The Sun’s total output power is unbelievably big. We refer to it with the symbol Ps and scientists say the Solar Power is Ps = 385 000 000 000 000 000 000 000 000 W. There is a power loss by the time the Sun’s rays reach the Earth. But still, we measure 170 000 TW here. And this is 10 000 x more than the Earth’s population uses!

Photovoltaics is the principle of converting light into electrical energy.

Spectrum of Solar Power

Like sound waves, light also comes in different frequencies. Some of these frequencies we interpret as colours. So a red colour has a different frequency than blue.

In fact, let’s picture it this way: frequencies are essentially pulses. There is a range of low frequencies, from 20Hz to 20,000 Hz that we can hear. There is also a range of frequencies that we can see: 400 – 790 THz. This is called visible light.

The Sun’s power is 10% Ultraviolet, 40% Visible and 50% Infrared.

Formula: Wavelength is speed over frequency

Formula: Wavelength, speed, frequency

Frequency means how many cycles happen in 1 second. In the case of sound: how many times does the speaker cone move out and back in in 1 second.

Wavelength relates to frequency by the above formula. Where:

  • v: speed of light
  • f: frequency
Spectrum of Solar Power

Figure 1: Spectrum of Solar Radiation on Earth. Source: commons.wikimedia.org

Looking at the spectrum above, we see that the visible light to the human eye is where the Irradiance is the largest. The Irradiance is much smaller in the UV and Infrared regions. This is because the water molecules in the air absorb these frequencies.

Humans evolved in water. This is why we see only the visible spectrum as the natural light is the strongest there. Birds evolved in the air. They see the UV and Infrared too!

Solar Irradiance

Solar Irradiance is a way of defining the incident solar power per square metre. The power that reaches the earth of course depends on the angle it hits the surface as well as atmospheric losses. But let’s ignore this for now.

The formula for the Solar Irradiance is:

Where:

  • Ps: the output power of the Sun.
    • It is a constant. Ps = 3.85 x 10^26 W
  • re: the distance between the Sun and the Earth

If we put the numbers in we get: I = 1.36 kW/m^2. However this value does not count with the above mentioned losses.

For simplicity, let’s ignore all the nitty-gritty and say with all losses included the average solar irradiance is:

I = 1 kW/m^2

Solar Power

Solar Panels convert solar energy to electrical energy. A large block of solar panel is made of smaller solar cells. These solar cells are made of Silicon.

Solar Power Generated per Area

The power is the product of the irradiance and the solar panel’s area. Of course solar panel’s can’t convert all solar energy that hits the panel to electricity. This is where the efficiency factor comes in the equation:

Formula: Solar Power is the product of the Irradiance, Area and Efficiency

Where:

  • η: efficiency
  • I: solar irradiance
  • A: area of solar panel

Efficiency

For a single junction solar panel, the maximum theoretical efficiency is 29%. For multi junction cells it is higher but they are also very expensive. We will cover what junction means below. The efficiency is limited by the following factors:

  • Thermodynamics
    • 86% due to the Carnot Limit which is related to heat energy.
  • Quantum Efficiency
    • Losses due to the ability of the panel’s absorption of photons and converting them to electricity.
  • Power Transfer Efficiency
    • Ohmic Losses due to the internal components’ resistance of the sola cell.

How Do Solar Cells Work ?

As we mentioned above, solar panels are made of smaller parts: solar cells. These solar cells consists of PN Junctions that is the key behind photovoltaics. PN Junctions are made of semiconductors: Silicon.

Silicon – what solar cells are made of

  • The second most abundant element on Earth after Oxygen.
  • It is a semiconductor
    • Semiconductors are nor good conductors, nor good insulators. Somewhere in between.
    • Their atoms are grouped together in a crystalline pattern called crystal lattice.
    • They have very few free electrons.

Essentially, semiconductors under special conditions can act as insulators or conductors.

Structure of the Silicon Atom

The Silicon has a Nucleus in the centre and electrons ‘flying’ around it. There are 14 positively charged protons and 14 neutrally charged neutrons in the Nucleus.

The 14 electrons are distributed in 3 different levels (layers) around the Nucleus. For us this outer layer (Valence Band) is the most important as the electrons situated there will form bonds with other atoms.

Atomic Structure of the Silicon Atom featuring 4 electrons at the outer shell

Figure 2: Atomic Structure of Silicon Atom

The Silicon Crystal Lattice

When multiple Silicon atoms are connected to each other they form a crystal lattice. Each of the 4 outer electrons of a Silicon atom will connect to another silicon atom’s outer electron.

Crystal Structure of Silicon

Figure 3: Crystal Structure of Silicon

N Type & P Type Silicon

The idea is simple. We have the Silicon lattice which is a semiconductor. We take out a Silicon atom and we put a different atom in place. This other type of atom can have either more or less electrons in the valence band. This valence band or in other words ‘outer electron shel’l is the most important to us remember?

Let’s get cleared up on some terms:

  • Impurity – the extra atom in the lattice
  • Donor – atom with 1 extra electron
    • usually Phosphorus (P)
    • has 5 electrones
  • Acceptor – atom with less electron than Si
    • usually Boron (B)
    • has 3 electrons

The way it works, is when we have an N-type semiconductor, there is an extra electron in the lattice that moves around freely.

In the case of a P-Type semiconductor, there is a hole. Picture holes as a placeholder for electron. Holes are not physical things, just places that electrons can snap to.

The image below shows an N-type and a P-type Silicon lattice respectively. We introduced a Phosphorus as the impurity in the first lattice and a Boron in the second.

N-Type Silicon Crystal Structure

Figure 4: N-Type Silicon – the crystal lattice has a Donor with 5 electrons, resulting the lattice having an extra, free electron.

P-Type Silicon Crystal Structure

Figure 5: P-Type Silicon – the crystal lattice has an Acceptor with only 3 electrons, resulting the lattice having a hole.

The PN Junction

To understand the principle behind solar panels, first, we need to cover what a PN junction is.

To make it simple to picture, a conventional LED light is made of a PN junction. We are doing the opposite at solar power: we are not emitting light using electricity, instead we are generating electricity from light.

PN Junction Before Recombination

PN Junction before recombination

Figure 6: PN Junction before recombination

A PN Junction is a P and an N-Type semiconductor (silicon) paired. The P side has impurities with less electron, thus there are holes. The N side has impurities with extra electrons, thus there are free electrons.

PN Junction After Recombination

PN Junction after recombination

Figure 7: PN Junction after recombination

When we connect a P-Type and an N-Type semiconductor, the first thing that is going to happen, is that the free electrons from the N side jump over to the positively charged P side to fill the holes.

Of course not all free electrons will jump, only the electrons from the edge. Why? Because P is not positively charged and does not attract the electrons. They wonder around and find holes. So, recombination will happen until a sort of equilibrium is reached. Where the recombination happens is called the Space Charge Region or in other name: Depletion Region.

Note, originally both the N-side and the P-side is electrically neutral. So, when electrons fill the holes, there going to be more electrons than protons. Now this will make the left side of the depletion region negatively charged. Similarly, we will have the right side positively charged.

This causes a potential difference in the Space Charge Region.

PN Junction Illuminated

PN Junction excited by sun light

Figure 8: PN Junction excited by sun light. Electrons are knocked off and pushed by the potential force towards N.

When the Sun shines, it has some energy. This energy is strong enough to knock some of these freshly re-combined electrons off again. Note, we still have the potential difference in the Depletion Region.

These electrons are negatively charged, thus, will be snapped toward the positively charged side. Connecting a bulb across our PN Junction will produce a current making the light bulb light up!

Solar Cell Cross-section

Cross-Section of a Solar Cell

Figure 9: Cross-section of a solar cell

A good video about PN Junctions: The PN Junction. How Diodes Work?
NEXT TOPIC: Renewables: Hydropower