1 00:00:05,210 --> 00:00:07,600 Welcome to week 5. 2 00:00:07,600 --> 00:00:13,260 Last week we have discussed the dominant PV technology in the current market, the PV technology 3 00:00:13,260 --> 00:00:15,200 based on c-Si wafers. 4 00:00:15,200 --> 00:00:21,020 I am confident that this technology will stay the dominant PV technology in the market 5 00:00:21,020 --> 00:00:23,529 for a long time to go. 6 00:00:23,529 --> 00:00:29,840 This week we will look into the alternative PV technologies like the thin-film technologies, 7 00:00:29,840 --> 00:00:34,620 also referred to as the second generation PV technology. 8 00:00:34,620 --> 00:00:39,870 Discussing these PV technologies in detail can easily fill an entire course. 9 00:00:39,870 --> 00:00:46,600 Due to time limitations, I will provide you with a general introduction into the various PV technologies. 10 00:00:46,719 --> 00:00:53,379 I will focus on the working principles of the various devices, the current status and 11 00:00:53,379 --> 00:00:56,820 the future challenges of the various technologies. 12 00:00:56,820 --> 00:01:02,579 I would like to start with the III-V PV technology. 13 00:01:02,579 --> 00:01:07,600 This is the solar cell technology which results in the highest conversion efficiencies 14 00:01:07,600 --> 00:01:13,790 under both 1 sun standard test conditions and concentrated sun conditions. 15 00:01:13,790 --> 00:01:19,070 It is mainly used in space technology and in concentrator technology. 16 00:01:19,070 --> 00:01:26,070 As some concepts use crystalline germanium and GaAs wafer as substrate, it might not 17 00:01:26,140 --> 00:01:33,900 be a real thin-film PV technology, like thin-film silicon, CdTe, CIGS or organics, 18 00:01:34,060 --> 00:01:40,200 which will discuss later in this course, however, the III-V based absorber layers itself 19 00:01:40,200 --> 00:01:45,939 can be considered as thin in reference to the crystalline silicon wafers. 20 00:01:45,939 --> 00:01:55,900 The III-V materials are based on III-valence electron elements like aluminum, gallium and indium 21 00:01:55,900 --> 00:02:02,130 and the V-valence electron materials as phosporous and arsenic. 22 00:02:03,140 --> 00:02:10,140 Various different semiconductor materials suchs as gallium arsenide, gallium phosphide, 23 00:02:10,490 --> 00:02:21,800 indium phosphide, indium arsenide, and more complex alloys like GaInAs, GaInP, AlGaInAs 24 00:02:21,800 --> 00:02:27,610 and AlGaInP have been explored. 25 00:02:27,610 --> 00:02:32,460 Let's consider for the moment the standard GaAs semiconductor material. 26 00:02:32,460 --> 00:02:38,150 Its lattice has a tetrahedral diamond lattice structure just like silicon. 27 00:02:38,150 --> 00:02:44,720 However, every gallium atom neighbors four arsenicum atoms, and every arsenic atom neigbors 28 00:02:44,720 --> 00:02:46,720 four gallium atoms. 29 00:02:46,720 --> 00:02:52,700 Compared to silicon it has a slightly larger lattice constant, and is significantly heavier than silicon. 30 00:02:53,520 --> 00:03:00,520 Both gallium and arsenic are roughly twice as heavy as silicon. 31 00:03:00,710 --> 00:03:04,380 Here we see the electronic band dispersion diagram again. 32 00:03:04,380 --> 00:03:11,330 We can easily see that the band gap of GaAs is a direct band gap material. 33 00:03:11,330 --> 00:03:16,550 The highest energy level in the valence band is vertically aligned with the lowest energy 34 00:03:16,550 --> 00:03:18,740 in the conduction band. 35 00:03:18,740 --> 00:03:29,000 It means that only transfer of energy is required to excite an electron from the valence to the conduction band. 36 00:03:29,400 --> 00:03:33,060 No transfer of momentum is required. 37 00:03:33,060 --> 00:03:41,000 The direct band gap transition is around 1.42 eV, which is the band gap of GaAs. 38 00:03:42,610 --> 00:03:47,340 Here we look again at the absorption coefficient versus the wavelength. 39 00:03:47,340 --> 00:03:56,300 As you can see here, the absorption coefficient of GaAs is significantly larger than for silicon. 40 00:03:56,450 --> 00:04:01,790 As example we also show the III-V material, InP. 41 00:04:01,790 --> 00:04:07,000 It means that to absorb the same amount of light in reference to a silicon wafer, 42 00:04:07,000 --> 00:04:13,480 the thickness of the GaAs film can be more than an order of magnitude thinner. 43 00:04:13,480 --> 00:04:19,790 Another advantage of the direct III-V semiconductor materials shown here is that the band gap 44 00:04:19,790 --> 00:04:21,370 is relatively sharp. 45 00:04:21,370 --> 00:04:26,800 The absorption coefficient increases quickly above the band gap energy. 46 00:04:26,800 --> 00:04:33,800 If we look at the utilization of the band gap energy, due to the direct band gap properties, 47 00:04:34,210 --> 00:04:40,900 radiative recombination processes can become an important recombination mechanism as well. 48 00:04:40,900 --> 00:04:46,229 The Shockley-Read-Hall recombination can be kept low as the typical epitaxy processes 49 00:04:46,229 --> 00:04:51,599 used to deposited III-V films, results in high purity films. 50 00:04:51,599 --> 00:04:58,599 Why does the III-V technology result in high conversion efficiencies? 51 00:04:59,229 --> 00:05:05,729 The III-V technology PV devices are based on the multi-junction concept. 52 00:05:05,729 --> 00:05:08,699 More than one band gap material is used. 53 00:05:08,699 --> 00:05:14,580 As discussed in week 4 using only one single band gap material, the theoretical efficiency 54 00:05:14,580 --> 00:05:17,680 is limited by the Shockley-Queisser limit. 55 00:05:17,680 --> 00:05:22,000 Either a large fraction of the energy of the energetic photons are lost as heat, 56 00:05:22,000 --> 00:05:27,680 or the photons below the band gap are lost as they are not absorbed. 57 00:05:27,680 --> 00:05:30,680 In this illustration you see a low band gap material. 58 00:05:30,680 --> 00:05:35,590 A large fraction of the energy carried by the photons is not used. 59 00:05:35,590 --> 00:05:41,080 However, if we use more band gaps, you see that the same amount of photons can be used, 60 00:05:41,080 --> 00:05:44,210 and less energy is wasted as heat. 61 00:05:44,210 --> 00:05:50,289 Consequently, simultaneously large parts of the solar spectrum and large parts of the 62 00:05:50,289 --> 00:05:57,289 energy in the solar spectrum can be utilized by using more than one p-n junction. 63 00:05:58,050 --> 00:06:03,259 Here you see an illustration of one of the typical III-V triple junctions. 64 00:06:03,259 --> 00:06:07,849 It is based on a low band gap material, a germanium substrate wafer. 65 00:06:07,849 --> 00:06:14,229 Note, that germanium has a band gap of 0.67 eV. 66 00:06:14,229 --> 00:06:20,699 The middle cell is based on GaAs and has a band gap of 1.4 eV. 67 00:06:20,699 --> 00:06:27,699 The cell at the top is based on GaInP2 with a band gap in the order of 1.86 eV. 68 00:06:28,729 --> 00:06:32,909 How does a multi-junction solar cell work? 69 00:06:32,909 --> 00:06:39,800 The p-n junction at the window layer at which the solar light enters the PV device is called the top cell. 70 00:06:40,060 --> 00:06:45,389 As the spectral part with the most energetic photons like blue light has the smallest penetration 71 00:06:45,389 --> 00:06:52,389 depth in materials, the high band gap junction always acts like the top cell. 72 00:06:52,699 --> 00:06:57,809 As the near infrared light outside the visible spectrum has the largest penetration depth, 73 00:06:57,809 --> 00:07:01,249 the bottom cell is the cell with the lowest band gap. 74 00:07:01,249 --> 00:07:06,610 This cell has to harvest the photons from spectral parts with the lower energetic photons. 75 00:07:06,610 --> 00:07:10,770 Here we see the J-V curve of three single p-n junctions. 76 00:07:10,770 --> 00:07:16,100 P-n junction 1 has the highest open-circuit voltage and the lowest short-circuit current density, 77 00:07:16,100 --> 00:07:21,020 this means this p-n junction has the highest band gap. 78 00:07:21,020 --> 00:07:26,000 P-n junction 3 has a low open-circuit voltage and a high current density, 79 00:07:26,000 --> 00:07:28,559 consequently it has the lowest band gap. 80 00:07:28,559 --> 00:07:31,490 P-n junction 2 has a band gap in between. 81 00:07:31,490 --> 00:07:36,600 If we would like to make a triple junction out of these three p-n junctions, 82 00:07:36,600 --> 00:07:42,000 p-n junction 1 will act as the top cell, p-n junction 2 will act as the middle cell 83 00:07:42,000 --> 00:07:46,000 and p-n junction 3 will act as the bottom cell. 84 00:07:46,500 --> 00:07:53,930 What would the J-V curve of a triple-junction look like? First we will consider the equivalent 85 00:07:54,169 --> 00:07:56,460 electric circuit. 86 00:07:56,460 --> 00:08:03,460 Here we show the equivalent electric circuit of an ideal single junction solar cell. 87 00:08:03,710 --> 00:08:09,539 Every p-n junction in a multi-junction solar cell can be represented by this circuit. 88 00:08:09,539 --> 00:08:18,200 However, how will these solar cell circuits be connected in the representative multi-junction circuit? 89 00:08:18,500 --> 00:08:22,000 Will the cells be connected in series? 90 00:08:22,000 --> 00:08:29,539 Or are the three cells connected in parallel? Take a few seconds to think and answer 91 00:08:34,060 --> 00:08:39,900 In a parallel circuit the voltage over all solar cells is equal and the currents do add up. 92 00:08:40,300 --> 00:08:45,390 However, the various solar cells have different band gaps and therefore different open-circuit 93 00:08:45,390 --> 00:08:50,530 voltages going from high voltage for the top cell down to a low voltage for the bottom cell. 94 00:08:51,850 --> 00:08:58,250 Secondly, the current density generated in all cells have to conduct through all three 95 00:08:58,250 --> 00:09:01,820 cells to be collected at the front and back contacts. 96 00:09:01,820 --> 00:09:08,900 As a result the multi-junction can be interpreted as a series connection of single junction solar cells. 97 00:09:09,200 --> 00:09:16,220 Now we return to the question: How does the J-V curve of the triple junction look like? 98 00:09:16,300 --> 00:09:22,740 The voltages of the individual cells add up in the triple junction like in a series connection. 99 00:09:22,740 --> 00:09:28,020 The current density in a series connection is equal over the entire solar cell. 100 00:09:28,020 --> 00:09:34,690 This means that the current density is determined by the p-n junction generating the lowest current. 101 00:09:34,690 --> 00:09:37,900 The resulting J-V curves look like this. 102 00:09:37,900 --> 00:09:43,910 The voltages add up and the current is determined by the cell delivering the lowest current. 103 00:09:43,910 --> 00:09:48,870 Let's look at a typical band diagram of such triple junction. 104 00:09:48,870 --> 00:09:53,370 Here you see the top cell with high band gap at the left hand side and the bottom cell 105 00:09:53,370 --> 00:09:55,460 at the right hand side. 106 00:09:55,460 --> 00:10:01,210 However, the band diagram would not look like this in reality. 107 00:10:01,210 --> 00:10:06,640 If we place three p-n junctions in series it means that the p-layer of for instance 108 00:10:06,640 --> 00:10:13,640 the top cell and the n-layer of the middle cell form a p-n junction as well. 109 00:10:14,800 --> 00:10:20,560 This p-n junction is in the reverse direction compared to the p-n junction forming the three 110 00:10:20,560 --> 00:10:23,060 single junction solar cells. 111 00:10:23,060 --> 00:10:28,690 This would significantly lower the voltage of the total triple junction. 112 00:10:28,690 --> 00:10:34,490 The creation of such reverse junction can be prevented by the inclusion of a so-called 113 00:10:34,490 --> 00:10:36,700 tunnel junction. 114 00:10:36,700 --> 00:10:42,850 It provides a low electrical resistance and has a high band gap to prevent any parasitic 115 00:10:42,850 --> 00:10:44,430 absorption losses. 116 00:10:44,430 --> 00:10:48,090 The high band gap tunnel junction is relative thin. 117 00:10:48,090 --> 00:10:53,850 It means that the valence band at one side is lined up with the conduction band at the 118 00:10:53,850 --> 00:10:55,740 other side of the tunnel junction. 119 00:10:55,740 --> 00:10:59,950 The depletion zone of such junction is extremely narrow. 120 00:10:59,950 --> 00:11:04,050 As a result the slope of the valence band and conduction band are so steep 121 00:11:04,050 --> 00:11:10,050 that the electrons from n-layer tunnel through the small barrier to the p-layer, 122 00:11:10,050 --> 00:11:14,120 where they recombine with the holes. 123 00:11:14,120 --> 00:11:21,000 Important is that the resistance of the tunnel junction is low resulting in a low voltage loss over the tunnel junction. 124 00:11:21,800 --> 00:11:26,050 Looking at a triple junction, it means that the holes in the p-layer of the top cell have 125 00:11:26,050 --> 00:11:30,450 to recombine with the electrons of the n-layer of the middle cell. 126 00:11:30,450 --> 00:11:34,570 The holes in the p-layer of the middle cell have to recombine with the electrons of 127 00:11:34,570 --> 00:11:36,440 the n-layer of the bottom cell. 128 00:11:36,440 --> 00:11:41,500 The electrons of the top cell are collected at the front contact and the holes in the p-layer 129 00:11:41,500 --> 00:11:44,000 at the back contact. 130 00:11:44,790 --> 00:11:49,330 This demonstrates that the recombination current at the tunnel junctions represent the current 131 00:11:49,330 --> 00:11:51,330 density of the triple junction. 132 00:11:51,330 --> 00:11:57,480 The cell producing the lowest current will determine the current density of the cell, 133 00:11:57,480 --> 00:12:01,750 just like we expected for a series connected cells. 134 00:12:01,750 --> 00:12:06,200 Let's move to the processing of III-V semiconductor materials. 135 00:12:06,200 --> 00:12:14,100 High-quality III-V semiconductor materials can be deposited using epitaxy deposition methods. 136 00:12:14,630 --> 00:12:20,540 Epitaxy is a deposition method in which a crystalline overlayer is deposited on a crystalline 137 00:12:20,540 --> 00:12:26,540 substrate, where the overlayer adopts the crystal lattice of its substrate. 138 00:12:26,540 --> 00:12:32,080 The precursor atoms are provided by various element sources. 139 00:12:32,080 --> 00:12:38,170 For instance like in this illustration, GaAs is epitaxial deposited. 140 00:12:38,170 --> 00:12:45,170 Gallium and arsenic atoms are directed to a growth surface under ultra-high vacuum conditions. 141 00:12:45,980 --> 00:12:49,260 The substrate is usually a germanium substrate. 142 00:12:49,260 --> 00:12:57,100 The GaAs crystalline lattice is grown layer-by-layer and adopts the structure of the crystalline substrate. 143 00:12:57,870 --> 00:13:03,740 The relatively slow layer-by-layer growth allows the deposition of compact materials without 144 00:13:03,740 --> 00:13:05,920 any vacancy defects. 145 00:13:05,920 --> 00:13:12,010 Furthermore, processing at high vacuum conditions prevents the incorporation of impurities. 146 00:13:12,010 --> 00:13:15,740 Dopants can be added to make it n-type or p-type. 147 00:13:15,740 --> 00:13:22,740 III-V semiconductors can be deposited up to a very high degree of purity. 148 00:13:24,060 --> 00:13:32,500 In general, metal-organic chemical vapor deposition, abbreviated as MOCVD, is used to deposit the 149 00:13:32,500 --> 00:13:35,000 III-V semiconductor layers. 150 00:13:35,000 --> 00:13:42,320 Typical precursor gasses are trimethyl-gallium, trimethyl-indium, trimethyl-aluminum, 151 00:13:42,320 --> 00:13:45,620 arsine gas and phosphine gas. 152 00:13:45,620 --> 00:13:50,670 Surface reactions of the metal-organic compounds and hydrides, containing the required metal 153 00:13:50,670 --> 00:13:57,010 chemical elements, creates the condition for the epitaxial crystalline growth. 154 00:13:57,010 --> 00:14:03,360 The deposition method is expensive, as similar techniques are used to make III-V based semiconductor 155 00:14:03,360 --> 00:14:05,770 devices on the scale of chips. 156 00:14:05,770 --> 00:14:11,740 A challenge of making these crystalline III-V semiconductor materials is that their lattice 157 00:14:11,740 --> 00:14:16,590 constants of the various materials are not the same. 158 00:14:16,590 --> 00:14:21,900 As you can see in this graph, with on the vertical axis the band gap and on the horizontal 159 00:14:21,900 --> 00:14:25,920 axis the lattice constant, every semiconductor material is unique. 160 00:14:25,920 --> 00:14:31,000 There are no two different semiconductor materials that have the same band gap and 161 00:14:31,000 --> 00:14:32,780 the same lattice constant. 162 00:14:32,780 --> 00:14:37,770 This means that interfaces between these materials show a mismatch. 163 00:14:37,770 --> 00:14:42,310 Not every valence electron is able to make a bond with a neighbor. 164 00:14:42,310 --> 00:14:49,310 However, the triple junction, I have introduced earlier based on GaInP, GaAs and germanium 165 00:14:49,410 --> 00:14:52,260 is a lattice matched solar cell. 166 00:14:52,260 --> 00:14:55,420 What do I mean by that? 167 00:14:55,420 --> 00:15:01,700 Let's look again at the phase diagram of semiconductors where the band gap is plotted versus the lattice constant. 168 00:15:01,930 --> 00:15:06,810 The triple junction is processed on a p-type germanium substrate. 169 00:15:06,810 --> 00:15:09,760 The bottom cell is a germanium cell. 170 00:15:09,760 --> 00:15:15,190 On top of this you would like to place a junction that has first the same lattice constant and 171 00:15:15,190 --> 00:15:18,290 secondly a higher band gap. 172 00:15:18,290 --> 00:15:24,510 As you can see, GaAs has exactly the same lattice constant as germanium. 173 00:15:24,510 --> 00:15:30,790 This means GaAs and germanium can make good interfaces without any coordination defects 174 00:15:30,790 --> 00:15:35,250 related to mismatched lattices. 175 00:15:35,250 --> 00:15:39,610 To have a reasonable current matching the desired band gap of the top cell should be 176 00:15:39,610 --> 00:15:41,890 around 1.8 eV. 177 00:15:41,890 --> 00:15:48,600 However, we see that no alloys, based on solely two elements, exist with a band gap of 1.8 eV. 178 00:15:48,600 --> 00:15:55,300 However, if we take a mixture of gallium, indium and phosphorous, we see that we can 179 00:15:55,300 --> 00:16:07,000 make a III-V alloy with a band gap of 1.8 eV and which has a matching lattice constant with GaAs and Ge. 180 00:16:07,010 --> 00:16:14,010 Consequently triple junctions based on GaInP2, GaAs and germanium can be fully lattice matched. 181 00:16:14,320 --> 00:16:21,320 Here you see some specs of a typical lattice matched triple junction under AM0 sun light 182 00:16:21,420 --> 00:16:28,150 exposure with an irradiance of 135 mW/cm^2 of Spectrolab. 183 00:16:28,150 --> 00:16:36,500 The total open-circuit voltage is 2.6 V, and the short-circuit current density is 17.8 mA/cm^2. 184 00:16:36,780 --> 00:16:43,700 This means that up to a band gap of 0.67 eV we have a spectral utilization expressed in 185 00:16:43,700 --> 00:16:48,480 current densities of 53.4 mA/cm^2. 186 00:16:48,480 --> 00:16:56,400 Theoretically, this spectral part can generate 62 mA/cm^2 of current with an EQE of 100%. 187 00:16:57,000 --> 00:17:04,090 This means that this triple junction of spectro-labs has an impressive averaged EQE of 86%. 188 00:17:04,270 --> 00:17:14,200 Having a high FF of 85% this cell has a 29.5% conversion efficiency for AM0 space conditions. 189 00:17:16,230 --> 00:17:18,839 The EQE of a triple junction would look like this. 190 00:17:18,839 --> 00:17:25,839 Note, that this graph is a simulation of a GaInP-GaAs-Ge triple junction solar cell. 191 00:17:25,899 --> 00:17:31,929 The spectral utilization of the various sub-cells approaches the shape of block functions. 192 00:17:31,929 --> 00:17:37,559 You have to realize that the fact that III-V materials have sharp band gaps and have high 193 00:17:37,559 --> 00:17:43,820 absorption coefficients, is really helpful in designing these triple junctions. 194 00:17:43,820 --> 00:17:50,039 As you can see, the bottom cell generates much more current than the middle and top cell. 195 00:17:50,039 --> 00:17:58,200 This ineffective use of the near infrared part can be improved when we move to quadruple junctions. 196 00:17:58,970 --> 00:18:05,970 Here you see the example of the EQE of a quadruple junction measured at the Fraunhofer Institute. 197 00:18:06,169 --> 00:18:12,000 As you can see an additional cell is placed between the middle and bottom cell of the triple junction. 198 00:18:12,500 --> 00:18:18,710 Currently the energy utilization out of the spectrum is further improved by moving to 199 00:18:18,710 --> 00:18:22,919 multi-junctions of 5 to 6 solar cells. 200 00:18:22,919 --> 00:18:30,000 A challenge with going to so many junctions is that the lattice matching can not be guaranteed anymore. 201 00:18:30,350 --> 00:18:35,809 These types of lattice mismatched multi-junctions are called metamorphic multi-junctions. 202 00:18:35,809 --> 00:18:42,330 It requires buffer layers that have a profiling in the lattice constant, going from the lattice 203 00:18:42,330 --> 00:18:47,850 constant of one p-n junction to the lattice constant of the next p-n junction. 204 00:18:47,850 --> 00:18:54,500 On the left you see a lattice matched triple junction and on the right you see a metamorphic triple junction. 205 00:18:54,860 --> 00:19:01,679 Boeing Spectrolab has demonstrated an impressive 37.8% conversion efficiency of a 5-junction 206 00:19:01,679 --> 00:19:04,779 solar cell under 1-sun solar conditions. 207 00:19:04,779 --> 00:19:10,300 As the III-V PV technologies are expensive, they are used for space applications 208 00:19:10,300 --> 00:19:13,629 and in concentrator technology. 209 00:19:13,629 --> 00:19:20,629 The $/Wp of a small expensive III-V multi-junction device can be improved by concentrating 210 00:19:21,100 --> 00:19:25,800 the irradiance of a large area on the small solar cell device. 211 00:19:25,800 --> 00:19:32,059 Advantage of concentrating sun light is that the open-circuit voltage is increased with 212 00:19:32,059 --> 00:19:40,800 the irradiance as well, as long as Auger recombination of light-excited charge carriers does not become dominant. 213 00:19:40,800 --> 00:19:46,919 Important is that the small solar cell devices have to be actively cooled. 214 00:19:46,919 --> 00:19:54,500 As will be discussed in weeks 7 and 8, the performance of PV devices goes down when they heat up. 215 00:19:55,100 --> 00:20:02,600 This loss in performance is a result of the increasing dark current of the diode component of a p-n junction. 216 00:20:03,000 --> 00:20:08,440 Furthermore, concentrator solar cells need sun-tracking systems. 217 00:20:08,440 --> 00:20:15,100 These systems make sure that the optical concentrator system tracks the sun and guarantees the optimal 218 00:20:15,100 --> 00:20:21,049 light concentration on the small PV device during the entire day. 219 00:20:21,049 --> 00:20:25,950 The non-modular costs of concentrator systems are consequently larger than that of conventional 220 00:20:25,950 --> 00:20:31,049 PV system based on c-Si or thin-film technologies. 221 00:20:31,049 --> 00:20:38,900 The current record conversion efficiency on lab-scale is 44.1% under 942 concentrated 222 00:20:38,929 --> 00:20:44,940 sunlight conditions as achieved by Solar Junction. 223 00:20:44,940 --> 00:20:49,000 This was a quick overview of the III-V PV technology. 224 00:20:49,000 --> 00:20:54,389 In the next block we will talk about thin-film silicon PV technology.