How Does a Solar Cell Turn Sunlight into Electricity? | How It Works

What is the short answer?

A solar cell is a thin slice of semiconductor, usually silicon, that turns light straight into electricity. Sunlight arrives as tiny packets of energy called photons. When the cell absorbs a photon with enough energy, an electron inside the silicon is set free.

A built-in electric field, made by joining two differently treated layers of silicon, pushes the freed electrons one way. They can only get back through an outside circuit, and that flow is the electric current.

Definition: The photovoltaic effect is the creation of a voltage and a current in a material when light falls on it. A solar cell, or photovoltaic (PV) cell, is a device built to use this effect.

How does light hand its energy to silicon?

Each photon carries a fixed amount of energy that depends only on its frequency, f, or its wavelength, λ:

E = hf = hc/λ

Here h is the Planck constant, 6.62607015 × 10⁻³⁴ joule seconds, and c is the speed of light, 299,792,458 metres per second; NIST lists both as exact. Physicists measure photon energies in electronvolts (eV): exactly 1.602176634 × 10⁻¹⁹ joules. A blue photon carries more energy than a red one, because its wavelength is shorter.

In a solid, electrons can only hold certain ranges of energy. In the valence band, they are held in the bonds between atoms. In the conduction band, they are free to carry current. The step between the two is the band gap. HyperPhysics at Georgia State University, citing Kittel's 1986 textbook, gives silicon's band gap as 1.11 eV at room temperature.

• Below the band gap: the photon cannot lift an electron across, so silicon does not absorb it.
• Above the band gap: the photon frees an electron, and the energy above the gap is lost as heat.

Each absorbed photon leaves an electron-hole pair: a free electron and a hole, the gap in a bond where the electron was. A hole behaves like a moving positive charge.

How does a p-n junction drive the current?

Left alone, a freed electron would soon fall back into a hole. A solar cell prevents this by doping: adding tiny amounts of other elements. Phosphorus has five outer electrons to silicon's four, so it adds free electrons and makes n-type silicon. Boron has three, so it creates holes and makes p-type silicon.

Where the two layers meet, a p-n junction forms. Electrons spread into the p side and holes into the n side, leaving fixed charged atoms behind: positive on the n side, negative on the p side. This thin zone, cleared of free carriers, is the depletion region, and its charges set up a built-in electric field.

The field pushes each freed electron towards the n side and each hole towards the p side. If the student joins the two metal contacts through a lamp, electrons flow round the outside circuit to meet holes on the p side.

• The open-circuit voltage is the largest voltage a cell gives, with no current flowing. The teaching site PVEducation says commercial silicon cells typically reach about 690 millivolts, and cells on high-quality single-crystal silicon up to 764.
• The short-circuit current flows when the contacts are joined directly, so the voltage is zero: about 28 to 35 milliamps per square centimetre for commercial silicon cells, the same site says.

How do cells become a working solar power system?

One silicon cell gives well under one volt, so many cells are wired together in a sealed module, often called a solar panel. Modules are joined into arrays on a roof or across a field.

Cells make direct current (DC), which flows one way. Homes use alternating current (AC). An inverter converts DC to AC; the US Department of Energy describes central, string and per-panel microinverters. How networks take in this output is covered by Energy Systems and the Power Grid.

A module's rating, in watts, is measured under standard test conditions (STC): sunlight of 1,000 watts per square metre, a cell temperature of 25 °C and a standard spectrum called air mass 1.5, as PVEducation sets out. Real sunlight is often weaker, and cells in full sun often run hotter.

What limits a solar cell's efficiency?

A cell's efficiency is the share of the sunlight's power that comes out as electricity. No cell reaches 100%, partly because of the band gap losses above.

In 1961, William Shockley and Hans Queisser calculated the ceiling for a cell with one junction, the Shockley-Queisser limit. It is about a third: MIT News, in 2013, gave 34 percent for a single, optimised junction. Germany's Fraunhofer Institute for Solar Energy Systems (Fraunhofer ISE) gives a lower physical limit for silicon cells: 29.4 percent.

Records go higher by stacking materials. In May 2022, Fraunhofer ISE reported 47.6% for a four-junction cell under sunlight concentrated 665 times. In February 2026, it reported record stacked modules of 34.2% on germanium, raised to 34.4% in June 2026, and 31.3% on silicon, and said commercially available modules already reach around 24 percent.

Heat lowers output too. PVEducation gives a temperature coefficient for silicon of about 0.4 to 0.5% less power for every degree Celsius, as the open-circuit voltage falls by about 2.2 millivolts per degree.

What kinds of solar cells are there?

• Crystalline silicon. Monocrystalline cells are cut from one large crystal; multicrystalline cells come from silicon cast as many small crystals. The US Department of Energy calls silicon by far the most common solar cell material.
• Thin film. A thin light-absorbing layer is laid on glass, plastic or metal. Cadmium telluride (CdTe) is the second most common PV material, the department says, and uses low-cost processes. CIGS (copper indium gallium selenide) reaches high lab efficiencies, but its four elements make manufacturing harder.
• Perovskites. Their efficiencies have improved faster than any other PV material's, the department says, but they must become stable enough to survive 20 years outdoors.
• Tandem cells. A tandem cell stacks two cells with different band gaps, each converting a different part of the spectrum. Fraunhofer ISE says a perovskite layer on silicon raises the theoretical limit from 29.4 to 43.3 percent; its tandem cells reached up to 33.1% in September 2025.

Where is solar power being built, and how fast?

Ember, an energy think tank, counted a record 647 gigawatts of new solar capacity worldwide in 2025, by the panels' DC rating. Solar generation rose from 2,143 terawatt hours in 2024 to 2,778 in 2025, overtaking wind power for the first time.

• China. Ember says China installed 378 of those 647 gigawatts. The National Energy Administration said in September 2026 that solar capacity had reached 1,286 gigawatts at the end of July 2026, just above coal-fired capacity of 1,285 gigawatts.
• United States. The Energy Information Administration (EIA) reports 30.8 gigawatts of utility-scale solar added in 2024 and 27.2 in 2025. Developers planned 43.4 gigawatts for 2026; the largest, Tehuacana Creek 1 in Texas, adds 837 megawatts of solar and 418 of battery storage.
• Germany. Preliminary figures from the Federal Network Agency (Bundesnetzagentur) show 16.4 gigawatts added in 2025, for a total of 117. Just under half the new capacity was on buildings, and about 430,000 plug-in balcony systems were registered.
• India. The Press Information Bureau reported in August 2026 that over 5 million households were benefiting from rooftop solar under the PM Surya Ghar: Muft Bijli Yojana scheme, with 14.8 gigawatts commissioned.

What are the debates about making and retiring solar panels?

• Supply chains. The International Energy Agency (IEA) found in July 2022 that China's share of every key stage of panel making, from polysilicon to modules, exceeded 80%. The IEA said Chinese policies helped cut solar costs by more than 80%, but said this level of concentration would be a considerable vulnerability and called for more diverse supply chains.
• Recycling. In June 2016, the International Renewable Energy Agency (IRENA) and the IEA's photovoltaic programme (IEA PVPS) estimated that panel waste, mostly glass, could total 78 million tonnes by 2050, with recoverable materials worth over USD 15 billion. Trade press reports of a newer IRENA study, published in July 2026, give over 200 million tonnes by 2050 in a scenario that limits warming to 1.5 °C.
• Land use. The US Department of Energy's Solar Futures Study projected that ground-mounted solar could need up to 0.5% of the contiguous United States by 2050. The department says siting affects communities, ecosystems and farmland, and that agrivoltaics, sharing land between panels and crops, can give farmers new economic opportunities.

How does this work with real numbers?

Both examples are illustrative; the constants, band gap, test conditions, module efficiency and temperature coefficient come from the sources above.

How does one photon's energy compare with silicon's band gap?

Take an illustrative green photon of 500 nanometres. Its frequency is f = c ÷ λ = 299,792,458 ÷ (500 × 10⁻⁹) ≈ 5.996 × 10¹⁴ hertz. Its energy is E = hf = 6.62607015 × 10⁻³⁴ × 5.996 × 10¹⁴ ≈ 3.97 × 10⁻¹⁹ joules, or about 2.48 eV.

That beats silicon's 1.11 eV band gap, so the photon frees an electron. The extra 1.37 eV (2.48 minus 1.11), about 55% of its energy, becomes heat.

The longest wavelength silicon can absorb is λ = hc ÷ band gap. With hc ≈ 1,239.8 eV nm, λ = 1,239.8 ÷ 1.11 ≈ 1,117 nm, in the infrared. An illustrative 1,200 nm photon carries about 1.03 eV and passes straight through.

How much electricity could one rooftop panel make in a day?

Table: Illustrative output of one rooftop panel

A peak sun hour is one hour at 1,000 W/m². If the cells run at an illustrative 45 °C, 20 degrees above test conditions, losing 0.4 to 0.5% per degree cuts output by 8 to 10%, to about 432 to 442 W.

Where can the student go deeper in World 101?

This page follows sunlight from a single photon to a current in a wire. The World 101 subject Renewable Energy places solar power beside the other sources that do not burn fuel, and looks at what it takes to build them on a large scale.

Three related subjects take these ideas further. Quantum Physics explains why light comes in photons and why electrons in solids sit in energy bands. Semiconductors and the Chip War follows purified silicon into computer chips and the contest over making them. Critical Minerals and Supply Chains asks where the materials for clean energy come from and who controls them.

Sources

• NIST, CODATA 2022 values for the Planck constant, speed of light and electronvolt.
• HyperPhysics, Georgia State University, band gap table from C. Kittel, 1986.
• PVEducation, C. Honsberg and S. Bowden, cell operation pages (764 mV result: A. Augusto and others, 2017), read 9 November 2026.
• US Department of Energy, pages on PV cells, inverters and solar siting, read 9 November 2026.
• W. Shockley and H. J. Queisser, Journal of Applied Physics, 1961.
• MIT News, D. L. Chandler, 18 April 2013.
• Fraunhofer ISE press releases, 30 May 2022, 5 September 2025, 17 February 2026 and 5 May 2026; news item, 10 June 2026.
• Ember, Global Electricity Review 2026, 21 April 2026 (2024 and 2025 data).
• Xinhua, citing China's National Energy Administration, 1 September 2026.
• US EIA, Today in Energy, 20 February 2026 (additions in 2024 and 2025).
• Bundesnetzagentur, renewable growth release, 8 January 2026.
• Press Information Bureau, India, PM Surya Ghar release, 4 August 2026.
• IEA, Solar PV Global Supply Chains, July 2022.
• IRENA and IEA PVPS, End-of-Life Management: Solar Photovoltaic Panels, June 2016 (projections to 2050); TaiyangNews and SolarQuarter, July 2026, reporting IRENA, End-of-life management for a circular economy: Solar PV panels.
• US Department of Energy, Solar Futures Study, September 2021 (land by 2050).

Common errors and misconceptions

• Misconception: Solar cells run on the sun's heat. Correct: They respond to light, and heat lowers a silicon cell's power.
• Misconception: Any light can make electricity in a silicon cell. Correct: Photons below the 1.11 eV band gap, beyond about 1,117 nm, pass through unabsorbed.
• Misconception: A photon with twice the energy frees twice as many electrons. Correct: In an ordinary cell, one absorbed photon frees one electron; the extra energy becomes heat.
• Misconception: Solar panels make the alternating current used in homes. Correct: Cells make direct current, which an inverter converts.
• Misconception: Better engineering could make a silicon cell convert nearly all sunlight. Correct: Fraunhofer ISE puts the physical limit for silicon at 29.4 percent; going higher needs tandem cells.

Key takeaways

• Light arrives as photons, each carrying energy E = hf = hc/λ, so blue photons carry more energy than red ones.
• Silicon absorbs photons with more energy than its 1.11 eV band gap; weaker photons pass through, and any extra energy becomes heat.
• Doping with phosphorus and boron makes a p-n junction, whose built-in field separates electrons and holes and drives current through a circuit.
• Cells are wired into modules and arrays and make direct current, which an inverter converts; ratings assume 1,000 W/m² and 25 °C.
• In ordinary sunlight, a one-junction cell cannot beat the Shockley-Queisser limit of about a third; silicon's limit is 29.4%, and the best commercial modules reach around 24%.
• Manufacturing concentration, recycling and land use are debated, with the IEA, IRENA and the US Department of Energy each stating their own findings.

Test yourself

Why can't silicon absorb a 1,200 nm photon?

Its energy, about 1.03 eV, is below silicon's 1.11 eV band gap, so it cannot free an electron.

What does the built-in field at a p-n junction do?

It pushes electrons towards the n side and holes towards the p side, so they flow through the outside circuit instead of recombining.

How do open-circuit voltage and short-circuit current differ?

The first is the largest voltage, with no current flowing; the second is the current when the contacts are joined and the voltage is zero.

Why does a panel on a hot roof make less than its rating?

The rating assumes cells at 25 °C, and hot silicon cells lose about 0.4 to 0.5% of their power per degree.

In the rooftop example, how was 2.4 kWh found?

2.0 m² × 1,000 W/m² × 0.24 gives 480 W; over 5 illustrative peak sun hours, that is 2,400 Wh.

How do tandem cells beat the single-junction limit?

They stack two cells with different band gaps, so each converts a different part of the spectrum and less of each photon's energy is lost as heat.