What Is Energy Converted Into In A Solar Cell?

A solar cell, also known as a photovoltaic cell, is an electrical device that converts sunlight directly into electricity. The basic principle of how solar cells work is the photovoltaic effect – when sunlight hits the solar cell, the photons from the sunlight dislodge electrons in the solar cell, creating an electric current that is captured and turned into usable electricity. Solar cells are made from semiconductors such as silicon and are combined together to form solar panels or modules. The purpose of solar cells is to harness the sun’s immense energy and convert it into a renewable electricity source that can power homes, buildings, and the electric grid.

Sunlight

Sunlight is composed of photons, which are packets of electromagnetic energy from the sun [1]. The sun produces an enormous amount of energy in the form of electromagnetic radiation. Approximately 63 million watts per square meter of sunlight reaches Earth’s outer atmosphere. When this solar radiation hits the atmosphere, about 30% is reflected back into space while the rest passes through the atmosphere. By the time sunlight reaches Earth’s surface, the intensity is reduced to about 1,000 watts per square meter on a perpendicular surface [2].

The photons in sunlight have a wide range of energies and wavelengths across the electromagnetic spectrum. However, the majority of the energy is concentrated in the visible light and infrared radiation bands. When photons from sunlight strike materials like solar cells, their energy can excite electrons in the material. This interaction between photons and electrons is key to how solar cells can convert sunlight into electricity.

Photovoltaic Effect

The photovoltaic effect is the process that allows solar cells to generate electricity from sunlight. It involves photons from sunlight being absorbed by materials like silicon, causing the energy levels of electrons in the material to increase. This results in electron excitation, where electrons move to a higher energy state. The electrons can then be captured and directed into an electric circuit, generating a flow of electricity.

Specifically, when photons from sunlight hit the solar cell, they transfer their energy to electrons in the solar cell’s semiconductor material like silicon. This extra energy frees the electrons from their normal positions associated with silicon atoms in the crystal lattice. The excited electrons can then move freely within the semiconductor material.

The solar cell has an electric field built into its design that forces these free electrons in one direction, creating an electric current. Metal conductive plates on the solar cell collect the electrons on the front and back, connecting the flow of electrons to the external circuit. This one-way flow of electrons extracted from the semiconductor material is the electric power produced by the photovoltaic effect.

Overall, the photovoltaic effect allows solar panels to absorb sunlight and convert it directly into usable electricity through excitation of electrons in the solar cell’s material. This process is crucial for generating renewable solar energy.

Source: https://energyeducation.ca/encyclopedia/Photovoltaic_effect

Solar Cell Structure

The basic structure of a solar cell consists of silicon semiconductor material sandwiched between two electrodes. The top electrode is a transparent conductive oxide layer that allows sunlight to pass through to the silicon. This is coated with an anti-reflective coating to reduce light reflection and allow more light to enter the cell. Underneath is an n-type silicon layer, a p-type silicon layer, and then the bottom electrode.

The silicon in a solar cell is normally 200-300 microns thick. The silicon wafer acts as the substrate, providing mechanical support. It is lightly doped compared to the other layers. The p-type silicon layer on the bottom contains boron impurities which give it a surplus of holes. The n-type silicon layer on top contains phosphorus impurities which provide extra electrons. The p-n junction between these two layers sets up an electric field.

On the very top is a thin anti-reflective coating made of silicon nitride or titanium oxide which increases light absorption by reducing reflection off the front surface. Under this is the transparent conductive oxide electrode layer which collects electrons. This is normally indium tin oxide. On the rear is a metal electrode layer which collects holes and channels electric current. This back contact is normally made of aluminum.

Excitation of Electrons

In a solar cell, photons from sunlight hit the electrons in the silicon layer causing them to become excited and jump from the valence band to the conduction band [1]. This process is known as the photovoltaic effect. When photons hit the electrons, they transfer their energy to the electrons. If the photon energy is equal to or greater than the band gap of the semiconductor, the electron will have enough energy to break free of its bound state and jump to the conduction band. This leaves behind a hole in the valence band. The excitation of electrons to the conduction band is what generates an electric current in a solar cell.

Electric Current

Inside the solar cell, the photons excite electrons in the silicon and give them enough energy to break free from their atoms. This creates free electron-hole pairs as the excited electrons move into the n-type silicon layer, leaving holes with positive charge behind in the p-type layer.https://www.eia.gov/energyexplained/solar/photovoltaics-and-electricity.php These free electrons in the n-type layer and holes in the p-type layer create an electric field across the junction between the two silicon layers. The electrons then flow from the n-type side to the p-type side, creating an electric current. This current can be captured and utilized before the electrons flow back to fill the holes in the p-type layer.

Inverters

Inverters play a critical role in solar power systems by converting the DC electricity produced by solar panels into usable AC electricity that can power homes and the grid. Solar panels produce DC current, but most household appliances and electrical equipment run on AC current. An inverter converts the DC output into 120-volt, 60-Hz AC power that is compatible with the electrical system.[1]

There are two main types of inverters used in solar systems: string inverters and microinverters. String inverters are central inverter units that convert the combined DC output from multiple solar panels into AC current. Microinverters are small units installed on each individual solar panel to convert the DC output of that panel to AC.[2]

When selecting an inverter, it’s important to properly size it to handle the solar array’s expected power output. Oversizing can lead to inefficiency while undersizing can cause instability or failure to convert all the DC power to usable AC electricity. Most solar installers will recommend the appropriately sized inverter for a given home solar system.

Output Electricity

The main purpose of a solar cell is to convert light energy from the sun in the form of photons into useful electrical energy that can then be used to power electrical devices, equipment, and grids (1). This conversion happens due to the photovoltaic effect, where photons from sunlight strike the solar cell and excite electrons in the semiconductor material to jump to a higher energy state. The movement of these high energy electrons in the solar cell produces an electric current that can then be captured and utilized (2).

While sunlight hitting the solar cell provides the energy input, the electrical output depends on factors like the efficiency of the solar cell, intensity of the sunlight, temperature, and presence of any shading. High-quality solar cells with higher efficiency will be able to convert a greater percentage of the sunlight into usable electricity. The intensity of the incident sunlight also impacts the electrical output. Solar cells produce more electricity on sunny days compared to cloudy days when the intensity of sunlight is lower. Operating temperature of the solar cell also plays a role. Solar cells work best within a certain optimal temperature range – too cold or too hot can negatively impact efficiency. Any shading, even partial shading, on the solar panels can also significantly reduce electrical output.

The direct current electricity generated by solar cells is fed into inverters, which convert the DC into alternating current or AC electricity. This AC power is then ready to be fed into electrical loads or grids for consumption, making the original light energy from the sun available as usable electric power (3).

(1) https://www.mrsolar.com/photovoltaic-effect/
(2) https://www.energy.gov/eere/solar/articles/photovoltaic-system-basics
(3) https://www.energy.gov/eere/solar/articles/how-do-solar-panels-work

Efficiency

The efficiency of a solar cell refers to the percentage of solar energy shining on the cell that is converted into usable electricity. Currently, most commercially available solar panels have efficiencies ranging from 15% to 22%.¹ However, researchers are continually working to improve solar cell efficiency.

Ongoing efforts to boost efficiency focus on advanced solar cell designs, novel materials, and new manufacturing techniques. Multi-junction solar cells, which have multiple layers to absorb different wavelengths of light, can achieve over 46% efficiency in lab conditions.² Other promising research directions include organic photovoltaics, quantum dot solar cells, and perovskite solar cells. While commercial applications lag behind laboratory achievements, higher efficiency solar panels could become economically viable in the future.

Conclusion

In summary, solar cells convert sunlight into electricity through the photovoltaic effect. Sunlight is composed of photons, which are particles of solar energy. When these photons strike the solar cell, they excite electrons in the semiconductor material used to make the solar cell. The excitation of electrons creates an electric current that can then be captured and utilized.

Solar cells play a critical role in renewable energy production. As concerns mount over climate change and fossil fuel depletion, solar power provides a clean and sustainable alternative for electricity generation. Solar cells will continue to become more efficient and cost-effective over time, allowing this renewable energy source to make up a larger percentage of the world’s energy mix. Widespread adoption of solar power will be an important step in fighting global warming and transitioning to a carbon-neutral economy.