1 00:00:05,650 --> 00:00:11,120 In this block we leave solar cell technology based on inorganic semiconductor materials 2 00:00:11,120 --> 00:00:18,120 like amorphous and nanocrystalline silicon, CdTe, CIGS and III-V semiconductor materials. 3 00:00:18,470 --> 00:00:25,080 We are going to look at organic solar cells, like polymer cells or dye-sensitized solar cells. 4 00:00:26,869 --> 00:00:32,990 The materials used are conductive organic polymers or organic molecules. 5 00:00:32,990 --> 00:00:38,800 All these materials can be considered as large conjugated systems. 6 00:00:38,800 --> 00:00:45,450 The organic polymers and molecules consist of large compounds based on carbon. 7 00:00:45,450 --> 00:00:52,450 The carbon may form cyclic or a-cyclic, linear or mixed compound structures. 8 00:00:52,950 --> 00:01:01,500 Here we have some example of organic materials used for PV applications: P3HT, phtalocyanine, 9 00:01:01,500 --> 00:01:06,400 PCBM and ruthenium dye N3. 10 00:01:06,400 --> 00:01:13,600 The conjugated system means that carbon atoms in the chain has an alternating single or 11 00:01:13,610 --> 00:01:18,860 a double bond and every atom in the chain has a p-orbital available. 12 00:01:18,860 --> 00:01:24,070 The classical example from chemistry classes is the benzene molecule which is a cyclic 13 00:01:24,070 --> 00:01:27,710 conjugated compound. 14 00:01:27,710 --> 00:01:31,850 These p-orbitals in the conjugated orbitals are delocalized. 15 00:01:31,850 --> 00:01:37,560 This means that they can form one big mixed orbital. 16 00:01:37,560 --> 00:01:43,420 The valence electron of the original p-orbital is shared over all the orbitals. 17 00:01:43,420 --> 00:01:46,280 Here you see the example of the benzene ring. 18 00:01:46,280 --> 00:01:50,659 This molecule has 6 carbon atoms and six p-orbitals. 19 00:01:50,659 --> 00:01:57,659 They mix forming two circle orbitals that are occupied by a total of 6 electrons. 20 00:01:57,909 --> 00:02:03,000 These electrons do not belong to one single atom, but to a group of atoms. 21 00:02:03,580 --> 00:02:10,809 A methane molecule, which is tretrahedrally coordinated has 4 equivalent sp-3 hydride bonds, 22 00:02:10,900 --> 00:02:15,120 with a bond angle of 109 degrees as we discussed in week 2. 23 00:02:15,120 --> 00:02:21,500 Here we see an ethene molecule, which has 3 equivalent sp-2 hybrid bonds 24 00:02:21,500 --> 00:02:27,209 with a bond angle of 120 degrees, plus an electron in a p-z orbital. 25 00:02:27,209 --> 00:02:33,529 Two neighboring p-z orbitals form a so-called pi-orbital. 26 00:02:33,529 --> 00:02:39,500 In week 2 we have discussed that two individual sp-3 hybrid orbitals of a Si atom, 27 00:02:39,500 --> 00:02:43,329 can make an anti-bonding and bonding state. 28 00:02:43,329 --> 00:02:49,669 The same is valid for the two p-z orbitals making a molecular pi-orbital. 29 00:02:49,669 --> 00:02:54,180 They make a bond and anti-bonding pi-state. 30 00:02:54,180 --> 00:02:59,689 Therefore, conjugated molecules can have similar properties as semiconductor materials. 31 00:02:59,689 --> 00:03:04,129 Most electrons are at room temperature in the bonding state, also referred to as 32 00:03:04,129 --> 00:03:08,079 the highest occupied molecular orbital (HOMO). 33 00:03:08,079 --> 00:03:15,079 The anti-bonding state can be considered as the lowest unoccupied molecular orbital (LUMO). 34 00:03:15,139 --> 00:03:20,859 As the conjugated molecules are getting longer, the HOMO and LUMO will broaden and act like 35 00:03:20,859 --> 00:03:24,019 a kind of valence and conduction band. 36 00:03:24,019 --> 00:03:30,000 The energy difference between the HOMO and the LUMO can be considered as the band gap of the polymer material. 37 00:03:30,139 --> 00:03:36,370 To discuss whether an organic material is p-type or n-type we have to discuss one concept, 38 00:03:36,370 --> 00:03:39,879 which I did not discuss so far: the vacuum level. 39 00:03:39,879 --> 00:03:46,290 The vacuum level refers to the energy of a free stationary electron that is outside of 40 00:03:46,290 --> 00:03:49,730 any material, or in other words in a vacuum. 41 00:03:49,730 --> 00:03:56,730 This level is often used as the level of alignment for the energy levels of two different materials. 42 00:03:57,209 --> 00:04:02,059 The ionization energy is the energy needed to excite an electron from the valence band 43 00:04:02,059 --> 00:04:05,889 or HOMO to the vacuum state. 44 00:04:05,889 --> 00:04:11,569 The electron affinity is the energy obtained by moving an electron from the vacuum just 45 00:04:11,569 --> 00:04:18,010 outside the semiconductor or conjugated polymer to the bottom of the conduction band or LUMO. 46 00:04:18,010 --> 00:04:24,490 It means when a molecular material has a low ionization potential, it can with a relatively 47 00:04:24,490 --> 00:04:31,080 ease release an electron out of the material, i.e. it can act as an electron donor. 48 00:04:31,080 --> 00:04:37,389 When a molecular material has a high electron affinity, it can easily accept an additional 49 00:04:37,389 --> 00:04:42,479 electron in the LUMO or conduction band. 50 00:04:42,479 --> 00:04:48,360 Another important aspect of exciting charge carriers in organic materials is that it differs 51 00:04:48,360 --> 00:04:52,580 from that of inorganic semiconductor materials discussed so far. 52 00:04:52,580 --> 00:04:58,500 In inorganic semiconductors we can excite an electron from the valence band and conduction band, 53 00:04:58,500 --> 00:05:00,680 leaving a hole in the valence band. 54 00:05:00,680 --> 00:05:07,169 In a semiconductor such hole-pair is weakly bound and both entities are easily separated 55 00:05:07,169 --> 00:05:09,979 and can diffuse away from each other. 56 00:05:09,979 --> 00:05:12,580 In organic materials this is not the case. 57 00:05:12,580 --> 00:05:19,189 Light excitation results in so-called excitons, which are excited electron and hole pairs, 58 00:05:19,189 --> 00:05:25,830 which are still in bound state, due to the mutual coulombic force between the particles. 59 00:05:25,830 --> 00:05:29,289 The exciton can diffuse through the material. 60 00:05:29,289 --> 00:05:35,500 These excitons have a low lifetime in organic materials, they recombine back to the ground state 61 00:05:35,500 --> 00:05:39,500 within a few nanoseconds meaning that the diffusion length of such excitons 62 00:05:39,500 --> 00:05:44,000 is in the order of only 10 nm. 63 00:05:44,919 --> 00:05:49,689 Let's make a solar cell out of the conjugated materials. 64 00:05:49,689 --> 00:05:54,800 Here we consider an organic PV device, based on an electron donor type 65 00:05:54,800 --> 00:05:58,000 and an electron acceptor type material. 66 00:05:58,000 --> 00:06:03,000 Similar like for semiconductor materials, a heterojunction based on two intrinsic materials 67 00:06:03,000 --> 00:06:05,000 can be constructed. 68 00:06:05,000 --> 00:06:11,460 The junction is based on two different semiconductor materials or different conjugated compounds. 69 00:06:11,460 --> 00:06:18,000 Here we line up the electron donor conjugated polymer and an electron acceptor conjugated polymer. 70 00:06:18,400 --> 00:06:23,060 The HOMO and LUMO of both polymers can be aligned considering their energy levels 71 00:06:23,060 --> 00:06:26,750 with reference to the vacuum level. 72 00:06:26,750 --> 00:06:32,259 At the interface between both layers we see a difference in the HOMO and LUMO levels. 73 00:06:32,259 --> 00:06:37,400 This difference between the HOMO and LUMO represents an electrostatic force 74 00:06:37,400 --> 00:06:39,610 between the two layers. 75 00:06:39,610 --> 00:06:45,310 The material can be chosen properly to make the difference large enough, so these local 76 00:06:45,310 --> 00:06:51,710 electric fields are strong, which may break up the excitons. 77 00:06:51,710 --> 00:06:54,659 An electron is injected in the electron acceptor 78 00:06:54,659 --> 00:06:59,639 and a hole remains in the electron donor material. 79 00:06:59,639 --> 00:07:04,659 A problem of this device concept is that the diffusion length of the exciton is only 10 nm. 80 00:07:05,300 --> 00:07:10,169 It means that in this simple configuration the thickness of the solar cell is limited 81 00:07:10,169 --> 00:07:15,389 by the diffusion length, while the thickness has to be at least 100 nm to absorb enough light. 82 00:07:15,849 --> 00:07:21,490 Therefore, the organic solar cells are based on bulk heterojunction photovoltaic devices. 83 00:07:21,490 --> 00:07:28,069 In such device the electron-donor and the electron-acceptor materials are mixed together. 84 00:07:28,069 --> 00:07:32,430 Typical length scales of the mixture of the blend equal to the exciton diffusion lengths 85 00:07:32,430 --> 00:07:34,199 can be achieved. 86 00:07:34,199 --> 00:07:39,469 As a result a large fraction of the excitons excited in the material due to light absorption 87 00:07:39,469 --> 00:07:44,419 can reach an interface, where they are separated into an electron and a hole. 88 00:07:44,419 --> 00:07:48,349 The electrons move though the acceptor material to the electrode. 89 00:07:48,349 --> 00:07:53,669 The holes move through the donor material to be collected at the other electrode. 90 00:07:53,669 --> 00:07:58,529 The holes are usually collected at a TCO electrode like ITO. 91 00:07:58,529 --> 00:08:02,430 The electrons are collected at a metal back electrode. 92 00:08:02,430 --> 00:08:07,099 The record organic solar cells are based on double junctions nowadays. 93 00:08:07,099 --> 00:08:12,909 Heliatek achieved a 12.0% solar cell efficiency on lab-scale. 94 00:08:12,909 --> 00:08:15,599 The stability of these cells is unknown. 95 00:08:15,599 --> 00:08:20,509 An advantage of organic solar cells is that they have a low production cost. 96 00:08:20,509 --> 00:08:25,710 Chemical engineering allows a large flexibility in band gap engineering. 97 00:08:25,710 --> 00:08:31,229 The organic solar cells can be integrated into flexible substrates. 98 00:08:31,229 --> 00:08:36,909 Important disadvantages are a low efficiency, low stability and low strength compared to 99 00:08:36,909 --> 00:08:39,460 inorganic PV cells. 100 00:08:39,460 --> 00:08:44,600 Although organic solar cells can be cheaply, the need of highly expensive encapsulation 101 00:08:44,600 --> 00:08:51,020 materials to stabilize the organic PV products limits the industrial application. 102 00:08:51,020 --> 00:08:55,520 To my knowledge, no company is producing organic solar cells at the moment. 103 00:08:55,520 --> 00:08:58,790 Konarka Technologies has been active in the past. 104 00:08:58,790 --> 00:09:02,600 The small solar modules had efficiencies in the range of 3 up to 5% 105 00:09:02,600 --> 00:09:06,000 and lasted only a couple of years. 106 00:09:06,000 --> 00:09:13,000 An alternative solar cell concept based on organic materials is the so-called dye-sensitized solar cell. 107 00:09:13,500 --> 00:09:17,580 It is a photoelectrochemical system. 108 00:09:17,580 --> 00:09:25,500 It contains TiO2 nanoparticles, dye-particles, an electrolyte and a platinum contact. 109 00:09:25,500 --> 00:09:31,540 In this illustration the dye-sensitized solar cell is schematically shown. 110 00:09:31,540 --> 00:09:36,570 Let's consider first the photoactive parts of the solar cell which consists of the photoactive 111 00:09:36,570 --> 00:09:41,290 dye-sensitizer which acts like an electron donor. 112 00:09:41,290 --> 00:09:48,190 The second material is TiO2 nanoparticles which acts like the electron acceptor. 113 00:09:48,190 --> 00:09:54,910 The dye-material is mixed with TiO2 material like organic bulk heterojunction solar cells. 114 00:09:54,910 --> 00:10:01,910 The photoactive material is the so-called dye-photosensitizer: ruthenium polypyridine. 115 00:10:03,230 --> 00:10:09,730 If a photon is absorbed by the ruthenium polypyridine, it can excite an electron from its ground 116 00:10:09,730 --> 00:10:15,470 state, referred to as S, to an excited state, referred to as S*. 117 00:10:15,470 --> 00:10:22,280 In this case the S can be considered as the HOMO and the S* state can be consider as the LUMO. 118 00:10:22,280 --> 00:10:31,200 The S* state lies higher in energy than the energy level of the conduction band of the TiO2. 119 00:10:32,300 --> 00:10:38,790 As a result the light-excited electrons are injected into the TiO2 nanoparticles. 120 00:10:38,790 --> 00:10:44,230 The photosensitizer molecule remains like a positively charged entity. 121 00:10:44,230 --> 00:10:51,210 The electrons in the TiO2 move to the TCO-based back contact. 122 00:10:51,210 --> 00:10:54,070 The transport mechanism is diffusion based. 123 00:10:54,070 --> 00:11:02,000 The electrons diffuse between the various TiO2 nanoparticles until they arrive at the TCO contact. 124 00:11:02,200 --> 00:11:07,180 Through the electric circuit the electrons move to the counter electrode, in other words 125 00:11:07,180 --> 00:11:10,890 the other contact. 126 00:11:10,890 --> 00:11:17,590 Between the counter electrode and the dye, a so-called electrolyte is placed. 127 00:11:17,590 --> 00:11:24,800 Electrolytes are solutions or compounds that contain ionized entities that can conduct electricity. 128 00:11:25,540 --> 00:11:29,670 Typical electrolyte contains iodine. 129 00:11:29,670 --> 00:11:36,670 The positively charged oxidized dye molecule is neutralized by a negatively charged iodide. 130 00:11:37,200 --> 00:11:42,700 Three negatively charged iodides neutralize two dye molecules and create one negatively 131 00:11:42,700 --> 00:11:46,000 charged triiodide. 132 00:11:47,000 --> 00:11:52,890 This negatively charged triiodide moves to the counter electrode where it is reduced 133 00:11:52,890 --> 00:11:58,660 using two electrons into three negatively charged iodines. 134 00:11:58,660 --> 00:12:05,660 These photoelectrochemical cells require a platinum back contact to facilitate the reactions. 135 00:12:06,100 --> 00:12:11,870 As a result the performance of a dye-sensitized solar cell depends on the HOMO and LUMO level 136 00:12:11,870 --> 00:12:18,310 of the dye material, the Fermi level of the TiO2 nanoparticles and the so-called redox potential 137 00:12:18,310 --> 00:12:22,660 of the iodide and triiodide reactions. 138 00:12:22,660 --> 00:12:29,170 The record efficiency of dye-sensitized PV device on lab-scale is currently 14.1% as 139 00:12:29,170 --> 00:12:33,570 achieved at EPFL in Switzerland. 140 00:12:33,570 --> 00:12:40,160 The advantage of dye-sensitized PV devices are the low cost price of producing the device. 141 00:12:40,160 --> 00:12:45,390 A disadvantage is the stability of the electrolyte under various weather conditions. 142 00:12:45,390 --> 00:12:49,050 At lower temperature the electrolyte can freeze. 143 00:12:49,050 --> 00:12:55,420 This stops the power production and it might result in physical damage. 144 00:12:55,420 --> 00:13:00,030 Higher temperatures result in significant expansion of the electrolyte, which make encapsulation 145 00:13:00,030 --> 00:13:03,250 of modules more complicated. 146 00:13:03,250 --> 00:13:08,230 The dye-sensitized PV technology is facing some more challenges, one is replacing the 147 00:13:08,230 --> 00:13:12,810 expensive platinum electrode material with other cheaper materials. 148 00:13:12,810 --> 00:13:19,570 The second one is the development of more stable and resistive electrolyte materials. 149 00:13:19,570 --> 00:13:25,000 The third one is the development of improved dyes, improving the spectral and band gap 150 00:13:25,000 --> 00:13:28,360 utilization of the solar cells. 151 00:13:28,360 --> 00:13:34,320 A dye-sensitized PV product is not yet available on the commercial market. 152 00:13:34,320 --> 00:13:39,450 In this week I have introduced you to various PV technologies. 153 00:13:39,450 --> 00:13:46,450 Next week, we will shortly discuss the so-called third generation PV concepts. 154 00:13:46,610 --> 00:13:52,790 In addition, we will look at alternative solar technologies, which not only produce electricity, 155 00:13:52,790 --> 00:13:57,100 but heat or solar fuels as well. 156 00:13:57,100 --> 00:13:58,500 See you next week!