How a Solar Cell Turns Sunlight Into Electricity

A solar cell, also called a photovoltaic or PV cell, turns light directly into electricity with no moving parts. Light knocks electrons loose inside a semiconductor, and an electric field built into the cell pushes them in one direction so they flow through a wire. This article explains the layers, why a silicon cell can convert only part of the light, and how many cells become a panel.

Light frees electrons

Sunlight is made of particles of energy called photons. In a semiconductor such as silicon, electrons sit in a lower energy band. When a photon with enough energy hits one, the electron jumps to a higher band, called the conduction band, where it can move. It leaves behind a gap, called a hole, that acts like a moving positive charge. Wikipedia describes the first step of any solar cell as absorbing light to create these electron-hole pairs.

The p-n junction

If nothing else happened, the freed electrons would simply fall back into their holes and give off heat. A solar cell prevents this with a p-n junction. Silicon is doped, meaning a tiny amount of another element is added. According to Wikipedia, one side is doped with boron to make p-type silicon, and the other with phosphorus to make n-type silicon.

Where the two meet, a built-in electric field forms. It sweeps electrons toward the n side and holes toward the p side. Metal contacts on the two faces collect them, and when a load such as a lamp is connected, the electrons flow through it and do useful work. Energy Education, a University of Calgary resource, describes the same junction as the key layer of the cell.

Why not all the light is used

Silicon has a band gap of 1.12 electron volts. Photons with less energy than that cannot free an electron and end up as heat. Photons with more energy than the gap free an electron but the extra energy is also lost as heat. Reflection and electrons that never reach the contacts cause further losses, which is why cells carry anti-reflection coatings.

In 1961 William Shockley and Hans Queisser worked out a theoretical ceiling for a single junction. Wikipedia gives about 33.16% for a silicon cell of this kind, and Energy Education gives about 33%. Real commercial panels are well below that. Cells stacked in several layers can go higher: Wikipedia reports a four-junction concentrator cell reaching 47.6% in 2022, under sunlight concentrated 665 times.

From cell to panel to house

One cell makes only a small voltage, so cells are wired together into modules, usually with a glass sheet on the sunny side. Wikipedia explains that wiring cells in series raises voltage and wiring them in parallel raises current. Bypass diodes let current go around a shaded cell so it does not hold back the rest of the string.

A group of modules forms an array. Cells produce direct current (DC), but homes and grids use alternating current (AC), so an inverter converts one to the other.

Types of silicon cell

• Monocrystalline: made from a single uniform crystal.
• Polycrystalline: made from many crystal grains, generally less uniform.
• Thin films and other materials: Energy Education also names gallium arsenide, cadmium telluride and CIGS, some made as flexible thin films.

Ageing

Cells slowly lose output as they age. Energy Education cites the US National Renewable Energy Laboratory (NREL) as reporting a median loss rate of 0.5% per year.

A short history

Edmond Becquerel demonstrated the photovoltaic effect in 1839. In 1954, Bell Laboratories publicly demonstrated the first practical solar cell, developed by Calvin Fuller, Daryl Chapin and Gerald Pearson. Solar cells powered the Vanguard satellite in 1958. This is general information about the technology, not advice on buying or installing a system.

Sources

• Solar cell, Wikipedia
• Photovoltaic cell, Energy Education (University of Calgary)