Solar cell

A solar cell, also known as a photovoltaic (PV) cell, is an electronic device that converts light energy directly into electricity. This conversion happens through a process called the photovoltaic effect. These cells serve as the fundamental building blocks for larger photovoltaic modules, which people commonly call solar panels. While they are most famous for generating power, they can also function as photodetectors. This means they can detect electromagnetic radiation near the visible light range or measure light intensity.

The mechanism of a PV cell relies on three essential stages to function. First, the device must absorb light. This absorption generates excitons, which are bound electron-hole pairs, or it creates unbound electron-hole pairs or plasmons. Second, the cell must achieve the separation of charge carriers. These carriers are the oppositely charged particles created by the light. Third, the cell must perform the separate extraction of these carriers into an external circuit. This flow of charges creates the electrical current we use.

Most modern solar cells are categorized by the materials they use. Approximately 95% of the commercial market uses crystalline silicon. The remaining market share is composed of cadmium telluride thin-film solar cells. A common single-junction silicon cell can produce a maximum open-circuit voltage of about 0.5 to 0.6 volts. There are also photoelectrolytic cells, or photoelectrochemical cells. These use light to excite electrons that are transported by a semiconductor. This process can actually use light to split water directly into hydrogen and oxygen for power generation.

The history of solar technology spans nearly two centuries of discovery. In 1839, a 19-year-old French physicist named Edmond Becquerel built the first photovoltaic cell. In 1873, Willoughby Smith described the effect of light on selenium. By 1883, Charles Fritts created a solid-state cell by coating selenium with gold, though it was only 1% efficient. A major breakthrough occurred in 1905 when Albert Einstein proposed a quantum theory of light. This explanation of the photoelectric effect earned him the Nobel Prize in Physics in 1921. In 1954, the first practical photovoltaic cell was demonstrated at Bell Laboratories by Calvin Souther Fuller, Daryl Chapin, and Gerald Pearson.

Solar cells have played a massive role in the history of space exploration. The Vanguard 1 satellite in 1958 was the first to use solar cells in space. This allowed the mission to last much longer than a battery-only system would. In 1959, the Explorer 6 satellite used large, wing-shaped arrays containing 9,600 Hoffman solar cells. By the 1960s, solar cells became the primary power source for most Earth-orbiting satellites. This was because they offered the best power-to-weight ratio for space travel. Modern spacecraft now use advanced III-V multijunction cells. These cells use different p–n junctions with varying bandgaps to capture a wider spectrum of solar energy.

Beyond space, solar cells have been applied to transportation and large-scale energy. Solar cars are electric vehicles that use solar panels to power electric motors. In 1987, the World Solar Challenge held a 3,000 km race across the Australian outback. General Motors won the race with their Sunraycer vehicle, which reached speeds over 40 mph.

On Earth, solar modules are often connected in series to increase voltage or in parallel to increase current. To prevent power loss from shade, modules use bypass diodes. These diodes allow current to skip over shaded or underperforming cells. This protection prevents damage from reverse bias applied by illuminated neighbors.

Today, the solar industry is a global system of interconnected technologies. Large arrays of modules create a solar array to produce direct current (DC). Most systems then use an inverter to convert this into alternating current (AC). The cost of these systems has changed significantly over time. In the United States, the cost per watt for utility-scale systems declined to $0.94 by 2020. This technology connects to many fields, from residential rooftop power to advanced satellite engineering. Whether they are flexible, rollable arrays in space or large silicon farms on Earth, solar cells continue to evolve.