Theory of Solar Cell - Coursework Example

Theory of Solar Cell Name: Course: Lecturer: Institution: City & State: Date: Theory of Solar Cell Introduction Solar cell refers to an electrical device that converts the light energy from the sun to useful electricity using photovoltaic effect. Solar cell uses the incident photons of energy from the sun radiation to generate electricity directly. During the hot day, crystalline solar cells are heated and this makes the atoms move faster as they vibrate, thereby creating the built in voltage. Kinetic energy of the electrons and holes is the key component to control their movement within the solar cell layers (Spurgeon, Atwater, & Lewis, 2008: 15). Solar panels are black, thereby when heated by the sun’s radiation they consume more solar energy thus creating higher temperatures than the ambient air. The normal operating solar temperature when the atmospheric temperature is  and the sun’s isolation is, it is between. Solar cells are made of different materials, which establish different efficiencies of the solar cell. These may include thermodynamic efficiency, efficiency based on charge carrier, conductive efficiency and reflectance efficiency. Silicon solar cells have the best features when used in developing effective solar cells (Nielsen, 1982: 821). This is due to the fact that silicon p-n connection has the lowest energy between the layers, which is 1.11 eV; this helps the charge carriers to have least energy during the electron transfer. Photogeneration of charge carriers On absorbing a photon, an electron releases its energy in the crystal lattice. This enables the electron to have the valence band, which tightly constrain the electrons from the atoms thus allowing them to move within the layers. The covalent bonds between the conduction band giving the photon excitation lattice. Therefore, this means electron pairs are created by the semiconductor photons. If an electron has a higher energy than the band gap, then it is able to stimulate the electron to jump to reach the conduction band from the valance band within the layer of the junction. Figure 1: Band of solar cell Light energy travels as packets of energy (photons). Inside the depletion zone of the p-n junction electric current is generated. The photoreceptor of the solar cell there is a bi-layer composed of the p-type and n-type doped silicon layers. This photoreceptor uses the concepts of photo conductance of the sun’s radiation. When the sun’s radiation is emitted to the surface of the solar cell, electrons become excited and move freely within the layers. When excited, electrons gain energy boost and they jump from one layer to the other using external force of excitation from the sun photons. Electrons gain sufficient power to go to the conduction band through grabbing energy from the photon that has the least band gap. In order to stimulate the electrons, the photons must have greater energy than the band gap to separate them from their electron pairs. Failure to have enough energy to leave these electrons, the photon simply does nothing and passes across the solar cell (Lorenzo, 1994: 122). On the other hand, if the photons have enough energy, electrons are separated from their pairs thus creating holes thus balancing their energy generated from heat. Crystalline structures, such as silicon have constant energy levels, so they have constant band gap of 1.11 eV. Charge carrier separation In a solar cell, there are two types charge carrier separation: Diffusion: in this mode of separation, charges diffuse randomly due to thermal motion until the electrical fields within the active region captures them. Drift: electric field across the device drives the charge carriers. Active regions are often lacking in thick solar cells. Therefore, diffusion is the only method of charge carrier separation. Thus, it is a requirement to have the diffusion length of the photo generated carriers is greater than the thickness of the cell. On the other hand, in film cells diffusion period is inadequate because of the defects present, so the charge separation is by drift mode, which uses electrostatic field to drive the charge carriers (Jenny, 2003: 56). P-n junctions The most common configuration for the solar cells is the silicon type with the p-n junction. When two types of semiconductors, extra holes (p-type) and (excess electrons) n-type, come into contact they make a p-n junction. Therefore, this is a joint interface that is surrounded by semiconductors. P-n junction is formed by in a single crystal that has each semiconductor doped with different dopants. The p-type semiconductor is boron-doped silicon, and it is quite conductive. On the other hand, the n-type semiconductor is phosphorous-doped silicon.The junction between the two semiconductors does not conduct so it is safe layer, which is called region of space charge. Formation of the non-conducting layer has its attributes because electrons in the two semiconductors diffuse to each other thereby eliminating their charges respectively. Due to the electron movement near the p-n junction, an intrinsic voltage is created, as the electrons tend to diffuse into the p-semiconductor. When these electrons leave the attached semiconductor, they leave holes that make the n-region. Similarly, when the holes near the p-n junction start to move into the n-semiconductor region, they leave negatively charged fixed electrons. Due to this reason, the space charge region is created when the two regions near the p-n junction lose their neutrality. The diffusion of the holes and electrons is as indicated below. Figure 2: p-n junction equilibrium The above diagram indicates the space generation and electric field generation by this space charge, which is in a counteracting process. Connection with external loads When the circuit is connected to both p-type and n-type sides to an external load, electrons flow from the n-type via a wire to the load. This flow continues to the p-type semiconductor, where they meet with a hole generated as an electron pair (Castellano, 2010: 6-7). This connection creates a potential difference between the two junctions and gives a potential difference of quasi Fermi levels. Solar cell circuit Figure 3: Equivalent circuit of a solar cell Figure 4: solar cell Characteristic equation Using the above solar cell circuit, current flow through the circuit is: Where:    Voltage across these elements is given as: Where:     Using Shockley diode equation diverted current is: Where:   Using the ohms law the shunt current is: Energy conversion efficiency of solar cells When solar radiation lands on n-silicon layer, it excites the electrons due to additional electromagnetic energy from the solar energy. Hence, the electrons move to the negative pole as the positive charges in the p-silicon layer move towards the positive pole.  Effect of temperature on solar cell There are two effects of temperature on the solar cell, that is, in exponential terms and indirect effect. Increasing the temperature T of the exponential factors, it results to a decrease in the size of the exponent factor. As the indirect saturation current increase, relative temperature T increases. Figure 5: Temperature on a current-voltage of a solar cell References and Bibliography Castellano, R., (2010). Solar Panel Processing, Paris: Archives contemporaines. P. 6-7. Jenny, N., (2003). The Physics of Solar Cells. New York: Imperial College Press. P. 56 Lorenzo, E., (1994), Engineering of Photovoltaic Systems: Solar Electricity, Paris: Progensa. P. 122 Nielsen, L. (1982). Series Resistance and Effects in Solar Cells", Transactions on Electron Devices, Volume 29, Issue 5, p. 821 – 827. Spurgeon, J.M., Atwater, H. A. & Lewis, N. S. (2008). Behavior of planar and nanorod array Photo electrodes. Journal of Physical Chemistry C, 112(15):6186–6193, John, P., (1999). From space to Earth: the story of solar electricity. New York: Earthscan. P. 79 Read More