What is a PN Junction Solar Cell?
PN Junction Solar cells are semiconductor devices that convert light energy to electrical energy. They are also known as PV(Photovoltaic) cells. Individual solar cells are combined to form modules known as solar panels. The solar panels are arranged in parallel and series according to the power requirement.
The first generation of PN Junction solar cells was produced using monocrystalline or polycrystalline silicon crystals. The recent generation of solar cells consists of concentrated solar cells, polymer-based, dye-sensitized, nanocrystal-based, and perovskite-based solar cells.
Construction of PN Junction Solar Cell
In practice, p-n junctions of silicon solar cells are made in this way, but rather by diffusing an n-type dopant into one side of a p-type wafer (or vice versa). A solar cell is made up of silicon which absorbs the photons. Silicon wafers are doped and the metal contacts are put in to connect each solar cell to another. After this these cells are given an anti-reflective coating. This coating is used to protect against sunlight losses. Finally, solar cells are encapsulated and placed in an Aluminum frame. The diagram gives the construction details of PN Junction solar cells.
Working Principle of PN Junction Solar Cell
Light reaches the p-n junction, in the form of photons, and supplies sufficient energy to the junction to create a number of electron-hole pairs. The thermal equilibrium condition of the junction is broken by incident light The free electrons in the depletion region can quickly come to the n-type side of the junction. Similarly, holes in the depletion region come to the p-type side of the junction. Once the newly created free electrons come to the n-type side and the newly created holes come to the p-type side can not further cross the junction because of the barrier potential of the junction.
Electrons concentration becomes higher on the n-type side of the junction and hole concentration becomes more on the p-type side of the junction, so the p-n junction starts behaving like a small battery cell. Thus voltage is set up which is photovoltage. Now when we connect a small load across the junction current flows through it.
Equivalent Circuit of PN Junction Solar Cell
The electronic behavior of a solar cell can be understood by its electrical equivalent and is based on discrete ideal electrical components. An ideal solar cell may be modeled by a current source in parallel with a diode; in practice no solar cell is ideal, so a shunt resistance and a series resistance component are added to the model. The equivalent circuit diagram of solar cells is shown below.
I = IL - ID - ISH
I = Output current
IL = Photogenerated current
ID= Diode current
ISH = Shunt current
VI Characteristics of PN Junction Solar Cell
The Solar Cell I-V Characteristic Curves show the current and voltage (I-V) characteristics of a particular photovoltaic cell. It gives a description of its solar energy conversion ability and efficiency.
With the solar cell open-circuited, the current is zero and the voltage across the cell is maximum, known as the solar cell's open-circuit voltage or Voc. On the other hand, when the solar cell is short-circuited, that is the positive and negative leads are connected together, the voltage across the cell is zero but the current flowing out of the cell reaches it's maximum, known as the solar cell short circuit current, or Isc.
The point at which the cell generates maximum electrical power is shown at the top right area of the rectangle. This is the “maximum power point” or MPP. The ideal operation of a solar cell is defined to be at the MPP. Below given is the VI characteristics of solar cells.