Silicon is a non-metallic element with the atomic number 14 and the symbol Si.
This hard and brittle, bluish-gray member of the carbon family is a crystalline solid at room temperature and relatively inert. It is a metalloid and a semiconductor besides being abundant and having a high melting point.
Are these the reasons why silicon is synonymous with semiconductors and solar cells? Why is silicon used in solar panels?
Let’s explore further and find out. To get a good understanding of this subject, we need to begin with the role of semiconductors in the photovoltaic effect.
Semiconductors and photovoltaic effect
Semiconductors are materials that conduct more electricity than insulators like glass or wood but conduct less electricity than absolute conductors like aluminum and copper. It’s also possible to tweak their properties to suit the needs of the situation.
The photovoltaic effect is the phenomenon that produces an electric current when certain materials are exposed to sunlight. When two types of semiconductors (p-type and n-type) are joined to form a p-n junction, the resultant material exhibits photovoltaic properties.
Among the discovered semiconductors, Silicon (Si), Germanium (Ge), and Gallium Arsenide (GaAs) are the ones suitable for use in photovoltaic cells.
GaAs crystals come with a high probability of defects which limits their potential. Being a compound semiconductor, GaAs is more difficult to create as a single crystal and faster to decompose. Its limited temperature range also works against its viability in solar cells.
However, both silicon and germanium score equally well as a semiconductor in PV cells. Still, silicon is preferred over germanium. Why?
Let’s find out.
Why is silicon preferred over germanium in solar cells?
Both silicon and germanium don’t have the limitations of gallium arsenide. But silicon is used more commonly as a semiconductor for its easy availability, cost-effectiveness, energy efficiency, nontoxicity, and favorable band gap.
Silicon in its pure form has been used as an electrical component for a long time now. So, it was naturally the preferred semiconductor when the first solar cell was manufactured in the 1950s.
Crystalline silicon (c-Si) is the most in-demand semiconductor in use even today. Today, silicon dominates the semiconductor scene, especially in the solar panel market.
However, the crystalline form of silicon is harder and more expensive to develop. So, in the effort to bring the cost down, other forms of silicon as well as other semiconductor materials are being utilized in the making of solar cells. Despite the presence of other choices, silicon continues to be the most preferred semiconductor for solar cells.
Here are the reasons for the popularity of silicon in solar panels.
1. Silicon is a perfect semiconductor.
Pure silicon in its crystalline form is a poor electrical conductor. To improve its conductivity, impurities are added to the crystal, thus increasing its capacity to absorb and convert sunlight into electricity.
For instance, gallium has electron deficiency compared to silicon, while arsenic comes with an extra one. When an arsenic layer comes between two silicon layers, it results in the formation of an electron-surplus structure. This helps in the creation of an electric field and the generation of electricity.
In a photovoltaic cell, when the sunlight falls on the n-type semiconductor, the photons in the sunlight impart energy to the surplus electrons, inducing them to jump across the p-n junction to the p-type semiconductor side. When these electrons are redirected back to the n-type semiconductor side externally, it will constitute a flow of electrons, otherwise known as electricity.
Only semiconductors can create a photovoltaic effect. Its conductivity lies somewhere between that of a good conductor and an insulator. While a semiconductor allows the flow of electricity in one direction, it acts as an insulator in the other direction.
2. Silicon is high on energy efficiency.
Single crystalline silicon solar cells come with the highest energy efficiency of above 20%. In real terms, this means that these silicon solar cells are capable of converting 20% of the sun’s energy incident on them.
Though 20% efficiency may sound low to a layperson, in the solar power scene, this is the highest efficiency level we have ever managed to achieve using the technology available to us. The PV cells made from other semiconductors are mostly much lower in energy efficiency.
However, it should be noted that there are semiconductors more efficient than silicon. As energy efficiency is not the only criterion for choosing a semiconductor for a solar cell, ultimately, silicon comes out the winner, as it scores well on other fronts.
3. Doping improves the energy efficiency of silicon.
While the n-type semiconductor comes with extra electrons, the p-type semiconductor has holes. During the doping process, the numbers of electrons and holes are increased, prompting a better flow of electrons.
Doping can be done by introducing a thin layer or n-type or p-type semiconductor. While an n-type semiconductor layer raises the conductivity by adding more electrons into the equation, a p-type semiconductor layer does the same by increasing more holes.
When silicon is doped with gallium and arsenic, the ability of the material to absorb and convert sunlight improves significantly, leading to high-efficiency solar cells.
4. Silicon is a non-toxic material.
It is always ideal if the material chosen for mass production is not damaging to the environment. Whether the production process itself harms the environment or this happens at a later stage such as during or after its lifetime, environmental impact is to be taken seriously.
Silicon is a downright winner on this count. It has almost zero environmental impact as the element itself is not toxic. However, this cannot be said about any of its alternatives available in the market.
Gallium Arsenic (GaAs), Copper-Indium Gallium-Diselenide (CIGS), Copper Indium Selenide (CIS), Cadmium Telluride (CdTe), Gallium Nitride (Gan), and Silicon Carbide (SiC) are some of the alternatives available today as a replacement to silicon in the manufacture of solar cells.
Unfortunately, these materials are all highly toxic, making their application as a semiconductor in solar cell manufacture harmful to the environment.
5. Silicon in crystalline form is stable.
Solar panels need to be able to survive the vagaries of weather as they are kept out in the open. This means the materials used in its manufacture have to be stable. Silicon fits this requirement perfectly.
Crystalline silicon solar cells survive the longest with a lifespan of 25-30 years. The payback period for solar panels is 7-10 years. The more years they continue to function after this, the more beneficial it is for the buyer.
A longer lifespan directly translates to cost-effectiveness as it means a lesser need for replacement. Besides the value for money spent, the long life of silicon solar panels also means less generation of waste from old unusable solar panels.
6. Silicon panels are cost-effective.
In the last decade or two, the cost of solar panels has come down drastically with improvements in technology. The future definitely looks bright for PV cells with technological advances bringing down their prices further.
With the impacts of climate change and depleting reserves of fossil fuels, the need to find a cost-effective replacement is gaining traction. Solar energy is the most lucrative among the choices available to us today.
7. Silicon is abundant.
Silicon is abundantly available in nature in the form of silicon dioxide (silica) or silicates, usually found in sand and rocks like quartz. Above 90% of the earth’s crust consists of silicate minerals. This makes silicon the second-most abundant element on the earth by mass, only next to oxygen.
Silicon is recovered from the silicon dioxide by a process known as carbon reduction.
8. Silicon is a good photoconductor.
Photoconductivity is described as the increase in electrical conductivity when exposed to light. This property of silicon is often used in light-sensitive devices to ascertain the presence of light and calculate its intensity. It also comes in handy to understand the internal mechanisms of these devices.
The excellent photoconductivity of silicon makes it an excellent choice for solar cells.
9. Silicon has the optimal band gap.
In a semiconductor, there is a small energy gap between the valence and conduction bands. This is called the band gap. The band gap is the minimum energy required to move an electron from the valence band to the conduction band where it can participate in conduction.
For a solar cell, the ideal band gap is 1.34 eV. This will help the electrons soak up more photons present in the sunlight. Silicon comes close to this with a band gap of 1.1 eV, while that of germanium is 0.7eV.
10. Silicon has high resistance to corrosion.
As silicon comes with an outer silicon dioxide layer, it is stable and long-lasting and doesn’t corrode easily. This is vital for use in solar cells as they are exposed to elements throughout their lifetime. They need to be strong enough to survive intense sunlight, high temperatures, and even highly corrosive saltwater.
11. Silicon is lightweight.
As silicon is less heavy than other semiconductors, it is suitable for use as a substrate. A substrate needs to be lightweight for the process of Thin Film Deposition. In this process, a thin film coating is deposited to improve its performance.
In electronics, a substrate is a thin wafer of a semiconductor made from a super-flat material with tiny irregularities on the surface. The flat surface is highly reflective, making it ideal as a substrate material. Silicon fits the bill better than any other semiconductor.
Types of silicon solar cells
Photovoltaic cells use two types of silicon – crystalline silicon and amorphous silicon. Although both are essentially silicon, they vary vastly in their physical features due to the variations in their atomic structure.
Pure silicon (c-Si) satisfies a majority of conditions required for use in PV cells. Especially, the fact that it is abundant, cost-effective, lightweight, durable, non-corrosive, and strong. It also comes with the ideal band gap and can be modified by doping it with small amounts of gallium, arsenic, boron, or phosphorus.
Crystalline silicon in its pure form doesn’t have all the characteristics to be used in solar cells. It has to be processed extensively to comply with all the criteria. For instance, pure crystalline silicon is not a good conductor of electricity. However, when doped with boron and phosphorus, we can improve its conduction.
Amorphous silicon (a-Si) is a non-crystalline allotrope of silicon. This semiconductor finds its use in thin-film solar panels as it can be cut into wafers 100 times thinner than pure crystalline silicon. This helps in reducing the quantity of material used, thereby bringing down its cost as well. However, the amorphous silicon used in thin-film solar cells comes with a lower efficiency level of around 12%.
Despite its lower efficiency levels, its affordability and flexibility in application make it a popular choice in certain situations. It is typically used in devices with low energy requirements. Recent advancements in technology have led to the wider application for amorphous silicon, the most notable being building-integrated photovoltaics (BIPV) applications.
Challenges for silicon solar cells
Pure crystalline silicon is the most preferred form of silicon for high-efficiency solar cells. The absence of grain boundaries in single crystalline silicon solar cells makes it easier for electrons to flow without hindrance.
However, this is not the case with polycrystalline silicon. The multiple grain boundaries present in it hamper the free flow of electrons resulting in lower efficiency levels.
R&D in the field is aimed at finding the right balance between cost and efficiency. With amorphous silicon, the affordability factor has improved at the cost of efficiency.
Let’s understand the challenges faced by the solar industry in detail.
Single crystalline silicon solar cells are made using the Czochralski process, an energy-consuming process. The purity of the silicon is paramount for the uniform formation of the crystalline structure. This means impurity concentration has to be reduced to 10% or below.
Besides this, it is vital to prevent the oxidation of silicon during the process as oxidized silicon is not a good conductor. This again adds to the processing cost.
During the manufacturing of crystalline silicon solar cells, silicon needs to be sliced to thicknesses in the range of 200-300mm to form wafers. An inner diameter saw with a blade with diamond particles is used for slicing.
While slicing, about 50% of silicon gets wasted in the form of sawdust.
Overcoming challenges when using silicon
Even though crystalline silicon solar cells have been popular in the last few decades and there have been constant technological advancements, serious challenges continue to exist, creating hurdles for the “Go Solar” campaign. These challenges pose a grave threat to the adoption of solar energy.
It’s a well-known fact that single crystalline silicon is ideal for high-efficiency solar cells. However, affordability is preventing its wholehearted acceptance. The cost factor takes away the advantage in efficiency.
Multiple crystalline silicon solar cells are more affordable but less efficient. With thin-film solar cells, the solar industry tried to overcome some of the major challenges but more research is needed to make this more workable and user-friendly.
Some of the serious challenges faced by the solar industry include tweaking the properties of silicon including its ability to capture sunlight and overcoming the challenges posed by grain boundaries. There are no simple ways to make this happen. Most of the current research is focusing on these.
Some of the new advancements in the solar scene are multijunction solar cells, heterojunction solar cells, and perovskite solar cells.
None of the choices available in the market today is perfect and devoid of challenges. We need to come up with better technologies to fully take advantage of the amazing photovoltaic technology and make it easier for the general public to embrace it without hesitation.
Final Thought on Using Silicon in Solar Cells
No doubt, silicon is the best choice for tapping into the free energy provided to us by the sun. It is the best suitable semiconductor for solar cell manufacture by virtue of its various properties including electronic, optical, thermal, and mechanical, not to leave out its easy availability and environmental advantage.
As we become more and more aware of the climate crisis and the imminent need to find a replacement for fossil fuels, the pace of research has gathered momentum in the past decade. The need of the hour is to find the perfect form of silicon that will tick all the boxes in solar cell production.
Let’s hope this happens sooner!
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