SOLAR CELLS

Solar  cells  that  we see  on calculators  and  satellites  are  photovoltaic  cells or modules (modules are simply a group of cells electrically connected and packaged in Photovoltaic,  as  the  word  implies  (photo  =  light,  voltaic  =  electricity),  convert directly into  electricity.  When  we consider  that  the  power  density  received from  at sea level is about 100 mW/cm (1kW/rn it is certainly an energy source that requires further research and development  to maximize the  conversion  efficiency from solar
to electrical energy.


The Fig shows the basic construction and cross section of solar cell. As shown
the top view, every effort is made to ensure that the surface area perpendicular to
the sun is maximum. The surface layer of p-type material is extremely thin so that
light   can;   penetrate   to   the   junction.   The   nickel-plated   ring   around  the   p-type material is the positive output terminal, and the plating at the bottom of the n-type   material is the negative output terminal. Usually, silicon is used as a semiconductor  material.

When light strikes the cell, certain portion of it is absorbed within the semiconductor material.  This  means  that  the  energy  of  the  absorbed  light  is  transferred  to  the
 semiconductor. This photon light energy collides with, a valence electron and imparts
to it sufficient energy to leave the parent atom. This results in the generation of free electrons and holes. This phenomenon will occur on each side of the junction. In the   p-type  material, the newly  generated  electrons  are  minority  carriers  and  will  move rather  freely  across  the  junction  as  explained  for  the  basic  p-n  junction.  A  similar discussion  is  true  for  the  holes  generated  in  the  n-type  material.  The  result  is  an increase in the minority carrier flow which is opposite in direction to the conventional forward current of a p-n junction.

The Fig shows the characteristics of solar cell. At vertical axis, V = 0 and it is short
circuit condition. The short circuit current is represented by notation Isc. Under open   circuit  condition  I  =  0  and  photovoltaic  voltage  V  will  result.  This  is  a  logarithmic function of the illumination, as shown in Fig.

Selenium  and  silicon  are  the  most  widely  used  materials  for  solar  cells,  although selenium arsenide, indium arsenide, and cadmium sulphide, among others, are also   used.


However, silicon has a higher  conversion  efficiency and greater stability and is  less subjected  to  fatigue.  It  also  has  excellent  temperature  characteristics.  That  is,  it withstands   extreme  high   or   low   temperatures   without   a  significant   drop-off   in efficiency.  The  efficiency  of  operation  of  a  solar  cell  is  determined  by  the  electrical power out divided by the power provided by the light source. It is given by

Typical levels of efficiency range from 10 to 40%.

Application of solar cell

Solar cells find many applications such as

• Satellites

• Automated street light

• Calculators

• Emergency light