What Is Photovoltaic Effect Mean?

What is the Photovoltaic Effect?

The photovoltaic effect is the process through which light is converted into electricity. It describes the interactions between light and matter that give rise to the production of voltage or electric current within a material. The photovoltaic effect was first discovered in 1839 by French physicist Alexandre-Edmond Becquerel.

Becquerel found that certain materials would produce small amounts of electric current when exposed to light. He showed that electrodes made of these materials, when placed in an electrolyte solution and exposed to light, could produce voltage and electricity. This demonstrated for the first time that light could directly be converted into electricity. The materials exhibiting this effect came to be known as photovoltaic materials, and the phenomenon itself became known as the photovoltaic effect.

In the photovoltaic effect, photons (particles of light) are absorbed by the atoms within a semiconductor material. This photon absorption excites electrons within the material to higher energy states. The electrons can then move freely through the material, producing an electric current. This current, along with the internal electric fields of the material, leads to the generation of voltage. Overall, the photovoltaic effect converts luminous energy into electrical energy.

How Does the Photovoltaic Effect Work?

The photovoltaic effect involves photons from sunlight knocking electrons into a higher state of energy to create electricity. When photons hit the solar cell, they transfer their energy to the electrons in the atoms of the semiconductor material. This energizes the electrons enough to break free of their atomic bonds and flow as electric current.

Solar cells are made of a positively charged (P-type) semiconductor layer and a negatively charged (N-type) semiconductor layer forming a P-N junction. The interface between the positive and negative layers forms an electric field. When photons strike the solar cell, electrons are knocked loose in both layers. The electrons want to flow from the N-type side to the P-type side, but the electric field at the junction separates the charges and creates voltage potential, forcing electrons out through metal contacts as electric current.

In summary, the photovoltaic effect converts sunlight directly into electricity through photons exciting electrons across the P-N junction in a solar cell. This generates usable electric current without any mechanical or heat conversion processes.

Applications of the Photovoltaic Effect

The photovoltaic effect has enabled the development of several important optoelectronic devices that convert light into electricity. The most well-known application is in solar cells, also known as photovoltaic cells. Solar cells contain semiconductor materials that absorb photons and generate an electric current. Individual solar cells are connected together to form solar panels or modules.

Solar cells and panels have a wide range of uses. They are commonly used to provide electricity in areas without access to an electrical grid, power satellites and space probes, and increasingly to supplement grid energy and feed into the grid through net metering programs. Solar power generation is growing rapidly due to improving efficiency and falling costs.

Photodiodes are another common application of the photovoltaic effect. Photodiodes are semiconductor devices that convert light into current. They are used for detecting optical signals and measuring light intensity in a variety of instruments. Applications include pulse oximeters, cameras, DVD players, and smoke detectors.

Phototransistors are a type of bipolar transistor that use light instead of a base current to control the collector current. They function similarly to a photodiode but with current gain. Phototransistors are commonly used in light detection, counting, and switching applications.

Factors Affecting Solar Cell Efficiency

The efficiency of a solar photovoltaic cell depends on several factors related to its materials, design, and operating conditions. Three major factors that influence solar cell efficiency are:

Semiconductor Materials

The semiconductor material used in the solar cell determines its ability to absorb sunlight and convert it into electricity. Materials like crystalline silicon, gallium arsenide, and cadmium telluride have different solar absorption properties that affect efficiency. Engineers aim to use materials with high light absorption for the solar spectrum.

P-N Junction Depth

Solar cells work due to the photovoltaic effect at the P-N junction. The depth of this junction below the surface determines how many photogenerated carriers are lost before they can be collected. Shallow junction depths help minimize surface recombination and improve efficiency. However, too shallow of a junction can increase resistance. Optimization is needed.

Level of Sunlight Exposure

The intensity and wavelength spectrum of sunlight exposure impacts the amount of electricity generated. Peak performance requires direct perpendicular exposure on a clear day. Cloud cover, pollution, sunrise/sunset angle, and other factors reduce the solar resource available to create current flow.

History of Solar Cell Development

The modern photovoltaic effect was first uncovered in 1839 by French physicist Alexandre-Edmond Becquerel. At just 19 years old, Becquerel conducted experiments exposing electrodes in electrolyte solutions to light and found that small amounts of electric current were generated. This discovery marked the first time that light was directly converted into electricity.

Building on Becquerel’s findings, in 1883 American inventor Charles Fritts created the very first solar cell made from selenium coated with an extremely thin layer of gold. Fritts’ solar cells had less than 1% efficiency, but paved the way for future improvements.

The next major development came in 1904 when Albert Einstein published a description of the photoelectric effect, for which he later won the Nobel Prize. Einstein’s paper explained the physics underpinning solar cells.

In 1916, American inventor William J. Bailey created a solar collector with copper coils and an insulated box. This was one of the first instruments to efficiently use the sun’s heat energy.

The modern silicon solar cell was invented in 1954 by researchers Gerald Pearson, Daryl Chapin, and Calvin Fuller while working at Bell Labs. They built on prior research to create the first practical solar cell, which had 6% efficiency.

Space programs drove rapid advances in solar technology in the 1950s-70s to provide power aboard satellites. Notable improvements came from NASA engineer Elliot Berman, who increased silicon solar cell efficiency to 10%

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In more recent decades, continued engineering has pushed lab solar cell efficiencies to over 40%. Ongoing research aims to improve manufacturing processes and make solar power more cost competitive with fossil fuels.

Current Solar Cell Technologies

There are several main types of solar cells in use today:

Crystalline Silicon Cells

Crystalline silicon cells are the most common type of solar cells, making up over 90% of the market. They are made from either single crystalline or multi-crystalline silicon. Single crystalline cells are more efficient but more expensive to produce. Multi-crystalline cells contain small crystals and defects but are cheaper.

Thin Film Cells

Thin film solar cells are made by depositing one or more thin layers of photovoltaic material onto a substrate. Types include amorphous and micromorph silicon, cadmium telluride (CdTe), and copper indium gallium selenide (CIGS). Thin film cells are less efficient than crystalline silicon but cheaper to manufacture.

Emerging Advanced Cells

Emerging solar cell technologies aim to improve efficiency and lower costs further. These include multi-junction or tandem cells that use multiple semiconductor layers to absorb a broader spectrum of light. Dye-sensitized solar cells use photosensitive dyes on a nano-porous surface. Perovskite cells are also promising due to low production costs and high efficiencies.

Solar Cell Efficiency Records

Researchers are constantly working to improve solar cell efficiency in order to increase the amount of electricity that can be generated from sunlight. In laboratory settings, scientists have achieved record-breaking solar cell efficiency levels as high as 47.1% for multi-junction concentrator solar cells as of 2020. However, these ultra-high efficiency solar cells use expensive materials and concentrators to focus sunlight, making them impractical for widespread commercial use.

For commercial silicon solar panels, the records are more modest but still impressive. The highest efficiency achieved for a monocrystalline silicon solar cell in a lab is 26.7%, while the record for a multicrystalline silicon cell is 22.3%. The most efficient commercially available silicon solar panels typically range from 15-22% efficiency.

While higher efficiency is desirable, it often comes with a tradeoff of higher manufacturing costs. So panel makers aim for an optimal balance between efficiency and affordability. Continuing to raise commercial solar cell efficiency levels while reducing costs is an ongoing focus of renewable energy research.

Solar Cell Manufacturing

The manufacturing process for solar cells generally involves several steps, starting with the production of very pure crystalline silicon. This silicon is then shaped into ingots or bricks and sliced into extremely thin wafers. Next, these wafers undergo surface etching and texturing to reduce reflections and increase light absorption. They are also doped with other elements like phosphorus and boron to form the P-N junction.

A common manufacturing technique is to print metal contacts directly onto the doped silicon using screen printing or photolithography. An anti-reflective coating is also typically applied. The individual cells are then interconnected and encapsulated to form a complete solar module. Automation and quality control systems are critical for high throughput and consistency.

The primary costs involved in solar cell production are the initial capital expenses of setting up factories with clean rooms, crystal growth furnaces, wafer saws, diffusion furnaces, metallization systems, and testing equipment. There are also ongoing costs for consumables like gases, electricity, water, and materials. Labor, maintenance, and overheads are additional expenses.

China has become the dominant player in solar cell and panel manufacturing. The country produces over two-thirds of the world’s solar photovoltaics today. Other major manufacturing hubs include Southeast Asia, Europe, and North America. Automation and economies of scale have steadily driven down costs, but further innovations in production techniques and supply chain management will be needed to make solar electricity more cost competitive.

Challenges for Wider Adoption of Solar Power

Despite the continuing improvements in solar cell technology and manufacturing, some key challenges remain for the wider adoption of solar power, especially when compared to established fossil fuel energy sources like coal, oil and natural gas.

One of the biggest barriers is still the higher cost of solar power on a kilowatt-hour (kWh) basis. The levelized cost of electricity from solar photovoltaics, even with declining prices, remains higher than fossil fuel generation in many markets. This is especially true when factoring in the cost of storage to address the intermittent nature of solar power.

The need for increased energy storage and transmission infrastructure is another key challenge. Solar energy can only be captured during daylight hours, requiring expensive batteries or other storage solutions. Transmitting solar electricity from point of generation to point of use also requires major grid infrastructure investments.

The large surface area required for solar installations also leads to land use and aesthetic concerns. Rooftop systems avoid land usage issues but leave many aesthetically opposed to the look of photovoltaic panels on housing and commercial developments. Large solar farms require clearing substantial land areas which could otherwise serve agricultural, ecological or other purposes.

Future Outlook

The potential for solar cell efficiency improvements continues to drive research and development in photovoltaics. Novel solar cell designs, advanced materials like perovskites, and nanotechnology applications could enable solar cells to reach over 50% efficiency in the future.

Emerging applications for solar cells also show promise, like building-integrated photovoltaics (BIPV). Solar window technology allows windows to generate electricity from natural light while remaining transparent. Widespread adoption of solar windows and other BIPV could greatly expand solar adoption in urban areas.

The growth of renewable energy worldwide is projected to accelerate in the coming decades. Solar power’s modular and scalable nature makes it an ideal source for much of this growth. Continued declines in solar cell costs coupled with efficiency improvements will enable solar photovoltaics to play a major role in the global transition to sustainable energy.