1 00:00:05,640 --> 00:00:10,879 Why do we need semiconductor materials for solar cells? 2 00:00:10,879 --> 00:00:16,680 This week I discuss the important properties of semiconductors and how these properties 3 00:00:16,680 --> 00:00:22,199 are being used in solar cells. This means that we have to take a tour in 4 00:00:22,199 --> 00:00:27,140 the world of physics. This week will be the most theoretical part 5 00:00:27,140 --> 00:00:33,329 of the course, but don't worry, I will guide you through it, using some visual figures 6 00:00:33,329 --> 00:00:38,349 and animations. Let's start with answering the question: 7 00:00:38,449 --> 00:00:44,120 What is a semiconductor material? Materials can be categorized in terms of their 8 00:00:44,120 --> 00:00:49,440 electrical properties. Let's consider metals. We know that metals 9 00:00:49,440 --> 00:00:55,940 can conduct electricity very well. The origin of the high conductivity of metals 10 00:00:55,940 --> 00:01:02,940 is based on the fact that the outer electrons of the atoms in a metal are weakly bound. 11 00:01:03,350 --> 00:01:10,350 This results in an ocean of free mobile electrons in the material indicated by the red dots. 12 00:01:11,720 --> 00:01:18,720 Electrons are negatively charged and therefore the conduction in a metal is based on mobile 13 00:01:19,009 --> 00:01:24,130 negative charge. These electrons move around in a background 14 00:01:24,130 --> 00:01:31,130 of atoms indicated by the blue dots. As many of the atoms donated an electron, 15 00:01:31,720 --> 00:01:38,720 they can be considered as fixed positively charged ions. 16 00:01:38,729 --> 00:01:43,270 Materials which do not conduct electricity are called insulators. 17 00:01:43,270 --> 00:01:49,570 In insulator materials the ocean of free moving electrons are missing. 18 00:01:49,570 --> 00:01:53,479 All electrons are bounded to the background atoms. 19 00:01:53,479 --> 00:01:58,619 As you can see in the figure, the red dots represent the electrons and are glued to 20 00:01:58,619 --> 00:02:04,119 the blue atoms. Semiconductors are materials which have a 21 00:02:04,119 --> 00:02:11,119 conductivity between that of a metal and insulator. The outer electrons of the atoms are more 22 00:02:12,010 --> 00:02:18,370 strongly bound to the background atoms than in metals, but under certain conditions some 23 00:02:18,370 --> 00:02:24,780 of the electrons can leave their background atoms and become freely mobile electrons as well. 24 00:02:25,780 --> 00:02:29,560 These few electrons that are separated from 25 00:02:29,560 --> 00:02:35,090 its atom, leave a positively charged entity behind. 26 00:02:35,090 --> 00:02:39,540 This positively charged entity is called a hole. 27 00:02:39,540 --> 00:02:45,060 The small blue dots in the illustration represent the holes. 28 00:02:45,060 --> 00:02:50,860 These holes are able to move around, just like electrons. 29 00:02:50,860 --> 00:02:57,860 As a result the charge transport in a semiconductor is facilitated by negatively charged electrons 30 00:02:58,739 --> 00:03:07,539 and positively charged holes. The properties of metals, insulators and semiconductors 31 00:03:07,560 --> 00:03:12,970 can easily be illustrated using the electronic band structure of a material. 32 00:03:12,970 --> 00:03:19,200 An electronic band reflects the potential energy levels an electron could occupy in 33 00:03:19,200 --> 00:03:24,830 the material. Metals have a broad electronic band which 34 00:03:24,830 --> 00:03:32,430 is not fully filled with electrons. This electronic band corresponds to the energy 35 00:03:32,489 --> 00:03:39,099 levels in which the electrons can freely diffuse around in the metal. 36 00:03:39,099 --> 00:03:46,499 An insulator and a semiconductor, to the contrary, have two distinct bands with a large forbidden 37 00:03:46,499 --> 00:03:53,049 gap between them. Most electrons fill the lower electronic band, 38 00:03:53,049 --> 00:03:58,570 the so-called valence band. If electrons reside in the valence band, 39 00:03:58,570 --> 00:04:05,010 they are not mobile, they are well bound to the atoms in the lattice. 40 00:04:05,010 --> 00:04:09,529 The upper electronic band is the so-called conduction band. 41 00:04:09,529 --> 00:04:16,650 Electrons in the conduction band are free and mobile and contribute to the conduction, 42 00:04:16,650 --> 00:04:23,650 just like the mobile electrons in a metal. The forbidden energy gap between the valence 43 00:04:24,040 --> 00:04:31,040 and conduction band is called the band gap. In this gap no energy states exist, 44 00:04:31,180 --> 00:04:38,780 which can be occupied by electrons. The difference between insulators and semiconductors 45 00:04:38,790 --> 00:04:46,190 is that the band gap of an insulator is much larger than the band gap of a semiconductor. 46 00:04:47,390 --> 00:04:53,550 The band gap of an insulator is typically larger than 3 eV. 47 00:04:53,550 --> 00:05:00,450 The larger the forbidden gap, the smaller the probability that an electron can have enough 48 00:05:00,450 --> 00:05:04,840 energy to occupy a state in the conduction band. 49 00:05:05,040 --> 00:05:12,520 For a semiconductor, the band gap is smaller. If the band gap of a material is smaller than 3 eV, 50 00:05:12,520 --> 00:05:18,320 the material can be considered to be a semiconductor. 51 00:05:18,420 --> 00:05:24,940 In a semiconductor, some electrons can have enough thermal energy to jump from the valence 52 00:05:24,940 --> 00:05:31,370 band to the conduction band. This energy can also be provided by light, 53 00:05:31,370 --> 00:05:36,870 if the photon has an energy equal or larger than the band gap. 54 00:05:36,870 --> 00:05:42,710 This means that when you shine light with photons having an energy larger than the band gap, 55 00:05:42,710 --> 00:05:49,410 you can make a semiconductor more conductive. The blue dots located in the valence band 56 00:05:49,410 --> 00:05:56,710 represent a positively charged hole. While electrons in the valence band are glued 57 00:05:57,080 --> 00:06:02,740 to their background atoms, the positively charged holes can diffuse through the valence band, 58 00:06:02,740 --> 00:06:05,930 just like the electrons in the conduction band. 59 00:06:05,930 --> 00:06:12,500 So, up to this point I have introduced the basic properties to describe semiconductor materials. 60 00:06:12,500 --> 00:06:15,870 The conductivity properties are determined 61 00:06:15,870 --> 00:06:20,270 by the electrons in the conduction band and the holes in the valence band. 62 00:06:20,270 --> 00:06:25,930 The energy gap between the conduction and valence band is called the band gap. 63 00:06:25,930 --> 00:06:32,820 The question now is: what is the principle behind the existence of the forbidden band gap 64 00:06:32,820 --> 00:06:40,620 between the valence and conduction band? Or in other words: what determines how strongly 65 00:06:40,670 --> 00:06:44,310 the electrons are bound to the atoms in the lattice? 66 00:06:44,310 --> 00:06:51,810 I will discuss that in the next two blocks. First, in the next block we will look at how 67 00:06:51,830 --> 00:06:56,270 electrons are bound to the nucleus within an atom.