How Solar Panels Convert Light to Electricity

01 The Photovoltaic Effect: Light as a Carrier of Energy

The story of solar panels begins not with silicon but with a nineteen-year-old French physicist. In 1839, Edmond Becquerel observed that illuminating certain materials with light produced a small electric current. He was experimenting with electrolytic cells — electrodes immersed in conductive solutions — when he noticed that shining light on one electrode increased the voltage between the two. He had no theory of electrons to explain what he saw (the electron would not be discovered for another 58 years), but he had identified what we now call the photovoltaic effect: the direct conversion of light into electricity at the atomic level.

The photovoltaic effect is not the same as the photoelectric effect that Einstein explained in 1905. In the photoelectric effect, light ejects electrons entirely from a material's surface into vacuum or air. In the photovoltaic effect, light excites electrons within a material — specifically within a semiconductor — raising them to higher energy states and creating a separation of charge that drives current through an external circuit. The distinction matters because photovoltaic conversion happens inside a solid crystal, not at its surface, and the internal structure of that crystal is what makes the process useful.

Light arrives at a solar cell as photons — discrete packets of electromagnetic energy. Each photon carries energy proportional to its frequency, described by E = hf, where h is Planck's constant and f is the light's frequency. Visible light photons carry roughly 1.5 to 3.5 electron-volts of energy. For a photon to contribute to electrical output, its energy must exceed the semiconductor's band gap — the energy needed to lift an electron from the valence band, where it is bound to an atom, to the conduction band, where it is free to move through the crystal. For crystalline silicon, that band gap is 1.12 electron-volts, corresponding to near-infrared light at about 1,100 nanometers wavelength.

02 The Semiconductor Junction: Where the Voltage Lives

A piece of pure silicon, no matter how carefully grown, will not generate electricity from light on its own. Photons can excite electrons into the conduction band, but without a built-in electric field to push those electrons in a consistent direction, they simply recombine with their holes — the vacancies they left behind — and the energy is lost as heat. The solar cell needs an internal asymmetry, and that asymmetry is created by the p-n junction.

Silicon has four valence electrons, meaning it forms four bonds with neighboring atoms in its crystal lattice. By introducing small amounts of phosphorus — which has five valence electrons — into one region, extra unbonded electrons become available: this is n-type silicon, with an excess of negative charge carriers. By introducing boron — which has three valence electrons — into another region, bond vacancies or "holes" appear: this is p-type silicon, with an excess of positive charge carriers. Where these two regions meet, electrons diffuse from the n-type side into the p-type side and holes diffuse the other way, creating a depletion region where mobile charge carriers are absent and a built-in electric field forms across the junction.

That built-in field is the engine of the solar cell. When a photon with sufficient energy strikes the silicon within or near the depletion region, it creates an electron-hole pair. The electric field immediately sweeps the electron toward the n-type side and the hole toward the p-type side. If external wires connect the two sides, electrons flow through the circuit from the n-type to the p-type side, recombining with holes — and that flow is electric current. The built-in field of a silicon p-n junction produces an open-circuit voltage of approximately 0.5 to 0.6 volts per cell, a value set by the band gap and junction physics.

Chart 1: Average solar PV module price per watt, 2010–2024. The ~95% cost decline made solar the cheapest source of electricity in history.

03 From Cell to Panel: The Manufacturing Chain

A single photovoltaic cell produces less than a watt of power at less than a volt — enough to light an LED, not a home. To produce useful electricity, cells are connected in series and parallel arrangements inside modules, commonly called solar panels. A typical residential panel contains 60 or 72 cells wired in series, producing an open-circuit voltage of roughly 40–45 volts and a short-circuit current of 9–11 amps under standard test conditions, yielding 350–450 watts of peak output.

The manufacturing chain begins with polysilicon, ultra-pure silicon melted and recrystallized. Polysilicon is produced by the Siemens process: metallurgical-grade silicon (98% pure, made from quartz in an arc furnace) is converted to trichlorosilane gas, distilled to semiconductor purity (99.9999999%), and deposited onto silicon rods in a hydrogen atmosphere at over 1,000°C. This polysilicon is then melted and grown into single crystals using the Czochralski process: a seed crystal is dipped into the molten silicon and slowly withdrawn while rotating, pulling a cylindrical ingot of monocrystalline silicon that can weigh 200 kilograms and measure 200 millimeters in diameter.

The ingot is sawn into wafers approximately 150 micrometers thick — thinner than a human hair. Diamond-wire sawing has largely replaced older slurry-based cutting, reducing silicon waste (kerf loss) and cutting time. Each wafer is then chemically textured to reduce surface reflection, doped with phosphorus on the front surface to create the n-type layer (the wafer itself is p-type, doped with boron during crystal growth), and coated with an anti-reflective layer — typically silicon nitride deposited by plasma-enhanced chemical vapor deposition — that gives solar cells their characteristic blue or black appearance. Metal contacts, usually screen-printed silver paste on the front and aluminum on the back, are fired through the anti-reflective coating to reach the silicon and provide electrical connection points.

04 Efficiency Limits: The Shockley–Queisser Ceiling

Not every photon that strikes a solar cell becomes electricity. Some are reflected from the surface, some pass through without being absorbed, and some create electron-hole pairs that recombine before contributing to current. But even for a perfect cell that captured every photon perfectly, there is a thermodynamic limit on how much of the sun's energy can be converted to electricity. That limit was calculated by William Shockley and Hans-Joachim Queisser in 1961, and for a single-junction cell under standard sunlight it is approximately 33.7%.

The Shockley–Queisser limit arises from three fundamental losses. First, photons with energy below the band gap cannot excite electrons at all — they pass through or are absorbed as heat. For silicon's 1.12 eV gap, this means all infrared light beyond 1,100 nanometers is wasted. Second, photons with energy above the band gap can only create one electron-hole pair regardless of how much excess energy they carry — the rest is lost as heat. A blue photon at 2.5 eV produces the same current as a red photon at 1.2 eV, with the extra 1.3 eV dissipated as thermal vibration. Third, the voltage a cell can produce is always less than its band gap because of the thermodynamic requirement that the cell must radiate some energy back as light to maintain equilibrium.

Commercial monocrystalline silicon panels now achieve 22–24% efficiency — roughly two-thirds of the Shockley–Queisser limit. The gap between theoretical and practical performance comes from reflection losses, contact shading, recombination at defects and surfaces, and resistive losses in the cell. PERC (Passivated Emitter and Rear Cell) technology, which adds a passivation layer on the cell's back surface to reflect unabsorbed photons back through the cell for a second chance at absorption, pushed commercial efficiency above 22% and has become the industry standard. TOPCon and heterojunction designs are pushing toward 25–26% in mass production.

Chart 2: Conversion efficiency by solar cell technology. The Shockley–Queisser limit caps single-junction silicon cells at 33.7%.

05 Inverters and Grid Integration

Solar panels produce direct current — electrons flowing in one direction. The electrical grid, and most household appliances, run on alternating current that reverses direction 50 or 60 times per second. Bridging this gap is the inverter, a power electronic device that converts DC to AC using high-speed switching transistors. Modern inverters use insulated-gate bipolar transistors (IGBTs) or silicon carbide MOSFETs to switch thousands of times per second, synthesizing a sine wave that matches the grid's voltage, frequency, and phase.

String inverters — the dominant architecture for residential systems — connect a series string of panels to a single inverter, typically mounted near the electrical panel. The inverter performs maximum power point tracking (MPPT), continuously adjusting the operating voltage to find the point where the panel string delivers maximum power under current irradiance and temperature conditions. Microinverters, mounted behind each panel, perform MPPT individually, eliminating the mismatch losses that occur when panels in a string experience different shading or orientation. Power optimizers offer a middle ground: DC optimizers at each panel with a single central inverter. The trade-offs among these architectures involve cost, reliability, shading tolerance, and monitoring granularity.

06 The Economics of the Cheapest Electricity

The cost decline of solar photovoltaics is without precedent in industrial history. In 2010, the average module cost approximately $2.00 per watt. By 2024, that figure had fallen to about $0.11 per watt — a 95% reduction driven by massive manufacturing scale, continuous incremental process improvements, and intense competition, primarily among Chinese manufacturers who now produce over 80% of the world's solar modules. This trajectory, sometimes called the "Swanson effect" (analogous to Moore's law), has seen module prices fall by roughly 20% for every doubling of cumulative installed capacity.

The levelized cost of energy (LCOE) from utility-scale solar — the total cost per kilowatt-hour over the system's lifetime, including construction, financing, maintenance, and fuel — has fallen below $0.04 per kWh in favorable locations, cheaper than electricity from any fossil fuel source and cheaper than most wind power. This is why annual installations have grown from about 17 gigawatts in 2010 to over 400 gigawatts in 2024, with cumulative global installed capacity exceeding 2 terawatts. Solar now accounts for the largest share of new electricity capacity additions worldwide each year, and its growth rate continues to outpace forecasts.

The remaining barriers are no longer cost but intermittency, grid integration, and land use. Solar produces electricity only when the sun shines, creating a diurnal mismatch with demand that requires storage, flexible loads, or complementary generation. Battery storage — particularly lithium-ion — is falling in cost on its own learning curve and increasingly pairs with solar to provide dispatchable renewable energy. Inverters and power electronics now provide grid services like voltage support and frequency response that were once the exclusive domain of synchronous generators. The technology that Edmond Becquerel stumbled upon in a Paris laboratory in 1839 has become, nearly two centuries later, the fastest-growing and cheapest source of electricity on Earth.