Light in, current out
A solar panel has no moving parts, no fuel and no exhaust. Light falls on it and an electric current comes out. The process behind this is called the photovoltaic effect, a name that combines the Greek word for light with the unit of voltage. The effect was first observed in the 1830s by the French physicist Edmond Becquerel, and practical silicon solar cells were developed in the 1950s.
Light as packets of energy
Light can behave as a stream of tiny packets of energy called photons. The energy of each photon depends on the color, or wavelength, of the light. Blue light photons carry more energy than red ones, and infrared photons carry less.
When a photon strikes a suitable material, it can transfer its energy to an electron. If the energy is large enough, the electron is freed from its place in the material and can move around.
Why silicon
Most solar panels use silicon, a semiconductor. A semiconductor sits between a conductor, which passes current easily, and an insulator, which does not. In pure silicon, electrons are held in the bonds between atoms. A photon with enough energy can knock one loose, leaving behind a gap called a hole, which behaves like a positive charge that can also move.
Freed electrons alone do not make a current. Without some direction, they simply wander around and then fall back into holes, releasing the energy as a bit of heat. The cell needs a way to push them in one direction.
The junction
That push comes from the way the cell is built. A solar cell has two thin layers of silicon that have been deliberately treated with small amounts of other elements, a process called doping.
- In the n-type layer, the added atoms, such as phosphorus, contribute extra electrons.
- In the p-type layer, the added atoms, such as boron, leave a shortage of electrons, creating extra holes.
Where the layers meet, called the p-n junction, some electrons from the n side move across to fill holes on the p side. This leaves a thin region with a built-in electric field, pointing in a fixed direction. The field acts like a one-way slope: when light creates electron-hole pairs near the junction, the field sweeps electrons toward the n side and holes toward the p side.
Metal contacts on the front and back of the cell collect these charges. If a wire connects the two sides through a device, the electrons flow through the wire to get back to the holes. That flow is the electric current delivered by the panel. The electrons are not used up; they are recycled around the circuit, while the energy comes from the light. The same cell cannot run backward as a battery, but the idea of electrons moving through a circuit is the same as in any electric current.
From cell to panel to house
A single silicon cell produces only a little over half a volt, which is not much. A panel therefore wires many cells in series to reach a more useful voltage, and panels are then connected into larger arrays.
The output of a solar cell is direct current, in which charge flows in one direction. Homes and the grid mostly use alternating current, so a device called an inverter converts one into the other. Many systems include battery storage so that daytime production can be used at night, while others send the extra to the grid.
How much of the light becomes power
Not all sunlight is converted. A photon with too little energy cannot free an electron, while a photon with more than enough loses the extra as heat. Some light reflects off the surface, and some electrons fall back before being collected. Together, these effects set a theoretical limit for a basic single-junction silicon cell, which is well below 100 percent.
Commercial panels typically convert roughly 15 to 25 percent of the sunlight that hits them into electricity, depending on the design, and laboratory cells using more complex designs can do better. These are rough ranges and vary by product and year. Panels also tend to produce slightly less as they get hotter, and they lose a small amount of output as they age.
Practical points
- Panels still produce some electricity under clouds, but much less than in direct sun.
- Output varies with the time of day, season, angle and shading. Even partial shade on a few cells can reduce the output of the whole string.
- Making panels requires energy and materials, though over their lifetimes they typically produce much more energy than went into making them. Exact payback times depend on the technology and location.
Solar is one of several choices described in the article on renewable energy, and the broader picture of how electricity gets to homes is covered in the overview of energy sources. Whether solar is a good fit for a specific building depends on local sunlight, costs, rules and roof conditions, which a qualified installer can assess.