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How does a Solar Cell works?

How does a Solar Cell work:

Solar cell converts one form of energy (sunlight) into another form of energy (electricity) without moving parts, noise, pollution, radiation or maintenance. It is the special properties of the semiconductor materials, which makes the conversion possible.



This diagram shows a typical crystalline silicon solar cell. The electrical current generated in the semiconductor is extracted by contacts to the front and rear of the cell. The top contact structure which must allow light to pass through is made in the form of optimally spaced thin Silver metal strips (called fingers) that supplies current to the large busbar. The cell is covered with a thin layer of dielectric material-the anti-reflection coating, ARC-to minimize the light reflection from the top surface. The Back surface is screen printed with Aluminium Alloy that lowers the effective surface recombination velocities and as well provides a deep p+ layer at the back, towards a better efficiency.

Semiconductors:

Most Solar cells are made from Silicon,. Silicon has properties of both metal and insulators. Atoms in a metal have loosely bound electrons that flow easily when voltage is applied. Atoms in an insulator have tightly bound electrons that cannot flow even when strong electric voltage is applied. Atoms in a semiconductor bind their electrons somewhat more tightly than metals, but more loosely than insulators. Their electrical conductivity can be modified by simply adding small amounts of impurities or dopants into the semiconductor structure. The two elements typically used to dope Silicon are Boron and Phosphorous.

Formation of Internal Electrostatic field:
Silicon atoms have four outer electrons, phosphorous atoms have five and Boron atoms have three. Silicon makes up most of the Solar cell, but some Boron atoms are present throughout the cell, and a very thin layer of Phophorous- doped (n-type) Silicon is created at the front surface of the cell, which forms the P-N junction. Although both materials are electrically neutral, n-type Silicon has excess electrons and p-type Silicon has excess holes. Sandwiching these together creates P-N junction at their interface, thereby creating an electric field.

When P-type and N-type semiconductors are sandwiched together, some of the extra outer electrons from the Phosphorous doped layer cross over and fills the holes in the Boron doped layer. This permanent displacement of electrons creates a fixed electric field, just under the front surface of the solar cell. It is this field that causes the electrons to jump from the semiconductor out towards the surface and makes them available for the electrical circuit. At this same time the hole moves in the opposite direction towards the positive surface, where the await incoming electrons. Hence this internal electrostatic field makes the solar cell work.

Converting sunlight to electricity:
At the atomic level light is composed of energy particles called photons ( Ephoton= hv; where h is Planck’s constant and v= frequency of its associated electromagnetic wave), that flows from the sun and strikes the solar cell. As each photon strikes a silicon atom, it ionizes the atom by transferring its energy to an outer electron, allowing it to break free of the atom. The energy of the photon is converted into electron movement energy called electric current.



Bandgap Matters:


Bandgap fundamentally limits the colour a solar cell can convert to electricity. Bandgap is basically the difference between the energy of the electrons in its filled valence band and the energy electrons would need to occupy its empty conduction band. Charge cannot flow either in a completely full or completely empty band, but doping a semiconductor provides “extra electron” or positively charged holes that can carry current.

Photons with just the right energy- the colour of light that matches the bandgap-creates electron hole pairs and allows current to flow across the junction between +vely and –vely doped layers. Photons with less energy than the bandgap (if solar spectrum are more near red or infrared), as they will not have enough energy to free the electrons; photons with too much energy (if solar spectrum are more blue or ultraviolet) are absorbed, but since each creates just one electron-hole pair, so the excess energy is wasted as heat.

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