Construction and Materials Used in Solar PhotoVoltaic Cells and Working
Q. How a PV Cell Works
Q. What is the working principle of photovoltaic cell?
Q. How does a solar cell work?
Q. Working Principle of Solar Cell or Photovoltaic Cell
Q. How Do Solar Panels Work?
- The most normal configuration for a solar cell to make a p-n junction semiconductor is shown In Figure. The junction of the ‘p-type’ and ‘n-type’ materials provide an Inherent electric field which separates the charge created by the absorption of sunlight.
- The p-n junction is usually obtained by putting a p-type base material into a diffusion furnace containing a gaseous n-type dopant such as phosphorus and allowing the n-dopant to diffuse Into the surface about 0.2 um.
- The junction is thus formed slightly below the planar surface of the cell and the light Impinges perpendicular to the junction. The positive and negative charges created by the absorption of photons are thus encouraged to drift to the front and back of the solar cell.
- The back is completely covered by a metallic contact to remove the charges to the electric load. The collection of charges from the front of the cell is aided by a fine grid of narrow metallic fingers.
- The surface coverage of the conducting collectors is typically about 5% In order to allow as much as light is possible to reach the active junction area. An anti-reflective coating is applied on the top of the cell.
- Figure represents how this p-n junction provides an electrical field that sweeps the electrons in one direction and the positive holes In the other. If the junction is in thermodynamic equilibrium, then the Fermi energy must be uniform throughout.
- Since the Fermi level is near the top of the gap of n-doped material and near the bottom of the p-doped side, an electric field must exist at the junction providing the charge separation function of the cell.
- Each of the Individual solar cells will produce power at about 0.5 V with the current directly proportional to the cells area. The Individual cells are connected in series-parallel combination to meet the required voltage, power and reliability requirements, of the particular application.
- Space cells are covered with transparent ‘cover-slips’ to absorb the high energy particles in space that could cause damage the cell and result in the degradation of output.
- For terrestrial applications, the solar cell panels have to be encapsulated to protect them from atmospheric degradation due to oxidation of the metal contacts, which would cause peeling and open circuit materials such as glass, acrylics or silicon epoxies are used to provide a dear, weather tight front covering for the panels.
- Modern solar cells make use of semiconductor materials, usually based on single crystal silicon. When doped with phosphorus, arsenic, or antimony, the silicon becomes an n-type semiconductor and when doped with boron, aluminium, indium or gallium, it forms a p-type semiconductor.
- If a p-type semiconductor Is brought into intimate contact with one of the n-type, they form a p-n or n-p junction. If the two semiconductor materials are derived from the same element or compound, such as silicon, the system is referred to as homo-junction.
- It is also possible for a p-n or n-p junction to be formed from two different semiconductor materials, such as cadmium sulfide (CdS) and cuprous sulfide (Cu2,S), this is known as ‘hetero- junction’.
- The general behaviour the junction, is the same regardless of type, the ‘Schottky junction consisting of the semiconductor and metal. This junction is formed by depositing a thin layer of a metallic conductor 1.e. platinum onto a p or n-type semiconductor. Schottky junction photovoltaic cells made with the so called amorphous silicon are more efficient than homo- junction p-n cells of the same material. Cost of It Is also less.
- The MIS (Metal Insulator Semiconductor) solar cell Is similar to Schottky type except that very thin layer of about 0.1 to 0.3 um of an Insulator is deposited between semiconductors or the semiconductor and the metallic conductor.
- A conversion efficiency of more than 17% has been reported for an MIS solar cell made with a single crystal silicon.
- The cells may be connected in parallel to achieve the desired current and then stacked in series to achieve the desired voltage. The optimum operating voltage of a cell is about 0.45 volts, at normal temperature and the current at full sunlight may be taken to be 270 amps./sq.m.
- A decrease in the solar radiation has little effect on the voltage, but the current and power are decreased proportionately.