How The Heck Do Solar Panels Work?

Here Comes The Sun

When you smash two atoms together, a couple of things happen. They combine into a new atom, and a ton of energy is released in the form of heat and light. This is the principle behind nuclear fusion.

The Sun has been doing this for billions of years at a completely incomprehensible scale. Hydrogen atoms get smashed together in the core to eventually form helium. When the Sun eventually runs out of hydrogen, it will start fusing helium into heavier elements like carbon and oxygen. Bigger stars repeat this process all the way up to iron, essentially producing every element that exists.

This is where Carl Sagan's famous saying “we are made of star stuff” comes from. The calcium in your bones, the iron in your blood, and the silicon in your smartphone all come from ancient stars.

This fusion process produces incredibly powerful electromagnetic radiation. Starting as gamma rays in the core, it takes hundreds of thousands of years for that energy to work its way to the surface of the Sun. From there, it takes another eight minutes to reach Earth as a mix of infrared, visible, and ultraviolet light.

Einstein, who is now officially a recurring character in this series (see the GPS piece), observed in 1905 that this light travels not just as a continuous wave, but also as a blast of trillions of tiny discrete particles called photons. Each photon carries a specific amount of energy, which is determined by its frequency. Lower energy photons have longer wavelengths and are redder, while higher energy photons have shorter wavelengths and are bluer.

It was this explanation of the photoelectric effect at the age of 26, not his work on relativity, that eventually won him the Nobel Prize in Physics.

Enter Silicon

Let's zoom all the way in.

Silicon (Si) is a chemical element found in great abundance on Earth. It makes up about 27% of the Earth's crust by weight. It's what sand is made of.

Its atomic number is 14, which means it has 14 protons in its nucleus and 14 electrons orbiting it.

The outermost four electrons are the valence electrons, which are responsible for chemical bonding. When two silicon atoms sit next to each other, they can each contribute one valence electron into the space between them, forming a covalent bond. Those shared electrons belong to both atoms at once, holding them together.

When silicon atoms bond in all four directions at once, they naturally arrange into a precise repeating three-dimensional structure called a lattice. This is what the dark blue cells you see on a rooftop solar panel are made of.

When The Photons Hit

Every material has a precise energy level where photons of that energy will knock valence electrons free. This is called the bandgap.

Insulators like glass have bandgaps so large that almost no photon can cross them. Conductors like copper have essentially no bandgap at all, so electrons flow freely through them. Silicon is somewhere in-between: a semiconductor with a bandgap of 1.12 eV, which is the energy carried by a near-infrared photon.

When photons hit the lattice, they can free electrons and generate heat as a byproduct. The name of the game is to maximize freed electrons and minimize heat, since the electrons are what we want to convert into electricity. Put another way, we want as much energy from the light as possible to go into freeing electrons and as little as possible to go into heat.

You can use the diagram below to see how the specific energies of photons, or color, interact with the lattice.

Playing with the diagram above shows that, theoretically, hitting the lattice with photons of a single precise frequency (essentially a laser), would produce the most freed electrons and the least heat. However, the cost of building a laser is much higher than the cost of building a solar panel. Instead, we handle the Sun's light as it naturally comes in.

Can We Do Better?

The Sun is a constant; it always sends us the same continuous mix of frequencies. We can experiment with different materials with different bandgaps, but it turns out silicon’s 1.12 eV is close to the theoretical optimum for converting sunlight into electricity. This works out well for us, given how abundant it is.

William Shockley and Hans Queisser worked out the math in 1961, seven years after the first working silicon solar cell was built at Bell Labs. They showed that for a cell tuned to a single bandgap (also called a single-junction cell), regardless of material or engineering, there's a hard theoretical ceiling on efficiency of around 33%. This is the Shockley-Queisser limit.

In practice, rooftop panels land between 20% and 23% efficiency. The gap between theory and reality comes from reflection off the surface, electrons recombining before they reach the junction, and other real-world losses.

There are ways to push past the single-junction limit. The most effective is stacking multiple layers of different semiconductor materials on top of each other, each with a different bandgap tuned to a different slice of the solar spectrum. These multi-junction cells can exceed 40% efficiency, but they are extraordinarily expensive to manufacture, so in practice they're mostly used in specialized applications where squeezing out every last bit of efficiency is more important than cost, like spacecraft.

One exciting near-term frontier is perovskites, a class of materials with a tunable bandgap that can be adjusted by tweaking their chemical composition. Layered on top of silicon, a perovskite-silicon tandem cell could theoretically push past 40% efficiency at a price point close to conventional silicon. Labs around the world are racing to make it work at scale. It all sounds a bit like science fiction to me, but it's real.

Doping The Lattice

Pure silicon is a poor conductor. It has very few free charge carriers in its natural state, so even when photons knock electrons loose, they quickly recombine and nothing particularly useful happens.

The fix is subtle. By introducing trace amounts of impurities into the crystal, roughly one foreign atom per million silicon atoms, you can completely change how it behaves electrically. This process, almost too simple to be true, is called doping.

The solar cells we commonly use are made of a single silicon crystal with two distinct regions side by side. One has a trace of phosphorus mixed in, giving it extra free electrons with nowhere to go: this is the n-type side. The other has a trace of boron, where each boron atom is missing one valence electron, leaving a positively charged vacancy called a hole: this is the p-type side. The foundation of semiconductors is that electrons naturally migrate toward these holes.

Where these two regions meet, a built-in electric field forms at the boundary. When a photon frees an electron nearby, the field grabs it and pulls it toward the wire before it can recombine. This is the photovoltaic effect: the direct conversion of light into electricity through a semiconductor material.

Solar's Steep Cost Curve

There's a theory in manufacturing called Wright's Law: for every doubling of cumulative production, costs fall by a fixed percentage. In other words, the more of something humanity has ever made, the better it gets at making it. Solar has been one of the most dramatic demonstrations of this principle in history. The cost of solar cells has dropped by roughly 20% for every doubling of global production.

A few things have made the curve even steeper than economic theory alone would predict. China made a strategic decision to invest heavily in solar manufacturing, driving prices down through sheer volume. And because solar cells are closely related to the semiconductors that power all of modern computing, advances in silicon purification and chip fabrication have spilled over into solar.

Too good to be true?

This piece has covered a lot of ground. We started with a nuclear reactor 93 million miles away, hundreds of thousands of years ago, and followed that energy all the way to rooftops around the world. It sounds too good to be true. So why isn't this technology everywhere?

For one, we have seen substantial growth. Global solar generation has grown over 10-fold since 2015 and now accounts for roughly 7% of global electricity. Nearly half of that comes from China, which made a strategic bet on solar manufacturing two decades ago and won.

But the cost picture is more complicated than the module price curve suggests. Solar cells themselves are now almost negligibly cheap, making up only about 13% of the total installed cost of a rooftop system in the United States. The inverter and other hardware add another third. The rest is everything else: installation labor, permitting fees, overhead, and profit.

These "soft costs" haven't fallen anywhere near as fast as the panels themselves, and in the US they're much higher than in other countries. An equivalent system costs less than a quarter of the US price in Australia, and the panels are identical. The difference is entirely in the soft costs.

Then there's the grid itself. The electricity network we rely on was designed around a small number of large, controllable power plants that could be dialed up and down on demand. Millions of small rooftop generators that all produce power only when it's light is a fundamentally different system. The best illustration of this is something called the duck curve.