What Is The Photovoltaic Effect In Solar Cells?

The photovoltaic effect is the basic physical process that enables solar cells to convert sunlight into electricity. It was first discovered in 1839 by French physicist Edmond Becquerel. He found that certain materials would produce small amounts of electric current when exposed to light.

In a solar cell, the photovoltaic effect refers specifically to the generation of voltage and electric current when sunlight hits the cell. Photons from the incoming light transfer their energy to electrons in the solar cell’s semiconductor material. This energy causes the electrons to break free from their atomic bonds and flow as electric current. This conversion of light (photons) to electricity (voltage and current) is the photovoltaic effect.

Understanding and harnessing the photovoltaic effect has enabled the development of solar cells and solar panels that provide renewable electricity from sunlight. It is a pivotal discovery that underpins photovoltaic power generation.

How Solar Cells Work

Solar cells are essentially semiconductor diodes made of materials that exhibit the photovoltaic effect. When sunlight is absorbed in the semiconductor material, it excites electrons to a higher energy state, creating electron-hole pairs. The built-in electric field of the P-N junction separates the electrons and holes, generating a flow of current through an external circuit. This process of converting sunlight into electricity through the photovoltaic effect is called the photovoltaic effect.

The basic components of a solar cell include:

• A P-type semiconductor layer with excess holes.
• An N-type semiconductor layer with excess electrons.
• A P-N junction between the P-type and N-type layers.
• Metal contacts on the top and bottom to collect current.

When sunlight strikes the solar cell, photons with enough energy are absorbed in the semiconductor, causing electrons to transition to a higher energy state. This frees them from their bonds and creates mobile electron-hole pairs. The P-N junction provides an internal electric field that separates and collects the electrons on the N-side, and holes on the P-side. This generates a voltage/current through the external circuit.

Photovoltaic Materials

The most common material used in solar cells is silicon. Crystalline silicon cells dominate the PV market, representing over 90% of solar cell production. Silicon cells come in two main types – monocrystalline and polycrystalline. Monocrystalline silicon is made from single crystal ingots, giving it a uniform look and higher efficiencies than polycrystalline silicon. Polycrystalline silicon is made from blocks of melted and recrystallized silicon, giving it a speckled blue appearance due to the random crystal orientations.

Other semiconductor materials are also used in solar cells, including:

• Cadmium telluride (CdTe) – Thin film solar cells made from cadmium telluride offer lower production costs compared to silicon. However, cadmium is toxic and supply is limited.
• Copper indium gallium selenide (CIGS) – Made from a thin film of copper, indium, gallium and selenium. CIGS cells have high absorption and stable performance but use rare materials.
• Gallium arsenide (GaAs) – Very high efficiency but high material costs limit GaAs to specialty applications like satellites.
• Perovskites – Emerging thin film made from perovskite crystal structured compounds. Perovskites have achieved lab efficiencies over 25% but durability issues remain.

Research continues into advanced thin film materials and nanostructures to improve solar cell efficiency and lower costs further.

Requirements for Photovoltaic Effect

For the photovoltaic effect to occur and generate electricity, solar cells require three main components: a light source, an absorbing material, and a p-n junction.

The light source is typically sunlight, as it provides a constant stream of photons across a wide range of wavelengths from ultraviolet to infrared. When these photons strike the absorbing material in a solar cell, they can transfer their energy to the electrons in that material.

The absorbing material, known as the photovoltaic material, is a semiconductor that can efficiently absorb photons and generate electron-hole pairs. Common photovoltaic materials include silicon, cadmium telluride, and copper indium gallium selenide. The absorbed photons must have energy greater than the band gap of the semiconductor to generate electron-hole pairs.

After the electron-hole pairs are generated, the built-in electric field of a p-n junction separates the electrons and holes, creating a voltage difference that can be used to power an external load and do electrical work. The p-n junction is therefore a critical component of any working solar cell.

Light Absorption Process

The photovoltaic effect begins when photons from sunlight hit the solar cell. Photons that have energy greater than the band gap of the semiconductor material are able to excite electrons up to the conduction band, leaving behind holes in the valence band. The band gap represents the minimum energy needed for an electron to break free from its bound state. Sunlight consists of photons with a wide range of energies, but only photons with energy exceeding the band gap contribute to the photovoltaic effect.

When a photon hits an electron, the energy is absorbed and transferred to the electron, giving it enough energy to move into the conduction band. The excitation of electrons to a higher energy state is the first step in converting sunlight into electrical energy. The photon essentially knocks the electron loose, allowing it to freely move within the material. Meanwhile, the absence of the electron leaves behind a positively charged hole in the valence band. This creation of electron-hole pairs through light absorption sets up the internal electric field that leads to current and voltage from the solar cell.

The photovoltaic materials used in solar cells are specially designed to have band gap energies that match the spectrum of sunlight. This allows for efficient absorption of photons and generation of electron-hole pairs. Without the right band gap, photons would pass right through the material without exciting electrons to produce electricity.

Electron-Hole Pairs

When photons from sunlight are absorbed by the photovoltaic material in a solar cell, their energy frees electrons within the material. The excited electrons leave behind positively charged spaces known as holes. These electron-hole pairs are created within the solar cell material when the photons are absorbed. The electrons, with their negative charge, are then able to move freely. The positively charged holes also act as charge carriers. But because the holes have a positive charge, they move in the opposite direction of the negatively charged electrons. Both the electrons and holes contribute to the electrical current produced by the photovoltaic effect in solar cells.

Electric Field Separation

When light is absorbed in the solar cell, electron-hole pairs are generated. For these electrons and holes to generate electricity, they must be separated and collected. This is achieved through the electric field created across the p-n junction in the solar cell. The p-type and n-type semiconductor materials that make up the solar cell have different charge carriers – the p-type has an abundance of positive holes, while the n-type has an abundance of negative electrons.

At the interface between the p-type and n-type materials, diffusion of carriers leads to a depletion region that has a strong built-in electric field. When the electron-hole pairs are generated by light absorption, this electric field causes the electrons to move toward the n-side and the holes to move toward the p-side. This separation of charge carriers across the p-n junction allows electrons to flow through an external circuit, generating electricity. Without this electric field to break apart the electron-hole pairs, the electrons and holes would quickly recombine and no current would be produced.

The electric field across the p-n junction is fundamental to the photovoltaic effect. By separating the light-generated electrons and holes, it enables their movement and the creation of usable electric current and voltage from the solar cell. The strength and design of this electric field is critical for achieving high efficiency solar energy conversion.

Solar Cell Efficiency

The efficiency of a solar cell refers to what percentage of the sunlight striking the cell gets converted into electricity. There are several factors that impact the efficiency.

Typical efficiencies for common commercial photovoltaic solar panels are in the 15% to 22% range. Top of the line experimental solar cells in laboratory conditions can reach over 40% efficiency.

The photovoltaic materials used plays a major role in determining solar cell efficiency. Materials like monocrystalline silicon, polycrystalline silicon, and gallium arsenide vary in their light absorption properties and electron behaviors, which directly impact efficiency.

Operating temperature also affects efficiency. Solar cell efficiency decreases as temperature increases. This is because higher temperatures excite more electrons into the conduction band, increasing electric current but decreasing voltage. Efficiency declines about 0.4% for every 1°C increase in temperature.

The orientation and intensity of sunlight can reduce efficiency if not optimal. Tilting solar panels at an angle helps maximize exposure to direct sunlight throughout the day. Solar cell materials are also optimized for the solar spectrum at Earth’s surface.

Ongoing research into advanced materials and nanostructures continues to push the boundaries of solar cell efficiency for both existing and novel photovoltaic materials.

Applications

The photovoltaic effect has enabled many practical applications that harness the power of sunlight. The most well-known application is solar panels, which convert sunlight into electricity for homes, businesses, utilities, and more. Solar panels can be mounted on rooftops or ground-mounted solar farms to generate clean renewable energy. Small portable solar panels are also used for charging consumer devices like phones, watches, and batteries for camping. Solar-powered calculators and watches rely on tiny solar cells to recharge their batteries and operate without external power sources.

Solar panels are a vital technology for powering satellites and space vehicles, enabling missions far from the sun’s energy. Every satellite launched today contains solar cells to generate power. The International Space Station orbits earth with nearly an acre of solar panels providing electricity. Future concepts for solar-powered spacecraft include solar sails propelled by sunlight. The Mars rovers Spirit, Opportunity, Perseverance and Curiosity have relied on solar panels to explore the Martian surface. Solar power allows satellites, space stations, and spacecraft to operate for years without fuel.

Off-grid solar power systems with battery storage provide electricity in remote areas without access to the grid. Portable folding solar panels allow camping and outdoor recreational activities powered by the sun. The photovoltaic effect makes it possible to utilize solar energy virtually anywhere the sun shines for an ever-widening array of practical renewable energy applications.

Future Outlook

The future looks bright for advancements in photovoltaic solar cell technology. Researchers are working on developing new materials and manufacturing processes to improve efficiency and lower costs.

Perovskite solar cells are a promising new material gaining attention. They are inexpensive to manufacture and can reach higher efficiencies than traditional silicon cells. With more research into making them durable and stable, perovskites could become a major player in solar technology.

Thin film cells made from materials like cadmium telluride are becoming more popular as a lightweight and flexible option. Manufacturing techniques like printing solar cells are being refined to enable mass production and widespread adoption.

Solar power generation is expanding beyond rooftops to integration into building materials, vehicles, and other devices. Creative applications will help solar deliver an increasing share of the world’s energy needs.

With solar already among the cheapest sources of electricity in many areas, ongoing innovations promise to make photovoltaic energy even more competitive and versatile for meeting future energy demands in a sustainable manner.