How a Solar Cell Turns Light Into Current

Sunlight arriving at a rooftop carries roughly one kilowatt of power per square metre at midday under clear skies. A modern silicon module converts somewhere between a fifth and a quarter of that into electricity. The remaining three quarters leaves as heat or is simply never absorbed. Understanding where each fraction goes is the difference between buying panels on a spec sheet and understanding what you own.

The photovoltaic effect in plain terms

A silicon atom in a crystal lattice shares its four outer electrons with neighbours. Those electrons are bound; they cannot wander. To free one you must supply enough energy to lift it across the band gap, which in crystalline silicon is about 1.12 electronvolts. A photon of sunlight carries energy inversely proportional to its wavelength, so red light at 700 nanometres carries about 1.77 eV and infrared beyond 1,100 nanometres carries less than the band gap.

This single number explains a great deal. Photons below 1.12 eV pass straight through the wafer and do nothing at all — roughly 19 percent of the solar spectrum's energy is lost this way. Photons well above the band gap free an electron but dump their surplus energy as lattice vibration, which is to say heat. That thermalisation loss accounts for another 33 percent or so. Together these two mechanisms set the Shockley-Queisser limit of about 33 percent for a single-junction cell, before you have made a single manufacturing compromise.

Why a cell needs a junction, not just silicon

Freeing an electron is not enough. In a uniform block of silicon the freed electron and the hole it leaves behind wander until they recombine, and the energy returns as heat. To harvest the pair you need a built-in electric field that pushes them in opposite directions.

That field comes from doping. One region of the wafer is doped with phosphorus, which contributes a spare electron and creates n-type material. The other is doped with boron, which contributes a spare hole and creates p-type material. Where the two regions meet, electrons diffuse across, holes diffuse back, and the resulting charge imbalance produces a depletion region with a field across it — typically a few tenths of a volt.

An electron-hole pair created within a diffusion length of that junction gets separated. The electron is swept toward the n-side, the hole toward the p-side, and if you connect the two sides through an external circuit you have current. Open the circuit and the charge piles up until the voltage cancels the built-in field. That voltage — around 0.6 to 0.75 volts for a good silicon cell — is why a 60-cell module produces roughly 40 volts open-circuit and not something more convenient.

From cell to module

A single cell producing 0.6 volts is useless on its own, so cells are soldered in series with ribbon or wire interconnects. Sixty cells in series gives a nominal maximum-power voltage near 32 volts; the 120 half-cells common today give a similar voltage at half the current, which halves resistive losses in the interconnects.

The stack is then laminated: tempered low-iron glass on the front, an encapsulant such as EVA or POE either side of the cells, and a polymer backsheet or a second sheet of glass behind. The laminate is pressed at around 150 °C so the encapsulant cross-links into a clear rubbery solid that bonds everything and keeps water out. An aluminium frame is bonded around the edge, and a junction box with bypass diodes is glued to the back.

What the bypass diodes are for

If one cell in a series string is shaded, it stops producing current but the other cells keep pushing current through it. The shaded cell then behaves as a resistor, dissipating the whole string's power as heat, and can reach temperatures high enough to delaminate the encapsulant — a hot spot. Bypass diodes, normally one per twenty or twenty-four cells, provide an alternate path so current routes around the shaded group. You lose that sub-string's output, not the module.

This is why partial shading costs far more than the shaded area suggests. A chimney shadow across one cell can knock out a third of a module.

Efficiency: where the rest of the light goes

For a commercial module rated at 21 percent, the losses stack roughly like this.

The industry's incremental gains come from attacking the smaller terms. Anti-reflective coatings and textured surfaces cut optical losses. Passivated emitter and rear cell (PERC) architecture adds a dielectric layer at the back that reflects unabsorbed infrared light back through the wafer and reduces rear-surface recombination. TOPCon and heterojunction designs go further, inserting thin passivating layers between the silicon and the contacts so that carriers are collected without recombining at the metal interface.

Temperature: the loss nobody sees on the datasheet

Modules are rated at Standard Test Conditions: 1,000 W/m², 25 °C cell temperature, and a defined spectrum. A roof in July does not resemble this. Cell temperature under full sun commonly reaches 55 to 65 °C in a flush-mounted array with limited airflow.

Every datasheet lists a temperature coefficient of maximum power, typically between −0.29 and −0.40 percent per degree Celsius. At −0.34 %/°C and a cell temperature of 60 °C, output falls by roughly 12 percent relative to nameplate. The effect is real and permanent for those hours, which is why annual yield models never simply multiply nameplate by sun hours.

Interestingly, cold clear days can push output above nameplate. Voltage rises as temperature falls, and inverter maximum input voltage must be sized against the record minimum temperature at the site, not the average. Getting this wrong is one of the most common design faults in cold-climate installations.

Degradation over twenty-five years

Modules are not static. A typical warranty promises no more than 2 percent loss in the first year and 0.45 to 0.55 percent per year thereafter, ending at 85 to 88 percent of nameplate at year twenty-five. The first-year drop is largely light-induced degradation, in which boron-oxygen complexes form in the wafer under illumination. Gallium doping, now widespread, mostly eliminates it.

Longer-term decline comes from encapsulant discolouration, solder joint fatigue from thermal cycling, and potential-induced degradation where leakage currents between cells and the grounded frame gradually strip performance. Well-built systems with correctly earthed frames and inverters that avoid sustained negative array potential rarely suffer the last of these.

Frequently asked questions

Do solar panels work on cloudy days? Yes, at reduced output. Diffuse light still reaches the cells, and a heavily overcast sky typically delivers 10 to 25 percent of clear-sky irradiance. Northern European systems earn a meaningful share of their annual yield from diffuse light.

Does a higher-efficiency panel produce more energy? Per square metre, yes. Per watt of nameplate, no. Two modules rated at 400 W produce the same energy in the same conditions regardless of efficiency; the efficient one is simply smaller. Efficiency matters when roof area is the binding constraint.

Why do modules have a lower rating than the sum of their cells? Because of cell-to-module losses: reflection off the glass, the small optical shading of ribbons, resistive loss in interconnects, and mismatch between cells that are never quite identical. Half-cell and multi-busbar designs reduce these losses, and some bifacial modules with white backsheets even show a small gain.

Should I worry about hail? Standard IEC 61215 qualification includes an impact test with 25 mm ice balls at 23 m/s. Larger hail does break modules, and in hail-prone regions it is worth checking whether a manufacturer has tested to a larger ball size and whether the insurer requires it.

What this means when you buy

Read the temperature coefficient, not just the efficiency figure. Check the year-25 warranty percentage rather than the headline "25-year warranty", since the guarantee covers a curve, not a number. Ask whether the module is gallium-doped if the first-year degradation clause matters to you. And if you are shading-constrained, plan for module-level electronics or a layout that keeps shadows off whole sub-strings, because the physics of series strings is unforgiving.

The technology is mature enough that the differences between reputable modules are small. The differences between a well-designed system and a careless one are not.