Silicon Solar Cells | ChemTalk

Core Concepts

This article provides an overview of the basics of a silicon-based solar cell, discussing what a silicon solar cell is, its components, how they function, and their underlying chemistry. This article also examines the advantages and disadvantages of silicon solar cells, as well as their practical applications. Lastly, the article touches upon one of the advancements made in the field of silicon-based solar cells.

Before looking into how it works, it is crucial to understand what silicon solar cells are and why silicon is the ideal material for photovoltaic applications.

What are Silicon Solar Cells?

The most prevalent type of photovoltaic cells is the silicon solar cells. These cells utilize purified silicon crystals to convert sunlight directly into direct current (DC) electricity. Light that strikes the cell may be absorbed, reflected, or simply pass right through the cell.

The photovoltaic cell is composed of a semiconductor material. A semiconductor is a material that can conduct electricity better than an insulator (like glass) but not as well as a conductor (like copper). The unique aspect of semiconductors is that their electrical behavior can be precisely controlled by factors such as composition, impurities (known as doping), and exposure to light or heat. Semiconductors are used in a solar cell because they can absorb light energy and convert it into moving electrical charges, such as electrons and “holes.” These charges can then be directed to produce an electric current. Researchers are examining various materials for use in solar cells, but silicon is the most prevalent.

A fundamental concept that determines how a semiconductor interacts with light is the band gap. In simple terms, the band gap is the minimum amount of energy required to excite (or move) an electron from a lower-energy, non-conducting state (the valence band) into a higher-energy, conducting state (the conduction band). When sunlight strikes a solar cell, photons possessing energy equivalent to or greater than the band gap can move electrons across this gap, creating charge carriers that generate electricity. The size of the band gap determines the wavelengths of light that can be absorbed and the efficiency with which the solar cell converts sunlight into usable electrical energy. Selecting or engineering materials with suitable band gaps is one of the primary challenges in designing solar cells.

Silicon is particularly well-suited for the commercial manufacturing of solar cells due to its atomic structure and bonding behavior, making it a powerful semiconductor. Since silicon has a tetravalent structure (four valence electrons in its outermost shell), it is considered to have a half-filled valence shell, which places it in a special category. Silicon can form stable covalent bonds with four neighboring silicon atoms. This way, they have eight electrons in their outermost shell, making them a stable, ordered diamond cubic lattice. This creates a narrow energy gap, allowing for controlled conductivity when the material is doped or energized.

The electrical properties of silicon can be easily tuned through doping.  Silicon can be engineered to have excess electrons (n-type) or electron “holes” (p-type) by introducing small amounts of other elements. This enables charge separation and electron flow when excited by light, making silicon ideal for forming the internal electric fields used in solar cells to collect charge carriers efficiently. Readers interested in a deeper explanation of doping, holes, and p–n junctions can explore ChemTalk’s dedicated semiconductor articles.

Silicon is also the second most abundant element in the Earth’s crust, ensuring a sustainable and cost-effective supply of this material. Silicon remains the most critical material in the solar industry, as it has been proven to be efficient, long-lasting, and easily scalable. This makes it the most reliable and commonly used material for photovoltaic cells. The efficiency of the cells is assessed by comparing the amount of electrical power they generate to the amount of energy they receive from sunlight. This helps determine how well the cell can transform energy from one form into another.

Now that we know the basics about how silicon solar cells function, let’s look at the parts that make them work.

Components of a Silicon Solar Cell

A silicon solar cell consists of an anti-reflective coating to maximize sunlight absorption and to prevent sunlight from reflecting off the surface of the cell, as you can see in Figure 1. This increases the efficiency of the solar cell. The front contact is a metal grid, a thin layer of metal on the top surface that collects electrons and allows current to flow out to an external circuit. The emitter, which is the n-type layer, consists of a thin layer of doped silicon that contains extra electrons. When the light hits the cell, the electrons are excited here and move toward the front contact. The base is a p-type layer consisting of a thicker silicon layer doped to create “holes”. These holes move toward the back contact when charge separation occurs. The internal electric field that divides the electrons and the holes forms at the p-n junction, which is a boundary between the p-type base and the n-type emitter. The rear contact is a metal contact at the bottom of the cell that collects holes and completes the circuit. Any device powered by a solar cell requires an external load through which electrons flow, generating electricity.  While these components define the structure of a silicon solar cell, understanding how they interact requires examining the physical principles that drive electron movement and current generation.

The Science Behind Silicon Solar Cells

A silicon cell produces electricity using the photovoltaic effect. This occurs when a semiconductor material absorbs photons from sunlight. When the photons are absorbed, they impart energy to the electrons in the silicon, causing them to move away from their original position. This movement of electrons creates positively charged areas, known as “holes”.

The energized electrons now begin to move, creating an electric current. At the p-n junction of the cell, a built-in electric field prevents electrons from immediately recombining with holes. The p-n junction is the interface between 2 types of semiconductor materials – the p-type, which has many holes and is positive, and the n-type material, which has many electrons and is negative, as shown in Figure 2.

The electric field causes electrons and holes to move in opposite directions, resulting in a usable voltage. As the electrons flow in one direction, they result in direct current. Although all silicon solar cells rely on this same fundamental mechanism, variations in material structure and design lead to differences in performance and efficiency, as discussed in the next section.

Types of Silicon Solar Cells

Polycrystalline and monocrystalline solar cells are the two main types of silicon cells. In the past, polysilicon solar panels were primarily manufactured using polycrystalline cells, with their efficiency ranging from 16% to 17%. As fabrication techniques improved, monocrystalline solar cells were developed, achieving efficiencies of approximately 19%-20%. More recently, mono-PERC half-cut solar cells, with efficiencies as high as 22.5%, have been introduced and are now widely used in modern monocrystalline silicon panels. The different types of silicon cells are explained in detail below (Figure 3):

1. Monocrystalline solar cells are made from a single, continuous silicon crystal and have a uniform black appearance. Because the crystal lattice is uninterrupted, excited electrons can move more freely through the material with fewer defects. This efficient charge transport allows more of the absorbed sunlight to be converted into electrical energy, resulting in higher efficiency.
2. Polycrystalline solar cells are produced from multiple silicon crystals fused and have a speckled blue appearance. The boundaries between these crystals act as sites where excited electrons are more likely to recombine before reaching the electrodes. This increased recombination reduces charge collection efficiency, thereby accounting for the lower efficiency of polycrystalline cells compared to monocrystalline cells. Although they are cheaper to manufacture, their decreased performance has led to a significant decline in market demand.
3. Mono-PERC half-cut silicon solar cells are a more advanced type of monocrystalline cells. They have a rear passivation layer (a thin coating at the back surface of a silicon solar cell) and a half-cut cell design. The passivation layer reduces energy loss at the rear surface and reflects unabsorbed light back into the silicon, allowing for additional absorption and improved efficiency. Meanwhile, the half-cut design minimizes the distance electrons must travel, which lowers energy losses and helps the solar panel perform better when part of it is shaded. Together, these features raise efficiencies to approximately 22.5%.
4. Amorphous silicon solar cells are another type of silicon-based cell used in thin-film solar panels. Amorphous silicon lacks a long-range crystal structure, unlike crystalline silicon, which introduces numerous defects. Although this structure allows for efficient light absorption in exceedingly thin layers, charge carriers exhibit reduced mobility and shorter lifetimes. As a result, amorphous silicon cells have lower efficiencies (around 11-12%) and shorter lifespans of 5-10 years, thereby rendering them impractical for the majority of rooftop solar installations, despite their lightweight and flexible nature.

These design differences have a direct impact on where and how silicon solar cells are utilized in real-world energy systems.

Real World Application

Silicon can be incorporated into thin films, wafers, and even newly developed hybrid structures because of its adaptability. As a result, silicon solar cells have numerous applications in the real world.  The most important applications of silicon solar cells are listed below:

• Homes and commercial buildings: To partially satisfy their electrical needs with solar energy, solar panels are installed on homes and commercial buildings.
• Solar farms: To generate electricity and supply the grid, these massive constructions combine vast arrays of solar panels.
• Mobility & transportation: Although this use is still in its infancy, several cars and trains use solar cells to charge their batteries.
• Urban infrastructure: Small photovoltaic panels are used in urban infrastructures like street lighting, mobile device charging stations, and other elements of street furniture.
• Power in space: Silicon solar cells have long been the primary energy source for satellites in Earth’s orbit. High-efficiency solar cells have been used to power the International Space Station (ISS) and surface rovers on the moon and Mars.

While silicon solar cells are widely used due to their versatility and reliability, their dominance is best understood by examining both their advantages and limitations.

Advantages of Silicon Solar Cells

The most evident benefit is that, after more than four decades of development, they have become a well-researched technology. They are inexpensive, dependable, long-lasting, and rich in silicon. Below is a list of primary benefits:

• Ubiquitous: Silicon is the second most common material on Earth. As a result, the raw materials for producing silicon solar cells are abundant. 
• Durable and lasting: With proper maintenance and upkeep, silicon solar panels can survive for more than 25 years. After 25 years, the majority of the cells still operate at 80% efficiency. 
• Withstanding harsh weather conditions: They’re designed to survive in severe weather.
• High efficiency: Modern silicon cells, such as mono-PERC cells, have an efficiency of up to 22.5%.
• Continuous, ongoing research: A significant amount of research is continually being conducted on silicon solar cells, including tandem solar cells and n-type cell designs such as Silicon Heterojunction (HJT) and Tunnel Oxide Passivated Contact (TOPCon), which are more efficient and perform better in low-light conditions compared to older designs.

Despite these strengths, silicon solar cells also face several limitations that motivate ongoing research and innovation.

Limitations of Silicon Solar Cells

The fact that silicon solar cells are made of a material that is rarely found in nature in its most basic, elemental form is the first major disadvantage. Beach sand, or silicon dioxide, is a plentiful source of silicon, but it takes a lot of energy to remove the oxygen from it. In an electrode arc furnace, silicon dioxide is typically melted between 1500 °C and 2000 °C. The cost of making silicon solar cells is fundamentally limited by the energy required to run these furnaces. During the purifying process, it also adds to greenhouse gas emissions.

For flexible and low-temperature applications, silicon solar cells are no longer ideal compared to emerging thin-film technologies.

The second disadvantage of silicon solar cells is their weight and stiffness. The best performance is achieved by flat silicon cells enclosed in large, hefty panels. However, these panels are primarily seen on rooftops due to the high expense of large-scale installations.

Their power conversion efficiency, which has stayed at 25% for the past 15 years, is the third drawback.

Compared to new thin-film technologies, silicon solar cells are no longer the best option for flexible and low-temperature applications. Addressing these challenges has led researchers to explore new strategies for improving efficiency and reducing losses in silicon-based solar cells.

Improving Efficiency of Silicon Solar Cells

Impurity doping, a technique used in conventional silicon solar cells, has several drawbacks that make advancements challenging. Nowadays, crystalline silicon wafers are used in the majority of solar cells. Atoms that have holes or extra electrons when they form bonds with silicon atoms are doped into the wafer itself and occasionally the layers deposited on it. The doping improves conductivity in both cases.

To effectively convert sunlight into current, two different kinds of doping atoms are required at the electrical contacts of the solar cell. This helps regulate the flow of electrons and holes.

Crystalline silicon-based solar cells with doped contacts can exceed 20% efficiency – more than 20 % of the sun’s energy is converted to electricity. A dopant-free silicon solar cell had not previously exceeded 14% efficiency.

A new study demonstrated a dopant-free silicon solar cell referred to as a DASH cell (dopant-free asymmetric heterocontact), with an average efficiency exceeding 19%. Researchers used a crystalline silicon core (or wafer) and applied layers of dopant-free silicon, called amorphous silicon, followed by ultrathin coatings of molybdenum oxide on the side of the solar cell facing the sun and lithium fluoride on the bottom surface. The two layers serve as dopant-free contacts for holes and electrons, respectively.

The research concluded that molybdenum is an efficient hole contact for crystalline solar cells, as it has numerous defects that are critical for the emerging properties of these cells. They also identified lithium fluoride as a good candidate for electron contacts to crystalline silicon coated with a thin amorphous layer. This layer complements the molybdenum oxide layer for hole contacts. Together, these advances highlight how incremental material and design improvements continue to push silicon solar technology closer to its theoretical limits.

Conclusion

Silicon solar cells combine semiconductor physics and large-scale manufacturing into one of the most successful technologies for renewable energy. With improvements being made in material manipulation, surface passivation, and next-generation tandem designs, the efficiency of solar cells is soaring. As these advances align with the increasing energy needs of the future, silicon solar cells will play a critical role in creating a more sustainable future.