1 00:00:06,180 --> 00:00:12,129 The next thin-film PV technology we will discuss today is based on CIGS. 2 00:00:12,129 --> 00:00:17,840 CIGS stands for copper indium gallium selenide sulfide. 3 00:00:17,840 --> 00:00:22,400 The typical CIGS alloys are heterogeneous materials. 4 00:00:22,400 --> 00:00:28,860 The physical properties of CIGS are rather complex and many different views exist on 5 00:00:28,860 --> 00:00:31,180 these properties among scientists. 6 00:00:31,180 --> 00:00:42,500 Some important compounds in the material are CuInSe2 with a band gap of 1.0 eV, CuInS2, 7 00:00:42,550 --> 00:00:51,000 having a band gap of 1.5 eV and CuGaSe2, having a band gap of 1.7 eV. 8 00:00:51,590 --> 00:00:57,730 These materials cover a wide range of both band gaps and lattice constants. 9 00:00:57,730 --> 00:01:05,700 The CIGS material is a direct band gap semiconductor material therefore it has a large absorption coefficient. 10 00:01:06,500 --> 00:01:13,300 It requires only a thickness of 1-2 microns to absorb a large fraction of the light 11 00:01:13,300 --> 00:01:15,880 above the band gap. 12 00:01:15,880 --> 00:01:22,080 Typical electron diffusion lengths are in the same order of a few microns. 13 00:01:22,080 --> 00:01:28,280 A variety of CIGS alloys exist, but the best performing layers have something in common 14 00:01:28,280 --> 00:01:33,210 - they contain a polycrystalline alpha-phase. 15 00:01:33,210 --> 00:01:37,350 The lattice atoms are tetrahedrally bonded. 16 00:01:37,350 --> 00:01:44,350 Such a lattice structure is a so-called chalsopyrite structure as you can see in this illustration. 17 00:01:45,020 --> 00:01:50,829 The heterogeneous material consists of a phase of copper indium selenide, often indicated 18 00:01:50,829 --> 00:01:57,110 by CIS, and copper indium gallium selenide. 19 00:01:57,110 --> 00:02:06,100 x=0 means it is a CuGaSe2 material and x=1 means it is a CuInSe2 material. 20 00:02:06,130 --> 00:02:15,900 It means using the Cu/Ga ratio x, the band gap can be tuned from the CuGaSe2 at 1.7 eV at x=0 21 00:02:15,900 --> 00:02:20,500 down to 1.0 eV for x=1.0. 22 00:02:21,000 --> 00:02:24,760 The CIGS absorber layer is a p-doped layer. 23 00:02:24,760 --> 00:02:31,900 The doping is a result of intrinsic defects in the material, related to Cu deficiencies. 24 00:02:32,000 --> 00:02:38,079 These vacancies efficiently act as an acceptor, it means electrons excited from the valence band 25 00:02:38,079 --> 00:02:40,340 can get easily trapped. 26 00:02:40,340 --> 00:02:46,609 As a result the holes become the majority charge carrier density. 27 00:02:46,609 --> 00:02:50,480 Let's look at a typical CIGS solar cell structure. 28 00:02:50,480 --> 00:02:53,590 The substrate is glass. 29 00:02:53,590 --> 00:02:59,799 On top of the glass a molybdenum layer with typical thickness of 1 micron is deposited, 30 00:02:59,799 --> 00:03:03,279 which acts as the back contact. 31 00:03:03,279 --> 00:03:11,500 On that the p-type CIGS absorber layer is deposited with thickness ranging from 2 up to 4 microns. 32 00:03:12,049 --> 00:03:19,049 The p-n junction is formed by a thin n-layer of around 15 nm on top of p-type CIGS 33 00:03:19,049 --> 00:03:23,089 that is based on CdS. 34 00:03:23,089 --> 00:03:27,430 This layer is referred to as the buffer layer. 35 00:03:27,430 --> 00:03:31,639 The n-type region is extended with an n-type TCO. 36 00:03:31,639 --> 00:03:38,639 First an intrinsic ZnO is placed followed by an Al-doped ZnO. 37 00:03:38,709 --> 00:03:43,779 The Al-doping makes the ZnO n-type. 38 00:03:43,779 --> 00:03:49,879 Similar to some concepts of the thin-film silicon technology, the aluminum-doped ZnO 39 00:03:49,879 --> 00:03:54,049 acts like a transparent front contact for the solar cell. 40 00:03:54,049 --> 00:03:59,389 On top of this transparent conductive oxide, anti-reflective coatings can be placed as discussed 41 00:03:59,389 --> 00:04:02,799 for the c-Si technology. 42 00:04:02,799 --> 00:04:07,549 Here we see the electronic band diagram of a CIGS solar cell. 43 00:04:07,549 --> 00:04:12,010 The light enters the cell from the left, at the ZnO side. 44 00:04:12,010 --> 00:04:22,700 The p-type CIGS absorber layers used in industrial modules have a typical band gap of 1.1-1.2 eV. 45 00:04:22,900 --> 00:04:30,500 This band gap is accomplished by a ratio of Ga to In of around 0.3. 46 00:04:31,169 --> 00:04:37,340 The n-type CdS buffer layer has a band gap of 2.5 eV. 47 00:04:37,340 --> 00:04:44,340 Since the band gap of the n- and p-type junction materials are different, this CIGS cell 48 00:04:44,550 --> 00:04:48,490 can be considered as a heterojunction. 49 00:04:48,490 --> 00:04:57,600 The light-excited minority electrons in the CIGS layers have to diffuse to the CdS/CIGS interface to be separated. 50 00:04:57,600 --> 00:05:03,520 The holes diffuse to the molybdenum back contact to be collected. 51 00:05:03,520 --> 00:05:10,520 Here, the holes recombine with the electrons supplied from this molybdenum back contact. 52 00:05:10,590 --> 00:05:14,289 ZnO acts like the front contact. 53 00:05:14,289 --> 00:05:21,289 The band gap of the ZnO is very large, minimizing the parasitic absorption losses in this device. 54 00:05:22,710 --> 00:05:28,669 The electrons have to be separated at the CIGS/CdS interface. 55 00:05:28,669 --> 00:05:34,840 As with every interface, this interface has more defects and could act as a loss mechanism 56 00:05:34,840 --> 00:05:37,189 to the minority electrons. 57 00:05:37,189 --> 00:05:44,189 This can be prevented by placing an n-type CIGS type of layer between the p-type CIGS 58 00:05:44,479 --> 00:05:51,479 and the CdS interface, which screens the CdS/CIGS interface from the holes. 59 00:05:52,120 --> 00:05:59,500 The n-type CIGS is an indium rich alloy, like Cu(In,Ga)3Se5. 60 00:05:59,500 --> 00:06:06,900 In Cu deficient p-type CIGS materials, the dominant recombination mechanism is Shockley-Read-Hall 61 00:06:06,990 --> 00:06:09,189 recombination in the bulk. 62 00:06:09,189 --> 00:06:21,100 In contrast, in Cu rich CIGS films the SRH recombination at the CIGS/CdS interface becomes dominant. 63 00:06:21,400 --> 00:06:27,699 One of the important aspects of CIGS solar cells is the role of sodium. 64 00:06:27,699 --> 00:06:34,699 Low contamination of sodium appears to increase the conductivity in the p-type CIGS materials, 65 00:06:34,939 --> 00:06:41,419 it leads to a welcome texture and an increase in the average grain size. 66 00:06:41,419 --> 00:06:46,849 Similar to multicrystalline silicon as discussed last week, the larger the grain size, 67 00:06:46,849 --> 00:06:52,099 the less grain boundaries and less recombination are present in the material. 68 00:06:52,099 --> 00:06:57,740 This results in higher band gap ultilization and higher open-circuit voltages. 69 00:06:57,740 --> 00:07:04,270 Typical optimum concentration of sodium in the CIGS layers is 0.1%. 70 00:07:04,270 --> 00:07:11,270 The sodium source in the growth mechanism can be the soda-lime glass used as substrate. 71 00:07:11,409 --> 00:07:18,409 In CIGS solar cell concepts, were this soda-lime glass is missing, the sodium has to be intentionally 72 00:07:18,900 --> 00:07:21,669 added during the deposition process. 73 00:07:21,669 --> 00:07:28,669 In the CIGS field the exact reason why sodium improves several properties of the CIGS 74 00:07:28,810 --> 00:07:30,860 is still under debate. 75 00:07:32,300 --> 00:07:38,740 In contrast to the thin-film silicon technology discussed earlier, CIGS films can be deposited 76 00:07:38,740 --> 00:07:42,229 using a variety of deposition technologies. 77 00:07:42,229 --> 00:07:48,599 As many of these activities are developed within companies, not much detailed information 78 00:07:48,599 --> 00:07:55,039 is available on many of these processing techniques. 79 00:07:55,039 --> 00:08:01,400 One of the processing techniques is co-evaporation or co-sputtering under vacuum conditions. 80 00:08:01,400 --> 00:08:07,400 Using various targets of copper, gallium and indium, the precursors in various steps are 81 00:08:07,400 --> 00:08:09,900 co-evaporated onto a substrate. 82 00:08:09,900 --> 00:08:13,300 Two approaches can be used. 83 00:08:13,300 --> 00:08:19,550 First is the sputtering and co-evaporation on a substrate at high temperatures. 84 00:08:19,550 --> 00:08:24,219 During the process there is an additional selenium source. 85 00:08:24,219 --> 00:08:27,590 During deposition a CIGS film is formed. 86 00:08:28,270 --> 00:08:34,060 The second approach is sputtering and co-evaporation on a substrate at room temperature. 87 00:08:35,110 --> 00:08:41,219 The deposited films on the cold substrate are thermally annealed in presence of a selenide 88 00:08:41,219 --> 00:08:44,320 vapor to form the final CIGS structure. 89 00:08:44,320 --> 00:08:51,500 Another option is to deposit a selenium-rich layer on top of the initial deposited alloy and this is annealed. 90 00:08:51,900 --> 00:08:58,900 Because of the variety and complexity of the reactions taking place during such 'selenization' process, 91 00:08:58,940 --> 00:09:03,750 the properties of CIGS are difficult to control. 92 00:09:03,750 --> 00:09:12,700 Companies that use or have used the co-evaporation process are Würth Solar, Global Solar and Ascent solar. 93 00:09:13,399 --> 00:09:20,800 Among CIGS companies using sputter approaches are Showa Shell, Solar Frontier, Avancis, 94 00:09:21,000 --> 00:09:24,100 Miasole and Honda Soltec. 95 00:09:24,100 --> 00:09:28,880 An alternative approach is based on a kind of wafer bonding technique. 96 00:09:28,880 --> 00:09:33,279 Two different films are deposited on a substrate and superstrate. 97 00:09:33,279 --> 00:09:36,800 The films are pressed together under high pressure. 98 00:09:36,800 --> 00:09:43,529 When heated the film is released from the superstrate and a CIGS film remains on a substrate. 99 00:09:43,529 --> 00:09:47,800 This processing technique is used by the company Heliovolt. 100 00:09:47,800 --> 00:09:53,540 Non-vacuum techniques are based on depositing nanoparticles of the precursor materials on 101 00:09:53,540 --> 00:09:57,910 a substrate after which the film is sintered. 102 00:09:57,910 --> 00:10:02,510 Sintering is a process in which films are made out of powder. 103 00:10:02,510 --> 00:10:07,300 The powder is heated up to a temperature below the melting point. 104 00:10:07,300 --> 00:10:11,060 Atoms in the particles can diffuse across the boundaries of the particles. 105 00:10:11,060 --> 00:10:15,600 As a result the particles fuse together, forming one big solid. 106 00:10:15,600 --> 00:10:22,250 An important advantage of the CIGS PV technology is that on lab-scale it has achieved the highest 107 00:10:22,250 --> 00:10:26,480 conversion efficiencies among the thin-film solar cells. 108 00:10:26,480 --> 00:10:34,300 Lab-scale CIGS solar cells processed on glass have a record efficiency of 19.9% as achieved 109 00:10:34,300 --> 00:10:37,500 by National Renewable Energy Lab in the US. 110 00:10:37,500 --> 00:10:45,399 Typical open-circuit voltages are close to 700 mV, FF of 81% and short-circuit current density 111 00:10:45,399 --> 00:10:52,380 between 35 and 36 mA/cm^2 have been achieved. 112 00:10:52,380 --> 00:10:56,940 The world record on flexible substrates has been obtained at the Swiss Federal Laboratories 113 00:10:56,940 --> 00:10:59,910 of Materials Science and Technology. 114 00:10:59,910 --> 00:11:08,500 The CIGS cell on a flexible polymer foil resulted in an impressive conversion efficiency of 20.4%. 115 00:11:08,500 --> 00:11:16,800 CIGS cells in a module are similarly interconnected as we have seen for thin-film silicon cells 116 00:11:16,860 --> 00:11:19,459 discussed earlier. 117 00:11:19,459 --> 00:11:25,220 First the molybdenum back contact is deposited on top of the glass substrate and the cell 118 00:11:25,220 --> 00:11:28,060 areas are defined by laser scribes. 119 00:11:28,060 --> 00:11:35,060 Then the CIGS p-layer and CdS n-layer are deposited including a laser scribe step. 120 00:11:36,209 --> 00:11:41,900 Finally the intrinsic and p-doped ZnO is deposited, followed by a final laser scribe step. 121 00:11:41,990 --> 00:11:49,100 Now the front TCO electrode is connected with the molybdenum back contact of the next solar cell. 122 00:11:51,259 --> 00:11:57,490 The record efficiencies of modules are significantly lower than that for the lab-scale cells. 123 00:11:57,490 --> 00:12:03,639 Defining conversion efficiencies we have to make a distinction between two types of numbers. 124 00:12:03,639 --> 00:12:09,300 The aperture area, which means that only the area of the PV active part is considered 125 00:12:09,300 --> 00:12:13,339 when the conversion efficiency is considered. 126 00:12:13,339 --> 00:12:19,700 Total area means that the entire module area is considered when calculating the conversion efficiency. 127 00:12:19,759 --> 00:12:26,740 This area includes the dead area created by interconnection and the edges of the module. 128 00:12:26,740 --> 00:12:31,800 The record efficiencies of 1 m^2 modules are in the order of 13%, 129 00:12:31,800 --> 00:12:39,200 whereas the aperture area efficiencies are just above 14% as confirmed by NREL. 130 00:12:39,209 --> 00:12:46,900 The German manufacturer Manz AG has presented a 15.9% aperture area efficiency 131 00:12:47,000 --> 00:12:51,319 and a total area efficiency of 14.6%. 132 00:12:51,319 --> 00:12:59,600 Solar Frontier in Japan claims a 17.8% aperture area efficiency on a very small module of 900 cm^2. 133 00:13:00,029 --> 00:13:07,600 With these results, CIGS has the highest conversion efficiency achieved among the thin-film PV technologies. 134 00:13:08,060 --> 00:13:15,200 However, as CIGS is a rather complex material with complex deposition processes over large areas, 135 00:13:15,200 --> 00:13:24,600 an important challenge for the CIGS PV industry is to achieve a high production yield of CIGS modules. 136 00:13:25,000 --> 00:13:31,860 Another challenge is that this technology includes the element indium. 137 00:13:31,860 --> 00:13:37,900 Here we see an illustration that shows the abundance of the various elements in the Earth's crust. 138 00:13:38,190 --> 00:13:44,040 The red line is an indication for the critical abundance of a source material to be used 139 00:13:44,040 --> 00:13:46,670 for a large-scale production. 140 00:13:46,670 --> 00:13:53,670 As you can see, indium is not a very abundant element as it lies below the red line. 141 00:13:53,670 --> 00:14:03,000 Therefore indium might be the limiting step to upscale the CIGS PV technology to future terawatt scales. 142 00:14:03,500 --> 00:14:08,100 In addition, the current display industry depends on indium as well, 143 00:14:08,100 --> 00:14:12,420 as ITO is integrated in many display screens. 144 00:14:12,420 --> 00:14:21,200 Consequently, the interest in copper zinc tin sulfide, referred to as CZTS, is increasing, 145 00:14:21,220 --> 00:14:24,000 to replace the CIGS absorber layer. 146 00:14:24,000 --> 00:14:31,040 CZTS is based on non-toxic and abundantly available elements. 147 00:14:31,040 --> 00:14:40,500 The current record efficiencies of CZTS solar cells on lab-scale is around 11% as achieved by IBM. 148 00:14:41,400 --> 00:14:48,160 In the next block we will discuss the thin-film CdTe PV technology, this is a PV technology, 149 00:14:48,160 --> 00:14:52,690 which has currently the lowest demonstrated cost price per Wp. 150 00:14:52,690 --> 00:14:54,519 See you in the next block!