Every hour, the Sun delivers more energy to Earth's surface than humanity uses in an entire year. Capturing even a tiny fraction of that energy could power our homes, cities and industries. Solar panels do exactly that, quietly converting sunlight into electricity on rooftops, in deserts and even on spacecraft. Solar power has become one of the fastest-growing and cheapest sources of electricity in much of the world. But how does a thin slab of silicon turn light into electric current, with no moving parts and no fuel? This explainer describes the science of solar cells, from photons to the power socket.
Light is energy
Sunlight is made of tiny packets of energy called photons. Each photon carries an amount of energy that depends on its wavelength: blue and ultraviolet photons carry more energy, red and infrared photons less. When photons strike a material, they can transfer their energy to electrons within it.
In 1905, Albert Einstein explained the photoelectric effect, in which light knocks electrons out of a metal surface. This work, which helped establish quantum theory, earned him the Nobel Prize in Physics in 1921. Solar cells use a related phenomenon called the photovoltaic effect, first observed by the French physicist Edmond Becquerel in 1839, in which light creates a voltage and current in a material.
Semiconductors: the heart of solar cells
Most solar cells are made of silicon, a semiconductor. Semiconductors conduct electricity better than insulators like glass but worse than metals like copper. Their special properties come from how their electrons are arranged.
In a silicon crystal, each silicon atom shares its four outer electrons with neighboring atoms, forming stable bonds. Normally, these electrons are held in place. But if an electron absorbs enough energy, for example from a photon, it can break free and move through the crystal, leaving behind a "hole," a missing electron that behaves like a positive charge. The minimum energy needed to free an electron is called the band gap. For silicon, it is about 1.1 electronvolts, corresponding to near-infrared light.
Doping and the p-n junction
Freeing electrons is not enough to make electricity. If nothing guides them, the freed electrons would simply wander and recombine with holes, releasing their energy as heat. Solar cells need a way to push the electrons in one direction. This is done by creating a p-n junction.
Engineers add small amounts of other elements to silicon, a process called doping:
- N-type silicon is doped with elements like phosphorus, which have five outer electrons. The extra electrons are free to move, giving the material extra negative charge carriers.
- P-type silicon is doped with elements like boron, which have three outer electrons. This creates extra holes, positive charge carriers.
When n-type and p-type silicon are joined, electrons from the n-side diffuse into the p-side near the boundary, and holes move the other way. This creates a thin region called the depletion zone, with a built-in electric field pointing from the n-side to the p-side.
How a solar cell generates electricity
- Absorption: Sunlight passes through an anti-reflective coating and is absorbed in the silicon.
- Excitation: Photons with enough energy free electrons, creating electron-hole pairs.
- Separation: The electric field at the p-n junction pushes electrons toward the n-side and holes toward the p-side.
- Collection: Thin metal contacts on the front and back of the cell collect the electrons.
- Current: When the cell is connected to an external circuit, electrons flow through the circuit, powering devices, and return to the p-side to recombine with holes.
A single silicon solar cell produces about 0.5 to 0.6 volts. To produce useful voltages and power, many cells are connected together in a solar panel or module, typically 60, 72 or more cells, protected by glass and an aluminum frame.
Direct and alternating current
Solar panels produce direct current (DC), where electricity flows in one direction. Homes and the electric grid use alternating current (AC), where the current reverses direction many times per second. An inverter converts DC from the panels into AC for household use. Systems can use a single central inverter, or small microinverters attached to each panel, which can improve performance when some panels are shaded.
Efficiency: why not 100 percent?
The efficiency of a solar panel is the percentage of sunlight energy it converts into electricity. Most commercial silicon panels today have efficiencies of about 20 to 23 percent. Several factors limit efficiency:
- Photons with too little energy below the band gap pass through without freeing electrons.
- Photons with excess energy free electrons, but the extra energy is lost as heat.
- Reflection: Some light reflects off the surface, although anti-reflective coatings and textured surfaces reduce this.
- Recombination: Some electrons recombine with holes before they can be collected.
- Resistance: Some energy is lost as electrons travel through the cell and wires.
For a single-junction silicon cell, physicists have calculated a theoretical maximum efficiency of about 29 to 33 percent, known as the Shockley-Queisser limit. Multi-junction cells, which stack layers of different materials that absorb different parts of the spectrum, can exceed this limit and reach efficiencies above 40 percent in laboratories, but they are expensive and used mainly on satellites and in concentrated solar systems.
Types of solar panels
| Type | Description | Typical efficiency |
|---|---|---|
| Monocrystalline silicon | made from single silicon crystals; uniform dark appearance | about 20 to 23 percent |
| Polycrystalline silicon | made from many small crystals; bluish, speckled look | about 15 to 18 percent, now less common |
| Thin-film | very thin layers of materials like cadmium telluride on glass or flexible surfaces | about 12 to 19 percent |
| Perovskite and tandem | emerging technologies, sometimes layered on silicon | laboratory records above 30 percent for tandem cells |
What affects solar panel output
- Sunlight intensity: Panels produce the most power in direct, strong sunlight, typically around midday.
- Angle and orientation: In the Northern Hemisphere, panels facing south and tilted at an angle close to the local latitude usually capture the most energy over a year.
- Clouds: Panels still produce electricity on cloudy days, though much less, often 10 to 25 percent of their full output.
- Shading: Even partial shade on part of a panel can significantly reduce output, depending on how the cells are wired.
- Temperature: Surprisingly, solar panels work less efficiently when hot. Their output typically drops by about 0.3 to 0.5 percent for every degree Celsius above 77 °F (25 °C). Cold, sunny days are ideal.
- Dirt and snow: Dust, pollen and snow block light. Rain often cleans panels naturally.
Storing solar energy
The Sun does not shine at night, and output varies with weather. To use solar energy when it is not sunny, systems can:
- Use batteries, such as lithium-ion home batteries, which store excess energy during the day for use in the evening.
- Feed electricity into the grid, which balances supply from many sources.
- Combine with other energy sources, like wind, hydropower and storage, in a mixed electricity system.
Lifespan and recycling
Solar panels are durable. Most manufacturers guarantee that panels will still produce about 80 to 90 percent of their original output after 25 years. Panels slowly degrade, typically by about 0.5 percent per year. At the end of their life, panels can be recycled to recover glass, aluminum, silicon and metals, although recycling infrastructure is still developing in many regions.
Solar power around the world
Solar energy has grown remarkably fast. The cost of solar modules has fallen by roughly 90 percent since around 2010, driven by mass production, better manufacturing and improved cell designs. Today, large solar farms are among the cheapest ways to generate new electricity in many countries. Solar panels power everything from homes and businesses to remote villages, water pumps, road signs and satellites. In sunny regions, rooftop panels can cover a large share of a household's electricity needs, while utility-scale plants spread across deserts and farmland feed power into national grids.
Solar energy also fits well with other technologies. Electric vehicles can charge during sunny hours, heat pumps can run when solar output is high, and smart systems can shift energy use, such as running a dishwasher or washing machine, to the middle of the day. As storage becomes cheaper, more solar electricity can be saved for evenings and cloudy periods.
Solar panels and the environment
Manufacturing solar panels requires energy and materials, including silicon, glass, aluminum and small amounts of silver and other metals. Studies of the full life cycle of panels show that a typical panel produces enough electricity to repay the energy used to make it within about one to three years, depending on location, and then continues producing clean electricity for decades. During operation, panels emit no greenhouse gases or air pollution and make no noise, which is why they play a central role in plans to reduce emissions from electricity generation.
Questions readers ask
Do solar panels work on cloudy days? Yes, but they produce less electricity because less sunlight reaches them.
Why are solar panels only about 20 percent efficient? Some photons have too little energy to free electrons, others have more energy than needed and lose it as heat, and some light is reflected or lost in other ways.
Do solar panels work at night? No. They need light. Batteries or the grid supply electricity at night.
Does heat make solar panels work better? No. Panels become less efficient as they get hotter. Bright, cool conditions are best.
How long do solar panels last? Typically 25 to 30 years or more, gradually losing a small amount of efficiency each year.
In short
Solar panels turn sunlight into electricity through the photovoltaic effect. Photons free electrons in silicon, and the electric field at a p-n junction pushes them in one direction, creating a current that flows through an external circuit. Many cells are combined into panels, and inverters convert their direct current into the alternating current used in homes. Efficiency is limited by physics to about 20 percent for typical panels, and output depends on sunlight, angle, shading and temperature. With batteries and the grid, solar power has become a clean, reliable and increasingly affordable way to harness the energy of the Sun.