Solar Power ~ Infinite ∞ Energy!

The Science behind Solar Power

What happens when light strikes on a solar cell? What is the process that causes the change from light into electricity?

The answer is photovoltaic effect.

A simple explanation on Photovoltaic Effect:

1. Sunlight hits on a solar cell.

2. Electrons in semiconductors are excited and start to move.

3. When electrons moves in a direction, electrics are generated.

Take a look inside the Solar Cell:

At a closer look;

Energy Contained in Sunlight

Sunlight, or any light, travels in forms of packets, which are photons. Each photon contains different amount of energy. We can only see some of the "light" that is emitted by Sun. For example, in the visible light, red had the lowest energy while violet has the highest energy. In terms of invisible "light", ultraviolet has higher energy while infrared has lower energy compared to visible lights.

When sunlight strikes on a solar cell, it is actually these photons that are striking on the solar cell. Some of the energy is absorbed and even more is lost. So where does these energies go?

Semiconductors

The energy absorbed goes to the semiconductors in the solar cell. Solar cells are mainly made of semiconductors. Why semiconductors?

P-Type and N-Type Semiconductor

Silicon is the most common semiconductor. However, a normal semiconductor is not enough. A process which is called "doping" is needed to create P-type semiconductor and N-type semiconductor.

Doping is a process to insert elements that has three valence electrons, for example Boron as shown below, into silicon and form P-Type semiconductor, or insert elements that has five valence electrons into silicon and form N-Type semiconductor.

From left to right: Basic Structure of N-Type Semiconductor and P-type Semiconductor

N-Type Semiconductor (Antimony - Sb)	P-Type Semiconductor (Boron - B)
1. Antimony has 5 valence electrons 2. Antimony act as donor atoms 3. Silicon has 4 valence electrons 4. An additional freely moving electron is "created"	1. Boron has 3 valence electrons 2. Boron act as acceptor atoms 3. Silicon has 4 valence electrons 4. An additional freely moving "hole" is "created"

With additional freely moving electron and additional electron "hole", electron can jump from atom to atom much more easily, resulting in higher electric conductivity.

Combining N-Type Semiconductor and P-Type Semiconductor

A "p-n junction" is formed when p-type semiconductor and n-type semiconductor is combined. This is due to the "holes" are positively charged while electrons are negatively charged. "Holes" and electrons start travelling to p-type and n-type semiconductors respectively.

When a substantial amount of "holes" and electrons travel to another semiconductor, they leave the opposite charges at their "home semiconductor". Positive charge will form at n-type semiconductor and negative charge will form at p-type semiconductor. When there is difference in charge, there is an electric field moving from positive to negative.

Since electron is n-type semiconductor is negatively charged, they will be repelled from the p-n junction. The same scenario works for the p-type semiconductor. Sometimes there will have some electrons or "holes" diffuse back into "home semiconductor" but eventually equilibrium is reached and the net flow of electron or holes in and out from p-n junction is the same.

This raises a question: "If net flow of electron is zero, which means net flow of current is zero, how do we generate electricity?"

That is why sunlight is needed to come into play!

Back to when Sunlight strikes the Solar Cells

When photons strikes on the solar cell with enough energy, some of the bond in the semiconductor can be broken. This happens in both p-type and n-type semiconductor. When a bond is broken, electron is released and at the same time, "holes" are created. There will be a lot of bond broken so there will be a lot of free electrons and "holes". Some of these electrons will recombine with their holes, which releases heat energy.

Other electrons and holes will generate electricity. As mentioned above, there is an electric field in the p-n junction, consisting the positive charges and negative charges. "Holes" will move towards negative charges while electrons will move towards positive charge. In short, "holes" and electrons are separated in different direction.

A solar cell does not only consist of semiconductors, we need to connect it with a circuit. The circuit will also be connected to the electrical appliances, for example light bulbs. Since the "holes" and electrons are separated due to electric field they will travel around in the circuit and go back to the solar cell.

Thus, electricity is generated!

To have an overall view on how does electricity is generated; it is advisable to watch the video below: