How Photovoltaics Work

To see how a cell works, first think about what sunlight actually is. Light behaves as a stream of tiny energy packets called photons. Each photon carries a specific amount of energy, and sunlight is a mixture of photons across the spectrum - the energetic blue and ultraviolet, the visible colours, the lower-energy infrared we feel as warmth. When light falls on any surface, it is really countless photons arriving every instant, each delivering its little parcel of energy to whatever it strikes. Most materials simply turn that energy into heat (which is what happens to a dark wall in the sun). A photovoltaic cell is engineered to do something far more useful with some of it: convert it directly into electrical energy.

The key is that a photon can, under the right conditions, hand its energy to an electron inside a material and knock it loose from its atom - freeing it to move. In most materials a freed electron just bounces around and quickly loses that energy back as heat, so nothing useful happens. The whole art of the solar cell is to catch those freed electrons *before* they settle back down and to channel them, all flowing the same way, out into a circuit. Do that, and a flow of electrons is exactly what electric current is.

This already tells us two honest things that matter later. First, only photons with enough energy can free an electron in a given material, and photons with more energy than needed waste the surplus as heat - so no cell can convert all of sunlight's energy, which is one deep reason real efficiencies are well under 100 percent (lesson 1.3). Second, because the cell is turning some light into electricity and inevitably some into heat, and because hot cells actually perform worse, temperature quietly becomes a real-world enemy of yield - a point that returns forcefully when we reach hot climates and poorly ventilated facades (Module 6). For now, hold the simple, powerful idea: sunlight is energy arriving in packets, and a solar cell is a device for turning some of those packets into moving electrons.

Freeing electrons is only half the trick; the cell must also make them all flow one way, and that is the job of a semiconductor - usually silicon - arranged as a p-n junction. A semiconductor is a material whose ability to conduct sits between a metal and an insulator, and, crucially, can be tuned. By deliberately adding tiny amounts of other elements - a process called doping - silicon is made into two flavours. N-type silicon is doped to have a surplus of loosely held electrons (negative charge carriers). P-type silicon is doped to have a shortage - a surplus of 'holes', places where an electron is missing, which behave like mobile positive charges. Neither is charged overall; each just has an excess of one kind of carrier.

Now bring the two into contact and something elegant happens on its own. Where the n-type and p-type meet - the p-n junction - some electrons drift across and fill holes, leaving behind a thin zone with a built-in electric field: an internal 'slope' pointing from one side to the other. Nothing is happening usefully yet in the dark; the junction is just primed. But it is now a one-way street for freed charges. When a photon frees an electron near the junction, that built-in field immediately sweeps the electron toward the n-side and the hole toward the p-side, pulling them apart before they can recombine. Connect the two sides with an external wire and the separated electrons pour through it - from the n-side, through your circuit and its load, back to the p-side - as electric current, for as long as the light keeps freeing charges.

That is the whole heart of the device: doping creates two kinds of silicon, joining them builds an internal field, and that field turns randomly freed electrons into an organised, directional flow. It is why a cell needs no moving parts and no fuel - the 'pump' driving the current is a static electric field frozen into the crystal, powered entirely by incoming light. The engineering that makes real cells efficient (textured surfaces, anti-reflection coatings, fine metal contacts, thin high-quality wafers) all serves this one purpose: get as many photons as possible to free electrons that the junction can then sweep out before they are lost.

Because the internal field always sweeps electrons the same direction, the current a cell produces is direct current (DC) - a steady one-way flow, not the alternating back-and-forth (AC) that mains electricity and most appliances use. A single silicon cell also produces only a small voltage, roughly half a volt, regardless of its size; making the cell bigger gives more current but not much more voltage. That is precisely why cells are wired together in series into modules to build up a useful voltage, and modules into arrays for useful power (lesson 1.3) - and why a PV system needs an inverter to convert its DC into the AC a building and grid actually use (Module 2). Keep that thread in mind: the cell's native output is low-voltage DC, and everything downstream exists to shape that into usable power.

The 'no moving parts' character is not a slogan; it has real consequences a designer should appreciate. With nothing rotating, rubbing or burning, a PV module is silent, produces no emissions in operation, and has few things that can mechanically wear out, which is why modules routinely carry very long performance warranties and degrade only slowly over decades. It also means a cell responds instantly to light: output tracks irradiance almost in real time, rising and falling as clouds pass - which ties straight back to the resource swings of lesson 1.1. Their output is a live mirror of the sunlight arriving.

But honesty, as always. 'No moving parts' does not mean 'no engineering' or 'no risk'. A PV system is a live electrical generator: it produces potentially dangerous DC voltage whenever light falls on it - it cannot simply be switched off by isolating it from the grid - which makes DC safety, wiring, fire behaviour and proper isolation genuinely serious matters, all of which belong to qualified electrical engineers and the governing codes (Module 7). And the cell itself is only the beginning of a system with inverters, wiring, protection and mounting that must all be designed correctly. The cell's elegance is real; the system around it is where the binding engineering, safety and yield live, and this course consistently defers those to the specialists, the manufacturers' data and the codes.

One distinction prevents a surprising amount of muddle: photovoltaic (PV) and solar-thermal are two completely different technologies that both use the sun, and they are constantly confused - including in casual talk of 'solar panels'. PV, the subject of this course, uses the photovoltaic effect to turn sunlight *directly into electricity* via semiconductors. Solar-thermal instead captures the sun's energy as *heat*: a collector (often a dark absorber with water or another fluid running through it, or in large power plants, mirrors concentrating sunlight) gets hot, and that heat is used directly - most familiarly a rooftop solar water heater - or, at utility scale, to raise steam and drive a turbine to make electricity indirectly. Same sun, entirely different physics and entirely different output: electrons versus heat.

Why does keeping them straight matter to a designer? Because they solve different problems and integrate into buildings differently. A solar water heater (solar-thermal) is often a very cost-effective way to cut a building's hot-water energy, but it produces no electricity and does nothing for lighting, cooling or plug loads. PV produces the versatile electricity that runs almost everything, and it is PV - not thermal - that BIPV integrates into facades, glazing and solar roofs to make the envelope a *power* generator. Confusing the two leads to real errors: expecting hot water from a PV array, or electricity from a thermal collector, or comparing their costs and yields as if they were the same thing.

There is a genuine hybrid worth knowing exists - photovoltaic-thermal (PVT) collectors that capture both electricity and heat from the same surface, partly because cooling a PV cell (carrying its waste heat away as useful hot water) also helps its electrical efficiency - and the course will note it later. But the clean mental filing is simple and worth locking in now: this course is about photovoltaics - sunlight to electricity, via the photovoltaic effect in a semiconductor. When you hear 'solar', always ask which: electricity (PV, our subject) or heat (thermal). That one habit will save you from a whole category of confident-sounding mistakes, and it keeps the rest of the course sharply focused on the envelope that generates *power*.