Why is silicon used for making solar cells?
Silicon is very often used in solar panels as a semiconductor because it is a cost-efficient material that offers good energy efficiency. Other than that it has high corrosion resistance, long-term durability, optimal thermal expansion properties, good photoconductivity, and low toxicity.
Silicon exists in nature abundantly so there is almost no shortage of raw material to make silicon crystals.
Pure silicon, which has been utilized as an electrical component for decades, is the basic component of a solar cell.
Silicon solar panels are frequently referred to as “first-generation” panels because silicon sun cell technology gained traction in the 1950s. Currently, silicon accounts for more than 90% of the solar cell market.
In addition to being one of the best-studied materials, crystalline silicon (c-Si) is the dominating semiconductor material in modern microelectronics.
Every year since the first integrated circuits were manufactured, the complementary metal-oxide-semiconductor (CMOS) integration density and operation frequency have increased fast.
Here are the primary reasons why silicone is popularly used in solar panels.
1. Silicon is a semiconductor
Because it is a semiconductor material at its core, pure crystalline silicon is a poor conductor of electricity.
To overcome this issue, the silicon in a solar cell contains impurities, which are other atoms that are purposely mixed in with the silicon atoms to improve silicon's capacity to capture and convert the sun's energy into electricity.
For example, a gallium atom has one fewer electron than a silicon atom, but an arsenic atom has one more electron. When arsenic atoms are sandwiched between many silicon atoms, the structure gains extra electrons, resulting in the formation of an electron-rich layer.
The layers in a solar cell are arranged so that an electric field is formed. When sunlight strikes a solar cell, it stimulates electrons, which cause holes to form.
Because of the presence of an electric field, they move to the cell's electrodes. Electricity is produced in this method.
Semiconductors have qualities that are intermediate between those of a conductor and an insulator. It possesses an electrical characteristic that allows it to be conductive in one direction while being insulating in the other.
2. Offers an good energy efficiency
Silicon solar cells have an efficiency of more than 20%. This means that silicon solar cells can convert up to 20% of the sunlight they encounter into electricity. Although this may seem to you to be a low efficiency, silicon solar cells are still more efficient than other types of photovoltaic cells.
Are there more efficient materials than silicon that can make a better job? Yes, silicon is not the most efficient and there are materials that can work more efficiently.
However, by looking at many factors we will discuss further silicon seems to be one of the best materials.
3. When doped its efficiency increases
When silicon is doped with impurities such as gallium and arsenic atoms, its capacity to capture and convert solar energy to electricity improves significantly.
4. Silicon is non-toxic
Another factor that influences the decision to use silicon over any of the other elements described above is its non-hazardous nature. Silicon has a very minimal environmental impact because silicon is a non-toxic material.
There are many alternatives for silicon that are being examined alongside the shift in manufacturing technology. Gallium Arsenic (GaAs), Cadmium Telluride (CdTe), copper indium: Diselenide (CIS), and Copper-Indium: Gallium-Diselenide (CIGS) are also being studied as silicon replacement materials in solar cells.
However, most of these materials have disadvantages that silicon does not have, such as toxicity.
5. Crystalline silicon is a stable material
PV modules using crystalline silicon solar cells have a long outdoor life (>20 years). This is critical for PV's cost competitiveness because investment now begins to pay off around the tenth year following the initial installation of the PV system.
6. Cost-effective
There is significant room for cost savings in the future. Although there have been projections that silicon PV has hit its cost minimum, the costs have decreased as a result of a learning curve with a learning rate of 20%. (20% cost savings for doubling total installed power), which will most certainly be extended in the future.
7. Readily available
Silicon is one of the most often utilized materials in photovoltaic cells. It's also abundant in nature as silicon dioxide in sand and quartz, from which it's recovered through carbon reduction. In actuality, silicon makes up roughly 26% of the earth's crust.
8. Good Photoconductivity
Photoconductivity is the increase in electrical conductivity of some materials when exposed to adequate energy light. It is frequently used to detect the presence of light and measure its intensity in light-sensitive devices, as well as to comprehend the internal processes in these materials.
Silicon has very high photoconductivity that makes it a popular choice for photovoltaic cells. Silicon's silicon dioxide layer absorbs energy when it is exposed to light and converts the photons from incident sunlight into free electrons that are then able to produce electricity.
9. Optimal band gap
Silicon has a bandgap of 1.1eV, which is close to the ideal value of 1.34eV for generating solar electricity. Silicon's optimum bandgap makes it a good choice for silicon solar cells because other semiconductors with similar band gaps are usually more expensive to manufacture.
10. High Corrosion resistance
Silicone has high corrosion resistance due to its silicon dioxide layer.
This makes it a good choice for silicon solar cells because they have to be durable in adverse environmental conditions like high temperatures, intense sunlight, and corrosive saltwater.
11. Lightweight material
Silicon is also lightweight which means it can be used as the substrate on which silicon solar cell materials are deposited.
It's not easy to find another lightweight material that could substitute silicon due to its chemical properties of being non-toxic, cost-effective, and corrosion-resistant.
Types of silicons used in photovoltaic cells
There are two types of silicons employed in photovoltaic cells: pure crystalline silicon and amorphous silicon. There are significant differences in physical attributes between pure crystalline silicon and amorphous silicon due to their structural differences.
Pure Crystalline Silicon
Pure crystalline silicon does not have the characteristics that are required for photovoltaic cells. As a result, pure crystalline silicon must undergo extensive processing in order to be used effectively in solar cells.
Although pure silicon is a poor conductor of electricity, it can be doped with phosphorous and boron to improve its conductivity.
All of these properties of silicon make it worthwhile to utilize in solar cells. Since silicon is used as the primary semiconductor material in practically all electronics and communication industries, there is currently a large scope and diverse set of technological instruments for processing and manufacturing silicon to meet specific needs.
Amorphous Silicon
Amorphous silicon (a-Si) is the semiconductor silicon's non-crystalline allotropic state.
It has a high absorption capacity and can thus be used in solar cells with very thin layer thicknesses (typically about a factor of 100 thinner than crystalline silicon), saving on material costs and compensating for performance deficiencies caused by its relatively low industry-maximum efficiency of about 13%.
Despite the fact that amorphous silicon solar cells have lower performance than c-Si, they are cheaper to manufacture and can be applied on surfaces besides just glass or plastic.
Silicon has primarily been used for thin-film-type solar cells in applications with low power requirements because of its simplified and cost-effective manufacturing process.
However, in recent years, improved manufacturing techniques and higher performance efficiency gains have resulted in a broader range of a-Si module applications, including building-integrated photovoltaics (BIPV) applications.
Challenges of using silicon in photovoltaic cells
Because of their efficiency, most solar cells are made of single crystalline silicon. The success of monocrystalline solar cells is mostly due to the fact that they lack grain boundaries due to their continuous structure, which means that excited electrons can flow around the silicon structure without being obstructed by grain boundaries.
On the other hand, many grain boundaries exist in polycrystalline cells, which hinder the continuous passage of excited electrons in the semiconductor, resulting in a drastic drop in efficiency to around 10-15%.
As a result, research has been performed to locate the solar cell that balances cost-effectiveness and performance. Even while amorphous silicon thin-film solar cells appear to be a good substitute, they suffer in terms of efficiency, owing to the lack of a uniform crystalline structure.
Some of these difficulties are addressed in greater detail below:
Cost of processing
The Czochralski process, which is used to create monocrystalline silicon cells, requires a significant amount of energy.
Because single crystal silicon must be pure in order for its crystalline structure to be very uniform, a significant amount of processing is required to achieve that level. Impurity concentrations in silicon utilized for single crystal silicon may need to be as low as 10%.
To produce an ingot with monocrystalline silicon throughout, the single-crystal silicon seed must be precisely controlled and balanced. Additionally, as oxygen mixes with the silicon and dopants on the ingot's surface during the operation, oxidation of the ingot is an unavoidable result.
Later, the combined oxygen impedes the flow of charge carriers/electrons in the cell, lowering efficiency. Overall, this complicates the procedure and hence raises the cost.
Loss of material:
To be modulus, single crystal silicon must be sliced into wafers with thicknesses varying from 200 to 300m. An inner diameter saw is used for this. This saw uses diamond particles on its blade to cut the ingots into tiny wafers.
It is difficult to utilize any type of saw mechanism since the wafers tend to shatter readily at the necessary thickness level (200-300m). However, when wafers are created using this method, around 50% of the generated silicon is wasted as sawdust, owing to the sawing in this process.
Coping with the challenges of using silicone
Despite the fact that silicone crystalline solar cells are widely employed in the market today, challenges related to silicon are keeping the demand for solar energy from increasing.
Though single crystalline silicon is particularly efficient in comparison to other types of solar cells, the cost factor outweighs its efficiency advantage. Polycrystalline silicon is less efficient since it saves money during the manufacturing phases by simply chilling molten silicon.
Though thin-film is viewed as a competitor to monocrystalline cells, research is still needed to address difficulties such as weak minority carriers and worse quantum efficiency in amorphous silicon.
Thus, in order to maximize the photovoltaic effect's potential, it is critical to discover a solution to the challenges connected with silicon or alternative materials that may implement the photovoltaic effect in unique and cost-effective ways.
There is still a lot of research being done in terms of enhancing silicon quality through overcoming difficulties. However, it is difficult to discover accessible solutions for difficulties connected to the material's properties, such as absorption and grain boundaries.
Because it necessitates the adoption of extremely complicated procedures in order to alter the natural features of the materials. As a result, the majority of research is focused on altering the process of producing solar cells.
