if you have a technology that is 151 00:06:47,080 --> 00:06:50,160 well-accepted by the market then you go to a bank and say, 152 00:06:50,160 --> 00:06:51,960 hey, I want to install some of those things 153 00:06:51,960 --> 00:06:53,751 and the bank says what are those things you 154 00:06:53,751 --> 00:06:55,330 say oh, hundreds of thousands of them 155 00:06:55,330 --> 00:06:56,180 have been installed already. 156 00:06:56,180 --> 00:06:56,700 It's OK. 157 00:06:56,700 --> 00:06:57,610 It's a proven technology. 158 00:06:57,610 --> 00:06:59,651 The bank says OK, I'll lower your interest rates. 159 00:06:59,651 --> 00:07:01,790 That means you pay less money on interest. 160 00:07:01,790 --> 00:07:04,030 Your capital is more cheap. 161 00:07:04,030 --> 00:07:07,490 It works better in your favor, and the opposite 162 00:07:07,490 --> 00:07:10,170 is true with an entirely new technology that's unproven. 163 00:07:10,170 --> 00:07:13,170 So that's really summing up why silicon. 164 00:07:13,170 --> 00:07:15,490 Momentum, forward motion if you will, 165 00:07:15,490 --> 00:07:18,500 some inherent intrinsic technological advantages, 166 00:07:18,500 --> 00:07:21,030 some of which are listed here, and I'll 167 00:07:21,030 --> 00:07:23,067 get to that in a second, scalability. 168 00:07:23,067 --> 00:07:24,900 To get back to the technological advantages, 169 00:07:24,900 --> 00:07:27,290 I think it's important to recognize what they are so 170 00:07:27,290 --> 00:07:30,350 that when you're thinking of a new material, 171 00:07:30,350 --> 00:07:32,492 you can cross check and say, gee, do I have these 172 00:07:32,492 --> 00:07:33,450 or do I not have these. 173 00:07:33,450 --> 00:07:35,040 If I don't have them, it's not the end of the world. 174 00:07:35,040 --> 00:07:37,530 You might have other advantages that overcome the ones 175 00:07:37,530 --> 00:07:40,740 that silicon doesn't have. 176 00:07:40,740 --> 00:07:43,360 Let's add some more into this list. 177 00:07:43,360 --> 00:07:45,940 Just stream of consciousness. 178 00:07:45,940 --> 00:07:48,390 Silicon has a very high refractive index 179 00:07:48,390 --> 00:07:50,030 near the band gap edge. 180 00:07:50,030 --> 00:07:52,330 So, near the band gap edge, it's absorbing light 181 00:07:52,330 --> 00:07:53,170 less efficiently. 182 00:07:53,170 --> 00:07:53,670 Right? 183 00:07:53,670 --> 00:07:56,660 It has a larger attenuation length 184 00:07:56,660 --> 00:07:59,350 of the light, a smaller optical absorption coefficient right 185 00:07:59,350 --> 00:08:01,180 as you approach the band gap. 186 00:08:01,180 --> 00:08:04,200 So silicon absorbs poorly in the infrared because it's 187 00:08:04,200 --> 00:08:05,840 an indirect band gap semiconductor, 188 00:08:05,840 --> 00:08:08,730 but it also has a very large optical, 189 00:08:08,730 --> 00:08:10,720 sorry, a very large real component 190 00:08:10,720 --> 00:08:12,355 of the refractive index. 191 00:08:12,355 --> 00:08:15,020 Does anybody remember what that refers to? 192 00:08:15,020 --> 00:08:17,360 Real component of refractive index. 193 00:08:17,360 --> 00:08:18,540 Lesson number two. 194 00:08:18,540 --> 00:08:20,185 What does that dictate? 195 00:08:20,185 --> 00:08:21,060 AUDIENCE: Reflection. 196 00:08:21,060 --> 00:08:22,390 PROFESSOR: Reflection, exactly. 197 00:08:22,390 --> 00:08:25,691 So, if I were to tailor and index of refraction grading 198 00:08:25,691 --> 00:08:27,190 on the front side of my device, so I 199 00:08:27,190 --> 00:08:30,470 allow the light to be absorbed efficiently, on the backside 200 00:08:30,470 --> 00:08:33,230 I can put a very large index of refraction mismatch 201 00:08:33,230 --> 00:08:35,020 so that the light bounces back. 202 00:08:35,020 --> 00:08:36,929 In other words, the light trapping silicon 203 00:08:36,929 --> 00:08:38,320 is benefited by the fact that you 204 00:08:38,320 --> 00:08:41,289 have this awesome reflection capability. 205 00:08:41,289 --> 00:08:43,319 The refractive index is around 3.6, 206 00:08:43,319 --> 00:08:45,110 the real component of the refractive index, 207 00:08:45,110 --> 00:08:47,600 in the infrared at around 1070 nanometers. 208 00:08:47,600 --> 00:08:49,590 Which means that if you design your cell right, 209 00:08:49,590 --> 00:08:51,714 you can get an extension of the optical path length 210 00:08:51,714 --> 00:08:54,700 by a factor of 50 over the thickness. 211 00:08:54,700 --> 00:08:56,414 So if your thickness of the device is d, 212 00:08:56,414 --> 00:08:58,330 the optical path length can be increased up to 213 00:08:58,330 --> 00:09:01,700 about [? 51d. ?] That's as a result 214 00:09:01,700 --> 00:09:03,880 of this great reflectance. 215 00:09:03,880 --> 00:09:07,090 Many other materials that are being explored as PV materials 216 00:09:07,090 --> 00:09:08,760 have refractive indices around two, 217 00:09:08,760 --> 00:09:10,676 which would mean that your optical path length 218 00:09:10,676 --> 00:09:12,080 extension is around 16. 219 00:09:12,080 --> 00:09:13,580 So that's one thing to keep in mind. 220 00:09:13,580 --> 00:09:15,663 even though it doesn't absorb light quite as well, 221 00:09:15,663 --> 00:09:16,870 it traps light fairly well. 222 00:09:16,870 --> 00:09:19,310 Another advantage of silicon is that it 223 00:09:19,310 --> 00:09:23,240 forms sp3 hybridized orbitals, for chemists, it 224 00:09:23,240 --> 00:09:25,207 forms-- it's tetrahedrally coordinated, 225 00:09:25,207 --> 00:09:27,040 in other words bond to four other neighbors, 226 00:09:27,040 --> 00:09:30,000 and most 3D transition metals don't do that. 227 00:09:30,000 --> 00:09:32,020 They don't bond in that configuration. 228 00:09:32,020 --> 00:09:35,890 Some do but many don't and, as a result, 229 00:09:35,890 --> 00:09:39,210 the solid's solubility in other words, the ability 230 00:09:39,210 --> 00:09:42,280 to incorporate impurities into a growing silicon crystal is low. 231 00:09:42,280 --> 00:09:45,790 It rejects the impurities from the solid into the melt, 232 00:09:45,790 --> 00:09:48,552 and you're able to purify the material very efficiently. 233 00:09:48,552 --> 00:09:50,510 That's not always the case with most materials. 234 00:09:50,510 --> 00:09:52,670 Sometimes they incorporate impurities very readily, 235 00:09:52,670 --> 00:09:54,296 up to a few atomic percents. 236 00:09:54,296 --> 00:09:56,170 The typical impurity concentration of silicon 237 00:09:56,170 --> 00:09:58,870 is in the order of parts per million, parts per billion, 238 00:09:58,870 --> 00:09:59,820 parts per trillion. 239 00:09:59,820 --> 00:10:01,660 Still can be enough, as you learned during your homework 240 00:10:01,660 --> 00:10:03,785 assignments, still could be enough to affect device 241 00:10:03,785 --> 00:10:06,220 performance but is very low. 242 00:10:06,220 --> 00:10:08,050 It would be a lot worse if silicon 243 00:10:08,050 --> 00:10:10,710 were able to absorb more impurities and so forth. 244 00:10:10,710 --> 00:10:14,120 So, there are a number of reasons why the silicon PV 245 00:10:14,120 --> 00:10:18,150 technology has gained the foothold that it has so 246 00:10:18,150 --> 00:10:21,000 to bump it out of its leadership position, 247 00:10:21,000 --> 00:10:24,290 one really has to be clever and the parameter of merit 248 00:10:24,290 --> 00:10:27,360 is performance per unit cost. 249 00:10:27,360 --> 00:10:31,340 Kilowatt hours per dollar, if you will. 250 00:10:31,340 --> 00:10:33,940 So, we're going to talk about the current manufacturing 251 00:10:33,940 --> 00:10:36,290 methods and materials because this will give you 252 00:10:36,290 --> 00:10:39,240 an insight into the dollars per kilowatt 253 00:10:39,240 --> 00:10:40,740 hour, the kilowatt hours per dollar. 254 00:10:40,740 --> 00:10:44,110 Essentially, the cost per unit energy produced. 255 00:10:44,110 --> 00:10:45,790 You can begin to seize opportunities 256 00:10:45,790 --> 00:10:48,331 within the crystal silicon world to improve the manufacturing 257 00:10:48,331 --> 00:10:50,560 process or you can begin to say OK, you know what, 258 00:10:50,560 --> 00:10:51,982 this is way too complicated. 259 00:10:51,982 --> 00:10:53,690 Let me take a completely different route. 260 00:10:53,690 --> 00:10:55,270 I'm going to develop a new technology instead 261 00:10:55,270 --> 00:10:57,436 that will overcome these manufacturing difficulties. 262 00:10:57,436 --> 00:10:59,090 So let's explore them in detail. 263 00:10:59,090 --> 00:11:00,950 First, the market. 264 00:11:00,950 --> 00:11:06,300 This is the evolution of market share from 1980 to mid 2000s. 265 00:11:06,300 --> 00:11:10,620 After mid 2000s, the market just continues growing at 68% a year 266 00:11:10,620 --> 00:11:12,570 and you really lose resolution to this portion 267 00:11:12,570 --> 00:11:16,660 down here so it's to 2006 so that we can actually see what's 268 00:11:16,660 --> 00:11:19,150 going on in the earlier days. 269 00:11:19,150 --> 00:11:22,110 In the earlier days, 1980, let's pick 1985, 270 00:11:22,110 --> 00:11:24,910 the market was split about a third-third-third between 271 00:11:24,910 --> 00:11:27,730 thin films, amorphous silicon namely, 272 00:11:27,730 --> 00:11:29,540 monocrystalline silicon, and a material 273 00:11:29,540 --> 00:11:31,280 called multicrystalline silicon. 274 00:11:31,280 --> 00:11:32,630 Now let's go piece by piece. 275 00:11:32,630 --> 00:11:36,320 What is monocrystalline silicon, multicrystalline silicon 276 00:11:36,320 --> 00:11:37,270 and thin films? 277 00:11:37,270 --> 00:11:40,230 Well thin films are materials that are usually 278 00:11:40,230 --> 00:11:43,530 between a few hundred nanometers up to about three, 279 00:11:43,530 --> 00:11:45,410 maybe five microns thick. 280 00:11:45,410 --> 00:11:47,070 To give you size perspective, your hair 281 00:11:47,070 --> 00:11:48,850 is about 50 microns in diameter, so we're 282 00:11:48,850 --> 00:11:51,480 talking about 1/50 the width of your hair. 283 00:11:51,480 --> 00:11:53,590 That's the active absorber layer and of course 284 00:11:53,590 --> 00:11:55,470 the plastics and encapsulates and everything else that 285 00:11:55,470 --> 00:11:57,150 go around them make it a bit thicker, 286 00:11:57,150 --> 00:11:59,130 but the absorber layer is very thin 287 00:11:59,130 --> 00:12:02,434 and so you're not spending much on your absorber layer. 288 00:12:02,434 --> 00:12:03,850 It absorbs light very efficiently, 289 00:12:03,850 --> 00:12:07,240 has a very large absorption coefficient, 290 00:12:07,240 --> 00:12:09,490 and is able to absorb photons efficiently. 291 00:12:09,490 --> 00:12:11,751 Crystalline silicon, on the other hand, 292 00:12:11,751 --> 00:12:14,125 does not absorb light as well as many thin film materials 293 00:12:14,125 --> 00:12:15,930 so we need about an order of magnitude 294 00:12:15,930 --> 00:12:19,380 to two orders of magnitude thicker substrates, 295 00:12:19,380 --> 00:12:22,212 and the crystalline silicon substrates today 296 00:12:22,212 --> 00:12:23,920 in commercial manufacturing are typically 297 00:12:23,920 --> 00:12:30,150 between 160 to 190 microns, with an average around 170, 180. 298 00:12:30,150 --> 00:12:32,870 So about four times the thickness of your hair. 299 00:12:32,870 --> 00:12:34,887 Monocrystalline silicon and multi. 300 00:12:34,887 --> 00:12:36,470 Let's talk about the difference there. 301 00:12:36,470 --> 00:12:38,670 So, monocrystalline silicon, folks 302 00:12:38,670 --> 00:12:41,390 are probably familiar seeing pictures, at least something 303 00:12:41,390 --> 00:12:41,890 like this. 304 00:12:41,890 --> 00:12:42,730 Right? 305 00:12:42,730 --> 00:12:46,900 So this right here is an example of a Cherkofsky silicon wafer. 306 00:12:49,690 --> 00:12:51,980 Appropriate for integrated circuit work. 307 00:12:51,980 --> 00:12:54,780 I'll pass this around so folks can get a sense. 308 00:12:54,780 --> 00:12:59,400 So this is an example of a monocrystalline silicon 309 00:12:59,400 --> 00:13:01,730 wafer for the integrated circuits industry. 310 00:13:01,730 --> 00:13:04,020 Let's analyze it in a little bit more detail. 311 00:13:04,020 --> 00:13:07,757 So, the front surface is polished, nicely polished. 312 00:13:07,757 --> 00:13:09,590 Polished to, I think, somewhere in the order 313 00:13:09,590 --> 00:13:12,280 of a few nanometers mean surface roughness. 314 00:13:12,280 --> 00:13:14,800 Using a chemical mechanical polishing mechanism. 315 00:13:14,800 --> 00:13:18,840 The thickness is around 700-- or 675 microns. 316 00:13:18,840 --> 00:13:20,120 Somewhere in that range. 317 00:13:20,120 --> 00:13:21,710 So very, very thick wafer. 318 00:13:21,710 --> 00:13:23,690 The objective is not to break. 319 00:13:23,690 --> 00:13:24,260 Right? 320 00:13:24,260 --> 00:13:25,830 If you're making integrated circuit, 321 00:13:25,830 --> 00:13:27,450 this entire wafer that I'm holding right here 322 00:13:27,450 --> 00:13:29,580 could be worth a few 10s or 100s thousands of dollars 323 00:13:29,580 --> 00:13:31,220 by the end of the processing sequence, 324 00:13:31,220 --> 00:13:33,594 so if one of these breaks, that's an awful lot of revenue 325 00:13:33,594 --> 00:13:34,680 that the company's losing. 326 00:13:34,680 --> 00:13:36,812 So the substrate is thick because they 327 00:13:36,812 --> 00:13:37,770 don't want it to break. 328 00:13:37,770 --> 00:13:40,450 Silicon is brittle at room temperature. 329 00:13:40,450 --> 00:13:42,660 If you were to manufacture solar cell out of this, 330 00:13:42,660 --> 00:13:44,210 you could but it would be very expensive. 331 00:13:44,210 --> 00:13:45,584 The chemical mechanical polishing 332 00:13:45,584 --> 00:13:47,260 that they use to flatten the surface out 333 00:13:47,260 --> 00:13:49,720 costs a lot of money, it's very time intensive, 334 00:13:49,720 --> 00:13:52,250 and the thickness of the silicon is above and beyond 335 00:13:52,250 --> 00:13:54,290 what is necessary to absorb light well. 336 00:13:54,290 --> 00:13:55,990 If anything, increasing the thickness 337 00:13:55,990 --> 00:13:58,215 is just increasing your emitter saturation current, 338 00:13:58,215 --> 00:14:00,340 since you have a higher recombination current being 339 00:14:00,340 --> 00:14:01,822 driven by bulk recombination. 340 00:14:01,822 --> 00:14:03,280 You have more recombination centers 341 00:14:03,280 --> 00:14:04,821 because you have a greater thickness, 342 00:14:04,821 --> 00:14:06,640 and it's driving a larger diffusion current 343 00:14:06,640 --> 00:14:08,740 from the emitter into the base. 344 00:14:08,740 --> 00:14:11,680 So making it this thick really doesn't make sense. 345 00:14:11,680 --> 00:14:15,310 So I'll pass this around so folks can kind of get a sense. 346 00:14:15,310 --> 00:14:17,390 Make sure this gets the entire round. 347 00:14:17,390 --> 00:14:20,397 I'll be recycling those platens. 348 00:14:20,397 --> 00:14:22,230 Please hold, if you're going to take it out, 349 00:14:22,230 --> 00:14:24,220 which you're welcome to do, please hold it 350 00:14:24,220 --> 00:14:26,900 like a photograph. 351 00:14:26,900 --> 00:14:31,720 What I don't want to have happen is folks put their fingerprints 352 00:14:31,720 --> 00:14:33,430 all over it. 353 00:14:33,430 --> 00:14:38,960 The wafers that are used in the PV industry 354 00:14:38,960 --> 00:14:40,850 are cut from the same ingot like that 355 00:14:40,850 --> 00:14:44,320 one, except that the ingots, essentially, 356 00:14:44,320 --> 00:14:47,770 if you were to pack circular wafers into a module, 357 00:14:47,770 --> 00:14:49,890 it would look something like this. 358 00:14:49,890 --> 00:14:52,259 Here's your module and, mind you, 359 00:14:52,259 --> 00:14:54,050 you're spending a lot of money on the glass 360 00:14:54,050 --> 00:14:56,760 and the encapsulates and the aluminum framing 361 00:14:56,760 --> 00:15:01,469 and so forth, and now your solar cells look like that. 362 00:15:01,469 --> 00:15:03,760 There's probably more of them that you can put in here, 363 00:15:03,760 --> 00:15:05,760 but what do you notice about this? 364 00:15:05,760 --> 00:15:07,810 What is the packing density, or packing fraction. 365 00:15:07,810 --> 00:15:08,830 It's very low, right? 366 00:15:08,830 --> 00:15:10,746 You're losing all of this material in between. 367 00:15:10,746 --> 00:15:13,370 All that space is just going to be blank space. 368 00:15:13,370 --> 00:15:16,690 Some of the earliest PV modules actually use circular wafers, 369 00:15:16,690 --> 00:15:18,390 but the more modern ones, what they do 370 00:15:18,390 --> 00:15:20,770 is a very complicated cost analysis 371 00:15:20,770 --> 00:15:29,550 where they say, OK, if I were to chop off the edges of my wafer 372 00:15:29,550 --> 00:15:32,330 and completely remove them, I'd be losing a lot of silicon 373 00:15:32,330 --> 00:15:34,970 but I'd be increasing the packing fraction. 374 00:15:34,970 --> 00:15:37,660 So in the limit that my module materials, the glass, 375 00:15:37,660 --> 00:15:40,080 the encapsulant, the framing materials 376 00:15:40,080 --> 00:15:43,070 are infinitely expensive and my silicon costs nothing, 377 00:15:43,070 --> 00:15:44,740 I want to do this. 378 00:15:44,740 --> 00:15:47,300 In the limit that my module materials are free 379 00:15:47,300 --> 00:15:51,010 and installation is free but the silicon is super expensive, 380 00:15:51,010 --> 00:15:52,980 I want to keep full round wafers, 381 00:15:52,980 --> 00:15:55,210 and the reality is that we're somewhere in between. 382 00:15:55,210 --> 00:15:57,760 And so, instead of making one or the other extreme, 383 00:15:57,760 --> 00:15:59,940 typically what you'll see is something 384 00:15:59,940 --> 00:16:02,750 like this chopped off, like that, 385 00:16:02,750 --> 00:16:06,480 where you have a pseudo-square. 386 00:16:06,480 --> 00:16:09,570 The wafer itself has flat edges on the sides 387 00:16:09,570 --> 00:16:13,000 but it also has kind of pseudo-rounded corners here, 388 00:16:13,000 --> 00:16:15,950 and Joe did we bring any of those in? 389 00:16:15,950 --> 00:16:18,810 The psuedo-squares, the monocrystalline psuedo-squares. 390 00:16:18,810 --> 00:16:20,190 These ones. 391 00:16:20,190 --> 00:16:20,690 OK. 392 00:16:20,690 --> 00:16:21,189 All right. 393 00:16:21,189 --> 00:16:21,776 No worries. 394 00:16:21,776 --> 00:16:23,150 I'll show them to you next class. 395 00:16:23,150 --> 00:16:25,390 So, the idea is to make-- cut it out 396 00:16:25,390 --> 00:16:27,200 of the same ingot as that one right 397 00:16:27,200 --> 00:16:31,550 there, but make it thinner, on the order of 170 microns thick, 398 00:16:31,550 --> 00:16:36,620 and to chop off part of the edge, and how much you chop off 399 00:16:36,620 --> 00:16:40,060 depends on the dynamic pricing of silicon versus module 400 00:16:40,060 --> 00:16:42,660 materials and installation and whether or not 401 00:16:42,660 --> 00:16:44,230 you can sell the module, if there's 402 00:16:44,230 --> 00:16:45,700 a certain threshold of performance 403 00:16:45,700 --> 00:16:47,908 that it needs to reach because obviously, if you have 404 00:16:47,908 --> 00:16:50,980 a bunch of dead space in here, you're losing that to-- you're 405 00:16:50,980 --> 00:16:52,430 not producing power out of that. 406 00:16:52,430 --> 00:16:55,230 So if somebody wants a module that's yay efficient, 407 00:16:55,230 --> 00:16:57,440 you might want to increase the packing density. 408 00:16:57,440 --> 00:16:59,490 So that's monocrystalline silicon. 409 00:16:59,490 --> 00:17:01,590 Multicrystalline silicon. 410 00:17:01,590 --> 00:17:03,110 Let's put it this way for now. 411 00:17:03,110 --> 00:17:05,026 We'll describe how multicrystalline silicon is 412 00:17:05,026 --> 00:17:06,569 made, but for now I'm going to say 413 00:17:06,569 --> 00:17:11,180 that multicrystalline silicon is a crystalline silicon 414 00:17:11,180 --> 00:17:14,099 variety that is comprised of many small grains. 415 00:17:14,099 --> 00:17:16,950 So if you look at a multicrystalline silicon wafer, 416 00:17:16,950 --> 00:17:19,250 something like, let's say, oh this 417 00:17:19,250 --> 00:17:21,430 is a perfect example right in here. 418 00:17:21,430 --> 00:17:23,720 If you look at a multicrystalline silicon wafer, 419 00:17:23,720 --> 00:17:30,050 you can see that it looks nice and-- here maybe, that's 420 00:17:30,050 --> 00:17:31,670 probably an OK view of it. 421 00:17:31,670 --> 00:17:32,920 You can see individual grains. 422 00:17:32,920 --> 00:17:33,510 Right? 423 00:17:33,510 --> 00:17:35,600 If you look closely at it. 424 00:17:35,600 --> 00:17:38,520 And those are grains of crystalline material 425 00:17:38,520 --> 00:17:40,387 that are joined by grain boundaries. 426 00:17:40,387 --> 00:17:41,970 So the grain orientation in one region 427 00:17:41,970 --> 00:17:43,780 might be pointing in this direction, the grain 428 00:17:43,780 --> 00:17:45,240 orientation in the neighboring region like that, 429 00:17:45,240 --> 00:17:47,220 and they come together at a grain boundary 430 00:17:47,220 --> 00:17:50,220 and, when we have polycrystalline materials 431 00:17:50,220 --> 00:17:55,310 like this, it's generally indicative of some faster 432 00:17:55,310 --> 00:17:58,550 growth that didn't allow for a nice homogeneous 433 00:17:58,550 --> 00:18:00,074 single crystal material to evolve, 434 00:18:00,074 --> 00:18:02,490 and that's indeed what happens during the multicrystalline 435 00:18:02,490 --> 00:18:03,680 silicon ingot growth. 436 00:18:03,680 --> 00:18:08,140 It's occurring under a slightly modified growth condition then, 437 00:18:08,140 --> 00:18:11,140 say, that beautiful single crystalline piece over there, 438 00:18:11,140 --> 00:18:14,160 and we'll explain how they're made in a second. 439 00:18:14,160 --> 00:18:16,930 So those are the technologies in general, the base absorber 440 00:18:16,930 --> 00:18:19,200 materials, and then there's ribbon silicon which 441 00:18:19,200 --> 00:18:21,180 is a really, really small fraction 442 00:18:21,180 --> 00:18:23,940 of the total production in decreasing, but at one time, 443 00:18:23,940 --> 00:18:27,019 ribbon silicon was viewed as the up and coming technology. 444 00:18:27,019 --> 00:18:29,060 Still today, there are about 20 startup companies 445 00:18:29,060 --> 00:18:31,450 around the United States working on some aspect of this 446 00:18:31,450 --> 00:18:34,569 and probably about a dozen more around the world. 447 00:18:34,569 --> 00:18:35,069 Yeah. 448 00:18:35,069 --> 00:18:36,488 AUDIENCE: I had a question about the multi. 449 00:18:36,488 --> 00:18:37,154 PROFESSOR: Yeah. 450 00:18:37,154 --> 00:18:39,892 AUDIENCE: So for the multi and micro 451 00:18:39,892 --> 00:18:43,110 and poly, is that different grain sizes? 452 00:18:43,110 --> 00:18:43,940 PROFESSOR: Sort of. 453 00:18:43,940 --> 00:18:47,140 So, multicrystalline silicon is a polycrystalline silicon 454 00:18:47,140 --> 00:18:48,000 material. 455 00:18:48,000 --> 00:18:49,750 The definition of multicrystalline silicon 456 00:18:49,750 --> 00:18:53,280 is that the average grain size is about a centimeter squared, 457 00:18:53,280 --> 00:18:56,670 or larger, and that's where multicrystalline came about. 458 00:18:56,670 --> 00:18:59,630 Polycrystalline silicon, in the silicon community, 459 00:18:59,630 --> 00:19:01,290 has a very specific meaning. 460 00:19:01,290 --> 00:19:05,430 It means, usually a plasma-enhanced chemical vapor 461 00:19:05,430 --> 00:19:08,670 deposited layer, so PCVD-deposited layer 462 00:19:08,670 --> 00:19:12,520 of silicon, that has on the order of one to five micron 463 00:19:12,520 --> 00:19:13,500 diameter grains. 464 00:19:13,500 --> 00:19:15,530 So very, very small grain material. 465 00:19:15,530 --> 00:19:17,830 About 1/50 the width of your hair. 466 00:19:17,830 --> 00:19:19,450 Maybe 1/10 the width of your hair 467 00:19:19,450 --> 00:19:21,575 and, to distinguish it from that really small grain 468 00:19:21,575 --> 00:19:23,570 material that will perform very poorly, 469 00:19:23,570 --> 00:19:27,140 one calls this multicrystalline silicon. 470 00:19:27,140 --> 00:19:29,309 AUDIENCE: And is there microcrystalline silicon? 471 00:19:29,309 --> 00:19:31,350 PROFESSOR: There is also microcrystalline silicon 472 00:19:31,350 --> 00:19:34,270 and microcrystalline silicon is actually 473 00:19:34,270 --> 00:19:37,670 at the phase transition between amorphous and polycrystalline 474 00:19:37,670 --> 00:19:38,530 silicon. 475 00:19:38,530 --> 00:19:40,880 So as you're going from an amorphous material 476 00:19:40,880 --> 00:19:43,320 increasing the temperature, let's say, of growth 477 00:19:43,320 --> 00:19:47,250 or increasing other parameters during the deposition process, 478 00:19:47,250 --> 00:19:49,870 as you begin to evolve from an amorphous material 479 00:19:49,870 --> 00:19:51,740 into a crystalline material, you transition 480 00:19:51,740 --> 00:19:53,400 through this microcrystalline regime 481 00:19:53,400 --> 00:19:54,937 which is a bit of a hybrid. 482 00:19:54,937 --> 00:19:56,520 It has some regions that are amorphous 483 00:19:56,520 --> 00:19:58,720 and other regions that are crystalline. 484 00:19:58,720 --> 00:20:02,190 In your assigned readings, this book was assigned, 485 00:20:02,190 --> 00:20:05,820 and I believe in the syllabus it says read chapter X. 486 00:20:05,820 --> 00:20:07,636 Unfortunately, there is no chapter X. 487 00:20:07,636 --> 00:20:09,700 I guess you could interpret it as 10, 488 00:20:09,700 --> 00:20:12,560 but the essence was that there are two versions of the book. 489 00:20:12,560 --> 00:20:14,630 One is version three, which was published 490 00:20:14,630 --> 00:20:16,840 about seven years ago, and the newest version just 491 00:20:16,840 --> 00:20:18,330 came out last year. 492 00:20:18,330 --> 00:20:20,630 The newest addition is addition three. 493 00:20:20,630 --> 00:20:24,450 So the chapters have rearranged slightly, but what I'll do 494 00:20:24,450 --> 00:20:26,830 is I'll highlight crystalline silicon solar cells 495 00:20:26,830 --> 00:20:28,660 and modules in here so that you can 496 00:20:28,660 --> 00:20:31,430 get a sense of what is in the chapter 497 00:20:31,430 --> 00:20:34,150 and you're welcome to go back and have a look. 498 00:20:34,150 --> 00:20:36,840 So I'll go ahead and highlight this chapter right here 499 00:20:36,840 --> 00:20:37,785 and pass it around. 500 00:20:37,785 --> 00:20:39,660 Feel free to glance through the book as well. 501 00:20:39,660 --> 00:20:40,440 It's a great read. 502 00:20:40,440 --> 00:20:41,940 It dives into great detail into each 503 00:20:41,940 --> 00:20:44,680 of the different technologies. 504 00:20:44,680 --> 00:20:45,180 OK. 505 00:20:45,180 --> 00:20:47,060 So, let's talk about feedstock refining. 506 00:20:47,060 --> 00:20:49,390 We're going to start the silicon value 507 00:20:49,390 --> 00:20:51,610 chain from the raw materials and work our way 508 00:20:51,610 --> 00:20:55,050 all the way to the final module at the end. 509 00:20:55,050 --> 00:20:57,620 So we'll start with the feedstocks themselves. 510 00:20:57,620 --> 00:21:00,420 Down here is a rough cost breakdown. 511 00:21:00,420 --> 00:21:03,170 Kind of think of it as wafer, cell, 512 00:21:03,170 --> 00:21:05,060 module being like a third-third-third 513 00:21:05,060 --> 00:21:07,410 of the total module cost and then balance the system 514 00:21:07,410 --> 00:21:09,340 components beyond that. 515 00:21:09,340 --> 00:21:11,190 So we'll start from our feedstocks 516 00:21:11,190 --> 00:21:12,690 and the raw materials in the ground, 517 00:21:12,690 --> 00:21:14,340 we'll wind up with systems on the roof, 518 00:21:14,340 --> 00:21:15,900 and we'll walk through each of the different steps 519 00:21:15,900 --> 00:21:17,760 of current manufacturing process. 520 00:21:17,760 --> 00:21:19,540 So raw materials. 521 00:21:19,540 --> 00:21:24,050 Shown here is quartz and coal, for a very good reason. 522 00:21:24,050 --> 00:21:28,280 The way feedstock refining occurs at the very first stage 523 00:21:28,280 --> 00:21:32,690 is to take oxidized silicon, silicon dioxide, quartz 524 00:21:32,690 --> 00:21:37,400 and to reduce it to silicon, say, silicon zero. 525 00:21:37,400 --> 00:21:41,440 Unoxidized silicon, which is also called silicon metal. 526 00:21:41,440 --> 00:21:43,797 It's called a metal because it is very low resistivity. 527 00:21:43,797 --> 00:21:46,005 It's very low resistivity because there's a very high 528 00:21:46,005 --> 00:21:47,210 impurity content still. 529 00:21:47,210 --> 00:21:49,920 The purity of this material coming out here is around 99, 530 00:21:49,920 --> 00:21:51,984 99.9% here. 531 00:21:51,984 --> 00:21:53,900 So, it sounds like a high purity but, if we're 532 00:21:53,900 --> 00:21:56,010 talking about parts per million of impurities, 533 00:21:56,010 --> 00:21:58,520 we have some further refining steps to do after this. 534 00:21:58,520 --> 00:21:59,880 So let's walk through this. 535 00:21:59,880 --> 00:22:02,740 We start with the raw materials in the upper left. 536 00:22:02,740 --> 00:22:04,360 It says raw material inputs. 537 00:22:04,360 --> 00:22:06,350 Carbon and SiO2. 538 00:22:06,350 --> 00:22:08,740 The SiO2 forms, usually, quartz. 539 00:22:08,740 --> 00:22:12,990 That can be some of high purity pegmatite, it could be, 540 00:22:12,990 --> 00:22:16,277 for example, a hydrothermal quartz, higher purity 541 00:22:16,277 --> 00:22:17,110 varieties of quartz. 542 00:22:17,110 --> 00:22:21,850 You could even use, maybe, a metamorphic quartzite material. 543 00:22:21,850 --> 00:22:22,980 Let me explain. 544 00:22:22,980 --> 00:22:25,550 So, some of the highest purity materials 545 00:22:25,550 --> 00:22:29,180 are coming from these veins of magma that float up 546 00:22:29,180 --> 00:22:33,430 and then phase separated during millennia. 547 00:22:33,430 --> 00:22:35,290 Some of the lowest purity quartz is 548 00:22:35,290 --> 00:22:38,310 coming from sand, essentially crushed rock that 549 00:22:38,310 --> 00:22:40,630 made its way into, say, a beach-like environment 550 00:22:40,630 --> 00:22:43,010 and then rock was deposited on top of that, 551 00:22:43,010 --> 00:22:46,510 pressure was increased, and this whole mixture 552 00:22:46,510 --> 00:22:50,710 of mica, feldspar, and of quartz got 553 00:22:50,710 --> 00:22:53,280 pushed together and formed a solid block. 554 00:22:53,280 --> 00:22:57,084 That would be your metamorphic quartz materials, 555 00:22:57,084 --> 00:22:58,750 and so you'd have a much higher impurity 556 00:22:58,750 --> 00:23:00,750 content in the metamorphic quartz than you would 557 00:23:00,750 --> 00:23:04,760 in, say, a high purity pegmatite or hydrothermal quartz. 558 00:23:04,760 --> 00:23:07,280 Regardless, depending on the feedstock source of the quartz, 559 00:23:07,280 --> 00:23:08,780 and there are people who study this. 560 00:23:08,780 --> 00:23:10,738 Believe it or not, there are entire departments 561 00:23:10,738 --> 00:23:13,111 dedicated to mining quartz and figuring out 562 00:23:13,111 --> 00:23:15,360 where the different veins of the highest purity quartz 563 00:23:15,360 --> 00:23:18,320 are, where you get them from. 564 00:23:18,320 --> 00:23:21,020 That's the SiO2 input and the C input over here 565 00:23:21,020 --> 00:23:23,350 on the left hand side, Carbon. 566 00:23:23,350 --> 00:23:26,130 So, typically what is used in the PV industry 567 00:23:26,130 --> 00:23:31,350 is either a fast-growing wood source like eucalyptus 568 00:23:31,350 --> 00:23:34,240 or southern pine, right? 569 00:23:34,240 --> 00:23:36,460 Northern pine tends to be slower growing, 570 00:23:36,460 --> 00:23:38,220 but eucalyptus and southern pine both 571 00:23:38,220 --> 00:23:40,050 tend to be fairly fast-growing. 572 00:23:40,050 --> 00:23:43,240 You can tell by the spacing in the rings, 573 00:23:43,240 --> 00:23:48,540 if you chop the tree down and do a cross section, or coal. 574 00:23:48,540 --> 00:23:50,380 So carbon, essentially. 575 00:23:50,380 --> 00:23:54,910 And the two react inside of this furnace right here 576 00:23:54,910 --> 00:23:57,860 and this furnace, just to give you a sense of scale, 577 00:23:57,860 --> 00:23:59,420 here's a human being. 578 00:23:59,420 --> 00:24:00,510 This is the furnace. 579 00:24:00,510 --> 00:24:03,430 So it's about five stories tall, 12 meters in diameter. 580 00:24:03,430 --> 00:24:05,180 It's a big, big, big creature. 581 00:24:05,180 --> 00:24:09,390 This furnace right here is what is producing 582 00:24:09,390 --> 00:24:11,640 the reduced silicon and what's happening 583 00:24:11,640 --> 00:24:14,770 is these feedstock chunks are being thrown in at the top 584 00:24:14,770 --> 00:24:19,800 and there's an arc going between the electrodes, usually 585 00:24:19,800 --> 00:24:23,950 some carbon-bearing material, and a base contact, 586 00:24:23,950 --> 00:24:26,510 and so that arc creates a very high temperature. 587 00:24:26,510 --> 00:24:28,610 Something in the order of up to 2000 degrees 588 00:24:28,610 --> 00:24:30,850 Celsius, near the arc, and the temperature 589 00:24:30,850 --> 00:24:33,050 decreases as you go further and further away, 590 00:24:33,050 --> 00:24:34,852 so up near the top here it might be even 591 00:24:34,852 --> 00:24:36,560 below the melting temperature of silicon, 592 00:24:36,560 --> 00:24:38,140 somewhere around 1,200 degrees. 593 00:24:38,140 --> 00:24:41,020 So this is an extremely inhomogeneous, messy system. 594 00:24:41,020 --> 00:24:44,750 This metallurgical grade silicon refining furnace right here, 595 00:24:44,750 --> 00:24:49,510 this arc furnace, also called a carbothermic reduction furnace, 596 00:24:49,510 --> 00:24:52,330 a very busy place. 597 00:24:52,330 --> 00:24:52,932 Lots going on. 598 00:24:52,932 --> 00:24:54,390 Extremely inhomogeneous if you were 599 00:24:54,390 --> 00:24:56,882 to take a cross section also in terms of temperature 600 00:24:56,882 --> 00:24:58,340 and in terms of the chemical states 601 00:24:58,340 --> 00:25:00,040 of the different constituents species, 602 00:25:00,040 --> 00:25:02,280 but the general reaction that happens 603 00:25:02,280 --> 00:25:03,900 is the carbon would much rather bond 604 00:25:03,900 --> 00:25:06,384 to the oxygen than silicon, and so the carbon steals 605 00:25:06,384 --> 00:25:08,800 the oxygen from the silicon reduces the silicon to silicon 606 00:25:08,800 --> 00:25:11,520 metal and CO2 is released. 607 00:25:11,520 --> 00:25:12,770 We'll get to that in a second. 608 00:25:12,770 --> 00:25:13,400 Flag that. 609 00:25:13,400 --> 00:25:14,525 Put an asterisk next to it. 610 00:25:14,525 --> 00:25:16,490 We'll come back to that in a second. 611 00:25:16,490 --> 00:25:18,030 Other byproducts of this reaction, 612 00:25:18,030 --> 00:25:20,220 so this is the liquid silicon metal coming out here 613 00:25:20,220 --> 00:25:20,803 at the bottom. 614 00:25:20,803 --> 00:25:23,840 It's essentially liquid molten silicon reduced, 615 00:25:23,840 --> 00:25:27,360 so silicon zero, not a silicon oxide, reduced silicon metal, 616 00:25:27,360 --> 00:25:30,530 and then finally it's poured into these buckets, 617 00:25:30,530 --> 00:25:34,660 also called ladles and solidified, crushed up to size, 618 00:25:34,660 --> 00:25:36,477 and then distributed at the end. 619 00:25:36,477 --> 00:25:38,310 Other byproducts coming out of this reaction 620 00:25:38,310 --> 00:25:40,761 include-- this is liquid silicon up hear. 621 00:25:40,761 --> 00:25:42,260 It's very high temperature and there 622 00:25:42,260 --> 00:25:45,870 are gases and a lot of oxygen because of the reduction 623 00:25:45,870 --> 00:25:51,730 process, and so silica, or SiO gas, can be produced 624 00:25:51,730 --> 00:25:55,470 and silica gas can begin aggravating and forming 625 00:25:55,470 --> 00:25:59,710 very small particles, almost like shards, 626 00:25:59,710 --> 00:26:03,030 of silicon oxide material, and these 627 00:26:03,030 --> 00:26:05,910 can be on the order of one to five microns 628 00:26:05,910 --> 00:26:08,320 and very rough and jaggedy around the edges. 629 00:26:08,320 --> 00:26:11,020 Now, who here has studied public health and knows anything about 630 00:26:11,020 --> 00:26:13,967 PM1 or PM1.5 denominations. 631 00:26:13,967 --> 00:26:14,800 Do they ring a bell? 632 00:26:14,800 --> 00:26:15,716 What are those Ashley? 633 00:26:15,716 --> 00:26:20,903 AUDIENCE: It's the size of particles that 634 00:26:20,903 --> 00:26:22,200 can get stuck in your lungs. 635 00:26:22,200 --> 00:26:22,850 PROFESSOR: Exactly! 636 00:26:22,850 --> 00:26:23,349 Right? 637 00:26:23,349 --> 00:26:26,630 So PM1 or PM1.5 would refer to the micron diameter, 638 00:26:26,630 --> 00:26:29,470 1 or 1.5 micron diameter particle that would get stuck 639 00:26:29,470 --> 00:26:32,000 in the [INAUDIBLE] and result, eventually, 640 00:26:32,000 --> 00:26:36,751 in edema or, probably, more of water filling up in the lungs 641 00:26:36,751 --> 00:26:38,750 as a result of the body trying to expunge these, 642 00:26:38,750 --> 00:26:40,150 and because they're jaggedy and pointy, 643 00:26:40,150 --> 00:26:42,900 they get stuck in there and they don't come out and eventually 644 00:26:42,900 --> 00:26:45,590 the people can even affixate as a result. 645 00:26:45,590 --> 00:26:48,149 So, before in the past, when we had these big smokestacks 646 00:26:48,149 --> 00:26:50,440 sitting on the top of these metallurgical grade silicon 647 00:26:50,440 --> 00:26:53,530 refineries that would just spew the silica dust into the air, 648 00:26:53,530 --> 00:26:55,650 the folks downstream would be affected 649 00:26:55,650 --> 00:26:59,010 and this actually did happen, to some degree, in, for, example, 650 00:26:59,010 --> 00:27:02,960 Kristiansand in Norway and, as a result, 651 00:27:02,960 --> 00:27:06,810 the refineries began putting in filters over here 652 00:27:06,810 --> 00:27:09,800 to prevent the silica dust from getting thrown 653 00:27:09,800 --> 00:27:11,580 and spewed out into the atmosphere 654 00:27:11,580 --> 00:27:14,430 and the filters are a very interesting contraption. 655 00:27:14,430 --> 00:27:17,000 A lot of work went into designing them just right 656 00:27:17,000 --> 00:27:19,900 to allow the air to go out but the particulate matter 657 00:27:19,900 --> 00:27:22,480 to stay behind and once every delta t, maybe 658 00:27:22,480 --> 00:27:26,360 in the order of an hour so, the airflow direction inverse 659 00:27:26,360 --> 00:27:28,810 and all the dust comes crashing down to the bottom 660 00:27:28,810 --> 00:27:30,569 and then gets collected inside of here. 661 00:27:30,569 --> 00:27:33,110 It's kind of like pushing air through the different direction 662 00:27:33,110 --> 00:27:35,970 through a sock, and all the dust comes out to the bottom, 663 00:27:35,970 --> 00:27:39,050 you collect it, and it's sold to the--? 664 00:27:39,050 --> 00:27:43,382 AUDIENCE: The footwear industry for absorbing-- 665 00:27:43,382 --> 00:27:44,340 PROFESSOR: It might be. 666 00:27:44,340 --> 00:27:46,400 I don't know, but I know that the majority of it 667 00:27:46,400 --> 00:27:52,100 goes to the cement industry and so, depending on the market 668 00:27:52,100 --> 00:27:54,830 rates of silicon, here at the bottom metallurgical grade 669 00:27:54,830 --> 00:27:58,230 silicon, versus what the cement industry is willing to pay, 670 00:27:58,230 --> 00:28:00,020 you might tune your process to optimize 671 00:28:00,020 --> 00:28:01,790 for one industry or another. 672 00:28:01,790 --> 00:28:05,520 So, this is to say that early on in refining processes, 673 00:28:05,520 --> 00:28:08,170 you're serving multiple industries with one plant 674 00:28:08,170 --> 00:28:11,177 and volatility of pricing is affected, 675 00:28:11,177 --> 00:28:13,260 in part, by what those other industries are doing. 676 00:28:13,260 --> 00:28:14,600 What the demand there is. 677 00:28:14,600 --> 00:28:15,850 It's something to be aware of. 678 00:28:15,850 --> 00:28:18,399 Let's go back to the CO2 real quick that's being emitted. 679 00:28:18,399 --> 00:28:20,440 So that is one of the byproducts of the reaction. 680 00:28:20,440 --> 00:28:23,530 In terms of total CO2 content from the production 681 00:28:23,530 --> 00:28:27,680 of solar cells, the CO2 produced during the reduction process 682 00:28:27,680 --> 00:28:30,777 is a small percentage, I think something under 5% or 10% 683 00:28:30,777 --> 00:28:32,360 is the number I pulled out of my head, 684 00:28:32,360 --> 00:28:35,260 it's a small percentage of the total CO2 emitted 685 00:28:35,260 --> 00:28:38,850 during solar cell manufacturing because the electricity that 686 00:28:38,850 --> 00:28:41,610 goes into producing the rest of the solar cells coming 687 00:28:41,610 --> 00:28:43,350 from fossil fuel based sources comprises 688 00:28:43,350 --> 00:28:46,270 the majority of CO2 emissions during fabrication 689 00:28:46,270 --> 00:28:47,380 of these devices. 690 00:28:47,380 --> 00:28:49,570 The electricity used to run these electrodes, 691 00:28:49,570 --> 00:28:51,520 for instance, the electricity used 692 00:28:51,520 --> 00:28:55,140 to melt this silicon byproduct here, 693 00:28:55,140 --> 00:28:57,830 or to gasify it in the subsequent reactions, that 694 00:28:57,830 --> 00:29:00,570 is the majority of the CO2 coming out of the process. 695 00:29:00,570 --> 00:29:02,740 Any questions so far about this? 696 00:29:02,740 --> 00:29:03,980 They're fun plants to see. 697 00:29:03,980 --> 00:29:07,460 We don't have too many of them in the US. 698 00:29:07,460 --> 00:29:10,680 Majority of these carbothermic reduction furnaces 699 00:29:10,680 --> 00:29:12,810 are either in China, Norway. 700 00:29:12,810 --> 00:29:15,460 Norway has a lot of cheap hydropower 701 00:29:15,460 --> 00:29:19,030 so the hydroplant is usually only a few 10s of kilometers 702 00:29:19,030 --> 00:29:21,690 away from the refinery and if you 703 00:29:21,690 --> 00:29:24,380 go to, say, [INAUDIBLE] in Norway, where they have 704 00:29:24,380 --> 00:29:25,980 a number of these plants, you'll see 705 00:29:25,980 --> 00:29:27,479 not only silicon being refined there 706 00:29:27,479 --> 00:29:31,080 but also magnesium, other elements, aluminum 707 00:29:31,080 --> 00:29:33,579 being smelted in the same peninsula-- 708 00:29:33,579 --> 00:29:34,620 the same industrial park. 709 00:29:34,620 --> 00:29:40,847 AUDIENCE: When general mining of silicon happens or silica, 710 00:29:40,847 --> 00:29:43,156 the Chinese have-- 711 00:29:43,156 --> 00:29:44,530 PROFESSOR: The reduction process, 712 00:29:44,530 --> 00:29:47,210 this carbothermic reduction process here, 713 00:29:47,210 --> 00:29:49,340 the majority of it happens at the same places 714 00:29:49,340 --> 00:29:53,140 like Norway or China-- places that have cheap electricity. 715 00:29:53,140 --> 00:29:55,239 There's also a feedstock refinery. 716 00:29:55,239 --> 00:29:57,030 I don't know if it extends all the way back 717 00:29:57,030 --> 00:29:58,880 to the metallurgical grade silicon refining, 718 00:29:58,880 --> 00:30:00,440 but there's a feedstock refining facility 719 00:30:00,440 --> 00:30:01,981 going up in the Middle East right now 720 00:30:01,981 --> 00:30:05,960 in Qatar, as a result of the cheap natural gas. 721 00:30:05,960 --> 00:30:08,204 So, wherever you have cheap access to energy, 722 00:30:08,204 --> 00:30:10,120 you can set one of these plants up and get off 723 00:30:10,120 --> 00:30:11,910 and running and your CO2 intensity 724 00:30:11,910 --> 00:30:14,650 will be dictated by the fuel source that you're using. 725 00:30:14,650 --> 00:30:16,570 Hydro, in that case, it might be low 726 00:30:16,570 --> 00:30:18,170 unless you take methane into account 727 00:30:18,170 --> 00:30:20,760 that might be emitted in the reservoir, 728 00:30:20,760 --> 00:30:24,250 if you have decaying biomass underneath the water, 729 00:30:24,250 --> 00:30:26,497 but if you would exclude that and if you 730 00:30:26,497 --> 00:30:28,580 look at the CO2 intensity of the fossil fuels that 731 00:30:28,580 --> 00:30:30,177 are being burned, it might be better 732 00:30:30,177 --> 00:30:32,760 to do it in, say, Norway, from an environmental point of view, 733 00:30:32,760 --> 00:30:35,386 than to, say, manufacture this stuff in China. 734 00:30:35,386 --> 00:30:35,886 Yeah. 735 00:30:35,886 --> 00:30:37,261 AUDIENCE: How many kilowatt hours 736 00:30:37,261 --> 00:30:38,874 are we talking [INAUDIBLE]? 737 00:30:38,874 --> 00:30:40,640 PROFESSOR: Okay, so what is the energy 738 00:30:40,640 --> 00:30:44,780 intensity of this process right here, in other words. 739 00:30:44,780 --> 00:30:46,876 Well, why don't I put a flag on that. 740 00:30:46,876 --> 00:30:48,750 Why don't we put a flag on that and come back 741 00:30:48,750 --> 00:30:51,990 with specific numbers for this process right here. 742 00:30:51,990 --> 00:30:54,232 I don't want to say something and regret it later. 743 00:30:54,232 --> 00:30:56,065 AUDIENCE: Well, we know the energy intensity 744 00:30:56,065 --> 00:30:57,350 of the solar panel itself. 745 00:30:57,350 --> 00:30:58,033 PROFESSOR: Yeah. 746 00:30:58,033 --> 00:30:59,116 AUDIENCE: But the energy-- 747 00:30:59,116 --> 00:31:01,680 PROFESSOR: But specifically what fraction 748 00:31:01,680 --> 00:31:04,760 comes from the MGSi refining, I'd rather not 749 00:31:04,760 --> 00:31:07,200 pull something out of my head. 750 00:31:07,200 --> 00:31:09,200 Any other questions? 751 00:31:09,200 --> 00:31:11,340 OK. 752 00:31:11,340 --> 00:31:14,000 So somewhere in the order of two million metric tons 753 00:31:14,000 --> 00:31:17,034 of metallurgical grade silicon are produced annually. 754 00:31:17,034 --> 00:31:18,950 Probably somewhere in the order of 10% of that 755 00:31:18,950 --> 00:31:21,600 is destined for the PV industry. 756 00:31:21,600 --> 00:31:23,950 The remainder gets split among a variety 757 00:31:23,950 --> 00:31:24,970 of different industries. 758 00:31:24,970 --> 00:31:26,428 So what I'm talking about here when 759 00:31:26,428 --> 00:31:29,080 I say metallurgical silicon, I'm referring to this right here. 760 00:31:29,080 --> 00:31:30,250 This stuff coming out. 761 00:31:30,250 --> 00:31:34,670 It has about 99% or 99.9% purity and it gets used 762 00:31:34,670 --> 00:31:36,030 in a variety of industries. 763 00:31:36,030 --> 00:31:38,174 So those industries are: the PV industry, 764 00:31:38,174 --> 00:31:40,090 and we'll explain how the rest of the refining 765 00:31:40,090 --> 00:31:43,370 happens, the integrated circuits industry, that's the wafer that 766 00:31:43,370 --> 00:31:45,680 just went around that's made its way back up here, 767 00:31:45,680 --> 00:31:51,510 and silicones those are-- so, a pet peeve of mine 768 00:31:51,510 --> 00:31:56,040 is hearing the word silicon and silicone used interchangeably. 769 00:31:56,040 --> 00:31:59,710 Silicon is this element-- is an element on the periodic table 770 00:31:59,710 --> 00:32:02,570 and it's the element that comprises this wafer right 771 00:32:02,570 --> 00:32:03,250 here. 772 00:32:03,250 --> 00:32:06,642 Silicone, on the other hand, is an organelle, 773 00:32:06,642 --> 00:32:09,100 I guess you could say, it's not exactly organelle metallic, 774 00:32:09,100 --> 00:32:12,300 silicon isn't a metal, but it would 775 00:32:12,300 --> 00:32:18,520 be a molecule that is comprised of carbon atoms and silicon-- 776 00:32:18,520 --> 00:32:21,300 silicon being in the middle and the carbon being on the sides-- 777 00:32:21,300 --> 00:32:25,440 and that is used as caulking or sealing agent in your showers, 778 00:32:25,440 --> 00:32:29,110 for instance, or in plumbing, round windows. 779 00:32:29,110 --> 00:32:32,320 It tends to be very flexible, compliant but yet impermeable, 780 00:32:32,320 --> 00:32:34,940 preventing the inflow of gases. 781 00:32:34,940 --> 00:32:37,140 So silicones, they're metal alloys 782 00:32:37,140 --> 00:32:39,430 including steel and aluminum. 783 00:32:39,430 --> 00:32:42,230 Why would you silicon there? 784 00:32:42,230 --> 00:32:45,085 What does it have to do with steel or aluminum? 785 00:32:45,085 --> 00:32:45,910 Let me ask this. 786 00:32:45,910 --> 00:32:48,760 Has anyone ever played with pure aluminum? 787 00:32:48,760 --> 00:32:52,300 Highly refined, ultra high purity aluminum. 788 00:32:52,300 --> 00:32:54,030 Say five nines or six nines. 789 00:32:54,030 --> 00:32:54,850 Yes! 790 00:32:54,850 --> 00:32:56,470 What happens to ultra-pure aluminum? 791 00:32:56,470 --> 00:32:57,941 AUDIENCE: It's really flexible. 792 00:32:57,941 --> 00:32:59,440 PROFESSOR: It's really flexible, you 793 00:32:59,440 --> 00:33:01,127 can dent it with your fingernail, 794 00:33:01,127 --> 00:33:02,710 and it wouldn't make very great boxes. 795 00:33:02,710 --> 00:33:03,590 Right? 796 00:33:03,590 --> 00:33:07,650 So we need it to be stronger and scratch-resistant and so 797 00:33:07,650 --> 00:33:09,990 we have these additives into the aluminum, silicon being 798 00:33:09,990 --> 00:33:12,364 one of them, that increases the strength of the aluminum, 799 00:33:12,364 --> 00:33:13,870 essentially preventing plasticity 800 00:33:13,870 --> 00:33:16,780 or preventing a dislocation flow into the material. 801 00:33:16,780 --> 00:33:20,630 So that's more or less how silicon-- metallurgical grade 802 00:33:20,630 --> 00:33:25,170 silicon, also called MGSi as shown up here at the very top-- 803 00:33:25,170 --> 00:33:27,770 that's how MGSi gs is distributed worldwide 804 00:33:27,770 --> 00:33:29,710 and that's the current production. 805 00:33:29,710 --> 00:33:31,440 Now let me ask another question. 806 00:33:31,440 --> 00:33:36,170 Steel and aluminum, where are those used the most? 807 00:33:36,170 --> 00:33:39,022 What industry uses steel, aluminum the most? 808 00:33:39,022 --> 00:33:39,980 AUDIENCE: Construction. 809 00:33:39,980 --> 00:33:43,110 PROFESSOR: Constructive industry, automotive industry. 810 00:33:43,110 --> 00:33:44,940 How fast are those growing annually? 811 00:33:48,540 --> 00:33:50,630 Let's estimate it from GDP. 812 00:33:50,630 --> 00:33:51,880 Annual-- worldwide GDP. 813 00:33:51,880 --> 00:33:54,270 What's the worldwide GDP growth look like. 814 00:33:54,270 --> 00:33:55,380 US is around 1%. 815 00:33:55,380 --> 00:33:55,942 China 8%. 816 00:33:55,942 --> 00:33:57,650 Let's pick a number somewhere in between. 817 00:33:57,650 --> 00:33:58,550 Four, right? 818 00:33:58,550 --> 00:33:59,050 All right. 819 00:33:59,050 --> 00:34:01,630 So, let's say 4%, 5% worldwide. 820 00:34:01,630 --> 00:34:03,487 Silicone's probably on that order. 821 00:34:03,487 --> 00:34:04,570 How about the PV industry. 822 00:34:04,570 --> 00:34:07,090 How fast is it going right now? 823 00:34:07,090 --> 00:34:09,615 Somewhere in the order of, it's a volatile year right now, 824 00:34:09,615 --> 00:34:11,489 this one year, but in the past, historically, 825 00:34:11,489 --> 00:34:14,510 it's been around 40% to 60% a year. 826 00:34:14,510 --> 00:34:16,560 So, where do you think the price pressure 827 00:34:16,560 --> 00:34:18,810 for metallurgical grade silicon is going to come from? 828 00:34:18,810 --> 00:34:19,585 What industry? 829 00:34:19,585 --> 00:34:20,710 It's going to come from PV. 830 00:34:20,710 --> 00:34:22,459 It's a small fraction of the pie right now 831 00:34:22,459 --> 00:34:23,630 but it's growing fast. 832 00:34:23,630 --> 00:34:25,510 Something to keep in mind. 833 00:34:25,510 --> 00:34:29,420 So that's why, if you look at pricing of metallurgical grade 834 00:34:29,420 --> 00:34:31,150 silicon, yes. 835 00:34:31,150 --> 00:34:33,870 Superimposed upon pricing is a function of time. 836 00:34:33,870 --> 00:34:37,010 You have the global macroeconomic situation. 837 00:34:37,010 --> 00:34:37,510 Right? 838 00:34:37,510 --> 00:34:40,020 So that's kind of the dampening function on top of it all, 839 00:34:40,020 --> 00:34:43,850 but there's just this general trend toward rising prices 840 00:34:43,850 --> 00:34:48,210 as you put increasing price pressure on metallurgical grade 841 00:34:48,210 --> 00:34:48,811 silicon. 842 00:34:48,811 --> 00:34:50,310 So additional refining capacity will 843 00:34:50,310 --> 00:34:54,070 be needed if the current growth keeps up in this industry. 844 00:34:54,070 --> 00:34:56,630 So let me talk about going from metallurgical grade 845 00:34:56,630 --> 00:35:00,400 silicon about two nines to three nines pure. 846 00:35:00,400 --> 00:35:02,970 What I mean two nines means 99%, three nines 847 00:35:02,970 --> 00:35:06,590 would be 99.9% pure, to silicon that we 848 00:35:06,590 --> 00:35:08,670 can use for solar cells, which typically 849 00:35:08,670 --> 00:35:11,730 has to be about six nines pure. 850 00:35:11,730 --> 00:35:15,010 And so this is called the Siemens process which 851 00:35:15,010 --> 00:35:18,087 is purification through gaseous distillation, 852 00:35:18,087 --> 00:35:19,920 and that's the method that is currently used 853 00:35:19,920 --> 00:35:22,132 to make most of our silicon. 854 00:35:22,132 --> 00:35:23,590 So the way this process works is we 855 00:35:23,590 --> 00:35:26,760 start with metallurgical grade silicon at the top, 856 00:35:26,760 --> 00:35:29,400 represented by a little sack of metallurgical grade silicon 857 00:35:29,400 --> 00:35:30,290 chunks. 858 00:35:30,290 --> 00:35:33,340 We produce silane gas out of that metallurgical grade 859 00:35:33,340 --> 00:35:34,150 silicon. 860 00:35:34,150 --> 00:35:37,800 We essentially- silane gas is SiH4. 861 00:35:37,800 --> 00:35:43,150 So it would essentially be this right here. 862 00:35:43,150 --> 00:35:46,320 So you'd have a silicon atom here, tetrahedrally coordinated 863 00:35:46,320 --> 00:35:50,720 with-- tetrahedrally meaning four bonds with hydrogen atoms 864 00:35:50,720 --> 00:35:56,170 on the side-- and this is silane gas-- well, silane-- 865 00:35:56,170 --> 00:35:58,070 which, at room temperature, is a gas 866 00:35:58,070 --> 00:36:01,660 and that's what happens in this step right here. 867 00:36:01,660 --> 00:36:05,170 We're forming-- we're gasifying the silicon. 868 00:36:05,170 --> 00:36:09,350 This process is the distillation process. 869 00:36:09,350 --> 00:36:11,650 To extract the pure silane gas, it's 870 00:36:11,650 --> 00:36:13,150 the distillation process that's used 871 00:36:13,150 --> 00:36:16,689 in large towers similar to fractional distillation where 872 00:36:16,689 --> 00:36:18,730 we might heat up the material and then, depending 873 00:36:18,730 --> 00:36:23,490 on its mass, it settles down to a certain height in that tower 874 00:36:23,490 --> 00:36:25,060 and we're able to extract it. 875 00:36:25,060 --> 00:36:27,210 The silane gas here has been sold 876 00:36:27,210 --> 00:36:28,680 to the photovoltaics industry. 877 00:36:28,680 --> 00:36:30,050 LCD. 878 00:36:30,050 --> 00:36:31,790 Liquid crystal display. 879 00:36:31,790 --> 00:36:32,390 Right? 880 00:36:32,390 --> 00:36:34,914 Thin film industries as well, they use silane. 881 00:36:34,914 --> 00:36:37,080 If you're depositing the polycrystalline and silicon 882 00:36:37,080 --> 00:36:39,880 for your LCDs or if you're making amorphous silicon 883 00:36:39,880 --> 00:36:41,954 solar cells, they use silane as well. 884 00:36:41,954 --> 00:36:43,370 So this little truck here might go 885 00:36:43,370 --> 00:36:44,995 to three different companies, depending 886 00:36:44,995 --> 00:36:47,730 on who's willing to pay more. 887 00:36:47,730 --> 00:36:50,840 Most of the silane is used for polysilicon. 888 00:36:50,840 --> 00:36:53,700 The gas has to be converted back into a solid, 889 00:36:53,700 --> 00:36:56,600 and that's where this particular process here, 890 00:36:56,600 --> 00:36:58,290 the Siemens process is used. 891 00:36:58,290 --> 00:37:00,370 Again, you have a current passing 892 00:37:00,370 --> 00:37:02,970 through some seed material and the gas 893 00:37:02,970 --> 00:37:04,910 is being cracked onto that seed. 894 00:37:04,910 --> 00:37:06,320 You form these rods. 895 00:37:06,320 --> 00:37:08,170 The rods are then cracked into chunks 896 00:37:08,170 --> 00:37:10,980 and then the chunks are loaded into ingot crucibles. 897 00:37:10,980 --> 00:37:11,480 Yes. 898 00:37:11,480 --> 00:37:17,560 AUDIENCE: So the silane gas is shipped as a gas in the trucks. 899 00:37:17,560 --> 00:37:18,466 PROFESSOR: Sure. 900 00:37:18,466 --> 00:37:20,410 AUDIENCE: Or on rails? 901 00:37:20,410 --> 00:37:23,550 PROFESSOR: Well it's pyrophoric, as you 902 00:37:23,550 --> 00:37:26,140 can guess from just glancing at this chemical structure 903 00:37:26,140 --> 00:37:26,640 right here. 904 00:37:26,640 --> 00:37:27,890 It's highly reactive. 905 00:37:27,890 --> 00:37:31,690 Pyrophoric means that it can combust at room temperature. 906 00:37:31,690 --> 00:37:34,650 It can catch on fire, meaning there are more stable compounds 907 00:37:34,650 --> 00:37:38,370 than this that can form when you react this gas with air 908 00:37:38,370 --> 00:37:42,170 and, during the early days of silane development, 909 00:37:42,170 --> 00:37:44,145 folks really didn't know much about it 910 00:37:44,145 --> 00:37:45,520 and there's some early research-- 911 00:37:45,520 --> 00:37:47,853 some of the earliest research done here at MIT, in fact. 912 00:37:47,853 --> 00:37:51,050 They would fill up an evacuated chamber with silane gas 913 00:37:51,050 --> 00:37:53,350 and spark and nothing would happen. 914 00:37:53,350 --> 00:37:55,100 Spark a second time, nothing would happen. 915 00:37:55,100 --> 00:37:56,500 Spark a third time, boom. 916 00:37:56,500 --> 00:37:57,110 OK. 917 00:37:57,110 --> 00:37:58,330 That's critical limit. 918 00:37:58,330 --> 00:37:59,220 Such and such amount. 919 00:37:59,220 --> 00:38:00,890 You know, they'd keep increasing the amount 920 00:38:00,890 --> 00:38:02,000 and finally it would go boom. 921 00:38:02,000 --> 00:38:03,150 Tell you what, lets repeat the experiment 922 00:38:03,150 --> 00:38:04,310 since we're good scientists. 923 00:38:04,310 --> 00:38:06,434 They'd repeat it and, at low concentrations, click. 924 00:38:06,434 --> 00:38:07,305 Boom. 925 00:38:07,305 --> 00:38:07,930 That's strange. 926 00:38:07,930 --> 00:38:09,260 That was much lower this time. 927 00:38:09,260 --> 00:38:10,880 Let's repeat the experiment one more time. 928 00:38:10,880 --> 00:38:11,210 Click. 929 00:38:11,210 --> 00:38:11,550 Click. 930 00:38:11,550 --> 00:38:11,880 Click. 931 00:38:11,880 --> 00:38:12,180 Click. 932 00:38:12,180 --> 00:38:12,470 Click. 933 00:38:12,470 --> 00:38:12,730 Click. 934 00:38:12,730 --> 00:38:12,960 Click. 935 00:38:12,960 --> 00:38:13,180 Click. 936 00:38:13,180 --> 00:38:13,390 Click. 937 00:38:13,390 --> 00:38:13,921 Boom. 938 00:38:13,921 --> 00:38:14,420 All right. 939 00:38:14,420 --> 00:38:16,090 I don't really understand this gas, 940 00:38:16,090 --> 00:38:18,830 but I'm going to say it's really dangerous so I'm 941 00:38:18,830 --> 00:38:21,200 going to have little warning bells that 942 00:38:21,200 --> 00:38:23,170 will detect the silane gas if it's leaking 943 00:38:23,170 --> 00:38:25,560 and tell people to get the heck out of the building 944 00:38:25,560 --> 00:38:29,180 if it starts being leaked. 945 00:38:29,180 --> 00:38:31,540 It's also toxic for humans, by the way. 946 00:38:31,540 --> 00:38:34,940 Very small dilute concentrations can kill you 947 00:38:34,940 --> 00:38:37,920 and so three buildings on campus, only three 948 00:38:37,920 --> 00:38:40,770 to my knowledge, are set up with the proper safety equipment 949 00:38:40,770 --> 00:38:42,900 to use silane gas in the laboratory. 950 00:38:42,900 --> 00:38:45,590 Building 13, which is the material science building, 951 00:38:45,590 --> 00:38:48,340 and then-- MTL and related. 952 00:38:48,340 --> 00:38:52,189 So we have this gas right here. 953 00:38:52,189 --> 00:38:52,980 Extremely powerful. 954 00:38:52,980 --> 00:38:55,980 There are variants thereof. 955 00:38:55,980 --> 00:39:00,020 You can replace some of the hydrogens 956 00:39:00,020 --> 00:39:10,280 with chlorine, like this and now you have trichlorosilane. 957 00:39:10,280 --> 00:39:11,930 It's all one word. 958 00:39:11,930 --> 00:39:15,330 So tricholorsilane, I've just replaced three of my silanes-- 959 00:39:15,330 --> 00:39:17,310 my hydrogens with chlorine and now I 960 00:39:17,310 --> 00:39:19,850 have a different molecule, still silicon bearing, 961 00:39:19,850 --> 00:39:22,480 still very reactive, but now reactive 962 00:39:22,480 --> 00:39:25,820 at different temperatures and I can modify my process 963 00:39:25,820 --> 00:39:29,310 by substituting out some of the hydrogens for chlorines. 964 00:39:29,310 --> 00:39:33,000 So we have the silane gas or trichlorosilane or the variants 965 00:39:33,000 --> 00:39:36,370 thereof, loaded into some transportation vehicle that 966 00:39:36,370 --> 00:39:40,030 is very safe, leak-proof and preventing accidents 967 00:39:40,030 --> 00:39:45,020 on the road, to deliver it to where it is going 968 00:39:45,020 --> 00:39:50,510 to be consumed, which are these so-called polysilicon, 969 00:39:50,510 --> 00:39:54,300 or Siemens reactor as shown here. 970 00:39:54,300 --> 00:39:56,350 Excuse me. 971 00:39:56,350 --> 00:39:59,240 What happens, or how the process actually flows, 972 00:39:59,240 --> 00:40:00,362 let me go back one step. 973 00:40:00,362 --> 00:40:02,820 We're going to start from up at the very top of the process 974 00:40:02,820 --> 00:40:04,830 and move all the way down, showing you 975 00:40:04,830 --> 00:40:07,520 what the manufacturing equipment looks like at each step. 976 00:40:07,520 --> 00:40:09,730 So, this is the distillation process 977 00:40:09,730 --> 00:40:11,960 used to create the silane and when 978 00:40:11,960 --> 00:40:15,740 you see one of these factories just think of a refinery. 979 00:40:15,740 --> 00:40:20,770 In fact, the people who don't like this particular process 980 00:40:20,770 --> 00:40:24,170 who aren't a fan of the silane refining process 981 00:40:24,170 --> 00:40:26,330 and opt for other ways of purifying their silicon, 982 00:40:26,330 --> 00:40:28,830 liken this to an oil refinery. 983 00:40:28,830 --> 00:40:32,190 The imagery is very stark there. 984 00:40:32,190 --> 00:40:36,040 The polysilicon production, this is the Siemens reactor, 985 00:40:36,040 --> 00:40:38,540 it's much smaller in comparison to the metallurgical grade 986 00:40:38,540 --> 00:40:39,620 silicon furnace. 987 00:40:39,620 --> 00:40:41,970 Much smaller than the carbothermic reduction furnace. 988 00:40:41,970 --> 00:40:44,850 Here, we have a small human or human next 989 00:40:44,850 --> 00:40:47,170 to the small contraption. 990 00:40:47,170 --> 00:40:49,500 Here are a series of them lined, almost 991 00:40:49,500 --> 00:40:52,720 like little pods and, out of this material, 992 00:40:52,720 --> 00:40:54,750 actually inside of the furnace, you 993 00:40:54,750 --> 00:41:00,940 have these rods that are passing current and heating up 994 00:41:00,940 --> 00:41:04,920 and the silicon is cracking onto the rods. 995 00:41:04,920 --> 00:41:07,220 So we wind up with six nines, usually 996 00:41:07,220 --> 00:41:11,390 called 6N solar grade silicon as a result of this process. 997 00:41:11,390 --> 00:41:16,180 We could also go up to, even, nine nines using the Siemens 998 00:41:16,180 --> 00:41:16,750 process. 999 00:41:16,750 --> 00:41:18,510 It could be very, very pure depending 1000 00:41:18,510 --> 00:41:21,011 on how fast you grow, what the purity of your silane gas is. 1001 00:41:21,011 --> 00:41:21,635 AUDIENCE: Yeah. 1002 00:41:21,635 --> 00:41:22,540 What is cracking. 1003 00:41:22,540 --> 00:41:23,654 What does that mean? 1004 00:41:23,654 --> 00:41:24,320 PROFESSOR: Sure. 1005 00:41:24,320 --> 00:41:27,330 So what it means is this gas molecule comes 1006 00:41:27,330 --> 00:41:31,970 in, sees a solid surface, the central atom right here, 1007 00:41:31,970 --> 00:41:34,740 the silicon atom, gets deposited onto the surface, 1008 00:41:34,740 --> 00:41:37,550 becomes an adatom, which means it's a surface atom, 1009 00:41:37,550 --> 00:41:41,370 it's scuttling around and the remaining elements 1010 00:41:41,370 --> 00:41:44,215 within this molecule are then free to move away as a gas. 1011 00:41:44,215 --> 00:41:45,840 AUDIENCE: So you've broken those bonds. 1012 00:41:45,840 --> 00:41:46,464 PROFESSOR: Yes. 1013 00:41:46,464 --> 00:41:49,600 Effectively, you've added the core constituent 1014 00:41:49,600 --> 00:41:53,740 of this molecule onto the surface. 1015 00:41:53,740 --> 00:41:56,580 It's joined the collective if you will and, in this matter, 1016 00:41:56,580 --> 00:41:59,530 the diameter of those rods grows with time. 1017 00:41:59,530 --> 00:42:01,480 So what I'm going to do is pass around 1018 00:42:01,480 --> 00:42:06,320 an example of a chunk coming from this Siemens rod. 1019 00:42:06,320 --> 00:42:07,960 Be very gentle with it please. 1020 00:42:07,960 --> 00:42:10,400 On the outside you can see a corrugated, rough, 1021 00:42:10,400 --> 00:42:12,520 cauliflower-like structure. 1022 00:42:12,520 --> 00:42:15,570 That's because you're optimizing for deposition speed, 1023 00:42:15,570 --> 00:42:17,120 not for beauty of the surface. 1024 00:42:17,120 --> 00:42:18,770 You don't really care how flat it 1025 00:42:18,770 --> 00:42:20,990 is, unless you're trying to grow a very specific type 1026 00:42:20,990 --> 00:42:23,210 of material called flotsam, which we get to the second, 1027 00:42:23,210 --> 00:42:25,180 but in general, if you're trying to crack it up and break it 1028 00:42:25,180 --> 00:42:27,138 into a smaller piece and into a chunk like this 1029 00:42:27,138 --> 00:42:28,970 and throw it into a big ingot furnace, 1030 00:42:28,970 --> 00:42:31,178 it doesn't really matter what the surface looks like. 1031 00:42:31,178 --> 00:42:33,590 On the inside, it's pretty dense silicon 1032 00:42:33,590 --> 00:42:36,310 and, if look very carefully, right in the middle there 1033 00:42:36,310 --> 00:42:37,410 you can see the rod. 1034 00:42:37,410 --> 00:42:38,910 The initial seeding rod. 1035 00:42:38,910 --> 00:42:40,812 It's a slightly different color. 1036 00:42:40,812 --> 00:42:42,770 So I'll pass these around and please be gentle. 1037 00:42:42,770 --> 00:42:45,270 AUDIENCE: Is the seeding rod just silicon? 1038 00:42:45,270 --> 00:42:46,900 PROFESSOR: It's actually doped silicon, 1039 00:42:46,900 --> 00:42:49,030 so it's lower resistivity so you can 1040 00:42:49,030 --> 00:42:52,330 pass more current through it. 1041 00:42:52,330 --> 00:42:57,600 This here is chunks, or smaller chunks of the polysilicon 1042 00:42:57,600 --> 00:42:59,840 so, essentially, just crushed polysilicon 1043 00:42:59,840 --> 00:43:02,610 and if you're trying to load a crucible with big chunks 1044 00:43:02,610 --> 00:43:04,730 like this you'll leave a lot of empty space 1045 00:43:04,730 --> 00:43:07,660 unless you crush some of this up and make finer grains out of it 1046 00:43:07,660 --> 00:43:08,930 and fill in the gaps. 1047 00:43:08,930 --> 00:43:11,120 So I'll pass these around right here 1048 00:43:11,120 --> 00:43:12,590 so you can have a look at them. 1049 00:43:12,590 --> 00:43:16,500 Those are examples of the Siemens grade polysilicon. 1050 00:43:16,500 --> 00:43:19,940 This is a bigger rod. 1051 00:43:19,940 --> 00:43:22,150 Here is the seed coming right through the middle. 1052 00:43:22,150 --> 00:43:23,650 Here's the surface where you can see 1053 00:43:23,650 --> 00:43:29,060 it's kind of rough and corrugated 1054 00:43:29,060 --> 00:43:32,570 and one of the biggest issues with this feedstock refining 1055 00:43:32,570 --> 00:43:34,580 process is that there are very large plants 1056 00:43:34,580 --> 00:43:36,070 and long lead times. 1057 00:43:36,070 --> 00:43:38,460 This is a plant construction going on right now, 1058 00:43:38,460 --> 00:43:39,720 you can see. 1059 00:43:39,720 --> 00:43:44,070 Typical lead times are between 18 and 24 months. 1060 00:43:44,070 --> 00:43:46,680 That's a long time between when the board says yes, we 1061 00:43:46,680 --> 00:43:50,360 will create new silicon refining capacity and product 1062 00:43:50,360 --> 00:43:52,040 starts to roll off the production line 1063 00:43:52,040 --> 00:43:53,360 and into customers' hands. 1064 00:43:53,360 --> 00:43:55,660 It's a long time and what this results in 1065 00:43:55,660 --> 00:43:58,090 are drastic oversupply and undersupply 1066 00:43:58,090 --> 00:43:59,500 conditions in the market. 1067 00:43:59,500 --> 00:44:01,310 So the silicon feedstock price goes 1068 00:44:01,310 --> 00:44:04,110 very high during periods of undersupply and very low 1069 00:44:04,110 --> 00:44:06,370 in periods of oversupply and we're in an oversupply 1070 00:44:06,370 --> 00:44:08,290 condition right now. 1071 00:44:08,290 --> 00:44:11,530 Five years ago, let me quantify this. 1072 00:44:11,530 --> 00:44:13,830 Five years ago if you went to the spot market-- 1073 00:44:13,830 --> 00:44:16,080 maybe four years ago-- if you went to the spot market, 1074 00:44:16,080 --> 00:44:20,480 you could pay $100 to $500 per kilogram of silicon. 1075 00:44:20,480 --> 00:44:23,070 That material that was just right there I bet one you 1076 00:44:23,070 --> 00:44:25,900 would put it into your bag and run away out the door right now 1077 00:44:25,900 --> 00:44:28,590 and be able to go to Mexico. 1078 00:44:28,590 --> 00:44:32,170 Now the polysilicon prices are much, 1079 00:44:32,170 --> 00:44:33,520 much lower on the spot market. 1080 00:44:33,520 --> 00:44:36,830 Somewhere in the order of $30 to $50 per kilogram. 1081 00:44:36,830 --> 00:44:39,680 About an order of magnitude lower. 1082 00:44:39,680 --> 00:44:43,241 AUDIENCE: Isn't lower cost silicon better for the PV 1083 00:44:43,241 --> 00:44:43,950 industry, though? 1084 00:44:43,950 --> 00:44:45,782 PROFESSOR: Is it better for the PV industry? 1085 00:44:45,782 --> 00:44:47,920 As a customer most definitely, it is good for you. 1086 00:44:47,920 --> 00:44:50,380 As an installer, it is most definitely good for you. 1087 00:44:50,380 --> 00:44:52,440 As a polysilicon producer who wants 1088 00:44:52,440 --> 00:44:54,440 to be a sustained industry presence, 1089 00:44:54,440 --> 00:44:55,530 it's not good for you. 1090 00:44:55,530 --> 00:45:00,710 So this wide oscillation between fat cat and scrawny 1091 00:45:00,710 --> 00:45:03,220 is not very good for any industry. 1092 00:45:03,220 --> 00:45:06,450 It's unpredictable and it causes some players to drop out. 1093 00:45:06,450 --> 00:45:06,992 AUDIENCE: OK. 1094 00:45:06,992 --> 00:45:09,241 PROFESSOR: And the investments are very large as well. 1095 00:45:09,241 --> 00:45:11,570 As you go from the early stage portions of the value 1096 00:45:11,570 --> 00:45:15,870 chain toward the module, the investments generally decrease 1097 00:45:15,870 --> 00:45:19,650 and so this is an outlook coming from last year-- 1098 00:45:19,650 --> 00:45:22,040 the numbers are still a little bit outdated-- polysilicon 1099 00:45:22,040 --> 00:45:25,500 production is buttressing up against 200,000 1100 00:45:25,500 --> 00:45:28,900 metric tons per year at this point in about 3/4 1101 00:45:28,900 --> 00:45:30,340 to the PV industry. 1102 00:45:30,340 --> 00:45:33,960 The cost of manufacturing is between $20 and $25 1103 00:45:33,960 --> 00:45:38,450 per kilogram and 2010 prices were around $50 to $70. 1104 00:45:38,450 --> 00:45:42,560 Now they're on $30 to $50 in 2011 1105 00:45:42,560 --> 00:45:46,150 and the 2008 prices were around $500 per kilogram in the spot 1106 00:45:46,150 --> 00:45:49,670 market and it really boils down to the inability 1107 00:45:49,670 --> 00:45:51,200 to adapt to demand. 1108 00:45:51,200 --> 00:45:53,550 If you have a very large contraption that 1109 00:45:53,550 --> 00:45:55,932 produces the feedstock materials and it takes a long time 1110 00:45:55,932 --> 00:45:57,390 to build the factories, you're just 1111 00:45:57,390 --> 00:46:00,870 not going to be able to adjust fast enough. 1112 00:46:00,870 --> 00:46:03,700 Here's supply and demand, demand being the red 1113 00:46:03,700 --> 00:46:05,210 and supply being the blue. 1114 00:46:05,210 --> 00:46:08,380 You can see how the oversupply-- the undersupply condition 1115 00:46:08,380 --> 00:46:13,740 of the mid 2000s really led to our current condition. 1116 00:46:13,740 --> 00:46:17,300 So, alternatives to solar grade silicon feedstock refining. 1117 00:46:17,300 --> 00:46:20,270 What are some people thinking in terms of other processes 1118 00:46:20,270 --> 00:46:21,690 that they can use? 1119 00:46:21,690 --> 00:46:26,300 These are two processes right here and, mind you, 1120 00:46:26,300 --> 00:46:30,180 when we were in this situation with this price 1121 00:46:30,180 --> 00:46:32,215 for the silicon, everybody and anybody 1122 00:46:32,215 --> 00:46:34,840 was coming up with new ideas of how to manufacture the silicon. 1123 00:46:34,840 --> 00:46:39,630 Now that we're barely selling at cost and in an oversupply 1124 00:46:39,630 --> 00:46:42,150 condition, many of these ideas are having a struggle-- 1125 00:46:42,150 --> 00:46:43,320 a hard time in the market. 1126 00:46:43,320 --> 00:46:44,936 They're struggling right now. 1127 00:46:44,936 --> 00:46:47,310 So fluidized bed reactor and upgraded metallurgical grade 1128 00:46:47,310 --> 00:46:49,260 silicon. 1129 00:46:49,260 --> 00:46:52,050 Let's talk about each of those in turn. 1130 00:46:52,050 --> 00:46:54,040 So what the fluidized bed reactor folks 1131 00:46:54,040 --> 00:46:57,550 realized was, gee, if we're depositing on a rod, 1132 00:46:57,550 --> 00:47:01,940 our surface area to volume ratio is really large-- sorry, 1133 00:47:01,940 --> 00:47:03,120 is really small. 1134 00:47:03,120 --> 00:47:06,480 Our surface area to volume ratio is going to be very small. 1135 00:47:06,480 --> 00:47:07,650 So think of it this way. 1136 00:47:07,650 --> 00:47:10,080 If we have a sphere, a sphere would 1137 00:47:10,080 --> 00:47:11,910 be the quintessential example where 1138 00:47:11,910 --> 00:47:15,880 we'd have a very large surface area to volume ratio. 1139 00:47:15,880 --> 00:47:18,270 If we had a plate, we would have, as well, 1140 00:47:18,270 --> 00:47:21,260 a very large surface area to volume ratio 1141 00:47:21,260 --> 00:47:26,130 and in the case of the condition prior, where you have this rod, 1142 00:47:26,130 --> 00:47:28,370 you really can't deposit that quickly and so what 1143 00:47:28,370 --> 00:47:30,390 these folks decided was, what we're going to do 1144 00:47:30,390 --> 00:47:35,290 is introduce small silicon granules into this vessel, 1145 00:47:35,290 --> 00:47:38,114 into this evacuated chamber, and-- here's 1146 00:47:38,114 --> 00:47:39,530 the evacuated chamber right here-- 1147 00:47:39,530 --> 00:47:45,482 and we're going to flow silane gas into the system right here 1148 00:47:45,482 --> 00:47:46,940 and the smaller particles are going 1149 00:47:46,940 --> 00:47:49,550 to go higher up because of this flow of gas 1150 00:47:49,550 --> 00:47:52,820 coming in the bottom and those will grow and eventually 1151 00:47:52,820 --> 00:47:56,230 settle down down here where we can extract the bottom. 1152 00:47:56,230 --> 00:47:58,800 So we'll wind up with these beautiful little silicon 1153 00:47:58,800 --> 00:47:59,585 granules. 1154 00:47:59,585 --> 00:48:01,710 These ones shown right here, which I'll pass around 1155 00:48:01,710 --> 00:48:04,480 as well, those are coming from a fluidized bed reactor, 1156 00:48:04,480 --> 00:48:08,020 and they're nice beautiful, spherical granules that 1157 00:48:08,020 --> 00:48:10,540 are grown a lot faster, I mean, a lot more silicon 1158 00:48:10,540 --> 00:48:14,240 is deposited per unit time than through the Siemens process 1159 00:48:14,240 --> 00:48:15,640 as shown there in the back. 1160 00:48:15,640 --> 00:48:17,920 As a result, the energy intensity is lower, 1161 00:48:17,920 --> 00:48:20,610 the cost is lower, there's a very tricky process 1162 00:48:20,610 --> 00:48:22,586 to nail to get just right, because you 1163 00:48:22,586 --> 00:48:24,085 have to get the gas flows right, you 1164 00:48:24,085 --> 00:48:27,820 have to design the chamber well, redo some purity contents. 1165 00:48:27,820 --> 00:48:29,520 It's a tricky process, and so this 1166 00:48:29,520 --> 00:48:31,670 is being produced right now, I believe, 1167 00:48:31,670 --> 00:48:32,980 by only a few companies. 1168 00:48:32,980 --> 00:48:35,510 REC has a capability of doing it. 1169 00:48:35,510 --> 00:48:37,930 MEMC, as well, has the capability 1170 00:48:37,930 --> 00:48:39,680 of doing this process. 1171 00:48:39,680 --> 00:48:41,370 By and large, most silicon is coming 1172 00:48:41,370 --> 00:48:44,770 from the Siemens process. 1173 00:48:44,770 --> 00:48:45,270 Yup. 1174 00:48:45,270 --> 00:48:46,936 AUDIENCE: Sorry, both of those companies 1175 00:48:46,936 --> 00:48:48,590 have the normal refining process? 1176 00:48:48,590 --> 00:48:49,760 PROFESSOR: They have the normal refining process. 1177 00:48:49,760 --> 00:48:50,801 AUDIENCE: The [INAUDIBLE] 1178 00:48:50,801 --> 00:48:53,640 PROFESSOR: Yup, and that's why they developed this new one. 1179 00:48:53,640 --> 00:48:55,600 They had these smaller, internal projects 1180 00:48:55,600 --> 00:48:57,220 that we're able to develop. 1181 00:49:00,750 --> 00:49:01,810 So, yeah. 1182 00:49:01,810 --> 00:49:03,734 I was just mentioning the energy intensity. 1183 00:49:03,734 --> 00:49:05,400 This is the kilowatt hours per kilogram, 1184 00:49:05,400 --> 00:49:07,690 going back to your question about energy intensity. 1185 00:49:07,690 --> 00:49:10,660 This is trichlorosilane based Siemens process, 1186 00:49:10,660 --> 00:49:12,600 silane based Siemens process. 1187 00:49:12,600 --> 00:49:15,370 They're more ore less comparable in terms of energy intensity. 1188 00:49:15,370 --> 00:49:18,400 And the silance based fluidized bed reactor process. 1189 00:49:18,400 --> 00:49:20,180 According to internal REC numbers, 1190 00:49:20,180 --> 00:49:21,945 which are little rosy, but never the less, 1191 00:49:21,945 --> 00:49:23,420 the trend is correct here. 1192 00:49:23,420 --> 00:49:25,560 It is lower somewhere in the order 1193 00:49:25,560 --> 00:49:29,080 of an order of magnitude energy intensity, 1194 00:49:29,080 --> 00:49:31,620 and cost is lower as well. 1195 00:49:31,620 --> 00:49:36,330 So let's move away from the silicon refining 1196 00:49:36,330 --> 00:49:38,270 by distillation process entirely. 1197 00:49:38,270 --> 00:49:41,817 Let's leave gaseous distillation aside and say, 1198 00:49:41,817 --> 00:49:44,150 what if we were to take this metallurgical-grade silicon 1199 00:49:44,150 --> 00:49:47,330 and, through liquid purification routes, 1200 00:49:47,330 --> 00:49:49,450 result in high purity silicon. 1201 00:49:49,450 --> 00:49:50,440 How would we do that? 1202 00:49:50,440 --> 00:49:52,660 Well, if we turn to other industries, the ones 1203 00:49:52,660 --> 00:49:56,770 that smelter aluminum or refine manganese and so forth, 1204 00:49:56,770 --> 00:49:59,910 we would see a multitude of different options 1205 00:49:59,910 --> 00:50:01,070 that we could borrow. 1206 00:50:01,070 --> 00:50:03,940 Slag refining, bleaching, leaching solidification. 1207 00:50:03,940 --> 00:50:06,780 Let me walk through them one by one. 1208 00:50:06,780 --> 00:50:09,430 Leaching-- that's fairly straightforward. 1209 00:50:09,430 --> 00:50:14,370 So if we put in some acid, for instance, 1210 00:50:14,370 --> 00:50:17,430 that dissolves the metals but doesn't dissolve the silicon 1211 00:50:17,430 --> 00:50:19,827 we could leach the metals out of the material, 1212 00:50:19,827 --> 00:50:21,410 and so that's the essence of leaching. 1213 00:50:21,410 --> 00:50:23,210 You might crush up your material, 1214 00:50:23,210 --> 00:50:27,270 in other ways other ways expose the metals, or impurities, 1215 00:50:27,270 --> 00:50:29,830 to the acids inside of your system. 1216 00:50:29,830 --> 00:50:31,820 Slag refining says, gee, what if we 1217 00:50:31,820 --> 00:50:34,210 were to introduce some material that could 1218 00:50:34,210 --> 00:50:36,240 absorb the metals into it? 1219 00:50:36,240 --> 00:50:37,920 The solubility of the metals would 1220 00:50:37,920 --> 00:50:39,930 be higher inside of the slag agent 1221 00:50:39,930 --> 00:50:41,490 than inside of the liquid. 1222 00:50:41,490 --> 00:50:45,210 Maybe we throw in calcium oxide or yttrium oxide or some, 1223 00:50:45,210 --> 00:50:47,810 usually it's a metal oxide that has a very high melting 1224 00:50:47,810 --> 00:50:51,910 temperature that remains a solid or, at least a glassy solid, 1225 00:50:51,910 --> 00:50:54,300 and we pour it on top of our silicon 1226 00:50:54,300 --> 00:50:57,730 and it's able to absorb, say, the phosphorus or the boron 1227 00:50:57,730 --> 00:51:00,920 that's inside of our silicon so that we reduce impurity content 1228 00:51:00,920 --> 00:51:02,480 and then we can add the phosphorous and boron later 1229 00:51:02,480 --> 00:51:04,146 intentionally, but to the concentrations 1230 00:51:04,146 --> 00:51:07,150 we want not to exuberantly high concentrations that 1231 00:51:07,150 --> 00:51:08,800 might be found in nature. 1232 00:51:08,800 --> 00:51:11,580 Solidification-- during this solidification process 1233 00:51:11,580 --> 00:51:13,455 you're taking your molten silicon 1234 00:51:13,455 --> 00:51:15,080 and you're solidifying it directionally 1235 00:51:15,080 --> 00:51:18,356 from the bottom up and, because the solubility of impurities 1236 00:51:18,356 --> 00:51:20,730 tends to be larger in the liquid than it is in the solid, 1237 00:51:20,730 --> 00:51:23,140 it's like dragging a comb through the entire material 1238 00:51:23,140 --> 00:51:24,400 dragging out the impurities. 1239 00:51:24,400 --> 00:51:28,610 Concentrating them in the liquid and leaving a more pure silicon 1240 00:51:28,610 --> 00:51:29,481 behind. 1241 00:51:29,481 --> 00:51:30,980 Obviously, at the very, very end you 1242 00:51:30,980 --> 00:51:32,980 have this highly concentrated region 1243 00:51:32,980 --> 00:51:35,680 of impurities which then you have to slice off and remove, 1244 00:51:35,680 --> 00:51:37,350 so the solicitation process doesn't 1245 00:51:37,350 --> 00:51:39,089 come without a yield penalty. 1246 00:51:39,089 --> 00:51:40,880 You still throw away some of your material. 1247 00:51:40,880 --> 00:51:43,244 So you can't repeat the solidification over and over 1248 00:51:43,244 --> 00:51:44,660 and over again, I guess you could, 1249 00:51:44,660 --> 00:51:47,140 but you'd be losing material every step. 1250 00:51:47,140 --> 00:51:49,720 So some combination of these processes here, and others. 1251 00:51:49,720 --> 00:51:51,450 Other trickery. 1252 00:51:51,450 --> 00:51:55,910 Low temperature eutectic formation with other elements, 1253 00:51:55,910 --> 00:51:57,230 for example. 1254 00:51:57,230 --> 00:52:00,260 Some combination of this is used to refine the silicon 1255 00:52:00,260 --> 00:52:02,000 without creating a gas out of it. 1256 00:52:02,000 --> 00:52:03,830 So wafer fabrication. 1257 00:52:03,830 --> 00:52:05,407 We're now going from feedstock, we're 1258 00:52:05,407 --> 00:52:06,990 leaving feedstocking behind, and we're 1259 00:52:06,990 --> 00:52:08,760 going to be talking about how do you go from the feedstock 1260 00:52:08,760 --> 00:52:10,718 materials that are being passed around the room 1261 00:52:10,718 --> 00:52:14,010 right now into a wafer that you can then manufacture 1262 00:52:14,010 --> 00:52:15,330 a solar cell device out of? 1263 00:52:15,330 --> 00:52:16,890 One of these for instance. 1264 00:52:16,890 --> 00:52:20,280 So let's talk about wafer fabrication right here. 1265 00:52:20,280 --> 00:52:21,980 So again, just to situate ourselves, 1266 00:52:21,980 --> 00:52:24,730 we've gone from raw materials to silicon feedstock 1267 00:52:24,730 --> 00:52:27,250 and now we're going to feedstocks to wafers. 1268 00:52:27,250 --> 00:52:30,240 Any questions right now before we dive into that? 1269 00:52:30,240 --> 00:52:30,740 Yeah. 1270 00:52:30,740 --> 00:52:32,470 AUDIENCE: Question about supply. 1271 00:52:32,470 --> 00:52:35,552 So silicon is very abundant but the high purity 1272 00:52:35,552 --> 00:52:38,900 silica deposits-- are they really abundant too? 1273 00:52:38,900 --> 00:52:40,180 PROFESSOR: Great question. 1274 00:52:40,180 --> 00:52:43,010 So the question was are the high purity silica deposits 1275 00:52:43,010 --> 00:52:45,400 as abundant as, say, silicon. 1276 00:52:45,400 --> 00:52:45,900 Certainly. 1277 00:52:45,900 --> 00:52:49,120 If you bend over and rub your fingers against the ground 1278 00:52:49,120 --> 00:52:52,080 you're probably going to come up with, probably, 1279 00:52:52,080 --> 00:52:55,930 millions of trillions of silicon atoms in your fingernails. 1280 00:52:55,930 --> 00:52:57,620 Those are not very purity. 1281 00:52:57,620 --> 00:53:01,260 So the highest security quartz deposits are more rare 1282 00:53:01,260 --> 00:53:03,620 and they are sought after, and so they're are known. 1283 00:53:03,620 --> 00:53:04,950 Their locations are known. 1284 00:53:04,950 --> 00:53:07,170 There's one specific one in Norway, 1285 00:53:07,170 --> 00:53:09,310 one specific one in North Carolina, 1286 00:53:09,310 --> 00:53:13,150 and so forth around the world and there-- in a sense, 1287 00:53:13,150 --> 00:53:14,140 they go to places. 1288 00:53:14,140 --> 00:53:16,830 People have adjusted their metallurgical-grade silicon 1289 00:53:16,830 --> 00:53:18,750 refineries and their subsequent down process 1290 00:53:18,750 --> 00:53:19,922 for that particular ore. 1291 00:53:19,922 --> 00:53:22,130 Once you run out of it, it's not that the world ends, 1292 00:53:22,130 --> 00:53:26,580 we just have to adjust for the next feedstock source. 1293 00:53:26,580 --> 00:53:29,640 So, in principle, there are people looking 1294 00:53:29,640 --> 00:53:31,080 at a variety of silicon inputs. 1295 00:53:31,080 --> 00:53:35,580 Anything from the dirtier, compressed, metamorphic quartz 1296 00:53:35,580 --> 00:53:36,600 that I mentioned. 1297 00:53:36,600 --> 00:53:39,390 Some people looking at rice husks, 1298 00:53:39,390 --> 00:53:41,206 which are silica rich as well. 1299 00:53:41,206 --> 00:53:42,580 Other people looking at seashells 1300 00:53:42,580 --> 00:53:44,996 which, mostly calcium carbonate, but other things as well. 1301 00:53:44,996 --> 00:53:47,860 I mean, there was a wide range. 1302 00:53:47,860 --> 00:53:50,690 When the price of silicon was $500 per kilogram, 1303 00:53:50,690 --> 00:53:52,520 you got a multitude of ideas. 1304 00:53:52,520 --> 00:53:54,850 When the price comes back down, people 1305 00:53:54,850 --> 00:53:57,744 tend to be more conservative. 1306 00:53:57,744 --> 00:54:02,100 AUDIENCE: Is silicon considered a renewable resource? 1307 00:54:02,100 --> 00:54:04,389 PROFESSOR: Is silicon considered a renewable resource. 1308 00:54:04,389 --> 00:54:06,180 It is not a renewable resource in the sense 1309 00:54:06,180 --> 00:54:09,150 that, once you mine it from the ground, 1310 00:54:09,150 --> 00:54:11,650 you've mined it from the ground and you used in some way. 1311 00:54:11,650 --> 00:54:14,740 The reason it's considered not an issue 1312 00:54:14,740 --> 00:54:16,920 is because there's so much of it. 1313 00:54:16,920 --> 00:54:20,610 Not all of it, though, is in the easy to access form. 1314 00:54:20,610 --> 00:54:21,110 Right? 1315 00:54:21,110 --> 00:54:22,651 Some of the silicon might be bound up 1316 00:54:22,651 --> 00:54:26,200 within heavily contaminated sources 1317 00:54:26,200 --> 00:54:30,800 and that's where the refining ingenuity comes into play. 1318 00:54:30,800 --> 00:54:34,210 As long as prices remain low, there's not too much interest, 1319 00:54:34,210 --> 00:54:36,100 say, for example, that mine in Peru 1320 00:54:36,100 --> 00:54:42,780 that has titanium oxide needles throughout their silicon 1321 00:54:42,780 --> 00:54:44,960 because why would you want heavily 1322 00:54:44,960 --> 00:54:47,070 titanium contaminated silicon? 1323 00:54:47,070 --> 00:54:50,030 But as the price of silicon, it probably will, 1324 00:54:50,030 --> 00:54:52,800 rise again then people might take another look at that mine 1325 00:54:52,800 --> 00:54:55,380 and say gee, how can we phase separate the rutile 1326 00:54:55,380 --> 00:54:57,240 and anatase from the quartz early 1327 00:54:57,240 --> 00:54:59,110 on in the process by crushing and etching 1328 00:54:59,110 --> 00:55:04,050 or something so we can access this feedstock material. 1329 00:55:04,050 --> 00:55:06,090 We'll see. 1330 00:55:06,090 --> 00:55:08,950 It really depends on how the market evolves, where people 1331 00:55:08,950 --> 00:55:11,385 go looking for their silicon, but there's a lot of it 1332 00:55:11,385 --> 00:55:12,260 in the earth's crust. 1333 00:55:12,260 --> 00:55:14,936 AUDIENCE: You're not concerned about silicon? 1334 00:55:14,936 --> 00:55:15,830 PROFESSOR: No. 1335 00:55:15,830 --> 00:55:16,850 Nope. 1336 00:55:16,850 --> 00:55:20,280 What is a bigger bottleneck are are the refining steps 1337 00:55:20,280 --> 00:55:21,090 in between. 1338 00:55:21,090 --> 00:55:23,854 First it was the reactors and soon it's 1339 00:55:23,854 --> 00:55:26,020 probably going to be the metallurgical-grade silicon 1340 00:55:26,020 --> 00:55:28,950 reactors as well. 1341 00:55:28,950 --> 00:55:29,600 All right. 1342 00:55:29,600 --> 00:55:30,840 Wafers. 1343 00:55:30,840 --> 00:55:33,020 How do we get to these from the raw feedstock 1344 00:55:33,020 --> 00:55:36,260 materials that are being passed around the room right now? 1345 00:55:36,260 --> 00:55:38,741 So single crystalline silicon ingot growth. 1346 00:55:38,741 --> 00:55:39,990 Let's walk through that first. 1347 00:55:39,990 --> 00:55:42,160 How do we get these beautiful ingots? 1348 00:55:42,160 --> 00:55:45,910 They're about half of all silicon market right now. 1349 00:55:45,910 --> 00:55:48,250 The biggest growth method, by far, 1350 00:55:48,250 --> 00:55:50,530 is called Czochralski growth and, named 1351 00:55:50,530 --> 00:55:54,920 after the Polish physicist there Jan Czochralski. 1352 00:55:54,920 --> 00:55:58,320 What you do is you have a bath of molten silicon. 1353 00:55:58,320 --> 00:55:59,840 A crucible, if you will. 1354 00:55:59,840 --> 00:56:01,480 This tends to be a circular crucible, 1355 00:56:01,480 --> 00:56:03,150 rounded at the bottom, usually made 1356 00:56:03,150 --> 00:56:05,195 of quartz with heaters on the outside 1357 00:56:05,195 --> 00:56:07,070 to heat up the molten silicon. 1358 00:56:07,070 --> 00:56:09,180 To heat up the silicon chunks in here. 1359 00:56:09,180 --> 00:56:10,640 Once everything is molten, looking 1360 00:56:10,640 --> 00:56:12,370 like a big bathtub of silicon, you 1361 00:56:12,370 --> 00:56:15,890 introduce a small crystalline silicon seed 1362 00:56:15,890 --> 00:56:18,450 into that molten silicon and then 1363 00:56:18,450 --> 00:56:20,880 you begin pulling while rotating that seed. 1364 00:56:20,880 --> 00:56:22,992 So the seed is a single crystal material 1365 00:56:22,992 --> 00:56:24,950 and what ends up happening is, as you introduce 1366 00:56:24,950 --> 00:56:27,020 the seed into the material and begin pulling, 1367 00:56:27,020 --> 00:56:29,000 you start pulling out this crystal. 1368 00:56:29,000 --> 00:56:30,900 Single crystalline crystal. 1369 00:56:30,900 --> 00:56:33,850 It's a thing of beauty and this seed is actually 1370 00:56:33,850 --> 00:56:35,790 very, very narrow in diameter. 1371 00:56:35,790 --> 00:56:38,370 It might be about that big around so pretty narrow 1372 00:56:38,370 --> 00:56:40,830 in diameter and it's being able to support 1373 00:56:40,830 --> 00:56:43,870 this ingot of a few, usually a few, 1374 00:56:43,870 --> 00:56:46,310 tens to hundreds of kilograms of mass underneath it 1375 00:56:46,310 --> 00:56:49,790 and that's because silicon is very strong even though it's 1376 00:56:49,790 --> 00:56:51,260 brittle. 1377 00:56:51,260 --> 00:56:55,100 So if you weren't to apply, say, for example, a shear 1378 00:56:55,100 --> 00:56:57,850 force on your silicon but just to apply an axial load, 1379 00:56:57,850 --> 00:57:00,570 you could support a very, large weight underneath it. 1380 00:57:00,570 --> 00:57:04,140 So the [INAUDIBLE] of silicon is grown from the bottom 1381 00:57:04,140 --> 00:57:06,640 and eventually you wind up with this nice ingot, 1382 00:57:06,640 --> 00:57:08,290 as shown right there. 1383 00:57:08,290 --> 00:57:12,690 The art that goes into growing this properly is amazing. 1384 00:57:12,690 --> 00:57:15,770 I'll highlight it with one small little example just 1385 00:57:15,770 --> 00:57:18,630 to illustrate the bigger picture that a lot of effort 1386 00:57:18,630 --> 00:57:21,340 goes into making these defect-free, quote unquote, 1387 00:57:21,340 --> 00:57:22,487 defect-free crystals. 1388 00:57:22,487 --> 00:57:24,820 They're called defect-free because they contain no grain 1389 00:57:24,820 --> 00:57:27,080 boundaries and no dislocations. 1390 00:57:27,080 --> 00:57:30,490 They have impurities, they have intrinsic point defects, 1391 00:57:30,490 --> 00:57:32,394 meaning vacancies or interstitial atoms, 1392 00:57:32,394 --> 00:57:34,560 but they don't have grain boundaries or dislocations 1393 00:57:34,560 --> 00:57:37,930 and so they're called defect-free silicon. 1394 00:57:37,930 --> 00:57:40,380 You introduce that seed down into the liquid melt. 1395 00:57:40,380 --> 00:57:41,370 Thermal stress happens. 1396 00:57:41,370 --> 00:57:41,870 Right? 1397 00:57:41,870 --> 00:57:47,690 Because you have the shock between the solid silicon seed 1398 00:57:47,690 --> 00:57:49,550 encountering the liquid for the first time. 1399 00:57:49,550 --> 00:57:51,040 So this locations [? form ?]. 1400 00:57:51,040 --> 00:57:54,040 And you have to pull the seed out in such a way, 1401 00:57:54,040 --> 00:57:57,040 you slowly rotate and make this shoulder. 1402 00:57:57,040 --> 00:57:59,130 The shoulder has to be as quick as possible 1403 00:57:59,130 --> 00:58:01,960 because you don't want to waste material. 1404 00:58:01,960 --> 00:58:05,140 Everything inside this shoulder right here gets thrown away. 1405 00:58:05,140 --> 00:58:07,920 So that little piece of material right there gets tossed out. 1406 00:58:07,920 --> 00:58:10,800 So you want to make the shoulders as narrow and as 1407 00:58:10,800 --> 00:58:12,450 quick as possible so you can utilize 1408 00:58:12,450 --> 00:58:14,140 the majority of your ingot but, at the same time, 1409 00:58:14,140 --> 00:58:15,690 you have to make it thick enough so 1410 00:58:15,690 --> 00:58:17,850 that the dislocations can move all the way 1411 00:58:17,850 --> 00:58:19,920 and propagate all the way to the outside 1412 00:58:19,920 --> 00:58:21,960 and end and terminate in the shoulder 1413 00:58:21,960 --> 00:58:24,420 before propagating into the crystal. 1414 00:58:24,420 --> 00:58:26,849 So that's just one example of the technology 1415 00:58:26,849 --> 00:58:28,140 that goes in the growing these. 1416 00:58:28,140 --> 00:58:30,660 Another might be, gee, we're PV industry, 1417 00:58:30,660 --> 00:58:34,220 we want to make the stuff fast whereas, in the IC industry 1418 00:58:34,220 --> 00:58:38,060 you can invest up to a few of dollars per gram of silicon 1419 00:58:38,060 --> 00:58:40,110 and still make a profit because you're selling 1420 00:58:40,110 --> 00:58:41,660 a computer at 1,000 bucks. 1421 00:58:41,660 --> 00:58:43,500 In the PV industry, we can invest, 1422 00:58:43,500 --> 00:58:46,122 at most, a few tens of cents per gram of silicon. 1423 00:58:46,122 --> 00:58:47,580 So we have to make this stuff fast. 1424 00:58:47,580 --> 00:58:48,617 We can't dilly dally. 1425 00:58:48,617 --> 00:58:50,450 You might want to crank up the growth speed, 1426 00:58:50,450 --> 00:58:52,480 then you run into issues with defect concentrations, 1427 00:58:52,480 --> 00:58:54,080 intrinsic point defect concentrations, 1428 00:58:54,080 --> 00:58:55,410 during the growth. 1429 00:58:55,410 --> 00:58:57,370 I'm illustrating this just to highlight 1430 00:58:57,370 --> 00:59:01,820 the complexity of the growth process of making these ingots, 1431 00:59:01,820 --> 00:59:04,560 and the latter example was one that the PV industry 1432 00:59:04,560 --> 00:59:05,460 is facing today. 1433 00:59:05,460 --> 00:59:06,925 It's actually a hot research topic. 1434 00:59:06,925 --> 00:59:07,425 Yeah. 1435 00:59:07,425 --> 00:59:08,091 And then Ashley. 1436 00:59:08,091 --> 00:59:11,074 AUDIENCE: The rotation speed does that just affect time, 1437 00:59:11,074 --> 00:59:12,490 or does it affect other things? 1438 00:59:12,490 --> 00:59:15,036 PROFESSOR: So it affects a multitude of things. 1439 00:59:15,036 --> 00:59:16,410 One of the things that it affects 1440 00:59:16,410 --> 00:59:19,660 is the flow of, the convective flow, of the melt. 1441 00:59:19,660 --> 00:59:21,540 So the liquid flow inside of the melt 1442 00:59:21,540 --> 00:59:24,470 is, in part, determining how much oxygen 1443 00:59:24,470 --> 00:59:26,430 gets transported from this crucible 1444 00:59:26,430 --> 00:59:28,360 here into the growing crystal. 1445 00:59:28,360 --> 00:59:31,290 If you manage to suppress that convective flow in the melt, 1446 00:59:31,290 --> 00:59:33,797 you will also suppress oxygen transport 1447 00:59:33,797 --> 00:59:35,630 since the fusion is going to be a lot slower 1448 00:59:35,630 --> 00:59:40,920 than turbulent transport or [INAUDIBLE] transport 1449 00:59:40,920 --> 00:59:42,811 or convective transport. 1450 00:59:42,811 --> 00:59:43,310 Yeah. 1451 00:59:43,310 --> 00:59:43,600 Question? 1452 00:59:43,600 --> 00:59:44,860 AUDIENCE: I just have two questions 1453 00:59:44,860 --> 00:59:46,510 so one is how fast do you rotate it 1454 00:59:46,510 --> 00:59:49,430 and the other is what that does control-- the diameter 1455 00:59:49,430 --> 00:59:51,286 because I've heard of 12 inch wafers 1456 00:59:51,286 --> 00:59:53,150 versus like 18 inch wafers. 1457 00:59:53,150 --> 00:59:53,980 PROFESSOR: Sure. 1458 00:59:53,980 --> 00:59:56,210 So one of the things that controls diameter 1459 00:59:56,210 --> 01:00:00,750 is the balance of heat extraction. 1460 01:00:00,750 --> 01:00:03,160 So if you cool something down, especially molten silicon, 1461 01:00:03,160 --> 01:00:04,560 it will freeze, it will grow. 1462 01:00:04,560 --> 01:00:06,330 If you heat it up, it will shrink. 1463 01:00:06,330 --> 01:00:08,930 So that's one of the components that controls the diameter. 1464 01:00:08,930 --> 01:00:12,850 The pull speed and how you grow that shoulder, 1465 01:00:12,850 --> 01:00:14,700 essentially how you heat up the material 1466 01:00:14,700 --> 01:00:17,020 and how fast you pull at those initial stages, 1467 01:00:17,020 --> 01:00:18,440 also dictates the diameter and you 1468 01:00:18,440 --> 01:00:20,110 can see in the ingots themselves, 1469 01:00:20,110 --> 01:00:21,140 they're not perfect. 1470 01:00:21,140 --> 01:00:23,260 They have a little bit of corregation 1471 01:00:23,260 --> 01:00:27,010 and that's the fluctuations of the temperature of the melt, 1472 01:00:27,010 --> 01:00:28,710 fluctuations of the heater output, 1473 01:00:28,710 --> 01:00:32,540 fluctuations of pull speed, maybe what's pulling 1474 01:00:32,540 --> 01:00:35,760 this entire contraption is kind of a stepper motor 1475 01:00:35,760 --> 01:00:38,500 that has a certain granularity to it. 1476 01:00:38,500 --> 01:00:40,770 Results in corregated edges. 1477 01:00:40,770 --> 01:00:42,530 It's not perfect and so there will 1478 01:00:42,530 --> 01:00:47,640 be some adjustment made to the form factor of the edge 1479 01:00:47,640 --> 01:00:50,910 to get this nice round wafer at the end of the day. 1480 01:00:50,910 --> 01:00:52,910 AUDIENCE: Does the seed rod [INAUDIBLE] 1481 01:00:52,910 --> 01:00:54,965 all the way to the ingot or just near the top? 1482 01:00:54,965 --> 01:00:55,840 PROFESSOR: All right. 1483 01:00:55,840 --> 01:00:57,540 So the entire ingot becomes pattern 1484 01:00:57,540 --> 01:00:59,200 or templated by the seed rod. 1485 01:00:59,200 --> 01:01:01,750 So this entire ingot has the same crystalline orientation 1486 01:01:01,750 --> 01:01:02,673 as a seed. 1487 01:01:02,673 --> 01:01:07,650 AUDIENCE: And is the seed doped differently than the silicon? 1488 01:01:07,650 --> 01:01:09,490 PROFESSOR: It might be but I'm not aware 1489 01:01:09,490 --> 01:01:11,600 that that affects the overall process. 1490 01:01:11,600 --> 01:01:13,930 It could be that it's one of the critical pieces 1491 01:01:13,930 --> 01:01:16,263 of the magic sauce that makes it work but I'm not aware. 1492 01:01:19,730 --> 01:01:20,530 Rotation speed. 1493 01:01:20,530 --> 01:01:23,620 It's not rotating like this it's a slow rotation 1494 01:01:23,620 --> 01:01:26,652 so I would-- let's see. 1495 01:01:26,652 --> 01:01:27,860 How many radians per second-- 1496 01:01:27,860 --> 01:01:28,910 AUDIENCE: Can you see it? 1497 01:01:28,910 --> 01:01:30,185 PROFESSOR: You can visually see it 1498 01:01:30,185 --> 01:01:31,518 if you looked at it long enough. 1499 01:01:31,518 --> 01:01:32,100 Yeah. 1500 01:01:32,100 --> 01:01:34,330 Yeah. 1501 01:01:34,330 --> 01:01:36,540 So one modification, one variant, 1502 01:01:36,540 --> 01:01:38,760 of the single crystalline growth method 1503 01:01:38,760 --> 01:01:40,100 is called float-zone growth. 1504 01:01:40,100 --> 01:01:44,100 You take a rod of poly, much like that right over there 1505 01:01:44,100 --> 01:01:49,720 that's inside of here, and you pass an RF coil, 1506 01:01:49,720 --> 01:01:52,220 radio frequency coil, next to the rod 1507 01:01:52,220 --> 01:01:54,140 and what that does is, essentially, heats up 1508 01:01:54,140 --> 01:01:56,870 the silicon, if it's doped highly enough. 1509 01:01:56,870 --> 01:01:59,600 It will melt the silicon locally. 1510 01:01:59,600 --> 01:02:03,230 Folks have probably heard of fancy high-end stoves 1511 01:02:03,230 --> 01:02:07,130 that we can only probably hope to afford in 10 or 15 years, 1512 01:02:07,130 --> 01:02:11,560 but these stoves that are inductive heaters. 1513 01:02:11,560 --> 01:02:12,060 Right? 1514 01:02:12,060 --> 01:02:15,334 They're not resistive heating elements, 1515 01:02:15,334 --> 01:02:17,250 they're inductive heating elements and the way 1516 01:02:17,250 --> 01:02:21,860 that works is you have a radio frequency source that then 1517 01:02:21,860 --> 01:02:25,870 is absorbed by, in the case of the RF heater, 1518 01:02:25,870 --> 01:02:28,630 I believe it's a specific type of iron 1519 01:02:28,630 --> 01:02:34,580 that the inductive heating ovens need. 1520 01:02:34,580 --> 01:02:38,160 And so this RF coil here is emitting energy, which 1521 01:02:38,160 --> 01:02:40,440 is absorbed by the silicon and melting it, 1522 01:02:40,440 --> 01:02:44,100 and you start with the polycrystalline rod coming 1523 01:02:44,100 --> 01:02:47,210 from the Siemens process and in that case, 1524 01:02:47,210 --> 01:02:51,540 this rough, corrugated material right here won't do. 1525 01:02:51,540 --> 01:02:52,070 Right? 1526 01:02:52,070 --> 01:02:54,360 This is too rough for that RF coil 1527 01:02:54,360 --> 01:02:57,620 to pass over and be a consistent distance away. 1528 01:02:57,620 --> 01:02:59,070 In the case of float-zone growth, 1529 01:02:59,070 --> 01:03:02,687 you actually have to modify your polysilicon production process. 1530 01:03:02,687 --> 01:03:04,270 You have to modify the Siemens process 1531 01:03:04,270 --> 01:03:08,030 so that you get a nice smooth rod, which 1532 01:03:08,030 --> 01:03:11,080 you can then pass the RF coil next to and melt 1533 01:03:11,080 --> 01:03:13,360 and you again start with a seed at the bottom, 1534 01:03:13,360 --> 01:03:16,600 your RF coil starts down here and then the RF coil moves 1535 01:03:16,600 --> 01:03:18,950 through the material, almost like a comb from the bottom 1536 01:03:18,950 --> 01:03:21,700 to the top, converting the polysilicon 1537 01:03:21,700 --> 01:03:27,240 into nice single crystalline material and, in the process, 1538 01:03:27,240 --> 01:03:29,560 it concentrates impurities in this liquid region. 1539 01:03:29,560 --> 01:03:31,893 Since the liquids have a higher solubility in the liquid 1540 01:03:31,893 --> 01:03:34,440 than they do in the solid, the impurities are then 1541 01:03:34,440 --> 01:03:37,550 aggregated inside of the liquid region and, again, like a comb, 1542 01:03:37,550 --> 01:03:40,050 they just get swept out of the material. 1543 01:03:40,050 --> 01:03:41,810 Not all of them, but a large percentage 1544 01:03:41,810 --> 01:03:43,510 of them, and so you can make multiple 1545 01:03:43,510 --> 01:03:46,605 passes with this RF coil to further concentrate 1546 01:03:46,605 --> 01:03:48,490 the impurities and the extremities 1547 01:03:48,490 --> 01:03:50,990 and remove them from the material. 1548 01:03:50,990 --> 01:03:52,240 So that's a float-zone method. 1549 01:03:52,240 --> 01:03:55,609 Very expensive material, very high purity. 1550 01:03:55,609 --> 01:03:57,150 One of the reasons it has high purity 1551 01:03:57,150 --> 01:04:00,310 is because you don't have this quartz crucible nearby, 1552 01:04:00,310 --> 01:04:01,760 you don't have this molten silicon 1553 01:04:01,760 --> 01:04:03,990 that's absorbing or dissolving the quartz 1554 01:04:03,990 --> 01:04:06,920 and transporting the oxygen into your crystal. 1555 01:04:06,920 --> 01:04:09,100 You have much lower carbon and oxygen concentrations 1556 01:04:09,100 --> 01:04:10,590 to [INAUDIBLE] float-zone material. 1557 01:04:10,590 --> 01:04:13,360 So if anybody is doing experiments with silicon, 1558 01:04:13,360 --> 01:04:16,090 for whatever reason, using it as a substrate material, 1559 01:04:16,090 --> 01:04:19,190 you want to think carefully about what type of silicon you 1560 01:04:19,190 --> 01:04:21,490 source and from where you source it. 1561 01:04:21,490 --> 01:04:23,792 You can find some very poor quality silicon 1562 01:04:23,792 --> 01:04:25,250 out there in the market, especially 1563 01:04:25,250 --> 01:04:26,930 if you going into the aftersale market, 1564 01:04:26,930 --> 01:04:28,764 and we know this from some-- 1565 01:04:28,764 --> 01:04:29,680 AUDIENCE: [INAUDIBLE]. 1566 01:04:29,680 --> 01:04:32,600 PROFESSOR: --very painful experiences. 1567 01:04:32,600 --> 01:04:35,055 And so there are some better sources 1568 01:04:35,055 --> 01:04:36,680 from which to get your wafers and we're 1569 01:04:36,680 --> 01:04:39,670 happy to talk about that offline. 1570 01:04:39,670 --> 01:04:42,019 So, again, single crystalline silicon. 1571 01:04:42,019 --> 01:04:43,560 We're going to venture into the world 1572 01:04:43,560 --> 01:04:46,240 of multicrystalline silicon ever so briefly here. 1573 01:04:46,240 --> 01:04:48,050 First, we'll start about cast material 1574 01:04:48,050 --> 01:04:50,200 and, just to emphasize here, we have 1575 01:04:50,200 --> 01:04:51,820 regions of crystalline material that 1576 01:04:51,820 --> 01:04:54,737 have grain boundaries separating the adjacent grains 1577 01:04:54,737 --> 01:04:56,820 and the reason we go into multicrystalline silicon 1578 01:04:56,820 --> 01:05:00,110 is really oftentimes, it is a lower cost method 1579 01:05:00,110 --> 01:05:03,221 of producing a silicon wafer although you have the grain 1580 01:05:03,221 --> 01:05:03,720 boundaries. 1581 01:05:03,720 --> 01:05:06,980 So, again, single crystalline, Czochralski and float-zone, you 1582 01:05:06,980 --> 01:05:10,030 wind up with round wafers, typically 1583 01:05:10,030 --> 01:05:12,880 single crystalline variety, and multicrystalline silicon wafers 1584 01:05:12,880 --> 01:05:19,680 tend to be more square-like and more visibly multi-grained, 1585 01:05:19,680 --> 01:05:21,180 if you will. 1586 01:05:21,180 --> 01:05:22,860 So let's talk about those for a minute. 1587 01:05:22,860 --> 01:05:26,880 How do you make a multicrystalline silicon wafer? 1588 01:05:26,880 --> 01:05:31,000 Again, you would start with the solar-grade silicon that 1589 01:05:31,000 --> 01:05:33,150 could either be coming from the Siemens process, 1590 01:05:33,150 --> 01:05:35,233 it could be coming from the fluidized bed reactor, 1591 01:05:35,233 --> 01:05:36,710 it could be coming from an upgraded 1592 01:05:36,710 --> 01:05:38,876 metallurgical-grade silicon, the liquid purification 1593 01:05:38,876 --> 01:05:41,870 route but, somehow, some way, you get chunks of silicon, 1594 01:05:41,870 --> 01:05:44,292 or granules of silicon, that have a high enough purity 1595 01:05:44,292 --> 01:05:45,750 for you to make solar cells out of, 1596 01:05:45,750 --> 01:05:47,270 and high enough purity is typically 1597 01:05:47,270 --> 01:05:50,510 in the order of one part per million impurity content. 1598 01:05:50,510 --> 01:05:53,500 So you put your solar-grade silicon into a crucible 1599 01:05:53,500 --> 01:05:55,750 and then you melt the silicon inside of it. 1600 01:05:55,750 --> 01:05:58,522 Silicon melts at 1,414 degrees Celsius. 1601 01:05:58,522 --> 01:05:59,730 It's a very high temperature. 1602 01:05:59,730 --> 01:06:04,600 So 1,414 degrees Celsius is the melting temperature of silicon. 1603 01:06:04,600 --> 01:06:05,940 And then it's cooled. 1604 01:06:05,940 --> 01:06:09,140 Not just randomly, but from the bottom up and the reason 1605 01:06:09,140 --> 01:06:11,790 it's cooled from the bottom up is because, 1606 01:06:11,790 --> 01:06:15,130 and here I guess you'll actually have to come up and see this 1607 01:06:15,130 --> 01:06:18,200 after class, it's rather difficult to see from here, 1608 01:06:18,200 --> 01:06:20,447 but this is a cross section of a small ingot. 1609 01:06:20,447 --> 01:06:22,030 This is the outside of the ingot where 1610 01:06:22,030 --> 01:06:25,210 it was contacting the wall, these little pieces 1611 01:06:25,210 --> 01:06:27,050 of white stuff that are flaking off, 1612 01:06:27,050 --> 01:06:32,290 this is the fused quartz silica that forms the crucible wall, 1613 01:06:32,290 --> 01:06:34,770 and the silicon nitride coating that 1614 01:06:34,770 --> 01:06:37,000 form the anti-stick coating that prevented 1615 01:06:37,000 --> 01:06:39,484 the silicon from sticking to the crucible, 1616 01:06:39,484 --> 01:06:41,150 and so it's kind of rough and corrugated 1617 01:06:41,150 --> 01:06:44,210 but, if we were to rotate this around and look at the inside, 1618 01:06:44,210 --> 01:06:47,269 this here is a cross section of the actual ingot 1619 01:06:47,269 --> 01:06:49,060 from the inside and, if you look carefully, 1620 01:06:49,060 --> 01:06:51,170 you'll see grains growing from the bottom to the top. 1621 01:06:51,170 --> 01:06:52,760 You probably can't see them from here, 1622 01:06:52,760 --> 01:06:54,450 you'll have to come up after class and take a look, 1623 01:06:54,450 --> 01:06:57,070 but the grains are growing from the bottom to the top 1624 01:06:57,070 --> 01:06:59,880 and that is called directional solidification, or the result 1625 01:06:59,880 --> 01:07:01,130 of directional solidification. 1626 01:07:01,130 --> 01:07:02,130 Directional solidification is when 1627 01:07:02,130 --> 01:07:03,870 you solidify from the bottom to the top 1628 01:07:03,870 --> 01:07:05,230 and, typically, your grain boundaries 1629 01:07:05,230 --> 01:07:06,771 are going to be running perpendicular 1630 01:07:06,771 --> 01:07:09,300 to the solid-liquid interface, so your grain boundaries 1631 01:07:09,300 --> 01:07:12,010 will be running up like this as you grow your material 1632 01:07:12,010 --> 01:07:13,660 from the bottom to the top. 1633 01:07:13,660 --> 01:07:16,660 If you were to do uncontrolled solidification 1634 01:07:16,660 --> 01:07:18,457 and all walls would freeze the same time, 1635 01:07:18,457 --> 01:07:20,290 you'd have grains growing in from the sides, 1636 01:07:20,290 --> 01:07:22,248 you'd have grains growing in through the bottom 1637 01:07:22,248 --> 01:07:24,740 and then, when you slice your wafer out horizontally, 1638 01:07:24,740 --> 01:07:26,990 the grain boundaries wouldn't be running perpendicular 1639 01:07:26,990 --> 01:07:27,710 to the surface. 1640 01:07:27,710 --> 01:07:30,260 They might be running parallel to the surface, in which case 1641 01:07:30,260 --> 01:07:32,610 they could wreck havoc on your minority care diffusion length. 1642 01:07:32,610 --> 01:07:34,155 Imagine you being an electron having 1643 01:07:34,155 --> 01:07:37,600 to travel across that grain boundary that's between you 1644 01:07:37,600 --> 01:07:39,270 and the P-N junction. 1645 01:07:39,270 --> 01:07:41,720 Whereas, if the grain boundaries are running perpendicular 1646 01:07:41,720 --> 01:07:43,390 to the surfaces, now they're only 1647 01:07:43,390 --> 01:07:47,570 affecting very small areas of the entire solar cell wafer. 1648 01:07:47,570 --> 01:07:49,170 So when I pick up a wafer like this, 1649 01:07:49,170 --> 01:07:53,200 this wafer was chopped from the ingot this way 1650 01:07:53,200 --> 01:07:55,090 or, to put it into perspective here, 1651 01:07:55,090 --> 01:07:58,240 this wafer was sliced out like that from this. 1652 01:07:58,240 --> 01:08:00,400 So the grain boundaries were running perpendicular 1653 01:08:00,400 --> 01:08:04,270 to the surfaces and that way they don't impede as much 1654 01:08:04,270 --> 01:08:06,690 with electron transport. 1655 01:08:06,690 --> 01:08:08,790 So the multicrystalline silicon ingot is formed. 1656 01:08:08,790 --> 01:08:11,230 The ingot is then chopped into these blocks, 1657 01:08:11,230 --> 01:08:17,330 usually between 16 and 24, that means four bricks to an edge 1658 01:08:17,330 --> 01:08:19,270 or five bricks to an edge. 1659 01:08:19,270 --> 01:08:23,010 Some folks are exploring six by six, so 36 bricks, 1660 01:08:23,010 --> 01:08:26,300 and then the bricks are rotated on their side 1661 01:08:26,300 --> 01:08:30,020 and then sliced into wafers and individual wafers come out. 1662 01:08:30,020 --> 01:08:34,689 So you can see the wafers I've sliced from the bricks 1663 01:08:34,689 --> 01:08:35,960 as I showed you right here. 1664 01:08:35,960 --> 01:08:38,700 Is this diagram clear to folks? 1665 01:08:38,700 --> 01:08:40,300 In general since-- any confusions? 1666 01:08:40,300 --> 01:08:41,859 Any questions? 1667 01:08:41,859 --> 01:08:42,359 No. 1668 01:08:42,359 --> 01:08:43,760 AUDIENCE: How do they cut the wafers? 1669 01:08:43,760 --> 01:08:45,176 PROFESSOR: How do they cut wafers! 1670 01:08:45,176 --> 01:08:48,460 So this is a process called wire sawing sign and this is 1671 01:08:48,460 --> 01:08:50,229 one of the most beautiful technologies 1672 01:08:50,229 --> 01:08:52,939 because it was invented in the PV industry 1673 01:08:52,939 --> 01:08:56,646 and transported back, adopted by the IC industry. 1674 01:08:56,646 --> 01:08:58,520 So it's one of the few examples of technology 1675 01:08:58,520 --> 01:08:59,760 that went the other way. 1676 01:08:59,760 --> 01:09:01,330 Let me get to that point. 1677 01:09:01,330 --> 01:09:02,890 AUDIENCE: How was it done before? 1678 01:09:02,890 --> 01:09:04,939 PROFESSOR: It was done by ID saws, 1679 01:09:04,939 --> 01:09:06,689 for instance, inner diameter saws, 1680 01:09:06,689 --> 01:09:10,974 that would slice off wafers like a wafer off of a salami. 1681 01:09:10,974 --> 01:09:13,604 AUDIENCE: So not a wire but like a disk? 1682 01:09:13,604 --> 01:09:14,729 PROFESSOR: Like a disk saw. 1683 01:09:14,729 --> 01:09:15,229 Yeah. 1684 01:09:15,229 --> 01:09:16,229 Exactly. 1685 01:09:16,229 --> 01:09:19,450 Like the inner diameter meaning your saw is like a rotating 1686 01:09:19,450 --> 01:09:22,351 blade and you're just using, you know-- Yeah. 1687 01:09:22,351 --> 01:09:22,850 OK. 1688 01:09:22,850 --> 01:09:25,880 So directional solidification of multicrystalline silicon. 1689 01:09:25,880 --> 01:09:29,569 This is a cross section of a furnace that is solidifying 1690 01:09:29,569 --> 01:09:30,470 an ingot right here. 1691 01:09:30,470 --> 01:09:31,470 Here's your ingot. 1692 01:09:31,470 --> 01:09:34,109 This is a liquid silicon and, essentially, it's 1693 01:09:34,109 --> 01:09:36,960 solidifying from the bottom to the top 1694 01:09:36,960 --> 01:09:39,540 and, hopefully, we'll have a tour of one of the world's 1695 01:09:39,540 --> 01:09:45,012 largest ingot solidification furnace manufacturing 1696 01:09:45,012 --> 01:09:45,970 companies in the world. 1697 01:09:45,970 --> 01:09:47,594 So, they don't manufacture the silicon, 1698 01:09:47,594 --> 01:09:50,619 they manufacture the furnace that manufactures the silicon. 1699 01:09:50,619 --> 01:09:51,655 If that makes sense. 1700 01:09:51,655 --> 01:09:52,810 AUDIENCE: Do they also make the crucible? 1701 01:09:52,810 --> 01:09:53,130 PROFESSOR: No. 1702 01:09:53,130 --> 01:09:54,180 That would be Vesuvius. 1703 01:09:54,180 --> 01:09:54,680 Yeah. 1704 01:09:54,680 --> 01:09:58,130 It would be other companies that make the crucibles. 1705 01:09:58,130 --> 01:10:01,090 And these are some of the furnaces right here. 1706 01:10:01,090 --> 01:10:02,230 The keyboard and monitor. 1707 01:10:02,230 --> 01:10:03,750 For size comparison, stairs. 1708 01:10:03,750 --> 01:10:05,630 So they're about two stories tall. 1709 01:10:05,630 --> 01:10:07,890 You can go up here to the top and look down into them. 1710 01:10:07,890 --> 01:10:09,077 It's pretty cool. 1711 01:10:09,077 --> 01:10:11,160 Using a little infrared lens to block out the heat 1712 01:10:11,160 --> 01:10:14,460 so you don't get blinded and the furnace itself-- 1713 01:10:14,460 --> 01:10:16,310 all the action happens inside of here. 1714 01:10:16,310 --> 01:10:19,910 The top can lift-- typically, they're the bottom loaded. 1715 01:10:19,910 --> 01:10:22,370 You'll see this little seal right here. 1716 01:10:22,370 --> 01:10:24,110 So this bottom part typically comes down 1717 01:10:24,110 --> 01:10:26,026 because you want to trap the heat inside of it 1718 01:10:26,026 --> 01:10:29,690 so you're not losing all that and the bottom is removed, 1719 01:10:29,690 --> 01:10:33,540 the forklift comes in, picks up this ingot and crucible 1720 01:10:33,540 --> 01:10:36,280 which could be a few of kilograms in mass-- up 1721 01:10:36,280 --> 01:10:39,210 to about 600, maybe even a ton-- and removes it 1722 01:10:39,210 --> 01:10:41,415 and places in the proper location. 1723 01:10:41,415 --> 01:10:43,310 It's a pretty dirty environment. 1724 01:10:43,310 --> 01:10:46,030 The operator will typically take a garden hose 1725 01:10:46,030 --> 01:10:48,240 and hose it down inside afterward. 1726 01:10:48,240 --> 01:10:51,150 It's really an antithesis of an IC 1727 01:10:51,150 --> 01:10:53,420 fab at this stage right here. 1728 01:10:53,420 --> 01:10:56,830 These are graphite insulation materials 1729 01:10:56,830 --> 01:11:01,310 on the sides of the crucible and this yellowish dust 1730 01:11:01,310 --> 01:11:03,830 that you see everywhere is silica, again. 1731 01:11:03,830 --> 01:11:09,270 That nice fine grained dust that's bad for you lungs. 1732 01:11:09,270 --> 01:11:12,350 The directional solidification process 1733 01:11:12,350 --> 01:11:15,450 can be, to some degrees, used interchangeably 1734 01:11:15,450 --> 01:11:18,030 with the so-called Bridgeman process. 1735 01:11:18,030 --> 01:11:20,000 It's also a name for a specific type 1736 01:11:20,000 --> 01:11:21,900 of directional solidification. 1737 01:11:21,900 --> 01:11:27,120 This is your ingot, this is the ingot chopped into bricks, 1738 01:11:27,120 --> 01:11:30,539 and then the bricks-- here's an ingot coming out of a furnace. 1739 01:11:30,539 --> 01:11:31,830 Those are the bricks over here. 1740 01:11:31,830 --> 01:11:32,970 This is a really tiny one. 1741 01:11:32,970 --> 01:11:34,330 It's like lab scale. 1742 01:11:34,330 --> 01:11:40,930 The big ones are about over a meter along the long edge. 1743 01:11:40,930 --> 01:11:43,250 And then, to saw them into wafers, 1744 01:11:43,250 --> 01:11:46,561 we use what's called wire sawing. 1745 01:11:46,561 --> 01:11:49,610 These are several kilometers of wires-- of wire. 1746 01:11:49,610 --> 01:11:51,270 One continuous wire, several kilometers 1747 01:11:51,270 --> 01:11:56,880 long, typically of a steel-based composite. 1748 01:11:56,880 --> 01:12:00,790 Running in these bricks right here, 1749 01:12:00,790 --> 01:12:03,330 in the presence of a glycol-based slurry, typically, 1750 01:12:03,330 --> 01:12:06,200 and silicon carbide or diamond grit, 1751 01:12:06,200 --> 01:12:08,540 and the grit is being pressured by the wire 1752 01:12:08,540 --> 01:12:09,850 against the silicon. 1753 01:12:09,850 --> 01:12:12,210 The grit is very small in size-- micron size-- 1754 01:12:12,210 --> 01:12:14,970 and it's, essentially, chipping out small pieces of silicon 1755 01:12:14,970 --> 01:12:16,720 as this wire is progressing through 1756 01:12:16,720 --> 01:12:19,080 and, over a period of around 6 to 8 hours, 1757 01:12:19,080 --> 01:12:22,690 you saw through the entire brick and you use, maybe, 1758 01:12:22,690 --> 01:12:24,650 four or eight of them at a time. 1759 01:12:24,650 --> 01:12:26,400 So if that wire were to snap about halfway 1760 01:12:26,400 --> 01:12:29,440 through the process, all those bricks are gone. 1761 01:12:29,440 --> 01:12:32,040 So it's very important that the wire be 1762 01:12:32,040 --> 01:12:35,890 very robust and able to support the sawing process 1763 01:12:35,890 --> 01:12:38,910 and, as I said, it's several kilometers long 1764 01:12:38,910 --> 01:12:42,650 and moving at a speed of a few meters per second. 1765 01:12:42,650 --> 01:12:44,850 So this is zinging along through your material 1766 01:12:44,850 --> 01:12:47,915 in the presence of very small grit and slurry, 1767 01:12:47,915 --> 01:12:50,540 and so the consumables that are used in the wire sawing process 1768 01:12:50,540 --> 01:12:53,150 are enormous, and you lose about half of your silicon 1769 01:12:53,150 --> 01:12:55,980 due to sawdust in this process right here. 1770 01:12:55,980 --> 01:12:59,220 So this is a prime candidate for replacement 1771 01:12:59,220 --> 01:13:01,910 of the manufacturing process, even though it's 1772 01:13:01,910 --> 01:13:04,050 so commonly used today. 1773 01:13:04,050 --> 01:13:06,460 What I'm going to do is give a quick pause right here 1774 01:13:06,460 --> 01:13:07,870 until our next class, where we'll 1775 01:13:07,870 --> 01:13:10,200 pick up and talk about ribbon growth, which 1776 01:13:10,200 --> 01:13:12,750 seeks to get around all the complexities 1777 01:13:12,750 --> 01:13:14,770 of multicrystalline silicon ingot growth 1778 01:13:14,770 --> 01:13:16,910 while still keeping the cost advantage. 1779 01:13:16,910 --> 01:13:19,640 So with that, thank you.