How Do Solar Cells Transfer Energy To Energy?

A solar cell, also called a photovoltaic (PV) cell, is a device that converts sunlight directly into electricity using the photovoltaic effect. Solar cells have become an increasingly important renewable energy technology over the past several decades.

The modern version of the solar cell was invented in 1954 at Bell Labs. This first solar cell was only able to convert sunlight into electricity with around 6% efficiency. Since then, ongoing research and development has steadily improved the efficiency and lowered the cost of solar cells.

Today, solar energy is one of the most promising and rapidly growing renewable energy sources. It offers a sustainable, clean alternative to fossil fuels. Solar cells and panels are now utilized worldwide in a wide variety of applications, from small consumer devices to large utility-scale solar power plants. The technology continues to advance each year, increasing solar energy’s importance for the future.

How Solar Cells Work

Solar cells, also called photovoltaic (PV) cells, convert sunlight directly into electricity through the photovoltaic effect. This process relies on materials that exhibit the properties of semiconductors. When light strikes the surface of a semiconductor, electrons absorb the energy and become free to move around.

PV cells consist of two layers of semiconductors, typically silicon, that form a p-n junction. One layer is doped with boron to give it a positive charge (p-type), while the other is doped with phosphorus to give it a negative charge (n-type). When sunlight strikes the solar cell, the p-n junction allows electrons freed by the photovoltaic effect to flow from the n-type side to the p-type side, generating an electric current.

The photovoltaic effect and the special electronic properties of semiconductors make it possible for PV cells to absorb sunlight and directly produce electricity. The p-n junction structure provides the internal electric field that enables the current generation.

Absorbing Light

The photovoltaic effect is fundamental to how solar cells work. When photons from sunlight strike the solar cell, they transfer their energy to electrons in the semiconductor material of the cell. This energizes the electrons, causing them to break free from their atoms and become mobile. The photon is absorbed in the process and its energy is transformed into electrical potential energy via the excited electron.

The energized electrons do not immediately become current. First, the photons generate bound electron-hole pairs called excitons. An exciton is a bound state of an electron and the hole it leaves behind in a solid. The electron is still weakly bound to the hole via Coulomb forces. A solar cell relies on these excitons to generate current.

Generating Current

Solar cells generate direct current electricity through a process called the photovoltaic effect. When photons from sunlight hit the solar cell, they transfer their energy to electrons in the semiconductor material. This energizes the electrons, causing them to break free of their atomic bonds and flow through the material as electric current.

The electrons can only move in one direction through the solar cell, creating a direct current or DC power output. Unlike alternating current that continuously changes direction, direct current provides a continuous, one-way flow of electrons. This direct current generated by solar cells is converted into usable electricity through power inverters.

The amount of current a solar cell generates depends on how much sunlight strikes the cell. More intense sunlight leads to more energetic photons and a greater electric current. Solar cells are wired together in modules to produce higher currents and voltages. These modules can then be linked to generate power at the scale required by the application, from small consumer devices to large-scale solar power plants.

PV Cell Structure

The basic structure of a solar cell consists of several key components layered on top of each other. This layered structure helps the cell efficiently absorb and convert sunlight into electricity.

Anti-reflective coating

The top layer is an anti-reflective coating that reduces reflection of sunlight and allows more light to penetrate the cell. Common coatings are silicon nitride or titanium dioxide.

Top contact

On top of the anti-reflective coating is a metallic contact grid that collects electrons freed by photons. The grid is interlaced fingers and a busbar structure made of materials like silver or aluminum.

N-type silicon

Below the metallic contact is a layer of n-type silicon, meaning it has extra electrons added through a process called doping. When sunlight hits this layer, electrons absorb photon energy and become free to move.

P-type silicon

Under the n-type silicon is a layer of p-type silicon, which has electron deficiencies or “holes” that act as positive charge carriers. This builds up an electric field at the junction between the two silicon layers.

Bottom contact

At the base is a bottom contact layer that collects positive charge carriers. It is also made of conductive metallic material like aluminum. The bottom contact completes the electrical circuit.

Connecting Solar Cells

Solar panels are made up of many individual solar cells wired together. There are a few main types of solar cells used in panels:

Monocrystalline Cells

Monocrystalline solar cells are made from a single crystal of silicon. This type of cell is the most efficient but also the most expensive to produce. Monocrystalline cells have a distinctive black color and perfectly even surface.

Polycrystalline Cells

Polycrystalline solar cells are made from fragments of silicon crystals melted together. The resulting cells have a speckled blue color and slightly less even surface. Polycrystalline cells are less efficient but cheaper to produce than monocrystalline.

Thin Film Cells

Thin film solar cells use a very thin layer of photosensitive material deposited on a substrate like glass or plastic. Because they use less raw material, thin film cells are cheaper but also less efficient than crystalline silicon cells.

Wiring

Individual solar cells generate relatively small amounts of electricity. To produce useful amounts of power, cells are connected together in series circuits and encapsulated behind a glass sheet to form a solar photovoltaic module or solar panel. Multiple panels can then be wired together to form an array sized to match the electrical load.

Efficiency Factors

The efficiency of a solar cell depends on how effectively it can utilize the solar spectrum, separate photogenerated charge carriers, and collect those carriers before they recombine. There are three main factors that impact efficiency:

Spectrum Utilization

Solar cells are only able to absorb and convert a portion of the solar spectrum into electricity. Photons with energies below the semiconductor bandgap are not absorbed, while photons with energies much greater than the bandgap lose their excess energy as heat. Matching the bandgap to the solar spectrum is key to absorbing the maximum amount of solar energy.

Charge Carrier Separation

The built-in electric field of the p-n junction separates photogenerated electrons and holes so they can be collected before recombining. Impurities, defects, and interfaces within the cell can allow charge carrier recombination, reducing efficiency. Careful manufacturing and materials selection minimizes opportunities for recombination.

Charge Carrier Collection

Photogenerated carriers need to reach the cell contacts in order to produce useful current. The mobility and lifetime of carriers impacts how far they can travel through the cell before recombining. Cell design, doping levels, and contact placement optimize charge collection.

Improving Efficiency

There are a few key ways solar cell technology continues to increase efficiency.

One method is developing multijunction cells. These solar cells contain multiple p-n junctions made up of different semiconductor materials. Each material is able to absorb a different wavelength of light, enabling more of the solar spectrum to be utilized. The current world record solar cell efficiency of 47.1% was achieved in 2020 using a six-junction solar cell.

Another approach is light trapping techniques. These methods maximize the amount of light absorbed by the solar cell by trapping the light inside the cell. Some examples include using textured surfaces on the top layer of the cell to reduce light reflection, and reflecting light that passes through the cell back to the active region. Light trapping can increase the optical path length of photons in the cell, enabling much thinner devices.

Real-World Applications

Solar cells have many practical uses in the real world. Here are some of the most common applications:

Grid-Tied Systems

Grid-tied systems, also called on-grid systems, are connected to the utility grid. They allow any excess electricity generated by the solar panels to be fed back into the grid. This helps reduce electricity bills. Grid-tied systems with net metering can even result in a credit for the household if they generate more than they use over a billing period.

Off-Grid Systems

Off-grid systems operate independently of the utility grid. They are popular for remote locations where connecting to the grid is impractical or very expensive. Off-grid systems require batteries to store excess electricity for use when the sun isn’t shining. They can power anything from single homes to villages to RV’s.

Solar Vehicles/Planes

Solar cells can directly convert sunlight into electricity to power vehicles and equipment. Solar-powered cars, boats, planes, and even spacecraft have been engineered. While sunlight is intermittent, improvements in energy storage technologies like batteries continue to advance solar transportation.

Future Outlook

The future looks very promising for solar cells and solar energy. There are several emerging technologies and trends that should lead to wider adoption and lower costs:

Emerging Technologies

– Perovskite solar cells use abundant, inexpensive materials and have shown efficiencies comparable to silicon photovoltaics. With more research, perovskite cells could be a game-changer.

– Organic solar cells use carbon-based materials that can be printed or coated in thin films over large areas. While efficiencies are currently low, organic PV offers lightweight, flexibility and potential for new applications.

– Concentrator photovoltaics (CPV) use lenses or mirrors to concentrate sunlight onto high-efficiency multi-junction solar cells. CPV can achieve very high efficiencies but requires direct sunlight.

Cost Reductions

– Economies of scale, manufacturing improvements and material innovations are expected to continue driving down solar PV costs. Costs have already dropped over 90% in the last decade.

– New financing and business models like solar leasing provide affordable access to solar energy with little to no upfront costs.

Projections

– Solar energy is one of the fastest growing renewable energy sources, with annual growth rates over 20%. It’s projected to become a mainstream electricity source and make up over 20% of global supply by 2050.

– With policy support, falling prices and technological improvements, solar PV adoption is poised for massive expansion worldwide.