How Do Solar Panels Work? A Junction That Sorts Charges
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Light hits a wafer of silicon, knocks electrons loose, and a built-in electric field inside the wafer pushes them all in one direction, which is a current. There are no moving parts, nothing is consumed, and the physics has been understood since the 1950s. What changed recently is not the science but the price, which has fallen by roughly ninety percent in little more than a decade.
The physics
A solar cell is a large semiconductor diode. Pure silicon has each atom bonded to four neighbours, with electrons held in place and unable to move freely. Doping the silicon by adding traces of other elements changes that: phosphorus supplies an extra electron per atom, giving n-type material with mobile negative charges, and boron leaves an electron short, giving p-type material with mobile positive vacancies called holes. Joining the two creates a junction across which charges diffuse and leave behind a fixed built-in electric field. When a photon with enough energy is absorbed, it lifts an electron out of a bond, creating a free electron and a hole, and the built-in field sweeps the electron one way and the hole the other before they can recombine. Collecting those at contacts on each face gives a voltage and, connected to a circuit, a current. A single silicon cell produces about half a volt regardless of size, which is why cells are wired in series into panels and panels into strings.
Why efficiency has a ceiling
Commercial silicon panels convert roughly twenty to twenty-three percent of incoming sunlight into electricity, and the limits are physical rather than a matter of manufacturing care:
- •Photons with less energy than the material's bandgap pass straight through and do nothing, which discards a large part of the infrared
- •Photons with more energy than the bandgap lift an electron and the excess is lost immediately as heat, which discards part of the blue and ultraviolet
- •Together those two effects impose the Shockley-Queisser limit, about thirty-three percent for a single-junction cell in unconcentrated sunlight, which is a theoretical ceiling no single-material cell can pass
- •Real cells lose more to reflection, to shading by the contact grid, to electrical resistance and to recombination at defects and surfaces
- •Output falls as the cell heats, by roughly a third of a percent per degree, so a hot roof produces less than a cool one at the same irradiance
- •Tandem cells stack materials with different bandgaps to catch different parts of the spectrum and exceed the single-junction limit, which is why silicon and perovskite tandems are the most watched research direction
The rest of the system
A panel alone is of limited use, because it produces direct current at a voltage that varies with light and temperature, while almost everything runs on alternating current at a fixed voltage. The inverter performs the conversion and also runs maximum power point tracking, continuously adjusting the electrical load to sit at the point on the panel's current and voltage curve that extracts the most power, which shifts constantly with conditions. Wiring panels in series means the weakest determines the string, so a single shaded panel can drag down its whole string, which is why bypass diodes are built into panels and why module-level electronics or microinverters are used where shading is unavoidable. Mounting angle and orientation matter substantially, and tracking mounts that follow the sun raise output at the cost of complexity and maintenance. Batteries store surplus for later, and the economics of adding them depend entirely on the difference between what exported electricity earns and what imported electricity costs.
The economics and the awkward parts
The cost collapse is the important fact: module prices have fallen by around ninety percent since 2010, driven by manufacturing scale, improved processes and fierce competition, with the result that new solar generation is now among the cheapest sources of electricity in much of the world on a levelised basis. The energy used to make a panel is repaid within roughly one to three years of operation, against a working life of twenty-five to thirty years or more, so the energy return is not in serious dispute. The genuine difficulties are about matching supply to demand. Solar output peaks in the middle of the day and disappears in the evening when demand rises, producing the duck-shaped daily net demand curve that grid operators now plan around, which makes storage, demand shifting and transmission the binding constraints rather than panel cost. Other real issues include the concentration of manufacturing in a small number of places, the energy intensity of producing purified silicon, and end-of-life recycling, which is technically feasible and not yet economic at scale.
The takeaway
A solar cell is a large diode in which absorbed light frees electrons and a built-in field at the junction between differently doped silicon sweeps them one way, producing a current. Efficiency is capped near thirty-three percent for a single junction because low-energy photons pass through and high-energy ones waste their excess as heat. An inverter converts to alternating current and continuously tracks the panel's maximum power point. Module prices fell around ninety percent since 2010, moving the problem to storage and timing.