Photovoltaic Solar Energy OPRE 6389 Lecture Note Compiled at 18:48 on Wednesday 1St March, 2017
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Photovoltaic Solar Energy OPRE 6389 Lecture Note Compiled at 18:48 on Wednesday 1st March, 2017 1 Science of Photovoltaics A photovoltaic device contains special materials that generate electricity from light. Materials used for a photovoltaic device are called semiconductors whose conducting properties can be deliberately altered by controlling the type and quantity of impurities added into the original semiconducting material. Semi- conducting materials can be classified as n(egative)-type and p(ositive)-type. Semiconductors that have an excess of electrons are called n-type semiconductors and semiconductors that have holes (lack of electrons) are called p-type semiconductors. When an n-type semiconductor is placed appropriately next to a p-type semiconductor and light is shined on them, electrons can flow from n-type to p-type. This phenomenon is known as photovoltaic effect. Conductors, Semiconductors and Insulators: The elements such as copper and aluminum are conduc- tors, which have a sea of free-roaming electrons somewhat independently of their nuclei. Moving these electrons away from their nuclei requires relatively less energy that can be supplied by light. Removal of the electrons from a metal by using light is called photoelectric effect. To generate electric current from light, what we want is controlled flow of electrons rather than their arbitrary dispersion into the space. Elec- trons of conductors are too free to control. At the other extreme, electrons of insulators such as plastics are bonded too strongly to their nuclei, hence their removal requires too much energy. Neither conductors nor insulators have the appropriate level of electron freedom to generate electricity from light. Electrons of semiconductors have less freedom than those of conductors and more freedom than those 28 of insulators. Semiconductors such as silicon crystal (made up of silicon Si14) have electrons that can be moved with light without causing these electrons to leave the surface of the semiconductors. This light-caused electron movement is an example of photovoltaic effect. Similar to silicon, elements of boron, germanium and arsenic are often used in semiconductor manufacturing; see Figure 1. Semi Metals: Boron, Silicon, Germanium, Arsenic, Antimony, Tellerium , Polonium Salt- Alkali Alkaline Transition forming Noble Metal Earth Metals Metals gases Gases ܋ܑܛ܉۰ ܛ܍ܛ܉۵ ଵ ଵ ସ ଶ ଵଶ ଵସ ଵ ଽ ଵଵ ଼ ଵଽ ଶ ଷ ସ ହ ଽ ଵ ଷଵ ଷଶ ଶଷ ଶସ ଶ଼ ଵହ ଵ ଷହ ସ ଵଵ ଵଶ ଶ ଵସ ଵ ଵ଼ ଵଷ ଽ ଷଽ ସ ସହ ସ଼ ହଵ ହଶ ହହ ହ ହଽ ହଽ ସ ହ ଷ ହ ଷସ ଼ ଼ହ ଵଽ ଶ ଶଵ ଶଶ ଶଷ ଶସ ଶହ ଶ ଶ ଶ଼ ଶଽ ଷ ଷଶ ଷଷ ଷହ ଷ ଷଵ ଼ହ ଼଼ ଼ଽ ଽଵ ଽଷ ଽ ଽଽ ଵଵ ଵ ଷ ଵ ଵ ଼ ଵଵଶ ଵଶ ଶ ଵଶ ଼ ଵଶ ଵଷଵ ଷ ଷ଼ ଷଽ ସ ସଵ ସଶ ସଷ ସସ ସହ ସ ସ ସ଼ ଵଵ ହ ଵଵ ଽ ହଵ ହଶ ହଷ ହସ ସଽ ହ ݐ݄ܽ ଵଽ ଵ଼ଵ ଵ଼ ସ ଵ଼ ଵଽ ଵଽଶ ଵଽହ ଵଽ ଶଵ ଶ ଽ ଶଵ ଶଶଶ݊ܽܮ ଵଷଷ ଵଷ ହହ ହ ଶ ଷ ସ ହ ଼ ଽ ଼ ଶ ସ ଶ ଶ ଽ ଼ସ ଼ହ ଼ ݊݅݀݁ݏ ଼ଵ ଼ଶ ଼ଷ ଶଶ ଷ ଶଶ ଶଵ ଶ ଶ ଶ ଶ ସ ଶଽ ଶ ଼ ଶଽ ଶଶ ଶ ǫǫǫ ǫǫǫ ଼ ݐ݅ ǫǫǫ ଶ଼ଽ ǫǫǫ ଶଽܿܣ ଼ ଼଼ ݊݅݀݁ݏ ଵସ ଵହ ଵ ଵ ଵ଼ ଵଽ ଵଵ ଵଵଵ ଵଵଶ ݏଵଵ ଵଵ଼ ଵଵଷ ଵଵସ ଵଵହ ଵଵ ݐ݄ܽ ଵଷଽ ଵସ ଵସଵ ଵସସ ଵସହ ଵହ ଵହଶ ଵହ ଵହଽ ଵଷ ଵହ ଵ ଵଽ ଵଷ ଵହ݊ܽܮ ݊݅݀݁ݏ ହ ହ଼ ହଽ ଵ ଶ ଷ ସ ହ ଼ ଽ ଵ ݐ݅ ଶଶ ଶଷଶ ଶଷଵ ଶଷ଼ ଶଷ ଶସସ ଶସଷ ଶସ ଶସ ଶହଵ ଶହସ ଶହ ଶହ଼ ଶ ଶଶܿܣ ݊݅݀݁ݏ ଼ଽ ଽ ଽଵ ଽଶ ଽଷ ଽସ ଽହ ଽ ଽ ଽ଼ ଽଽ ଵ ଵଵ ଵଶ ଵଷ Figure 1: Periodic table of elements shows semimetals between metals and gases. Photovoltaic Solar Energy by Metin C¸akanyıldırım In a semiconductor material, each electron resides in its regular orbit (valence band) around its nucleus. While residing in its valence band, an electron has a certain amount of energy. Energy of an electron increases upon receiving photovoltaic energy through a light photon; see Figure 2. A rise in an electron’s energy, if small, warms up the material containing the electron. If the energy rises by a large amount, it moves an electron from its valence band to the conduction band. Conduction band is further away from the nucleus and accommodates electrons that are ready to break away from their nucleus. If an electron in the conduction band is not directed through a circuit to come out away from its nucleus, it can drop back to its valence band. In the process of falling back, the electron may emit light. For example, fluorescent lamps excite gas electrons in the lamp with electricity and the electrons go up to their conduction band, fall back to their valence band and emit light. Fluorescent effect is the opposite of the photovoltaic effect. Energy of an electron increase upon receiving photovoltaic energy (through a light Valence band If an electron in the conduction band is not directed through a circuit to come out Conduction band Figure 2: Light increases the energy of an electron possibly from valence band to conduction band. To facilitate a single directional movement of electrons under photovoltaic effect, electron-rich (n-type) 28 and electron-lacking (p-type) regions of a semiconductor can be manufactured. Silicon Si14 has 14 electrons; 10 are in lower stable orbits while 4 are in the external orbit. These 4 electrons for each silicon atom makes up chemical bonds among the atoms in the silicon crystal. It is not easy to move electrons in this pure silicon crystal structure, so the crystal is unpurified slightly. Negative and positively charged regions of crystal can be obtained by using some other elements than silicon. Here are two examples: • 31 Fosfor (phosphorus P15) immediately to the right of silicon in Figure 1 is an ideal element to load up the crystal with extra electrons because fosfor has 5 electrons in its external orbit. If silicon is replaced with fosfor in a region of the crystal, that region has extra electrons and becomes relatively n(egative)-type. • 11 Boron (B5 ) to the left of silicon in Figure 1 is an ideal element to reduce the number of electrons because boron has 3 electrons in its external orbit. If silicon is replaced with boron in a region of the crystal, that region lacks electrons and becomes relatively p(ositive)-type. The elements used instead of silicon are called dopants. The concentration of dopant atoms can range from 1012 to 1020 per cm3 and affect the conductivity of the semiconductor. The most fundamental electronic device constructed using semiconductor materials is a diode. A diode is essentially an electrical device that allows current to move through in primarily one direction. A photovoltaic device is the most basic type of diode, where the photo-electric effect is used in combination with a p-n junction to build a photo- diode. In this diode, a p-type semiconducting material is brought in contact with an n-type semiconducting material to form a p-n junction, which is then exposed to sunlight. This is described in Figure 3 in steps. • In Step 1, we observe a p-n junction where the n-type material has an excess of electrons and the p-type material has excess holes in the absence of light. • In Step 2, the excess electrons present in the p-type material combine with the holes in the interface region due to opposite charge attraction and create the depletion region observed in Step 2 of Figure 3. This depletion region is well defined and it occurs only at the interface of the P-N junction. • In Step 3, when light shines on this semiconductor device, mobile electrons are created as a result of the photoelectric effect. The energy to mobilize electrons come from the photons in the light. Some electrons accelerate towards the n-type material or, said differently, some holes accelerate towards the p- 2 Photovoltaic Solar Energy by Metin C¸akanyıldırım Figure 3: Multi-step physics of a photovolatic device. A better name for Step 3 is photovoltaic effect. P: Phosphorus, B: Boron, e: electron, h: hole. type material. • In Step 4, the acceleration in the previous step forms a high concentration of mobile electrons in the n-type semiconductor and mobile holes in the p-type semiconductor. • In Step 5, by merely connecting the p-type and the n-type with a conductor, a unidirectional flow of electrons is observed across the p-n junction. The efficiency of this photovoltaic device is directly dependent on the wavelength of the incident light, the intensity of the incident light and the bandgap of the semiconducting material used to build the pho- tovoltaic device. Besides this, the reflectance of the material, the thermodynamic efficiency of the solar cell and conductive efficiency of the solar cell also play a strong interdependent role in defining the efficiency of a photo-voltaic device. Semiconductor materials commonly used to build photovoltaic devices need to have bandgaps that match the solar spectrum. Silicon is the most commonly used semiconducting material in solar energy harvesting. Affordability and the processing experience from the electronic device industry (transistors, LEDs etc.) have given silicon a competitive advantage over other materials. Some other materials used in photovoltaic devices are Galium Arsenide (GaAs), Cadmium Telluride, Copper Indium Gallium Selenide (CIGS), etc. 2 Photovoltaic Cell Types and Manufacturing Each photovoltaic (PV) device is called a photovoltaic cell or a solar cell.