How Does Photovoltaics Work?

Photovoltaics (PV) is a fascinating technology that converts sunlight directly into electrical energy — without moving parts, without noise, without emissions. In this article we explain the physical principles clearly and visually, from quantum physics to a complete solar system.

The Photoelectric Effect

The foundation of every solar cell is the photoelectric effect, which Albert Einstein explained in 1905. This groundbreaking work earned him the Nobel Prize in Physics in 1921 — not, as many assume, the theory of relativity.

What happens at the atomic level?

Sunlight consists of tiny energy packets called photons. When a photon strikes certain materials, it can transfer its energy to an electron. If this energy is large enough, the electron is freed from its bond within the material and can move freely.

Animation: From Photon to Current

Watch how a photon generates electricity. The animation shows the process step by step:

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Photon trifft auf

Ein Photon aus dem Sonnenlicht trifft auf die Oberfläche der Solarzelle.

Animation: Vom Photon zum elektrischen Strom in 5 Schritten

Detailed Solar Cell Animation

The following animation shows the complete structure of a silicon solar cell — from the crystal lattice through the anti-reflection layer to the external circuit. Start the animation to follow the electricity generation process step by step.

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Sonnenlicht trifft ein

Photonen aus dem Sonnenlicht treffen auf die Antireflexschicht der Solarzelle und dringen in das Silizium-Kristallgitter ein.

Interaktive Animation: Vom Sonnenlicht zum elektrischen Strom in 5 Schritten

The 5 Steps of Electricity Generation

1. Photon hits the cell: Sunlight reaches the surface of the solar cell
2. Absorption in the semiconductor: The photon transfers its energy to an electron
3. Electron excitation: The electron is lifted from the valence band to the conduction band
4. Charge separation at the p-n junction: The electric field separates the electron and the “hole”
5. Current flow in the circuit: Electrons flow through the external load

Structure of a Solar Cell

A crystalline silicon solar cell consists of several precisely layered components. Click on the individual layers to learn more:

Antireflexschicht

n-dotierte Schicht

p-n-Übergang

p-dotierte Schicht

Rückseitenkontakt

Klicken Sie auf eine Schicht für mehr Informationen

The Key Components

Layer	Function	Material
Anti-reflection coating	Minimises reflection, increases light absorption	Silicon nitride (Si3N4)
n-doped layer	Electron surplus, negative charge carriers	Silicon + Phosphorus
p-n junction	Creates electric field, separates charges	Interface
p-doped layer	Hole surplus, positive charge carriers	Silicon + Boron
Rear contact	Current collection and light reflection	Aluminium

Semiconductor Physics: The Bandgap

The secret of the solar cell lies in the special properties of semiconductors like silicon. Unlike metals (good conductors) and insulators (non-conductors), semiconductors can change their conductivity.

What is the bandgap?

In every solid, electrons can only occupy certain energy states. These form so-called “bands”:

• Valence band: This is where the bound electrons sit (fully occupied)
• Conduction band: This is where free electrons can move (normally empty)
• Bandgap (Eg): The “forbidden” energy range in between

Silizium (Si)Galliumarsenid (GaAs)Cadmiumtellurid (CdTe)Perowskit

Photonenenergie1.50 eV

InfrarotSichtbarUV

Modultemperatur25°C

-10°C (Winter)25°C (STC)80°C (Sommer)

Was bedeutet die Bandlücke?

Die Bandlücke (Eg) ist der Energieunterschied zwischen Valenzband und Leitungsband. Ein Photon muss mindestens diese Energie haben, um ein Elektron anzuregen.

• Zu kleine Bandlücke: Infrarotlicht kann genutzt werden, aber viel Energie geht als Wärme verloren.
• Zu große Bandlücke: Nur energiereiches Licht wird genutzt, rote Photonen gehen verloren.
• Optimal: ~1,3 eV für Single-Junction-Zellen (Shockley-Queisser-Limit).

Temperatureffekt: Bei 25°C hat Silizium (Si) einen Temperaturkoeffizienten von -0.45%/°C. Höhere Temperaturen verringern die Effizienz leicht.

Material	Bandlücke (eV)	Temp.-Koeff.
Silizium (Si)	1.12	-0.45%/°C
Galliumarsenid (GaAs)	1.42	-0.35%/°C
Cadmiumtellurid (CdTe)	1.45	-0.25%/°C
Perowskit	1.55	-0.2%/°C

Why is the bandgap important?

The size of the bandgap determines:

1. Which light can be used: Photons need at least the bandgap energy
2. How much energy is lost: Excess energy is converted to heat
3. The theoretical maximum efficiency: The Shockley-Queisser limit (~33% for 1.34 eV)

The p-n Junction

The p-n junction is the heart of the solar cell. It forms when two differently “doped” semiconductor layers are brought together.

Doping — what is it?

Pure silicon conducts electricity poorly. By deliberately introducing foreign atoms, this changes:

Comparison of semiconductor doping types

Doping	Foreign atom	Effect	Charge carriers
n-doping	Phosphorus (5 valence electrons)	Excess of electrons	Electrons (negative)
p-doping	Boron (3 valence electrons)	Excess of "holes"	Holes (positive)

How does the p-n junction generate electricity?

1. At the interface, electrons diffuse from n to p and holes from p to n
2. A depletion region with an electric field is created
3. This field separates the electron-hole pairs generated by light
4. The separation creates a voltage — the open-circuit voltage of the cell

From Cell to System

A single silicon solar cell generates about 0.5-0.7 volts at maximum solar irradiance. To achieve usable voltages, many cells are combined:

Single cell0,6 V

String (60 cells)36 V

Module40 V

String (10 modules)400 V

Typical System Sizes

System type	Power	Area	Annual yield
Balcony power station	300-800 Wp	1-2 m2	250-800 kWh
Single-family home	5-10 kWp	25-50 m2	4,500-10,000 kWh
Multi-family building	15-30 kWp	75-150 m2	13,500-30,000 kWh
Commercial roof	50-500 kWp	250-2,500 m2	45,000-500,000 kWh

Efficiency and Losses

Not all solar energy can be converted into electricity. The theoretical maximum efficiency (Shockley-Queisser limit) is approximately 33% for single-junction cells.

Where does the remaining energy go?

• Electrical energy20
• Photons with too little energy25
• Excess energy converted to heat35
• Reflection4
• Recombination10
• Other losses6

Frequently Asked Questions about Solar Physics

Why are solar cells usually blue or black?

The blue colour comes from the anti-reflection coating (silicon nitride), which is optimised to capture as much light as possible. Black or dark grey cells use a thicker coating or special surface textures for even less reflection. Monocrystalline cells often appear black, polycrystalline cells tend to look bluish.

Do solar cells work on cloudy days?

Yes! Solar cells also use diffuse (scattered) light. In overcast conditions they typically reach 10-30% of their maximum output. Even under heavy cloud cover, electricity is still generated — just significantly less than under direct sunlight.

Why can’t solar cells use 100% of the light?

This is due to physical limits:

• Photons with too little energy (infrared): Cannot excite an electron
• Photons with too much energy (UV): The excess energy is converted to heat
• Recombination: Some electron-hole pairs recombine
• Reflection and absorption: At the glass, contacts and surfaces

These losses are fundamental and cannot be completely eliminated.

What are tandem solar cells?

Tandem cells stack multiple semiconductors with different bandgaps on top of each other. The upper layer captures high-energy (blue) photons, while the lower layer captures lower-energy (red) ones. This allows more photons to be used efficiently. In the lab, tandem cells have already exceeded 47% efficiency!

How long does the energy payback take?

The energy needed to manufacture a solar cell is recouped after approximately 1-2 years. With a lifespan of 25-30+ years, it produces 15-30 times more energy than was required for its manufacture. That is an excellent energy balance!

Is there a difference between a solar cell and a solar panel?

Yes:

• Solar cell: A single unit (approx. 15x15 cm), generates 0.5-0.7 V
• Solar module/panel: Multiple cells (e.g. 60 or 72) connected and framed
• PV system: Multiple modules plus inverter and wiring

The term “solar panel” is colloquially often used for the entire module.

Conclusion

Photovoltaics uses the photoelectric effect to convert sunlight directly into electricity. The key concepts are:

• Photons transfer their energy to electrons
• The bandgap of the semiconductor determines which light can be used
• The p-n junction separates the charge carriers and generates voltage
• The theoretical maximum efficiency is ~33% (single-junction)
• Modern crystalline silicon cells achieve 20-24% efficiency

The technology is mature, reliable and continuously being improved. With new materials like perovskites and tandem concepts, even higher efficiencies are possible.

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Further reading:

• Efficiency of different module types compared
• Solar modules: Mono vs. Poly vs. Thin-film
• How does an inverter work?
• What affects the yield of a PV system?