How Do Photovoltaic Cells Convert Sunlight Into Electricity?
Solar Power and Cell
Solar energy or solar power is the most abundant form of energy available on the earth. The net solar radiation which is incident on the surface of the earth is much more than what the world currently needs to meet its energy requirement. If this energy is optimally harnessed, sunlight as a renewable source of energy has the potential of sustaining the energy needs of the future generations. It is inexhaustible, at least for the next 4 billion years to come. Contrarily, increased dependence on the conventional sources of energy is sure to cause an energy crisis, due to their limited availability. Our reservoirs of fossil fuels such as coal and petroleum are expected to last for a few more decades before running out. On the other hand, solar energy is clean and causes no pollution. Investment and research in solar energy can avert the impending energy crisis, and help in achieving sustainable development goals.
The drawback of solar energy is its low intensity, because the radiation is spread over the entire geographical area of the earth. The atmosphere, along with the clouds absorb/scatter almost 54% of the incoming radiation. The light that is incident on the surface is composed of visible light (50%), infrared rays (45%) and traces of ultraviolet rays, along with some other forms of rays.
However, the countries that are situated between the Tropic of Cancer and the Tropic of Capricorn, in the tropical zone receive plenty of sunshine throughout the year. These countries (India being one of them) have enormous potential to channelize solar energy.
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Photovoltaic Cell
Photovoltaic effect is a process in which a photovoltaic cell, when exposed to sunlight, is capable of producing voltage or electricity.
A photovoltaic cell is a technology to harness solar energy and convert it to electric energy. It is made up of two types of semiconductors- a p-junction and an n-junction. Together, they create a p-n junction. When these two semiconductors are joined, an electric field develops at the junction, causing the electrons to move towards the p-side, and the holes to move towards the n-sides. Under the effect of the electric field, the negatively charged and the positively charged particles move in opposite directions.
Light is a form of electromagnetic radiation, which is made up of small packets or bundles of energy called photons. When a radiation of the required wavelength falls on these cells, the energy of the photon is transferred to an electron of the semiconductor. As the electron gains this energy, it becomes excited and jumps to a higher energy level. It is this motion of the electron through the material in an excited state that induces a current in the cell.
Advantages Of Photovoltaic Cells
They produce clean energy. The source of energy is the radiation from the sun, which is totally non-polluting. Hence, no pollutants are released in the process and there is no danger of environmental degradation. Using solar energy, we can immensely cut down our carbon emissions.
They are reliable and are based on green technology. They can last for long periods of time with minimum loss of efficiency.
They do not produce any noise and have no moving/mechanical parts. So, there is no disturbance produced.
The operating and maintenance costs involved are minimal. Just cleaning the surface of solar panels is sufficient to maintain their efficiency.
Photovoltaic cells can be of great help in rural and remote areas, which are yet not electrified. Solar resources are already abundantly present. The power loss that occurs due to the long-distance transmission of electricity can be drastically reduced.
Disadvantages of Photovoltaic Cells
They are less efficient as compared to other renewable sources of energy.
They can operate only in the presence of sunlight, which implies that they will not be of any use against the unpredictable weather. On a cloudy or a rainy day, an alternative may be needed.
Solar energy cannot be transmitted over long distances. As photovoltaic cells produce direct current, its conversion to alternating current will involve more equipment and make the process quite arduous.
They are quite susceptible to damage and have to be maintained with care.
Though the maintenance charges are quite nominal, the entire setting up of a solar panel may be expensive.
Did You Know?
After the energy crisis of 1970, the interest in solar energy began rising. Research, technological developments and industrial progress have made the use of photovoltaic cells and solar energy viable. As the production and demand increased, the costs began decreasing. However, we still have a long way to go and tap the solar potential.
FAQs on Solar Energy and Photovoltaic Cell: Complete Guide
1. What is the working principle of a photovoltaic cell?
A photovoltaic cell operates on the principle of the photovoltaic effect. When photons from sunlight strike the semiconductor material of the cell, they transfer their energy to electrons. If the photon's energy is greater than the semiconductor's band gap, it excites an electron, creating a free electron and a hole (an electron-hole pair). The internal electric field of the cell's p-n junction then separates these charge carriers, directing electrons to the n-side and holes to the p-side. This separation creates a voltage, and when an external circuit is connected, a direct current (DC) flows, converting light energy directly into electrical energy.
2. What is the basic structure of a typical silicon solar cell?
A standard silicon solar cell is essentially a large-area p-n junction diode. Its structure consists of several layers:
Top Metal Contact (Grid): A metallic grid on the top surface collects the electrons but is thin enough to allow most sunlight to pass through.
Anti-reflective Coating: A thin layer applied to the top to minimise light reflection and maximise absorption.
n-type Semiconductor: A thin layer of silicon doped with a pentavalent impurity (like phosphorus) to create an excess of free electrons.
p-type Semiconductor: A thicker base layer of silicon doped with a trivalent impurity (like boron) to create an excess of holes.
Back Metal Contact: A metallic layer covering the entire back surface, which acts as the other terminal for current collection.
3. How does the p-n junction in a solar cell help in generating electricity?
The p-n junction is crucial for a solar cell's function. When the p-type and n-type layers are joined, a depletion region with a built-in electric field is formed at the interface. This field acts like a one-way gate for charge carriers. When sunlight creates electron-hole pairs, this intrinsic electric field forcefully separates them before they can recombine. It sweeps the electrons to the n-side and the holes to the p-side, causing a build-up of negative charge on one side and positive charge on the other. This charge separation is what establishes the voltage (potential difference) across the cell, enabling it to drive a current through an external circuit.
4. Why is silicon the most commonly used semiconductor for solar cells?
Silicon is the preferred material for solar cells for several key reasons:
Abundance: Silicon is the second most abundant element in the Earth's crust, making it relatively inexpensive and widely available.
Optimal Band Gap: Its band gap of about 1.1 eV is well-matched to the solar spectrum, allowing it to efficiently absorb a significant portion of sunlight.
Mature Technology: The semiconductor industry has decades of experience in processing and purifying silicon to a very high grade, making its manufacturing process well-understood and reliable.
Stability: Silicon is a stable and robust material that does not degrade quickly under exposure to the elements, ensuring a long operational life for solar panels.
5. What is the difference between the photovoltaic effect and the photoelectric effect?
While both effects involve electrons gaining energy from photons, they are distinct phenomena. The photoelectric effect is the emission or ejection of electrons from the surface of a material (usually a metal) when it is illuminated by light of a suitable frequency. In contrast, the photovoltaic effect occurs within the bulk of a semiconductor material at a p-n junction. The electrons are not ejected from the material; instead, they are excited to a higher energy level and guided by an internal electric field to produce a voltage and current.
6. What are the main types of photovoltaic cells available?
Photovoltaic cells are primarily categorised based on the type of silicon used:
Monocrystalline Silicon Cells: Made from a single, pure crystal of silicon. They are highly efficient and have a uniform black appearance but are the most expensive to produce.
Polycrystalline Silicon Cells: Made by melting and solidifying multiple silicon fragments. They are slightly less efficient than monocrystalline cells but are more affordable. They have a characteristic blue, marbled look.
Amorphous (Thin-Film) Silicon Cells: Made by depositing a thin layer of non-crystalline silicon onto a substrate. They are flexible and cheaper but have lower efficiency and a shorter lifespan compared to crystalline types.
7. How is a solar panel constructed from individual photovoltaic cells?
A single photovoltaic cell produces a very small voltage (around 0.5V) and current. To generate useful amounts of power, multiple cells are connected together to form a solar panel or module. The cells are typically connected in series to increase the overall voltage and in parallel to increase the overall current. These interconnected cells are then encapsulated between a transparent top layer (like tempered glass) and a protective backsheet, all held together by an aluminium frame to ensure durability and weather resistance.
8. What key factors affect the efficiency of a photovoltaic cell in the real world?
The actual performance and efficiency of a photovoltaic cell are influenced by several external factors:
Temperature: Higher operating temperatures reduce the efficiency of a solar cell. For every degree Celsius rise above 25°C, a cell's efficiency can drop by approximately 0.4-0.5%.
Light Intensity (Irradiance): The power output is directly proportional to the intensity of sunlight hitting the cell. Cloudy days, shading, or poor orientation significantly reduce output.
Shading: Even partial shading of a single cell in a panel can disproportionately reduce the power output of the entire string of cells.
Cleanliness: Dust, dirt, snow, or bird droppings on the surface of the panel can block sunlight and reduce energy generation.
9. What are some common applications of solar energy and photovoltaic cells?
Photovoltaic cells are used in a wide range of applications, from small-scale devices to large power plants. Common examples include:
Small Electronics: Powering devices like calculators, watches, and portable chargers.
Residential and Commercial Rooftops: Generating electricity for homes and businesses to reduce reliance on the grid.
Off-Grid Systems: Providing electricity in remote areas for lighting, water pumping, and telecommunication towers.
Space Exploration: Powering satellites, space probes, and the International Space Station (ISS).
Utility-Scale Solar Farms: Large arrays of solar panels that generate electricity for the power grid.
10. What happens if a solar cell is illuminated with light of energy less than its band gap energy?
If a photon striking the solar cell has energy that is less than the semiconductor's band gap energy, it cannot excite an electron from the valence band to the conduction band. The photon will simply pass through the material without being absorbed and without creating an electron-hole pair. Consequently, this light energy is not converted into electrical energy and does not contribute to the cell's current output. This is one of the fundamental reasons why no solar cell can be 100% efficient, as it can only convert a specific portion of the solar spectrum.
