Photoelectric cells are devices that convert light into electricity. They work by exploiting a fundamental property of certain materials: when photons (particles of light) strike their surface, they either knock electrons free or change the material’s ability to conduct current. This simple principle powers everything from solar panels on rooftops to the automatic doors at your grocery store.
How Photoelectric Cells Work
Every photoelectric cell relies on the same core physics. When light hits a semiconductor material, photons transfer their energy to electrons inside that material. If a photon carries enough energy, it frees an electron from its position, creating both a loose electron and a “hole” where that electron used to be. This electron-hole pair is the foundation of every photoelectric device.
The critical detail is that not just any light will do. Each material has a threshold frequency, a minimum energy level the incoming light must meet before it can knock electrons loose. Light below that threshold won’t eject a single electron no matter how bright it is. Above that threshold, brighter light frees more electrons, and higher-energy photons give those electrons more kinetic energy. Albert Einstein described this relationship in a Nobel Prize-winning equation: the energy an ejected electron carries equals the photon’s energy minus the binding energy (also called the work function) holding that electron in the material.
There’s an efficiency ceiling built into this process. Photons with less energy than the material’s threshold pass right through without doing anything useful. Photons with more energy than needed do free electrons, but the excess energy just converts to heat. This is why no single-material cell can capture 100% of sunlight’s energy.
Three Main Types
Photoelectric cells come in three broad categories, each using a slightly different mechanism to turn light into a useful electrical signal.
Photovoltaic Cells
These generate voltage directly when light hits them, with no external power source needed. The cell operates at zero bias, meaning no outside voltage pushes current through it. Sunlight creates electron-hole pairs at the junction between two semiconductor layers, and the built-in electric field at that junction drives current through an external circuit. Solar panels are the most familiar example.
Photoconductive Cells
Instead of generating their own voltage, photoconductive cells change their electrical resistance when exposed to light. Photons hitting the semiconductor free electrons to flow, which drops the material’s resistance. An external circuit supplies voltage, and the changing resistance modulates the current. Light-dependent resistors in streetlights and camera light meters use this principle. The brighter the light, the lower the resistance, and the more current flows.
Photoemissive Cells
These are the most direct application of Einstein’s photoelectric effect. Photons striking a metal surface physically eject electrons into a vacuum, and a positively charged plate collects them to form a current. Photoemissive cells are less common in consumer products but remain important in scientific instruments and light-detection equipment where extreme sensitivity matters.
Materials That Make Them Work
Silicon dominates the photoelectric cell market, accounting for roughly 95% of solar modules sold today. It’s abundant, well understood, and can be manufactured in crystalline wafers or thinner polycrystalline sheets. Most rooftop solar panels use silicon-based cells.
Beyond silicon, two thin-film semiconductors have carved out significant market share. Cadmium telluride is the second most common photovoltaic material and can be manufactured at lower cost than silicon. Copper indium gallium diselenide offers high efficiency in a thin, flexible format. Both are used in commercial solar panels, particularly in large-scale installations where weight and flexibility matter.
More specialized applications use different materials entirely. Organic photovoltaic cells built from carbon-rich compounds can be tuned for specific properties like transparency or color, opening up possibilities like solar windows. Multijunction cells stack multiple semiconductor layers (typically from columns III and V of the periodic table) to capture different wavelengths of light, pushing efficiency higher than any single material can achieve. The tradeoff is cost: these cells are primarily used in satellites and concentrated solar systems where maximum output per square centimeter justifies the expense.
What Different Cells Can Detect
Not all photoelectric cells respond to the same wavelengths. The material’s band gap determines which portion of the light spectrum it can convert into electricity. Silicon-based detectors work well from about 400 to 1,150 nanometers, covering visible light and the near-infrared range. Gallium indium arsenide detectors extend that range out to around 1,300 nanometers, deeper into the infrared. Specialized thermophotovoltaic cells can respond to wavelengths from 400 all the way to 2,800 nanometers, capturing energy from heat radiation that silicon cells would miss entirely.
This spectral range matters for practical applications. A photoelectric cell designed to detect visible light for a camera sensor needs different materials than one designed to harvest infrared radiation from industrial heat sources. Pyroelectric detectors, which respond to temperature changes rather than photons directly, offer a flat response from ultraviolet through far infrared, making them useful as calibration references.
Modern Efficiency Levels
The efficiency of a photoelectric cell tells you what percentage of incoming light energy it converts to electricity. For standard silicon technology, the current record sits at 27.81%, achieved by a hybrid interdigitated back contact cell from Longi. Other high-performance silicon designs cluster close behind: heterojunction back contact cells at 27.3%, TOPCon cells between 26.7% and 27.1%, and PERC cells (the workhorse of today’s residential solar) at 24.1%.
Newer materials are closing the gap. Halide perovskite cells have reached 26.9% in the lab, remarkable for a technology that barely existed a decade ago. Perovskite mini-modules, which are closer to real-world products, have hit 23.7%. Kesterite cells, made from more earth-abundant elements, trail at 14.1% but continue improving.
These numbers represent lab records under controlled conditions. The panels on your roof typically convert 18% to 22% of sunlight into electricity. The gap between lab and rooftop comes from real-world factors: temperature fluctuations, dust, wiring losses, and the difference between a small champion cell and a full-sized production module.
Common Everyday Uses
Solar energy generation is the most visible application, but photoelectric cells show up in places you might not expect. Automatic doors use photoemissive or photoconductive sensors to detect when someone breaks a light beam. Smoke detectors rely on photoelectric cells that register when smoke particles scatter a light beam onto a sensor. Light meters in cameras measure scene brightness to set the correct exposure. Industrial sorting machines use arrays of photoelectric sensors to detect objects on conveyor belts at high speed.
In infrastructure, photoelectric cells power remote weather stations, highway signs, and telecommunications equipment in locations where running power lines would be impractical. Satellites depend almost exclusively on high-efficiency multijunction photovoltaic cells, where every watt per kilogram counts. Even calculators with small solar strips on their face use amorphous silicon photoelectric cells to supplement or replace batteries.