How do Solar cells work?

Summary

This video script offers an insightful overview of how solar energy, derived from the sun's abundant energy, is harnessed through photovoltaic cells. It details the transformation of sand into pure silicon crystals, the creation of silicon wafers, and the functioning of the crucial PN junction that facilitates the flow of electrons. The script further explains the structure of a solar panel, the series and parallel connections of cells, and the differences between polycrystalline and monocrystalline panels. It also touches on the challenges of solar energy's global contribution, the use of storage solutions in homes, and how solar power plants integrate with the electrical grid.

Takeaways

• 🌞 Solar energy contribution to global energy supply has significantly increased over the last two decades.
• 💧 Sand, the second most abundant element on Earth, is crucial for producing solar cells through a purification process to create pure silicon crystals.
• 🔄 The conversion of raw silicon into gaseous silicon compounds and subsequent mixing with hydrogen results in highly purified polycrystalline silicon.
• 🔩 Silicon ingots are reshaped into thin slices called silicon wafers, which are essential for photovoltaic cells.
• 🏃‍♂️ Electrons in a silicon structure are initially unable to move freely, but when doped with phosphorus, some electrons become free to move.
• 🔋 The creation of a PN junction through doping processes is necessary to generate a driving force for the unidirectional flow of electrons.
• 🌐 When light strikes the PN junction, it generates electron-hole pairs in the depletion region, leading to a potential difference and resulting in a flow of current.
• 📊 Solar panels consist of interconnected PV cells, with series and parallel connections to increase current and voltage to usable levels.
• 🛠️ Polycrystalline and monocrystalline solar panels differ in their internal crystalline lattice structure, with the latter offering higher electrical conductivity but at a higher cost.
• 🔌 Home solar panels can store electricity with the help of batteries and solar charge controllers, while solar power plants typically connect to the electrical grid system due to storage limitations.

Q & A

How has solar energy contributed to the world's total energy supply in the last two decades?

In the last two decades, the contribution of solar energy to the world's total energy supply has grown significantly, becoming an important part of the global shift towards renewable energy sources.

What is the role of sand in the production of solar cells?

Sand, which is rich in silicon, is the second most abundant element on Earth and is crucial in the production of solar cells. It needs to be converted into 99.999% pure silicon crystals, which are then used in the creation of photovoltaic cells.

What process does raw silicon undergo to become suitable for use in solar cells?

Raw silicon undergoes a complex purification process, during which it is converted into a gaseous silicon compound. This compound is then mixed with hydrogen to produce highly purified, polycrystalline silicon.

What is the significance of the silicon wafer in a photovoltaic cell?

The silicon wafer is the heart of a photovoltaic cell. It is a very thin slice made from silicon ingots and plays a crucial role in the conversion of sunlight into electricity within the solar cell.

How does the structure of silicon atoms affect the movement of electrons?

In the structure of silicon atoms, the electrons are bonded together, which restricts their movement. This is similar to how people lose their freedom when bonded with someone else. To facilitate the movement of electrons, a 2D structure of silicon crystals is often considered for easier analysis.

What is the purpose of doping silicon with phosphorus in the creation of solar cells?

Doping silicon with phosphorus, which has five valence electrons, introduces free electrons into the silicon structure. This allows for one electron per phosphorus atom to move freely, which is essential for the functioning of the solar cell.

How does a PN junction create a driving force for electron flow in a solar cell?

A PN junction is created by joining n-type doped material (with extra electrons) and p-type doped material (with holes). When these materials join, electrons from the n-side migrate to fill the holes in the p-side, creating a depletion region. This results in an electric field that provides the necessary driving force for unidirectional electron flow.

What happens when light strikes the PN junction of a solar cell?

When light strikes the PN junction, it penetrates the N region and reaches the depletion region. The photon energy is sufficient to generate electron-hole pairs there. The electric field in the depletion region then drives these electrons and holes apart, leading to a potential difference and the flow of electrons through a connected load as direct current.

How are PV cells interconnected within a solar panel?

PV cells are interconnected through a series connection, where the negative side of one cell is connected to the back side of the next cell using copper strips. These series connections are then paralleled to form the solar panel, which increases the current and voltage values to a usable range.

What are the differences between polycrystalline and monocrystalline solar panels?

Polycrystalline solar panels consist of randomly oriented multi-crystals, while monocrystalline cells are made from a single crystal structure. Monocrystalline cells offer higher electrical conductivity, but they are more expensive and thus not as widely used as polycrystalline panels.

How do solar panels adapt to the electrical grid system?

Solar panels are connected to the electrical grid system with the help of power inverters that convert the direct current (DC) produced by the solar cells into alternating current (AC), which is compatible with the grid. This allows the solar power plant to feed electricity into the grid just like other conventional power plants.

Outlines

00:00

🌞 Understanding Solar Cell Technology

This paragraph delves into the significant growth of solar energy's contribution to the global energy supply over the last two decades. It explains the process of electricity production by solar or photovoltaic cells, emphasizing the importance of sand's conversion to ultra-pure silicon crystals. The script outlines the purification process of sand to silicon, the creation of silicon wafers, and the fundamental structure of silicon atoms. It introduces the concept of doping with phosphorus for n-type material and boron for p-type material, leading to the formation of a PN junction that provides the driving force for electron movement. The paragraph further describes how light striking the PN junction generates electron-hole pairs, creating a flow of direct current. It also discusses the structure of a solar panel, the series and parallel connections of PV cells, and the advantages of mono crystalline cells over polycrystalline ones, despite the former's higher cost and less widespread use. The limitations of solar voltaic's global energy contribution due to capital costs and efficiency constraints are mentioned, as well as the potential for home solar panels to store electricity with the help of batteries and solar charge controllers.

05:02

🔌 The Functionality and Limitations of Solar Panels

This paragraph focuses on the functionality of solar panels, starting with the fact that a single PV cell produces a low voltage, which is made usable through series and parallel connections. It highlights the protective role of the EVA sheeting in solar panels against environmental factors. The differences in appearance between polycrystalline and monocrystalline solar panels are attributed to their internal crystalline lattice structures. The paragraph points out that despite the higher electrical conductivity of monocrystalline cells, their costliness limits their widespread adoption. It addresses the global energy contribution of solar voltaic, which remains relatively low due to initial costs and efficiency limitations. The paragraph also discusses the practicality of storing electricity from home solar panels versus the challenges faced by solar power plants. It concludes with the mention of power inverters that convert DC to AC for grid integration and a call to support Learn Engineering's educational activities and subscribe to their channel.

Keywords

💡Photovoltaic cell

A photovoltaic cell, commonly known as a solar cell, is a device that converts light into electricity using the photovoltaic effect. This is central to the video's theme as it explains the process from converting sand into silicon wafers to creating these cells. The video illustrates this by detailing the conversion of silicon into a usable form for photovoltaic cells, highlighting their fundamental role in solar energy production.

💡Silicon

Silicon is the second most abundant element on earth and is crucial for producing photovoltaic cells. The video describes the complex process required to purify sand into 99.999% pure silicon crystals, which are then used in solar cells. This highlights silicon's essential role in harnessing solar energy, emphasizing its abundance and utility in solar technology.

💡PN junction

A PN junction is formed at the boundary between two types of semiconductor material, p-type and n-type, which are doped differently to create an electric field. In the context of the video, the PN junction is described as the key to directing electron flow and generating electricity in solar cells. The formation of this junction and its function in creating a driving force for electron movement are crucial for the operation of photovoltaic cells.

💡Depletion region

The depletion region is an area within a PN junction where no free charge carriers (electrons or holes) exist. This region's formation and role are critical as discussed in the video, which explains how the electron-hole pairs generated by photon energy are driven out by the electric field in this region, facilitating electricity generation in solar cells.

💡Electron-hole pairs

Electron-hole pairs are created when photons from sunlight provide sufficient energy to break the bonds of electrons in the silicon atoms within a solar cell. The video uses this concept to explain how light energy is converted into electrical energy, with the electron-hole pairs being crucial for creating a current when they are separated by the electric field in the depletion region.

💡Doping

Doping refers to the process of adding impurities to semiconductor materials to change their electrical properties. The video explains n-type and p-type doping, which introduce free electrons and holes, respectively. This is essential for forming the PN junction that directs electron flow in solar cells, highlighting doping's role in enhancing the conductivity and functionality of photovoltaic cells.

💡Direct current (DC)

Direct current (DC) is a type of electrical flow where electrons move in one direction around a circuit. The video discusses how a solar cell generates DC, which can then be converted to alternating current (AC) using an inverter for use in the power grid. This conversion is crucial for integrating solar energy into the existing infrastructure.

💡Monocrystalline vs. Polycrystalline

These terms refer to the types of silicon used in solar panels. Monocrystalline silicon is made from a single crystal structure, offering higher efficiency and conductivity but at a higher cost. Polycrystalline silicon consists of multiple crystals and is cheaper but less efficient. The video compares these to explain the different performances and costs associated with solar panel technologies.

💡Energy conversion efficiency

Energy conversion efficiency in solar panels measures how effectively they convert sunlight into electricity. The video highlights efficiency constraints as a major challenge for solar technology, comparing it unfavorably with conventional energy sources. This context helps explain ongoing research and development efforts to improve solar technology.

💡Electrical grid

The electrical grid is the network that delivers electricity from producers to consumers. The video mentions that unlike smaller solar setups that can store energy in batteries, solar power plants often feed directly into the grid, similar to conventional power plants. Understanding this integration is key to realizing the role of solar energy in larger energy systems.

Highlights

Solar energy contribution to the world's total energy supply has grown significantly in the last two decades.

Solar cell or photovoltaic cell is showcased to demonstrate how electricity is produced from sunlight.

Sun's energy is the most abundant and freely available energy source on Earth.

Sand, the second most abundant element on Earth, is crucial for solar cell production through a complex purification process.

Silicon crystals are produced from sand to be used in solar cells, requiring 99.999% purity.

Gaseous silicon compounds are created and mixed with hydrogen to achieve highly purified polycrystalline silicon.

Silicon ingots are reshaped into thin slices known as silicon wafers, which are essential for photovoltaic cells.

Electrons in silicon atoms are bonded and lack freedom of movement, similar to how bonding restricts freedom.

2D structure of silicon crystals is considered with phosphorus atoms to illustrate free movement of one electron.

A simplified solar cell is made using the described material, where light enables electron movement.

Random electron movement does not produce current; a PN junction is needed for unidirectional electron flow.

PN junction is created by combining n-type doped material with p-type doped material, forming a depletion region.

An electric field between the charges of the depletion region creates the necessary driving force for electron movement.

Light striking the PN junction generates electron-hole pairs, which are driven by the electric field to produce current.

Practical solar cells have a thin, heavily doped N layer and a thick, lightly doped P layer for increased performance.

The structure of a solar panel is composed of interconnected PV cells, series and parallel connections, and protective layers.

Polycrystalline and monocrystalline solar panels differ in their internal crystalline lattice structure and electrical conductivity.

Solar voltaic's global energy contribution is only 1.3 percent due to capital costs and efficiency constraints.

Solar panels on homes can store electricity with the help of batteries and solar charge controllers, unlike solar power plants.

Solar power plants are connected to the electrical grid system and use power inverters to convert DC to AC for grid feeding.

Transcripts

In the last two decades the contribution of solar energy to the world's total energy supply has grown significantly.

This video will show how solar cell or photovoltaic cell produce electricity.

Energy from the Sun is the most abundant and absolutely freely available energy on planet earth.

In order to utilize this energy we need help from the second most abundant element on earth, sand.

The sand has to be converted to 99.999% pure silicon crystals to use in solar cells.

To achieve this, the sand has to go through a complex purification process as shown.

The raw silicon gets converted into a gaseous silicon compound form.

This is then mixed with hydrogen to get highly purified

polycrystalline silicon.

These silicon ingots are reshaped,

and converted into very thin slices called silicon wafers. The silicon wafer is the heart of a photovoltaic cell.

When we analyze the structure of the silicon atoms you can see they are bonded together.

When you are bonded with someone you lose your freedom.

Similarly the electrons in the silicon structure also have no freedom of movement.

To make the study easier let's consider a 2d structure of the silicon crystals.

Assume that phosphorus atoms with five valence electrons are injected into it.

Here one electron is free to move.

In this structure when the electrons get sufficient energy they will move freely.

Let's try to make a highly simplified solar cell only using this type of material.

When light strikes them, the electrons will gain photon energy and will be free to move.

However this movement of the electrons is random. It does not result in any current through the load.

To make the electron flow unidirectional a driving force is needed.

An easy and practical way to produce the driving force is a PN junction.

Let's see how a PN Junction produces the driving force.

Similar to n-type doping if you inject boron with three valence electrons into pure silicon there will be one hole for each atom.

This is called p-type doping.

If these two kinds of doped materials join together,

some electrons from the N side will migrate to the P region and fill the holes available there.

This way a depletion region is formed where there are no free electrons and holes.

Due to the electron migration the N-side boundary becomes slightly positively charged.

And the P side becomes negatively charged.

An electric field will definitely be formed between these charges.

This electric field produces the necessary driving force.

Let's see it in detail.

When the light strikes the PN Junction something very interesting happens.

Light strikes the N region of the PV cell and it penetrates and reaches up to the depletion region.

This photon energy is sufficient to generate electron hole pairs in the depletion region.

The electric field in the depletion region drives the electrons and holes out of the depletion region.

Here we observe that the concentration of electrons in the N region and holes in the P region

become so high that a potential difference will develop between them.

As soon as we connect any load between these regions, electrons will start flowing through the load.

The electrons will recombine with the holes in the P region after completing their path.

In this way a solar cell continuously gives direct current.

In a practical solar cell you can see that the top N layer is very thin and heavily doped.

Whereas the P layer is thick and lightly doped.

This is to increase the performance of the cell.

Just observe the depletion region formation here.

You should note that the thickness of the depletion region is much higher here compared to the previous case.

This means that due to the light striking the electron hole pairs are generated in a wider area compared to the previous case.

This results in more current generation by the PV cell.

The other advantage is that due to the thin top layer, more light energy can reach the depletion region.

Now let's analyze the structure of a solar panel. You can see the solar panel has different layers.

One of them is a layer of cells. You will be amazed to see how these PV cells are interconnected.

After passing through the fingers the electrons get collected in busbars.

The top negative side of this cell is connected to the back side of the next cell through copper strips.

Here, it forms a series connection.

When you connect these series connected cells parallel to another cell series you get the solar panel.

A single PV cell produces only around 0.5 voltage. The combination of series and parallel connection of the cells

increases the current and voltage values to a usable range.

The layer of EVA sheeting on both sides of the cells is to protect them from shocks, vibrations, humidity and dirt.

Why are there two different kinds of appearances for the solar panels?

This is because of the difference in the internal crystalline lattice structure.

In polycrystalline solar panels multi crystals are randomly oriented.

If the chemical process of silicon crystals is taken one step further, the polycrystalline cells will become mono crystalline cells.

Even though the principles of operation of both are the same, mono crystalline cells offer higher electrical conductivity.

However mono crystalline cells are costlier and thus not widely used.

Even though running costs of PV cells are negligible.

The total global energy contribution of solar voltaic is only 1.3 percent.

This is mainly because of the capital costs and the efficiency constraints of solar voltaic panels,

which do not match conventional energy options.

Solar panels on the roofs of homes have the option to store electricity with the help of batteries and solar charge controllers.

However in the case of a solar power plant the massive amount of storage required is not possible.

So generally they are connected to the electrical grid system in the same way that other conventional power plant outputs are connected.

With the help of power inverters, DC is converted to AC and fed to the grid.

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