procedure sheet for the experiment " SOLAR CELL"
- 1. Solar Cell salman January 29, 2017 AIM : To draw the I-V characteristics of a solar cell and to find the efficiency and fill factor of a solar cell. APPARATUS : Solar cell, Light source, Basic circuit , connecting wires etc. PRINCIPLE : Solar cells are the semiconductor devices which produce electric voltage across their terminals when light is incident on it(by photovoltaic effect). THEORY : Solar cell is a p-n junction . We know that a built in voltage exists across a p-n junction but this voltage cannot deliver current in a external circuit. If light is shown on the junction, there is current in the circuit. This effect is called the Photovoltaic effect. Let the junction be illuminated. Under this condition many excess electron-hole pairs are generated in the regions on the either side of the junc- tion via absorption of photons. When sunlight or other sufficiently energetic light is incident upon the photodiode, the electrons present in the valence band absorbs energy and being excited jump to the conduction band and become free. These excited electron diffuce and some reach the rectifying junction (usually a p-n junction).When they are accelerated into a different material by a built in potential, this generates an electro motive force and thus some of the light energy is converted into electrical energy. The photo- voltaic effect can also occur when two photons are absorbed simultaneously in a process called two photon photovoltaic effect. The Photovoltaic effect was first observed by French physicist A.E Becquerel in 1839. As the field 1
- 2. within the p-n junction , the excess minority carriers thus generated diffuse to the junction where they are carried across and become majority carriers. The holes and electrons generated on either side of the junction move to- wards opposite side of the junction. If the junction is now open circuited the majority carrier excess charge will build on both sides of the junction tending to lower the build in voltage to some value as shown in figure. This charge in build in voltage i.e v0 appears as a measurable potential difference across the junction, which there by behaves as a source of voltage v0. If the external circuit is closed, the current will therefore flow there in. This current will continue as long as there is diffusion of excess electrons from n-side and excess holes from p-side. That means the current flow as long as the semi-conductor regions are illuminated. In most photovoltaic applications, the radiation is sunlight and the devices are called solar cells. In the case p-n junction solar cell, illuminating the material creates an elec- tric current as excited electrons and remaining holes are swept in different directions by the build in electric field of the depletion region. FORMULA : Efficiency(η): In a solar cell , the light energy is converted into electrical energy. The fraction of the light energy converted into electrical energy is the efficiency of the solar cell. It is given by, η = pmpp/Ac.E Where Ac = surface area of the solar cell E = w/d2= light intensity in Wm−2 W = power of the light source = 60W d = distance between the source and the cell. For the given solar cell, Ac = πr2 r = radius of the solar cell FILL FACTOR: This is the measure of the number of photo junction inside the solar cell which is effectively contributing to the photo current.Also,it can be defined as the ratio of maximum obtainable power to the product of the open-circuit voltage and short-circuit current.Since not all the junctions are effective in the photo voltaic phenomenon. Hence the fill factor decides amount of junction contributions. If all the junctions participate FF=1 Otherwise FF<1. Fill Factor is given by, FF= ηAc.E/Isc.Voc 2
- 3. CIRCUIT DIAGRAM: PROCEDURE: • Set up the circuit as shown in the figure. The solar cell power supply is connected to the bulb. The supply and cooling fan are switched on the solar cell is kept at a distance d from the source. • The rheostat is shorted and the short circuited current is noted (Isc approx 50 mA). • Both the terminals of the rheostat are disconnected from the ammeter voltmeter junction and the open circuit voltage is noted in the mille voltmeter (Voc approx 500 mV). • By varying the resistance values as 10 ohm, 22 ohm, 47 ohm, 56 ohm, 68 ohm, ..upto 1 kilo ohm correspondingly note down the values of voltage and current; power is calculated • Calculate and record the Pmpp value from the above table. Also calculate the efficiency and fill factor. • Draw a curve with voltage along axis and current along y axis to get the characteristics of the solar cell. Draw one more graph between power and voltage to show power variation. • Repeat the experiment for at least three more distances. 3
- 4. Tabular column Model graphs Precautions • Light from source should fall vertically on the solar cell. • Short circuits should not be done for a longer duration. While taking down the reading of Isc leads to damage permanently. RESULT : 4