INSTRUMENTATION & CONTROL

Theory
Silicon junction photovoltaic cells are for the direct conversion of solar energy to electric energy. No external bias source is required in photovoltaic type diode. The junction generate the voltage and the current depending upon the illumination and the load. This is the voltage mode of opearation for photodiodes. The photovoltaic or solar cell are able to generate an emf of 0.4 to 0.5v per cell. The individual cell can be connected in parallel and series into solar batteries or panels which have a relatively high efficiency ( up to 20%) and can deliver a power output of several kilowatts. The photovoltaic cells are made of p-type selenium and N-type cadmium oxide as shown in fig. The N-type region is exposed to the incident radiation.

Principal of operation of photovoltaic cell: (Photovoltaic type diade)
Photons incident on the PN junction and the adjacent give rise to the generation of electron- hole pairs. The electron and holes created an the N-region and P-region respectively, diffuse towards the junction and if they no time to recombine in the transit, they fall under the influence of the internal electric field exiting in the junction as shown in fig. this field also acts on the carriers created in the junction itself. It sweeps the electrons into the N- region. Similarly, the holes ae swept by the field out of the N-region into the P-region. With regard to the majority carriers i.e the holes in the P-region , however , the field og the junction is a decelerating one. These carriers stay in their respective regions, i.e the holes remain in P-region andelectron in N –Region as shown in figure. As a result , access majority carriers accumulate in their respective regions. The electron and holes build up charges in their respective region, and a potential difference called the photo-emf appears. This photo emf is build up nonlinearly with increasing luminous flux, i.e irradiance as shown in figure. The solar cell operates in the quadrent where Ip is negative and V is positive as shown in figure. Under increasing levels of illumination, the curve is progressively shifted downwards.