How Do Solar Cells Work? Light to Electricity and Its Limits

Where you see it: from rooftops to calculators

The black or dark blue boards on house roofs and in large fields are solar panels. Each panel holds dozens of solar cells, each about the size of your palm. There is no fuel and no moving part. Electricity comes out simply when light shines on it.

The thin strip on the face of a calculator is a solar cell too. So are the wing-like boards on satellites. They differ in size, but they all make electricity from light in the same way.

How it works 1: particles of light make electron–hole pairs

Light behaves like a stream of tiny packets of energy. Each packet is called a photon. To keep things simple, we will describe light here as "a stream of particles." A photon's energy depends on the color of the light. Blue light carries more energy per photon, and red light carries less.

In silicon, electrons are normally busy holding the atoms together, so they cannot move. To move freely, an electron must climb an energy "step" of a fixed size. This step is called the band gap. In silicon, it is about 1.1 eV. The eV (electron volt) is a unit for very small amounts of energy.

When a photon with more energy than the step hits, an electron climbs the step and becomes free to move. The electron leaves an empty seat behind. This empty seat acts like a particle that carries positive charge, so it is called a hole. In this way, one photon makes one electron–hole pair.

But you cannot take out electricity yet. After a while, the electron and the hole meet again and join back together. They give up the energy they gained, as heat and in other forms. If you just shine light on a plain piece of silicon, it only warms up a little. To get electricity, you must pull the electrons and holes apart, in opposite directions.

How it works 2: the force at the boundary separates electrons and holes

The job of pulling them apart is done by joining p-type and n-type silicon. Both are silicon with small amounts of other elements added. N-type silicon gets extra electrons this way, and p-type silicon gets extra holes. The boundary where the two meet is called a PN junction. A solar cell is one large PN junction that covers the whole area of the cell.

At the boundary, some electrons from the n-type side and some holes from the p-type side cross over, meet and disappear. This leaves a thin layer with almost no free electrons or holes. This layer is called the depletion layer. The n-type side, which lost electrons, becomes positively charged. The p-type side, which lost holes, becomes negatively charged. So an electric force is always at work here, pushing electrons toward the n side and holes toward the p side.

When electrons and holes made by light enter this layer, the electrons are sent to the n side and the holes to the p side. In fact, much of the light is absorbed far from the boundary. Pairs made there are still separated if they wander to the boundary. (Pairs that meet and disappear before they get there are lost.) Once separated, they cannot go back, because the force at the boundary blocks them. Electrons build up on the n side and holes on the p side, so a voltage appears between the two sides.

This effect, where light creates a voltage, is called the photovoltaic effect. Put metal contacts on the n-type and p-type surfaces, and connect them with an outside circuit. The electrons then travel through the circuit back to the p side. There they meet holes and join with them. As long as light keeps shining, this flow keeps going. This is the current from a solar cell.

How this differs from an image sensor

Phone camera image sensors use the same idea. The photodiode in each pixel is also a PN junction that collects electrons made by light. The difference is the purpose. An image sensor stores and counts the electrons to measure the amount of light. A solar cell keeps sending the electrons into an outside circuit to get power. (For the camera side, see "How Do Image Sensors Work?")

How it works 3: from cells to panels, and panels to rows

A single solar cell is called a cell. A commercial crystalline silicon cell gives about 0.7 volts when nothing is connected to it. That is only about half of an ordinary 1.5-volt battery. So dozens of cells are connected in series to raise the voltage.

Cells are laid out side by side and protected with glass and a frame. The finished board is called a module. What people usually call a "panel" is this module.

Panels are measured and compared under the same conditions, set by international standards. For example, the light is 1,000 watts per square meter and the cell temperature is 25 °C. These are called standard test conditions (STC). A rating such as "400 W" in a catalog is the value under these conditions.

Why only part of the light becomes electricity

Sunlight is a mix of many colors, from ultraviolet through visible light to infrared. In other words, it mixes high-energy and low-energy photons. A silicon solar cell cannot use all of them equally.

First, a photon with less energy than the step cannot free an electron. In silicon, this applies to infrared light with wavelengths longer than about 1,100 nm. Most of it passes straight through. A nanometer (nm) is one millionth of a millimeter.

Second, even a photon with more energy than the step gives at most the step's worth of electricity. That is because a freed electron quickly sheds its extra energy. The energy it sheds becomes heat. For example, a photon of blue light (wavelength about 450 nm) carries about 2.8 eV, so more than half of it becomes heat (our calculation).

The efficiency limit: a wall at about 34%

The share of the light's energy that turns into electricity is called the conversion efficiency. The two losses in the last section depend on the size of the material's step. With a small step, the cell can use more photons, but it gets less electricity from each one. With a large step, it gets more from each photon, but more photons pass straight through.

In a 1961 paper, Shockley and Queisser in the US worked out this trade-off. Shockley was one of the inventors of the transistor. They showed that a solar cell made with one PN junction has a theoretical upper limit on efficiency. This is called the Shockley–Queisser limit.

Using today's standard sunlight data, the limit is highest, about 33.7%, when the step is about 1.34 eV. But this number depends on the assumptions. It assumes an ideal cell, standard sunlight at ground level, and no lenses to concentrate the light. Concentrating the light raises the limit.

Silicon's step (about 1.1 eV) is a little smaller than the best value. Electrons and holes also disappear naturally inside silicon. Counting that loss too, the limit for silicon is about 29.4% (Fraunhofer Institute in Germany, 2013). In September 2026, LONGi of China announced 28.29% for a research silicon cell. The value was confirmed by ISFH, a German research institute, and it is about 96% of this limit.

Why heat and weather change the output

A solar cell produces less power as its temperature rises. When it is hot, more electrons and holes climb the step through heat alone, even without light. Then the electrons and holes separated at the boundary leak back more easily, and the voltage drops. This explanation is greatly simplified.

The rate of this drop is called the temperature coefficient. In 2024 results from PVEL, an independent testing lab, the averages for the three main panel types were about 0.26 to 0.32% per °C. At 0.3% per °C, a panel 35 °C hotter than 25 °C loses about 10% of its output (our calculation). For the same brightness, a cool day produces more power.

On cloudy days, output falls in line with the amount of light that reaches the panel. Under thick clouds, it is much lower than on a sunny day.

From the panel to your wall outlet

A solar cell makes direct current (DC), which always flows in the same direction. But the electricity in home outlets and power lines is alternating current (AC), which keeps switching direction. So a device that turns DC into AC is needed. In home solar systems, this unit is often called a solar inverter. In Japan, it is known as a "power conditioner."

At the heart of this unit is an inverter circuit, which turns DC into AC. Inside, switches controlled by electricity (power semiconductors) turn on and off thousands of times a second or more. "What Is a Power Semiconductor?" explains how this works. The conversion loses little. Home units convert about 96 to 97% of the power (examples from Sharp, a Japanese maker).

Always searching for the point with the most power (MPPT)

The power you can draw from a solar cell depends on how much current you let it push. Draw a lot of current, and the voltage falls. Try to keep the voltage high, and the current falls. Power is voltage times current, so somewhere in between there is a "sweet spot" where power is greatest.

This point moves when the light or the temperature changes. So the inverter keeps nudging the voltage a little and checking whether the power went up. In this way, it keeps searching for the top of the hill. This is called maximum power point tracking (MPPT).

Types of material: from silicon to new materials

Crystalline silicon: nearly all of today's cells

Almost all solar cells made today are crystalline silicon. A summary by the same Fraunhofer Institute puts its share at about 98% of 2024 production. Its strengths are that it can use technology developed for making chips, and that silicon is plentiful. New silicon panels made in 2024 had an average efficiency of about 22% (same institute).

Thin film: a thin layer absorbs the light

Instead of a slice of silicon, some cells use a thin layer of light-absorbing material on glass or another base. These are called thin-film cells. The main types are cadmium telluride (CdTe) and a compound of copper, indium, gallium and selenium (CIGS). Thin film makes up the remaining 2% or so of production.

Perovskite: a newcomer you can coat on

Over the past ten-plus years, perovskite solar cells have quickly drawn attention. "Perovskite" is the name of a pattern in which atoms are arranged. Compounds with this pattern, made of lead, iodine and other elements, can be turned into a liquid. That liquid can be coated or printed to form a film. In 2009, Tsutomu Miyasaka and colleagues at Toin University of Yokohama, Japan, reported using them in a solar cell, with an efficiency of 3.8%.

After that, the efficiency of small lab cells rose fast. In January 2026, a Chinese company announced 27.87% for a cell smaller than 0.1 square centimeters. Perovskite films can be thin, light and even bendable. So they are expected to be used on walls and roofs that cannot easily carry heavy panels. In Japan, for example, the Ministry of Economy, Trade and Industry (METI) set a goal in a November 2024 strategy: about 20 GW installed by 2040.

The challenge is making them last. Perovskite materials are easily damaged by moisture and heat. It is still an open question whether they can work outdoors for decades. Many of these materials also contain lead, and how to handle it is another challenge.

Tandem cells: stacking cells with different steps

Another approach stacks solar cells with different steps. A material with a large step goes on top and handles the high-energy blue light. The red and infrared light that passes through is used by a material with a smaller step below. This is called a tandem cell. It cuts the "change" from each photon (the part that becomes heat).

In July 2026, LONGi announced 35.5% for a cell with perovskite on top and silicon below. The value was confirmed by a test facility of the European Commission's Joint Research Centre (JRC). Because it cuts the heat loss a lot, it can beat the limit for a single junction (about 33.7%). Turning it into products is still ahead.

It started in 1954

The first practical silicon solar cell was announced in 1954 at Bell Labs in the US. Three researchers, Chapin, Fuller and Pearson, made it, and its efficiency was about 6%. In 1958, the US satellite Vanguard 1 was launched carrying solar cells. In the roughly 70 years since, even mass-produced panels have passed 20% efficiency.

Common myths

By the numbers

The light under standard test conditions is 1,000 watts per square meter. So a panel with 22% efficiency gives about 220 watts per square meter (our calculation). Records are for research cells, and panels on sale are a few points lower.

Learn more

• Compare in a table: n-type vs p-type semiconductors, direct vs indirect band gap semiconductors, Converter vs Inverter
• See it in a figure: in the PN junction diagram, you can move things and watch the depletion layer at the boundary
• Terms for the next step: built-in potential (the source of the force at the boundary), diffusion length (how far an electron or hole can travel before it disappears), quantum efficiency (the share of photons that become electrons)
• Try the numbers: band gap and wavelength calculator (the step and the light it can use), PN junction depletion calculator (how thick the boundary layer is)
• Related reading: "What Is a Semiconductor?", "How Do Image Sensors Work?", "What Is a Power Semiconductor?"