What Is Solar Cell And Its Working?

What is a Solar Cell?

A solar cell, also known as a photovoltaic cell, is an electrical device that converts energy from the sun directly into electricity. Solar cells are made from semiconducting materials that exhibit the photovoltaic effect – meaning when sunlight hits the cell, the absorbed energy knocks electrons loose, allowing them to flow and generate an electric current. The photovoltaic effect was first observed in 1839 by French physicist Edmond Becquerel.

Solar cells are different from solar thermal collectors, which absorb sunlight to generate heat. They are also different from solar panels or modules, which are sets of interconnected solar cells encased in a weatherproof module. While the terms are sometimes used interchangeably, solar panels contain multiple solar cells that combine to generate solar electricity.

So in short, a solar cell is a single device made of semiconductors that converts sunlight into electricity due to the photovoltaic effect. Solar panels contain many solar cells working together to provide usable levels of electricity.

History of Solar Cells

The history of solar cells began in the 19th century with early experimentation and discovery of the photovoltaic effect. In 1839, French physicist Antoine-César Becquerel first observed the photovoltaic effect while experimenting with electrolytic cells made up of two metal electrodes placed in an electricity-conducting solution. He noticed that small amounts of electricity were produced when light fell upon the cell.

In 1883, American inventor Charles Fritts created the first true solar cell by coating selenium with an extremely thin layer of gold. This formed the first solar cell capable of converting sunlight into electricity at 1% to 2% efficiency. However, the expense of selenium cells limited their practical use.

In 1905, Albert Einstein published a paper that explained the photoelectric effect, pivotal to the development of solar PV, by defining light as packets of energy called photons.

Bell Laboratories developed the first usable photovoltaic module in 1954. It was billed as a solar battery and was mostly just a curiosity as it was too expensive to gain widespread use. The space industry began using solar modules built with high cost solar cells made from single crystal silicon in the 1960s.

The oil crisis of the 1970s drove the development of practical solar cells for terrestrial applications. Japanese manufacturer Sharp introduced the first photovoltaic modules packaged as solar panels in 1963. Improvements in manufacturing and solar cell efficiency helped drive down costs over the next decades.

The solar PV market grew rapidly in the late 1990s and early 2000s with Japan and Germany leading the way. Solar cell technology has continued to advance with the development of thin-film solar cells, multi-junction cells, organic cells, and nanotechnology based designs.

How Solar Cells Work

Solar cells work through the photovoltaic effect, which is the process of converting light energy into electricity. When light hits the solar cell, it excites the electrons in the semiconductor material used to make the cell. This excitation gives the electrons enough energy to break free of their atoms and flow through the material, generating electricity.

Solar cells are made of specially treated semiconductor materials like silicon that have two layers – a p-type layer which has an excess of positive charge carriers called holes, and an n-type layer which has an excess of negatively charged electrons. The interface between these two layers forms a natural electric field.

When sunlight hits the solar cell, the photons are absorbed by the semiconductor atoms, transferring their energy to the electrons. The excited electrons in the p-type layer then flow into the n-type layer where they are attracted to the extra electrons. By connecting the p-type layer to the n-type layer with an external circuit, this electron flow can be captured as electric current.

The electric field at the p-n junction separates the electrons and holes, creating a voltage differential across the solar cell terminals. The more light that hits the solar cell, the more electricity that is generated. The photovoltaic effect continuously generates electricity as long as light shines on the solar cell.

Solar Cell Design

Solar cells require a few key components in order to convert sunlight into electricity. Here are the basic parts of a solar cell:

• Front contact: This transmits current to the external circuit. It is made of very thin layers of metallic coating like silver, aluminum or copper.



• Semiconductor silicon layers: This includes layers of both N-type (negatively doped) and P-type (positively doped) silicon. These layers form the electric field that generates current.
• Back contact: This electrode completes the electrical circuit. It is made of aluminum, titanium, silver or other metals.
• Antireflection coating: This is a thin coating of silicon nitride or titanium oxide which reduces reflection from sunlight, improving efficiency.
• Encapsulant: This protects the cell from environmental damage. It is commonly made of EVA (ethylene vinyl acetate).

The most common material used in solar photovoltaic cells is crystalline silicon, either mono-crystalline or multi-crystalline. Thin film solar cells use amorphous silicon, cadmium telluride, copper indium gallium selenide and other thin-film technologies which reduce manufacturing costs. However, thin film cells tend to have lower efficiency rates.

There are a few key factors that affect the efficiency and performance of a solar cell:

• Band gap of the semiconductor material
• Reflection or absorption of light from the cell surface
• Recombination rate of electron-hole pairs
• Resistance electrical losses
• Quality of semiconductor material

By optimizing these parameters through solar cell design, manufacturers continue to increase the efficiency and lower the costs of photovoltaic devices.

Types of Solar Cells

There are several main types of solar cells, which are classified by the materials used in their construction and manufacturing process. The most common types are:

Crystalline Silicon Cells

This is the most common type, representing about 90% of the solar panel market. They are made from silicon wafers, which are either monocrystalline or polycrystalline. Monocrystalline silicon is a single continuous crystal structure, making it highly efficient but more expensive. Polycrystalline silicon contains small crystals fused together, which reduces efficiency slightly but also costs less.

Thin Film Solar Cells

These use very thin layers of photovoltaic materials deposited on substrates like glass, plastic or metal. Thin film cells tend to have lower efficiencies than crystalline silicon, but their flexible shape and thin profile allow more diverse applications. Common semiconductors used include cadmium telluride (CdTe), copper indium gallium selenide (CIGS), and amorphous silicon (a-Si).

Organic Solar Cells

Also known as organic photovoltaics (OPV), these use thin films of organic semiconductors like organic dyes or polymers. They are lightweight and easy to manufacture, although durability and efficiency is still quite low compared to inorganic cells. Ongoing research is working to improve their stability and performance.

Multi-Junction Cells

These special cells utilize multiple semiconductor layers to absorb light at different wavelengths, allowing much higher efficiencies by minimizing spectrum losses. The layers are stacked in descending order of band gap to absorb light effectively across the solar spectrum. Multi-junction cells are used for concentrated photovoltaics and space applications.

Solar Cell Efficiency

Solar cell efficiency refers to the percentage of sunlight energy that is converted into electricity by the solar cell. It is one of the key metrics used to rate the performance of different solar cell technologies.

Current records for solar cell efficiency in lab testing are over 47% for multi-junction concentrated photovoltaic cells. However, mass-produced commercial solar panels typically have efficiencies of 15-20% for crystalline silicon solar cells and 10-15% for thin film technologies like CIGS and CdTe.

There are several challenges involved in improving solar cell efficiency further:

• Thermodynamic efficiency limits – there are theoretical limits to the maximum efficiency possible for single junction solar cells.
• Impurity effects – defects and impurities in the semiconductor material can diminish efficiency.
• Surface recombination – electrons and holes recombining at the surface reduces efficiency.
• Cost – higher efficiency cells often require more expensive materials and manufacturing techniques.

Research is ongoing to surpass these limitations through approaches like multi-junction cells, nanotechnology, spectrum splitting, and advanced light management.

Solar Cell Manufacturing

The manufacturing process for solar cells generally consists of the following steps:

Crystalline Silicon Solar Cells

Purification and Crystallization – High purity silicon is obtained and grown into crystalline ingots or ribbons.

Slicing – The ingots are sliced into thin wafers using wire saws or other specialized cutting equipment.

Surface Texturing – The wafers are textured to reduce light reflectance and improve absorption.

Cleaning – Wafers are cleaned to remove contaminants from sawing and texturing steps.

Doping – Dopants like phosphorus and boron are diffused into the wafer to form the p-n junction.

Anti-Reflection Coating – Anti-reflection layer is applied to increase light absorption.

Screen Printing – Contacts are printed onto the wafer using metal pastes.

Firing – Printed metal pastes are fired at high temperature to form metal contacts.

Testing and Sorting – Completed cells are tested and sorted based on performance.

Module Assembly – Cells are interconnected and encapsulated into modules.

Thin Film Solar Cells

Deposition – Thin layers of photovoltaic material are deposited onto substrates.

Patterning – The films are laser-patterned to isolate individual cells.

Add Contacts – Transparent conducting oxide top contacts are added.

Laser Scribing – The films are scribed with lasers to interconnect cells.

Testing and Sorting – Completed modules are tested and sorted.

Framing – Modules are assembled into protective frames.

Applications of Solar Cells

Solar cells have many practical uses and are commonly found in a wide range of technologies and consumer products. Some of the most common applications of solar cells include:

Calculators

Small solar cells are frequently used to power simple handheld calculators. These cells convert sunlight into enough electricity to run the calculator’s display and electronics.

Satellites

Satellites and spacecraft often rely on solar cells for power generation. Solar panels made up of multiple solar cells can convert sunlight into electricity even while in orbit high above Earth’s atmosphere.

Solar-Powered Vehicles

Electric vehicles such as cars, boats, planes, and even spacecraft can be powered by electricity from onboard solar panels. Solar race cars in particular are designed to be powered solely by solar energy.

Off-Grid Power

Solar panels are also used to provide electricity in remote locations and developing areas without access to an electrical grid. Individual solar cells can be wired together into solar panels to provide electricity for off-grid homes, buildings, and communities.

Future of Solar Cell Technology

Researchers continue to push solar cell technology forward to increase efficiency and lower costs. Some key areas of cutting-edge solar cell research include:

Perovskite Solar Cells

Perovskite solar cells are a promising new technology made from low-cost materials that can be processed at low temperatures. Perovskites have seen rapid efficiency gains, jumping from 3% in 2006 to over 25% today. Perovskites can capture more light wavelengths than silicon, potentially enabling efficiencies beyond 30%. However, perovskites currently have issues with stability and lead toxicity that need to be resolved before widespread commercial use.

Organic Photovoltaics (OPV)

OPVs use organic electronics made from carbon-based materials like plastics and dyes. Benefits include flexibility, light weight, transparency, and easy processing. However, OPV efficiencies are currently below 15% and lifetimes are short compared to silicon. But ongoing research on new organic compounds and device architectures aims to improve OPV performance.

Solar Cell Limitations

Despite great progress, solar cells have fundamental limits related to the Shockley-Queisser limit, which constrains how much electrical energy can be extracted from sunlight. Ongoing areas of research to push past these limits include multi-junction cells, solar concentrators, upconversion and downconversion techniques, and hot carrier extraction.

In summary, researchers continue to innovate on new solar cell technologies like perovskites and organics, as well as push against practical efficiency limits. The future looks bright for solar cells with greater efficiency and lower costs on the horizon.

FAQs

What is a solar cell?

A solar cell, also called a photovoltaic (PV) cell, is an electrical device that converts sunlight directly into electricity. Solar cells are made of semiconductor materials like silicon and can generate electricity from sunlight without any mechanical or thermal energy conversion.

How does a solar cell work?

Solar cells work through the photovoltaic effect. When sunlight hits the solar cell, photons from the sunlight are absorbed by the semiconductor material, causing electrons to be freed from their atomic bonds. This generates electron-hole pairs that create an electric current as they move through the semiconductor material to metal contacts on the solar cell, producing electricity.

What are the different types of solar cells?

The main types of solar cells are silicon cells, thin-film cells, and organic cells. Silicon cells use monocrystalline or polycrystalline silicon and are the most common. Thin-film cells use cadmium telluride (CdTe) or copper indium gallium selenide (CIGS). Organic cells use organic electronics like polymers or dyes.

How efficient are solar cells?

Typical commercial silicon solar cells have efficiencies around 15-20%. The highest efficiency silicon cells can reach over 25%. Thin-film technologies tend to have lower efficiencies around 10-13%. Lab solar cells have reached efficiency records over 40%, but these have not been commercialized.

How are solar cells manufactured?

Manufacturing solar cells involves purifying semiconductor materials, adding dopants, slicing wafers, and adding electrical contacts. Cells are interconnected and encapsulated into solar modules. Automated production lines handle many processing steps to reduce costs. Quality control testing ensures cells and modules meet efficiency and durability standards.

What are solar cells used for?

Solar cells are used for electricity in many applications. They power calculators, houses, satellites, and even entire power plants. Individual cells are connected together to form modules and arrays to generate power. Solar panels on rooftops are common for providing residential power.