In this tutorial, we are going to learn about Solar Cell.
- It is semiconductor device which generates voltage when illuminated with a light.
- The functioning of a solar cell is similar to a photodiode. It is a photodiode which is unbiased and connected to a load.
- The solar cell is different from photodiode in the sense which works on a wide spectrum unlike photodiode which works on narrow range of wavelength; it has wide area of expose to light as compared to a photodiode. For a photo diode quantum efficiency is prime where as power conversion efficiency is prime for a solar cell.
Working of Solar Cell –
When the junction of an circuited photodiode is illuminated with light there is generation of electron hole pairs in depletion layer. The photo generated additional electrons drift through depletion layer from P-side to N-side and holes from N-side to P-side under influence of built in potential and results in reduction of potential barrier. The reduction in potential barrier appears across the terminals of diode as induced voltage called photo voltage.
- Current through a photo diode from p to n side in the diode is given by,
I = -IDC + Ia V
- When photodiode is used as photovoltaic or solar cell the terminal of the diode are open circuited and junction is illuminated with light source. Under such conditions the current in diode is zero and voltage induced across the terminal is called photovoltaic potential.Replacing V by V and setting 1=0 in equation we have,
→ -IDC + Ia Ia V
(envr -1) =0
Io e nvr = Isc + Io
Vphoto = nVT In (1+ Isc)
- When, Jsc is short circuit current or photo current of the cell. Here V photo is open circuited voltage so it is also denoted by Voc .The photo voltage or open circuited voltage of solar cell can also be written in terms of current density as under,
|Vphoto = nVT In (1+Jsc) Jo|
Where Jsc is short circuit current density and Jo is reverse current density.
Note: Vphoto also relates to optical generation rate and thermal generation rate as under,
|Vphoto =nVT In gas go|
Below Circuit of obtaining VI characteristics of Solar cell-
- When V = 0, the current through the cell is maximum called short circuit or photo current. When biasing voltage V is increased the current in the circuit starts decreasing. When voltage across the terminals becomes equal to Vphoto or VOC the diode current becomes equal to zero , Vphoto is maximum or open circuited voltage induced across the solar cell.
V-I characteristics of solar cell-
- Photo voltaic cell or solar cell works in fourth quadrant of V-I characteristic.
- A solar cell behaves like constant current source in 3nd quadrant of VI characteristic i.e. under reverse biased mode.
- The photo voltaic potential for Si is 0.5 V and for Ge is 0.I V.
- For getting the higher voltages multiple cells are connected series.
- For getting current rating higher than short circuited current Ix of a cell, multiple photo voltage cells are connected in parallel.
- Photo voltaic cell is the only cell which can work even under short circuited condition.
- Photo voltaic cell is called solar cell. The photo voltage cell converts light energy to electrical energy:
Output Power, Conversion Efficiency and Fill Factor of Solar cell-
- When a solar cell is directly connected across a load resistance R1 then the output power of solar cell can be given by.
Solar cell connected to a load, R1.
P = VI = V Ix + In (I –e V )
- Where V is voltage induced in the cell and I is current supplied by the cell.
P = FRL = V2
- When a solar cell is directly connected across a load resistance RL the output power is zero when RL is replaced either by a short circuit or by an open circuit or by an open circuit. The power supplied by the cell is maximum (Pm) only at some optimum value of RL Let the voltage and current corresponding to maximum power are Vm and Im respectively.
Maximum Power rectangle of solar cell-
- Fill factor of solar cell is defined as maximum output power of cell to the product Vas Io’
Fill factor = PT = VmIm
- The conversion efficiency of a solar cell is defined as ratio of output power of to the input solar power to the cell. The efficiency is maximum when the output power is maximum.
TIC = Pout x 100
Maximum efficiency of solar cell
TIC = Pout x 100