1 00:00:05,930 --> 00:00:11,150 A third thin-film technology we will discuss is the cadmium telluride. 2 00:00:11,150 --> 00:00:15,859 This thin-film technology which has currently demonstrated the lowest cost price per Wp 3 00:00:15,859 --> 00:00:18,539 among all PV technologies. 4 00:00:18,539 --> 00:00:22,530 Let's start with the physical properties of CdTe. 5 00:00:22,530 --> 00:00:27,400 This semiconductor material consists of the II-valence electron element cadmium 6 00:00:27,400 --> 00:00:30,699 and the VI-valence element tellurium. 7 00:00:30,699 --> 00:00:37,800 Its network is a cubic tetrahedrally lattice structure, where every Cd atom is bonded to a Te atom. 8 00:00:37,940 --> 00:00:43,400 A lot of the research activities on CdTe have taken place on industrial level and 9 00:00:43,400 --> 00:00:48,809 First Solar is the leading company in the CdTe technology. 10 00:00:48,809 --> 00:00:56,800 The band gap of CdTe is 1.44 eV, a value which lies within the optimal range of band gaps 11 00:00:56,800 --> 00:00:59,890 for a single junction solar cell. 12 00:00:59,890 --> 00:01:08,000 CdTe has a direct band gap, consequently only a few microns of CdTe is required to absorb 13 00:01:08,079 --> 00:01:12,000 all the photons with an energy higher than the band gap. 14 00:01:12,400 --> 00:01:18,070 It means that the diffusion length for the charge carriers has to be in the same order 15 00:01:18,070 --> 00:01:23,530 to have the light-excited charge carriers collected at the contact. 16 00:01:23,530 --> 00:01:31,500 N-doping of CdTe can be achieved by replacing the II-valence atom Cd with a III-valence 17 00:01:31,500 --> 00:01:36,650 electron atom like aluminum, gallium and indium. 18 00:01:36,650 --> 00:01:40,470 These elements act as shallow donors. 19 00:01:40,470 --> 00:01:48,800 N-doping is achieved as well by replacing the VI-valence tellurium atom with a VII-valence 20 00:01:48,800 --> 00:01:55,840 electron element like fluorine, chlorine, bromine and iodine atoms. 21 00:01:56,170 --> 00:02:00,500 They act as shallow acceptors. 22 00:02:00,500 --> 00:02:05,240 A tellurium vacancy acts like a donor as well. 23 00:02:05,240 --> 00:02:14,300 P-doping of CdTe can be achieved by replacing the II-valence atom Cd with a I-valence electron 24 00:02:14,300 --> 00:02:18,260 atom like copper, silver or gold. 25 00:02:18,260 --> 00:02:21,469 These elements act as a shallow acceptor. 26 00:02:21,469 --> 00:02:28,469 P-doping is achieved as well by replacing a VI-valence tellurium atom with a V-valence 27 00:02:28,800 --> 00:02:34,719 electron element like nitrogen, phosporous, arsenicum. 28 00:02:34,719 --> 00:02:38,849 They act as shallow acceptors. 29 00:02:38,849 --> 00:02:42,629 A cadmium vacancy acts like an acceptor as well. 30 00:02:42,629 --> 00:02:53,000 In solar cells p-doped CdTe is used, however, it is difficult to give CdTe a very high doping level. 31 00:02:54,100 --> 00:02:58,999 The structure of a typical CdTe solar cell looks like this. 32 00:02:58,999 --> 00:03:05,309 On a glass the transparent front contact is deposited. 33 00:03:05,309 --> 00:03:12,309 This can be tin oxide or cadmium stannate, which are CdSnO alloys. 34 00:03:12,719 --> 00:03:18,049 On top of that, the n-layer is deposited which is a cadmium sulfide layer, similar to the 35 00:03:18,049 --> 00:03:21,829 n-buffer layer in CIGS solar cells. 36 00:03:21,829 --> 00:03:29,200 On top of that, a p-type CdTe absorber layer is deposited with typical thickness of a few microns. 37 00:03:29,980 --> 00:03:36,849 Making a good back contact on CdTe is rather challenging, the material properties of CdTe 38 00:03:36,849 --> 00:03:41,779 do not allow a large choice of acceptable metals. 39 00:03:41,779 --> 00:03:47,169 Heavily doping the contact area with a semiconductor material improves the contact, however, achieving 40 00:03:47,169 --> 00:03:51,669 high doping levels in CdTe is problematic. 41 00:03:51,669 --> 00:03:56,499 Copper containing contacts have been used as back contacts, however, in long time scales 42 00:03:56,499 --> 00:04:03,189 they may face instability problems due to the diffusion of copper through the CdTe layer 43 00:04:03,189 --> 00:04:06,449 up to the CdS buffer layer. 44 00:04:06,449 --> 00:04:12,919 Nowadays a stable antimony telluride layer in combination with molybdenum is used. 45 00:04:12,919 --> 00:04:17,989 Here you see the band diagram of a CdTe solar cell. 46 00:04:17,989 --> 00:04:28,400 The p-type semiconductor CdTe has a band gap of 1.45 eV, whereas the n-type CdS has a bandgap of 2.4 eV. 47 00:04:28,800 --> 00:04:34,200 Consequently, the junction is a heterojunction, similar to the CIGS PV device. 48 00:04:34,200 --> 00:04:40,220 The light-excited minority electrons in the p-layer are separated at the heterojunction 49 00:04:40,220 --> 00:04:43,550 and collected at the TCO-based front contact. 50 00:04:43,550 --> 00:04:47,949 The holes are collected at the back contact. 51 00:04:47,949 --> 00:04:53,560 An important concept I did not discuss for thin-film solar cells is the two types of 52 00:04:53,560 --> 00:05:00,500 solar cell configurations: the superstrate and the substrate configuration. 53 00:05:01,240 --> 00:05:06,840 A superstrate configuration is a cell concept in which the substrate on which the solar cell 54 00:05:06,840 --> 00:05:13,840 is processed acts as the front window at which the light enters the solar cell. 55 00:05:14,250 --> 00:05:20,620 A substrate configuration is that either the substrate acts like a back contact or the 56 00:05:20,620 --> 00:05:24,139 back contact is deposited on the substrate. 57 00:05:24,139 --> 00:05:28,539 Consequently, no light will pass through the substrate. 58 00:05:28,539 --> 00:05:35,539 The light enters through a TCO layer deposited on top of the n-type CdS layer. 59 00:05:35,620 --> 00:05:39,430 Compare this to, for instance the thin-film silicon we discussed earlier. 60 00:05:39,430 --> 00:05:51,000 A p-i-n junction is considered as a superstrate configuration, whereas a n-i-p junction is a substrate configuration. 61 00:05:51,379 --> 00:05:58,379 The CdS/CdTe layers are in general processed using the closed space sublimation method. 62 00:05:59,819 --> 00:06:05,669 In a closed space sublimation method, the source and the substrate are placed at a short 63 00:06:05,669 --> 00:06:12,150 distance from each other, like a few mms up to cms under vacuum conditions. 64 00:06:12,150 --> 00:06:16,539 Both, the source and the substrate are heated. 65 00:06:16,539 --> 00:06:21,639 The source can be granulates or powders of CdTe. 66 00:06:21,639 --> 00:06:26,430 An inert carrier gas like argon or nitrogen can be used. 67 00:06:26,430 --> 00:06:32,930 The source is at a higher temperature as the substrate, and induces driven force on the 68 00:06:32,930 --> 00:06:36,810 precursors, which are deposited on the substrate. 69 00:06:36,810 --> 00:06:40,550 The bulk p-type CdTe is formed. 70 00:06:40,550 --> 00:06:48,400 First Solar and Antec are companies producing the CdTe solar modules using the closed space sublimation method. 71 00:06:48,600 --> 00:06:56,500 Among new start-ups moving into the CdTe PV technology are Calyxo, Prime Star Solar from General Electric, 72 00:06:56,600 --> 00:06:58,969 and Abound Solar. 73 00:06:59,080 --> 00:07:06,871 However, First Solar is by far the largest CdTe manufacturer in the world nowadays. 74 00:07:06,900 --> 00:07:15,900 From 2008, First Solar has an annual production rate of 500 MW and more and was in 2006 and 2007 75 00:07:15,949 --> 00:07:20,620 one of the biggest solar module manufacturers in the world. 76 00:07:20,620 --> 00:07:29,000 The record conversion efficiency of lab-scale solar cells is 18.7% as obtained by First Solar in 2013. 77 00:07:29,000 --> 00:07:39,000 The open-circuit voltage of the record cell is 852 mV, the short-circuit current density is 28.6 mA/cm^2 78 00:07:39,000 --> 00:07:43,000 with a FF of 76.7%. 79 00:07:43,000 --> 00:07:49,000 General Electric achieved in the same year an efficiency of 18.3%. 80 00:07:49,000 --> 00:07:58,000 NREL has confirmed a new record conversion efficiency for a CdTe solar module of First Solar of 16.1%. 81 00:07:59,000 --> 00:08:06,000 The current cost price per Wp of the First Solar products is in the order of 68 to 70 dollar cents per Wp 82 00:08:06,000 --> 00:08:13,500 and is expected to drop to 40 dollar cent per Wp in the future, keeping the cost price per Wp lower 83 00:08:13,500 --> 00:08:17,500 than the solar modules based on crystalline wafers. 84 00:08:17,500 --> 00:08:25,500 An important aspect to be addressed is that the CdTe solar cells contain the toxic material Cd, 85 00:08:25,500 --> 00:08:34,000 however, the insoluble Cd compounds like CdTe and CdS are much less toxic. 86 00:08:34,000 --> 00:08:39,000 It is important to prevent cadmium entering into the ecosystem. 87 00:08:39,000 --> 00:08:45,500 The question is whether the CdTe modules would be a main source of Cd pollution. 88 00:08:45,500 --> 00:08:52,500 A 2 GW/year production capacity, as installed by First Solar at the moment, would take up around 89 00:08:52,500 --> 00:09:00,000 2% of the total Cd consumption by the industry and would not yet be a dominant contributor. 90 00:09:00,000 --> 00:09:07,000 Nevertheless, recycling schemes have been set up for installed CdTe solar modules. 91 00:09:07,000 --> 00:09:14,000 For instance, First Solar has a recycling scheme in which a deposit of 5 dollar cents per Wp is included, 92 00:09:14,000 --> 00:09:19,500 which covers the cost for the recycling at the end of the modules lifetime. 93 00:09:19,500 --> 00:09:25,500 Maybe the biggest challenge for the CdTe will be the supply of Te. 94 00:09:25,500 --> 00:09:32,000 Here we see again the illustration that shows the abundance of the various elements in the Earth's crust. 95 00:09:32,000 --> 00:09:40,000 As you can see, tellurium is not a very abundant element, so tellurium supply might be 96 00:09:40,000 --> 00:09:46,000 the limiting step to upscale the CdTe PV technology to future terawatt scales. 97 00:09:46,000 --> 00:09:52,300 On the other hand, tellurium as source material has only had a few users, 98 00:09:52,300 --> 00:09:57,300 and therefore a dedicated mining of tellurium has not been explored. 99 00:09:57,300 --> 00:10:04,300 In addition, new supplies of tellurium-rich ores have been found in Xinju in China. 100 00:10:04,300 --> 00:10:08,700 So at this moment it is not clear to which extent the CdTe PV technology 101 00:10:08,700 --> 00:10:11,400 might be limited by the tellurium supply. 102 00:10:11,400 --> 00:10:17,400 So far we have discussed the inorganic thin-film semiconductor materials 103 00:10:17,400 --> 00:10:22,800 like amorphous and nanocrystalline silicon, CIGS and CdTe solar cells. 104 00:10:22,800 --> 00:10:28,800 In the next block we are going to look at organic and dye-sensitized solar cells. 105 00:10:28,800 --> 00:10:29,800 See you in the next block.