700c so that the metals in the paste diffuse both at

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700°C so that the metals in the paste diffuse both at the front as well as on the back to make contact with the silicon. An anti-reflection coating of silicon nitride or titanium dioxide, having a thickness of about 0.1 μ m is applied on the top surface to complete the cell. A typical cell develops a voltage of 0.5-0.7 V and a current density of 20-40 mA/cm. In order to obtain higher voltages and currents, individual cells are fixed side by side on a suitable back-up board and connected in series and parallel to form a module. The cells are encapsulated in a thin transparent Metal electrode finger on front side on back side Fig. 5.1 Cross-sectional diagram of a silicon cell
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37 material in order to protect them from the environment and support them in the module. A number of modules are interconnected to form an array. Earlier the cells used to be circular in shape with diameters ranging from 6 to 15 cm. Now they are often rectangular in shape, resulting in more compact modules. Apart from single crystal silicon, silicon solar cells are now also made in large numbers from multi crystalline silicon and amorphous silicon. 3.14.2 Principle of Working of a Solar Cell Two important steps are involved in the principle of working of a solar cell. These are, 1. Creation of pairs of positive and negative charges (called electron-hole pairs) in the solar cell by absorbed solar radiation. 2. Separation of the positive and negative charges by a potential gradient within the cell. For the first step to occur, the cell must be made of a material which can absorb the energy associated with the photons of sunlight. The energy (E) of a photon is related to the wavelength (A) by the equation E = hc/λ - - - 1 where h = Planck's constant = 6.62×10 -27 erg-s and c = velocity of light = 3×10 8 m/s Substituting these values, we get E = 1.24/λ - - - 2 where E is in electron-volts (eV) and λ is in μm. Materials suitable for absorbing the energy of the photons of sunlight are semiconductors like silicon, cadmium telluride, gallium arsenide, etc. In a semiconductor, the electrons occupy one of two energy bands, the valence band and the conduction band. The valence band has electrons at a lower energy level and is fully occupied, while the conduction band has electrons at a higher energy level and is not fully occupied. The difference between the minimum energy of electrons in the conduction band and the maximum energy of the electrons in the valence band is called conduction the band gap energy E. Photons of sunlight having energy E greater than the band gap energy E g are absorbed in the cell material and excite some of the electrons. These electrons jump across the band gap from the valence band to the conduction band leaving behind holes in the valence band. Thus electron-hole pairs are created.
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38 The electrons in the conduction band and the holes in the valence band are mobile. They can be made to flow through an external circuit (thereby executing the second step of the photovoltaic effect) if a potential gradient exists within the cell, in the case of silicon, the
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