Sat. Jan 28th, 2023
Making tea with the sun in Tibet.
Enlarge / Making tea with the sun in Tibet.

While our recent look at residential solar might lead you to believe that harnessing power is a newer initiative, people are exploiting solar power for thousands years to heat their homes, cook and produce hot water. Some of the earliest written references to technology deliberately designed to capture the sun’s rays come from ancient Greece. Socrates himself said: “In houses facing south, in winter the sun penetrates through the portico, while in summer the path of the sun is directly above our heads and above the roof, so that there is shade.” He describes how Greek architecture utilized the sun’s different paths through the sky at different times of the year.

By the fifth century BCE, the Greeks were grappling with an energy crisis. Their main fuel, charcoal from trees, was scarce because they cut down their forests for cooking and heating their homes. Wood and charcoal were rationed and olive groves needed protection from the citizenry. The Greeks solved their energy shortage by carefully planning the layout of their cities so that each house could benefit from the sunshine in the way Socrates described. The combination of technology and enlightened government policy worked, and a crisis was avoided.

Technologies for harnessing the thermal energy in sunlight have only continued to grow over time. New England settlers borrowed the ancient Greek techniques for building houses to keep warm in the harsh winters. Simple passive solar water heaters, little more than a barrel painted black, were introduced in late 19e century. And more elaborate solar heating systems were developed to pass water through absorbing and/or focusing panels. The hot water is stored in an insulated tank until needed. In climates subject to freezing, a two-fluid system is used, where the sun heats a water/antifreeze mixture that passes through coils embedded in the storage tank, which doubles as a heat exchanger.

Today, several advanced commercial systems are available for domestic water and space heating. Solar thermal systems are deployed around the world, with the largest installed base per capita in Austria, Cyprus and Israel.

A rooftop solar water heating system in Washington, DC.
Enlarge / A rooftop solar water heating system in Washington, DC.

But modern solar energy really begins in 1954 with the discovery of a practical way to make electricity from light: Bell Labs discovered that silicon could make a photovoltaic material. This finding laid the foundation for today’s solar cells (essentially the devices that convert light energy into electricity) and ushered in a new era of solar energy. Aided by intensive research since then, it is an era that continues today as solar power appears poised to become the dominant energy source in the future.

What is a solar cell?

The most common type of solar cell is a semiconductor device made of silicon — a cousin of the solid-state diode. The known solar panels are made of a number of solar cells that are connected together to create the desired output voltage and current. Those cells are surrounded by a protective package and covered with a glass window.

Solar cells generate electrical power using the photovoltaic effect, a fact that does not come from Bell Labs. Instead, this was first discovered in 1839 by French physicist Alexandre-Edmond Becquerel (son of physicist Antoine Cesar Becquerel and father of physics, Nobel laureate Henri Becquerel, the discoverer of radioactivity). Just over a century later, Bell Labs made its solar cell breakthrough, laying the foundation for the most common solar cells.

In the language of solid state physics, a solar cell is formed from a pn junction in a silicon crystal. The compound is made by “doping” different parts of the crystal with small amounts of various impurities; the interface between these regions is the junction. The n side is a conductor with electrons as current carriers, and the p side has “holes,” or areas of missing electrons that act as current carriers in the crystal. In the region near the interface, the diffusion of charges creates a local “built-in voltage” across the interface. When a photon enters the crystal, if it has enough energy, it can detach an electron from an atom, creating a new electron-hole pair.

The pn junction of a standard solar cell.
Enlarge / The pn junction of a standard solar cell.

By Bhpaak / CC BY-SA 4.0 via Wikimedia Commons

The newly released electrons are attracted to the holes on the other side of the junction, but the built-in voltage prevents them from passing through. However, if a path is provided through an external circuit, the electrons can travel through it, illuminating our homes along the way. When they reach the other side, they rejoin the holes. This process can continue as long as the sun keeps shining.

The energy required to convert a bound electron into a free electron is called the “band gap”. It is key to understanding why photovoltaic (PV) cells have an intrinsic limit on efficiency. The band gap is a fixed property of the crystal material and its dopants. Those dopants are tuned so that solar cells have a bandgap close to the energy of a photon in the visible region of the spectrum. This is a practical choice, because visible light is not absorbed by the atmosphere (in other words, we humans have evolved to see in most common wavelengths).

Photons come in fixed amounts of energy, meaning their energy is quantized. That also means that a photon with energy smaller than the bandgap (for example, one in the infrared part of the spectrum) will not create a charge carrier. It just heats the panel. Two infrared photons together won’t fare any better, even if their combined energy were enough to bridge the gap. A photon with excess energy (such as an ultraviolet photon) will knock an electron loose, but the excess energy is also lost.

Since efficiency is defined as the ratio of the light energy incident on the panel divided by the electrical energy extracted – and since much of this light energy is necessarily lost – the efficiency cannot be 100 percent.

The band gap of a silicon PV solar cell is 1.1 electron volts (eV). As shown in the electromagnetic spectrum diagram shown here, the visible spectrum is just above it, so visible light of any color will produce electrical current. But this also means that for every photon absorbed, excess energy is wasted and converted to heat.

The result is that even if the PV panel is manufactured flawlessly and the conditions are ideal, the theoretical maximum efficiency is about 33 percent. Commercially available solar panels typically achieve an efficiency of about 20 percent.

Perovskites

Most commercially deployed solar panels are made from the silicon cells described above. But research into other materials and strategies is ongoing in laboratories around the world.

Some of the most promising recent research on silicon alternatives involves materials mentioned perovskites. The mineral perovskite (CaTiO3) was named in 1839 in honor of Count Lev Aleksevich Perovsky (1792-1856), a Russian mineralogist. It can be found on every continent and in the clouds of at least one exoplanet. The word “perovskite” is also used for synthetic compounds that have the same orthorhombic crystal structure as the naturally occurring mineral (or a closely related one) and share a structurally similar chemical formula.

Crystal structure of natural perovskite.

Crystal structure of natural perovskite.

Solid state | CC BY-SA 3.0

Depending on which elements are used, perovskites can exhibit a wide variety of useful properties, such as superconductivity, giant magnetoresistance and photovoltaic activity. Their use in PV cells has sparked much optimism as they have shown an unprecedented increase in efficiency from 3.8 percent to 20.1 percent over the past seven years of lab research. This rapid pace of progress inspires confidence that further gains are likely, especially as the factors limiting efficiency become increasingly apparent.

By akfire1

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