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Module 5 : Electrochemistry

Lecture 23 : Batteries and Fuel Cells

23.6

Photovoltaic(PV) Solar Cells

A very important application of light energy for the generation of electrical power comes from the photovoltaic (PV) solar cell. A PV solar cell is made up of n- and p-type materials of either the same or different compounds. For example, photovoltaic solar cells made out of polycrystalline n-Si and p-Si are commercially available. Pure silicon material is made n-type (electron donating) by doping it with phosphorous and p-type by doping it with boron. When n- and p-type Si are joined together, an electric field is developed at their interface due to the difference in the energy of their electrons. This field makes the n-type semiconductor negatively charged while the p-type semiconductor becomes positively charged. When the junction is illuminated with photons of energy greater than the band gap of Si, excited electrons due to the presence of the electrical field are forced towards n-type and holes (generated due to the creation of vacancy in the valence band) are forced to reach the p- type material. As a result of this, both ends of n- and p- type semiconductors become more charged than what they were before the illumination. If these two ends are short circuited by a connecting a wire through a load, electrons flow from n- type to p-type semiconductor, giving us electrical energy. In Figure 23.5, the mechanism of electron/hole transfer across the interface of p- and n-type semiconductors is shown.

Figure 23.5 A schematic diagram of energy levels and electron/hole transfer across the p:n junction and creation of excess charges at the two ends of the n and p-type semiconductors. The flow of current through the load and the junction is also shown. 

Table 23.1 some characteristics of silicon solar cells.

Parameters	Values
PV module efficiency (%)	15
Module cost (Rs / peak watt)	80
Plant life time	30 years
Energy cost (Rs/ kWh)	6
Photocurrent at peak power for a 12cm ²cell	0.33 A
Photovoltage	0.44 V

The PV cells are normally made of 12 cm² area and generally, polycrystalline solar cells are circular in dimension, though square types are also being made. From these individual solar cells, a power module is made to get the desired wattage. For example, to get a power of 7.7 kW, the total number of solar cells will be 53000, of which 250 would be connected in series and 212 in parallel. Such a combination of solar cells is called a PV module. The module as explained above would need a total area 1028 ft ² of which solar cells would occupy an area of 685 ft²

(The circular solar cells will need more space than its own area i.e., 685 ft² to accommodate 53000 solar cells in a rectangular shape). In Figure 23.6, schematic arrangement of silicon solar module with load is shown to give an idea about the entire system of power utilization by solar energy.

The efficiency of a solar cell is calculated by measuring the total power generated by the solar cell divided by total power of light falling on the solar cells. For these calculations, normally solar intensity at noon is taken as a standard value, which is known as the peak power measured in the units of watt and is normally taken as 100 milliwatt /cm ².