How silicon solar cells generate electricity?

25/6/2015

The sun radiates the electromagnetic energy and it reaches the surface of the earth in around 8.33 minutes, covering almost 150 million kilometres with a speed of 3,00,000 km/sec. The silicon solar cells absorb this electromagnetic energy and convert it into electrical energy. The most of the solar energy is present in the visible spectrum of the light.

Solar spectrum

The solar spectrum looks like as shown below:

But If you look closer you will find the dark lines in the spectrum, just like as shown below:

These dark lines are formed by the atoms in the earth’s atmosphere by absorbing energy corresponding to the precise wavelength of light. These patterns of lines are unique and are called solar absorption spectra. The electronic structure of any atom depends on how it absorbs and releases specific amount of energy. The spectral lines are the effective tool to learn the electronic structure of any atom.

Energy equation

In order to jump to the particular orbit a specific amount of energy is required. Therefore we can say that photon’s energy should match with the energy required by the electron to jump to the specific orbit.

Energy = hc/ wavelength

Where,

h = Plank's constant
c = Velocity of light

As earth revolves around the sun due to the gravitational force; an electron revolves around the heavy nucleus (filled with positive protons and neutrons) in the centre and its attractive electric force holds the electron in the orbit. The electrons require energy to jump from lower orbit to the higher orbit. The electrons absorb the photons of light and jump to the higher energy levels or orbits. Similarly, when electron falls back to the lower energy level then it must release the same amount of energy.
See how the photons of light with different energy can excite the electron to jump to the higher energy levels. Each color is associated with different energy levels.

Understanding the electronic structure of Silicon

Silicon is a semi-conductor, it means that it shares some of the properties of metal and some of the insulator. The four electrons in the outer most shell of the silicon are called valence electrons. They can interact with the electrons of the other atoms to make chemical bonds.

Atomic structure of Silicon

The atomic structure of silicon is shown above, which is having atomic number 14. The electrons shown in blue colors are revolving around nucleus (red). (The first orbit can hold up to 2 electrons, second and third orbits/shells can accommodate up to 8 electrons). The first two orbits have 2 and 8 electrons respectively are completely filled and neutralized while the third orbit has 4 electrons in its valence shell. The electrons in the valence shell has tendency to neutralize themselves and they find ways to do the same. They try to go into stable state through bonding called the covalent bond.
Now, when the photons of light falls on the surface of the silicon, the valence electrons absorb the photons and jump into higher energy conduction band. In conduction band the electrons are free to move and it is an unstable state.
Now, as electrons leave the valence band it creates the hole or it makes that region positive. Those electrons which are promoted to the conduction band have tendency to fall back to the valence band by releasing the absorbed energy and recombine with the hole to neutralize. If we want to run our electrical appliances then we must pass these electrons through the electrical circuit before their recombination with the holes.
Or we can say that we need some force or electric field which can pull electrons present in the conduction band to pass through our electrical appliances.

Doping

Now the question is how to create this force or electric field?
The answer to this question is Doping of silicon with another impurity with similar electronic structure. Doping is a process where replacing small number of silicon atoms with other atoms like Boron or phosphorous.
When we bring to atoms (Silicon with Boron or Phosphorous) close to each other, the valence electrons of each atom will be attracted by the positive nucleus of the other atom. In this way the pair of electrons is shared forming the covalent bond structure
One is called P type which is made by adding impurities like Boron and other is called N type, which is formed by adding impurities like Phosphorous.
The atomic number of Boron is 5 and its electronic structure is shown below:

The P-Type Semiconductor

The outermost shell of Boron has 3 electrons, which is one less than Silicon. When Boron is added as an impurity with Silicon, then it forms 3 complete covalent bonds with the surrounding silicon atoms and leaves one incomplete bond. Now this incomplete bond pulls one electron from the valence electrons of other surrounding silicon atom. This process leaves the hole or net positive charge in the silicon atom hence makes it a P type semiconductor.

Atomic structure of Boron

P-type semi-conductor

The formation of N-Type semiconductor

The atomic number of Phosphorous is 15 and the outermost shell of Phosphorous has 5 electrons in its valence shell, which is one more than the Silicon. When we add Phophorous as an impurity in the silicon, it forms four complete covalent bonds with the neighboring silicon atoms. The extra electrons left in the valence band of the Phosphorous orbits around it. Now, as Phosphorous donates one extra electron to the silicon crystal hence it is called N- type semiconductor.

Atomic structure of Phosphorous

N type semi-conductor

The creation of electric field through PN junction

The creation of electric field through PN junction: Now, when the P type semi conductor and N type semiconductor are attached then a junction is formed which is called PN junction. Here electrons and holes are diffused across the PN junction and recombine. The overall result of this process is the formation of net negative charge on the P side and net positive charge on the n side, forming a electric field across the junction.

Now, when the photon of light falls on the PN junction, the electrons in valence band will get excited and promoted to conduction band, leaving holes behind. Now, before the recombination of the electrons with the holes, they are pulled apart and get separated by the electric field created by the PN junction. The electrons with negative charge move towards the positive field whereas the holes with the positive charge are attracted by the negative field. This electric field has prevented the recombination of the electrons with hole in the valence band. This results in the generation of electric current which can pass through our appliances and run them.
On the silicon solar panel, the surface of silicon is covered and doped with thin wafer of Boron in order to make it P type semiconductor. After that the atoms of Phosphorous are diffused on the surface creating a PN junction. An anti-reflective layer is spread to prevent the reflection losses and increase the amount of absorption. Now, when you connect the electrical load across the panel under sunlight, the electrons reaches the conduction band after absorbing the photons of light leaving positive holes in the valence band. When the electrons are prompted to fall back and recombine with the holes, the electric field in the PN junction separates them and made them to travel through the electrical load connected across the panel and then return back to recombine with the holes.

Final note

The energy of photon is utilized in exciting electrons which is further used to power the electrical devices. In this whole process no extra energy is created but it is only converted into one form to another form. Hence, validating the law of conservation of energy that energy can neither be created nor be destroyed but it can only be transformed to one form to another.
So, we come to conclusion that silicon doped with impurities forms the PN junction which further creates a electric field and which in turn helps electrons to flow through the electrical appliances and run them.