On average, every square meter of Earth’s surface receives 164 watts of solar energy (a figure we’ll explain in more detail in a moment). In other words, you could stand a really powerful (150 watt) table lamp on every square meter of Earth’s surface and light up the whole planet with the Sun’s energy! Or, to put it another way, if we covered just one percent of the Sahara desert with solar panels, we could generate enough electricity to power the whole world. That’s the good thing about solar power: there’s an awful lot of it—much more than we could ever use.
But there’s a downside too. The energy the Sun sends out arrives on Earth as a mixture of light and heat. Both of these are incredibly important—the light makes plants grow, providing us with food, while the heat keeps us warm enough to survive—but we can’t use either the Sun’s light or heat directly to run a television or a car. We have to find some way of converting solar energy into other forms of energy we can use more easily, such as electricity. And that’s exactly what solar cells do.
A solar cell is an electronic device that catches sunlight and turns it directly into electricity. It’s about the size of an adult’s palm, octagonal in shape, and colored bluish black. Solar cells are often bundled together to make larger units called solar modules, themselves coupled into even bigger units known as solar panels (the black- or blue-tinted slabs you see on people’s homes—typically with several hundred individual solar cells per roof) or chopped into chips (to provide power for small gadgets like pocket calculators and digital watches).
Just like the cells in a battery, the cells in a solar panel are designed to generate electricity; but where a battery’s cells make electricity from chemicals, a solar panel’s cells generate power by capturing sunlight instead. They are sometimes called photovoltaic (PV) cells because they use sunlight (“photo” comes from the Greek word for light) to make electricity (the word “voltaic” is a reference to Italian electricity pioneer Alessandro Volta, 1745–1827).
We can think of light as being made of tiny particles called photons, so a beam of sunlight is like a bright yellow fire hose shooting trillions upon trillions of photons our way. Stick a solar cell in its path and it catches these energetic photons and converts them into a flow of electrons—an electric current. Each cell generates a few volts of electricity, so a solar panel’s job is to combine the energy produced by many cells to make a useful amount of electric current and voltage. Virtually all of today’s solar cells are made from slices of silicon (one of the most common chemical elements on Earth, found in sand), although as we’ll see shortly, a variety of other materials can be used as well (or instead). When sunlight shines on a solar cell, the energy it carries blasts electrons out of the silicon. These can be forced to flow around an electric circuit and power anything that runs on electricity. That’s a pretty simplified explanation!
Silicon is the stuff from which the transistors (tiny switches) in microchips are made—and solar cells work in a similar way. Silicon is a type of material called a semiconductor. Some materials, notably metals, allow electricity to flow through them very easily; they are called conductors. Other materials, such as plastics and wood, don’t really let electricity flow through them at all; they are called insulators. Semiconductors like silicon are neither conductors nor insulators: they don’t normally conduct electricity, but under certain circumstances we can make them do so.
A solar cell is a sandwich of two different layers of silicon that have been specially treated or doped so they will let electricity flow through them in a particular way. The lower layer is doped so it has slightly too few electrons. It’s called p-type or positive-type silicon (because electrons are negatively charged and this layer has too few of them). The upper layer is doped the opposite way to give it slightly too many electrons. It’s called n-type or negative-type silicon. (You can read more about semiconductors and doping in our articles on transistors and integrated circuits.)
When we place a layer of n-type silicon on a layer of p-type silicon, a barrier is created at the junction of the two materials (the all-important border where the two kinds of silicon meet up). No electrons can cross the barrier so, even if we connect this silicon sandwich to a flashlight, no current will flow: the bulb will not light up. But if we shine light onto the sandwich, something remarkable happens. We can think of the light as a stream of energetic “light particles” called photons. As photons enter our sandwich, they give up their energy to the atoms in the silicon. The incoming energy knocks electrons out of the lower, p-type layer so they jump across the barrier to the n-type layer above and flow out around the circuit. The more light that shines, the more electrons jump up and the more current flows.
A solar cell is a sandwich of n-type silicon (blue) and p-type silicon (red). It generates electricity by using sunlight to make electrons hop across the junction between the different flavors of silicon:
That’s a basic introduction to solar cells—and if that’s all you wanted, you can stop here. The rest of this article goes into more detail about different types of solar cells, how people are putting solar power to practical use, and why solar energy is taking such a long time to catch on.
A basic rule of physics called the law of conservation of energy says that we can’t magically create energy or make it vanish into thin air; all we can do is convert it from one form to another. That means a solar cell can’t produce any more electrical energy than it receives each second as light. In practice, as we’ll see shortly, most cells convert about 10–20 percent of the energy they receive into electricity. A typical, single-junction silicon solar cell has a theoretical maximum efficiency of about 30 percent, known as the Shockley-Queisser limit. That’s essentially because sunlight contains a broad mixture of photons of different wavelengths and energies and any single-junction solar cell will be optimized to catch photons only within a certain frequency band, wasting the rest. Some of the photons striking a solar cell don’t have enough energy to knock out electrons, so they’re effectively wasted, while some have too much energy, and the excess is also wasted. The very best, cutting-edge laboratory cells can manage 46 percent efficiency in absolutely perfect conditions using multiple junctions to catch photons of different energies.
In theory, a huge amount. Let’s forget solar cells for the moment and just consider pure sunlight. Up to 1000 watts of raw solar power hits each square meter of Earth pointing directly at the Sun (that’s the theoretical power of direct midday sunlight on a cloudless day—with the solar rays firing perpendicular to Earth’s surface and giving maximum illumination or insolation, as it’s technically known). In practice, after we’ve corrected for the tilt of the planet and the time of day, the best we’re likely to get is maybe 100–250 watts per square meter in typical northern latitudes (even on a cloudless day). That translates into about 2–6 kWh per day (depending on whether you’re in a northern region like Canada or Scotland or somewhere more obliging such as Arizona or Mexico). Multiplying up for a whole year’s production gives us somewhere between 700 and 2500 kWh per square meter (700–2500 units of electricity). Hotter regions clearly have much greater solar potential: the Middle East, for example, receives around 50–100 percent more useful solar energy each year than Europe.
Unfortunately, typical solar cells are only about 15 percent efficient, so we can only capture a fraction of this theoretical energy. That’s why solar panels need to be so big: the amount of power you can make is obviously directly related to how much area you can afford to cover with cells. A single solar cell (roughly the size of a compact disc) can generate about 3–4.5 watts; a typical solar module made from an array of about 40 cells (5 rows of 8 cells) could make about 100–300 watts; several solar panels, each made from about 3–4 modules, could therefore generate an absolute maximum of several kilowatts (probably just enough to meet a home’s peak power needs).
Some people are concerned that solar farms will gobble up land we need for real farming and food production. Worrying about land-take misses a crucial point if we’re talking about putting solar panels on domestic roofs. Environmentalists would argue that the real point of solar power is not to create large, centralized solar power stations (so powerful utilities can go on selling electricity to powerless people at a high profit), but to displace dirty, inefficient, centralized power plants by allowing people to make power themselves at the very place where they use it. That eliminates the inefficiency of fossil fuel power generation, the air pollution and carbon dioxide emissions they make, and also does away with the inefficiency of transmitting power from the point of generation to the point of use through overhead or underground power lines. Even if you have to cover your entire roof with solar panels (or laminate thin-film solar cells on all your windows), if you could meet your entire electricity needs (or even a large fraction of them), it wouldn’t matter: your roof is just wasted space anyway. According to a 2011 report [PDF] by the European Photovoltaic Industry Association and Greenpeace, there’s no real need to cover valuable farmland with solar panels: around 40 percent of all roofs and 15 percent of building facades in EU countries would be suitable for PV panels, which would amount to roughly 40 percent of the total electricity demand by 2020.
It’s important not to forget that solar shifts power generation to the point of power consumption—and that has big practical advantages. Solar-powered wristwatches and calculators theoretically need no batteries (in practice, they do have battery backups) and many of us would relish solar-powered smartphones that never needed charging. Road and railroad signs are now sometimes solar powered; flashing emergency maintenance signs often have solar panels fitted so they can be deployed in even the remotest of locations. In developing countries, rich in sunlight but poor in electrical infrastructure, solar panels are powering water pumps, phone boxes, and fridges in hospitals and health clinics.
Real-world domestic solar panels might achieve an efficiency of about 15 percent, give a percentage point here or there, and that’s unlikely to get much better. First-generation, single-junction solar cells aren’t going to approach the 30 percent efficiency of the Shockley-Queisser limit, never mind the lab record of 46 percent. All kinds of pesky real-world factors will eat into the nominal efficiency, including the construction of the panels, how they are positioned and angled, whether they’re ever in shadow, how clean you keep them, how hot they get (increasing temperatures tend to lower their efficiency), and whether they’re ventilated (allowing air to circulate underneath) to keep them cool.