Solar cell vs. Photodiode: Are they different?

We are all familiar with the terms Solar Cell and Photodiode, which are semiconductor devices. A semiconductor is a material with conductivity between a conductor and an insulator.

Semiconductor devices are important electronic devices because of their unique characteristics. Solar cells are photovoltaic cells which are basically semiconductor diodes. But they are not quite equal to a Photodiode.

We all agree that both the solar cell and photodiodes are diodes, but they are different from the functional view. Each of them efficiently fulfills two different purposes. This blog will find out what Solar cells and Photodiode are and how they differ. 

What are Solar Cells?

Solar cells are a ubiquitous device. From satellites to even in a calculator, you can find solar cells. They are basically PN junction semiconductor devices without any biasing. That means no voltage is directly applied across the junction.

A solar cell absorbs light and changes it into electrical energy. They don’t need any fuel or any chemical reaction to produce energy. The semiconductor materials used in solar cells can produce free electrons when exposed to light.

Did you know that these cells can utilize artificial light as well? PV cells can also have electric power from lamps or any light source or sunlight. 

The symbol (electronic) of a Solar Cell is:

What are photodiodes?

Photodiodes are PN junction diodes with the reverse biased condition. When the p side of the diode is connected to the battery’s positive terminal and the n-side to the negative terminal, the diode is said to be in reversed bias condition.

You can detect light quickly by photodiodes. For the diode’s fast functioning, you need low capacitance. So photodiodes have a small area of silicon.

Photodiodes absorb light, and the photons inside them get the energy to escape. This results in generating electric current. Photodiodes are a type of light sensor. 

The symbol (electronic) of a Photodiode is:

Key differences between Solar cell and Photodiode

Although both are semiconductor diodes, they are not invariably the same! Let’s find out what makes them different from one another. 

Material

Solar cells are most of the time made of semiconductors like silicon. Silicon has an extremely high internal resistance, producing a great deal of power for relatively low voltages and currents. But you’ll be surprised to know the large variety of materials used to make solar cells. 

III-V semiconductor solar cells are an example. Elements like Boron, Aluminium, Gallium, etc., have three (III) electrons in their outermost (valence) shell. Nitrogen, Arsenic, and Phosphorus have five (V) electronics in their valence shell. Together, they make a crystal structure used to make solar cells, and you can call them III-V Solar cells. 

We also have thin-film solar cells, dye-sensitized organic, and even amorphous Si solar cells with their own pros and cons. 

Photodiodes are profusely light sensors in reserved bias conditions. Any semiconductor material having a bandgap of 1.5 eV or lower is preferable for making photodiodes. The most used materials are silicon and gallium arsenide (GaAs). But you can use germanium, indium, lead sulfide, etc., for photodiode making. 

Structure

Electronic construction of a Solar cell: 

Generally, Solar cells are arranged in a particular manner called ‘arrays.’ Electrically connected solar cells are what we know as solar panels or modules. 

Inside a solar battery, two plates of doped silicon are electrically connected. N-type (dominant of negative charge carriers, i.e., electrons) silicon and P-type ( dominant of positive charge carriers, i.e., holes) silicon. These two plates are again externally connected to complete a circuit.

Electronic construction of a Photodiode:

Photodiodes have the same structure as the p-n junction diode. Just, in addition, the p-n junction is made of light-sensitive semiconductor materials. One primary requirement in the design of a Photodiode is that you have to make sure the maximum amount of light reaches the intrinsic layer of the device.

Working Principle

Solar Cells

We know that solar cells work as PN junction diodes. When two silicon plates with P-type and N-type doping are connected, the electrons rush to the holes. But due to barrier potential, all electrons can’t meet holes, and an equilibrium arrives.

When light falls on the surface, the energetic photons break the electron-hole pairs. Each photon freeing one electron creates one hole as well.

Now, suppose you connect the two sides of the silicon plates, forming an external path for the electrons to flow to the holes for recombination. In that case, they will flow back to the holes. The flow of electrons provides us with the electric current.

The cell’s electric field creates a potential difference which means voltage. Hence, the solar cells produce DC power. This is how a PV cell works.

The I-V characteristics of a solar cell give an overview of where the Maximum PowerPoint is. This means we can get the best out of our solar cells at which voltage and current values. The below curve shows it as the MPP point.

We can also see that when the solar cell is not connected to any loan, the voltage is maximum, corresponding to zero current, which is known as open-circuit voltage. Another extreme from the characteristics curve is the short-circuited current. When the current is maximum, the voltage across the cell is zero. 

Photodiodes

Photodiodes are p-n junction diodes with transparent packaging rather than having opaque packaging like ordinary diodes. This is how the p-n junction is exposed to the light. In complete darkness, a photodiode will behave just like an ordinary diode.

As a light ray falls on it, it gets the required energy that sets the electrons off to the conduction band from the valence band. This gives you the idea that photocurrent is proportional to the illumination intensity. The I-V characteristics of the diode show that in reverse biasing, this device is a good detector of light. 

From the characteristics, we can see as illumination increases gradually from zero, the operating point moves linearly with the increasing voltage (potential energy of light, i.e., photons). Also, the amount of current is higher for highly energetic light (high potential energy).

Not to mention the load capacity is much higher in solar cells than in photodiodes.

Application

Solar cells are used in our solar power system to absorb sunlight and make electricity. They are also used in spacecraft to produce huge amounts of power from solar energy. Even our International space station also has sets of solar panels. 

Photodiodes are light detectors or sensors best used for UV, infrared spectrum, and visible light spectrum.

Photodiodes can detect ‘UV radiation’ ‘ionizing radiation’ by photoconductivity. That can help us get ourselves out of any radioactive contamination. 

Conclusion

All semiconductor devices are important and are being used in our daily life. Among them, diodes are the most recognized. Even in your Solar Power System, diodes are used. 

Yes, the solar cell or the photovoltaic cells are typically semiconductor diodes that entrap light to produce an electric current.

On the other hand, photodiodes are sensitive to light and produce electricity in the presence of light. But Photodiodes are not Photovoltaic cells. 

In photodiodes, we primarily want a quick response to detect light. We don’t have anything to do with the amount of current so far. We are good if the current is significant enough to make the photodiode work as a sensor.

Well, you can imagine solar cells as a large area photodiode. But in solar cells, we want to absorb as much sunlight as possible. We want the absorption efficiency over response time.