Lesson 1 of 12 · Chapter 1
The Physics of Photovoltaics: From Photon to Power
Before a rail is levelled or a wire is landed, every solar job starts with one question: what exactly is arriving at the roof? This lesson follows sunlight from photon to current: why only some light is useful, why a rating is a benchmark and not a promise, how a junction splits charge, why power lives at the knee of a curve, and why a leaf and a cold morning both matter.
Choose how you want to learn
You can change this any time. Every level does the same hands-on steps. Press Listen on any part to hear it read aloud.
- 1 · Photons and the bandgap
- 2 · A benchmark, not a promise
- 3 · The p-n junction
- 4 · Voltage, current and power
- 5 · Series, shade and cold
- 6 · Try it at home
- 7 · Your card
- 8 · Check yourself
Part 1
Sunlight is a stream of photons
Sunlight arrives as tiny packets of energy called photons.
Bluer, shorter-wavelength light carries more energy per photon than redder light.
Silicon can only use photons that carry at least about 1.12 electron-volts, its bandgap. Weaker photons make no current.
The relationship is energy equals Planck’s constant times frequency, E = h f, or E = h c divided by wavelength. You do not need the constants to be a good installer. You do need the idea: a module is fed by a mix of photon energies, and it can only harvest the ones above its threshold.
A photon above the bandgap can create an electron-hole pair, but its extra energy does not become extra voltage. Most of it becomes heat. So the same sunlight both powers the module and warms it, and heat lowers voltage.
“Sunlight is not just brightness.”
“Energy equals Planck’s constant times frequency.”
“It is sorting photons by energy.”
Read the whole chapter free in the book reader.
The Photon Sorter. New photons every round: decide which ones silicon can use.
Part 2
A rating is a benchmark, not a promise
Module ratings are measured at standard test conditions (STC): 1,000 W/m² of sunlight, a 25°C cell and the AM1.5 spectrum.
Real roofs run hotter, and sunlight is rarely a steady 1,000 W/m².
So a 400 W module is a fair way to compare modules, not a promise of what one will make on your roof.
Irradiance is power per area. The atmosphere filters sunlight: ozone takes much of the ultraviolet, water vapour and carbon dioxide take parts of the infrared, and dust scatters blue light. So the spectrum changes with the time of day, the season, humidity and altitude.
Two moments can have the same total irradiance and a different photon mix, so the module responds differently. On bright overcast days much of the light is diffuse, arriving from many directions, and modules still produce. Under thick grey cloud there are simply far fewer usable photons.
“STC is not a promise of rooftop output.”
“But spectrum and irradiance are not the same.”
Three questions about ratings and real sunlight.
Part 3
The p-n junction splits the charge
A useful photon lifts an electron free and leaves a hole behind: an electron-hole pair.
Left alone, they would fall back together (recombination) and make only heat.
The p-n junction, where boron-doped and phosphorus-doped silicon meet, has a built-in field that pushes them apart. The metal contacts collect them. That flow is current.
Phosphorus has five outer electrons, so it adds loosely held electrons: n-type. Boron has three, so it leaves holes: p-type. Where they meet, charges drift across and leave fixed ions behind, and those ions create the electric field that works like a one-way slope for charge.
Crystal defects are places where electrons and holes reunite without doing work, which is one reason monocrystalline cells tend to beat polycrystalline ones. Coatings, texturing and passivation layers (you will hear PERC, TOPCon, heterojunction) all fight losses so more of each photon event reaches the contacts.
“Recombination is a constant opponent inside every solar cell.”
“The solar cell is not ‘making electrons.’”
“Separation is the first step toward current.”
Six events inside a cell. Where does each belong in the chain?
Part 4
Voltage, current, and the knee of the curve
With the leads open, a module shows its highest voltage, Voc, but no current flows, so there is no power.
Shorted, it gives its highest current, Isc, at almost no voltage.
Real power lives between the two, at the maximum power point, near the knee of the curve. The inverter hunts for it all day.
More light means more current, roughly in proportion. Voltage moves much less with light, but it is sensitive to temperature: heat lowers it and cold raises it. Current rises slightly with heat, but not nearly enough to make up for the voltage lost, so hot modules make less power.
A module can sit in full sun and deliver nothing if the circuit is open. That is why a healthy Voc reading on a meter does not prove a module is producing. Maximum power point tracking keeps adjusting the operating point as clouds pass and the roof heats up.
“Maximum power point tracking is not marketing fluff; it is the control strategy that keeps the array operating near the knee as light and temperature change.”
“Voltage decreases as cell temperature rises.”
The Current-Voltage Bench. Change the light and the cell temperature and watch Voc, Isc and the maximum power point move.
Part 5
Series strings, shade, and the cold morning
Cells, and then modules, are wired in series: voltages add, and the same current must flow through every one.
So one shaded cell can hold back the whole string. Bypass diodes give the current a way around, at the cost of that group’s voltage.
Voltage is highest when cells are cold, so string voltage is checked against the coldest expected morning.
Without a bypass path, the lit cells push a shaded cell into reverse bias, where it resists and turns power into heat: a hot spot. That is why a small leaf can cost far more than its share of the glass. Modules that differ slightly also drag on each other, which is one reason some systems use microinverters or optimizers.
For code and equipment ratings you never size a string on a hot-day voltage. The coldest expected condition sets the highest Voc, and a careless design can exceed the inverter’s limit or the conductor insulation rating. Always use the numbers printed for your module and your local code.
“Currents add in parallel, voltages add in series.”
“The physics that drops voltage in heat is the same physics that raises it in cold.”
The Shade and Bypass Bench. Shade one or more cell groups and switch the bypass diodes on and off.
Part 6
Try it at home
Do these at home. Nothing here asks you to go on a roof or touch a live system. Tick each one when it is done.
Part 7
Your “Datasheet Decoder” card
The numbers that describe a module, from the datasheet you found. Nothing you type here leaves this device.
My Datasheet Decoder
Global Sovereign University · Solar PV Installers, lesson 1
Physics on one card
- Only photons above the bandgap make current.
- STC = 1,000 W/m², 25°C, AM1.5: a benchmark.
- The junction separates; recombination steals.
- Voc has no current; power lives at the knee.
- Series: one current. Shade holds it back.
- Cold raises voltage: check the coldest morning.
Design with the module’s own datasheet and your local code.
Part 8
Check yourself
Six real situations. Pick what you would do. You will see why after each one.
Lesson 1 complete.
You can now follow sunlight from photon to current, read a rating as a benchmark, explain the junction, find power on the curve, and say why shade and cold mornings matter. Next: Lesson 2, safety at heights and with high voltage.
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Solar Photovoltaic (PV) Installers by Dr. Gene A Constant. This lesson comes from chapter one. Read the whole book free in the reader, or keep a copy.
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GSU is a 501(c)(3) nonprofit and book sales are its only income. A purchase is not a tax-deductible donation. This lesson is free and always will be.