What Is Pv Cell?

Photovoltaic (PV) cells, also known as solar cells, are devices that convert sunlight directly into electricity. PV cells have become an indispensable source of renewable power that will play a critical role in the world’s transition to clean energy.

The history of solar cells began in 1839 when the French physicist Edmond Becquerel first discovered the photovoltaic effect while experimenting with metal electrodes in electrolyte solutions. However, it took over a century until the first silicon solar cell capable of generating usable electrical power was invented in 1954 by researchers at Bell Labs.

Today’s solar cells are made of semiconductor materials like silicon that exhibit the photovoltaic effect – meaning when sunlight hits the cell, photons are absorbed by the semiconductor material which frees up electrons. The freed electrons then flow through the material producing electric current. This electricity can be used to power electrical loads or fed back into the grid.

PV cells are Distinctively known for their ability to produce electricity anywhere the sun shines, without needing to be connected to an electrical grid. By arranging many solar cells together into solar photovoltaic modules and panels, PV technology can be used to generate electricity for a wide variety of applications – from powering single homes to large scale solar farms.

How PV Cells Work

Photovoltaic (PV) cells generate electricity through the photoelectric effect. When sunlight strikes the cell, photons are absorbed by the semiconductor material, transferring their energy to electrons. This frees electrons from their atomic bonds, allowing them to flow through the material and produce an electric current.

The basic structure of a PV cell consists of a top and bottom layer of semiconductor material, typically silicon, with an electric field in between. The top layer is doped with impurities to create an excess of electrons. The bottom layer is doped to create a deficit of electrons, leaving behind “holes” with positive charge. The junction between these two layers forms the electric field.

When a photon hits the PV cell, its energy frees an electron in the top layer. The electric field accelerates the electron out of the cell into the circuit before it can recombine. The leftover hole then migrates to the bottom layer. The movement of the electrons from the top to bottom layer produces an electric current through an external load. Metallic contacts on the top and bottom collect the current for power usage.

The individual PV cells are wired together into modules, which are then connected into solar arrays. The amount of current and voltage supplied depends on the cell material, size, and lighting conditions. Bypass diodes are added between cells to route current in case of shading or damage.

Types of PV Cells

There are three main types of photovoltaic cells in use today:

Crystalline Silicon Cells

Crystalline silicon PV cells are the most common type, accounting for over 90% of the solar panel market. They are made from silicon ingots which are sliced into wafers about 0.2mm thick. The wafers are doped with impurities to form a p-n junction capable of generating electricity when exposed to sunlight. Crystalline silicon cells come in two main types:

• Monocrystalline – Made from single crystal silicon ingots, resulting in high efficiencies of 15-20%. However, they are expensive to produce.
• Polycrystalline – Made from cast square silicon ingots, cheaper to produce but slightly less efficient at 13-16%.

Thin Film Cells

Thin film PV cells are made by depositing extremely thin layers of photosensitive materials onto surfaces like glass, stainless steel or plastic. This makes them lighter and more flexible than crystalline silicon cells. The thin-film materials include:

• Amorphous silicon – Low efficiency of 6-9% but used in some consumer products.
• Cadmium telluride (CdTe) – Efficiency of 11-12% making it comparable to polysilicon.
• Copper indium gallium selenide (CIGS) – Efficiency of up to 22% but high material costs.

Emerging and Novel Types

Some novel and emerging PV technologies that are still in early stages of commercialization include:

• Perovskites – Hybrid organic-inorganic lead material with quickly rising efficiencies already exceeding 20%.
• Quantum dots – Tiny nanoparticles tuned to absorb specific wavelengths of light.
• Organic photovoltaics – Carbon-based cells using conductive organic polymers.

Efficiency and Performance

The efficiency of a PV cell refers to the percentage of sunlight energy it can convert into electricity. Typical conversion efficiencies for commercially available PV cells range from 15% to 22%. The most efficient mass-produced PV cells today are monocrystalline silicon cells, with lab cell efficiencies approaching 27%.

There are several factors that affect the performance and efficiency of PV cells:

• Cell material and manufacturing – Semiconductors like monocrystalline silicon tend to have higher efficiencies than thin-film materials like cadmium telluride.
• Solar cell temperature – Conversion efficiency decreases as temperature increases. Cooler ambient temperatures improve efficiency.
• Solar spectrum and intensity – Performance is best with direct intense sunlight across solar spectrum.
• Shading – Even small amounts of shading can greatly reduce output.

The lifespan of a typical PV panel is around 25-30 years. Output capacity can decrease about 0.5% per year. Lifespan depends on manufacturing quality, climate conditions, and proper maintenance.

Applications and Installation

PV cells have a wide variety of applications, from small rooftop systems on homes to large utility-scale solar farms covering acres of land. Some key installation types for PV include:

Rooftop Solar

Rooftop solar arrays, also known as distributed generation, are installed on the roofs of homes and businesses. These small-scale systems typically range from 2-10 kilowatts in size. They can provide supplemental power to the building and reduce electricity bills without completely disconnecting from the grid. Rooftop solar is attractive for its simplicity, ease of permitting and installation, and proximity to where the power is used. Homes and businesses with large, unshaded roofs and high electricity costs are ideal candidates for rooftop PV.

Solar Farms

Also known as utility-scale or commercial solar, large ground-mounted solar farms generate power for utility grids rather than individual buildings. They range anywhere from 5-500+ megawatts in capacity and cover very large areas of land. Many are developed and owned by utilities themselves. These massive centralized solar plants feed into transmission lines and distribute power across the grid. Utility-scale solar is attractive for its economies of scale but requires large initial investments.

Residential vs. Commercial Installations

In residential applications, PV panels are typically mounted on rooftops to offset home energy use. System components include the panels, inverter, wiring and mounting hardware. Permits are required but installation is straightforward. In commercial settings, rooftop systems are typically larger and roof space limited. Often PV canopies and carports are used for shaded parking. Ground mounts allow flexibility in panel orientation. Electrical interconnections and safety regulations are more complex for commercial sites. Professional installation and maintenance services are recommended.

Manufacturing and Materials

The vast majority of PV cells today are made from silicon. Purifying and growing silicon crystals is an important part of the manufacturing process.

Silicon is purified through processes like the Siemens process, where raw silicon is exposed to hydrochloric acid to form trichlorosilane gas. The trichlorosilane is then distilled to produce ultra-pure silicon for crystal growth.

The purified silicon is melted and seeded to grow into a large single crystal ingot through the Czochralski process. The ingot is then sliced into thin wafers to make PV cells.

Other semiconductor materials like gallium arsenide and cadmium telluride are also used in some types of PV cells. These materials have different properties that make them suitable for certain applications.

PV cells are fabricated through processes like diffusion, etching, metallization and anti-reflective coating. Automation and quality control during manufacturing is critical for high performance and consistency.

Costs

The cost of photovoltaic systems has decreased significantly over the past few decades, making solar power more affordable and accessible. There are several components that factor into the overall cost of a PV system.

Module Costs Over Time

The PV module, which converts sunlight into electricity, represents the largest share of a solar system’s cost. Module prices have declined around 90% since 2010, from $3.50-4.00/watt to around $0.25-0.40/watt today. This dramatic price drop is attributed to economies of scale, technology improvements, and a global oversupply of panels.

Balance of System Costs

In addition to the modules, a solar system requires inverters, mounting equipment, wiring, labor, permits and other hardware, known as balance of system (BOS) costs. While module costs have fallen sharply, BOS costs have declined more gradually over time, now representing up to half the price of a fully installed system.

Installed Costs per Watt

Looking at overall system prices, the cost per watt of an installed residential PV system has decreased from around $7-8/watt in 2010 to around $2.50-3.50/watt today, depending on system size and location. For large utility-scale solar farms, installed costs are even lower, in the range of $1/watt. Further reductions in soft costs and BOS components will be important for continued solar cost declines.

Challenges and Improvements

Despite their many benefits, PV cells also face some key challenges that hinder more widespread adoption. One major issue is intermittency – solar energy can only be captured when the sun is shining. This makes PV cells unable to provide continuous baseload power like fossil fuels or nuclear. However, pairing PV systems with energy storage solutions like batteries can help mitigate this.

Research is also ongoing to extend the typical 25-year lifespan of PV panels and improve their conversion efficiency. Current lab records exceed 46% efficiency, but commercial panels operate around 20%. Longer-lasting and more efficient panels would increase the return on investment for PV systems. Novel manufacturing techniques using new materials like perovskites also show promise for next-gen solar cells.

With increased scale and advances in storage and materials, many of the current challenges with PV technology can be addressed. But solar power already provides a clean, renewable energy source that empowers homes and businesses to generate their own electricity. Continued improvements will only solidify PV cells as a critical player in building a sustainable energy future.

Environmental Impact

The environmental impact of PV cells has been a topic of much discussion. Here are some of the key considerations around their carbon footprint, use of potentially toxic materials, and recyclability.

Carbon Footprint

PV cells themselves do not produce any carbon emissions during their operation. This gives them an advantage over fossil fuel energy sources. However, there are emissions associated with their manufacturing, transportation, and installation. Most lifecycle analyses find that the energy needed to produce a PV module is paid back in 1-4 years of operation. After that, they generate energy with very low carbon emissions for their remaining 20-30 year lifespan.

Toxic Materials

Some PV cells contain potentially toxic materials like lead and cadmium. This has led to restrictions on their use and disposal in some regions. Manufacturers are increasingly shifting to less toxic materials, but toxicity remains a concern. Proper recycling and safe disposal at end of life is important.

Recyclability

With PV cells having lifespans of 20-30 years, recycling is an important consideration. They contain valuable materials like silicon, silver, and copper that can be recovered through recycling. Some manufacturers and regions now have established recycling programs. But more infrastructure is needed to recycle PV cells at a global scale and ensure safe disposal.

Future Outlook

The future looks bright for photovoltaic technology. Here are some key trends emerging in the market:

Projected growth

Solar PV is one of the fastest growing renewable energy sources in the world. Total global solar PV capacity is expected to reach over 1,500 gigawatts by 2025, a tripling of capacity compared to 2020 levels. Much of this growth will come from large-scale utility projects as well as distributed rooftop solar on homes and businesses. Key markets leading this growth include China, the United States, India, Japan and Europe.

Emerging markets and applications

While growth has been strong in developed countries, emerging markets will play a greater role going forward. Countries like Brazil, South Africa, Saudi Arabia, Egypt and others are investing heavily in solar PV to meet rising energy demands. New applications are also emerging, like building-integrated PV, solar vehicles, PV-powered drones and spacecraft, and off-grid solar systems for remote areas.

Advancements on the horizon

Research is ongoing to improve PV efficiency and flexibility while reducing costs. Emerging solar cell technologies like perovskites, organic PV and quantum dots show promise to dramatically improve efficiency beyond traditional silicon. New manufacturing techniques, installation methods, solar tracking systems and balance-of-system components will also help drive down costs. With continued innovation, solar PV will become an increasingly attractive energy source worldwide.