How a PV Cell Works: The Junction and the Half-Volt Rule

A photovoltaic (PV) cell is the smallest unit that converts light into electricity. A panel is a collection of these cells wired together, and understanding what happens inside a single cell explains why panels behave the way they do: why they produce a characteristic voltage, why they are wired in series, and why they lose output on hot days.

The heart of a PV cell is a thin slice of silicon, a semiconductor. Pure silicon has a crystal lattice where each atom shares electrons with its neighbours. This structure has no free charge carriers, so pure silicon is a poor conductor. To make it useful, the silicon is doped: small amounts of other elements are added.

One side of the cell is doped with atoms that have one more electron than silicon, such as phosphorus. This creates an excess of free electrons, making it n-type (negative). The other side is doped with atoms that have one fewer electron, such as boron. This creates ‘holes’ — missing electrons that can move as if they were positive charges, making it p-type (positive). The boundary where the two types meet is called the p-n junction.

At the junction, electrons from the n-side diffuse into the p-side and fill holes, leaving a region depleted of free carriers. This creates an electric field that acts as a one-way barrier: it blocks further diffusion and, importantly, separates any charge pairs that are generated nearby.

When a photon of light hits the cell, it can be absorbed by a silicon atom, giving an electron enough energy to break free. This creates an electron-hole pair. If this happens near the junction, the electric field pushes the electron to the n-side and the hole to the p-side. This separation of charge creates a voltage difference between the two sides. Connect a wire between them, and electrons flow — that is the current.

The Half-Volt Rule

A silicon PV cell produces roughly 0.5 to 0.6 volts under standard test conditions (STC: 1000 W/m² irradiance, 25°C cell temperature). This voltage is determined by the physics of the silicon band gap — the energy needed to free an electron — and not by the size of the cell. A small cell and a large cell produce the same voltage; the larger one produces more current because it captures more photons.

This fact explains why panels are wired in series. A single cell cannot produce enough voltage to charge a 12-volt battery or feed an inverter. By connecting cells in series, the voltages add: 36 cells give about 18 volts, 60 cells give about 30 volts, and 72 cells give about 40 volts. The current is the same through each cell in series, so the panel’s current is limited by the weakest cell — a shaded or damaged cell can reduce the whole string’s output.

The Effect of Temperature

PV cells are semiconductors, and their behaviour changes with temperature. As the cell gets hotter, the voltage decreases. The temperature coefficient for voltage is typically around -0.3% per °C for silicon cells. This means that on a hot day, a panel that produces 30 volts at 25°C might produce only 28.5 volts at 40°C. The current increases slightly with temperature, but the voltage drop dominates, so the overall power output decreases. This is why panels are rated at 25°C, and why a hot roof can reduce output by 10% or more.

What People Get Wrong

A common misunderstanding is that a bigger cell produces a higher voltage. Because voltage is set by the material, a larger cell produces more current, not more voltage. This leads to confusion when reading a panel’s specifications: a 400-watt panel and a 300-watt panel might have the same voltage, but the 400-watt one has a higher current rating.

Another mistake is thinking that a panel can be ‘switched off’ when it is disconnected. A PV cell generates electricity whenever there is light, regardless of whether it is connected to a load. Even a disconnected panel has voltage across its terminals, and a shaded panel can still produce enough voltage to shock. The only way to make a panel safe is to cover it completely with an opaque material.

A third error is assuming that a panel’s output is proportional to the amount of light. While it is roughly true that current is proportional to irradiance, the voltage stays relatively constant until the light drops to a very low level. This is why a panel can produce a significant voltage even on a cloudy day, but a very low current.

How Cells Are Connected

In a typical panel, cells are connected in series to build voltage. A standard 60-cell panel has three strings of 20 cells, each string producing about 10 volts. The cells are interconnected with flat metal ribbons that are soldered to the front of one cell and the back of the next. The whole assembly is laminated between glass and a backsheet for protection.

Bypass diodes are placed across groups of cells to protect against hot spots. If a cell is shaded, it stops producing current and becomes a resistance. Without a bypass diode, the current from the other cells would be forced through the shaded cell, heating it and potentially damaging it. The diode provides an alternative path, allowing current to flow around the shaded group.

Practical Implications

Understanding the cell-level physics helps in interpreting a panel’s specifications and in troubleshooting.

When a panel’s output seems lower than expected, the cause is often a mismatch between the actual operating conditions and the STC ratings. For example, a panel rated at 300 watts will rarely produce 300 watts in real life because the sun is rarely at exactly 1000 W/m² and the cell temperature is usually above 25°C.

Another practical point is the effect of dirt and debris. A layer of dust reduces the light reaching the cells, which reduces the current. The voltage is largely unaffected, so a dirty panel may still show a high voltage but a low current, which can be misleading if only voltage is measured.

Wiring Systems: Series and Parallel

Panels are combined in series to increase voltage, and in parallel to increase current. A series string of panels adds their voltages, but the current is limited by the lowest-current panel in the string. Parallel strings add currents, but the voltage is the same across all strings. In a typical residential system, panels are wired in series to reach the inverter’s minimum voltage, and multiple strings are then combined in parallel.

This arrangement has implications for shading. If one panel in a series string is shaded, the whole string’s current is reduced, and the shaded panel can become a hot spot. Bypass diodes within the panel help, but they only bypass a group of cells, not the whole panel. For this reason, some systems use microinverters or power optimizers to mitigate the effect of shading on individual panels.

Safety Considerations

A PV array produces dangerous DC voltage whenever there is light on it. The voltage from a single cell is low, but a series string can easily reach several hundred volts. Never open a combiner box, probe a live string, or disconnect an energised connector. Work on the DC side should be done by a qualified person. Roof work and mains connections are also licensed activities in most jurisdictions.

Summary of Key Points

• A PV cell is a p-n junction that separates light-generated charge carriers to create a voltage.
• Each silicon cell produces about 0.5 volts, regardless of size; larger cells produce more current.
• Panels are wired in series to build voltage, and in parallel to build current.
• Cell voltage decreases with temperature, which is why panels produce less on hot days.
• A panel cannot be switched off; it generates voltage whenever it is illuminated.
• Bypass diodes protect against hot spots caused by shading, but they do not eliminate all losses.