Your turn
Each lab is one step from the video. Press, slide and flip, and see what happens.
Each lab isolates one part of the physics. The models are simplified to show the idea.
Tap the buttons and slide the sliders.
Switch the extra atoms off. What happens to the free electrons and the one-way slide?
Without phosphorus and boron there are no free carriers and no junction field. Toggle the dopants.
Tap the button. Where do the extra electrons go?
Atoms are drawn huge and flat. A real crystal is 3D and far finer.
Fire photons, then switch the field off. Does the bulb still light?
With the junction field off, freed carriers recombine instead of reaching the circuit. Compare both.
Tap to send sunshine. Does the lamp light up?
A simplified model: real cells free electrons all through the silicon, and holes move too.
Switch the inverter off and on. How does the flow into the house change?
DC flows one way; the inverter switches its direction rapidly to make AC for the home and grid.
Move the Sun, tilt the panel and add clouds. Can you get the meter to the top?
Output follows the angle between the rays and the panel. Add a low Sun or cloud and watch the diffuse part.
Slide the Sun and the panel. When does the panel make the most?
The meter is a simplified model of how angle, air and clouds change the light on a panel.
Switch to night. What keeps the light on when there's no sunlight?
No sunlight on the panels, no current: storage or the grid covers the night.
Tap for night time. What keeps the lamp on?
Solar panels are the dark blue squares you see on some roofs. They make electricity from sunshine. They have no fire inside, and nothing in them moves.
Each square is a thin slice of silicon. Silicon comes from sand! Its top layer has extra tiny bits called electrons. Its bottom layer has empty spots, called holes. Where the layers meet, there is a slide that only goes one way: up.
Sunshine comes in tiny bits of light. When one hits the silicon, it bumps an electron loose. The slide pushes the electron up. It can't slide back down, so it goes home around the wire. Electrons going round the wire are electricity!
A panel makes the most when it faces the Sun. On a cloudy day it still works. It just makes less.
At night there is no sunshine, so the panel rests. A battery can save power for later.
A solar panel is made of many solar cells, and each cell is a thin slice of silicon built as a two-layer sandwich. Where the two layers meet there is a built-in electric field.12 When sunlight hits the cell, its energy knocks electrons free from their atoms. This is called the photovoltaic effect.9 The field pushes those free electrons toward one side of the cell, so one side becomes negative and the other positive, like the two ends of a battery.12 Connect a wire from one side to the other, and the electrons flow through it and back into the cell. That flow is an electric current.9
There's no fuel, no burning and no moving parts.11 The electrons were already in the silicon. Sunlight gives them energy, and the field gives them a direction.
Almost all solar panels are made from silicon. It's the second most common element in Earth's crust, after oxygen, and it's the same semiconductor used in computer chips.1 Here's how a panel is made, step by step:5
A finished cell is often thinner than four human hairs.2 Plain silicon is shiny and reflects more than 30% of the light that hits it, so cells get an anti-reflection coating. That's why solar panels look dark blue or black.4
In a pure silicon crystal, every electron is held in place by the bonds between atoms, so none of them can move around.12 To make a solar cell, engineers "dope" the silicon, which means adding a tiny amount of another element:13
| Layer | Added atom | What it does |
|---|---|---|
| n-type (negative) | Phosphorus | Brings a spare electron that is free to move |
| p-type (positive) | Boron | Leaves a gap where an electron is missing, called a hole |
Where the n-type and p-type layers touch, an electric field forms. This boundary is called the p-n junction. Free electrons can cross it easily in one direction but not the other, a bit like a one-way slide.1213
Sunlight is made of tiny packets of energy called photons. When photons hit a cell, some bounce off, some pass straight through, and some are absorbed. Only the absorbed ones make electricity.94
light frees electron → field pushes it one way → wire is the only road back = current
One cell only makes about 1 to 2 watts, enough for a calculator or a wristwatch.29 So cells are wired together into panels (also called modules), and panels are wired together into an array.2
Solar cells make direct current (DC), which flows in one direction. Homes and the power grid use alternating current (AC), which flows back and forth. A box called an inverter converts DC to AC by switching the direction of the current back and forth very rapidly.3
| Part | Job |
|---|---|
| Solar cells | Turn light into DC electricity |
| Panels / array | Wire many cells together for useful power |
| Inverter | Turns DC into AC for the home and grid |
| Battery or grid | Stores or takes extra power, and supplies it when the Sun is down |
When the panels make more than the home needs, the extra can charge batteries or flow into the grid.7
Most of the sunlight that reaches a cell doesn't become electricity. Some is reflected, some passes through, and some turns into heat.4 Commercial panels have improved a lot: under 10% efficient in the mid-1980s, about 15% by 2015, and now close to 25% for the best modules. Some experimental cells reach nearly 50%.914 A good panel today turns roughly a fifth of the light into power.
Heat is actually bad for solar cells: they work best when they're cool, because higher temperatures lower their voltage.4
A panel makes the most electricity when it faces the Sun head on.9 When it's tilted away, the same beam of light is spread over a slant, so less light lands on each cell.13 A low Sun in the morning, evening or winter is weaker too, because its light travels a longer path through the air. The most solar energy arrives around solar noon.6
That's why, in the northern hemisphere, panels usually face south (north in the southern hemisphere), tilted at roughly the local latitude to collect the most energy over a year.97 You can test this in chapter 5 of the explainer above.
Cloudy days: yes, but less. Thick clouds can cut the direct beam of sunlight by up to 100%, but flat panels still catch the diffuse light scattered through the clouds.610
At night: no. With no sunlight on the panels, they make no useful current (moonlight is far too weak). That's what batteries and the grid are for.79
Silicon panels are expected to last 25 years or more and still make over 80% of their original power.1 The average lifespan has grown from about 20 years in 2007 to 25–35 years today, and recycling processes for old panels exist.8
The first practical solar cell was made by researchers at Bell Telephone in 1954. From the late 1950s, solar cells powered satellites in space.9
Each solar cell is a thin sandwich of two kinds of silicon with an electric field built in where they meet. When light hits the cell, it knocks electrons free. The field pushes them one way, and the only way for them to get back is through a wire you connect. That flow of electrons is an electric current.
Yes, but they make less power. Thick clouds can block nearly all of the direct sunlight, but some light is scattered through the clouds, and flat solar panels can still use that diffuse light.
No. With no sunlight on the panels, too few photons arrive to free electrons (moonlight is far too weak), so they make no useful current. Homes with solar panels use electricity from batteries or from the grid at night.
Yes. Solar cells actually work better when they are cool, because heat lowers their voltage. Winter output is lower mainly because days are shorter and the Sun is lower in the sky, so its light travels through more air and hits the panel at a slant.
Good commercial panels turn roughly a fifth of the sunlight that hits them into electricity, and the best modules are approaching 25%. In the mid-1980s it was under 10%. Some experimental cells reach nearly 50%.
Silicon solar panels are expected to last 25 years or more and still produce more than 80% of their original power. The U.S. Department of Energy says the average lifespan has risen to about 25 to 35 years.
Mostly silicon, which comes from quartz sand. Thin silicon wafers are treated with small amounts of other atoms such as phosphorus and boron, given thin metal contact lines, wired together with copper ribbons, and sealed under glass in an aluminium frame.